Linear mixed modeling on the effects of varus knee surgery on the ankle joint weight-bearing axis

•After varus knee surgery, the weight-bearing axis from the femoral head to the calcaneus moved laterally at the ankle joint.•The results suggest that the subtalar joint included within the weight-bearing axis is likely to change too.•The weight-bearing axis at the ankle joint moved laterally by 0.9% per degree of postoperative HKA angle decrease.