A Finite Element Analysis of the Effects of Bone Loss on Cementless Total Hip Arthroplasty

Fatigue fractures of the femoral component of cementless hip prostheses are rare occurrences at present, but may become more widespread in the future as the demand increases for porous coated cementless hip arthroplasty in young, active, and heavy patients. Whereas the fatigue fracture of high strength forged alloys is unlikely, a risk of fatigue fracture may exist for sintered porous coated stems since the fatigue strength of these implants is relatively low. The low fatigue strength of porous coated stems is due to the combined effects of the use of cast alloys, the exposure of the cast alloy to high temperatures during the sintering process, and the presence of stress concentrations where the porous coating beads contact the substrate surface. Furthermore, due to load sharing between the bone and prosthesis, bone loss may increase the risk of stem fracture. Recent dual energy xray absorptiometry measurements have demonstrated substantial and progressive bone loss around cementless prostheses which lead to stem fatigue fracture. The objective of this research was to develop clinical guidelines to identify combinations of spatial patterns of bone loss, patient factors, and prosthesis design factors that create a high risk of stem fatigue fracture in the long-term (10-15 years) for an AML-type hip prosthesis. These clinical guidelines would help to minimize the future incidence of fatigue fractures of sintered stems by guiding bone density evaluation for monitoring patients who currently possess implants and prosthesis selection for future patients. To accomplish this, a detailed series of finite element stress analyses of an AML cementless prosthesis were performed. The material property distributions of the bone were varied to simulate various hypothetical and clinical patterns of bone remodeling. The finite element results were then generalized by using simpler composite beam theory analyses for a range of applied diaphyseal bending moments, periosteal bone diameters, prosthesis diameters, and prosthesis materials. The finite element analyses of a well-fit 15 mm diameter AML prosthesis in an average sized bone indicated that bone loss around the central third of the prosthesis controlled the risk of fracture and that typical clinical patterns of bone loss increased the stem stresses beyond the fatigue strength of the substrate. The composite beam theory analyses predicted that the risk of fracture for small stem sizes was more sensitive to reductions in bone modulus and bone size than large stem sizes. Although only small diameter cobaltchrome prostheses were predicted to be at risk for fatigue fracture based on analyses using values of bone modulus which neglect implant-induced bone loss, titanium and larger diameter cobalt-chrome prostheses are also at risk for fracture when there is a reduction in bone modulus around the central third of the prosthesis in the long-term. Taken together, it is important to consider the effects of long-term bone loss in addition to stem diameter, patient body weight, and patient activity levels when evaluating the risk of fracture for sintered prosthesis stems. Thesis Supervisor: Wilson C. Hayes Maurice E. Mueller Professor of Biomechanics, Harvard Medical School, Harvard-MIT Division of Health Sciences and Technology