Design rationale for intraoperatively machined uncemented femoral components in total hip replacement

Abstract

First-generation uncemented femoral components failed to improve the results of total hip replacement. Insteadof maintaining the health and strength of the femur, the proximal femur was often injured either by stress-shielding, resulting from extensive coatings on the stems, or by osteolysis, resulting from unimpeded migration of particulate (primarily polyethylene) debris. Further, the incidence of thigh pain, intraoperative fractures, and early loosening increased. although the designs and surgical technique for cemented femoral components have improved, these stems still result in dramatic reductions and alterations in proximal femoral stresses. Disuse of the proximal femur will eventually have adverse consequences for the bone even if the primary implants remain well fixed. Therefore, the goals of providing long-term, stable fixation while maintaining nearly normal, healthy proximal femur have not been met with either first-generation uncemented stems or modern cemented components. although first-generation uncemented components have failed to achieve their desired goals, proximally coated, collarless press-fit stems do achieve the most normal proximal bone strains. Uncemented femoral components, therefore, still hold great promise if stable fixation, impedence of debris migration, and elimination of thigh pain can be achieved. To achieve the goals of long-term bone health and stable fixation, the flaws in design and surgical technique of first-generation uncemented stems must be identified and addressed.Based on early experiences, second-generation uncemented femoral component systems must include several essential features. The femur should primarily be prepared with accurate machine tools instead of broaches to improve initial implant-bone apposition and reduce the risk of intraoperative fracture. The component should not be extensively coated as this has clearly resulted in stress shielding.at this point, femoral components should be made of titanium, as cobalt-chromium stems have clearly been associated with higher incidences of both stress-shielding and thigh pain.The addition of flutes distally is essential because they dramatically reduce initial micromotion to the level of a cemented stem before osseointegration and have not been associated with stress shielding.Some method of stem stiffness reduction is also important. The addition of a coronal slot in the stem tip, for example, has been shown to reduce the incidence of thigh pain dramatically. Finally, stems that are not circurnferentially coated proximally have been associated with a high incidence of endosteolysis and must be avoided so that particulate debris migration can be inhibited.Preliminary evidence suggests that macrotextured-Ha components inhibit polyethylene debris migration to a level that is comparable with or better than cemented stems while maintaining and increasing bone and density proximally. In summary, second-generation uncemented components should be intraoperatively machined, avoid extensive coating, encourage proximal stress transfer, be made of titanium alloy, inhibit particulate migration, and include mechanisms for proximal and distal rotational control. Preliminary experience with such a second-generation stem suggests that these essential design features do result in excellent immediate implant stability, early osseointegration, and the eliminations of intraoperative fractures and activity-related thigh pain.