The effect of femoral rotation on the projected femoral neck-shaft angle.

Femoral neck-shaft angle (NSA) was measured in a series of anteroposterior (AP) hip radiographs of a cadaveric femur in varying degrees of rotation. A mathematical model was developed to predict NSA on an AP radiograph in varying degrees of femoral rotation. The predictions of the model were found to correlate well with the experimental data (correlation coefficient = 0.94). Based on the mathematical model, a wide range of patient positioning was found to result in a <10 degrees error in the measurement of femoral NSA. Coxa vara, increased femoral anteversion, cerebral palsy, and developmental dislocation of the hip resulted in a more restricted range of acceptable femoral positioning. External rotation of the femur should be avoided during patient positioning because as little as 7 degrees can cause a > 10 degrees change in the apparent NSA. For all patient populations, internally rotating the femur will allow for determination of the femoral NSA to within 10 degrees .