Aims

Focal knee arthroplasty is an attractive alternative to knee arthroplasty for young patients because it allows preservation of a large amount of bone for potential revisions. However, the mechanical behaviour of cartilage has not yet been investigated because it is challenging to evaluate in vivo contact areas, pressure, and deformations from metal implants. Therefore, this study aimed to determine the contact pressure in the tibiofemoral joint with a focal knee arthroplasty using a finite element model.

Hip joint biomechanics can be altered by abnormal morphology of the acetabulum and/or femur. This may affect load distribution and contact stresses on the articular surfaces, hence, leading to damage and degradation of the tissue. Experimental hip joint simulators have been used to assess tribology of total hip replacements and recently methods further developed to assess the natural hip joint mechanics. The aim of this study was to evaluate articular surfaces of human cadaveric joints following prolonged experimental simulation under a standard gait cycle. Four cadaveric male right hips (mean age = 62 years) were dissected, the joint disarticulated and capsule removed. The acetabulum and femoral head were mounted in an anatomical hip simulator (Simulation Solutions, UK). A simplified twin peak gait cycle (peak load of 3kN) was applied. Hips were submerged in Ringers solution (0.04% sodium azide) and testing conducted at 1 Hertz for 32 hours (115,200 cycles). Soft tissue degradation was recorded using photogrammetry at intervals throughout testing. All four hips were successfully tested. Prior to simulation, two samples exhibited articular surface degradation and one had a minor scalpel cut and a small area of cartilage delamination. The pre-simulation damage got slightly worse as the simulation continued but no new areas of damage were detected upon inspection. The samples without surface degradation, showed no damage during testing and the labral sealing effect was more obvious in these samples. The fact that no new areas of damage were detected after long simulations, indicates that the loading conditions and positioning of the sample were appropriate, so the simulation can be used as a control to compare mechanical degradation of the natural hip when provoked abnormal conditions or labral tissue repairs are simulated

Reorientating pelvic osteotomies are performed to improve femoral head coverage and secondary degenerative arthritis. A rectangular triple pelvic innominate osteotomy (3PIO) is performed in symptomatic cases. However, deciding optimal screw fixation type to avoid complications is questionable. Therefore, this study aimed to investigate the biomechanical behavior of two different acetabular screw configurations used for rectangular 3PIO osteosynthesis. It was hypothesized that bi-directional screw fixation would be biomechanically superior to mono-axial screw fixation technique. A rectangular 3PIO was performed in twelve right-side artificial Hemi-pelvises. Group 1 (G1) had two axial and one transversal screw in a bi-directional orientation. Group 2 (G2) had three screws in the axial direction through the iliac crest. Acetabular fragment was reoriented to 10.5° inclination in coronal plane, and 10.0° increased anteversion along axial plane. Specimens were biomechanically tested until failure under progressively increasing cyclic loading at 2Hz, starting at 50N peak compression, increasing 0.05N/cycle. Stiffness was calculated from machine data. Acetabular anteversion, inclination and medialization were evaluated from motion tracking data from 250-2500 at 250 cycle increments. Failure cycles and load were evaluated for 5° change in anteversion. Stiffness was higher in G1 (56.46±19.45N/mm) versus G2 (39.02±10.93N/mm) but not significantly, p=0.31. Acetabular fragment anteversion, inclination and medialization increased significantly each group (p≤0.02) and remained non-significantly different between the groups (p≥0.69). Cycles to failure and failure load were not significantly different between G1 (4406±882, 270.30±44.10N) and G2 (5059±682, 302.95±34.10N), p=0.78. From a biomechanical perspective, the present study demonstrates that a bi-directional screw orientation does not necessarily advantageous versus mono-axial alignment when the latter has all three screws evenly distributed over the osteotomy geometry. Moreover, the 3PIO fixation is susceptible to changes in anteversion, inclination and medialization of the acetabular fragment until the bone is healed. Therefore, cautious rehabilitation with partial weight-bearing is recommended

Shoulder arthroplasty humeral stem design has evolved to accommodate patient anatomy characteristics. As a result, stems are available in numerous shapes, coatings, lengths, sizes, and vary by fixation method. This abundance of stem options creates a surgical paradox of choice. Metrics describing stem stability, including a stem's resistance to subsidence and micromotion, are important factors that should influence stem selection, but have yet to be assessed in response to the diametral (i.e., thickness) sizing of short stem humeral implants. Eight paired cadaveric humeri (age = 75±15 years) were reconstructed with surgeon selected ‘standard’ sized short-stemmed humeral implants, as well as 2mm ‘oversized’ implants. Stem sizing conditions were randomized to left and right humeral pairs. Following implantation, an anteroposterior radiograph was taken of each stem and the metaphyseal and diaphyseal fill ratios were quantified. Each humerus was then potted in polymethyl methacrylate bone cement and subjected to 2000 cycles of 90º forward flexion loading. At regular intervals during loading, stem subsidence and micromotion were assessed using a validated system of two optical markers attached to the stem and humeral pot (accuracy of <15µm). The metaphyseal fill ratio did not differ significantly between the oversized and standard stems (0.50±0.06 vs 0.50±0.10; P = 0.997, Power = 0.05); however, the diaphyseal fill ratio did (0.52±0.06 vs 0.45±0.07; P < 0.001, Power = 1.0). Neither fill ratio correlated significantly with stem subsidence or micromotion. Stem subsidence and micromotion were found to plateau following 400 cycles of loading. Oversizing stem thickness prevented implant head-back contact in all but one specimen with the least dense metaphyseal bone, while standard sizing only yielded incomplete head-back contact in the two subjects with the densest bone. Oversized stems subsided significantly less than their standard counterparts (standard: 1.4±0.6mm, oversized: 0.5±0.5mm; P = 0.018, Power = 0.748;), and resulted in slightly more micromotion (standard: 169±59µm, oversized: 187±52µm, P = 0.506, Power = 0.094,). Short stem diametral sizing (i.e., thickness) has an impact on stem subsidence and micromotion following humeral arthroplasty. In both cases, the resulting three-dimensional stem micromotion exceeded, the 150µm limit suggested for bone ingrowth, although that limit was derived from a uniaxial assessment. Though not statistically significant, the increased stem micromotion associated with stem oversizing may in-part be attributed to over-compacting the cancellous bed during broaching, which creates a denser, potentially smoother, interface, though this influence requires further assessment. The findings of the present investigation highlight the importance of proper short stem diametral sizing, as even a relatively small, 2mm, increase can negatively impact the subsidence and micromotion of the stem-bone construct. Future work should focus on developing tools and methods to support surgeons in what is currently a subjective process of stem selection

Reverse shoulder arthroplasty (RSA) is commonly used to treat patients with rotator cuff tear arthropathy. Loosening of the glenoid component remains one of the principal modes of failure and is the main complication leading to revision. For optimal RSA implant osseointegration to occur, the micromotion between the baseplate and the bone must not exceed a threshold of 150 µm. Excess micromotion contributes to glenoid loosening. This study assessed the effects of various factors on glenoid baseplate micromotion for primary fixation of RSA. A half-fractional factorial experiment design (2k-1) was used to assess four factors: central element type (central peg or screw), central element cortical engagement according to length (13.5 or 23.5 mm), anterior-posterior (A-P) peripheral screw type (nonlocking or locking), and bone surrogate density (10 or 25 pounds per cubic foot [pcf]). This created eight unique conditions, each repeated five times for 40 total runs. Glenoid baseplates were implanted into high- or low-density Sawbones™ rigid polyurethane (PU) foam blocks and cyclically loaded at 60 degrees for 1000 cycles (500 N compressive force range) using a custom designed loading apparatus. Micromotion at the four peripheral screw positions was recorded using linear variable displacement transducers (LVDTs). Maximum micromotion was quantified as the displacement range at the implant-PU interface, averaged over the last 10 cycles of loading. Baseplates with short central elements that lacked cortical bone engagement generated 373% greater maximum micromotion at all peripheral screw positions compared to those with long central elements (p < 0.001). Central peg fixation generated 360% greater maximum micromotion than central screw fixation (p < 0.001). No significant effects were observed when varying A-P peripheral screw type or bone surrogate density. There were significant interactions between central element length and type (p < 0.001). An interaction existed between central element type and level of cortical engagement. A central screw and a long central element that engaged cortical bone reduced RSA baseplate micromotion. These findings serve to inform surgical decision-making regarding baseplate fixation elements to minimize the risk of glenoid loosening and thus, the need for revision surgery

Aims. A functional anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) has been assumed to be required for patients undergoing unicompartmental knee arthroplasty (UKA). However, this assumption has not been thoroughly tested. Therefore, this study aimed to assess the biomechanical effects exerted by cruciate ligament-deficient knees with medial UKAs regarding different posterior tibial slopes. Methods. ACL- or PCL-deficient models with posterior tibial slopes of 1°, 3°, 5°, 7°, and 9° were developed and compared to intact models. The kinematics and contact stresses on the tibiofemoral joint were evaluated under gait cycle loading conditions. Results. Anterior translation increased in ACL-deficient UKA cases compared with intact models. In contrast, posterior translation increased in PCL-deficient UKA cases compared with intact models. As the posterior tibial slope increased, anterior translation of ACL-deficient UKA increased significantly in the stance phase, and posterior translation of PCL-deficient UKA increased significantly in the swing phase. Furthermore, as the posterior tibial slope increased, contact stress on the other compartment increased in cruciate ligament-deficient UKAs compared with intact UKAs. Conclusion. Fixed-bearing medial UKA is a viable treatment option for patients with cruciate ligament deficiency, providing a less invasive procedure and allowing patient-specific kinematics to adjust posterior tibial slope. Patient selection is important, and while AP kinematics can be compensated for by posterior tibial slope adjustment, rotational stability is a prerequisite for this approach. ACL- or PCL-deficient UKA that adjusts the posterior tibial slope might be an alternative treatment option for a skilled surgeon. Cite this article: Bone Joint Res 2022;11(7):494–502

Aims

One of the main causes of tibial revision surgery for total knee arthroplasty is aseptic loosening. Therefore, stable fixation between the tibial component and the cement, and between the tibial component and the bone, is essential. A factor that could influence the implant stability is the implant design, with its different variations. In an existing implant system, the tibial component was modified by adding cement pockets. The aim of this experimental in vitro study was to investigate whether additional cement pockets on the underside of the tibial component could improve implant stability. The relative motion between implant and bone, the maximum pull-out force, the tibial cement mantle, and a possible path from the bone marrow to the metal-cement interface were determined.

Aims

This study investigates head-neck taper corrosion with varying head size in a novel hip simulator instrumented to measure corrosion related electrical activity under torsional loads.

Aims

The aim of our study was to investigate the effect of asymmetric crosslinked polyethylene liner use on the risk of revision of cementless and hybrid total hip arthroplasties (THAs).

Aims

Dual mobility implants in total hip arthroplasty are designed to increase the functional head size, thus decreasing the potential for dislocation. Modular dual mobility (MDM) implants incorporate a metal liner (e.g. cobalt-chromium alloy) in a metal shell (e.g. titanium alloy), raising concern for mechanically assisted crevice corrosion at the modular liner-shell connection. We sought to examine fretting and corrosion on MDM liners, to analyze the corrosion products, and to examine histologically the periprosthetic tissues.

Aims

To determine whether half-threaded screw holes in a new titanium locking plate design can substantially decrease the notch effects of the threads and increase the plate fatigue life.

Aims. This combined clinical and in vitro study aimed to determine the incidence of liner malseating in modular dual mobility (MDM) constructs in primary total hip arthroplasties (THAs) from a large volume arthroplasty centre, and determine whether malseating increases the potential for fretting and corrosion at the modular metal interface in malseated MDM constructs using a simulated corrosion chamber. Methods. For the clinical arm of the study, observers independently reviewed postoperative radiographs of 551 primary THAs using MDM constructs from a single manufacturer over a three-year period, to identify the incidence of MDM liner-shell malseating. Multivariable logistic regression analysis was performed to identify risk factors including age, sex, body mass index (BMI), cup design, cup size, and the MDM case volume of the surgeon. For the in vitro arm, six pristine MDM implants with cobalt-chrome liners were tested in a simulated corrosion chamber. Three were well-seated and three were malseated with 6° of canting. The liner-shell couples underwent cyclic loading of increasing magnitudes. Fretting current was measured throughout testing and the onset of fretting load was determined by analyzing the increase in average current. Results. The radiological review identified that 32 of 551 MDM liners (5.8%) were malseated. Malseating was noted in all of the three different cup designs. The incidence of malseating was significantly higher in low-volume MDM surgeons than high-volume MDM surgeons (p < 0.001). Pristine well-seated liners showed significantly lower fretting current values at all peak loads greater than 800 N (p < 0.044). Malseated liner-shell couples had lower fretting onset loads at 2,400 N. Conclusion. MDM malseating remains an issue that can occur in at least one in 20 patients at a high-volume arthroplasty centre. The onset of fretting and increased fretting current throughout loading cycles suggests susceptibility to corrosion when this occurs. These results support the hypothesis that malseated liners may be at risk for fretting corrosion. Clinicians should be aware of this phenomenon. Cite this article: Bone Joint J 2020;102-B(7 Supple B):20–26

Introduction. The use of bone cement as a fixation agent has ensured the long-term functionality of THA implants . 1. However, some studies have shown the undesirable effect of wear of stem-cement interface, due to the release of metals and polymeric debris lead to implant failure . 2,3. Debris is generated by the micromotion together with a severely corrosive medium present in the crevice of stem-cement interface . 3,4. FEA studies showed that micromotion can affect osseointegration and fretting wear . 5,6. The aim of this research is to investigate if the micromotions measures from in silico analysis of the stem-cement correlate with the fretting-corrosion damage observed on in vitro testing. Methods. The in vitro fretting-corrosion testing was made with positioning and loading based on ISO 7206-4 and ISO 7206-6. It was used Exeter stems embedded in bone cement (PMMA) and immersed in a saline solution (9.0 g/L of NaCl). A fatigue testing system (Instron 8872, USA) was used to conduct the test, applying a sinusoidal cyclic load at 5.0 Hz. The tests were finished after 10 million cycles and images of stem surfaces were taken with a photographic camera (Canon EOS Rebel T6i, Japan) and a stereoscope (Leica M165C, Germany). For the computational analysis, the same testing configurations were modeled on software ANSYS. The analysis was performed using linear isotropic elasticity for both stem (E=193GPa; ⱱ=0.27; σ. y. =400MPa) and PMMA cement (E=2.7GPa; ⱱ=0.35; σ. u. =76MPa). 7,8. . A second-order tetrahedral element was used to mesh all components with a size of 0.5 mm in the stem-cement contact area, increasing until 1.0 mm outside from them. A frictional contact (µ=0.25) with an augmented Lagrange formulation was used. The third cycle of loading was evaluated and a variation of sliding distance less than 10% was set as convergence criteria. The micromotion was measured as the sliding distance on the stem-cement interface. Results and Discussion. The in silico analysis showed the presence of areas almost without micromotion in the proximal lateral and distal medial regions. In these regions, there is no evidence of fretting-corrosion after the in vitro testing. The lack of micromotion is caused by the debonding due to testing configurations and implant design. The absence of contact doesn't allow wear by abrasion or third body, avoiding the fretting-corrosion damage. For the regions distal lateral and proximal medial, it is possible to observe fretting-corrosion due to micromotions, which is supported by the in silico analysis results. The region proximal medial had the highest micromotion on computational analysis and the fretting-corrosion was more severe on laboratory testing, reinforcing the relevance of micromotion in the fretting-corrosion damage on the stem-cement interface. Conclusion. The results indicate a correlation of micromotion calculated by in silico analysis and fretting-corrosion damage observed on in vitro testing. The developed FEA model may be a useful tool to predict the fretting-corrosion damage on the THA implants on pre-clinical testing. Additional efforts are needed to apply this tool on bone-implant systems to predict fretting-corrosion damage observed in vivo. For any figures or tables, please contact authors directly

Objectives. Unicompartmental knee arthroplasty (UKA) is an alternative to total knee arthroplasty with isolated medial or lateral compartment osteoarthritis. However, polyethylene wear can significantly reduce the lifespan of UKA. Different bearing designs and materials for UKA have been developed to change the rate of polyethylene wear. Therefore, the objective of this study is to investigate the effect of insert conformity and material on the predicted wear in mobile-bearing UKA using a previously developed computational wear method. Methods. Two different designs were tested with the same femoral component under identical kinematic input: anatomy mimetic design (AMD) and conforming design inserts with different conformity levels. The insert materials were standard or crosslinked ultra-high-molecular-weight polyethylene (UHMWPE). We evaluated the contact pressure, contact area, wear rate, wear depth, and volumetric wear under gait cycle loading conditions. Results. Conforming design inserts had the lower contact pressure and larger contact area. However, they also had the higher wear rate and volumetric wear. The improved wear performance was found with AMD inserts. In addition, the computationally predicted volumetric wear of crosslinked UHMWPE inserts was less than half that of standard UHMWPE inserts. Conclusion. Our results showed that increasing conformity may not be the sole predictor of wear performance; highly crosslinked mobile-bearing polyethylene inserts can also provide improvement in wear performance. These results provide improvements in design and materials to reduce wear in mobile-bearing UKA. Cite this article: Bone Joint Res 2019;8:563–569

Objectives

The aim of this study was to determine the polyethylene wear rate of Phase 3 Oxford Unicompartmental Knee Replacement bearings and to investigate the effects of resin type and manufacturing process.

Objectives. The aim of this study was to investigate the biomechanical effect of the anterolateral ligament (ALL), anterior cruciate ligament (ACL), or both ALL and ACL on kinematics under dynamic loading conditions using dynamic simulation subject-specific knee models. Methods. Five subject-specific musculoskeletal models were validated with computationally predicted muscle activation, electromyography data, and previous experimental data to analyze effects of the ALL and ACL on knee kinematics under gait and squat loading conditions. Results. Anterior translation (AT) significantly increased with deficiency of the ACL, ALL, or both structures under gait cycle loading. Internal rotation (IR) significantly increased with deficiency of both the ACL and ALL under gait and squat loading conditions. However, the deficiency of ALL was not significant in the increase of AT, but it was significant in the increase of IR under the squat loading condition. Conclusion. The results of this study confirm that the ALL is an important lateral knee structure for knee joint stability. The ALL is a secondary stabilizer relative to the ACL under simulated gait and squat loading conditions. Cite this article: Bone Joint Res 2019;8:509–517

Objectives

This study investigated the biomechanical performance of decellularized porcine superflexor tendon (pSFT) grafts of varying diameters when utilized in conjunction with contemporary ACL graft fixation systems. This aimed to produce a range of ‘off-the-shelf’ products with predictable mechanical performance, depending on the individual requirements of the patient.

Aims

The aim of this study was to evaluate fretting and corrosion in retrieved oxidized zirconium (OxZr; OXINIUM, Smith & Nephew, Memphis, Tennessee) femoral heads and compare the results with those from a matched cohort of cobalt-chromium (CoCr) femoral heads.

Objectives

Unicompartmental knee arthroplasty (UKA) is an alternative to total knee arthroplasty for patients who require treatment of single-compartment osteoarthritis, especially for young patients. To satisfy this requirement, new patient-specific prosthetic designs have been introduced. The patient-specific UKA is designed on the basis of data from preoperative medical images. In general, knee implant design with increased conformity has been developed to provide lower contact stress and reduced wear on the tibial insert compared with flat knee designs. The different tibiofemoral conformity may provide designers the opportunity to address both wear and kinematic design goals simultaneously. The aim of this study was to evaluate wear prediction with respect to tibiofemoral conformity design in patient-specific UKA under gait loading conditions by using a previously validated computational wear method.