Femoral rotational asymmetry (FRA) is the difference in rotational alignment between the trochlear groove and posterior condyles. We hypothesize that FRA increases as the tibial plateau becomes more varus due to internal rotation of the posterior condyles and external rotation of the trochlear groove to ensure a vertical trochlear groove at 90o knee flexion.
Seventy lower limb Computed Tomography (CT) scans were reviewed by two examiners. Comparisons were made between both the sulcus line (SL) and posterior condylar line (PCL) relative to the surgical epicondylar axis (SEA). Femoral and tibial coronal alignment were measured on CT scanograms and 3D reconstructions. Correlation analysis was performed to identify associations between FRA, SL and PCL and the coronal alignment of the tibia and femur.
The mean FRA was +2.9° (SL externally rotated to PCL) (−2.4° to +7.7°, SD 2.2o). FRA greater than four degrees occurred in (17/70) 24% of knees. A statistically significant correlation was found between the degree of FRA and proximal tibial varus (MPTA) (R2 = 0.67, p < 0.001). Furthermore, there were significant correlations between the SL and the MPTA (p < 0.001, R2 = 0.77) and the PCL and the MPTA (p < 0.001, R2 = −0.41).
Native femora are frequently rotationally asymmetrical. As the tibial plateau becomes increasingly varus there is an increase in external rotation of the SL and internal rotation of the PCL. The effect is to maintain a more vertical trochlear groove during flexion in the presence of a varus tibia.