Influence of stem geometry on the stability of polished tapered cemented femoral stems

Polished, tapered stems are now widely used for cemented total hip replacement and many such designs have been introduced. However, a change in stem geometry may have a profound influence on stability. Stems with a wide, rectangular proximal section may be more stable than those which are narrower proximally. We examined the influence of proximal geometry on stability by comparing the two-year migration of the Exeter stem with a more recent design, the CPS-Plus, which has a wider shoulder and a more rectangular cross-section. The hypothesis was that these design features would increase rotational stability. Both stems subsided approximately 1 mm relative to the femur during the first two years after implantation. The Exeter stem was found to rotate into valgus (mean 0.2 degrees, SD 0.42 degrees) and internally rotate (mean 1.28 degrees, SD 0.99 degrees). The CPS-Plus showed no significant valgus rotation (mean 0.2 degrees, SD 0.42 degrees) or internal rotation (mean -0.03 degrees, SD 0.75 degrees). A wider, more rectangular cross-section improves rotational stability and may have a better long-term outcome.