EFFECT OF FEMORAL ROTATION ON BONE-MINERAL DENSITY-MEASUREMENTS WITH DUAL-ENERGY X-RAY ABSORPTIOMETRY

Precision in femoral neck scans with dual energy X-ray absorptiometry (DXA) is affected by variability in positioning and subsequent reposit ioning of the femur for repeated scans. To study the in vitro effect o f femoral rotation on the bone mineral density (BMD), four fresh-froze n cadaveric femurs were fixed in a specially designed jig which allows for rotation of the femurs. BMD measurements of the femurs were done in neutral position (0 degrees) i.e., with the femoral neck axis paral lel to the surface of the couch and at 15 degrees, 30 degrees, and 45 degrees of internal and external rotation. In vivo precision of the fe moral neck scan was determined in five normal male subjects. The scans were first done with the legs positioned using the manufacturer's foo t block. Five scans were performed, with repositioning, on the left hi p of each subject. The procedure was then repeated with the legs posit ioned using a custom-designed positioning jig to minimize the rotation of the hips during a scan. In the in vitro study, the femoral neck BM D value was minimum at neutral position (0 degrees) and increased when the femur was rotated internally or externally. In vivo precision err or of the femoral neck scan was reduced by almost 50% with the use of the positioning jig when compared with the manufacturer's foot block. Femoral rotation was shown to have a significant effect on BMD measure ments, and proper positioning of the femur during a scan can improve p recision significantly.