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 var ying degrees of femoral rotation. The predictions of the model were found t o 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 developmenta l dislocation of the hip resulted in a more restricted range of acceptable femoral positioning. External rotation of the femur should be avoided durin g patient positioning because as little as 7 degrees can cause a >10 degree s change in the apparent NSA. For all patient populations, internally rotat ing the femur will allow for determination of the femoral NSA to within 10 degrees.