Range of change of measured BMD in the femoral neck and total hip with rotation in women.

To obtain a quantitative assessment of the range of variation of bone mineral density (BMD) measurements in the femoral neck and total hip with rotation of the hip, 200 women, ages 21-86 years, were scanned by DXA in the neutral position and with 25% internal rotation of the leg. The difference in BMDs (neutral minus internal rotation) was > or =0 in about 65% of the patients, whereas the remaining 35% were <0. In terms of absolute change, the femoral neck median BMD of 0.025 g/cm2 was significantly greater than the total hip, 0.016 g/cm2. Percent absolute femoral neck change was 3.13% compared to 1.79% for the total hip. Absolute change of the total hip correlated positively with age, while the femoral neck showed a positive trend that was not statistically significant at the P = 0.05 level. Despite the total hip's lower precision error and smaller BMD change with rotation, the number of patients exceeding the 95% confidence limit for each site was virtually the same, 11% for the femoral neck and 13% for the total hip. This finding underlines the need to achieve confirmed repositioning accuracy in longitudinal studies, whether at zero femoral neck axis anteversion or otherwise, to appreciate the relative advantages of total hip measurement. The question that remains is what method can be employed to obtain this confirmation simply and economically in a busy clinical service facility with the usual turnover in personnel.