Abstract:
Total hip arthroplasty (THA) is one of the most successful interventions in any form of surgery. In the 21st century patients with degenerative hip disease can expect to return to a very high level of pain free function when their joint is replaced. Despite technologic advances in materials we have not yet created the perfect implant, that is, one that does not alter the biomechanics of the skeleton or suffer mechanical failure during the patients lifetime. Bone loss, or osteolysis, secondary to local and systemic physiologic effects will continue to present management challenges to the arthroplasty surgeon. While osteolytic defects are commonly observed in long-‐term follow-‐up, how such lesions alter stress distribution is less clear. Many patients with osteolysis are asymptomatic with medical comorbidity that makes surgical intervention risky. Can we safely observe these lesions or do we place patients at an increased risk of fracture? Finite element (FE) models of skeletal tissue are useful in allowing us to predict how changes in bone architecture and material properties can alter load transfer. The specific focus of this thesis is the analysis of load distribution in the pelvis when defects arise in the retro-‐acetabular region using FE models. Critical to the success of any computational analysis is accuracy. In the study presented herein, development of conceptual knowledge began with two-‐dimensional models before a process of verification and validation was conducted using three-‐dimensional models. Initial work with a simple composite beam demonstrated that assigning material properties to Gauss points within large elements using cubic Hermite basis functions predicts surface deformation and internal shear stress similar to experimental and theoretical results. This approach was enhanced in a validation study using a sawbone pelvis implanted with a cementless acetabular component and also, novel to this thesis, an acetabular defect. FE model predictions were highly correlated to experimental surface strains. The validated FE model was applied to a population of patients with variable volume cancellous, and in some cases cortical defects, in the retro-‐acetabular region. Loads occurring during walking and a fall onto the side were assessed. A fracture algorithm was applied to determine differences in load and site of failure in the presence of defects. Predictions of cortical stress magnitude and distribution were consistent with those previously reported in the literature. Defects caused increases in cortical stress during loads consistent with routine activity but these were consistently well below theoretical yield stress. During a fall onto the side, defects were observed to cause lower load to failure in the smaller females pelvis and those with larger volume defects (>20cm3). To my knowledge this thesis represents one of the first pelvis FE studies that includes bone defects of this type, and implant and incorporates a population-‐based investigation. The modeling experiments have generated a large amount of data, only a small proportion of which has been assessed and presented in this thesis. With constant improvements in the efficiency of building patient specific models, and ongoing validation work it is hoped that the current data set can be expanded with further exploration of bone remodeling in the periprosthetic environment.