Measurement accuracy of proximal femoral geometry using biplanar radiography

Experimental testing indicates that the neck-shaft angle of the proximal femur can be reliably measured from biplanar radiographs, but torsion cannot. Calculated torsion varies significantly from true torsion as a function of the error created by femoral rotation, while the calculated neck-shaft angle is invariant with respect to rotation even though the calculated neck-shaft angle is mathematically dependent on calculated torsion. Theoretical considerations support this finding and demonstrate for the first time why it is true. This study demonstrates that clinical measurements of frontal plane hip deformities using existing biplanar radiography are quite accurate, but clinical measurements of torsional abnormalities are unreliable. These errors can be minimized if investigators specify the angle of the lateral X-ray beam on the anteroposterior film using methods described in this paper and if patient position is radiographically verified. The magnitude and types of errors inherent in biplanar radiography should be considered in planning preoperative strategies and in postoperative assessments of hip geometry.