Mechanical stability of custom and anatomical femoral stems: an experimental study in human femora.

In this study we have compared the mechanical stability of custom (n=8) and anatomical (n=8) uncemented femoral components, following insertion into human cadaveric femurs, during simulated single leg stance and stair climbing. In the custom group two specimens were excluded from the study due to detachment of the greater trochanter during cyclical loading. As a consequence of their mechanical behaviour both types of stems could be divided into subgroups of "unstable" and "stable" implants. In the course of one thousand loading cycles three anatomical stems and one custom stem migrated more than 1 mm, which was interpreted as mechanical loosening. This difference in rate of mechanical loosening was not significant. However, the majority of the stems were remarkably stable and showed micromotion of less than 18 m and migration of less than 35 m at the proximal implant-bone interface. The corresponding figures for the tip of the stems were 243 m and 170 m, respectively. During torsional loading the custom stems showed less rotatory motion than the anatomical stem (p<0.05). There were no significant differences in the magnitude of cyclical micromotion or migration for the two types of femoral stems. (Hip International 2002; 12: 263-73).