Precision of the measurements of periprosthetic bone mineral density in hips with a custom-made femoral stem

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 bef ore 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 bet ween 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 th e operated hip before and at ten days after operation to determine the effe ct 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 modi fied coefficient of variation of 0.6% for the non-operated side in 15 patie nts and of 0.6% for the operated femur in 20 patients. The effect of rotation on the precision of the measurements after implantat ion of an uncemented femoral stem was determined in ten explanted femora an d for the operated side in ten patients at 10 degrees rotation and in 20 pa tients at 30 degrees rotation, Rotation within 30 degrees influenced the pr ecision 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 pr ospective cross-sectional measurements, since the effect of the difference in preoperative BMD, as well as the apparent increase in BMD after implanta tion of an uncemented stem, is not known from retrospective studies, The DE XA method is a reliable procedure for determining periprosthetic BMD when p ositioning and rotation are strictly controlled.