Underestimation of varus angulation in knees with flexion deformity

In order to investigate the change in limb alignment in cases of knee flexion deformity, 17 male volunteers aged from 25 to 35 years with no complaints and no knee disorders were selected. The femorotibial angle (FTA), which is the lateral angle at the intersection between the femoral axis and the tibial axis in an anteroposterior radiograph, was employed as an index of limb alignment. FTA in standing on one leg with and without knee flexion was measured. The mean standing FTA was 179.4+/-19 degrees in full extension, which decreased to 176.9+/-2.5 degrees in a mean flexion position of 18+/-3.3 degrees (range 15-25 degrees ) (P<0.002), and in all cases, standing FTA in flexion was smaller (more valgus-angulated) than that in full extension. There was a significant positive relationship with a straight regression line (D=0.101KFA + 0.296, R=0.8505) between the angle of knee flexion (KFA) and the difference (D) in standing FTA between full extension and knee flexion. We conclude that limb alignment became more valgus-angulated with an increase in flexion deformity.