Introduction:

 Resurfacing Total Hip Arthroplasty (RTHA) may preserve the femoral neck bone-stock post-operatively. Bone Mineral Density (BMD), could theoretically be affected by the hip-position, and bias longitudinal studies. We aimed to investigate BMD precision dependency on type of ROI and position of hip.

 

Method and Materials

 We DXA scanned the femoral neck of 15 resurfacing patients twice with the hip in 3 different rotations; 15° internal, neutral, and 15° external. For each position BMD was analyzed with 3 different surface area models. One model measured BMD in the total femoral neck, the second model divided the neck in two and the third model had 6 divisions.

 

Results

 When all hip positions were pooled a mean Coefficient of variation (CV) of 3.1%, 3.6% and 4.6% was found in the 1, 2 and 6-region models, respectively, The external rotated hip position was less reproducible. When the hip was rotated in increments of 15° and 30°, the mean CVs rose to 7.2%, 7.3% and 11.8%.  Rotation affected the precision most in the model that divided the neck in 6 sub regions, predominantly in the lateral and distal regions. For larger-region models, some rotation could be allowed without compromising the precision.

 

Konklusion

 If hip rotation is strictly controlled, DXA can reliably provide detailed topographical information about the BMD changes around a RTHA. As rotation strongly affects the precision of the BMD measurements in small regions, we suggest applying a less detailed analyzing model in studies where the leg position has not been firmly controlled.