Improving the fit of press-fit hip stems.

The implant-bone fit is critical to the longevity of press-fit total hips. Maximum contact on the internal cortical bone surface, especially proximally, produces more normal strain values and reduces micromotion and sinkage. Stem design computer software was developed to design both individualized hip stems that maximize implant-bone contact for a unique anatomy, and average anatomic hip stems in which the mean error of fit is minimized for many patients. Cadaver femurs were used to test the following two hypotheses: (1) Average anatomic stems, designed using the optimal-fit software for an average femoral geometry, improve implant-bone fit when compared with standard stems; and (2) Individualized optimal-fit stems provide an even better fit than average anatomic stems. Results showed that the average anatomical stems fit better than the standard stems. However, the individualized hip stems, designed from individual three-dimensional geometry data, fit the best.