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Objectives Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure. Materials and Methods A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix). Results For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy. Conclusion We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure.

IntroductionOsteoporosis is the most common bone disease, contributing to over 3.5 million fragility fractures each year in the EU [1]. Managing these fractures costs €37 billion each year with a 25% increase predicted by 2025. Fracture fixation in reduced density osteoporotic bone is a surgical challenge and failures are common, occurring in 15-40% of cases [2]. It is hypothesised that the torque used to fasten bone screws during fracture fixation significantly influences the holding strength of screws and thus the likelihood of fixation failure. The aim of this study was to develop a validated finite element model capable of predicting the effect of different insertion torques on the holding strength of cortical bone screws for a range of bone densities and cortical thicknesses.MethodsExperimental tests (n=100) were conducted to evaluate cortical screw pull-out strength using bovine tibiae (4-5 months). The bone density (assessed using CT-images) and cortical thickness of each specimen were recorded. Screws were inserted using a range of insertion torques, and the pullout strength was measured for each (Instron 5967, High Widcomb, UK). Additionally, the effect of stress-relaxation was investigated by imaging the surface of the bone surrounding the screw immediately after screw fastening at logarithmic intervals up till 17 hours post-fixation. Strains were evaluated using digital image correlation (Ncorr v1.2, G.I.T., USA). An axisymmetric finite element (FE) model was developed based on the experimental tests. An idealized screw geometry incorporating screw threads was based on CT-images (Simpleware ScanIP, Exeter, UK). Using a maximum tensile strain failure criteria, (considering the tensile yield strain of cortical bone to be 1.0%), the FE model was used to predict the pull-out strength for the variables considered (Ansys 15.0, USA).ResultsSignificant stress relaxation was found to occur post-fixation, with the strain distribution appearing to be related to the proximity of the thread to the surface. There was good agreement in predicted and measured pullout force versus mean cortical thickness.DiscussionAlthough relatively simple, the developed validated model is capable of predicting cortical bone screw pullout strength for a range of bone densities and cortical thicknesses. This study found that significant stress-relaxation occurs post-fixation, in line with previous literature [3]. Currently, the effect of screw insertion and stress-relaxation is not included in the models. The relationship between fastening torque and axial screw preload is well defined [4], thus screw insertion torque can be reasonably approximated in this way. We aim to incorporate both of these effects to understand the post-fixation screw mechanics and further improve our modelling predictions. Our intention is to develop this model into a patient-specific tool providing surgeons with guidance regarding appropriate screw fastening torques for different bone qualities. This tool has the potential to enhance construct stability and reduce the incidence of failures. The developed model could also be used as a design tool for screws.References1. Hernlund et al, 2013, Arch Osteoporosis 8:136.2. Broderick et al., 2013, The Scientific World Jn. 5151973. Iyo et al. 2004. Journal of Biomechanics 37:1433–14374. Exp Meth in Ortho Biomech. Ch.8. Ed. Zdero, 2017.

When using locked plating for bone fracture fixation, screw loosening is reported as one of the most frequent complications and is commonly attributed to an incorrect choice of screw configuration. Choosing a patient‐optimized screw configuration is not straightforward as there are many interdependent variables that affect device performance. The aim of the study was to evaluate the influence that locking screw configuration has on loosening risk and how this is influenced by bone quality. This study uses finite element models that incorporate cortical bone heterogeneity, orthotropy, and geometrical nonlinearity to examine the effect of screw configuration on variables associated with loosening and interfragmentary motion. Strain levels within the bone were used as indicators of regions that may undergo loosening. The study found that, in healthy bone under axial loading, the most important variables influencing strain levels within the bone were the size of the bridging span (working length) and the plate rigidity. Unlike healthy bone, osteoporotic bone was found to be particularly sensitive to the spacing of the screws within the plate. Using two empty screw holes between the screws closest to the fracture was found to reduce the strain levels at the first screw by 49% in osteoporotic bone (compared to only 2.4% in healthy bone). The study also found that under torsional loading the total number of screws used was the most important variable with a 59% reduction in the strain around the screws closest to the fracture when using six rather than four screws in osteoporotic bone.

Introduction: Femoral head collapse is a possible complication after surgical treatment of femoral neck fractures. The purpose of this study was to examine whether implantation of a Sliding Hip Screw (SHS) or an X-Bolt could increase the risk of femoral head collapse. Similar to traditional hip screws, the X-Bolt is implanted through the femoral neck; however, it uses an expanding cross-shape to improve rotational stability. The risk of collapse was investigated alongside patient factors, such as osteonecrosis.Materials & Methods: This numerical study assessed the risk of femoral head collapse using linear eigenvalue buckling (an established method [1]), and also from the maximum von Mises stress within the cortical bone. The femoral head was loaded using the pressures reported by Yoshida et al. for a patient sitting down (reported to put the femoral head at greatest risk of collapse [2]), with a peak pressure of 9.4 MPa and an average pressure of 1.59 MPa. The femur was fixed in all degrees of freedom at a plane through the femoral neck. The X-Bolt and SHS were implanted in accordance with the operative techniques. The femoral head and implants were meshed with quadratic tetrahedral elements, and cortical bone was meshed with triangular thin shell elements. A converged mesh seeding density of 1.2 mm was used. All models were create and solved using ABAQUS finite element software (version 6.12, Simulia, Dassault Systèmes, France). The influence of implant type and presence was examined alongside a variety of patient factors: osteonecrosis (modelled as a cone of bone of varying angle, and varying modulus values), cortical thinning, reduced cortical modulus, and femoral head size. Twenty-two finite element models were run for each implant condition (intact; implanted with the X-Bolt; implanted with a SHS), resulting in a total of 66 models. The finite element models were validated using experimental tests performed on five 4th generation composite Sawbones femurs (Malmö, Sweden), and verified against previously published results [1].Results: No significant difference was found between the X-Bolt and the SHS, for either critical buckling pressure (p=0.964), or the maximum von Mises stress (p=0.274), indicating no difference in the risk of femoral head collapse. The maximum von Mises stress (and therefore the risk of collapse) within the cortical bone was significantly higher for the intact femoral head compared to both implants (X-Bolt: p=0.048, SHS: p=0.002). Of the factors examined, necrosis of the femoral head caused the greatest increase in risk.Discussion: The study by Volokh et al. [1] concluded that deterioration of the cancellous bone underneath the cortical shell can greatly increase the risk of femoral head collapse, and the results of the present study support this finding. Interestingly the presence of either an X-Bolt or SHS implant appeared to reduce the risk of femoral head collapse.References: [1] Volokh, K. Y. et al. “Prediction of femoral head collapse in osteonecrosis.” (2006) J Biomech Eng 128, p467-470.[2] Yoshida, H. et al. “Three-dimensional hip contact area and pressure distribution during activities of daily living” (2005) J Biomech 39, p1996-2004.

Introduction The majority of experimental biomechanical studies relating to the proximal femur use synthetic composite test specimens because of the advantages in terms of cost, availability and preservation. Studies report significantly (p