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) c hanges over time in seven zones of interest around uncemented Zweymuel ler total hip arthroplasty (THA) components, using dual-energy X-ray a bsorptiometry (DEXA) combined with a radiographical analysis of the di stal femoral cortices, close to the tip of the stem. In 36 women (aver age age 55 years) operated on for primary hip osteoarthritis, BMC of t he hips was measured preoperatively and 2 weeks, 1 year and 4 years po stoperatively. In particular, a significant decrease of the preoperati ve values (ranging from 35% to 42.53%; P = 0.05 to 0.01) was noted imm ediately postoperatively in zones 2-7. Thereafter, an additional signi ficant increase of BMC was observed between the first and last observa tions at the greater (30%, P < 0.05) and lesser (35.48%, P < 0.01) tro chanter. 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 remov ed 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 a nd 7 remained practically unchanged. The fact that a significantly del ayed BMC increase was found in the region of the greater and minor tro chanter 4 years later may be due to an increasing, continuous bone tur nover in the intertrochanteric area. Thus, the Zweymueller screw socke t becomes definitively anchored immediately postoperatively in the med ial and distal thirds of the acetabulum and remains stable over time, whereas the Zweymueller stem is mainly anchored within the distal femo ral cortex as well as within the mass of the greater and minor trochan ter, and was still stable at the 4-year follow-up.