[Axial lower limb alignment and knee geometry in patients with osteoarthritis of the knee]

We conducted a retrospective radiographic study to identify and quantify medial and lateral misalignments in candidates for total knee arthroplasty.

We studied a discontinuous series of 101 patients (164 knees) among candidates for total knee arthroplasty seen between 1990 and 2002. We selected a random sample for study. Exclusion criteria were any history of fracture, surgical treatment, rheumatoid disease, or congenital disorder. A first group of 136 genu varum knees was identified in 82 patients (mean age 72 years, mean weight 79 kg). A second group of 28 genu valgum knees was identified in 19 patients (mean age 67 years, mean weight 71 kg). The standard radiograms were digitalized for semi-automatic measurements using the Metros software. We recorded overall deformation, femoral valgus, tibial varus, HKA angle, HKS angle (mechanical/anatomic axis of the femur) and angle C (horizontal inclination of the ankle to the ground). All measures were compared between each other for each patient to search for relations.

In the genu varum group (136 knees), mean overall deformation was 9 degrees , mainly due to tibial misalignment (5.5 degrees ) with a 1 degrees femoral valgus component. The HKS angle was 6.3 degrees , and lateral joint gap 4.3 degrees ; the C angle was 4 degrees . Overall varus followed tibial varus (p<0.01). Ankle inclination followed tibial varus (p<0.01). In the genu valgum group (28 knees), overall deformation was 4.2 degrees , mainly due to femoral misalignment (5.6 degrees) with 1 degrees tibial varus and a medial joint gap of 2 degrees . The HKS angle was 4.7 degrees and the mean C angle -4 degrees . Overall valgus was related to femoral valgus (p<0.001). Tibial varus was greater with greater femoral valgus (p<0.01). In both groups, the HKS angle was smaller with greater femoral valgus.

Semi-automatic measurements on digitalized films enabled excellent reproducibility. Causes of error were related to limb rotation at acquisition. This study showed that genu varum worsens because of tibial wear followed by ligament distension, while for genu valgum, worsening predominantly results from femoral wear. The ankle joint line was nearly horizontal in one-third of the limbs, even when there was a major misalignment of the lower limb. The HKS angle was quite variable in both morphotypes, but was smaller with greater femoral valgus in both groups.

Measuring all components involved in medial and lateral knee misalignment is very useful for understanding the morphotype of each individual knee before performing osteotomy or implanting a knee prosthesis.