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 < 0.0001). The approximation plane was aligned more parallel to the ground under WB conditions than under NWB conditions in healthy (p < 0.0001) and osteoarthritic knees (p < 0.0001). CONCLUSIONS The parallel phenomenon in the medial proximal tibia was confirmed for healthy and varus osteoarthritic knees. The medial proximal tibia plays an important role in the parallel phenomenon, assumingly associated with varus alignment and varus thrust. The inclination of the medial proximal tibia may become a new parameter for imaging investigations. LEVEL OF EVIDENCE III.

中文翻译：

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在健康和内翻型骨关节炎膝盖中，在负重条件下，内侧近端胫骨的关节表面在三维空间中平行于地面对齐。

目的测试以下假设：在负重（WB）条件（平行现象）下，在近端胫骨上的倾斜关节表面在三维（3D）空间中的对齐方式比在非WB（NWB）条件下更平行于地面健康和内翻性骨关节炎的膝关节疾病。方法我们检查了55个健康的膝盖（26名女性，29名男性；平均年龄70±6岁）和108例膝内翻性骨关节炎（66名女性，16名男性；平均年龄74±7岁）。为了在WB条件下进行评估，使用3D到2D图像配准技术在双平面长腿X射线照片和3D骨骼模型上使用了3D评估系统。另外，使用最小二乘法确定近似平面。计算内侧胫骨近端关节表面近似平面的法线向量与胫骨或世界坐标系的每个轴之间的角度。结果从形态上看，骨关节炎膝关节的倾斜面比健康膝关节的倾斜度更大（p <0.0001）。在健康条件下（p <0.0001）和骨关节炎膝盖（p <0.0001），WB条件下的近似平面比NWB条件下的平面更平行于地面。结论健康胫骨和膝内翻性骨关节炎的胫骨内侧近端均出现平行现象。胫骨内侧近端在平行现象中起重要作用，假设与内翻对准和内翻推力有关。内侧近端胫骨的倾斜度可能成为影像学检查的新参数。证据级别III。