Structural integrity and life of hip implants: review of case studies

Hip implants are nowadays typically high-quality and reliable components which rarely fail. Nevertheless, they still sometimes do, due to static, impact or amplitude loading, under hostile environment such as our bodies. Therefore, we still have to analyse factors that may affect both hip implant integrity and life, such as body weight, implant geometry and material. In this paper, a review of recent investigation of hip implant fracture and fatigue behaviour is presented with focus on the effects of body weight, implant geometry and material. It is shown that body weight plays an important role, especially for fatigue life, since the relation between remaining life and stress (proportional to weight) is exponential (typically m = 3 in Paris equation). Regarding implant geometry, it is demonstrated that the main issue is stress analysis. In the case of modular (full cross-section) implants, stress state is more favourable than in the case of implant with holes for fine adjustment according to patient bone configuration. It is also shown that stress level in this case can be surprisingly high, especially with younger, active patients. Finally, the interaction between influencing factors should be taken into account, being a relatively simple task when numerical methods are used.