Posterior condylar cartilage may distort rotational alignment of the femoral component based on posterior condylar axis in total knee arthroplasty

Among several anatomical references, the posterior condylar axis is the most apparent landmark to decide the femoral rotation for total knee arthroplasty. External rotation based on the posterior condylar axis requires condylar twist angle in order to set the femoral component parallel to trans-epicondylar axis although the angle is not constant. The angle during surgery differs from the pre-operative measurement on epicondylar view, because X-rays do not show the posterior condylar residual cartilage thickness. The thickness should be measured for the accurate femoral rotation. We investigated the two twist angles on preoperative X-ray and during surgery, and the impact of residual cartilage on the setting of rotational angle of the femoral component in 184 knees in 112 patients with varus osteoarthritis (mean femorotibial angle: 185 +/- A 6.9 from 169 to 205A degrees). The twist angle during surgery was 5.2 +/- A 1.3A degrees (1.5-8.5A degrees) and the angle on X-ray was 6.5 +/- A 2.3A degrees (0.6-13.5A degrees). The rotational angle influenced by the residual cartilage was calculated to be 1.7 +/- A 1.3A degrees (0.0-4.6A degrees). The discrepancy in the two twist angles was close to the rotational angle. There were, however, wide variations in all angles. The results suggested the importance of considering the influence of the residual cartilage and the individual variation in determining the femoral component setting. Multiple reference frames for femoral component rotation or combination with gap technique may help to minimize malalignment which may lead to poor clinical outcome.