Weight-bearing computed tomography findings in varus ankle osteoarthritis: abnormal internal rotation of the talus in the axial plane

To assess the incidence of abnormal internal rotation of the talus in the axial plane in patients with varus ankle osteoarthritis, and to determine whether this incidence differs from the severity of varus ankle osteoarthritis (moderate versus severe). We retrospectively evaluated weight-bearing computed tomography (CT) and plain radiographs of 52 ankles with no abnormalities (control group) and 96 ankles with varus osteoarthritis (varus-OA group), which were further stratified into a moderate-OA subgroup (50 ankles) and a severe-OA subgroup (46 ankles). A new radiographic parameter on weight-bearing CT, the talus rotation ratio, was used to assess the rotation of the talus in the axial plane. The normal range of the talus rotation ratio was defined as the 95% prediction interval for talus rotation ratio values in the control group. Abnormal internal rotation of the talus was defined for talus rotation ratio values above the normal range. We determined the incidence of abnormal internal rotation of the talus in the varus-OA group, moderate-OA subgroup, and severe-OA subgroup. In the varus-OA group, the incidence of abnormal internal rotation of the talus was 45% (43 ankles), which corresponded to an incidence of 32% (16 ankles) in the moderate-OA subgroup and 59% (27 ankles) in the severe-OA subgroup (p = 0.013). Our study demonstrates that abnormal internal rotation of the talus occurs in patients with varus ankle osteoarthritis, and is more frequently noted in severe than in moderate varus ankle osteoarthritis.