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Introduction: Bone mineral density (BMD) measured by dual energy x-ray absorbtiometry (DEXA) is widely used in management of patients with osteoporosis. Factors which are specific to machine or operator, can influence the accuracy and precision of BMD estimations. In this study we investigated the effect of leg rotation and femoral neck on densitometry of femur. Materials and Methods: In a before and after, interventional study on 200 women between 30-70 years old, without metabolic bone diseases, densitometry was done first in standard position and then in 15ْ rotation of leg from standard position and then after changing femoral neck angle from 90ه to 80ْ in relation to central femoral neck axis . Density of femoral neck, ward triangle, greater trochanter, shaft of femur and total hip was measured and data was analyzed by means comparison test. Results: External rotation of leg by 15 ه from the customary position, increased the average BMD in femoral neck, wards area, trochanter and shaft (p<0.001, p<0.04, p<0.001, p<0.008 respectively).While no significant change was observed in average BMD of total hip. Change of femoral neck area angle from 90 ه to 80 ه in relation to central femoral axis decreased BMD in femoral neck (p<0.001), and significantly increased BMD in wards triangle, trochanter, shaft and total hip (p<0.001, p<0.001, p<0.001, p<0.04 respectively). Conclusion: Malrotation of hip can be an important confounding factor when interpreting serial BMD values. Regarding the less effect of malpositioning on BMD of total hip, this area may be prefered for assessment of densitometry result.

Keywords: Bone mineral density, technical error, densitometry, femoral neck

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