There is continued concern over complication rates (20–30% of cases) in locked proximal humeral plating. The most common sequelae of this is screw penetration of the humeral head. This is associated with natural settling of the fracture, malreducition in varus, insufficent medial support of the fracture. The proximity of the screws to the articular surface can also be influential on outcome if collapse occurs. Our operative technique is to establish the rotation of the humeral head where the drill appears closest to the articular margin (by sequential xray screening) and subtract from this to avoid intra-articular penetration of the humeral head. 55 Consecutive patients of average age 56.4 years (14.7–86.1), 17 male and 38 females, who underwent PHILOS plating were identified using Bluespier database. Xrays were analysed for fracture pattern, restoration of neck-shaft angle, plate positioning, number and configuration of screws and presence of screw penetration both intra-operatively and at postoperative follow-up. There were 6.07 screws used per head (total 330). There was one intraoperative screw penetration and 3 patients had evidence of screw penetration at follow-up, which required implant removal (total screw penetration rate of 7%). There was one case of AVN. The mean neck shaft angle was 137 degrees (anatomical 135 degrees). Accurate reduction of fractures and placement of screws in the humeral head using image intensifier can act to minimise risk of screw penetration and make some of the complications of locked proximal humeral plating avoidable

This study compared the effect of a computer-assisted and a traditional surgical technique on the kinematics of the glenohumeral joint during passive abduction after hemiarthroplasty of the shoulder for the treatment of fractures. We used seven pairs of fresh-frozen cadaver shoulders to create simulated four-part fractures of the proximal humerus, which were then reconstructed with hemiarthroplasty and reattachment of the tuberosities. The specimens were randomised, so that one from each pair was repaired using the computer-assisted technique, whereas a traditional hemiarthroplasty without navigation was performed in the contralateral shoulder. Kinematic data were obtained using an electromagnetic tracking device. The traditional technique resulted in posterior and inferior translation of the humeral head. No statistical differences were observed before or after computer-assisted surgery. Although it requires further improvement, the computer-assisted approach appears to allow glenohumeral kinematics to more closely replicate those of the native joint, potentially improving the function of the shoulder and extending the longevity of the prosthesis

Glenoid and humeral head bone defects have long been recognized as major determinants in recurrent shoulder instability as well as main predictors of outcomes after surgical stabilization. However, a universally accepted method to quantify them is not available yet. The purpose of the present study is to describe a new CT method to quantify bipolar bone defects volume on a virtually generated 3D model and to evaluate its reproducibility. A cross-sectional observational study has been conducted. Forty CT scans of both shoulders were randomly selected from a series of exams previously acquired on patients affected by anterior shoulder instability. Inclusion criterion was unilateral anterior shoulder instability with at least one episode of dislocation. Exclusion criteria were: bilateral shoulder instability; posterior or multidirectional instability, previous fractures and/or surgery to both shoulders; congenital or acquired inflammatory, neurological, or degenerative diseases. For all patients, CT exams of both shoulders were acquired at the same time following a standardized imaging protocol. The CT data sets were analysed on a standard desktop PC using the software 3D Slicer. Computer-based reconstruction of the Hill-Sachs and glenoid bone defect were performed through Boolean subtraction of the affected side from the contralateral one, resulting in a virtually generated bone fragment accurately fitting the defect. The volume of the bone fragments was then calculated. All measurements were conducted by two fellowship-trained orthopaedic shoulder surgeons. Each measurement was performed twice by one observer to assess intra-observer reliability. Inter and intra-observer reliability were calculated. Intraclass Correlation Coefficients (ICC) were calculated using a two-way random effect model and evaluation of absolute agreement. Confidence intervals (CI) were calculated at 95% confidence level for reliability coefficients. Reliability values range from 0 (no agreement) to 1 (maximum agreement). The study included 34 males and 6 females. Mean age (+ SD) of patients was 36.7 + 10.10 years (range: 25 – 73 years). A bipolar bone defect was observed in all cases. Reliability of humeral head bone fragment measurements showed excellent intra-observer agreement (ICC: 0.92, CI 95%: 0.85 – 0.96) and very good interobserver agreement (ICC: 0.89, CI 95%: 0.80 – 0.94). Similarly, glenoid bone loss measurement resulted in excellent intra-observer reliability (ICC: 0.92, CI 95%: 0.85 – 0.96) and very good inter-observer agreement (ICC: 0.84, CI 95%:0.72 – 0.91). In conclusion, matching affected and intact contralateral humeral head and glenoid by reconstruction on a computer-based virtual model allows identification of bipolar bone defects and enables quantitative determination of bone loss

To quantify bone-nail fit in response to varying nail placements by entry point translation in straight antegrade humeral nailing using three-dimensional (3D) computational analysis. CT scans of ten cadaveric humeri were processed in 3D Slicer to obtain 3D models of the cortical and cancellous bone. The bone was divided into individual slices each consisting of 2% humeral length (L) with the centroid of each slice determined. To represent straight antegrade humeral nail, a rod consisting of two cylinders with diameters of 9.5mm and 8.5mm and length of 0.22L mm and 0.44L mm respectively joined at one end was modelled. The humeral head apex (surgical entry point) was translated by 1mm in both anterior-posterior and medio-lateral directions to generate eight entry points. Total nail protrusion surface area, maximum nail protrusion distance into cortical shell and top, middle, bottom deviation between nail and intramedullary cavity centre were investigated. Statistical analysis between the apex and translated entry points was conducted using paired t-test. A posterior-lateral translation was considered as the optimal entry point with minimum protrusion in comparison to the anterior-medial translation experiencing twice the level of protrusion. Statistically significant differences in cortical protrusion were found in anterior-medial and posterior-lateral directions producing increased and decreased level of protrusion respectively compared to the apex. The bottom anterior-posterior deviation distance appeared to be a key predictor of cortical breach with the distal nail being more susceptible. Furthermore, nails with anterior translation generated higher anterior-posterior deviation (>4mm) compared to posterior translation (<3mm). Aside from slight posterolateral translation of the entry point from the apex, inclusion of a distal posterior-lateral bend into current straight nail design could improve nail fitting within the curved humeral bone, potentially improving distal working length within the flat and narrow medullary canal of the distal humeral shaft

Proximal humerus fractures (PHF) are the third most common fractures in the elderly. Treatment of complex PHF has remained challenging with mechanical failure rates ranging up to 35% even when state-of-the-art locked plates are used. Secondary (post-operative) screw perforation through the articular surface of the humeral head is the most frequent mechanical failure mode, with rates up to 23%. Besides other known risk factors, such as non-anatomical reduction and lack of medial cortical support, in-adverse intraoperative perforation of the articular surfaces during pilot hole drilling (overdrilling) may increase the risk of secondary screw perforation. Overdrilling often occurs during surgical treatment of osteoporotic PHF due to minimal tactile feedback; however, the awareness in the surgical community is low and the consequences on the fixation stability have remained unproved. Therefore, the aim of this study was to evaluate biomechanically whether overdrilling would increase the risk of cyclic screw perforation failure in unstable PHF. A highly unstable malreduced 3-part fracture was simulated by osteotomizing 9 pairs of fresh-frozen human cadaveric proximal humeri from elderly donors (73.7 ± 13.0 ys, f/m: 3/6). The fragments were fixed with a locking plate (PHILOS, DePuy Synthes, Switzerland) using six proximal screws, with their lengths selected to ensure 6 mm tip-to-joint distance. The pairs were randomized into two treatment groups, one with all pilot holes accurately predrilled (APD) and another one with the boreholes of the two calcar screws overdrilled (COD). The constructs were tested under progressively increasing cyclic loading to failure at 4 Hz using a previously developed setup and protocol. Starting from 50 N, the peak load was increased by 0.05 N/cycle. The event of initial screw loosening was defined by the abrupt increase of the displacement at valley load, following its initial linear behavior. Perforation failure was defined by the first screw penetrating the joint surface, touching the artificial glenoid component and stopping the test via electrical contact. Bone mineral density (range: 63.8 – 196.2 mgHA/cm3) was not significantly different between the groups. Initial screw loosening occurred at a significantly lower number of cycles in the COD group (10,310 ± 3,575) compared to the APD group (12,409 ± 4,569), p = 0.006. Number of cycles to screw perforation was significantly lower for the COD versus APD specimens (20,173 ± 5,851 and 24,311 ± 6,318, respectively), p = 0.019. Failure mode was varus collapse combined with lateral-inferior translation of the humeral head. The first screw perforating the articular surface was one of the calcar screws in all but one specimen. Besides risk factors such as fracture complexity and osteoporosis, inadequate surgical technique is a crucial contributor to high failure rates in locked plating of complex PHF. This study shows for the first time that overdrilling of pilot holes can significantly increase the risk of secondary screw perforation. Study limitations include the fracture model and loading method. While the findings require clinical corroboration, raising the awareness of the surgical community towards this largely neglected risk source, together with development of devices to avoid overdrilling, are expected to help improve the treatment outcomes

Tear pattern and tendon involvement are risk factors for the development of a pseudoparalytic shoulder. However, some patients have similar tendon involvement but significantly different active forward flexion. In these cases, it remains unclear why some patients suffer from pseudoparalysis and others with the same tear pattern show good active range of motion. Moment arms (MA) and force vectors of the RC and the deltoid muscle play an important role in the muscular equilibrium to stabilize the glenohumeral joint. Biomechanical and clinical analyses were conducted calculating different MA-ratios of the RC and the deltoid muscle using computer rigid body simulation and a retrospective radiographic investigation of two cohorts with and without pseudoparalysis and massive RC tears. Idealized MAs were represented by two spheres concentric to the joints centre of rotation either spanning to the humeral head or deltoid origin of the acromion. Individual ratios of the RC /deltoid MAs on antero-posterior radiographs using the newly introduced Shoulder Abduction Moment (SAM) Index was compared between the pseudoparalytic and non-pseudoparalytic patients. Decrease of RC activity and improved glenohumeral stability (+14%) was found in simulations for MA ratios with larger diameters of the humeral head which also were consequently beneficial for the (remaining) RC. Clinical investigation of the MA-ratio showed significant risk of having pseudoparalysis in patients with massive tears and a SAM Index <0.77 (OR=11). The SAM index, representing individual biomechanical characteristics of shoulder morphology has an impact on the presence or absence of pseudoparalysis in shoulders with massive RC tears

Latarjet procedure (transfer of coracoid process to the anterior glenoid rim) has been widely used for severe anterior shoulder instability. The purpose of the present study was to investigate the intraarticular stress distribution after this procedure to clarify the pathomechanism of its postoperative complications. CT-DICOM data of the contralateral healthy shoulder in 10 patients with unilateral anterior shoulder instability (9 males and 1 female, age: 17–49) was used for the present study. Three-dimensional finite element models of the glenohumeral joint was developed using software, Mechanical Finder (RCCM, Japan). In each shoulder, a 25% bony defect was created in the anterior glenoid cavity, where coracoid process was transferred using two half-threaded screws. The arm position was determined as 0-degree and 90-degree abduction. While medial margin of the scapula was completely constrained, a standard compressive load (50 N) toward the centre of the glenoid was applied to the lateral wall of the greater tuberosity. A tensile load (20N) was also applied to the tip of coracoid process along the direction of conjoint tendon. Then, elastic analysis was performed, and the distribution pattern of Drucker-Prager equivalent stress was investigated in each model. The proximal half of the coracoid represented significantly lower equivalent stress than the distal half (p < 0.05). In particular, the lowest mean equivalent stress was seen in its proximal-medial-superficial part. On the other hand, a high stress concentration newly appeared in the antero-inferior aspect of the humeral head exactly on the site of coracoid bone graft. We assumed that the reduction of mean equivalent stress in the proximal half of the coracoid was caused by the stress shielding, which may constitute one of the pathogenetic factors of its osteolysis. A high stress concentration in the humeral head may eventually lead shoulder joint to osteoarthritis

Abstract. Objectives. A fibril reinforced multiphasic cartilage model was developed to improve the understanding of the depth-dependent cartilage internal structure and its through thickness biomechanical response. The heterogeneous model of cartilage was validated against full-field strain measurement obtained via Digital Image Correlation (DIC) during free swelling experiments. Methods. Hemi-cylindrical cartilage cores of 5mm diameter were obtained from porcine femoral condyles and humeral heads. The full field behaviour of these samples was monitored using DIC during an osmotic free swelling experiment performed following a standardised protocol [1]. Computational models were created in FEBio (version 2.8, . febio.org. ). The cartilage, submerged in saline solution was represented by a 1×1mm cube [2] with geometry and constrains set up to mimic the experimental conditions. Cartilage was modelled as a multiphasic material represented by one inhomogeneous layer with depth-dependent Young's modulus [3], zonally varied water content and zonally oriented collagen fibrils [4]. Experimental and predicted strain maps were compared to each other both qualitatively and quantitatively. Results. The numerical strain map showed high strain localisation close to the cartilage surface, with strain in this region reaching 40% and 12% for femoral and humeral samples respectively, this finding was confirmed in our experimental results. Strain magnitude gradually decreased with depth, reaching near-zero at around 200μm. This behaviour also matched experimental observations. Conclusions. Both sets of computational strain results exhibited very good agreement with experimental data, both in terms of cartilage through-thickness swelling behaviour and strain magnitude. Our results show the importance of including cartilage structural inhomogeneities and inclusions of collagen fibrils when simulating through-thickness cartilage swelling. These findings highlight the crucial role of collagen fibrils on both tissue solute transport properties as well as the overall biomechanical response of 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

To date, the fixation of proximal humeral fractures with angular stable locking plates is still insufficient with mechanical failure rates of 18% to 35%. The PHILOS plate (DePuy Synthes, Switzerland) is one of the most used implants. However, this plate has not been demonstrated to be optimal; the closely symmetric plate design and the largely heterogeneous bone mineral density (BMD) distribution of the humeral head suggest that the primary implant stability may be improved by optimizing the screw orientations. Finite element (FE) analysis allows testing of various implant configurations repeatedly to find the optimal design. The aim of this study was to evaluate whether computational optimization of the orientation of the PHILOS plate locking screws using a validated FE methodology can improve the predicted primary implant stability. The FE models of nineteen low-density (humeral head BMD range: 73.5 – 139.5 mg/cm3) left proximal humeri of 10 male and 9 female elderly donors (mean ± SD age: 83 ± 8.8 years) were created from high-resolution peripheral computer tomography images (XtremeCT, Scanco Medical, Switzerland), using a previously developed and validated computational osteosynthesis framework. To simulate an unstable mal-reduced 3-part fracture (AO/OTA 11-B3.2), the samples were virtually osteotomized and fixed with the PHILOS plate, using six proximal screws (rows A, B and E) according to the surgical guide. Three physiological loading modes with forces taken from musculoskeletal models (AnyBody, AnyBody Technology A/S, Denmark) were applied. The FE analyses were performed with Abaqus/Standard (Simulia, USA). The average principal compressive strain was evaluated in cylindrical bone regions around the screw tips; since this parameter was shown to be correlated with the experimental number of cycles to screw cut-out failure (R2 = 0.90). In a parametric analysis, the orientation of each of the six proximal screws was varied by steps of 5 in a 5×5 grid, while keeping the screw head positions constant. Unfeasible configurations were discarded. 5280 simulations were performed by repeating the procedure for each sample and loading case. The best screw configuration was defined as the one achieving the largest overall reduction in peri-screw bone strain in comparison with the PHILOS plate. With the final optimized configuration, the angle of each screw could be improved, exhibiting significantly smaller average bone strain around the screw tips (range of reduction: 0.4% – 38.3%, mean ± SD: 18.49% ± 9.56%). The used simulation approach may help to improve the fixation of complex proximal humerus fractures, especially for the target populations of patients at high risk of failure

Objectives. All-suture anchors are increasingly used in rotator cuff repair procedures. Potential benefits include decreased bone damage. However, there is limited published evidence for the relative strength of fixation for all-suture anchors compared with traditional anchors. Materials and Methods. A total of four commercially available all-suture anchors, the ‘Y-Knot’ (ConMed), Q-FIX (Smith & Nephew), ICONIX (Stryker) and JuggerKnot (Zimmer Biomet) and a traditional anchor control TWINFIX Ultra PK Suture Anchor (Smith & Nephew) were tested in cadaveric human humeral head rotator cuff repair models (n = 24). This construct underwent cyclic loading applied by a mechanical testing rig (Zwick/Roell). Ultimate load to failure, gap formation at 50, 100, 150 and 200 cycles, and failure mechanism were recorded. Significance was set at p < 0.05. Results. Overall, mean maximum tensile strength values were significantly higher for the traditional anchor (181.0 N, standard error (. se). 17.6) compared with the all-suture anchors (mean 133.1 N . se. 16.7) (p = 0.04). The JuggerKnot anchor had greatest displacement at 50, 100 and 150 cycles, and at failure, reaching statistical significance over the control at 100 and 150 cycles (22.6 mm . se. 2.5 versus 12.5 mm . se. 0.3; and 29.6 mm . se. 4.8 versus 17.0 mm . se. 0.7). Every all-suture anchor tested showed substantial (> 5 mm) displacement between 50 and 100 cycles (6.2 to 14.3). All-suture anchors predominantly failed due to anchor pull-out (95% versus 25% of traditional anchors), whereas a higher proportion of traditional anchors failed secondary to suture breakage. Conclusion. We demonstrate decreased failure load, increased total displacement, and variable failure mechanisms in all-suture anchors, compared with traditional anchors designed for rotator cuff repair. These findings will aid the surgeon’s choice of implant, in the context of the clinical scenario. Cite this article: N. S. Nagra, N. Zargar, R. D. J. Smith, A. J. Carr. Mechanical properties of all-suture anchors for rotator cuff repair. Bone Joint Res 2017;6:82–89. DOI: 10.1302/2046-3758.62.BJR-2016-0225.R1

Proximal humerus fractures are the third most common fragility fractures with treatment remaining challenging. Mechanical fixation failure rates of locked plating range up to 35%, with 80% of them being related to the screws perforating the glenohumeral joint. Secondary screw perforation is a complex and not yet fully understood process. Biomechanical testing and finite element (FE) analysis are expected to help understand the importance of various risk factors. Validated FE simulations could be used to predict perforation risk. This study aimed to (1) develop an experimental model for single screw perforation in the humeral head and (2) evaluate and compare the ability of bone density measures and FE simulations to predict the experimental findings. Screw perforation was investigated experimentally via quasi-static ramped compression testing of 20 cuboidal bone specimens at 1 mm/min. They were harvested from four fresh-frozen human cadaveric proximal humeri of elderly donors (aged 85 ± 5 years, f/m: 2/2), surrounded with cylindrical embedding and implanted with a single 3.5 mm locking screw (DePuy Synthes, Switzerland) centrally. Specimen-specific linear µFE (ParOSol, ETH Zurich) and nonlinear explicit µFE (Abaqus, SIMULIA, USA) models were generated at 38 µm and 76 µm voxel sizes, respectively, from pre- and post-implantation micro-Computed Tomography (µCT) images (vivaCT40, Scanco Medical, Switzerland). Bone volume (BV) around the screw and in front of the screw tip, and tip-to-joint distance (TJD) were evaluated on the µCT images. The µFE models and BV were used to predict the experimental force at the initial screw loosening and the maximum force until perforation. Initial screw loosening, indicated by the first peak of the load-displacement curve, occurred at a load of 64.7 ± 69.8 N (range: 10.2 – 298.8 N) and was best predicted by the linear µFE (R. 2. = 0.90), followed by BV around the screw (R. 2. = 0.87). Maximum load was 207.6 ± 107.7 N (range: 90.1 – 507.6 N) and the nonlinear µFE provided the best prediction (R. 2. = 0.93), followed by BV in front of the screw tip (R. 2. = 0.89). Further, the nonlinear µFE could better predict screw displacement at maximum force (R. 2. = 0.77) than TJD (R. 2. = 0.70). The predictions of non-linear µFE were quantitatively correct. Our results indicate that while density-based measures strongly correlate with screw perforation force, the predictions by the nonlinear explicit µFE models were even better and, most importantly, quantitatively correct. These models have high potential to be utilized for simulation of more realistic fixations involving multiple screws under various loading cases. Towards clinical applications, future studies should investigate if explicit FE models based on clinically available CT images could provide similar prediction accuracies

Osteoporosis is a worldwide disease with a high prevalence in elderly population; it results in bone loss and decreased bone strength that lead to low-energy fractures. Since antiresorptive treatments could lead to long-term adverse effects, the ERC BOOST project aims to propose a biomimetic 3D-printed scaffold reproducing the architecture and chemistry of healthy bone. In this study, the structural parameters of healthy bone were studied in order to reproduce them through 3D printing; furthermore, structural and mechanical differences between healthy and osteoporotic (OP) bones were assessed. Healthy and OP humeral heads discarded during surgical interventions (following ethical approval by Istituto Ortopedico Rizzoli-Italy) were tomographically analysed to obtain bone structural parameters. Successively, 8 mm diameter biopsies were harvested from the heads and underwent compression and nanoindentation tests to investigate macroscopic and microscopic mechanical properties, respectively. XRD measurements were performed on bone fragments. OP bone samples exhibited inferior mechanical properties to their less interconnected and more anisotropic structure, with thinner trabeculae and larger pores. On the other hand, nanoindentations performed on OP trabeculae showed increased Young Modulus compared to healthy samples probably due to their increased hydroxyapatite crystal size, as revealed by XRD. Osteoporosis causes the weakening of the trabecular structure that leads to a decrease of bone mechanical properties. However, OP trabeculae are stiffer due to increased dimensions of hydroxyapatite crystals

The medial periosteal hinge plays a key role in fractures of the head of the humerus, offering mechanical support during and after reduction and maintaining perfusion of the head by the vessels in the posteromedial periosteum. We have investigated the biomechanical properties of the medial periosteum in fractures of the proximal humerus using a standard model in 20 fresh-frozen cadaver specimens comparable in age, gender and bone mineral density. After creating the fracture, we displaced the humeral head medial or lateral to the shaft with controlled force until complete disruption of the posteromedial periosteum was recorded. As the quality of periosteum might be affected by age and bone quality, the results were correlated with the age and the local bone mineral density of the specimens measured with quantitative CT. Periosteal rupture started at a mean displacement of 2.96 mm (. sd. 2.92) with a mean load of 100.9 N (. sd. 47.1). The mean maximum load of 111.4 N (. sd. 42.5) was reached at a mean displacement of 4.9 mm (. sd. 4.2). The periosteum was completely ruptured at a mean displacement of 34.4 mm (. sd. 11.1). There was no significant difference in the mean distance to complete rupture for medial (mean 35.8 mm (. sd. 13.8)) or lateral (mean 33.0 mm (. sd. 8.2)) displacement (p = 0.589). The mean bone mineral density was 0.111 g/cm. 3. (. sd. 0.035). A statistically significant but low correlation between bone mineral density and the maximum load uptake (r = 0.475, p = 0.034) was observed. This study showed that the posteromedial hinge is a mechanical structure capable of providing support for percutaneous reduction and stabilisation of a fracture by ligamentotaxis. Periosteal rupture started at a mean of about 3 mm and was completed by a mean displacement of just under 35 mm. The microvascular situation of the rupturing periosteum cannot be investigated with the current model

Background. While total shoulder arthroplasty (TSA) is a generally successful procedure, glenoid loosening remains a common complication. Though the occurrence of loosening was related to patient-specific factors, biomechanical factors related to implant features may also affect the fixation of the glenoid component, in particular increased glenohumeral mismatch that could result in eccentric loads and translations. In this study, a novel test setup was used to quantify glenohumeral pressures for different motion patterns after TSA. Methods. Six cadaveric human shoulders were implanted with total shoulder replacements (Exactech, Inc., USA) and subjected to cyclic internal-external, flexion-extension and abduction-adduction rotations in a passive motion testing apparatus. The system was coupled to a pressure sensor system (Tekscan, Inc., USA) to acquire joint loads and to a Zebris system (Zebris Medical, GmbH, Germany) to measure joint kinematics. The specimens were subjected to a total of 2160 cycles and peak pressures were compared for each motion pattern. Results. It was shown that during abduction the contact area between the humeral head and the glenoid component shifts from a posterior to an anterior position, while also moving inferiorly. For internal-external rotation a mean peak pressure of 8.37 ± 0.22 MPa was registered, while for flexion-extension a pressure of 9.37 ± 0.38 MPa and for abduction-adduction a pressure of 9.88 ± 0.07 MPa were obtained. Conclusion. This study showed how glenohumeral pressures after TSA vary during simulated internal-external, flexion-extension and abduction-adduction rotations in a cyclic testing setup. It showed that peak loads are mainly obtained in abduction, and that these occurred mainly near the anterior part of the glenoid. Future steps involve implantation of other type of anatomical glenoid components to obtain different levels of glenohumeral mismatch and relating the 3D measurements of motion patterns to contact pressures

Introduction. The standard treatment of proximal humerus fractures includes pre-contoured metal plates and up to nine cortical and trabecular screws. Frequent failures are reported, especially in case of poor bone quality. The scope of this study was to assess the strength of an innovative reconstruction technique (Cement-and-screws) based on a commercial plate, with a reduced number of screws compared to the standard, and with the injection of a beta-TCP additivated acrylic bone cement (Cal-Cemex, Tecres, Italy). The focus was on a four-fragment fracture of the proximal humerus, in combination with a bone defect. For comparison, also a standard technique, based on a commercial system of plate and screws was tested (Screws-only). Materials and Methods. Six pairs of cadaveric humeri were obtained through an ethically-approved donation program. The humeri were osteotomized to simulate a reproducible four-fragment fracture with the aid of a dedicated jig. Preparation included the simulation of a bone defect in the humeral head. One humerus of each pair was randomly assigned to one of two reconstruction techniques: (i) cement-and-screws humeri were repaired with a commercial fixation plate, 2 cortical and 3 trabecular screws (Philos, DePuy Synthes), and with injection of an acrylic cement additivated with beta-TCP (Cal-Cemex, Tecres); (ii) for comparison, screws-only humeri were prepared with the same commercial plate, 2 cortical and 6 trabecular screws. The reconstructed humeri underwent a biomechanical test. An axial force was cyclically applied, where the load magnitude started at 140 N and increased by 1% at each cycle. Failure was defined as fragment motion exceeding 8 mm. Results. As expected, the displacement increased monotonically as the amplitude of the load increased. Progressive failure was observed in all specimens, and initiated at a force significantly lower than the final peak. The proposed cement-and-screws reconstruction was over 5 times stronger than the traditional screws-only (paired t-test, p<0.005). For all of the pairs, the failure force of the cement-and-screws reconstruction was larger than for the contralateral screws-only reconstruction. Conclusions. The innovative repair technique based on a lower number of screws and cement injection seems very promising in terms of biomechanical strength and is a potentially alternative for the treatment of comminuted fractures. Acknowledgements. This study was funded by Tecres SpA, Italy

Introduction. Glenoid loosening, still a main complication for shoulder arthroplasty, was suggested to be related implant design, surgical aspects, and also bone quality. However, typical studies of fixation do not account for heterogeneity in bone morphology and density which were suggested to affect fixation failure. In this study, a combination of cyclic rocking horse tests on cadaver specimens and microCT-based finite element (microFE) analysis of specimens of a wide range of bone density were used to evaluate the effects of periprosthetic bone quality on the risks of loosening of anatomical keeled or pegged glenoid implants. Methods. Six pairs of cadaveric scapulae, scanned with a quantitative computer tomography (QCT) scanner to calculate bone mineral density (BMD), were implanted with either cemented anatomical pegged or keeled glenoid components and tested under constant glenohumeral load while a humeral head component was moved cyclically in the inferior and superior directions. Edge displacements were measured after 1000, 4000 and 23000 test cycles, and tested for statistical differences with regards to changes or implant design. Relationships were established between edge displacements and QCT-based BMD below the implant. Four other specimens were scanned with high-resolution peripheral QCT (82µm) and implanted with the same 2 implants to generate virtual models. These were loaded with constant glenohumeral force, varying glenohumeral conformity and superior or inferior load shifts while internal stresses at the cement-bone and implant-cement interfaces were calculated and related to apparent bone density in the periprosthetic zone. Results. Mean displacements at the inferior and superior edges showed no statistical difference between keeled and pegged designs (p>0.05). Compression and distraction were however statistically different from the initial reference measurement at even 1000 and 4000 cycles for both implant designs (p<0.05). For both implant designs, superior and inferior distractions were generally highest at each measurement time in specimens where BMD below the lifting edge was lower, showing a trend of increased distraction with decreased BMD. Moreover, the microFE models predicted higher bone and cement stresses for specimens of lower apparent bone density. Finally, highest peak stresses were located at the cement-bone interface, which seemed the weaker part of the fixation. Discussion. With this combined experimental and numerical study, it was shown that implant distraction and stresses in the cement layer are greater in glenoids of lower bone density for both implant designs. This indicates that fixation failure will most likely occur in bone of lower density, and that fixation design itself may play a secondary role. These results have important impact for understanding the mechanisms of glenoid component failure, a common complication of total shoulder arthroplasty

Background. Osteoporotic fracture fixation in the proximal humerus remains a critical challenge. While the biomechanical benefits of screw augmentation with bone cement are established, minimising the cement volume may help control any risk of extravasation and reduce surgical procedure time. Previous experimental studies suggest that it may be sufficient to only augment the screws at the sites of the lowest bone quality. However, adequately testing this hypothesis in vitro is not feasible. Methods. This study systematically evaluated the 64 possible strategies for augmenting six screws in the humeral head through finite element simulations to determine the relative biomechanical benefits of each augmentation strategy. Two subjects with varying levels of local bone mineral density were each modeled with a 2-part and 3-part fracture that was stabilised with a PHILOS plate. The biomechanical fixation was evaluated under physiological loads (muscle and joint reaction forces) that correspond to three different motions: 45 degrees abduction, 45 degrees abduction with 45 degrees internal rotation, and 45 degrees flexion. Results. The higher risk cases (low bone quality or 3-part fracture) exhibited greater peri-implant bone strains and derived greater benefits from screw augmentation. When selecting four screws to augment, the biomechanical benefits ranged from a 25% reduction in bone strain to a 59% reduction in bone strain, depending on the choice of screws. Further, the relative benefits of each augmentation strategy varied between patients and under different loading conditions. Correlations between local bone mineral density and benefits of augmentation were not significant. Conclusions. An optimal augmentation strategy is likely patient-specific and a larger cohort, modeled under a variety of conditions, would be required to elucidate any patient-specific factors (e.g. morphology or bone quality) that may dictate the relative benefits of each augmentation strategy

Introduction. Stemless shoulder implants have recently gained increasing popularity. Advantages include an anatomic reconstruction of the humerus with preservation of bone stock for upcoming revisions. Several implant designs have been introduced over the last years. However, only few studies evaluated the impact of the varying designs on the load transfer and bone remodeling. The aim of this study was to compare the differences between two stemless shoulder implant designs using the micro finite element (µFE) method. Materials and Methods. Two cadaveric human humeri (low and high bone mineral density) were scanned with a resolution of 82µm by high resolution peripheral quantitative computer tomography (HR-pQCT). Images were processed to allow virtual implantation of two types of reverse-engineered stemless humeral implants (Implant 1: Eclipse, Arthrex, with fenestrated cage screw and Implant 2: Simpliciti, Tornier, with three fins). The resulting images were converted to µFE models consisting of up to 78 million hexahedral elements with isotropic elastic properties based on the literature. These models were subjected to two loading conditions (medial and along the central implant axis) and solved for internal stresses with a parallel solver (parFE, ETH Zurich) on a Linux Cluster. The bone tissue stresses were analysed according to four subregions (dividing plane: sagittal and frontal) at two depths starting from the bone-implant surface and the distal region ending distally from the tip of Implant 1 (proximal, distal). Results. Medial loads produced higher bone tissue stresses when loading was applied along the implant axis. This was more prominent in the lower density bone, causing more than 3 times higher stresses in the highest region for both implants. Bone tissue stresses were also shown to be higher in the low density specimen, especially in the distal zone. The maximum bone tissue stress ratio for low/high density bone reached 4.4 below Implant 1 and 2.2 below Implant 2, occurring both with a medially-directed load. For both implants, the highest bone tissue stresses were predicted in the distal region than in the proximal region, with larger distal-to-proximal stress ratios below Implant 1 than Implant 2 (3.8 and 1.7, respectively). Discussion. Our µFE analyses show that the implant anchorage design clearly influences load transfer to the periprosthetic bone. The long fenestrated cage screw of Implant 1 showed more direct distal stress transfer, which may lead to stress shielding in the proximal region, in a larger extent than Implant 2 which tends to distribute loads more evenly. Furthermore, periprosthetic bone quality appears to be an important factor for load transfer, causing dramatic changes due to different loading condition and implant geometry. These findings will help further improve anchorage design for stemless humeral heads in order to minimize bone remodeling and the long-term fixation of these implants

The morphology of the proximal part of the humerus varies largely. Morphometric features characterizing the three-dimensional geometry of the proximal humerus have revealed a wide difference within individuals. These parameters include head size, radius of curvature, inclination angle, retroversion angle, offsets and neck-shaft angle. Different implant designs have been adapted so as to make provision for these anatomical variations. However, the optimal design criteria are yet to be established. Implant design is one of the main factors determining the success of Total Shoulder Arthroplasty (TSA) since slight modifications in the implant anatomy could have significant biomechanical effects. Therefore, this study investigates the three-dimensional morphometric parameters of the South African proximal humerus which will serve as a basis for designing a new Total Shoulder Prosthesis for the South African population. Sixteen South African (SA) fresh cadaveric humeri (8 left, 8 right; 8 paired) were used in this study. The data consisted of 6 men and 2 women with ages ranging from 32 to 55 years (43.13 ±8.51). The humeri were scanned using a Computer Tomography (CT) scanner. The Digital Imaging and Communications in Medicine (DICOM) files from the CT data were imported into medical modelling software, MIMICS for reconstruction. The 3D reconstructed model of the humeri as an STL file was used for further processing. The STL data were generated as a cloud of points in a CAD software, SolidWorks. These were then remodeled by defining the detailed Referential Geometric Entities (RGEs) describing the anatomical characteristics. Anatomical reference points were defined for the anatomical neck plane, the epiphyseal sphere and the metaphyseal cylinder. Also, axes were defined which comprises of the humeral head axis and the metaphyseal axis. Thereafter, the posterior offsets medial offsets and the inclination angles were measured based on the RGEs. The posterior offset varied from 0.07 mm to 2.87 mm (mean 1.20 mm), the medial offset varied from 4.40 mm to 8.45 mm (mean 6.50 mm) while the inclination angle varied from 114.00º to 133.87º (mean 121.05º). The outcome of the study showed that the shape and dimensions of the proximal humerus varies distinctively. The articular surface is not a perfect sphere and differs independently with respect to the inclination angles. In addition, variations were noticeable in the medial and lateral offsets. The morphometric data on the African shoulder is very limited and this study will significantly contribute to the shoulder data repository for the SA population. The morphometric parameters measured in this study will be useful in designing a South African shoulder prosthesis that mimics the native shoulder hence eliminating post-surgical complications