Loosening pattern in a cementless custom-made hip stem: X-ray analysis, finite-elements and photoelasticity measurements

Thirty-three X-press cementless stems (Depuy) manufactured according to standardized X-rays were inserted from 1992 to 1994. The patients’ mean age was 49 (range 15–79) years with a mean follow-up of 32 (+/–6) months. A characteristic radiographic pattern of aseptic loosening with erosion of the medial cortex by the tip of the stem occurred in 28 patients and a valgus shift of the implant in 14 cases. A radiolucent line with increased sclerosis below the tip (zone Gruen 4) was observed in 17 cases. Four stems were revised due to histologically confirmed aseptic loosening. Biomechanical investigation of one of the revised stems with the typical pattern of valgus angulation and medial cortex erosion included photoelasticity and finite-element analysis. The intertrochanteric fit and fill obviously resulted in an unfavorable distribution of contact areas, including peaks of high stress on the medial tip of the stem. These experimental findings are even evident for a postulated rotational stability. The clinical and radiographic results of the cementless X-press stems do not seem to support the fixation concept of intertrochanteric fit and fill of femoral components.