[Femoral shaft bowing in valgus knees: an anatomic study]

In a previous anatomic study of healthy knees, we observed that femoral valgus cannot be attributed to hypoplasia of the lateral femoral condyle. In the present study, in an attempt to determine the site of the femoral deformation, we examined femoral shaft bowing in the frontal plan.

This cadaver study included 41 lower limbs of healthy Caucasian subjects aged over 65 years. The following anatomic landmarks were identified: center of the femoral head (H), center of the intercondylar notch (K), center of the talar dome (A), center of the femoral shaft half way between the apex of the greater trochanter and the middle of the intercondylar notch (S), the tangent line of the femoral condyles (I). Three angles were analyzed: HKA, HKI and SKI. There were 23 normal knees (HKA = 179.1 +/- 1.6 degrees) and 18 valgus knees (HKA = 182.7 +/- 0.8 degrees). Skeletal analysis (the skeleton of an object being defined as the median points of the object) was used to describe the morphology of the femoral shaft then to shape it with a second degree function (y = fx2 + bx + c). The protocol was repeated seven times to achieve accurate measurement. Accuracy was 1 degrees for the HKA angle and 0.45 degrees for the HKI and SKI angles. This accuracy was comparable to that reported in the literature. The Mann and Whitney U test was used to compare means. Spearman's t test was used to search for correlations. The first order risk was set at 0.05.

The HKI angle of valgus knees (95.5 +/- 1.1 degrees) was greater than for the normally aligned knees (93.6 +/- 2.4 degrees), confirming the femoral origin of the valgus. The form parameter f for the normal knees (1.33 10(-5) +/- 1.41 10(-5)) was greater than for the valgus knees (5.71 10(-6) +/- 5.27 10(-6)). There was a correlation between the form parameter f and the HKI angle for valgus knees, reflecting a relationship between frontal bowing of the femoral shaft and femoral valgus in this group.

The difference observed between the two groups of knees regarding the form parameter f and the correlation between f and the HKI angle in the valgus knees led us to consider that a considerable part of constitutional valgus knees can be attributed to the femoral shaft. Thus for equivalent anatomic valgus (SKI), minimal bowing (f) of the femoral shaft in valgus knees leads to greater mechanical valgus (HKI). These results confirm those obtained in our earlier study where we concluded that hypoplasia of the lateral femoral condyle does not contribute to constitutional valgus knees. We hypothesize that the same could be true for degenerative disease of constitutional valgus knees. For surgical cure, the origin of the misalignment in valgus knees dictates the rotation position of the femoral component of total knee arthroplasty and the lengthening technique for the lateral structures.