Frontal deformation of the knee is certainly not the only factor involved in the occurrence of lateralised tibio-femoral arthrosis. The aim of the study was to analyze if any kind of tibial torsion or femoral torsion could be able to induce lateralized arthrosis.
Femoral torsion, tibial torsion and tibio-femoral index (tibial torsion minus femoral torsion) have been measured on 59 knees with lateral arthrosis (8 knees) or with medial arthrosis (51 knees). For each knee, two frontal deformations were measured: 1) the actual arthrosis deformation was calculated on a hip knee ankle radiograph, 2) the pre arthrosis deformation is the arthrosis deformation minus the angle made by the femoral condyle tangent and the tibial plateau tangent. A knee has no frontal deformation if the angle between the mechanical axis of the femur and the mechanical axis of the tibia is between 178 degrees and 182 degrees; there is a varus deformity if the angle is inferior to 178 degrees; there is a valgus deformation if the angle is superior to 182 degrees.
Out of the 8 knees with lateral arthrosis, 2 showed initially no frontal deformation and 6 had a valgus deformation; out of the 51 knees with medial arthrosis, 34 showed initially no frontal deformation, 6 had a valgus deformity and 11 a varus deformity. The tibio-femoral index in lateral FT arthrosis was statistically different from those in medial FT arthrosis (p 0.0001). When a lateral arthrosis appeared whatever the pre arthrosis deformation was the index was always negative (tibial torsion lower than femoral torsion); when a medial FT arthrosis appeared, whatever the pre arthrosis deformation was, the index (except for two cases) was always positive (tibial torsion higher than femoral torsion).
Femoral and tibial torsions play a part in lateralised arthrosis occurrence together with frontal mechanical factors. Perhaps troubles in torsion explain some spontaneous or post-therapeutic evolutions not explained by frontal mechanical factors.