An anatomical stem should be short enough to avoid distal locking and distal load transfer but long enough to ensure adequate primary stability of the stem. In this in vitro study, the cortical strains in the femur and the primary stability of the stem were measured after insertion of Anatomic Benoist Girard-I anatomical stems with gradually reduced stem length in six human cadaver femurs in order to find the optimal stem length. A shortening of 40-50 mm, corresponding to a stem extending 30-40 mm below the lesser trochanter, did not affect stem stability but nearly normalized the load distribution in the lower metaphysis and upper diaphysis. The large strain shielding observed in the calcar region was not influenced by shortening of the stem.
Keywords: Hip prosthesis; femoral stem; micromotion; primary stability; stem design; stem length; strain; stress shielding.