Articular surface of the medial proximal tibia is aligned parallel to the ground in three-dimensional space under weight

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Articular surface of the medial proximal tibia is aligned parallel to the ground in three‑dimensional space under weight‑bearing conditions in healthy and varus osteoarthritic knees Tomoharu Mochizuki1   · Yoshio Koga2 · Takahiro Mori3 · Katsutoshi Nishino4 · Koichi Kobayashi5 · Osamu Tanifuji1 · Takashi Sato6 · Ryota Katsumi1 · Hiroshi Koga1 · Go Omori7 · Yuji Tanabe3 Received: 15 November 2019 / Accepted: 11 December 2019 © European Society of Sports Traumatology, Knee Surgery, Arthroscopy (ESSKA) 2019

Abstract Purpose  To test the hypothesis that an inclined articular surface on the medial proximal tibia is aligned more parallel to the ground in three-dimensional (3D) space under weight-bearing (WB) conditions (parallel phenomenon) than under non-WB (NWB) conditions in healthy and varus osteoarthritic knees. Methods  We examined 55 healthy knees (26 women, 29 men; mean age, 70 ± 6 years) and 108 varus osteoarthritic knees (66 women, 16 men; mean age, 74 ± 7 years). For the evaluation under WB conditions, a 3D assessment system was used on biplanar long-leg radiographs and 3D bone models using a 3D-to-2D image registration technique. In addition, the least square method was used to determine the approximation plane. The angles between the normal vector for the approximation plane of an articular surface on the medial proximal tibia and each axis of the tibial or world coordinate system were calculated. Results  Morphologically, the inclination of the approximation plane was steeper in osteoarthritic knees than in healthy knees (p