Deviation from Native Posterior Tibial Slope and Dose‒Response Relationships with Forgotten Joint Score-12 in Robot-Assisted Total Knee Arthroplasty: A Retrospective Study

Abstract Background Under the premise of high-precision osteotomy enabled by robot-assisted total knee arthroplasty (TKA), this study compared functional outcomes between a population-wide optimal posterior tibial slope (PTS) target and individualized restoration of the native anatomical PTS. It also quantified the dose‒response relationship between PTS deviation from the native angle and the Forgotten Joint Score-12 (FJS-12). Methods This retrospective analysis included 240 patients who underwent primary unilateral TKA in our hospital from August 2021 to June 2024. Patients were divided into conventional and robot-assisted groups (120 patients each) according to surgical method. All patients received the same brand of posterior-stabilized prosthesis. The PTS was measured on pre- and postoperative CT scans, and the osteotomy error (|actual PTS – target PTS|) was calculated. At the 1-year follow-up, the primary outcomes were the maximum active flexion angle and FJS-12 score; the secondary outcomes included the Knee Society Score, Oxford Knee Score, WOMAC score, anterior knee pain VAS score, tibial anterior translation distance, and complication rate. Multivariate linear and quadratic regression models were used to assess associations between postoperative PTS and clinical function, adjusting for age, sex, BMI, and preoperative flexion. Results Baseline characteristics were comparable between the groups (P > 0.05). The incidence of osteotomy error was significantly lower in the robot-assisted group (P < 0.05). At 1 year, the robot-assisted group achieved greater maximum flexion (121.5° ± 9.2° vs 118.3° ± 10.1°) and FJS-12 (68.7 ± 15.8 vs 64.2 ± 16.3) than did the conventional group (both P < 0.05). Multivariate regression revealed a positive linear correlation between postoperative PTS and maximum flexion (β = 1.9, 95% CI 1.2–2.6, P < 0.05) and a significant quadratic (inverted U-shaped) relationship between PTS and FJS-12, peaking at approximately 5.3°. Subgroup analysis indicated that patients with |actual PTS – native PTS| ≤ 2° had markedly higher FJS-12 scores than those with > 2° deviation (P < 0.05). Conclusions Under precise osteotomy conditions, the postoperative PTS is nonlinearly associated with subjective functional outcomes, with a population-level optimum of approximately 5.3°. For patients whose native PTS approximates this value, population-based and individualized targets converge; however, greater PTS deviation from native anatomy independently impairs FJS-12 scores, suggesting that individualized native PTS restoration is prioritized to improve patient satisfaction. These findings warrant validation in prospective randomized controlled trials.