Effect of rotation on periprosthetic bone mineral measurements in a hip phantom.

The effect of rotation on the measurement of periprosthetic bone mineral content and bone mineral density using dual energy xray absorptiometry was studied in 9 anatomic specimen femora implanted with noncemented titanium hip prostheses. An apparatus was constructed to permit accurate femoral rotation through an arc of 30° internal to 30° external rotation in 5° increments. Two scans were obtained at each increment of rotations for a total of 26 scans for each implanted femur. Change in bone mineral content and bone mineral density was measured for each of 7 contiguous periimplant regions (Gruen zones). The precision of the paired bone mineral density measurements was 1.7%. In all Gruen zones, the means of bone mineral content and bone mineral density measurements varied within 5% between 15° internal and 15° external rotation. Variation in limb rotation, as might occur with temporal studies of periimplant bone remodeling, can be tolerated within easily controllable limits without excessive error in bone mineral measurements.