Course of bone mineral content changes around cementless Zweymueller total hip arthroplasty. A 4-year follow-up study.

In this prospective study we followed the bone mineral content (BMC) changes over time in seven zones of interest around uncemented Zweymueller total hip arthroplasty (THA) components, using dual-energy X-ray absorptiometry (DEXA) combined with a radiographical analysis of the distal femoral cortices, close to the tip of the stem. In 36 women (average age 55 years) operated on for primary hip osteoarthritis, BMC of the hips was measured preoperatively and 2 weeks, 1 year and 4 years postoperatively. In particular, a significant decrease of the preoperative values (ranging from 35% to 42.53%; P = 0.05 to 0.01) was noted immediately postoperatively in zones 2-7. Thereafter, an additional significant increase of BMC was observed between the first and last observations at the greater (30%, P < 0.05) and lesser (35.48%, P < 0.01) trochanter. The BMC changes over time were not related to the age of the patients. The BMC reduction observed immediately after implantation of the Zweymueller THA is probably related to the intraoperatively removed bone from (1) the medial and distal inner surface of the acetabulum and (2) the inner femoral cortex. Thereafter, the BMC in zones 2, 6 and 7 remained practically unchanged. The fact that a significantly delayed BMC increase was found in the region of the greater and minor trochanter 4 years later may be due to an increasing, continuous bone turnover in the intertrochanteric area. Thus, the Zweymueller screw socket becomes definitively anchored immediately postoperatively in the medial and distal thirds of the acetabulum and remains stable over time, whereas the Zweymueller stem is mainly anchored within the distal femoral cortex as well as within the mass of the greater and minor trochanter, and was still stable at the 4-year follow-up.