Extra-articular location of the three-dimensional mechanical axis in advanced knee osteoarthritis: an upright computed tomography study

Purpose One of the most widely used benchmarks of lower-limb alignment is the mechanical axis (MA), which passes through the centers of the femoral head and the ankle in the weight-bearing position. However, where the MA passes through three-dimensionally (3D) is unclear. We investigated the MA in 3D (3D-MA) in knee osteoarthritis (OA) using upright computed tomography (CT). Materials and methods This study included 66 varus OA knees from 38 patients [age 70.0 (64.8–77.0) years; median (interquartile range)]. The 3D-MA was determined using upright CT data and compared among Kellgren–Lawrence (KL) grades. Further, correlations between the 3D-MA and other parameters were evaluated. Results The 3D-MA was located at 5.3 (1.3–14.4)% medially and 7.1 (0.7–15.3)% posteriorly on the tibial plateau in KL-1, and was translated medioposteriorly with increased KL grade. The 3D-MA in KL-3 [30.6 (22.6–42.6)% medially and 50.9 (45.8–80.2)% posteriorly] and KL-4 [56.7 (48.5–62.9)% medially and 92.3 (50.2–127.1)% posteriorly] was located extra-articularly. The mediolateral position of the 3D-MA correlated with the femorotibial angle [correlation coefficient (CC) = − 0.85, p < 0.001], and the anteroposterior position of the 3D-MA correlated with the knee flexion angle (CC = − 0.93, p < 0.001). Conclusion Our analysis demonstrated that the 3D-MA in low-grade OA knees passes slightly medial and posterior to the knee center, and the 3D-MA is translated medioposteriorly with the progression of knee OA. Further, the 3D-MA is translated medially with varus progression and posteriorly with the progression of knee flexion contracture.