Change in Axial Rotation of the Femur in the Resting Supine Position Following Total Hip Arthroplasty.

Stem anteversion in the setting of total hip arthroplasty (THA) is usually measured by referencing the posterior condylar line (PCL). However, stem anteversion may not express the functional torsion of the femur if a consistent horizontal tilt in the PCL is observed after THA. Herein, the horizontal tilt of the PCL at the resting neutral position of the hip was retrospectively measured in 53 THA patients from three computed tomography (CT) datasets: (i) preoperative (preop-CT), (ii) 3 weeks after THA (1st postop-CT), and (iii) more than 2 years after THA (2nd postop-CT). Factors including sex, surgical approach, change in femoral anteversion, leg length, leg lateral length, and pelvic sagittal tilt, were analyzed to determine if they were predictors for the rotational change in PCL, and the number of cases in which the horizontal tilt of the PCL remained within 10° in the three CT images was calculated. Comparing the preop-CT to the 1st postop-CT demonstrated that the PCL underwent internal rotation by (mean ± standard deviation) 13.1° ± 8.6°; only leg lateralization was significantly correlated to the change in PCL (β = 0.278, P = 0.046). Comparing the preop-CT to the 2nd postop-CT demonstrated that the PCL was internally rotated by 9.7° ± 9.3°; no factors were correlated to the change in PCL. There were only 12 hips (23%) in which the horizontal tilt of the PCL remained within 10°. In conclusion, the PCL in the resting position was internally rotated shortly after THA and remained internally rotated from the preoperative position at two or more years following THA. Thus, stem anteversion measured by referencing the PCL may not represent functional torsion of the femur.