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

O ur 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 1999 Measurements 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 11 We 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 uncemented stems. Materials and Methods Measurements were made by dual-energy x-ray absorptiometry (DEXA) using the Lunar DPX-L instrument (Lunar Corporation, Madison, Wisconsin) and 'orthopaedic hip software'. This was the 'fast scan mode' with 66 kV, 3 mA and a resolution of 0.6 * 1.2 mm. The scans began 1 cm distal to the tip of the prosthesis and were continued to 3 cm proximal to the tip of the greater trochanter. A bag equivalent to soft tissue was positioned lateral to the uppe