Strains applied to bone can stimulate its development and adaptation. High strains and rates of strain are thought to be osteogenic, but the specific dose response relationship is not known. In vivo human strain measurements have been performed in the tibia to try to identify optimal bone strengthening exercises for this bone, but no measurements have been performed in the distal radial metaphysis, the most frequent site of osteoporotic fractures. Using a strain gauged bone staple, in vivo dorsal metaphyseal radial strains and rates of strain were measured in ten female patients during activities of daily living, standard exercises and falls on extended hands. Push-ups and falling resulted in the largest compression strains (median 1345 to 3146 με, equivalent to a 0.1345% to 0.3146% length change) and falling exercises in the largest strain rates (18 582 to 45 954 με/s). On the basis of their high strain and/or strain rates these or variations of these exercises may be appropriate for distal radial metaphyseal bone strengthening

The objective of this study was to use patient-specific finite element modeling to measure the 3D interfragmentary strain environment in clinically realistic fractures. The hypothesis was that in the early post-operative period, the tissues in and around the fracture gap can tolerate a state of strain in excess of 10%, the classical limit proposed in the Perren strain theory. Eight patients (6 males, 2 females; ages 22–95 years) with distal femur fractures (OTA/AO 33-A/B/C) treated in a Level I trauma center were retrospectively identified. All were treated with lateral bridge plating. Preoperative computed tomography scans and post-operative X-rays were used to create the reduced fracture models. Patient-specific materials properties and loading conditions (20%, 60%, and 100% body weight (BW)) were applied following our published method.[1]. Elements with von Mises strains >10% are shown in the 100% BW loading condition. For all three loading scenarios, as the bridge span increased, so did the maximum von Mises strain within the strain visualization region. The average gap closing (Perren) strain (mean ± SD) for all patient-specific models at each body weight (20%, 60%, and 100%) was 8.6% ± 3.9%, 25.8% ± 33.9%, and 39.3% ± 33.9%, while the corresponding max von Mises strains were 42.0% ± 29%, 110.7% ± 32.7%, and 168.4% ± 31.9%. Strains in and around the fracture gap stayed in the 2–10% range only for the lowest load application level (20% BW). Moderate loading of 60% BW and above caused gap strains that far exceeded the upper limit of the classical strain rule (<10% strain for bone healing). Since all of the included patients achieved successful unions, these findings suggest that healing of distal femur fractures may be robust to localized strains greater than 10%

Abstract. Objectives. Ultrasound speckle tracking is a safe and non-invasive diagnostic tool to measure soft tissue deformation and strain. In orthopaedics, it could have broad application to measure how injury or surgery affects muscle, tendon or ligament biomechanics. However, its application requires custom tuning of the speckle-tracking algorithm then validation against gold-standard reference data. Implementing an experiment to acquire these data takes months and is expensive, and therefore prohibits use for new applications. Here, we present an alternative optimisation approach that automatically finds suitable machine and algorithmic settings without requiring gold-standard reference data. Methods. The optimisation routine consisted of two steps. First, convergence of the displacement field was tested to exclude the settings that would not track the underlying tissue motion (e.g. frame rates that were too low). Second, repeatability was maximised through a surrogate optimisation scheme. All settings that could influence the strain calculation were included, ranging from acquisition settings to post-processing smoothing and filtering settings, totalling >1,000,000 combinations of settings. The optimisation criterion minimised the normalised standard deviation between strain maps of repeat measures. The optimisation approach was validated for the medial collateral ligament (MCL) with quasi-static testing on porcine joints (n=3), and dynamic testing on a cadaveric human knee (n=1, female, aged 49). Porcine joints were fully dissected except for the MCL and loaded in a material-testing machine (0 to 3% strain at 0.2 Hz), which was captured using both ultrasound (>14 repeats per specimen) and optical digital image correlation (DIC). For the human cadaveric knee (undissected), 3 repeat ultrasound acquisitions were taken at 18 different anterior/posterior positions over the MCL while the knee was extended/flexed between 0° and 90° in a knee extension rig. Simultaneous optical tracking recorded the position of the ultrasound transducer, knee kinematics and the MCL attachments (which were digitised under direct visualisation post testing). Half of the data collected was used for optimisation of the speckle tracking algorithms for the porcine and human MCLs separately, with the remaining unseen data used as a validation test set. Results. For the porcine MCLs, ultrasound strains closely matched DIC strains (R. 2. > 0.98, RMSE < 0.59%) (Figure 1A). For the human MCL (Figure 1B), ultrasound strains matched the strains estimated from the optically tracked displacements of the MCL attachments. Furthermore, strains developed during flexion were highly correlated with AP position (R = 0.94) with strains decreasing the further posterior the transducer was on the ligament. This is in line with previously reported length change values for the posterior, intermediate and anterior bundles of the MCL. Conclusions. Ultrasound speckle tracking algorithms can be adapted for new applications without ground-truth data by using an optimisation approach that verifies displacement field convergence then minimises variance between repeat measurements. This optimisation routine was insensitive to anatomical variation and loading conditions, working for both porcine and human MCLs, and for quasi-static and dynamic loading. This will facilitate research into changes in musculoskeletal tissue motion due to abnormalities or pathologies. Declaration of Interest. (a) fully declare any financial or other potential conflict of interest

Background. In vivo evaluation of IVD strains is crucial to better understand normal and pathological IVD mechanics, and to evaluate the effectiveness of treatments. This study aimed to 1) develop a novel in vivo technique based on 3T MRI and digital volume correlation (DVC) to measure strains within IVDs and 2) to use this technique to resolve 3D strains within IVDs of healthy volunteers during extension. Methods. This study included 40 lumbar IVDs from eight healthy subjects. The optimal MR sequence to minimise DVC uncertainties was identified by scanning one subject with four different sequences: CISS, T1VIBE, T2SPACE, and T2TSE. To assess the repeatability of the strain measurements in spines with different anatomical and morphological variations four subjects were scanned with the optimal sequence, and uncertainties of the strain measurements were quantified. Additionally, to calculate 3D strains during extension, MRIs were acquired from six subjects in both the neutral position and after full extension. Results. Measurement errors were lowest when using the T2TSE sequence (precision=0.33 ± 0.10%, accuracy=0.48 ± 0.11%). The largest average maximum tensile and shear strains were seen at the L2-L3 level in all volunteers (7.2 ± 1.5% and 6.8 ± 1.1%, respectively), while the L5-S1 level experienced the lowest average tensile and shear strains (3.5 ± 1.0% and 3.9 ± 0.7%, respectively). Conclusion. The findings of this study establish clinical MRI-based DVC (MRI-DVC) as a new tool for in vivo strain measurement within human IVDs. MRI-DVC successfully provided internal strain distributions within IVDs and has great potential to be used for a wide range of clinical applications. Conflict of interest: No conflicts of interest. Source of funding: This work was supported by the EPSRC, New Investigator Award, EP/V029452/1

Silk fibroin (SF) has been used as a scaffold for cartilage tissue engineering. Different silkworms strain produced different protein. Also, molecular weight of SF depends on extraction method. We hypothesised that strain of silkworm and method of SF extraction would effect biological properties of SF scaffold. Therefore, cell viability and chondrogenic gene expression of human chondrogenic progenitor cells (HCPCs) treated with SF from 10 silkworm strains and two common SF extraction methods were investigate in this study. Twenty g of 10 strains silk cocoons were separately degummed in 0.02M Na2CO3 solution and dissolved in 100๐C for 30 minutes. Half of them were then dissolved in CaCl2/Ethanol/H2O [1:2:8 molar ratio] at 70±5๐C (method 1) and other half was dissolved in 46% w/v CaCl2 at 105±5๐C (method 2) for 4 hours. HCPCs were cultured in SF added cultured medial according to strain and extraction method. Cell viability at day 1, 3, and 7, were determined. Expression of collagen I, collagen II, and aggrecan at day 7 and 14, was studied. All experiment were done in triplicated samples. Generally, method 1 SF extraction showed higher cell viability in all strains. Cell viability from Nanglai Saraburi, Laung Saraburi and Nangtui strains were higher than those without SF in every time point while Wanasawan and J108 had higher viability at day 1 and decreased by time. Expression in collagen 1, collagen 2 and aggrecan in method 1 are higher at day 7 and day 14. Collagen 1 expression was highest in Nangnoi Srisaket, followed by Laung Saraburi and Nanglai Saraburi in day 7. Nangnoi Srisaket also had highest expression at day 14, followed by Nanglai Saraburi and Laung Saraburi respectively. Nangseaw had highest collagen 2 expression, follow by Laung Saraburi and Nangnoi Srisaket respectively. Higher aggrecan gene expression of Tubtimsiam, Wanasawan, UB 1 and Nangnoi Srisaket was observed at day 7 and increased expression of all strains at day 14. SF extraction using CaCl2/Ethanol/H2O offered better cell viability and chondrogenic expression. Nangseaw, Laung Saraburi and Nangnoi Srisaket strains expressed more chondrogenic phenotype

Biomechanical stability is important for fracture healing. With standard plate and screw constructs, longer plates with screws well spaced, near and far from the fracture site, are biomechanically superior. Newer locked plates have been shown to be superior to conventional plating for difficult fractures. The ideal screw configuration for fixation with locked plates has yet to be addressed. This study investigates the effects of screw position on construct stiffness as well as strain in both the plate and bone during fixation of a diaphyseal comminuted fracture using a locking plate with bicortical fixation. A composite cylinder (Sawbones) was machined to produce two models:. (a) comminuted model (4mm gap) and. (b) whole model (no gap) to simulate the remodelling phase. Five strain gauges were mounted to the bone models and one between the center holes of the locking plate. Four different configurations of screw number and position were evaluated using a twelve-hole locking plate (Smith & Nephew Perilock). Plate holes were numbered on each side of the gap from one to six. Screw configuration 654321, 621, 654 and 321 were tested in four-point bending on an MTS 858 Mini-Bionix. Force (N) and displacement (mm) as well as strain readings were recorded at 10 Hz. Plate strain in the gap model did not vary significantly for the different configurations. Construct stiffness of the 654 model (all screws far from gap) showed a 30% decrease in stiffness as compared to other screw configurations (p< 0.001). In the whole bone model, the maximal bone strain was outside the farthest screw from the center of the plate (stress shielding) and bone strain at the fracture site in 654 was significantly higher than in 621 (p< 0.001). Results showed that three screw fixation produced similar construct stiffness to a six screw construct when well spaced. Three screws placed far from the fracture gap (654) as compared to three screws evenly spaced (621) showed decreased stability in the comminuted model but resulted in increased bone strain at the fracture site in the whole bone model. All configurations produced similar plate strain

Aims. Periprosthetic hip fractures (PPFs) after total hip arthroplasty are difficult to treat. Therefore, it is important to identify modifiable risk factors such as stem selection to reduce the occurrence of PPFs. This study aimed to clarify differences in fracture torque, surface strain, and fracture type analysis between three different types of cemented stems. Methods. We conducted biomechanical testing of bone analogues using six cemented stems of three different types: collarless polished tapered (CPT) stem, Versys Advocate (Versys) stem, and Charnley-Marcel-Kerboull (CMK) stem. Experienced surgeons implanted each of these types of stems into six bone analogues, and the analogues were compressed and internally rotated until failure. Torque to fracture and fracture type were recorded. We also measured surface strain distribution using triaxial rosettes. Results. There was a significant difference in fracture torque between the three stem types (p = 0.036). Particularly, the median fracture torque for the CPT stem was significantly lower than that for the CMK stem (CPT vs CMK: 164.5 Nm vs 200.5 Nm; p = 0.046). The strain values for the CPT stem were higher than those for the other two stems at the most proximal site. The fracture pattern of the CPT and Versys stems was Vancouver type B, whereas that of the CMK stem was type C. Conclusion. Our study suggested that the cobalt-chromium alloy material, polished surface finish, acute-square proximal form, and the absence of a collar may be associated with lower fracture torque, which may be related to PPF. Cite this article: Bone Joint Res 2022;11(5):270–277

Abstract. OBJECTIVES. An unresolved challenge in osteoarthritis research is characterising the localised intra-tissue mechanical response of articular cartilage. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) and digital volume correlation (DVC) permit non-destructive visualisation of three-dimensional (3D) strain fields in human articular cartilage. METHODS. Human articular cartilage specimens were harvested from the knee (n=4 specimens from 2 doners), mounted into a loading device and imaged in the loaded and unloaded state using a micro-CT scanner. Strain was calculated throughout the volume of the cartilage using the CT image data. RESULTS. Strain was calculated in the 3D volume with a spatial resolution of 75 µm, and the volumetric DVC calculated strain was within 5% of the known applied stain. Variation in strain distribution between the superficial, middle and deep zones was observed, consistent with the different architecture of the material in these locations. CONCLUSIONS. The DVC method is suitable for calculating strain in human articular cartilage. This method will be useful to generate chondral repair scaffolds that that seek to replicate the strain gradient in cartilage. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Introduction and Objective. The patients with a total hip arthroplasty is growing in world manly in Europe and USA, and this solution present a high success at 10years in several orthopaedic registers. The application of total press-fit hip fixation presents the most used solution, but presents some failures associated to the acetabular component fixation, associated to the load transfer and bone loss at long term. The aim of this work is to investigate the influence of different acetabular bone loss in the strain distribution in iliac bone. To evaluate implant fixation, an experimental study was performed using acetabular press-fit component simulating different acetabular bone loss and measuring the strain distribution. Materials and Methods. The experimental samples developed was based in an iliac bone model of Sawbones supplier and a acetabular component Titanium (Stryker) in a condition press-fit fixation and was implanted according surgical procedure with 45º inclination angle and 20º in the anteversion angle. Were developed five models with same initial bone, one with intact condition simulating the cartilage between bones and four with different bone loss around the acetabular component. These four models representing the evolution of bone support of acetabular components presented in the literature. The evolution of bone loss was imposed with a CAD CAM process in same iliac bone model. The models were instrumented with 5 rosettes in critical region at the cortical bone to measure the strain evolution along the process. Results. The results of strain gauges present the influence of acetabular component implantation, reducing the bone strains and presented the effect of the strain shielding. The acetabular component works as a shield in the load transfer. The critical region is the posterior region with highest principal strains and the strain effect was observed with different bone loss around acetabular component. The maximum value of principal strain was observed in the intact condition in the anterior region, with 950μ∊. In the posterior superior region, the effect of bone loss is more important presenting a reduction of 500% in the strains. The effect of bone loss is presented in the strains induced with acetabular implantation, in the first step of implantation the maximum strain was 950μ∊ and in the last model the value was 50μ∊, indicating lower press-fit fixation. Conclusions. The models developed allows study the effect of bone loss and acetabular implant fixation in the load transfer at the hip articulation. The results presented a critical region as the anterior-superior and the effect of strain shielding was observed in comparison with intact articulation. The results of press-fit fixation present a reduction of implant stability along bone loss. The process of bone fixation developed present some limitation associated to the bone adhesion in the interface, not considered. Acknowledgement. This work was supported by POCI-01-0145-FEDER-032486,– FCT, by the FEDER, with COMPETE2020 - (POCI), FCT/M

Objectives. Elevated proximal tibial bone strain may cause unexplained pain, an important cause of unicompartmental knee arthroplasty (UKA) revision. This study investigates the effect of tibial component alignment in metal-backed (MB) and all-polyethylene (AP) fixed-bearing medial UKAs on bone strain, using an experimentally validated finite element model (FEM). Methods. A previously experimentally validated FEM of a composite tibia implanted with a cemented fixed-bearing UKA (MB and AP) was used. Standard alignment (medial proximal tibial angle 90°, 6° posterior slope), coronal malalignment (3°, 5°, 10° varus; 3°, 5° valgus), and sagittal malalignment (0°, 3°, 6°, 9°, 12°) were analyzed. The primary outcome measure was the volume of compressively overstrained cancellous bone (VOCB) < -3000 µε. The secondary outcome measure was maximum von Mises stress in cortical bone (MSCB) over a medial region of interest. Results. Varus malalignment decreased VOCB but increased MSCB in both implants, more so in the AP implant. Varus malalignment of 10° reduced the VOCB by 10% and 3% in AP and MB implants but increased the MSCB by 14% and 13%, respectively. Valgus malalignment of 5° increased the VOCB by 8% and 4% in AP and MB implants, with reductions in MSCB of 7% and 10%, respectively. Sagittal malalignment displayed negligible effects. Well-aligned AP implants displayed greater VOCB than malaligned MB implants. Conclusion. All-polyethylene implants are more sensitive to coronal plane malalignments than MB implants are; varus malalignment reduced cancellous bone strain but increased anteromedial cortical bone stress. Sagittal plane malalignment has a negligible effect on bone strain. Cite this article: I. Danese, P. Pankaj, C. E. H. Scott. The effect of malalignment on proximal tibial strain in fixed-bearing unicompartmental knee arthroplasty: A comparison between metal-backed and all-polyethylene components using a validated finite element model. Bone Joint Res 2019;8:55–64. DOI: 10.1302/2046-3758.82.BJR-2018-0186.R2

In-situ assessment of collateral ligaments strain could be key to improving total knee arthroplasty outcomes by improving the ability of surgeons to properly balance the knee intraoperatively. Ultrasound (US) speckle tracking methods have shown promise in their capability to measure in-situ soft tissue strain in large tendons but prior work has also highlighted the challenges that arise when attempting to translate these approaches to the in-situ assessment of collateral ligaments strain. Therefore, the aim of this project was to develop and validate an US speckle tracking method to specifically assess in-situ strains of both the MCL and LCL. We hypothesize that coefficients of determination (R. 2. ) would be above 0.90 with absolute differences below 0.50% strain for the comparison between US-based and the reference strain, with better results expected for the LCL compared with the MCL. Five cadaveric legs with total knee implants (NH019 2017-02-03) were submitted to a varus (LCL) and valgus (MCL) ramped loading (0 – 40N). Ultrasound radiofrequency (rf) data and reference surface strains data, obtained with 3D digital image correlation (DIC), were collected synchronously. Prior to processing, US data were qualitatively assessed and specimens displaying substantial imaging artefacts were discarded, leaving five LCL and three MCL specimens in the analysis. Ultrasound rf data were processed in Matlab (The MathWorks, Inc., Natick, MA) with a custom-built speckle tracking approach adapted from a method validated on larger tendons and based on normalized cross-correlation. Digital image correlation data were processed with commercial software VIC3D (Correlated Solutions, Inc., Columbia, SC). To optimize speckle tracking, several tracking parameters were tested: kernel and search window size, minimal correlation coefficient and simulated frame rate. Parameters were ranked according to three comparative measures between US- and DIC-based strains: R. 2. , mean absolute error and strains differences at 40N. Parameters with best average rank were considered as optimal. To quantify the agreement between US- and DIC-based strain of each specimen, the considered metrics were: R. 2. , mean absolute error and strain differences at 40N. The LCL showed a good agreement with a high average R. 2. (0.97), small average mean absolute difference (0.37%) and similar strains at 40N (DIC = 2.92 ± 0.10%; US = 2.99 ± 1.16%). The US-based speckle tracking method showed worse performance for the MCL with a lower average correlation (0.55). Such an effect has been observed previously and may relate to the difficulty in acquiring sufficient image quality for tracking the MCL compared to the LCL, which likely arises due to structural or mechanical differences; notably MCL is larger, thinner, more wrapped around the bone and stretches less. However, despite these challenges, the MCL tracking still showed small average mean absolute differences (0.44%) and similar strains at 40N (DIC = 1.48 ± 0.06%; US = 1.44 ± 1.89%). We conclude that the ultrasound speckle tracking method developed is ready to be used as a tool to assess in-situ strains of LCL. Concerning the MCL strain assessment, despite some promising results in terms of strain differences, further work on acquisition could be beneficial to reach similar performance

Aim. Staphylococcus aureus is the leading pathogen in fracture-related infection (FRI). Virulence factors vary between different strains, which may have a decisive influence on the course of infection. Previous in vitro experiments, in vivo testing in wax moth larvae, and genomic analysis of S. aureus isolates from FRI identified a low- and high-virulent strain. These findings correlated with the acute course of FRI induced by the high-virulent pathogen, whereas the low-virulent strain caused a chronic FRI in its human host. However, the role of bacterial virulence in FRI is not completely understood. Therefore, the present study aimed to compare the identified high- and low-virulent S. aureus isolates in a murine FRI model. Method. Skeletally mature C57Bl/6N mice received a femoral osteotomy stabilized by titanium locking plates. FRI was established by inoculation of either high-virulent S. aureus EDCC 5458 or low-virulent S. aureus EDCC 5464 in the fracture gap. Mice were euthanized 4 and 14 days after surgery, respectively. Severity and progression of infection were assessed in terms of clinical presentation, quantitative bacteriology, semiquantitative histopathologic evaluation, and serum cytokine profile. Results. Quantitative bacteriological results 4 days after surgery revealed a higher bacterial load in soft tissue samples in high-virulent infected animals (p =0.026). Mice infected with the high-virulent strain also displayed higher rates of organ dissemination (24/36 organs in high-virulent, versus 5/36 organs in low-virulent infected animals; p <0.0001). In the histopathological assessment, bacterial agglomerations at the fracture ends were present to a greater extent in the high-virulent cohort and barely detectable in low-virulent infected mice. In both cohorts, no bone healing was observed after 4 days. On day 14, bone healing at the fracture site was visible in low-virulent infected animals, whereas callus formation was observed in only one animal from the high-virulent infected cohort. Furthermore, osteonecrosis and osteolysis were increased in high-virulent infected animals. Regarding serum cytokines, innate immune markers were elevated in both groups at day 4. By day 14, a more pronounced proinflammatory response indicated by increased serum cytokine levels of IFN-γ, IL-1β, and IL-6 was observed in high-virulent infected animals. Conclusions. The present study demonstrated distinct bacteriological and histopathological differences between two different virulent S. aureus strains previously shown to have different courses in human patients. While host physiology is often considered to have a major impact on the course of FRI, this study highlights the critical influence of the invading pathogen and its virulence characteristics

Objectives. Unicompartmental knee arthroplasty (UKA) is a demanding procedure, with tibial component subsidence or pain from high tibial strain being potential causes of revision. The optimal position in terms of load transfer has not been documented for lateral UKA. Our aim was to determine the effect of tibial component position on proximal tibial strain. Methods. A total of 16 composite tibias were implanted with an Oxford Domed Lateral Partial Knee implant using cutting guides to define tibial slope and resection depth. Four implant positions were assessed: standard (5° posterior slope); 10° posterior slope; 5° reverse tibial slope; and 4 mm increased tibial resection. Using an electrodynamic axial-torsional materials testing machine (Instron 5565), a compressive load of 1.5 kN was applied at 60 N/s on a meniscal bearing via a matching femoral component. Tibial strain beneath the implant was measured using a calibrated Digital Image Correlation system. Results. A 5° increase in tibial component posterior slope resulted in a 53% increase in mean major principal strain in the posterior tibial zone adjacent to the implant (p = 0.003). The highest strains for all implant positions were recorded in the anterior cortex 2 cm to 3 cm distal to the implant. Posteriorly, strain tended to decrease with increasing distance from the implant. Lateral cortical strain showed no significant relationship with implant position. Conclusion. Relatively small changes in implant position and orientation may significantly affect tibial cortical strain. Avoidance of excessive posterior tibial slope may be advisable during lateral UKA. Cite this article: A. M. Ali, S. D. S. Newman, P. A. Hooper, C. M. Davies, J. P. Cobb. The effect of implant position on bone strain following lateral unicompartmental knee arthroplasty: A Biomechanical Model Using Digital Image Correlation. Bone Joint Res 2017;6:522–529. DOI: 10.1302/2046-3758.68.BJR-2017-0067.R1

INTRODUCTION. The magnitude of principal strain is indicative of the risks of femoral fracture,. 1,2. while changes in femoral strain energy density (SED) after total hip arthroplasty (THA) have been associated with bone remodeling stimulus. 3. Although previous modeling studies have evaluated femoral strains in the intact and implanted femur under walking loads through successfully predicting physiological hip contact force and femoral muscle forces,. 1,2,3. strains during ‘high load’ activities of daily living have not typically been evaluated. Hence, the objective of this study was to compare femoral strain between the intact and the THA implanted femur under peak loads during simulated walking, stair descent, and stumbling. METHODS. CTs of three cadaveric specimens were used to develop finite element (FE) models of intact and implanted femurs. Implanted models included a commercially-available femoral stem (DePuy Synthes, Warsaw, IN, USA). Young's moduli of the composite bony materials were interpolated from Hounsfield units using a CT phantom and established relationships. 4. Peak hip contact force and femoral muscle forces during walking and stair descent were calculated using a lower extremity musculoskeletal model. 5. and applied to the femur FE models (Fig. 1). While maintaining the peak hip contact forces, muscle forces were further adjusted using an iterative optimization approach in FE models to reduce the femur deflection to the reported physiological range (< 5 mm). 2. Femoral muscle forces during stumbling were estimated utilizing the same optimization approach with literature-reported hip contact forces as input. 6. Maximum and minimum principal strains were calculated for each loading scenario. Changes in SED between intact and THA models were calculated in bony elements around the stem. RESULTS. As expected, high loads during stumbling resulted in the highest peak principal strains along femoral diaphysis (THA: 3179±523 and −4559±629 με; intact: 4232±818 and −5853±204 με) compared to stair descent and typically evaluated gait loads (THA: 1741±363 and −1893±76 με; intact: 2256±887 and −2509±493 με; Fig. 2). Principal strains in THA models peaked close to the tip of the femoral stem across three activities, compared with proximally located peak principal strains in the intact models (Fig. 2). Bony elements located medially and laterally to the femoral stem showed decreased SED after THA, while increased SED was observed in elements distal to the femoral stem (Fig. 3). DISCUSSION. Using appropriately distributed muscle forces, our model predicted similar peak principal strains and SED differences compared with reported values during walking (peak principal strains: ±1500 to ±2000 με. 1,2. ; SED differences: ± 0.02 MPa. 3. ). In addition to the close to failure level principal strains, stumbling showed the most noticeable changes in SED compared with the other two activities. Results suggest iterative bone remodeling simulations should include a composite of activities-of-daily-living loading conditions as well as appropriately distributed muscle forces. For any figures or tables, please contact authors directly

Abstract. OBJECTIVES. To determine if force measured using a strain gauge in circular external fixation frames is different for 1) different simulated stages of bone healing, and for 2) fractures clinically deemed either united or un-united. METHODS. In a laboratory study, 3 similar Ilizarov frame constructs were assembled using a Perspex bone analogue. Constructs were tested in 10 different clinical situations simulating different stages of bone healing including with the bone analogue intact, with 1,3 and 50mm gaps, and with 6 materials of varying stiffness's within the 50mm gap. A Bluetooth strain gauge was inserted across the simulated fracture focus, replacing one of the 4 threaded rods used to construct the frame. Constructs were loaded to 700N using an Instron testing machine and maximum force during loading was measured by the strain gauge. Testing was repeated with the strain gauge replacing each of the 4 threaded rods in turn, with measurements being repeated 3 times, across all 3 frame constructs for all 10 simulated clinical situations (n=360). Force measurements between the situations were compared using a Kruskal-Wallis test (KW) and a post-hoc Steel test was used for multiple comparison against control (intact bone model). Additionally, a pilot study has been initiated to assess clinical efficacy of the strain gauge measurement in patients with circular frames. The strain gauge replaced the anterior rod across the fracture focus for each patient. Patients were asked to step on a weighing scale with their affected limb, and maximum weight transfer through the limb and maximal force measured in the frame were recorded. This was repeated 3 times and a mean ratio of force to weight through affected limb was calculated for each patient. The clinical situation at each measurement was designated as united or un-united by one of the senior authors for analysis. Force measurements between the situations were compared using a Wilcoxon-Mann-Whitney test. RESULTS. In the laboratory study, including all constructs with the strain gauge in all positions, a statistically significant relationship between model stability and force measured was identified (KW test for overall relationship p<0.0001). The largest force was measured in the model with a 50mm gap (median 170N, IQR 155–192, range 83–213) and the smallest in the intact bone model (median 3N, IQR 1–8, range 0–11). Multiple comparison testing found a significant difference between intact bone and all the unstable situations (p=0.002 or better). Examining initial results from our pilot clinical study, nine measurements were available in seven patients. Three of these were taken in patients with fractures yet to unite, six in patients where union has since been confirmed clinically. The median force measured was significantly greater where the fracture was not united (median 1.66 N/kg, range 1.07–1.99 vs 0.12 N/kg, range 0.05–0.73, p=0.02). CONCLUSIONS. This laboratory study demonstrates that force measurement may be different at different stages of healing, and although only limited data was available, a pilot clinical study showed a significant relationship between the force measured and clinical union of the patient's fracture. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Aims. This study aimed to identify the effect of anatomical tibial component (ATC) design on load distribution in the periprosthetic tibial bone of Koreans using finite element analysis (FEA). Methods. 3D finite element models of 30 tibiae in Korean women were created. A symmetric tibial component (STC, NexGen LPS-Flex) and an ATC (Persona) were used in surgical simulation. We compared the FEA measurements (von Mises stress and principal strains) around the stem tip and in the medial half of the proximal tibial bone, as well as the distance from the distal stem tip to the shortest anteromedial cortical bone. Correlations between this distance and FEA measurements were then analyzed. Results. The distance from the distal stem tip to the shortest cortical bone showed no statistically significant difference between implants. However, the peak von Mises stress around the distal stem tip was higher with STC than with ATC. In the medial half of the proximal tibial bone: 1) the mean von Mises stress, maximum principal strain, and minimum principal strain were higher with ATC; 2) ATC showed a positive correlation between the distance and mean von Mises stress; 3) ATC showed a negative correlation between the distance and mean minimum principal strain; and 4) STC showed no correlation between the distance and mean measurements. Conclusion. Implant design affects the load distribution on the periprosthetic tibial bone, and ATC can be more advantageous in preventing stress-shielding than STC. However, under certain circumstances with short distances, the advantage of ATC may be offset. Cite this article: Bone Joint Res 2022;11(5):252–259

Introduction. The ankle cartilage has an important function in walking movements, mainly in sports; for active young people, between 20 and 30 years old, the incidence of osteochondral lesions is more frequent. They are also more frequent in men, affecting around 21,000 patients per year in USA with 6.5% of ankle injuries generating osteochondral lesions. The lesion is a result of ankle sprain and is most frequently found in the medial location, in 53% of cases. The main objective of this work was to develop an experimental and finite element models to study the effect of the ankle osteochondral lesion on the cartilage behavior. Materials and Methods. The right ankle joint was reconstructed from an axial CT scan presenting an osteochondral lesion in the medial position with 8mm diameter in size. An experimental model was developed, to analyze the strains and influence of lesion size and location similar to the patient. The experimental model includes two cartilages constructed by Polyjet™ 3D printing from rubber material (young modulus similar to cartilage) and bone structures from a rigid polymer. The cartilage was instrumented with two rosettes in the medial and lateral regions, near the osteochondral region. The fluid considered was water at room temperature and the experimental test was run at 1mm/s. The Finite element model (FE) includes all the components considered in the experimental apparatus and was assigned the material properties of bone as isotropic and linear elastic materials; and the cartilage the same properties of rubber material. The fluid was simulated as hyper-elastic one with a Mooney-Rivlin behavior, with constants c1=0.07506 and c2=0.00834MPa. The load applied was 680N in three positions, 15º extension, neutral and 10º flexion. Results. The experimental strain measured in the cartilage in the rosettes presents similar behavior in all experiments and repetitions. The maximum value observed near the osteochondral lesion was 3014(±5.6)µε in comparison with the intact condition it was 468 (±1.95)µε. The osteochondral lesion increases the strains around 6.5 times and the synovial liquid reduces the intensity of strain distribution. The numerical model presents a good correlation with the experiments (R2 0.944), but the FE model underestimates the values. Discussion and conclusion. As a first conclusion, the size of the osteochondral lesion is important for the strains developed in cartilage. The size of lesion greater than 10mm is critical for the strains concentration. The synovial fluid present an important aspect in the strains measured, it reduces the strains in the external surface of cartilage and induces an increase in the lower part. This phenomenon should be addressed in more studies to evaluate this effect

We previously reported that osteoblasts at the curve apex in adolescent idiopathic scoliosis (AIS) exhibit a differential phenotype, compared to non-curve osteoblasts(1). However, the Hueter-Volkmann principle on vertebral body growth in spinal deformities (2) suggests this could be secondary to altered biomechanics. This study examined whether non-curve osteoblasts subjected to mechanical strain resemble the transcriptomic phenotype of curve apex osteoblasts. Facet spinal tissue was collected perioperatively from three sites, (i) the concave and (ii) convex side at the curve apex and (iii) from outside the curve (non-curve) from six AIS female patients (age 13–18 years; NRES 19/WM/0083). Non-curve osteoblasts were subjected to strain using a 4-point bending device. Osteoblast phenotype was determined by RNA sequencing and bioinformatic pathway analysis. RNAseq revealed that curve apex osteoblasts exhibited a differential transcriptome, with 1014 and 1301 differentially expressed genes (DEGs; p<0.05, fold-change >1.5) between convex/non-curve and concave/non-curve sites respectively. Non-curve osteoblasts subjected to strain showed increased protein expression of the mechanoresponsive biomarkers COX2 and C-Fos. Comparing unstimulated vs strain-induced non-curve osteoblasts, 423 DEGs were identified (p<0.05, fold-change >1.5). Of these DEGs, only 5% and 6% were common to the DEGs found at either side of the curve apex, compared to non-curve cells. Bioinformatic analysis of these strain-induced DEGs revealed a different array of canonical signalling pathways and cellular processes, to those significantly affected in cells at the curve apex. Mechanical strain of AIS osteoblasts in vitro did not induce the differential transcriptomic phenotype of AIS osteoblasts at the curve apex

Abstract. Introduction. Ankle arthritis is estimated to affect approximately 72 million people worldwide. Treatment options include fusion and total ankle replacement (TAR). Clinical performance of TAR is not as successful as other joint replacement and failure is poorly understood. Finite element analysis offers a method to assess the strain in bone implanted with a TAR. Higher strain has been associated with microfracture and alters the bone-implant interface. The aim of this study was to explore the influence of implant fixation on strain within the tibia when implanted with a TAR through subject-specific models. Methods. Five cadaveric ankles were scanned using a Scanco Xtreme CT. The Tibia and Talus were segmented from each scan and virtually implanted with a Zenith TAR (Corin, UK) according to published surgical technique. Patient specific models were created and run at five different positions of the gait cycle corresponding to peak load and flexion values identified from literature. Bone material properties were derived from CT greyscale values and all parts were meshed with linear tetrahedral elements. The implant-bone interface was adjusted to fully-fixed or frictionless contact, representing different levels of fixation post-surgery. Strain distributions around the tibial bone fixation were measured. Results. Initial results showed clear differences in strain distributions both between different ankle specimens and fixation levels, with highest strain occurring within the bone at the tip of the tibial stem. Frictionless contact gave higher strain outputs than fully-fixed for all specimens with a range 0.12–0.3% and 0.07%–0.13% respectively. Conclusions. In all specimens, strain was higher in the frictionless contact, which may be considered representative of no bony ingrowth, highlighting fixation may be a critical factor in TAR failure. Differences observed between specimens highlights that TAR may not be a suitable intervention for all patients, due to variation in bone quality and anatomy. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Incidence of intraoperative fracture during cementless Total Hip Arthroplasty (THA) is increasing. This is attributed to factors such as an increase in revision procedures and the favour of cementless fixation. Intraoperative fractures often occur during the seating of cementless components. A surgical mallet and introducer are used to generate the large impaction forces necessary to seat the component, sometimes leading to excessive hoop strain in the bone. The mechanisms of bone strain during impaction are complex and occur over very short timeframes. For this reason experimental and simulation models often focus on strain shortly after the implant is introduced, or seat it quasi-statically. This may not produce a realistic representation of the magnitude of strain in the bone and dangerously under-represent fracture risk. This in-vitro study seeks to determine whether strain induced during impaction is similar both during the strike (dynamic strain) and shortly after the strike has occurred (post-strike strain). It is also asked whether post-strike strain is a reliable predictor of dynamic strain. A custom drop tower was used to seat acetabular components in 45 Sawbones models (SKU: 1522–02, Malmo, Sweden), CNC milled to represent the acetabular cavity. Ten strikes were used to seat each cup. 3 strike velocities (1.5 m/s, 2.75 m/s, 4 m/s) and 3 impact masses (600 g, 1.2 kg, 1.8 kg) were chosen to represent 9 different surgical scenarios. Two strain gages per Sawbone were mounted on the surface of the block, 2 mm from the rim of the cavity. Strain data was acquired at 50 khz. Each strain trace was then analysed to determine the peak dynamic strain during mallet strike and the static strain post-strike. A typical strain pattern was observed during seating. An initial pre-strike strain is followed by a larger dynamic peak as the implant is progressed into the bone cavity. Strain subsequently settles at a lower (tensile) value than peak dynamic post-strike, but higher than pre-strike strain. Over the 450 strikes conducted dynamic strain was on average 3.39 times larger than post-strike strain. A statistically significant linear relationship was observed between the magnitude of post-strike and dynamic strain (adjusted R. 2. =0.391, p<0.005). This indicates that, for a known scenario, post-strike strain can be used as an indicator for dynamic peak strain. However when only the maximum dynamic and post-strike strains were taken from across the 10 strikes used to seat the implant, the relationship between the two strains was not significant (R. 2. =0.300, p=0.73). This may be due to the fact that the two maximums did not often occur on the same strike. On average, max dynamic strain occurred 1.7 strikes after max post-strike strain. We conclude that peak dynamic strain is much larger than the strain immediately post-strike in a synthetic bone model. It is shown that post-strike strain is not a good predictor of dynamic strain when the max strain during any strike to seat the component is considered, or variables (such as mallet mass or velocity) are changed. It is important to consider dynamic strain in bone as well as post-strike strain in experimental or simulated bone models to ensure the most reliable prediction of fracture

Aims. There is a lack of biomaterial-based carriers for the local delivery of rifampicin (RIF), one of the cornerstone second defence antibiotics for bone infections. RIF is also known for causing rapid development of antibiotic resistance when given as monotherapy. This in vitro study evaluated a clinically used biphasic calcium sulphate/hydroxyapatite (CaS/HA) biomaterial as a carrier for dual delivery of RIF with vancomycin (VAN) or gentamicin (GEN). Methods. The CaS/HA composites containing RIF/GEN/VAN, either alone or in combination, were first prepared and their injectability, setting time, and antibiotic elution profiles were assessed. Using a continuous disk diffusion assay, the antibacterial behaviour of the material was tested on both planktonic and biofilm-embedded forms of standard and clinical strains of Staphylococcus aureus for 28 days. Development of bacterial resistance to RIF was determined by exposing the biofilm-embedded bacteria continuously to released fractions of antibiotics from CaS/HA-antibiotic composites. Results. Following the addition of RIF to CaS/HA-VAN/GEN, adequate injectability and setting of the CaS/HA composites were noted. Sustained release of RIF above the minimum inhibitory concentrations of S. aureus was observed until study endpoint (day 35). Only combinations of CaS/HA-VAN/GEN + RIF exhibited antibacterial and antibiofilm effects yielding no viable bacteria at study endpoint. The S. aureus strains developed resistance to RIF when biofilms were subjected to CaS/HA-RIF alone but not with CaS/HA-VAN/GEN + RIF. Conclusion. Our in vitro results indicate that biphasic CaS/HA loaded with VAN or GEN could be used as a carrier for RIF for local delivery in clinically demanding bone infections. Cite this article: Bone Joint Res 2022;11(11):787–802

Aims. Vertebrates have adapted to life on Earth and its constant gravitational field, which exerts load on the body and influences the structure and function of tissues. While the effects of microgravity on muscle and bone homeostasis are well described, with sarcopenia and osteoporosis observed in astronauts returning from space, the effects of shorter exposures to increased gravitational fields are less well characterized. We aimed to test how hypergravity affects early cartilage and skeletal development in a zebrafish model. Methods. We exposed zebrafish to 3 g and 6 g hypergravity from three to five days post-fertilization, when key events in jaw cartilage morphogenesis occur. Following this exposure, we performed immunostaining along with a range of histological stains and transmission electron microscopy (TEM) to examine cartilage morphology and structure, atomic force microscopy (AFM) and nanoindentation experiments to investigate the cartilage material properties, and finite element modelling to map the pattern of strain and stress in the skeletal rudiments. Results. We did not observe changes to larval growth, or morphology of cartilage or muscle. However, we observed altered mechanical properties of jaw cartilages, and in these regions we saw changes to chondrocyte morphology and extracellular matrix (ECM) composition. These areas also correspond to places where strain and stress distribution are predicted to be most different following hypergravity exposure. Conclusion. Our results suggest that altered mechanical loading, through hypergravity exposure, affects chondrocyte maturation and ECM components, ultimately leading to changes to cartilage structure and function. Cite this article: Bone Joint Res 2021;10(2):137–148

Introduction. Ring breakage is a rare but significant complication requiring revision surgery and prolonging the course of treatment. We have encountered three cases with Taylor Spatial Frames (TSF) with breakage at the half ring junction of the distal ring. This experimental study examines the strains produced at different locations on the distal ring during loading and the effects of altering the construct in order to develop techniques to minimise the risk of breakage. Materials and Methods. We mounted different TSF constructs on tibia sawbone models. Construct 1 reproducing the configuration of cases where failure was seen, Construct 2 with different wire and half pin configuration and construct 3 with the distal ring rotated 60 degrees. Strain Gauges were attached to different locations and measurements were collected during loading. Statistical analysis was subsequently performed. Results. The highest strain values were recorded at the half ring junction of constructs 1,2 (>600 microstrains in tension). Rotating the ring 60 degrees significantly reduces the strain observed at the half ring junction (300 microstrains) (p=.000). Strain is increased in areas close to where a half pin attaches to the ring. Conclusions. The highest strains are observed in the half ring junction as the two half rings are subjected to different modes of loading. This area is at higher risk of failure as the thickness of the half rings is halved and their second moment of area significantly reduced. Positioning this junction close to the half pin frame interface increases the strain produced. This interface is dictated by the safe zone in the mid-distal diaphysis of the tibia. Rotating the distal ring 60 degrees has a protective effect by significantly reducing the strain. This simple technical tips should be taken into consideration in order to reduce the risk of breakage at the half ring junction

As many as 25% to 40% of unicompartmental knee replacement (UKR) revisions are performed for pain, a possible cause of which is proximal tibial strain. The aim of this study was to examine the effect of UKR implant design and material on cortical and cancellous proximal tibial strain in a synthetic bone model. Composite Sawbone tibiae were implanted with cemented UKR components of different designs, either all-polyethylene or metal-backed. The tibiae were subsequently loaded in 500 N increments to 2500 N, unloading between increments. Cortical surface strain was measured using a digital image correlation technique. Cancellous damage was measured using acoustic emission, an engineering technique that detects sonic waves (‘hits’) produced when damage occurs in material. Anteromedial cortical surface strain showed significant differences between implants at 1500 N and 2500 N in the proximal 10 mm only (p < 0.001), with relative strain shielding in metal-backed implants. Acoustic emission showed significant differences in cancellous bone damage between implants at all loads (p = 0.001). All-polyethylene implants displayed 16.6 times the total number of cumulative acoustic emission hits as controls. All-polyethylene implants also displayed more hits than controls at all loads (p < 0.001), more than metal-backed implants at loads ≥ 1500 N (p < 0.001), and greater acoustic emission activity on unloading than controls (p = 0.01), reflecting a lack of implant stiffness. All-polyethylene implants were associated with a significant increase in damage at the microscopic level compared with metal-backed implants, even at low loads. All-polyethylene implants should be used with caution in patients who are likely to impose large loads across their knee joint. . Cite this article: Bone Joint J 2013;95-B:1339–47

Introduction. Non-unions often arise because of high strain environments at fracture sites. Revision fixation, bone grafting and biologic treatments to treat long bone fracture non-union can be expensive and invasive. Percutaneous strain reduction screws (PSRS) can be inserted as a day-case surgical procedure to supplement primary fixation at a fraction of the cost of traditional treatments. Screw insertion perpendicular to the plane of a non-union can resist shear forces and achieve union by modifying the strain environment. A multi-centre retrospective study was undertaken to confirm the results of the initial published case series, ascertain whether this technique can be adopted outside of the developing institution and assess the financial impact of this technique. Materials and Methods. Retrospective analysis was performed for all PSRS cases used to treat un-united long bone fractures in four level 1 trauma centres from 2016 to 2020. All patients were followed up until union was achieved or further management was required. Demographic data was collected on patients, as were data about their injuries, initial management and timings of all treatments received. A comparative cost analysis was performed comparing patients treated with PSRS and with traditional non-union surgery methods. Results. 51 patients were treated with the PSRS technique. 45 (88%) patients achieved union at a median time of 5.2 months (range 1.0 – 24.7). Comparable results were seen between the developing institution and independent units. No patients experienced adverse events beyond failure to achieve union. PSRS appears to offer savings of between £2,957 to £11,231 per case compared with traditional methods of non-union surgery. Conclusions. PSRS is a safe, cost-effective treatment for long bone non-union. The promising results of the initial case series have now been replicated outside of the developing institution

We have developed a decellularised porcine superflexor tendon (pSFT), which has shown promising regenerative capacity in an ovine model of anterior cruciate ligament (ACL) repair. This study investigated the strain rate dependent and dynamic mechanical properties of native and decellularised pSFTs. Decellularisation was carried out using a previously established procedure, including antibiotic washes, low concentration detergent (0.1% sodium dodecyl sulphate) washes and nuclease treatments. Three different strain rates were employed: 1, 10 & 100%s-1 (n=6 for all groups). Toe-region modulus (E0), linear-region modulus (E1), transition coordinates (εT, σT), tensile strength (UTS) and failure strain were calculated. For DMA, specimens were loaded between 1 & 5MPa with increasing frequency up to 2Hz. Dynamic (E*), storage (E') and loss (E'') moduli, and tan delta were calculated for native and decellularised groups (n=6). Data was analysed by 2-way ANOVA and Tukey post-hoc test (p<0.05). For decellularised tendons, altering the strain rate did not affect any of the static tensile properties. For native pSFTs, the UTS, failure strain and E1 were not affected by changing the strain rate. Increasing the strain rate significantly increased E0 (1% vs 10% and 1% vs 100%) and σT (1% vs 100%) and decreased εT (1% vs 10% and 1% vs 100%) for native pSFT. E*, E' and E'' were all significantly reduced in decellularised specimens compared to native controls across all frequencies investigated. No significant differences were found for tan delta. Evidence of strain rate dependency was witnessed in the native pSFTs by increase of the toe region modulus and displacements of the transition point coordinates. This response was not seen in the tissue following decellularisation. DMA demonstrated a reduction in dynamic, storage and loss moduli. Tan delta (E''/E') remained unchanged, indicating reductions in solid and fluid components are interlinked

Initial stability of cementless components in bone is essential for longevity of Total Hip Replacements. Fixation is provided by press-fit: seating an implant in an under-reamed bone cavity with mallet strikes (impaction). Excessive impaction energy has been shown to increase the risk of periprosthetic fracture of bone. However, if implants are not adequately seated they may lack the stability required for bone ingrowth. Ideal fixation would maximise implant stability but would minimise peak strain in bone, reducing the risk of fracture. This in-vitro study examines the influence of impaction energy and number of seating strikes upon implant push-out force (indicating stability) and peak dynamic strain in bone substitute (indicating likelihood of fracture). The ratio of these factors is given as an indicator of successful impaction strategy. A custom drop tower with simulated hip compliance was used to seat acetabular cups in 30 Sawbone blocks with CNC milled acetabular cavities. 3 impaction energies were selected; low (0.7j), medium (4.5j) and high (14.4j), representing the wide range of values measured during surgery. Each Sawbone was instrumented with strain gauges, secured on the block surface close to the acetabular cavity (Figure 1). Strain gauge data was acquired at 50 khz with peak tensile strain recorded for each strike. An optical tracker was used to determine the polar gap between the cup and Sawbone cavity during seating. Initially 10 strikes were used to seat each cup. Tracking data were then used to determine at which strike the cups progressed less than 10% of the final polar gap. This value was taken as number of strikes to complete seating. Tests were repeated with fresh Sawbone, striking each cup the number of times required to seat. Following each seating peak push-out forces of the cups were recorded using a compression testing machine. 10, 5 and 2 strikes were required to seat the acetabular cups for the low, medium and high energies respectively. It was found that strain in the Sawbone peaked around the number of strikes to complete seating and subsequently decreased. This trend was particularly pronounced in the high energy group. An increase in Sawbone strain during seating was observed with increasing energy (270 ± 29 µε [SD], 519 ± 91 µε and 585 ± 183 µε at low, medium and high energies respectively). The highest push-out force was achieved at medium strike energy (261 ± 46N). The ratio between push-out and strain was highest for medium strike energy (0.50 ± 0.095 N/µε). Push-out force was similar after 5 and 10 strikes for the medium energy strike. However push-out recorded at ten strikes for the high energy group was significantly lower than for 2 strikes (<40 ± 19 N, p<0.05). These results indicate that a medium strike energy with an appropriate number of seating strikes maximizes initial implant stability for a given peak bone strain. It is also shown that impaction with an excessive strike energy may greatly reduce fixation strength while inducing a very high peak dynamic strain in the bone. Surgeons should take care to avoid an excessive number of impaction strikes at high energy. For any figures or tables, please contact the authors directly

Abstract. Introduction. All-tissue quadriceps tendon (QT) is becoming an increasingly popular alternative to hamstrings tendon (HT) and bone-tendon-bone (BTB) autograft for anterior cruciate ligament (ACL) reconstruction. The relatively short graft length however dictates that one, or both, ends rely on suture fixation. The strength of this construct is therefore extremely important. This study evaluates whether the use of a novel fixation technique can improve the tensile properties of the construct compared to a Krackow suture, and a looped tendon (suture free) gold standard. Methods. Eighteen porcine flexor tendons were tested, across three groups; suture-tape Krackow, looped tendon, and the novel ‘strain suture’. Biomechanical testing simulated the different stages of ACL graft preparation and loading (60N preload for 10 minutes, 10 cycles from 10N to 75N, and 1000 cycles from 100N to 400N). Elongation and load to failure were recorded, and stiffness calculated for each construct. Results. The mean elongation was significantly improved for the strain suture compared to the suture tape Krackow for preload, 10 cycle and 1000 cycle testing protocols respectively (1.36mm vs 4.93mm, p<001; 0.60mm vs 2.72mm, p<0.001; 2.95mm vs 29.08mm, p<0.001). Compared with the looped tendon, the strain suture demonstrated similar results for preload and 10 cycle elongation, but greater elongation during the 1000 cycle stage. Stiffness of the latter two constructs was similar. Conclusions. Augmentation of the suture fixation using this novel technique provides a construct that is significantly superior to currently practised suture techniques, and similar in elongation and stiffness to a looped graft

It is nowadays widely recognized that patient satisfaction following knee arthroplasty strongly depends on ligament balancing. To obtain this balancing, the occurring ligament strain is assumed to play a crucial role. To measure this strain, a method is described in this paper that allows full field 3D evaluation of the strains. The latter is preferred over traditional measurement techniques, e.g. displacement transducers or strain gauges, as human soft tissue is not expected to deform uniformly due to its highly inhomogeneous and anisotropic properties. To facilitate full field strain measurements, the 3D digital image correlation (DIC) technique was adopted. This technique was previously validated by our research group on human tissue. First, a high contrast speckle pattern was applied on the sMCL. Therefore, the specimens are first coated with a small layer of methylene blue. Following, a random white speckle pattern is applied. During knee flexion, two cameras simultaneously take pictures of the deforming region at predefined flexion angles. Using dedicated software, the captured images are eventually combined and result in 3D full field strains and displacements. Using this method, the strain distribution was studied in six cadaveric knees during flexion extension movement. Therefore, the femur was rigidly fixed in a custom test rig. The tibia was left unconstrained, allowing the six degrees of freedom in the knee. A load was applied to all major muscles in physiological directions of each muscle by attaching a series of calibrated weights (Farahmand et al., J Orthop Res., 1998;16(1)). The direction of the pulling cables was controlled using a digital inclinometer for each specimen. As a result, a statically balanced muscle loading of the knee was obtained. From these cadaveric experiments, it is observed that on average the sMCL behaves isometrically between 0° and 90° of flexion. However, high regional differences in strain distribution are observed from the full field measurements. The proximal region of the sMCL experiences relatively high strains upon flexion. These strains are positive (tension) in the anterior part and negative (compression) in the posterior region. In contrast, the distal region remains approximately isometric upon knee flexion (see Figure 1). It is accordingly concluded that the sMCL behaves isometric, though large regional differences are observed. The proximal region experiences higher strains. Furthermore, the DIC technique provided valuable insights in the deformation of the sMCL. This technique will therefore be applied to study the impact of knee arthroplasty in the near future. Caption with figure 1: Full field strain distribution in the sMCL's longitudinal direction for specimen in 45° (a) and 90° (b) of knee flexion

Aims. Surgeons and most engineers believe that bone compaction improves implant primary stability without causing undue damage to the bone itself. In this study, we developed a murine distal femoral implant model and tested this dogma. Methods. Each mouse received two femoral implants, one placed into a site prepared by drilling and the other into the contralateral site prepared by drilling followed by stepwise condensation. Results. Condensation significantly increased peri-implant bone density but it also produced higher strains at the interface between the bone and implant, which led to significantly more bone microdamage. Despite increased peri-implant bone density, condensation did not improve implant primary stability as measured by an in vivo lateral stability test. Ultimately, the condensed bone underwent resorption, which delayed the onset of new bone formation around the implant. Conclusion. Collectively, these multiscale analyses demonstrate that condensation does not positively contribute to implant stability or to new peri-implant bone formation. Cite this article:Bone Joint Res. 2020;9(2):60–70

Bone surface strains were measured in cadaver femora during loading prior to and after resurfacing of the hip and total hip replacement using an uncemented, tapered femoral component. In vitro loading simulated the single-leg stance phase during walking. Strains were measured on the medial and the lateral sides of the proximal aspect and the mid-diaphysis of the femur. Bone surface strains following femoral resurfacing were similar to those in the native femur, except for proximal shear strains, which were significantly less than those in the native femur. Proximomedial strains following total hip replacement were significantly less than those in the native and the resurfaced femur. These results are consistent with previous clinical evidence of bone loss after total hip replacement, and provide support for claims of bone preservation after resurfacing arthroplasty of the hip

Strain is a robust indicator of bone failure initiation. Previous work has demonstrated the measurement of vertebral trabecular bone strain by Digital Volume Correlation (DVC) of µCT scan in both a loaded and an unloaded configuration. This project aims to improve previous strain measurement methods relying on image registration, improving resolution to resolve trabecula level strain and to improve accuracy by applying feature based registration algorithms to µCT images of vertebral trabecular bone to quantify strain. It is hypothesised that extracting reliable corresponding feature points from loaded and unloaded µCT scans can be used to produce higher resolution strain fields compared to DVC techniques. The feature based strain calculation algorithm has two steps: 1) a displacement field is calculated by finding corresponding feature points identified in both the loaded and unloaded µCT scans 2) strain fields are calculated from the displacement fields. Two methods of feature point extraction, Scale Invariant Feature Transform (SIFT) and Skeletonisation, were applied to unloaded (fixed) and loaded (moving) µCT images of a rat tail vertebra. Spatially non-uniform displacement fields were generated by automatically matching corresponding feature points in the unloaded and loaded scans. The Thin Plate Spline method and a Moving Least Squares Meshless Method were both tested for calculating strain from the displacement fields. Verification of the algorithms was performed by testing against known artificial strain/displacement fields. A uniform and a linearly varying 2% compressive strain field were applied separately to an unloaded 2D sagittal µCT slice to simulate the moving image. SIFT was unable to reliably match identified feature points leading to large errors in displacement. Skeletonisation generated a more accurate and precise displacement field. TPS was not tolerant to small displacement field errors, which resulted in inaccurate strain fields. The Meshless Methods proved much more resilient to displacement field errors. The combination of Skeletonisation with the Meshless Method resulted in best performance with an accuracy of −405µstrain and a detection limit of 1210µstrain at a strain resolution of 221.5µm. The DVC algorithm verified using the same validation test yielded a similar detection limit (1190µstrain), but with a lower accuracy for the same test (2370µstrain) for a lower resolution strain field (770µm) (Hardisty, 2009). The Skeletonisation algorithm combined with the Meshless Method calculated strain at a higher resolution, but with a similar detection limit, to that of traditional DVC methods. Future improvements to this method include the implementation of subpixel feature point identification and adapting this method of strain measurement into a 3D domain. Ultimately, a hybrid DVC/feature registration algorithm may further improve the ability to measure trabecular bone strain using µCT based image registration

Aims. Biofilm-related infection is a major complication that occurs in orthopaedic surgery. Various treatments are available but efficacy to eradicate infections varies significantly. A systematic review was performed to evaluate therapeutic interventions combating biofilm-related infections on in vivo animal models. Methods. Literature research was performed on PubMed and Embase databases. Keywords used for search criteria were “bone AND biofilm”. Information on the species of the animal model, bacterial strain, evaluation of biofilm and bone infection, complications, key findings on observations, prevention, and treatment of biofilm were extracted. Results. A total of 43 studies were included. Animal models used included fracture-related infections (ten studies), periprosthetic joint infections (five studies), spinal infections (three studies), other implant-associated infections, and osteomyelitis. The most common bacteria were Staphylococcus species. Biofilm was most often observed with scanning electron microscopy. The natural history of biofilm revealed that the process of bacteria attachment, proliferation, maturation, and dispersal would take 14 days. For systemic mono-antibiotic therapy, only two of six studies using vancomycin reported significant biofilm reduction, and none reported eradication. Ten studies showed that combined systemic and topical antibiotics are needed to achieve higher biofilm reduction or eradication, and the effect is decreased with delayed treatment. Overall, 13 studies showed promising therapeutic potential with surface coating and antibiotic loading techniques. Conclusion. Combined topical and systemic application of antimicrobial agents effectively reduces biofilm at early stages. Future studies with sustained release of antimicrobial and biofilm-dispersing agents tailored to specific pathogens are warranted to achieve biofilm eradication. Cite this article: Bone Joint Res 2022;11(10):700–714

The cortical strains on the femoral neck and proximal femur were measured before and after implantation of a resurfacing femoral component in 13 femurs from human cadavers. These were loaded into a hip simulator for single-leg stance and stair-climbing. After resurfacing, the mean tensile strain increased by 15% (95% confidence interval (CI) 6 to 24, p = 0.003) on the lateral femoral neck and the mean compressive strain increased by 11% (95% CI 5 to 17, p = 0.002) on the medial femoral neck during stimulation of single-leg stance. On the proximal femur the deformation pattern remained similar to that of the unoperated femurs. The small increase of strains in the neck area alone would probably not be sufficient to cause fracture of the neck However, with patient-related and surgical factors these strain changes may contribute to the risk of early periprosthetic fracture

Aims. Staphylococcus aureus is a major cause of septic arthritis, and in vitro studies suggest α haemolysin (Hla) is responsible for chondrocyte death. We used an in vivo murine joint model to compare inoculation with wild type S. aureus 8325-4 with a Hla-deficient strain DU1090 on chondrocyte viability, tissue histology, and joint biomechanics. The aim was to compare the actions of S. aureus Hla alone with those of the animal’s immune response to infection. Methods. Adult male C57Bl/6 mice (n = 75) were randomized into three groups to receive 1.0 to 1.4 × 10. 7. colony-forming units (CFUs)/ml of 8325-4, DU1090, or saline into the right stifle joint. Chondrocyte death was assessed by confocal microscopy. Histological changes to inoculated joints were graded for inflammatory responses along with gait, weight changes, and limb swelling. Results. Chondrocyte death was greater with 8325-4 (96.2% (SD 5.5%); p < 0.001) than DU1090 (28.9% (SD 16.0%); p = 0.009) and both were higher than controls (3.8% (SD 1.2%)). Histology revealed cartilage/bone damage with 8325-4 or DU1090 compared to controls (p = 0.010). Both infected groups lost weight (p = 0.006 for both) and experienced limb swelling (p = 0.043 and p = 0.018, respectively). Joints inoculated with bacteria showed significant alterations in gait cycle with a decreased stance phase, increased swing phase, and a corresponding decrease in swing speed. Conclusion. Murine joints inoculated with Hla-producing 8325-4 experienced significantly more chondrocyte death than those with DU1090, which lack the toxin. This was despite similar immune responses, indicating that Hla was the major cause of chondrocyte death. Hla-deficient DU1090 also elevated chondrocyte death compared to controls, suggesting a smaller additional deleterious role of the immune system on cartilage. Cite this article: Bone Joint Res 2022;11(9):669–678

Aims. The aim of the study was to investigate whether the primary stability of press-fit acetabular components can be improved by altering the impaction procedure. Methods. Three impaction procedures were used to implant acetabular components into human cadaveric acetabula using a powered impaction device. An impaction frequency of 1 Hz until complete component seating served as reference. Overimpaction was simulated by adding ten strokes after complete component seating. High-frequency implantation was performed at 6 Hz. The lever-out moment of the acetabular components was used as measure for primary stability. Permanent bone deformation was assessed by comparison of double micro-CT (µCT) measurements before and after impaction. Acetabular component deformation and impaction forces were recorded, and the extent of bone-implant contact was determined from 3D laser scans. Results. Overimpaction reduced primary acetabular component stability (p = 0.038) but did not significantly increase strain release after implantation (p = 0.117) or plastic deformations (p = 0.193). Higher press-fits were associated with larger polar gaps for the 1 Hz reference impaction (p = 0.002, R. 2. = 0.77), with a similar trend for overimpaction (p = 0.082, R. 2. = 0.31). High-frequency impaction did not significantly increase primary stability (p = 0.170) at lower impaction forces (p = 0.001); it was associated with smaller plastic deformations (p = 0.035, R. 2. = 0.34) and a trend for increased acetabular component relaxation between strokes (p = 0.112). Higher press-fit was not related to larger polar gaps for the 6 Hz impaction (p = 0.346). Conclusion. Overimpaction of press-fit acetabular components should be prevented since additional strokes can be associated with increased bone damage and reduced primary stability as shown in this study. High-frequency impaction at 6 Hz was shown to be beneficial compared with 1 Hz impaction. This benefit has to be confirmed in clinical studies. Cite this article: Bone Joint J 2023;105-B(3):261–268

To quantify the variation in strain between the deep and superficial layers of the supraspinatus tendon, ten cadaveric shoulders were tested on a purpose built rig. Differential Variable Reluctance Transducers (DVRTs) were inserted into the superficial and deep aspects of the tendon spanning the critical zone. DVRTs accurately measured linear displacement and from this strain was calculated. The strain was measured for two aspects of supraspinatus action, abduction from 0 to 120 degrees with a tensile load (100 Newtons) and static load increases at zero abduction (20, 50, 100, 150 and 200 Newtons). After preconditioning, ten sets of results were recorded for each load/position. The hypothesis, there is a statistically significant difference in strain between the superficial/deep supraspinatus tendon during abduction and with static loading, was tested using a one way ANOVA. During abduction a statistically significant difference in strain was measured between the layers of the supraspinatus tendon at thirty degrees (p=0.000428) and this increased with further abduction. Tensile loading increased tendon strain more in the deep layer of the tendon. This was statistically significant at loads greater than 150N (p= 0.007). The variation in properties between the superficial and deep layers of the supraspinatus tendon has been proposed as a cause of differential strain (1). This study confirms statistically different strains between the superficial and deep tendon layers. It is proposed that the resulting shearing effect initiates intratendinous defects and ultimately tears

Aims. Delayed postoperative inoculation of orthopaedic implants with persistent wound drainage or bacterial seeding of a haematoma can result in periprosthetic joint infection (PJI). The aim of this in vivo study was to compare the efficacy of vancomycin powder with vancomycin-eluting calcium sulphate beads in preventing PJI due to delayed inoculation. Methods. A mouse model of PJI of the knee was used. Mice were randomized into groups with intervention at the time of surgery (postoperative day (POD) 0): a sterile control (SC; n = 6); infected control (IC; n = 15); systemic vancomycin (SV; n = 9); vancomycin powder (VP; n = 21); and vancomycin bead (VB; n = 19) groups. Delayed inoculation was introduced during an arthrotomy on POD 7 with 1 × 10. 5. colony-forming units (CFUs) of a bioluminescent strain of Staphylococcus aureus. The bacterial burden was monitored using bioluminescence in vivo. All mice were killed on POD 21. Implants and soft-tissue were harvested and sonicated for analysis of the CFUs. Results. The mean in vivo bioluminescence in the VB group was significantly lower on POD 8 and POD 10 compared with the other groups. There was a significant 1.3-log. 10. (95%) and 1.5-log. 10. (97%) reduction in mean soft-tissue CFUs in the VB group compared with the VP and IC groups (3.6 × 10. 3. vs 7.0 × 10. 4. ; p = 0.022; 3.6 × 10. 3. vs 1.0 × 10. 5. ; p = 0.007, respectively) at POD 21. There was a significant 1.6-log. 10. (98%) reduction in mean implant CFUs in the VB group compared with the IC group (1.3 × 10. 0. vs 4.7 × 10. 1. , respectively; p = 0.038). Combined soft-tissue and implant infection was prevented in 10 of 19 mice (53%) in the VB group as opposed to 5 of 21 (24%) in the VP group, 3 of 15 (20%) in the IC group, and 0% in the SV group. Conclusion. In our in vivo mouse model, antibiotic-releasing calcium sulphate beads appeared to outperform vancomycin powder alone in lowering the bacterial burden and preventing soft-tissue and implant infections. Cite this article: Bone Joint J 2024;106-B(6):632–638

In order to investigate the osteoinductive properties of allograft used in impaction grafting and the effect of strain during impaction on these properties, we designed an in vitro experiment to measure strain-related release of bone morphogenetic protein-7 (BMP-7) from fresh-frozen femoral head allograft. A total of 40 10 mm cubes of cancellous bone were cut from ten samples of fresh-frozen femoral head. The marrow was removed from the cubes and the baseline concentrations of BMP-7 were measured. Specimens from each femoral head were allocated to four groups and subjected to different compressive strains with a material testing machine, after which BMP-7 activity was reassessed. It was present in all groups. There was a linear increase of 102.1 pg/g (95% confidence interval 68.6 to 135.6) BMP-7 for each 10% increase in strain. At 80% strain the mean concentration of BMP-7 released (830.3 pg/g bone) was approximately four times that released at 20% strain. Activity of BMP-7 in fresh-frozen allograft has not previously been demonstrated. This study shows that the freezing and storage of femoral heads allows some maintenance of biological activity, and that impaction grafting provides a source of osteoinductive bone for remodelling. We have shown that BMP-7 is released from fresh-frozen femoral head cancellous bone in proportion to the strain applied to the bone. This suggests that the impaction process itself may contribute to the biological process of remodelling and bony incorporation

Introduction: Cartilage is an anisotropic material whose structure and tensile properties vary with the depth from the articular surface. Further, ultrastructural changes of articular cartilage under strain are poorly understood. The aim of this study therefore was to visualize the zonal variations in ultrastructural changes of cartilage when subjected to a range of tensile strains to failure. Materials and Methods: 3 osteochondral plugs were harvested from the femur of a 3 years old bovine with a cylindrical reamer. Cartilage was cut parallel to the articular surface into the superficial, middle and deep layers, 300μm thick each and then each was cut normal to the surface into dumbbell shaped specimen 10 mm long. Each specimen (9 in total) was clamped in an individual mini tension device and subjected to a specific strain, then fixed and processed whilst still under strain within its tension device for observation with SEM. Results: When specimens were observed in en face view under no strain, a fibrillar meshwork was seen to run parallel to the articular surface in the superficial layer, randomly in the middle layer and perpendicular to the articular surface in the deep layer. Under strain the fibrillar meshwork began to reorient parallel to strain (tangential to the surface) in each layer. At 20% strain the whole fibrillar meshwork was reoriented and formed bundles in the superficial layer. In the middle layer almost whole of the fibrillar meshwork was reoriented at 40% strain. In the deep layer the fibrillar mesh-work was reoriented parallel to the strain in some areas, while in the other areas it was still seen perpendicular to the surface even at 70% strain. Conclusions: The collagen meshwork of cartilage was reorganised under strain and this appears to play an important role in cartilage extension. Thus the more rapid reorientation in the superficial layer may result in its reduced extensibility compared to that of the deeper layers

Introduction. After TKR, excessive tension within the lateral retinaculum can lead to joint instability, component wear, stiffness and pain. The spatial distribution of strain in the lateral retinculum is unknown, both in the native knee and after TKR. In this study we measure the magnitude and distribution of mechanical strain in the lateral retinaculum with knee flexion, both in the native knee and after TKR. We hypothesize that: . 1. Strain in the lateral retinaculum will increase as a function of flexion. 2. Some regions of the lateral retinaculum experience greater strain than others. 3. TKR will affect the magnitude and location of strain during knee flexion. Materials and Methods. A fiduciary grid of approximately 40–70 markers was attached to the exposed lateral retinacula of five fresh frozen cadaveric knees in order to allow tracking of soft-tissue deformation. Each knee was flexed from 0–120° in a 6 degree-of-freedom custom activity simulator that physiologically loaded the knee during a squatting maneuver. During simulation, the displacement of each fiduciary point was measured using visible-light stereo-photogrammetry. The fiduciary grid divided into four distinct regions for strain analysis. Using the grid of the native knee in full extension as the initial state, the average principal strain in each region was calculated as a function of flexion. Measurements were repeated after TKR was performed using a contemporary implant system. Results. In the native knee, average retinacular strain increased dramatically with knee flexion (30°: 12% vs 120°: 25%; p = 0.007). The greatest strain was observed in the supero-lateral region in high flexion (34% at 120°). No significant change in strain with flexion was seen in the infero-medial region bordering the patellar ligament (10% at 30° to 15% at 120°; p > 0.05). After TKR, retinacular strains increased by an average of 13% in extension when compared to the native knee. In flexion, strains decreased following arthroplasty by an average of 4% at 30° and 6% at all other angles. The largest strains were observed in the supero-lateral region and were comparable to strain observed in the native knee (34% at 120°). The greatest decrease in strain after TKR was observed in the supero-medial region (26% vs 16% at 90°). Conclusions. In the native knee, average lateral retinacular strains are greatest mid- to high-flexion as the retinaculum tightens to constrain patellar motion. The superior regions of the retinaculum, where the iliotibial band-patellar fibers are located, experience the most strain, especially in higher flexion. After TKR, strain in the supero-medial region decreases while strain in the supero-lateral region remains comparable to the native knee, suggesting the geometry of the native knee along

Introduction:. The subscapularis muscle experiences significant strain as it accommodates common movements of the shoulder. Little is known about what happens with this obligatory strain once the subscapularis insertion is disrupted and repaired in the course of shoulder arthroplasty. Subscapularis failure is a serious known complication after shoulder arthroplasty. It is not known what the effect of increasing the thickness of the shoulder head will have on subscapularis strain. It is our hypothesis that the use of large or expanded humeral heads during shoulder replacement will cause increased tension in the repaired subscapularis. The primary purpose of this study was to identify the optimal manner to perform a passive range of motion (PROM) program without invoking a significant increase in strain in the repaired subscapularis. The secondary purpose was to determine the impact of varying the thickness of the humeral head on subscapularis strain using the same PROM protocol. Methods:. Eight fresh-frozen, forequarter cadaver (four female, four male) specimens were obtained following IRB approval. An extended deltopectoral incision was performed so that the subscapularis insertion site could be well visualized. PROM exercises with the following motions were evaluated: external rotation, abduction, flexion and scaption. An optical motion analysis system was used to measure strain in the subscapularis. The same measurement protocol was repeated after performing a subscapularis osteotomy and after placement of an anatomic hemiarthroplasty of three different thicknesses (short, tall, expanded). Results:. A decrease in joint laxity (less strain but more tension on the subscapularis) was observed in abduction, external rotation, and forward flexion, following implantation of the shoulder arthroplasty components. For abduction and forward flexion, we observed a trend of decreasing laxity with increasing humeral head component thickness. For external rotation, all components displayed a similar reduction in joint laxity. With the short humeral head, strain was similar to native joint with passive scaption and flexion but not with external rotation or abduction. Discussion:. The PROM that tends to minimize tension on the subscapularis is forward flexion and scaption whereas external rotation and abduction will stress the subscapularis repair. Therefore passive forward flexion or scaption do not need to be limited but clearly external rotation should have passive limits and abduction should probably be avoided. The subscapularis muscle is under greater preload tension after shoulder joint arthroplasty. Even the short head size humeral component demonstrated decreased laxity compared to the intact joint. This suggests that even the shortest head size available may not be anatomical and perhaps a thinner humeral head size would be more representative of the normal anatomy

There is an established link between bone quality and fracture risk. It has been suggested that reduced bone quality will also reduce the toughening mechanisms displayed during loading at a high strain rate. We hypothesised that partially decalcified bone will not demonstrate an increase in force required to cause failure when comparing low and high strain rate loading. Mechanical properties were defined by the maximum force at failure. Bone quality was defined by the mineral content. This was altered by subjecting the bones to ultrasonically assisted decalcification in 10M EDTA to achieve an average 18% mineral reduction (A 70 yr old woman has approx 18% of her peak bone mass). 20 pairs of sheep femurs were harvested and split into four equal groups: normal bone quality, fast strain rate (NF); normal bone quality, slow strain rate (NS); low bone quality, fast strain rate (LF) and low bone quality, slow strain rate (LS). All mechanical testing was carried out by means of 3-point bending. Load representing the slow strain rate was applied by a mechanical testing machine (Zwick) at a rate resulting in a deflection of 1mm/s. The dynamic loading was applied by a custom designed pneumatic ram at a mean rate of deflection between the specimens of 2983 mm/s (±SD 1155), this equates to strain rates experienced in a road traffic accident. The following results for force at failure were found (mean ± SD). NF: Force 5503N (± 1012); NS: Force 3969N (± 572); LF: Force 3485N (± 772); LS: Force 3165N (± 605). Groups were compared using a Mann-Whitney U test. Significant results were found between the following groups: Normal bone quality, strain rate compared (NF-NS) p<0.002; Fast strain rate, bone quality compared (NF-LF) p=0.008; Slow strain rate, bone quality compared (NS-LS) p=0.02. No statistical significance was found when comparing low bone quality, strain rate compared (LF-LS) p=0.47. These results show that normal healthy bone has an ability to withstand higher strain rates which protects it against fracture. This ability to withstand high strain rates is lost in decalcified bone making it more susceptible to fracture. The results of this study indicate the importance of strain rate reduction as well as energy absorption in the design of hip protectors and in environmental modifications

Aim: The aim of this study is to visualize the structural changes of both the matrix collagen meshwork and the chondrocyte cytoskeleton of articular cartilage when it is subjected to tensile strain. Materials and Methods: Dumbbell-shape specimens were harvested from the articular surface of the femur. Specimens were placed with the articular surface uppermost each in individual mini tension device and subjected to a graded series of tensile strains, whilst being observed with phase contrast light microscopy. Thereafter each specimen was fixed in its particular position of strain, and stained with fluorochrome conjugated primary antibodies specific for actin and vimentin and with DAPI for nuclear staining for observation by confocal laser scanning microscopy (CLSM). Results: Phase contrast microscopy visualized the reorganization of the matrix which became aligned parallel to the direction of strain, resulting in the deformation of the chondrocyte and their nuclei into an elliptical shape. CLSM demonstrated the reorganization of the matrix and chondrocyte cytoskeleton; at no strain, the vimentin meshwork spanned the cytoplasm from plasma membrane to nuclear membrane. At 20% strain, the vimentin meshwork became aligned parallel to the direction of strain and the nucleus was deformed into elliptical shape. Discussions: There are two possibilities to explain the structural changes in the chondrocyte under tensile strain. 1.The collagen meshwork becomes aligned parallel to the direction of tensile strain, squeezing the chondrocyte into the observed elliptical shape subsequently with the cytoskeleton reorganizing in response to it. 2.The collagen meshwork transfers the tensile strain through the plasma membrane to the vimentin meshwork which reorganizes and subsequently results in the changes in chondrocyte morphology. Further explanation is required to test the above two hypotheses

Aims. Periprosthetic joint infections (PJIs) are rare, but represent a great burden for the patient. In addition, the incidence of methicillin-resistant Staphylococcus aureus (MRSA) is increasing. The aim of this rat experiment was therefore to compare the antibiotics commonly used in the treatment of PJIs caused by MRSA. Methods. For this purpose, sterilized steel implants were implanted into the femur of 77 rats. The metal devices were inoculated with suspensions of two different MRSA strains. The animals were divided into groups and treated with vancomycin, linezolid, cotrimoxazole, or rifampin as monotherapy, or with combination of antibiotics over a period of 14 days. After a two-day antibiotic-free interval, the implant was explanted, and bone, muscle, and periarticular tissue were microbiologically analyzed. Results. Vancomycin and linezolid were able to significantly (p < 0.05) reduce the MRSA bacterial count at implants. No significant effect was found at the bone. Rifampin was the only monotherapy that significantly reduced the bacterial count on implant and bone. The combination with vancomycin or linezolid showed significant efficacy. Treatment with cotrimoxazole alone did not achieve a significant bacterial count reduction. The combination of linezolid plus rifampin was significantly more effective on implant and bone than the control group in both trials. Conclusion. Although rifampicin is effective as a monotherapy, it should not be used because of the high rate of resistance development. Our animal experiments showed the great importance of combination antibiotic therapies. In the future, investigations with higher case numbers, varied bacterial concentrations, and changes in individual drug dosages will be necessary to be able to draw an exact comparison, possibly within a clinical trial. Cite this article: Bone Joint Res 2022;11(3):143–151

Aim: To compare the strain pattern in intact and resurfaced femurs with and without abductor force using validated third generation composite femurs and rosette strain gauges. Methods: Rosette strain gauges were applied to an intact and a resurfaced third generation composite femur at three sites; narrowest part of the lateral surface of the neck, narrowest part of the medial surface of the neck and medial surface at the level of lesser trochanter. The femurs were loaded with axial loads of 600N, 800N and 1000N sequentially. The tests were repeated thrice for each femur. Maximum and minimum principal strains were calculated. Further tests were carried out in which an abductor load was included in the model. Testing was done at 600N and repeated thrice for each femur. The principal strains were calculated and compared with the principal strains without the abductor load. Results: The maximum principal strains in the resurfaced femur were approximately 50% higher in the lateral surface of the neck and about 30% higher in the lesser trochanteric region when loaded without including an abductor force. Inclusion of the abductor force decreased the strain particularly at the lateral surface of the neck by approximately 45% in the intact femur and approximately 25% in the implanted femur. Even with the inclusion of the abductor load, the strain in the resurfaced femur remained more than 50% higher at the lateral surface of the neck and 20% higher in the lesser trochanteric region. Conclusion: Our study suggests that proximal femoral stress protection will not occur following surface replacement of the hip. The increased strain at the lateral surface of the neck could result in fracture, particularly if there is notching of the neck or if abductor function has been compromised, which can happen with the direct lateral approach

Aims. Bone regeneration during treatment of staphylococcal bone infection is challenging due to the ability of Staphylococcus aureus to invade and persist within osteoblasts. Here, we sought to determine whether the metabolic and extracellular organic matrix formation and mineralization ability of S. aureus-infected human osteoblasts can be restored after rifampicin (RMP) therapy. Methods. The human osteoblast-like Saos-2 cells infected with S. aureus EDCC 5055 strain and treated with 8 µg/ml RMP underwent osteogenic stimulation for up to 21 days. Test groups were Saos-2 cells + S. aureus and Saos-2 cells + S. aureus + 8 µg/ml RMP, and control groups were uninfected untreated Saos-2 cells and uninfected Saos-2 cells + 8 µg/ml RMP. Results. The S. aureus-infected osteoblasts showed a significant number of intracellular bacteria colonies and an unusual higher metabolic activity (p < 0.005) compared to uninfected osteoblasts. Treatment with 8 µg/ml RMP significantly eradicated intracellular bacteria and the metabolic activity was comparable to uninfected groups. The RMP-treated infected osteoblasts revealed a significantly reduced amount of mineralized extracellular matrix (ECM) at seven days osteogenesis relative to uninfected untreated osteoblasts (p = 0.007). Prolonged osteogenesis and RMP treatment at 21 days significantly improved the ECM mineralization level. Ultrastructural images of the mineralized RMP-treated infected osteoblasts revealed viable osteoblasts and densely distributed calcium crystal deposits within the extracellular organic matrix. The expression levels of prominent bone formation genes were comparable to the RMP-treated uninfected osteoblasts. Conclusion. Intracellular S. aureus infection impaired osteoblast metabolism and function. However, treatment with low dosage of RMP eradicated the intracellular S. aureus, enabling extracellular organic matrix formation and mineralization of osteoblasts at later stage. Cite this article: Bone Joint Res 2022;11(5):327–341

The causes of spine disease are often biomechanical ones (e.g. disc degeneration, vertebral fracture). Currently, a deep investigation of the spine biomechanics is missing, due to the high complexity of the spine system (Fung 1980, Brandolini, Cristofolini et al. 2014): vertebrae and intervertebral discs. Recently, the Digital Image Correlation allowed measuring in vitrothe displacement and strain on the surface of soft and hard tissues, upon a specific non-invasive preparation of their surface with a speckle pattern (Palanca, Tozzi et al. 2016). The aim of this explorative work was to evaluate the deformation on spine segments, being able to distinguish between hard and soft tissue in the elastic regime and up to fracture. Segment of four vertebrae were extracted from porcine spines. All ligaments and muscles were removed, without damaging the spine segment (vertebrae and intervertebral discs). A suitable non-conventional white-on-black speckle pattern was prepared on the surface with airbrush airgun to track the movements of the specimen with DIC (Lionello, Sirieix et al. 2014). The endplates of the extreme vertebrae were potted in poly-methyl-methacrylate. The spine segments were tested in pure axial loading with cycles of increasing magnitude, up to fialure. A commercial 3D-DIC (Dantec Dynamics, Denmark) was used. In the present configuration, it allowed a resolution of 30 micrometers. It was used to measure the displacements and strains in a full-field and contactless way on the frontal surface of the spine segments. DIC allowed measuring with success the displacement and strain during the entire test, in the elastic regime and up to failure. The displacements and strains could be measured on the entire specimen, both in the vertebrae (hard tissue) and in the intervertebral discs (soft tissue). The axial strain evaluated prior to failure was close to 10’000 microstrain on the vertebral body surface and exceed 70’000 microstrain on the intervertebral discs, where failure was localized. The pattern, prepared in a dedicated way showed its suitability for both the bone and the disc. The evaluated failure strains were in agreement with the literature (Bayraktar, Morgan et al. 2004) (Spera, Genovese et al. 2011). To the authors' best knowledge, this kind of measurement including strain on soft and hard tissue simultaneously has never been performed before. This work showed the capability of DIC in providing full-field measures on the surface with complex geometry, such as the spine. The assertion of these potentialities could open the way to further application of DIC to study the behaviour of human spines, including improvement of spinal fixation devices

Introduction: Surface replacement of the hip has been proposed as an alternative to total hip replacement, particularly in young active patients. The suggested benefits include preservation of bone stock for future revision surgery and avoidance of proximal femoral stress protection, which can cause bone resorption. However, following femoral head resurfacing, femoral neck fracture can occur. The aim of this study was to compare the strain pattern in intact and resurfaced femurs using validated third generation composite femurs and rosette strain gauges. Methods: Rosette strain gauges were applied to an intact and a resurfaced third generation composite femur at three sites; narrowest part of the lateral surface of the neck, narrowest part of the medial surface of the neck and medial surface at the level of lesser trochanter. The femurs were loaded with axial loads of 600N, 800N and 1000N sequentially. The tests were repeated thrice for each femur. Maximum and minimum principal strains were calculated. Further tests were carried out in which an abductor load was included in the model. Testing was done at 600N and repeated thrice for each femur. The principal strains were calculated and compared with the the principal strains without the abductor load. Results: The maximum principal strains in the resurfaced femur were approximately 50% higher in the lateral surface of the neck and about 30% higher in the lesser trochanteric region when loaded without including an abductor force. Inclusion of the abductor force decreased the strain particularly at the lateral surface of the neck by approximately 45% in the intact femur and approximately 25% in the implanted femur. Even with the inclusion of the abductor load the strain in the resurfaced femur remained more than 50% higher at the lateral surface of the neck and 20% higher in the lesser trochanteric region. Conclusion: Our study suggests that proximal femoral stress protection will not occur following surface replacement of the hip. The increased strain at the lateral surface of the neck could result in fracture, particularly if there is notching of the neck or if abductor function has been compromised, which can happen particularly with the direct lateral approach

Introduction: By characterising ACL strain behaviour in intact and posteromedial deficient knees under a variety of external loading conditions the aim of this work was to demonstrate whether posteromedial corner insufficiency could increase strain in an ACL reconstruction graft. Materials and Methods: 15 fresh cadaveric knees were mounted on a materials testing machine. A miniature extensometer was implanted onto the anteromedial bundle (AMB) of the ACL. The knees were loaded in: Anterior draw (150N), varus/valgus rotation (5Nm) and internal/external rotation (5Nm) at 0°, 15°, 30°, 60° & 90° flexion. The posteromedial corner structures – posteromedial capsule, superficial MCL and deep MCL – were cut sequentially and the effect AMB strain measured. Results: Strain data for analysis was available for 11 intact knees: Tibial internal rotation produced increased strain in the AMB at all angles of knee flexion (p< 0.05). Tibial external rotation reduced ACL strain at 0° to 30° (p< 0.05) and 60° to 90° knee flexion (p> 0.05). Anterior loading of the tibia increased AMB strain. With the tibia free to rotate, strain was highest at 90 degrees knee flexion (5.3%) and lowest at 0 degrees (1.6%). Fixed internal and external tibial rotation reduced AMB strain produced by a 150 N anterior drawer force at all knee flexion angles. Strain data for analysis was available for 6 Posteromedial Corner deficient knees:. With the tibia free to rotate or when locked in internal rotation, cutting the posteromedial structures had no effect on AMB strain with a 150 N anterior drawer force applied to the tibia. However, with the tibia locked in external rotation, cutting the posteromedial structures increased AMB strain at 60 and 90 degrees flexion. This difference however did not reach statistical significance. Conclusions: The findings that division of the posteromedial structures may cause increased AMB strain and that there is significant load sharing by the peripheral ligamentous structures, suggests that valgus and rotational stresses to the knee in a patient with posteromedial corner insufficiency could lead to increased strain in the ACL graft, that would otherwise have been restrained by the posteromedial corner complex. It would also therefore seem to be appropriate to recommend the use of a collateral ligament brace in the post-operative period when combining a repair of the posteromedial structures and the ACL, to again prevent excessive graft strains

Hydroxyapatite-coated standard anatomical and customised femoral stems are designed to transmit load to the metaphyseal part of the proximal femur in order to avoid stress shielding and to reduce resorption of bone. In a randomised in vitro study, we compared the changes in the pattern of cortical strain after the insertion of hydroxyapatite-coated standard anatomical and customised stems in 12 pairs of human cadaver femora. A hip simulator reproduced the physiological loads on the proximal femur in single-leg stance and stair-climbing. The cortical strains were measured before and after the insertion of the stems. Significantly higher strain shielding was seen in Gruen zones 7, 6, 5, 3 and 2 after the insertion of the anatomical stem compared with the customised stem. For the anatomical stem, the hoop strains on the femur also indicated that the load was transferred to the cortical bone at the lower metaphyseal or upper diaphyseal part of the proximal femur. The customised stem induced a strain pattern more similar to that of the intact femur than the standard, anatomical stem

Aim. It has been suggested that the use of a pilot-hole may reduce the risk of fracture to the lateral cortex. Therefore the purpose of this study was to determine the effect of a pilot hole on the strains and occurrence of fractures at the lateral cortex during the opening of a high tibial osteotomy (HTO) and post-surgery loading. Materials and Methods. A total of 14 cadaveric tibias were randomized to either a pilot hole (n = 7) or a no-hole (n = 7) condition. Lateral cortex strains were measured while the osteotomy was opened 9 mm and secured in place with a locking plate. The tibias were then subjected to an initial 800 N load that increased by 200 N every 5000 cycles, until failure or a maximum load of 2500 N. Results. There was no significant difference in the strains on the lateral cortex during HTO opening between the pilot hole and no-hole conditions. Similarly, the lateral cortex and fixation plate strains were not significantly different during cyclic loading between the two conditions. Using a pilot hole did not significantly decrease the strains experienced at the lateral cortex, nor did it reduce the risk of fracture. Conclusions. The nonsignificant differences found here most likely occurred because the pilot hole merely translated the stress concentration laterally to a parallel point on the surface of the hole. Cite this article: K. Bujnowski, A. Getgood, K. Leitch, J. Farr, C. Dunning, T. A. Burkhart. A pilot hole does not reduce the strains or risk of fracture to the lateral cortex during and following a medial opening wedge high tibial osteotomy in cadaveric specimens. Bone Joint Res 2018;7:166–172. DOI: 10.1302/2046-3758.72.BJR-2017-0337.R1

Six pairs of human cadaver femora were divided equally into two groups one of which received a non-cemented reference implant and the other a very short non-dependent experimental implant. Thirteen strain-gauge rosettes were attached to the external surface of each specimen and, during application of combined axial and torsional loads to the femoral head, the strains in both groups were measured. After the insertion of a non-cemented femoral component, the normal pattern of a progressive proximal-to-distal increase in strains was similar to that in the intact femur and the strain was maximum near the tip of the prosthesis. On the medial and lateral aspects of the proximal femur, the strains were greatly reduced after implantation of both types of implant. The pattern and magnitude of the strains, however, were closer to those in the intact femur after insertion of the experimental stem than in the reference stem. On the anterior and posterior aspects of the femur, implantation of both types of stem led to increased principal strains E1, E2 and E3. This was most pronounced for the experimental stem. Our findings suggest that the experimental stem, which has a more anatomical proximal fit without having a distal stem and cortex contact, can provide immediate postoperative stability. Pure proximal loading by the experimental stem in the metaphysis, reduction of excessive bending stiffness of the stem by tapering and the absence of contact between the stem and the distal cortex may reduce stress shielding, bone resorption and thigh pain

There has been an unprecedented increase in total knee replacement in recent years. The UK national joint registry recorded over 80,000 total knee replacements per year with a generally successful outcome. Improvements in modern knee replacement designs and surgical techniques has resulted in more and more young and active patients having knee replacements. Their more active lifestyles and increased life expectancy is also leading to a rise in revision knee surgery. The most common reason for revision knee replacement is for loosening as a result of wear and/or bone resorption. Revision knee tibial components typically use long stems to increase the stability in the presence of the proximal bone loss associated with implant removal and loosening. The stem design has been cited as a possible cause of the clinically reported pain at the stem end region. The aim of this study was to experimentally validate a finite element (FE) model and the analysis different load conditions and stem orientations in a stemmed tibial component. CT-scans of a composite tibia (Sawbones) were utilized to form a multi-body solid consisting of cortical bone and cancellous bone with an intramedullary canal. A fully cemented tibial component (Stryker) was virtually implanted in the composite tibia with the stem-end centred in the cancellous bone. The tibial compartment loads were distributed with a 60:40 (Medial: Lateral) and 80:20 ratio to simulate a normal and varus type knee. Several stem-end positions were developed with the modification of the tibias proximal resection angle. An experimental study using strain gauges applied to the same composite tibia was used to compare the results with the FE-model. The model was validated with the strain gauged experimental test specimens demonstrating a similar pattern and magnitude of predicted strains. The simulation of different stem-end orientations revealed an increase in strain to the posterior cortex below the stem-end with the stem in direct contact to the posterior cortical bone. A tibial stem fully surrounded by cancellous bone demonstrated a small increase to the proximal strains. The simulation of a varus aligned knee with a 80:20 (Medial: Lateral) load distribution shifted strain overall to the medial side and revealed a large increase of strain to the posterior-medial in the proximity of the stem-end. The intensification of the load on one side of the tibial plateau, associated with a varus aligned knee, developed the largest increase in strain beneath the stem-end region and is possibly a factor in the reported pain after surgery. The stem in close proximity to the posterior cortical bone is also a possible contributing factor to pain due to the increase of strain in the vicinity of the stem-end

Introduction. Tendinopathies are among the most common musculoskeletal injuries. Nowadays, part of its diagnosis is established through subjective qualitative evaluation of 2D ultrasound (US). This enables limited diagnostic differentiation or therapeutic optimization and has limited added value to diagnosis in an earlier stage. It is generally accepted that extra diagnostic information can be obtained via strain evaluation. The accurate validation of strain estimation is challenging due to the lack of a ground-truth. Therefore we evaluate the repeatability of displacement and strain estimations in the longitudinal direction, using an easy, fast and interactive application to estimate local strain during dynamic loading of the tendon. Materials and Methods. One healthy volunteer laid in a prone position with the foot fixed to an isokinetic device. Three sets of passive movement between −10° plantarflexion and +10° dorsiflexion were performed and repeated the following day. During this, US images with a spatial resolution of 0.02mm × 0.09mm were acquired at a frame-rate of 100Hz. The US system used was the Vevo2100 with a MS250 linear array transducer with a center frequency of 20MHz. After image collection, consecutive pairs of 2D images were registered in a multi-resolution scheme, using an affine and b-spline transformation optimized by the minimization of the sum-of-squared differences, to obtain deformation vector fields. Lastly the interactive application allows local analysis of tissue displacement and strain within selected regions of interest. Mean and standard deviation of the intra- and inter-day relative differences were calculated. Results. The results show a mean intra-day relative difference of 13.71%±4.76% in displacement and of 16.29%±5.17% in strain. For inter-day comparison, the relative difference was 16.98%±14.62% in displacement and was 16%±13.51% in strain. Results show physiologically meaningful and similar strain tendencies when grouping proximal and distal regions. Discussion. This work shows promising preliminary data that suggest that with our method strain and deformation can be measured in a reproducible way using high-frequency US, with little effect of slight variations in acquisition conditions. This brings the application of US based strain estimation in clinical scenarios closer to reality. However, further tests are needed to confirm these conclusions

Background: Resurfacing hip arthroplasty has re-emerged as an option in total hip arthroplasty and by 2008 these prostheses constituted 7.8% of the total number of primary hip replacements in Australia. In the Scandinavian countries the use of resurfacing prostheses is substantially less, reported from 0.6–2.8% in the different national arthroplasty registries. The resurfacing implant preserves proximal bone stock and is expected to retain a physiological load transfer in the proximal femur. Mid-term results for the resurfacing implants are promising, but periprosthetic neck fractures remains the most frequent complication. Finite element analyses have suggested increased strains in the femoral neck area after resurfacing arthroplasty. This has not yet been proved in a cadaver model. Purpose: This study compared the strain pattern of the femoral neck and the proximal femur in cadaver femurs before and after insertion of a resurfacing femoral component. Material and method: When load transfers trough the hip joint to the femur, the bone undergoes a deformation, which can be measured by strain gauges. In this study, ten strain gauge rosettes were distributed on the femoral neck and proximal femur of thirteen human cadaver femurs. The femurs were loaded in a hip simulator for single leg stance and stair climbing. Cortical strains were measured on the femoral neck and proximal femur before and after implantation of a resurfacing femoral component (DePuy ASRTM). Results: After resurfacing the mean tensile strain increased by 15 % (CI: 6 – 24%, p=0.003) on the lateral femoral neck, and mean compressive strain increased by 11 % (CI: 5 – 17%, p=0.002) on the medial femoral neck during single leg stance simulation. On the anterior side of the femoral neck the strain increased up to 16%, however this difference was not found statistically significant. On the proximal femur the deformation pattern remained similar to the strains measured on the unoperated femurs. Discussion: Both patient related factors such as female gender, obesity and high age, and surgical factors such as notching, lack of seating and varus-orientation of the implant have been associated with increased risk of neck fracture after resurfacing arthroplasty. We asked ourselves if there could be a biomechanical factor contributing to the risk of periprosthetic fracture. The small increase of strains in the neck area would probably not alone be sufficient to cause a neck fracture. Acting together with patient-specific and surgical factors it may however contribute to the risk of early periprosthetic fracture

Total shoulder arthroplasty is a well-tested procedure that offers pain relief and restores the joint function. However, failure rate is still high, and glenoid loosening is pointed as the main reason in orthopedic registers. In order to understand the principles of failure, the principal strain distributions after implantation with Comprehensive® Total Shoulder System of Biomet® were experimental and numerically studied to predict bone behavior. Fourth generation composite left humerus and scapula from Sawbones® were used. These were implanted with Comprehensive® Total Shoulder System (Biomet®) with a modular Hybrid® glenoid base and Regenerex® glenoid and placed in situ by an experienced surgeon. The structures were placed in order to simulate 90º abduction, including principal muscular actions. Muscle forces used were as follows: Deltoideus 300N, Infraspinatus 120N, Supraspinatus 90N, Subscapularis 225N. All bone structures were modeled considering cortical and the trabecular bone of the scapula. The components of prosthesis were placed in the same positions than those in the in vitro models. Geometries were meshed with tetrahedral linear elements, with material properties as follows: Elastic modulus of cortical bone equal to 16 GPa, elastic modulus of trabecular bone equal to 0.155 GPa, polyethylene equal to 1GPa and titanium equal to 110 GPa. The assumed Poisson's ratio was 0.3 in all except for polyethylene where we assumed a value of 0.4. The prosthesis was considered as glued to the adjacent bone. The finite element model was composed of 336 024 elements. At the glenoid cavity, the major influence of the strain distributions was observed at the posterior-superior region, in both cortical and trabecular bone structures. The system presents critical region around holes of fixation in glenoid component. At the trabecular bone, the maximum principal strains at the posterior-superior region ranged from 2250 µε to 3000 µε. While at the cortical bone, the maximum principal strains were 300 µε to 400 µε. The results observed evidence some critical regions of concern and the effect of implant in the bone strains mainly at the posterior-superior region of the glenoid cavity is pronounced. This indicates that this region is more affected by the implant if bone remodeling is a concern and it is due to the strain-shielding effect, which has been connected with loosening of the glenoid component

Osteoarthritis is one of the most common musculoskeletal diseases. It involves degeneration and loss of articular cartilage, leading to a painful bone on bone articulation during movement. Numerical FEA models exist to predict the mechanical behaviour of degenerated cartilage. One of the limitations of these models arises from the poor validation that can be attained with traditional experimental data. This typically relies on comparison with global mechanical quantities such as total tissue strain, which mask the individual contributions originating from the different layers. In order to improve on this, an experimental method was developed to visualise the through-thickness behaviour of articular cartilage. Four experiments were performed on hemi-cylindrical cartilage plugs, harvested from a porcine femoral head, and immersed in a fluid solution. An Indian ink speckle pattern was applied to the flat surface of each hemi-cylinder. The specimens were equilibrated in 2.5M NaCl solution, transferred to a custom designed testing rig, and a reference image of the tissue cross-section was taken. The solution concentration was then decreased to 0.15M and, predictably, the tissue thickness changed. Images of the tissue cross section were taken every 60s for the duration of the experiment (3600s). All images were analysed using a DIC algorithm (Ncorr open-source 2D digital image correlation matlab program), and documented the strain changes through the tissue thickness as a function of time. The measured total strain in the tissue was consistent with that reported by Lai et al. (1991). However the present technique allows to quantify the strain contribution from any of the tissue layers or sublayer. This poses a significant advantage over traditional methods as resulting information can further the understanding of the factors contributing to the mechanical behaviour of the tissue and provides an ideal platform for validating more and more refined models of tissue behaviour

Aim. To investigate the effects of strain rate and mineral level on the stress at failure, stiffness and toughness of whole bones. Methods. 40 ovine femurs were harvested and subjected to either slow [8.56 × 10−3 s−1 (± 1.42 × 10−3 SD)] or dynamic [17.14 s−1 (± 8.20 SD)] loading. Half the bones were demineralised by 20% compared to the original mineral content. These were allocated evenly between the high and low strain rate groups. Dynamic loading was achieved by custom designed comminution device. Slow rate testing was carried out on a Zwick/Roell z005 testing machine. Results. Strain rate was found to increase the Young's modulus in both the normal and demineralised bone. Additionally the toughness of the bones at failure was found to reduce with increasing strain rate. When comparing bone of normal quality the stress at failure was found to increase with strain rate. However, this effect was greatly reduced when comparing the effect of strain rate on the stress occurring in demineralised bones. Discussion. These results show that bone has an ability to withstand higher than normal stresses if these are applied quickly and for a short duration (such as would occur in a traumatic event). This ability is greatly reduced when the bone is of reduced mineral content, such as is found in aged or diseased bone

Introduction. The role of soft tissue balancing in optimizing functional outcome and patient satisfaction after total knee arthroplasty surgery is gaining interest. This is due in part to the inability of pure alignment to demonstrate excellent functional outcomes 6. Consistent soft tissue balancing has been aided by novel technologies that can quantify loads across the joint at the time of surgery 4. In theory, compressive load equilibrium should be correlated with ligamentous equilibrium between the medial and lateral collateral ligaments. The authors propose to use the Collateral Ligaments Strain Ratio (CLSR) as a functional tool to quantify and track surgical changes in laxity of the collateral ligaments and correlate this ratio to validated functional scores and patient reported outcomes. The relationship with intra-operative balancing of compartmental loads can then be scrutinized. The benefits of varus-valgus balancing within 2o include increased range of motion 7, whereas pressure imbalance between the medial and lateral joint compartments has been linked to condylar liftoff and abnormal kinematics post-TKA 8. Methods. The study is a prospective IRB approved clinical study with three cohorts of 50 patients each: (1) a surgical prospective study group (2) a matched control group of non-operated high function patients; (3) a matched control group of high function knee arthroplasty recipients. Standard statistical analysis method is applied. The testing of the CLSR is performed using a validated Smart Knee Brace developed by the authors and previously reported 1. The output variables consist of the maximum angular change of the knee in the coronal plane at 10 degrees of flexion produced by a controlled torque application in the varus and valgus (VV) directions. This creates measureable strain on the lateral and medial collateral ligaments, which is reported as a ratio (CLSR). The New Knee Society Score is used to track outcomes. The intra-operative balance is achieved by means of an instrumented tibial tray (OrthoSensor, Inc). Results. Pre-operative scatter graphs (Fig 1) demonstrate a wide distribution of absolute VV values, reflecting the spectrum of pathological states. The relative distribution of strain after surgery trends towards consolidation. The median CLSR is 0.55 with a SD of 0.20 at 4 weeks post-operative. This asymmetrical value indicates a shift toward a tighter medial side as noted in the non-operated cohort. Scatter graphs demonstrate post-operative clustering similar to that reported by the authors for kinetic loads after soft tissue balancing (Fig 2)3. The overall displacement values range from 0 −4 degrees. Discussion. The angular changes under standard torque appear to correlate with previously reported linear displacement values 3. Past studies do indicate a shift toward a tighter medial side in healthy older individuals, with an average CLSR in extension and flexion of 0.55 5. Success in achieving soft tissue balancing of the knee at the time of arthroplasty surgery may be predicted by a defined collateral ligament strain ratio under controlled VV testing. This study demonstrates clustering of the strain ratio in slight medial tightness with a range of absolute angular displacements of 0–4 degrees

The fourth lumbar vertebrae and L4-5 discs from six cadaveric lumbar spines were subjected to detailed strain gauge analysis under conditions of controlled loading. With central compression loads, maximal compressive strain was found to occur near the bases of the pedicles and on both superficial and deep surfaces of the pars interarticularis, which emphasises the importance of the posterior elements of lumbar vertebrae in transmitting load. Radial bulge and tangential strain of the disc wall were maximal at the posterolateral surface, in agreement with the fact that disc degeneration and prolapse commonly occur there. Under posterior offset loads simulating extension, both compressive and tensile strains were found to be increased on both surfaces of the pars interarticularis, which suggests that hyperextension may lead to stress fractures and spondylolisthesis. Posterior offset loads also increased the radial bulge of the posterior disc wall and tangential strain at the anterior surface of the disc. Anterior offset loads simulating flexion increased the radial bulge of the anterior disc wall and tangential strain at the posterior surface of the disc. These findings are compatible with movement of the nucleus pulposus within the disc during flexion and extension. This hypothesis was supported by post-mortem discography

Aim: This study investigated the difference in proximal tibial cortical strain distribution using a fixed or mobile bearing design for TKA. Methods: Eight fresh frozen human cadaver tibias were used. The strain magnitude and distribution on the anterior cortex of the proximal tibia during axial and rotational loading of the knee were measured with a quantitative full-field strain measurement technique (Electronic Speckle Pattern Interferometry). First, strain distributions of the intact knee were acquired. Subsequently, strain distributions after implementation of conventional and mobile bearing PCL retaining total knee implants (Scorpio®) were measured. Results: Under each loading condition, the minimum principal strain was greater in magnitude as compared to the maximum principal strain. Under 1′500 N axial loading, the resulting minimum principal strain magnitude and orientation was nearly identical between the mobile bearing configuration (500 ± 287 με), and the fixed bearing configuration (500 ± 286 μ ε). In response to 10° internal rotation, this strain increased to 782 ± 371 μ ε and 1000 ± 389 μ ε for the mobile and fixed tibial component, respectively. In response to 10° external rotation, minimal principal strain decreased to 421 ± 233 μ ε for the mobile bearing, but increased to 632 ± 293 μ ε for the fixed bearing. These differences between mobile and fixed bearing scenarios were statistically highly significant. Conclusion: For this in-vitro study under exact controlled loading conditions the mobile bearing design induced less strain in the proximal tibia as the fixed bearing tibial component. The difference in strain levels may be of importance to understand bone remodeling and osseointegration

Purpose: Previous research has reported that increasing the posterior tibial slope through an opening wedge osteotomy results in an anterior shift in the position of the tibia relative to the femur. However, the effect of this on anterior cruciate ligament (ACL) strain remains insufficiently understood. The purpose of this study was to examine the relationship between tibial slope and tibial translation, as well as between tibial slope and ACL strain. It was hypothesized that increasing the posterior tibial slope would result in an increase in anterior tibial translation thereby increasing strain in the ACL. Methods: Five cadaveric knees were subjected to a randomized experimental design study. One knee was excluded due to failure of a strain gauge during experimentation, resulting in data for four knees. The femoral and tibial portions of the knee were potted with PMMA and fixed using fixation pins. An anterior-based osteotomy was performed with no osteotomy plate present. A strain gauge was then placed in the anteromedial bundle of the ACL. Each knee was mounted at a flexion angle of 15° and loaded with various combinations of A-P loads (18N, 108N, 209N) and axial loads (216N, 418N), according to the study design. Osteotomies of 5mm and 10mm were then performed and measurements of strain and tibial translation were taken after each according to the study design. Tibial slopes were determined through lateral fluoroscopic imaging. Results: As posterior tibial slope increased, anterior tibial translation increased as anticipated. However, contrary to expectations, as posterior slope increased, ACL strain decreased. One explanation for this result could be that by performing the osteotomy, the insertions sites of the ACL were being moved closer together resulting in increased ACL laxity. At higher slope angles, translation levels off, suggesting constraint of some tissue besides the ACL. Conclusions: Although increasing the tibial slope through opening wedge osteotomy leads to an anterior tibial translation, there is no increase in strain on the ACL. Further studies are needed to examine the effect of opening wedge osteotomy on other soft tissue restraints of the knee

Mechanical loading during physical activity produces strains within bones. It is thought that these forces provide the stimulus for the adaptation of bone. Tibial strains and rates of strain were measured in vivo in six subjects during running, stationary bicycling, leg presses and stepping and were compared with those of walking, an activity which has been found to have only a minimal effect on bone mass. Running had a statistically significant higher principal tension, compression and shear strain and strain rates than walking. Stationary bicycling had significantly lower tension and shear strains than walking. If bone strains and/or strain rates higher than walking are needed for tibial bone strengthening, then running is an effective strengthening exercise for tibial bone

To compare strains measured in a whole rat-tail vertebra by image registration (IM) with those predicted by solid finite element analysis (FEA). Quantification of bone strain allows better understand fracture risk, bone healing and turnover. The sixth caudal vertebra of an rnu/rnu rat was μCT scanned (17.5×17.5×17.5μm/voxel) while loaded (27N axial compression) and unloaded. IM was used to calculate strain and displacement fields in the bone due to the applied load by finding a spatial mapping between the two scans. Strain was resolved to varying spatial resolution; high strain gradient regions (ie growth plates) were analyzed to higher spatial resolutions. A FE model was created of the unloaded vertebra, consisting of tetrahedral elements with transversely isotropic material properties. Elements were assigned elastic moduli based upon μCT image intensities. Growth plate moduli ranged from 0–150kPa and the bone moduli ranged from 0.2–15000MPa. Vertebral geometry was created through segmentation of μCT images. Displacement boundary conditions were obtained by matching cranial and caudal surfaces in the unloaded and loaded scans. The displacement fields of the two methods were compared by using the fields calculated to deform the unloaded scan to match the loaded scan. The strains were compared by plotting FEA measured axial strain against IM calculated axial strain. The displacement fields calculated by both methods were able to spatially align the unloaded scan to the loaded scan (Mean Voxel Intensity Difference: FEA=441HU, IM=328HU, Unregistered=969HU). IM and FEA show very limited agreement in axial strain measurement (R2=0.388, Slope=0.75, X-Intercept=0.0037) although both calculated high axial strains in the growth plates and low axial strains in the trabecular and cortical bone. Good agreement was found in the mean axial strain measured by both methods (IM= −0.044, FEA=−0.037). IM was better able to deal with difficulties in quantifying bone strain due to the growth plate than FEA. IM presents advantages over FEA in measuring strain in complex whole bone trabecular structures, however has lower spatial resolution than is possible with FEA

Polymer foams have been extensively used in the testing and development of orthopaedic devices and computational models. Often these foams are used in preference to cadaver and animal models due to being relatively inexpensive and their consistent material properties. Successful validation of such models requires accurate material/mechanical data. The assumed range of compressive moduli, provided in the sawbones technical sheet, is 16 MPa to 1.15 GPa depending on the density of foam. In this investigation, we apply two non-contact measurement techniques (digital volume correlation (DVC) and optical surface extensometry/point-tracking) to assess the validity of these reported values. It is thought that such non-contact methods remove mechanical extensometer errors (slippage, misalignment) and are less sensitive to test-machine end-artifacts (friction, non-uniform loading, platen flexibility). This is because measurement is taken directly from the sample, and hence material property assessment should be more accurate. Use of DVC is advantageous as full field strain measurement is possible, however test time and cost is significantly higher than extensometry. Hence, the study also sought to assess the viability of optical extensometry for characterising porous materials. Testing was conducted on five 20 mm cubic samples of 0.32g/cc (20 pcf) solid rigid polyurethane foam (SAWBONESTM). The strain behaviour was characterised by incremental loading via an in situ loading rig. Loading was performed in 0.1 mm increments for 8 load steps with scans between loading steps. Full field strain measurement was performed on one sample by micro focus tomography (muvis centre, Southampton) and subsequent DVC (DaVis, Lavision). Average strains in each direction were then calculated to enable modulus and Poisson's ratio calculation. These results were subsequently corroborated by use of optical point-tracking (MatchID). To account for heterogeneities, axial strain measurements were averaged from six points on the front and rear surfaces (fig.2). In each test compressive displacement was applied to 900N (∼2MPa) to remain within the linear elastic region. Significant variability of individual strain measurements were observed from point couples on the same sample, indicating non-uniform loading did occur in all samples. However, by averaging across multiple points, linear loading profiles were ascertained (fig.2). For all non-contact methods the calculated elastic moduli were found to range between 331–428 MPa whilst the approximated modulus based on cross head displacement was ∼210 MPa, similar to the manufacturer's quoted value (220MPa). The point-tracking gave a significantly higher modulus (p = 0.047) than the DVC results as only surface measurements were made. It is thought that a correction factor may be ascertained from the finite element method to correct this. Both the DVC and point-tracking results (p = 0.001) indicated a substantially higher compressive modulus than the manufacturer provided properties. This study demonstrates that methods of measuring displacement data on cellular foams must be carefully considered, as artefacts can lead to errors of up to 70% compared to optical and x-ray based techniques

Introduction. In this study, three-dimensional (3D) digital image correlation (DIC) was adopted to investigate the strain in the superficial medial collateral ligament (sMCL) of the human knee. To our knowledge, no reports or validation of 3D DIC measurement on human collagenous tissue exists. The first part of this research project focused on the validation of 3D DIC (1) as a highly accurate tool for non-contact full field strain analysis of human collagenous tissue. In the second part, 3D DIC was used to measure the strain patterns in the superficial medial collateral ligament (sMCL) of the native knee (2). In a third part, the strain pattern in the sMCL after total knee arthroplasty (TKA) in an ‘optimal’ (3) and with a proximalised joint line (4) was analysed. Methods. (1) Six fresh frozen human Achilles tendon specimens were mounted in a custom made rig for uni-axial loading. The accuracy and reproducibility of 3D DIC was compared to two linear variable differential transformers (LVDT's). (2) The strain pattern of the sMCL during the range of motion (ROM) was measured using 3D DIC in six fresh frozen cadaveric knees. The knees were mounted in a custom made rig, applying balanced tension to all muscle groups around the knee. The experiment was repeated after computer navigated implantation of a single radius posterior stabilised (PS) TKA in ‘optimal’ (3) and with a 4 mm proximalised joint line (4). Results. (1) Accuracy analysis revealed that the scatter was very low for all specimens (0,03%) and a spatial resolution of 0,1 mm for strain measurement was reached. When compared to the LVDT, DIC showed excellent correlation (R = 0.99). (2) Overall, the sMCL behaved isometrically between 0° and 90° of flexion showing less 1% slackening in all specimens. Further slackening was seen in deeper flexion. Significant regional inhomogeneity was observed (fig 1). The highest strains (up to 5% lengthening) were seen in the proximal part. The middle and distal part were near isometric between 0° and 90° of flexion. (3) A significant alteration of the strain pattern was seen after TKA with an increased strain in all parts of the sMCL from 90° to deeper flexion (fig 2). (4) This effect became significantly more pronounced with joint line proximalisation. Discussion. Strain in the native sMCL proved to be inhomogeneously distributed with significant differences between proximal, middle and distal part during the ROM. The higher baseline strain in the proximal part might be the explanation for the fact that most of the sMCL lesions are seen in that region. A single radius TKA failed to reproduce the native sMCL strain pattern from 90° to deeper flexion. This effect became even more pronounced with joint line proximalisation. These higher sMCL strains might compromise deeper flexion after TKA. Conclusion. The strain pattern of the sMCL in the native knee showed important regional differences during the ROM and significant alterations after TKA implantation and joint line proximalisation

Rotator cuff repair is performed to treat shoulder pain and disability. Failure of the tendon repair site is common; one strategy to improve healing is to enforce a period of post-operative immobilisation. Immobilisation may have unintended effects on tendon healing. Tenocytes under uniaxial strain form more organised collagen and up regulate expression of proliferative genes. Vitamin C (ascorbic acid), an anti-oxidant that is a co-factor for collagen synthesis, has also been reported to enhance collagen deposition and organisation. The purpose of this study was to compare human tenocyte cultures exposed to uniaxial cyclical strain with or without slow-release ascorbic acid (ascorbyl-2 phosphate) to determine their individual and combined effects on tissue remodelling and expression of tissue repair genes. Rotator cuff tissues were collected from degenerative supraspinatus tears from eight patients. Tenocytes were incorporated into 3D type I collagen culture matrices. Cultures were divided into four groups: 1) ascorbic acid (0.6mMol/L) + strain (1%–20% uniaxial cyclic strain at 0.1 Hz), 2) ascorbic acid unstrained, 3) strain + vehicle 4) unstrained + vehicle. Samples were fixed in paraffin, stained with picrosirius red and analysed with circular polarising light. A second set of cultures were divided into three groups: 1) 0.5mM ascorbic acid, 2) 1mM ascorbic acid, 3) vehicle cultured for 24, 72, 120 and 168 hours. Cell-free collagen matrix was used as a control. Tenocyte proliferation was assessed using the water soluble tetrazolium-1 (WST1) assay and f tissue repair gene expression (TGFB1, COL1A1, FN1, COLIII, IGF2, MMP1, and MMP13), were analysed by qPCR. The data were analysed using a Split model ANOVA with contrast and bonferroni correction and a one-way ANOVAs and Tukey's test (p<0.05 was significant). Our results indicated that unstrained cultures with or without exposure to slow release ascorbic acid exhibited greater picrosirius red birifringency and an increase in collagen fiber deposition in a longitudinal orientation compared to strained tenocytes. We found that slow release ascorbic acid promoted significant dose and culture-time dependent increases in tenocyte proliferation (p<0.05) but no obvious enhancement in collagen deposition was evident over cultures without ascorbic acid supplementation. Based on these data, applying strain to tenocytes may result in less organised formation of collagen fibers, suggestive of fibrotic tissue, rather than tendon remodelling. This may indicate that a short period of immobilisation post-rotator cuff repair is beneficial for the healing of tendons. Exposure to slow release ascorbic acid enhanced tenocyte proliferation, suggesting that supplementation with Vitamin C may improve tendon repair post-injury or repair. Future studies will assess levels of tissue repair-associated proteins as well as comparing traumatic and degenerative rotator cuff tears to healthy uninjured rotator cuff tissue

Background: Locking plates are used frequently in distal tibial fractures. We tested two different types of locking compression plates (LCP): the metaphyseal plate (MP) and the distal tibial plate (DTP). We evaluated the strain imposed on an experimental tibial osteotomy, and the stability of plate-tibia (composite bone) construct using LCP-MP and LCP-DTP. Materials and methods: Twin strain gauged special composite tibial bones were used to simulate the human tibiae. We tested 5 tibiae: one was used as control, two tibiae were tested using LCP-MP, and two with LCP-DTP. Strain was measured by subjecting each construct to a cyclic load of 700 N at 3 Hz in neutral, flexion, extension and torsion to simulate the normal walking cycle. Results: When compared with the control tibia, strain during the neutral moment at the proximal and distal strain gauge site in the LCP-MP and LCP-DTP constructs decreased by 6.4%/−41.5% and −39%/−47%, respectively. In flexion, the strain increased consistently in both the proximal and distal strain gauge sites using the LCP-MP by 34% and 109%. Using the LCP-DTP, the strain at the proximal strain gauge site decreased by 0.2% and increased by 18% at the distal strain gauge site. In extension, strain decreased by 25% at the proximal strain gauge site, and by 60% at the distal strain gauge site in the LCP –MP construct. In the LCP-DTP construct, the strain decreased by 13% at the proximal strain gauge site, and by 21% at the distal strain gauge site. There were no statistically significant torsional differences between LCP-MP and LCP-DTP group (P=0.121). In this experimental setup, the LCP-DTPs offer greater control of strain than LCP-MPs. They also confer greater resistance to fracture macro-movements, and improved stiffness consistently in neutral, flexion, and torsion than LCP-MPs. Conclusion: The strain from osteotomised tibiae stabilised with LCP-MPs and LCP-DTPs were close to the strain of the control tibia. Both these locking plates were equally good and conferred greater stiffness in all loading positions

Objective: To examine the effect of varying the thickness of the cement mantle on the strain distribution near the bone-cement interface. Background: An insufficient cement mantle is thought to generate cement fractures near the bone-cement interface. Debonding at the bone-cement interface may accompany such fractures, and, mechanical failure of the prosthesis may follow. In this study, we aim to analyse the relationship between the cement mantle thickness and the acetabular strain distribution near the bone-cement interface. Experimental model: Four hemi-pelvic saw bones specimens were implanted with six protected precision strain gauges. All specimens were prepared to receive a 53/28 cemented polyethylene cup (Depuy Charnley Elite). Methods: We simulated hip joint force relative to the cup during normal walking for quasi-static tests on an Instron 1603 testing machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated based on the readings of strains from a 32 channel digital acquisition system. Results: Statistically significant differences in the total strains per gait cycle (p< 0.001) have been noted at all gauge locations. In the principal load bearing quadrants, the recorded tensile strains are reduced by 50% as a result of the thicker mantle, while the transmission of compressive strain is enhanced. Conclusion: A cement mantle thickness of 5-6mm may preserve the structural integrity of the principal load bearing quadrants of the acetabulum better than a mantle thickness of 2-3mm, by minimising the acetabu-lar strains. This maybe desirable in total hip replacements for conditions such as rheumatoid arthritis and osteoporosis, where the poorer quality bone can be assisted by recruitment of a larger surface area to participate in load bearing. Keywords: Principal strains; Cement mantle; Mantle thickness; Bone-cement interface; Acetabular strains

We implanted titanium and carbon fibre-reinforced plastic (CFRP) femoral prostheses of the same dimensions into five prosthetic femora. An abductor jig was attached and a 1 kN load applied. This was repeated with five control femora. Digital image correlation was used to give a detailed two-dimensional strain map of the medial cortex of the proximal femur. Both implants caused stress shielding around the calcar. Distally, the titanium implant showed stress shielding, whereas the CFRP prosthesis did not produce a strain pattern which was statistically different from the controls. There was a reduction in strain beyond the tip of both the implants. This investigation indicates that use of the CFRP stem should avoid stress shielding in total hip replacement

This study investigated the difference in proximal tibial cortical strain distribution using a fixed or mobile bearing design for TKA. Eight fresh frozen human cadaver tibias were used. The strain magnitude and distribution on the anterior cortex of the proximal tibia during axial and rotational loading of the knee were measured with a quantitative full-field strain measurement technique (Electronic Speckle Pattern Interferometry). First, strain distributions of the intact knee were acquired. Subsequently, strain distributions after implantation of conventional and mobile bearing PCL retaining total knee implants (Scorpio®) were measured. Under each loading condition, the minimum principal strain was greater in magnitude as compared to the maximum principal strain. Under 1,500 N axial loading, the resulting minimum principal strain magnitude and orientation was nearly identical between the mobile bearing configuration(500 ± 287m;e;), and the fixed bearing configuration (500 ± 286m;e;). In response to 10° internal rotation, this strain increased to 782 ± 371m;e; and 1000± 389m;e; for the mobile and fixed tibial component, respectively. In 10° external rotation, minimal principal strain decreased to 421 ± 233m;e; for the mobile bearing, but increased to 632 ± 293m;e; for the fixed bearing. These differences between mobile and fixed bearing scenarios were highly statistically significant. For this in-vitro study under exact controlled loading conditions the mobile bearing design induced less strain in the proximal tibia than the fixed bearing tibial component. The difference in strain levels may be of importance for bone remodeling and osseointegration

Bones can adapt in response to mechanical stimuli; higher rates of loading have been associated with greater bone formation rates. This study determined where bone accretion was localized in response to high loading rates. Non-invasive loads were applied to mice tibiae at one of three rates for four week. It was found, via calcein labels, that adaptation on the periosteal, but not endosteal, surface exhibited a dose-response relation with loading rate; periosteal and endosteal adaptation was localized to regions of high strain gradients. Understanding the stimuli bone responds to may underpin the development of non-pharmacological treatments to enhance bone mass. Bones can adapt to mechanical stimuli; higher rates of loading have been linked with greater bone formation rates (BFR). The purpose of this research was to determine if bone accretion associated with higher loading rates occurs in regions of high strain gradients or strain rates and if adaptation is similar on periosteal and endosteal surfaces. Periosteal but not endosteal surfaces displayed a dose-response relation with loading rate. Adaptation on both periosteal and endosteal surfaces was localized to sectors with high strain gradients. Understanding the precise stimuli by which bone responds may underpin the development of non-pharmacological treatments to enhance bone mass. Tibia loaded at the high rate had significantly greater periosteal BFR, relative to tibiae loaded at medium (> 48 %) and low (> 104 %) rates; adaptation was localized to posterior sectors (high strain gradients). Endosteally, adaptation was localized to regions of high strain gradients (anterior sectors), but did not display a dose-response relation with loading rate. Forty-three skeletally mature C57BL/6 mice were randomly assigned to one of three groups, based on loading rate; low (0.004 ε/s; n = 14), medium (0.020 ε/s; n = 15), and high (0.100 ε/s; n = 14). Loads were applied so that tibiae experienced non-invasive medio-lateral cantilever bending (peak strain = 1000 με) at 1 Hz, 60 s, 5 d/w, for 4 wk. Calcein bone labels were administered on d one and eighteen. A standardized sector of the tibial middiaphyeal shaft was digitally divided into 45° radial sectors, and prepared for histomorphometry. Funding: Funded in part by NSERC, and CIHR

Objective: To develop in-vitro experiments that measure the strain distributions at the bone-implant and bone-cement interface of the acetabular region under physiological loading conditions for cemented and cementless sockets. Experimental model: Four hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges, two specimens were prepared to receive a cemented polyethylene cup (Depuy Charnley Elite 53/28). Another two specimens were prepared and implanted with un-cemented Duraloc 58/28 cups. Press-fit technique was validated by torque measurements. Background: Symptoms associated with prosthetic migration result from osteoclast induced bone resorption at the interface adjacent to bone. We aim to develop a new and more accurate method of measuring strains at this critical interface. Methods: To simulate quasi-static loading, selected variables of hip joint force relative to the cup during normal walking was used for quasi-static tests on an Instron 1603 testing machine. The magnitude and orientation of the principal strains (maximum and minimum) were calculated based on the readings of strains from a 32 channel digital acquisition system. Results: The magnitude and distribution of acetabular trabecular bone strains are dependent on the type of cup material (un-cemented/cemented) implanted. At the position of maximum load, the maximum principal strain in the un-cemented specimens was 14.4 times higher than that for the cemented specimens (T-value = −96.40, P-value = 0.007). The highest recorded tensile strains in these specimens were localised to the acetabular rim of the posterior-superior quadrant. For the cemented specimens, the maximum principal strains are highest in the dorsal acetabulum, at a location that approximates to the centre of rotation of the replaced hip joint. Shear strains in the posterior-superior quadrant of both cementless and cemented acetabuli surpass the maximum principal strains. Conclusion: In both cemented and un-cemented specimens, the maximum shear and principal strains magnitude show similar spatial and statistical distribution. As indicators of local failure prospect within the acetabulum, these strains suggest that the posterior-superior quadrant is the most likely site for load-induced micro-fractures, in both cemented and cementless acetabuli

Introduction. Each year, a large number of total hip arthroplasties (THA) are performed, of which 60 % use cementless fixation. The initial fixation is one of the most important factors for a long lasting fixation [Gheduzzi 2007]. The point of optimal initial fixation, the endpoint of insertion, is not easy to achieve, as the margin between optimal fixation and a femoral fracture is small. Femoral fractures are caused by peak stresses induced during broaching or by the hammer blows when the implant is excessively press-fitted in the femur. In order to reduce the peak stresses during broaching, IMT Integral Medizintechnik (Luzern, Switzerland) designed the Woodpecker, a pneumatic broach that generates impulses at a frequency of 70 Hz. This study explores the feasibility of using the Woodpecker for implant insertion by measuring both the strain in the cortical bone and the vibrational response. An in vitro study is presented. Material and Methods. A Profemur Gladiator modular stem (MicroPort Orthopedics Inc. Arlington, TN, USA) and two artificial femora (composite bone 4th generation #3403, Sawbones Europe AB, Malmö, Sweden) were used. One artificial femur was instrumented with three rectangular strain gauge rosettes (Micro-Measurements, Raleigh, NC, USA). The rosettes were placed medially, posteriorly and anteriorly proximally on the cortical bone. Five paired implant insertions were repeated on both artificial bones, alternating between standard hammering and Woodpecker insertions. During the insertion processes the vibrational response was measured at the implant and Woodpecker side (fig. 1) using two shock accelerometers (PCB Piezotronics, Depew, NY, USA). Frequency spectra were derived from the vibrational responses. The endpoint of insertion was defined as the point when the static strain stopped increasing during the insertion. Results. Peak stress values calculated out of the strain measurement during the insertion showed to be significantly (p < 0.05) lower at two locations using the Woodpecker compared to the hammer blows at the same level of static strain. However, the final static strain at the endpoint of insertion was approximately a factor two lower using the Woodpecker compared to the hammer. During the last hammer insertion a fracture occurred, which was clearly visible in the frequency spectra. Figure 2 shows the sudden change between the spectra of the hit prior and after the fracture. Discussion/Conclusion. Peak stresses showed to be lower using the Woodpecker compared to hammer insertion, which is a promising result concerning fracture prevention. However it needs to be taken into account that it was not possible to reach the same level of static strain using the Woodpecker as with the hammer insertion. It is expected that the Woodpecker in its actual design is not able to reach a similar level of press-fit as hammer blows. Using vibrational data showed to be promising for fracture detection, as fractures are not always visible due to the soft tissue. For figures, please contact authors directly

25–40% of unicompartmental knee replacement (UKR) revisions are performed for unexplained pain possibly secondary to elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on cancellous bone strain in a finite element model (FEM) of a cemented fixed bearing medial UKR, validated using previously published acoustic emission data (AE). FEMs of composite tibiae implanted with an all-polyethylene tibial component (AP) and a metal backed one (MB) were created. Polyethylene of thickness 6–10mm in 2mm increments was loaded to a medial load of 2500N. The volume of cancellous bone exposed to <−3000 (pathological overloading) and <−7000 (failure limit) minimum principal (compressive) microstrain (µ∊) and >3000 and >7000 maximum principal (tensile) microstrain was measured. Linear regression analysis showed good correlation between measured AE hits and volume of cancellous bone elements with compressive strain <−3000µ∊: correlation coefficients (R= 0.947, R2 = 0.847), standard error of the estimate (12.6 AE hits) and percentage error (12.5%) (p<0.001). AP implants displayed greater cancellous bone strains than MB implants for all strain variables at all loads. Patterns of strain differed between implants: MB concentrations at the lateral edge; AP concentrations at the keel, peg and at the region of load application. AP implants had 2.2 (10mm) to 3.2 (6mm) times the volume of cancellous bone compressively strained <−7000µ∊ than the MB implants. Altering MB polyethylene insert thickness had no effect. We advocate using caution with all-polyethylene UKR implants especially in large or active patients where loads are higher

INTRODUCTION:. The purpose of this study was to determine if a short femoral stem (Lima Corporate, Udine, Italy) would result in a strain distribution which mimicked the intact bone better than a traditional length stem, thereby eliminating the potential for stress-shielding. METHODS:. A 2 mm thick moldable plastic (PL-1, Vishay Micromeasurements, Raleigh, NC) was contoured to six fourth-generation composite femoral bones (Pacific Research Laboratories, Vashon, WA). The intact femurs were then loaded (82 kg) in a rig which simulated mid-stance single limb support phase of gait (Figure 1). During testing, the femurs were viewed and video recorded through a model 031 reflection polariscope. Observing the photoelastic coating through the polariscope, a series of fringes could be seen, which represented the difference in principal strain along the femur. The fringes were quantified using Fringe Order, N, as per the manufacturers technical notes. In order to analyze the strain distribution, the femur was separated into 6 zones, 3 lateral and 3 medial, and the maximum fringe order determined. Upon completion of testing of the intact femur, the short length femoral stem was inserted and tested, and finally the traditional length femoral stem was inserted and tested. Anterior and lateral radiographs were obtained of the femur with each femoral stem in order to confirm proper alignment. RESULTS:. Fringes formed in a similar pattern for all femurs, intact and with stems. The fringes first occurred medially and laterally in a proximal-distal direction and radiated outward, decreasing in fringe order, toward the neutral axis of bending (anterior and posterior). The magnitude of the fringe order, N, remained the same or increased in the proximal to distal direction. This became more prominent, particularly on the lateral side, with the traditional length femoral stem, when a distal migration of the fringes was seen compared to the intact femur (Figures 2 and 3). Medially, with the traditional length femoral stem, the fringes remained but were of a lower magnitude than the intact femur. The femoral strain pattern, resulting from implantation of the short length femoral stem, was found to closely match the intact femur. X-rays confirmed proper alignment of all implants. CONCLUSIONS:. The distal migration of strain seen with the traditional length femoral stem was indicative of potential stress shielding. As an alternative, this study suggests that the short length femoral stem most closely replicates the strain distribution of the intact femur and may limit this type of failure

Introduction. Traditional applied loading of the knee joint in experimental testing of RTKR components is usually confined to replicating the tibiofemoral joint alone. The second joint in the knee, the patellofemoral joint, can experience forces of up to 9.7 times body weight during normal daily living activities (Schindler and Scott 2011). It follows that with such high forces being transferred, particularly in high flexion situations such as stair climbing, it may be important to also represent the patellofemoral joint in all knee component testing. This research aimed to assess the inclusion of the patellofemoral joint during in vitro testing of RTKR components by comparing tibial strain distribution in two experimental rigs. The first rig included the traditional tibiofemoral joint loading design. The second rig incorporated a combination of both joints to more accurately replicate physiological loading. Five implanted tibia specimens were tested on both rigs following the application of strain gauge rosettes to provide cortical strain data through the bone as an indication of the load transfer pattern. This investigation aimed to highlight the importance of the applied loading technique for pre-clinical testing and research of knee replacement components to guide future design and improve patient outcomes. Methods. Five composite tibias (4th Generation Sawbones) were prepared with strain gauge rosettes (HBM), correctly aligned and potted using guides for repeatability across specimens. The tibias were then implanted with Stryker Triathlon components according to surgical protocol. The first experimental rig was developed to replicate traditional knee loading conditions through the tibiofemoral joint in isolation. The second experimental rig produced an innovative method of replicating a combination of the tibiofemoral and patellofemoral joint loading scenarios. Both rigs were used to assess the load distribution through the tibia using the same tibia specimens and test parameters for comparison integrity (Figure 1). The cortical strains were recorded under an equivalent 500 N cyclical load applied at 10° of flexion by a hydraulic test machine. Results. The average results comparing both experimental rigs at three strain gauge locations are shown in Figure 2. Paired t-tests were performed on all results and a p value of p<0.05 was considered significant. No significant differences were found between the rigs. There was a trend towards a reduction in proximal principal strain with the inclusion of the patellofemoral joint (p=0.058). Discussion. The results of this study indicate that there is no significant difference in tibial load transfer between the traditional and novel applied loading techniques at small flexion angles. There is a trend towards a reduction in proximal strain when including the patellofemoral joint. This reduction may be linked to the patella tendon force counteracting the effect of tibiofemoral loading at this small flexion angle. At high flexion angles the patellofemoral reaction load increases significantly relative to the tibiofemoral load. This will have a significant effect on tibial strains and so it is recommended that testing at higher flexion angles should be performed in a combined loading rig

In the past it has been widely accepted that bone remodelling of the proximal femur after cementless total hip replacement is a result of the altered mechanical environment. Usually, there is are distribution of the stresses in the bone, and subsequently bone mass, from the metaphysis to the proximal part of the diaphysis. The design rationale for some cementless stems is to transmit load to the proximal femur and thus to preserve the bone mineral content in this area. The aim of the present study was to investigate the relationship between postoperative strain shielding of the proximal femur and the bone remodelling after insertion of two different cementless femoral stems. Experimental study: Twelve pairs of human cadaveric femurs were instrumented with strain gauge rosettes in Gruen zones2 to 7 and the cortical strains were measured during simulation of one leg stance before and after insertion of a custom stem (Unique, SCP) or an anatomic stem (ABG, Stryker-Howmedica). Clinical study: In a prospective, randomized study including 80 patients, the same types of stems were inserted and the bone mineral density (BMD) was measured during the first two years postoperatively using DEXA. Then, the pattern of remodelling was compared with the gradient of strain shielding in each of the Gruen zones in the frontal plane. In Gruen zone 7 the relative cortical strain shielding was45% in the femurs with a custom stem and 87% in the femurs with an anatomic stem. In zone 6 the corresponding figures were 2% and 38%, in zone 5 0% and15% and in zone 3 0% and 20%. The DEXA measurements showed a decrease in BMD in zone 7 of 22% and 23% for the two stems, respectively. In the other zones the bone loss was smaller and there was no difference between the stems. In the proximal zones there was a highly significant difference in strain shielding between femurs receiving a customor an anatomic stem. However, there was no difference in the pattern of bone remodelling. The bone remodelling around these two stems does not seem to mirror the gradient of strain shielding

In vitro femoral studies have demonstrated the addition of hydroxyapatite (HA), to morcellised bone graft (MBG) decreases femoral prosthesis subsidence. However, with an increased risk of femoral fracture during the impaction of a MBG:HA mixture, possibly due to greater force transmission to the femoral cortex via the HA. The aim was to compare the hoop strains and subsidence of a 1:1 mixture of MBG:HA with pure bone allograft during impaction and subsequent endurance testing in a revision hip arthroplasty model. Materials and methods Large Sawbone femurs were prepared to represent a femur with bone loss (Sawbones, Sweden). 12 uniaxial strain gauges were attached to each femur at 0, 90, 180 and 270 degrees, at distal, midshaft, proximal points to measure hoop strain. Impaction grafting was performed using X-Change 2 instruments and an Instron servohydaulic machine for 2 distal impactions and 4 proximal impactions for 60 impactions each. Study groups. The study consisted of four experimental groups: 1)Pure MBG, force of 1.98 kN 2)Pure MBG, force 3.63kN. 3)1:1 mixture of MBG: porous HA (pHA), 4)1:1 mixture MBG: non porous HA (npHA). 6 samples of each group were performed. Endurance testing. The potted femur was loaded in a manner representing the walking cycle (1.98kN) at 1 Hz for 50 000 cycles. The displacement of the femoral head during loading was measured by two displacement transducers (LVDT) were mounted on aluminum brackets to measure vertical displacement and rotation. Statistical analysis. Statistical analysis was performed using a Mann – Whitney U test for total subsidence and prosthesis cyclical movement at 6 hours between the control MBG1.98 and the other experimental groups. Subsequent analysis compared pHA and npHA mixes with and HA to MBG3. 65kN. Level of significance was taken at p<0.05. Results. Distal strain gauges:. All experimental groups had a significantly greater distal hoop strain than MBG1.98 kN (p=0.004). Middle strain gauges:. No significant differences between the HA groups and MBG1.98. MBG3.65 was significantly greater (p=0.02). Top strain gauges:. No significant differences, p=0.9. Endurance testing There was a non significant trend towards decreased subsidence in the HA groups compared to the bone groups. (p=0.7) but significantly less cyclical subsidence between HA groups and MBG 1.98 (p=0.02). No difference between the MBG 3.65 kN and HA groups for the cyclical subsidence. Statistically significant difference in cyclical rotation between both the MBG1.98 and the pHA (p=0.02) and npHA(p=0.02) and the bone 3.65 and pHA and npHA groups(p=0.04). Conclusion. The addition of HA to MBG decreases total and cyclical subsidence. There were significantly greater hoop strains in the distal and midshaft strain gauges with greater impaction force and in the distal gauges with the use of HA. Endurance testing demonstrated a significant decrease in cyclical motion and cyclical rotational stability of the prosthesis with a trend to a decrease in total subsidence during endurance testing with HA groups. The addition of HA might have longer term benefits in terms of prosthesis stability and subsequent graft healing but caution is needed during impaction

Revision knee prostheses are often augmented with intramedullary stems to provide stability following bone loss. However, there are concerns with the use of such stems, including loosening caused by strain-shielding, end-of-stem pain, and removal of healthy bone surrounding the medullary canal. Extracortical fixation plates may present an alternative. The aim of the study was to quantitatively evaluate and compare strain-shielding in the tibia following implantation of a knee replacement component augmented with either a conventional intramedullary stem (design1), or extracortical plates (design2) on the medial and lateral surfaces. Eight composite synthetic tibiae were implanted with one of the two designs, painted with a speckle pattern, loaded in axial compression (peak 2.5 kN) using a materials test machine, and imaged with a 5-megapixel digital image correlation (DIC) system throughout loading. Bone loss was simulated in all models by removing a volume of metaphyseal bone. For four tibiae, the tibial tray was augmented with a cemented stem (∼150 mm). The others were augmented by extracortical plates (maximum 90 mm long) along the medial and lateral surfaces (Fig. 1). Strains were computed using an ARAMIS 5M software system between loaded and unloaded states in the longitudinal direction, for the medial, posterior and lateral surfaces of the tibiae. Strains were checked locally by use of strain gauge rosettes at three levels on medial, lateral and posterior aspects. The bone strains measured on the posterior surfaces were reported in three regions; proximal (0–70 mm, where the medial extracortical plate lies), middle (70–130 mm, the stem is present but not the extracortical plates), and distal (130–200 mm, beyond the stem). Mean longitudinal strains for both implant types were comparable in the distal region, and were greater than in the other regions (Fig 2). The mean strains differed considerably in the middle region: 565–715 μstrain with stemmed components 1050–1155 μstrain with plated components. Strains followed a similar pattern in the proximal region, particularly very close (20 mm) to the tibial tray component, where the stemmed component bones (775 ± 160 μstrain) displayed less surface strain than the plated component bones (1210 ± 180 μstrain). Strain-shielding was observed for both designs. The side plates were shorter than the intramedullary rods, so the region of the bone distal to the plates was not strain-shielded, while the same region was strain-shielded when a stemmed component was implanted. It was also shown that in the region of bone just distal of the tibial tray component, design1 shielded the bone from strain 56% more on average than design2. From these results, it can be speculated that the use of extracortical plate rather than intramedullary stems may lead to improved long-term results of revision TKA, assuming the plates and screws provide adequate stability. The extramedullary fixation system preserves more bone than IM fixation, and has the advantage of allowing use of primary TKA components, cemented over the subframe. Similar components have been developed for the femur

Introduction: The success of impaction-grafting depends on mechanical stability and adequate bony incorporation of the graft. Full incorporation of this type of graft has been demonstrated histologically and depends on many factors including the biological activity of the graft. Bone morphogenic proteins (BMPs) are known to play a central role in bone formation and their presence reflects the biological activity of a graft material. The aim of this study was to determine the activity of fresh frozen femoral head (FFH) grafts by analysing BMP-7 release after milling and during strain imposed by the impaction process. Methods: 10mm cancellous bone cubes were cut from 5 samples of FFH. The cubes were washed, centrifuged and washed again to remove the marrow contents. Specimens from each femoral head were allocated to five groups and subjected to strains of 0%, 20%, 40%, 60% and 80% with a material testing machine. The cubes were washed again and the wash fluid analysed for bmp-7 activity using a commercially available elisa kit. Additionally, samples of bone were taken after standard milling of FFH, washed and the fluid analysed for bmp-7 activity. Results: bmp-7 activity was found to be present in all groups. Release of bmp-7 was found to increase with increasing strain in a linear relationship. At 80% strain the mean concentration of bmp-7 released (2.2 ng/g bone) was approximately double that released at 20% strain. Discussion: activity of bmp-7 in FFH has not previously been demonstrated. This study shows that the freezing and storage of femoral heads allows some maintainance of biological activity. Furthermore we have shown that bmp-7 may be released from FFH cancellous bone in proportion to the strain applied to the bone. This may go some way to explaining the full bony incorporation often seen after impaction-grafting

In vitro loading of the proximal femur has improved our understanding of stress shielding after total hip arthroplasty. However, previous load simulators often use simplified loading regimens that may not produce physiologic baseline strains. The purpose of this study was to compare the femoral strain levels produced when using simplified and more complex loading. A mechanical load simulator was developed which could simultaneously apply a spinal load and nine of eleven available muscle loads to the proximal femur in heelstrike and stair climbing modes. Computer controlled electromechanical actuators were attached to a strain gauged fresh cadaver femur (donor body weight 39 6kg) with metal cables. A spinal load of 668 N (SPL) was applied alone and in combination with individual muscle loads of 267 N to determine the effect of each muscle on femoral strain. The magnitude and direction of the joint reaction force (JRF) was monitored in real time by a three-dimensional force transducer proximal to a metal acetabulum. Anterior, middle and posterior portions of the gluteus medius (ABD), iliotibial band (ITB), short external rotators (SER), vastus lateralis, adductors, rectus femoris, hamstrings, iliopsoas, and gluteus maximus were simulated. SPL was applied and ABD and ITB were adjusted to produce a JRF magnitude of 2.0 BW. SPL was applied with two combinations of nine muscle loads adjusted in heelstrike mode to produce a JRF magnitude of 2.0 and 2.5 BW and JRF trajectory aligned within one degree of the radiographically determined compression trabecular stream axis. Both nine-muscle combinations produced lower medial compression strains and substantially lower lateral tension strains than SPL+ABD+ITB in heelstrike and stair climbing. Simplified loading caused a bending moment in the proximal femur resulting in higher strains. Combined loading at 2.5 BW produced compression at 10 of 12 gauges in heelstrike mode and 9 of 12 gauges in stairclimbing

Chondrocyte sensitivity to strain depends on signal transduction pathways which include integrin-dependent increases in intracellular calcium. Human articular chondrocytes were cultured as monolayers in silicone dishes. After loading the cells with the calcium-fluorescent dye Fluo-3/AM the dishes were mounted in a 4-point bending apparatus and then fixed to a laser scanning confocal microscope. Biaxial substrate strain (15 000e) was applied to the silicone dish via a hand operated cam rotated at ~60 RPM (1 Hz) for 10 or for 50 cycles. Changes in intracellular calcium in single cells were determined by measuring the mean pixel values in the basal and stimulated images taken at different time points. The data reported for 50 cycle treatments represent 49 single cells of six independent cell isolations. The data for 10 cycle strain treatment are from a single experimental setup. Increases in intracellular calcium were consistently observed in chondrocytes exposed to 15 000me for 50 cycles in a range from 1.3- to 4.0-fold with an average of 2.3-fold (SD=0.79). Few cells responded before 30 minutes but most of the responses occurred 30–60 minutes after strain. Consistent intracellular Ca++-increases were also seen after 10 strain cycles, however responses were detected within 5 minutes post-strain. The relative increase (2.7-fold ± 1.7) was similar in magnitude to 50 cycle responses. Intracellular Ca++-fluxes in chondrocytes and other cells occur by at least two different mechanisms: through stretch-activated channels in the plasma membrane permit immediate Ca++-influx during strain application or by Ca++-efflux from intracellular compartments stimulated by slower acting second messengers. Our results suggest that the early response to 10 strain cycles is due to Ca++-influx via membrane channels while the later response to 50 cycles is due to Ca++-efflux from intracellular compartments, probably mediated by cytokines released in response to an initial Ca++-influx from the medium

For younger patients many surgeons recommend femoral neck endoprostheses as alternative to stemmed implants in THA. Due to metaphyseal anchorage several advantages are quoted, e.g. preservation of the femoral diaphysis for a revision implant. Determinant factor for long-term implant stability is the load transmission to the bone. Because so far only few information about the load transfer of femoral neck endoprostheses exist, a photoelastic analysis was performed. Aim of the study was the comparison of bony strain pattern before and after implantation of a femoral neck endoprosthesis. ‘Composite-femurs’ (Pacific Research Labs) were used due to of their mechanical characteristics close to human femurs but better reproducibility. Three femurs were coated with photoelastic material. The femurs were loaded prior and post implantation of a femoral neck endoprosthesis type Cigar (ESKA Implants). Test load consisted of the resulting hip joint force and muscle forces (abductors, tractus iliotibialis). Load was applied statically by a universal testing machine and additional weights. Bony strain was measured along the medial, ventral, lateral and dorsal cortex. Statistical analysis of the implant related strain alterations was based on a 99% confidence interval. The unresected femurs showed an excellent match of bony strain patterns. Implantation of femoral neck endoprostheses caused highly significant strain changes at the trochanteric region. Greatest differences were observed at the lateral cortex. Above the implant’s traction screw former areas of tension changed to compression. Along the medial cortex below the resection plane strain reductions were measured but disappeared at the latest at 40 mm below. No significant changes in strain were detected at the ventral and dorsal cortex. Implant related bony strain alterations were limited to the trochanteric region of the femur. A marked strain alteration at the lateral trochanteric aspect was measured. Whether this is of clinical importance can not be answered yet

Introduction: Unicomparmental knee replacements have a long clinical history of success as well as failure. Recently, in Australia some 40% of knee surgery performed consists of unicompartmental knees for the treatment of medial compartment OA. This increased use of unicompartmental knees is in part due to advances in surgical technique through a minimally invasive approach. Loading conditions at the tibia-implant interface will play an important role in the stress/strain distributions at the proximal tibia. The use of an all PE tibial insert versus a metal backed component may provide a different strain disribution to the proximal tibia. This study examined the influence of metal backed and polyethylene tibial components in unicompartmental knee replacements with and without cement fixation on the initial strain distributions under various loading conditions. Materials and Methods: Three cadaveric tibias (mean age 47 years old) were cleaned of all soft tissue and strain gauged. Rosette strain gauges (TML Ltd., Tokyo, Japan) were placed at 2 levels on the tibial cortex. The intact tibia were embedded in a low melting point alloy at a standard height and tested using an MTS 858 Bionix testing machine (MTS Systems, Min., MI). The tibia were tested in nuetral, varus and valgus positions at zero and sixty degrees of flexion. A 1500N was applied for 15 seconds and the strains measured. A K-Scan sensor (Tekscan, Boston, MA) was used to confirm the varus and valgus loading positions and to obtain a contact footprint and pressure for the intact and reconstructed tibias under the loading conditions (Fig. 1). Following intact testing, the tibias were templated and reconstructed by a surgeon familiar with the technigue. The implants were investigated with and without cement fixation and compared to their respective all polyethylene component if it was available using the same loading regime as the intact tibias. Principal strains were calculated. Results: Tibial cortical strain distributions were significantly different at the proximal and distal sites under the loading conditions examined. The strain distribution for metal backed components was greater than the all PE design. Increasing flexion angle shifted the peak strains posteriorly. Metal backing and all PE tibial inserts presented different strain distributions on the medial side under nuetral and varus loading. Lateral compartment strains did not differ between designs, were higher proximal and decreased dramatically at the distal gauges. Cementless fixation tended to overload compared to the intact condition. Figure 2 presents the strain distribution for a typical metal backed and all poly unicompartmental knee in the nuetral position. Discussion: Metal backed unicompartmental components overloaded the proximal cortex of the tibia. All polyethylene tibial inserts did not overload the proximal cortex and had similar strain distribution to the intact tibia. Cemented fixation allows the transfer of load to the distal tibial cortex via the proximal cortex and subchondral bone, provided that the bone cement has inter-digitised the subchondral bone

The screw fastening torque applied during bone fracture fixation has a decisive influence on subsequent bone healing. Insufficient screw tightness can result in device/construct instability; conversely, excessive torques risk damaging the bone causing premature fixation failure. This effect is even more prominent in osteoporotic bone, a condition associated annually with almost 9 million fractures worldwide. During fracture fixation, screw tightening torque is applied using subjective feel. This approach may not be optimal for patient”s recovery, increasing risk of fixation failure, particularly in osteoporotic bone, and potentially require revision surgical interventions. Besides bone density, various factors influence the performance of screw fixation. These factors include bone geometry, cortical thickness and time-dependant relaxation behaviour of the bone. If the influence of screw fastening torque on the bone and relationships between these factors was better understood, the surgical technique could be optimised to reduce the risk of complications. Within this study, we developed an axisymmetric finite element (FE) model of bone screw tightening incorporating viscoelastic behaviour of the cortical bone such as creep and stress relaxation. The model anticipated time-dependent behaviour of the bone for different bone thickness and density after a typical bone fixation screw had been inserted. The idealised model has been developed based on CT scans of bones with varying densities and inserted screws. The model was validated through a series of experiments involving bovine tibiae (4–5 months) to evaluate the evolution of surface strains with time (Ncorr v1.2). Stress distribution was assessed in photoelastic experiments using acrylic analogues. Relaxation tests have been performed in aqueous environment for up to 48 hours to ensure the relaxation would be complete. The creep behaviour (maximum principal strain) was compared against computational predictions. Our early simulations predicted relaxation strains on the surface of the bone to be 1.1% within 24 hours comparing favourably to 1.3% measured experimentally. Stress distribution patterns were in agreement with photoelastic results. Using experimentally derived viscoelastic properties, the model has the potential to predict creep and stress relaxation patterns after screw insertion with different fastening torques for bones with varying density and geometry. We aim to develop this into a planning tool providing guidance to surgeons for optimal tightening when using screw fixation, particularly in reduced quality bone

Purpose of study: To study the effect of an additional locking screw on fracture strain and stability in tibias undergoing intramedullary nailing. Methods: An additional locking hole was drilled into four tibial nails, 185 mm from the proximal end of the 8 mm x 315 mm solid tibial nails. The nails were locked proximally and distally into a triple strain-gauged sawbone. An osteotomy was created distal to the additional hole, and the construct loaded axially, in flexion and extension, and in torsion with and without the extra locking screw. With the additional locking screw in place, strain increased at the proximal strain gauge site during loading in neutral by 17% (139 mϵ, 91–198) (p=0.01) and flexion by 8% (65 mϵ, 60–73) (p< 0.005). Strain decreased on loading in extension by 10% (141 mϵ, 62–243) (p=0.0497). The extra locking screw decreased strain at the gauge closest to the osteotomy site in all loading positions. Strain showed an overall increase with axial loading of 14% (47 mϵ, 4–105) (p=0.16), an increase with loading in flexion of 2% (9 mϵ, −38 to 62) (p=0.75) but a decrease of 47% (254 mϵ, 6–549) (p=0.18) with loading in extension. A significant reduction in angular motion at the osteotomy site occurred with the addition of the extra locking screw (21° at 34.5 Nm without the screw, 13° at 34.5 Nm with the screw, p=0.001). Additional hole in the shaft of the nail lead to increase the stress from 29 – 48 mPa (29 – 48 N/mm2) but did not fail when vertically loaded with 450 Newtons applied at rate of 5Hz sinusoidal waves for 2 million cycles. Conclusion: Nails with additional locking options, by altering strain and motion at the fracture site, may have the clinical potential to affect fracture healing with relatively low risk of implant failure

Joint registries report that 25–40% of UKR revisions are performed for pain. Proximal tibial strain and microdamage are possible causes of this “unexplained” pain. The aim of this study was to examine the effect of UKR implant design and material on proximal tibial cortical strain and cancellous microdamage. Composite Sawbone tibias were implanted with cemented UKR components: 5 fixed bearing all-polyethylene (FB-AP), 5 fixed bearing metal backed (FB-MB), and 5 mobile bearing metal backed implants (MB-MB). Five intact tibias were used as controls. Tibias were loaded in 500N increments to 2500N. Cortical surface strain was measured using digital image correlation (DIC). Cancellous microdamage was measured using acoustic emission (AE), a technique which detects elastic waves produced by the rapid release of energy during microdamage events. DIC showed significant differences in anteromedial cortical strain between implants at 1500N and 2500N in the proximal 10mm only (p<0.001) with strain shielding in metal backed implants. AE showed significant differences in cancellous microdamage (AE hits), between implants at all loads (p=0.001). FB-AP implants displayed significantly more hits at all loads than both controls and metal backed implants (p<0.001). FB-AP implants also differed significantly by displaying AE hits on unloading (p=0.01), reflecting a lack of implant stiffness. Compared to controls, the FB-AP implant displayed 15x the total AE hits, the FB-MB 6x and the MB-MB 2.7x. All-polyethylene medial UKR implants are associated with greater cancellous bone microdamage than metal backed implants even at low loads

Background: Stress shielding in the proximal femur is a widely recognized sequel of total hip replacement. It is due to a discrepancy in the stiffness between the implant and the proximal femur. The strain characteristics of implants of differing materials on the femur have previously been demonstrated in the laboratory using strain gauges and photo-elastic techniques. These are however relatively crude techniques giving limited results. Finite element analysis has also been used, but this provides only a theoretical analysis. Digital Image Correlation is an extremely accurate technique for strain analysis previously used in micro and nano engineering research. Methods: A stainless steel, a titanium, and a carbonfibre reinforced plastic (CFRP) femoral prosthesis of the same dimensions, were implanted without cement into 5 prosthetic femora. A 1kN load was applied using a compression device. The process was repeated with 5 other prosthetic control femora. Digital Image Correlation was used to give an extremely detailed 2D strain map of inner cortex of the proximal femora during laboratory simulated static physiological loading conditions. Results: All implants caused stress shielding in the proximal calcar area. In Gruen zones 6–4, both the stainless steel and titanium implants caused statistically significant stress shielding, whereas the femora implanted with the CFRP prosthesis did not show a statistically different strain pattern from the control group. There was a reduction in strain experienced by the medial cortex of the femora beyond the tip of all of the implants. Conclusion: Digital Image Correlation is a novel method for strain measurement within Orthopaedic research which produces extremely accurate strain maps and data that can be reliably used for statistical analysis. Using this technique, this laboratory based investigation indicates that a carbon-fibre reinforced plastic stem is a good candidate to avoid stress shielding in total hip replacement surgery

Introduction: Proximal bone resorption is a common problem after total hip arthroplasty. This has been attributed to stress shielding and has been reported to be more pronounced for cemented than for uncemented implants. Aim: To investigate the cortical strain distribution of a new proximal “fit and fill” cementless, titanium, femoral, hip prosthesis based on the SROM design. Methods: Strain gauges were mounted on five fresh-frozen cadaveric and five saw-bone femora and checked against a template for the prosthesis. The strain gauges were placed at four levels on the anterior, posterior, medial and lateral cortices corresponding to the Gruen zones. Two extra strain gauges were placed on the proximal posteromedial cortex. Loading was applied to the intact and reconstructed femora in the ISO 7206–4 orientation and single legged stance in an MTS servo-hydraulic testing machine. Data were analysed using analysis of variance. Results: The strain distributions following reconstruction and multi-axis loading (ISO 7206–4 orientation) approximated the strains in an intact femur in the diaphysis. The proximal posteromedial cortical strains were approximately 50% of those of the intact femur. Conclusions: The strains observed in the proximal femur following reconstruction in the present study are considerably higher than most others reported in the literature. A number of factors may contribute to the high proximal strains observed. This study has illustrated that geometric design and material selection along with surgical technique may allow for greater loading to proximal bone and enhance the long term integrity of this type of implant

Purpose. As human soft tissue is anisotropic, non-linear and inhomogeneous, its properties are difficult to characterize. Different methods have been described that are either based on contact or noncontact protocols. In this study, three-dimensional (3D) digital image correlation (DIC) was adopted to examine the mechanical behaviour of the human Achilles tendon. Despite its wide use in engineering research and its great potential for strain and displacement measurements in biological tissue, the reported biomedical applications are rather limited. To our knowledge, no validation of 3D DIC measurement on human tendon tissue exists. The first goal of this study was to determine the feasibility to evaluate the mechanical properties of the human Achilles tendon under uniaxial loading conditions with 3D Digital Image Correlation. The second goal was to compare the accuracy and reproducibility of the 3D DIC against two linear variable differential transformer (LVDT's). Methods. Six human Achilles tendon specimens were prepared out of fresh frozen lower limbs. Prior to preparation, all limbs underwent CT-scanning. Using Mimics software, the volume of the tendons and the cross sectional area at each level could be calculated. Subsequently, the Achilles tendons were mounted in a custom made rig for uni-axial loading. Tendons were prepared for 3D DIC measurements with a modified technique that enhanced contrast and improved the optimal resolution. Progressive static loading up to 628,3 N en subsequent unloading was performed. Two charge-coupled device camera's recorded images of each loading position for subsequent strain analysis. Two LVDT's were mounted next to the clamped tendon in order to record the displacement of the grips. Results. 3D DIC strain measurement proved to be technical feasible on human tendon tissue if an adapted preparation protocol is used. A spatial resolution of 0,1 mm was reached. Accuracy analysis shows a very low scatter, comparable to that obtained in steel applications (0,03%). When compared to the LVDT measurements, DIC showed excellent correlation (R = 0.99). Apart from the longitudinal strain component, an important transverse strain component was revealed in all specimens (fig 1). Also a significant amount of slip was observed at the clamps. Through the non-contact nature of the measurement, this could be quantified and the analysis became independent of any slip (fig 2). The strain distribution was of a strongly inhomogeneous nature, both within the same specimen (fig 1) and amongst different specimens. Conclusion. 3D DIC is a very promising technique for strain measurement of human collagenous tissue. Accuracy analyses indicate a very low scatter, comparable to that obtained in traditional steel applications. The major advantages of the DIC technique over the LVDTs is the 3D, non-contact, full-field nature of the analysis and the possibility to analyse multidirectional strain, without disturbing slip effects in the grips

Background: It is thought that the forces transmitted across the hip joint produce migration of the prosthesis by failure at either the bone-cement or the prosthesis-cement interface. As symptoms associated with such motions often result from failure at the cement-bone interface, it is this interface and its sub-surfaces that are the critical areas of prosthesis loosening. Our aim is to produce a new and more accurate method of measuring strains at this critical interface. Objective: To develop in-vitro experiments to measure the strain distributions near the bone-cement interface of the acetabulum region under physiological, quasi-static loading conditions. Experimental Model: Two hemi-pelvic specimens of saw bones were used. Following careful placement of six protected precision strain gauges (4.6 x 6.4mm, tri-axial EA-13-031RB-120/E). One specimen was prepared to receive a cemented polyethylene cup (Depuy Charnley Ogee LPW 53/22). An uncemented 58mm Duraloc cup was implanted into a second specimen. Methods: Hip joint force relative to the cup during normal walking (Bergmann, G., 2001. HIP98) was used for quasi-static tests on a Llody LR30K loading machine. The magnitude of the maximum and minimum principal strains, and the orientation of the maximum principal strains were calculated from a 32 channel digital acquisition system. Results: For both specimens, the maximum principal strains at the maximum loading were highest in the medial wall (dome area) of the acetabulum. The tensile strain from the dome of the uncemented specimen at the maximum loading was twice that of the cemented specimen. In the cemented specimen, the compressive strains in the medial wall were almost twice the tensile strains at the maximum load. Within the acetabular quadrants, the highest strains were recorded in the posterio-inferior quadrant. Compressive strains in the posterio-inferior wall of the acetabulum seem to be comparable to those in the anterior-superior wall. Conclusion: The critical areas for load transfer in the acetabulum are the medial wall (dome area), the posterio-inferior and the anterior-superior quadrants. The uncemented cup appears to provide a better load transfer mechanism than the cemented cup

We have compared the changes in the pattern of the principal strains in the proximal femur after insertion of eight uncemented anatomical stems and eight customised stems in human cadaver femora. During testing we aimed to reproduce the physiological loads on the proximal femur and to simulate single-leg stance and stair-climbing. The strains in the intact femora were measured and there were no significant differences in principal tensile and compressive strains in the left and right femora of each pair. The two types of femoral stem were then inserted randomly into the left or right femora and the cortical strains were again measured. Both induced significant stress shielding in the proximal part of the metaphysis, but the deviation from the physiological strains was most pronounced after insertion of the anatomical stems. The principal compressive strain at the calcar was reduced by 90% for the anatomical stems and 67% for the customised stems. Medially, at the level of the lesser trochanter, the corresponding figures were 59% and 21%. The anatomical stems induced more stress concentration on the anterior aspect of the femur than did the customised stems. They also increased the hoop strains in the proximomedial femur. Our study shows a consistently more physiological pattern of strain in the proximal femur after insertion of customised stems compared with standard, anatomical stems

The Mayo Conservative uncemented stem (Zimmer, Warsaw, USA) is designed to conserve proximal bone stock by virtue of a minimal neck resection and to maintain proximal femoral stress transfer, thereby reducing problems associated with stress shielding. This study was performed to evaluate proximal femoral strain after implantation of the Mayo stem, in cadaveric femora. Eight fresh-frozen cadaveric femora (each selected at random from within a pair) of known bone mineral density were prepared and coated with photoelastic materials (Measurements Group, Raleigh NC). Strain patterns of the intact bone were determined using a reflection polariscope, and recorded photographically, while under load. Quantitative measurements were taken from set points of the proximal femur. The femoral head was then replaced using a Mayo femoral prosthesis. Under the same loading conditions strain patterns were re-examined and measurements taken from the same set points. The strain patterns following insertion of the Mayo stem closely matched those seen in intact femora except in two areas. Strain was reduced in the region of the lesser trochanter (53% of normal), although more proximal than this strain in the neck was closer to intact values (78% of normal). Previous studies have found that implantation of diaphyseal press fit stems in particular have led to significant reductions in shear strain values in the calcar region and distally along the medial border of the femur. This study documents the strain pattern in the proximal femur after implantation with a new “conservative” short stem cementless prosthesis. The hypothesis that the Mayo femoral stem maintains proximal femoral stress transfer and may thus prevent stress shielding in vivo remains to be proven, but is supported by the results of this study

The design of the femoral prosthesis in cementless total hip arthroplasty is known to affect the initial strains in the cortex during implantation and in the early postoperative time period. High strains have a direct influence on periprosthetic fracture. This study compares the existing ABGII stem, which is proximally coated with a grit blasted titanium surface with hydroxyapatite coating with a prototype that has a rougher titanium plasma spray proximal coating. The Australian National Joint registry results 2011 reported the ABG2 femoral component cumulative percent revision (CPR) of 6.5 (93.5% survival), which compares favourably with equivalent stems with 10 year CPR data such as the Taperloc 6.6 and Corail 7.3. Six pairs of fresh-frozen cadaveric femurs were mounted in blocks according to ISO guidelines in single leg stance setup. Five strain gauges were attached around the neck of the femur and then prepared according to routine operative techniques to accept the femoral prosthesis. Cortical strains were measured during insertion of the prosthesis with an instrumented mallet attached to an accelerometer. Subsequently, force-displacement readings were taken during cyclical loading on a servo-hydraulic machine and finally the stems were tested to failure. Our results showed significantly less strain during cyclical loading of the stem with increased surface roughness (p < 0.05). They also showed no significant differences loads/strains during impaction (p = 0.159), no significant difference in micromotion (p = 0.148) and no significant difference in load-to-failure (p = 0.37)

Introduction. Low back pain is a major public health problem in our society. Degeneration of intervertebral disc (IVD) appears to be the leading cause of chronic low-back pain [1]. Mechanical stimulations including compressive and tensional forces are directly implicated in IVD degeneration. Several studies have implicated the cytoskeleton in mechanotransduction [2, 3], which is important for communication and transport between the cells and extracellular matrix (ECM). However, the potential roles of the cytoskeletal elements in the mechanotransduction pathways in IVD are largely unknown. Methods. Outer annulus fibrosus (OAF) and nucleus pulposus (NP) cells from skeletally mature bovine IVD were either seeded onto Flexcell¯ type I collagen coated plates or seeded in 3% agarose gels, respectively. OAF cells were subjected to cyclic tensile strain (10%, 1Hz) and NP cells to cyclic compressive strain (10%, 1Hz) for 60 minutes. Post-loading, cells were processed for immunofluorescence microscopy and RNA extracted for quantitative PCR analysis. Results. F-actin reorganisation was evident in OAF and NP cells subjected to tensile and compressive strain respectively and is likely due to load-induced differential mRNA expression of actin-binding proteins. The vimentin network was also more intricately organised in loaded NP cells. Compressive strain increased type II collagen and aggrecan transcription in NP cells, whereas levels decreased in OAF cells under tension. mRNA levels of ECM-degrading enzymes were significantly reduced in both cell populations after loading. Conclusion. Tensile and compressive strains induce different mechano-responses in the organisation/expression of cytoskeletal elements and on markers of IVD metabolism. Differential mechano-regulation of anabolic and catabolic ECM components in the OAF and NP populations reflects their respective mechanical environments in situ

Introduction. A new conservative hip stem has been designed to address the complex problem of total hip arthroplasty in the younger population. Objectives. To assess the stability and strain distribution of a new conservative hip stem. Materials and Methods. The prosthesis is tapered and collared and made from titanium (Ti6Al4V) with a titanium porous plasma spray to encourage bony ingrowth (Figure.1). It is circular-trapezoidal in cross-section to provide optimal ‘fit and fill’ in the femoral neck. (i) Finite Element Analysis (FEA). Computed tomography scans of an intact femur were modelled using MARC software and consisted of 161390 elements and 174881 nodes. The implant was modelled (Unigraphics) as a titanium alloy stem with a cobalt-chrome alloy head and consisted of 93440 hexahedral elements and 101133 nodes. This study compared the strains in the femoral calcar of an intact femur with a stem ‘implanted’ in neck shaft angles of 125°, 135°, and 145°. The head of all models received a load of 2.3KN at 7 degrees medially. (ii) Photoelastic Coating. A photoelastic coating was moulded around the medial cortices of ten third generation femora Sawbones. Strain before and after prosthesis insertion was measured at one-centimetre intervals down the medial cortex of the bones using a polariscope. The bones were positioned in a simplified single leg stance (7° physiological alignment), and loaded at 2.3 KN with strain recorded. (iii) Linear Variable Differential Transducers (LVDT's). Micromotion and migration of the prosthesis was measured using LVDT's. The femoral heads were cyclically loaded with 2.3KN at 1Hz for 2,500 cycles and held in a single leg stance. The bones were then repositioned at 70° of flexion to produce torsional (stair climbing) forces and loaded with 0.5KN for 2,500 cycles. Statistical analysis of non-parametric data was performed using a two-tailed Wilcox signed rank test (p<0.05). Results. The FEA analysis revealed strains in the neutral position most closely resembled that of an intact femur (Figure.2). Photoelastic strain readings for intact bone and following insertion were paired and statistically analysed using the Wilcox signed rank test (two tailed). The composite bones with prostheses inserted at 125° and 145° demonstrated a significant difference to the intact bones, whereas those at 135° showed no significant difference in the surface strain pattern of the femur following prosthetic insertion (Figure.3). Under single leg stance loading all prostheses produced axial micromotion of less than 200 µm and 50 µm in the varus-valgus direction. Implants inserted at 135° and 125° produced the least micromotion, the implants inserted at 145° had the greatest magnitude of motion and may be more susceptible to loosening. Under torsional load the same was true with the 135° and 125° producing the least micromotion while with the angulation of 145° micromotion increased over the test period – again suggesting loosening. Conclusion. This design transfers load in a physiological manner and the prosthesis is most stable in the neutral position. The findings from this study have been translated into clinical practice with the prosthesis implanted into two patients with promising results

Injuries to the sciatic nerve are an occasional complication of surgery to the hip and acetabulum, and traction is frequently the causative mechanism. In vitro and animal experiments have shown that increased tensile strain on peripheral nerves, when applied for prolonged periods, impairs nerve function. We have used video-extensometry to measure strain on the human sciatic nerve during total hip replacement (THR). Ten consecutive patients with a mean age of 72 years undergoing primary THR by the posterior approach were recruited, and strains in the sciatic nerve were measured in different combinations of flexion and extension of the hip and knee, before dislocation of the hip. Significant increases (p = 0.02) in strain in the sciatic nerve were observed in flexion of the hip and extension of the knee. The mean increase was 26% (19% to 30%). In animal studies increases of this magnitude have been shown to impair electrophysiological function in peripheral nerves. Our results suggest that excessive flexion of the hip and extension of the knee should be avoided during THR

Purpose: Glenoid component loosening is a common reason for failed total shoulder arthroplasty. Multiple factors have been suggested as causes for component loosening including asymmetric loading of the glenoid prosthesis by the humeral head (rocking horse phenomenon). A novel technique was employed to measure in-vitro strain in the subchondral bone adjacent to a cemented all polyethylene pegged glenoid prosthesis. The purpose of the study was to develop and validate a testing protocol to investigate load transfer in the polyethylene glenoid implant and bone construct. Method: Eight polyethylene components were implanted using standard cementing techniques in eight cadaveric specimens. Loading was performed with a pneumatic actuator capable of applying loads at various angles. A dynamic 10 N/s force was applied for a total of 15 seconds producing a maximum force of 150N at angles of 0, 10, 20, 30, 40 and 50o. Strain gauges were placed around the implant 1mm proximal to the bone-cement interface at the four quadrants. The humeral head was simulated with a custom steel ball with a non-conforming diameter in relation to the prosthesis that is typical in total shoulder arthroplasty. Results: During pure compressive loading, tension was observed in the superior and inferior quadrants of the glenoid. Superior and inferior loading caused increasing same side (ipsilateral) tension, occurring from 0 to 30o and 0 to 20o, respectively. Compression was recorded superiorly when loading was applied at 40o and 50o in the superior direction while contralateral tension was recorded in the inferior gauges. Strain measurements were less consistent in the anterior and posterior glenoid quadrants and varied between tension and compression. Conclusion: Tension measurements in the ipsilateral direction at lower angles were unexpected. This observation differs from the previous assumption that applied loads at relatively perpendicular angles to the implant should dissipate as compression. Tension at the bone cement interface is unfavorable. The identification of tension in some quadrants of the implant in this study, therefore, may have revealed a mechanism of implant loosening. Our data support the previously described rocking horse phenomena and also illustrate a new umbrella type effect of polyethylene flexure, which causes the periphery of the glenoid implant to flex upwards superiorly and inferiorly. These findings have the potential to influence future designs of total shoulder arthroplasty perhaps leading to increased implant survival