Our aim was to determine the precision of the measurements of bone mineral density (BMD) by dual-energy x-ray absorptiometry in the proximal femur before and after implantation of an uncemented implant, with particular regard to the significance of retro-and prospective studies.We examined 60 patients to determine the difference in preoperative BMD between osteoarthritic and healthy hips. The results showed a preoperative BMD of the affected hip which was lower by a mean of 4% and by a maximum of 9% compared with the opposite side. In addition, measurements were made in the operated hip before and at ten days after operation to determine the effect of the implantation of an uncemented custom-made femoral stem. The mean increase in the BMD was 8% and the maximum was 24%. Previous retrospective studies have reported a marked loss of BMD on the operated side.The precision of double measurements using a special foot jig showed a modified coefficient of variation of 0.6% for the non-operated side in 15 patients and of 0.6% for the operated femur in 20 patients.The effect of rotation on the precision of the measurements after implantation of an uncemented femoral stem was determined in ten explanted femora and for the operated side in ten patients at 10°r otation and in 20 patients at 30° rotation. Rotation within 30° influenced the precision in studies in vivo and in vitro by a mean of 3% and in single cases in up to 60%.Precise prediction of the degree of loss of BMD is thus only possible in prospective cross-sectional measurements, since the effect of the difference in preoperative BMD, as well as the apparent increase in BMD after implantation of an uncemented stem, is not known from retrospective studies. The DEXA method is a reliable procedure for determining periprosthetic BMD when positioning and rotation are strictly controlled.
J Bone Joint Surg [Br] 2000;82-B:1065-71.
Received 8 January 1999; Accepted after revision 27 August 1999Measurements of the bone mineral density (BMD) of the proximal femur after arthroplasty of the hip have been made for a number of years, mainly in retrospective studies [1][2][3][4][5] in which a reduction of up to 50% has been observed compared with the non-operated side. Prospective studies could not predict a reduction in BMD by this amount. The maximum reduction of BMD two years after arthroplasty was 38% compared with the immediate postoperative density of the operated femur. 6-10 Direct comparison of retroand prospective studies is not possible since the implant time of the prostheses is markedly different and it has not been possible to identify the degree to which the BMD of both femora differ because of individual differences, and the effect of implantation of an uncemented stem.
11We have compared the preoperative with the immediate postoperative BMD and examined the effect of implantation. The method of measurement and the influence of rotation in vitro and in vivo after implantation of an uncemented custom-made femoral stem were compared with those in standard uncemente...