Validation of Neck-Shaft Angle Correction After Cephalomedullary Nail Fixation

To validate a neck-shaft angle (NSA) correction formula using the known angle between the medullary nail and lag screw of an indwelling cephalomedullary nail (CMN).

Pertrochanteric femur fractures were created in 7 synthetic femurs and reconstructed with a CMN. Anteroposterior x-rays were taken with the proximal femur in 0, 15, 30, and 45 degrees of internal/external rotation and 10 and 20 degrees of flexion with neutral rotation. The femoral NSA and the nail angle were measured on anteroposterior x-rays. The corrected NSA was obtained by multiplying the known nail angle by the quotient of the measured NSA over the measured nail angle.

With increasing degrees of internal or external rotation, the difference between the actual and the measured NSA increased. After correction using the known angle of the nail, the measured NSA was reduced to within 3 degrees of the actual NSA over all degrees of rotation. At neutral rotation, 15 degrees of external rotation, and 10 and 20 degrees of hip flexion, there was minimal difference between the actual NSA and the measured NSA, negating the effect of the correction formula.

The measured NSA after CMN of intertrochanteric fractures is altered by hip rotation. This study validates the correction formula, which determines this difference by using the known angle between the lag screw and the nail.