Increased external rotation of the osteoarthritic knee joint according to the genu varum deformity

Purpose To evaluate the rotational profile of the lower extremity using computed tomography (CT) in accordance with the degree of varus deformity in medial condyle-affected knee joint osteoarthritis (OA). Methods This retrospective study included 1036 patients (872 lower extremities) with end-stage knee OA. The coronal alignment of the lower extremity was measured using standing anteroposterior radiography. The CT parameters of femoral anteversion and tibial torsion were assessed in relation to the knee joint. The axes were the femoral neck axis; the distal femoral axis, which was composed of the anterior trochlear axis, the clinical transepicondylar axis, and the posterior condylar axis; the axis of the proximal tibial condyles; and the bimalleolar axis. Results There was a tendency for increased external rotation of the knee joint parameters in relation to the hip and ankle joints as varus deformity of the lower extremity increased. The relative external rotational deformity of the knee joint in relation to the hip joint had a positive value with a good correlation. The relative external rotational deformity of the knee joint in relation to the ankle joint also demonstrated a positive value with a good correlation. Conclusion The distal femur and proximal tibia (knee joint) tended to rotate externally in relation to the hip and ankle joint, respectively, as the degree of varus deformity increased. This study identified the relationship between lower extremity varus deformity and rotational deformity of knee joints with OA. Level of evidence III.