Deviation from native medial posterior tibial slope and dose-response relationships with forgotten joint score-12 in Robot-assisted versus conventional manual total Knee arthroplasty: A retrospective cohort study

Robot-assisted total knee arthroplasty (TKA) enables high-precision osteotomy, but the clinical choice between pursuing a population-wide optimal posterior tibial slope (PTS) and individualizing the restoration of the native anatomical PTS remains debated. This study quantified the dose-response relationship between PTS deviation from native anatomy and the Forgotten Joint Score-12 (FJS-12), comparing outcomes between robot-assisted and conventional manual TKA. A retrospective cohort study analyzed 240 patients who underwent primary unilateral posterior-stabilized TKA, divided equally into conventional manual ( n = 120) and robot-assisted ( n = 120) groups. Postoperative PTS and absolute osteotomy errors were measured via CT scans. At the 1-year follow-up, the primary outcomes assessed were maximum active flexion and FJS-12 scores. Multivariate linear and quadratic regressions were utilized to map the association between postoperative PTS, absolute native deviation, and functional outcomes. Subgroup analysis was conducted using a 2° PTS deviation (Δ) threshold. The robot-assisted group demonstrated a significantly lower absolute osteotomy error than the conventional group (1.2° ± 0.9° vs. 2.8° ± 1.6°, P < 0.001). Although the robotic cohort showed statistically greater maximum flexion and FJS-12 scores, objective functional metrics (KSS, OKS, WOMAC) did not significantly differ between groups. Regression analysis revealed a linear correlation between PTS and flexion ( β = 1.9) and a quadratic (inverted U-shaped) relationship between PTS and FJS-12, peaking at approximately 5.3°. Crucially, greater deviation from the native PTS independently impaired FJS-12 scores ( β = − 2.4). Subgroup analysis confirmed that patients with a precise reconstruction (Δ ≤ 2°) achieved distinctly higher FJS-12 scores compared to high-deviation groups (Δ > 2°), regardless of the surgical modality employed. The postoperative PTS exhibits a nonlinear relationship with subjective joint awareness, with a population-level optimum of approximately 5.3°. However, anatomical deviation from the native PTS is an independent predictor of worsening FJS-12 scores. While robotic assistance is highly effective at minimizing extreme biomechanical outliers, surgical planning should prioritize the precise restoration of individualized native anatomy over universally fixed targets to maximize patient satisfaction.