Theoretical and finite element method investigation of hip resurfacing biomechanics

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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Hip resurfacing is a popular alternative to total hip replacement for patients suffering from osteoarthritis. It is proposed as a bone conserving alternative for younger, heavier and more active patients for whom a total hip replacement is less suitable. Despite its increasing acceptance and use by orthopaedic surgeons there still remains controversy and disagreement over whether it should be used or not. One of the main concerns is the incidence of early, post-operative, hip fracture. There are several anatomical and surgical parameters that affect the incidence and likelihood of hip fracture. One of the most commonly cited surgical parameters leading to increased risk of post operative fracture is the valgus-varus (angular) orientation of the femoral implant. Although clinical observation and evidence exists to support surgical planning decisions and recommendations that aim to reduce the risk of fracture, there has been a lack of strong mathematical and biomechanical reasoning and explanation for them. The aim of the research presented in this thesis was to make a significant contribution to the understanding of the biomechanics of hip resurfacing through theoretical biomechanical analysis coupled with detailed finite element analysis. The main surgical and biomechanical parameters investigated were implant angular alignment and offset. A kinematic model of the anatomical hip that simulates possible changes in, and correlations between, various components of hip joint functional anatomy was developed. This model, importantly, incorporated changes in hip joint functional anatomy due to variations in neck shaft angle and served as the basis from which changes to hip joint biomechanics due to hip resurfacing could be assessed. Finite element models were developed to more accurately assess the stress and strain in the intact and resurfaced hip and also to evaluate the predictive value of theoretical biomechanical models. Comparison of finite element results with theoretical biomechanical calculations in this study has shown that using the cross-section at the implant rim and perpendicular to the stem axis is inadequate to determine the stress in the resurfaced femoral head due to the variability in geometry produced by changing the implant varus-valgus angle. Not only is it unable to accurately predict the magnitude of stress, but importantly, it is unable to predict the trend of stress variation due to changes in varus-valgus orientation. Relatively small changes in offset and varus-valgus angle can change the subcapital head/neck geometry appreciably. These geometry changes result in significant changes to the stress distribution such that they no longer resemble the stress distribution due to simple bending theory and axial loading. A major finding of the research is that the effect of the femoral implant offset on the state of stress and strain, and hence risk of fracture, in the resurfaced femoral head and neck is not insignificant or negligible as has been previously believed or assumed and it may, in fact, be more important than the implant valgus-varus orientation. The theoretical biomechanical calculations and finite element model results in this study have shown that the reduced stress and strain in the superolateral femoral neck, and the larger fracture load, in valgus oriented resurfaced femurs can be attributed more to a reduced femoral offset than the valgus angle itself. This research has made a significant contribution to the understanding of the biomechanics of hip resurfacing. It has provided an explanation for some of the inconsistencies seen in published results and has identified and corrected an implicit misinterpretation of stress analysis that has persisted for 30 years. Recommendations have been made that, if followed, will facilitate better understanding, interpretation and comparison of results between clinical, finite element and experimental studies of hip resurfacing. Perhaps most importantly, the improved understanding of hip resurfacing biomechanics will ultimately lead to a reduced incidence of hip fracture in patients undergoing this procedure.
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