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Purpose: Understanding the native posterior tibial slope (PTS) is important especially in cruciate ligament retaining total knee arthroplasty (TKA) and uni-compartmental knee arthroplasty (UKA). This study investigated the efficacy of PTS with lateral knee radiograph (radiographic PTS) by comparing it with PTS of medial and lateral tibial condyles (MPTS and LPTS) by computed tomography (CT). Methods: In patients who underwent TKA and UKA on varus knee osteoarthritis, radiographic PTS, MPTS, and LPTS were measured. Spearman's correlation was used to compare the radiographic PTS and PTS on CT. More than 3° delta PTS (CT PTS - radiographic PTS) were defined as positive outliers. Results: Three-hundred and twenty-six osteoarthritic knees (278 TKAs and 48 UKAs) were evaluated. The mean radiographic PTS, MPTS, LPTS was 8.0 ​± ​2.9°, 10.2 ​± ​4.2°, and 8.3 ​± ​3.3°, respectively. Plain radiograph tended to underestimate steep MPTS, and its correlations to LPTS (ρ ​= ​0.65, p ​

Abstract Poor clinical outcomes are reported in excessive flexion of the femoral component in total knee arthroplasty (TKA), but their mechanisms have not yet been elucidated. This study aimed to investigate the biomechanical effect of flexion of the femoral component. Cruciate-substituting (CS) and posterior-stabilised (PS) TKA were reproduced in a computer simulation. The femoral component was then flexed from 0° to 10° with anterior reference, keeping the implant size and the extension gap. Knee kinematics, joint contact, and ligament forces were evaluated in deep-knee-bend activity. When the femoral component was flexed 10° in CS TKA, paradoxical anterior translation of the medial compartment was observed at mid-flexion. The PS implant was best stabilised with a 4° flexion model in mid-flexion range. The medial compartment contact force and the medial collateral ligament (MCL) force increased with the flexion of the implant. There were no remarkable changes in the patellofemoral contact force or quadriceps in either implant. In conclusions, excessive flexion of the femoral component yielded abnormal kinematics and contact/ligament forces. Avoiding excessive flexion and maintaining mild flexion of the femoral component would provide better kinematics and biomechanical effects in CS and PS TKA.