Micromotion measurements with hip center and modular neck length alterations.

Hip center relocation often is necessary because of acetabular deformity or in revision surgery. Superolateral relocation of the acetabular component increases the hip joint reaction forces and has been associated with early femoral implant loosening. In addition, relocation can necessitate the use of extended femoral neck lengths. The purpose of this study was to compare the initial stability (micromotion) of an anatomically placed femoral component with that of a superolaterally relocated component and with a component having an extended neck length. A six-degree of freedom device was constructed to measure three-dimensional micromotion at the proximal and distal regions of the femoral component. The instrumented femur was loaded using a unique loading device that included musculature necessary to simulate stairclimbing. Results showed that superolateral relocation of the hip center (25 mm) only moderately increased femoral component micromotion (13%). However, it was found that extending the neck length 12.5 mm produced a dramatic increase in micromotion (38%). Clinically this suggests that hip center lateralization and the use of long modular neck lengths should be avoided.