Posterior Tibial Slope Is Highly Variable and Asymmetric in Asian Osteoarthritic Knees: A Three-Dimensional CT Analysis

Purpose: Posterior tibial slope (PTS) is an important anatomical parameter that influences knee kinematics and clinical outcomes following total knee arthroplasty (TKA). Recent advances in CT-based robotic-assisted TKA (RA-TKA) have enabled accurate three-dimensional (3D) evaluation of knee morphology. However, limited data are available regarding PTS and its associated coronal and sagittal alignment factors in Asian patients with knee osteoarthritis. The purpose of this study was to quantify medial and lateral PTS and to identify alignment factors associated with PTS, thereby clarifying the relevance of individualized 3D assessment for TKA planning. Methods: This retrospective study evaluated 236 knees from 236 Asian patients with knee OA undergoing primary RA-TKA (MAKO®, Stryker, Kalamazoo, MI, USA). Radiological parameters, including medial posterior tibial slope (MPTS), lateral posterior tibial slope (LPTS), medial proximal tibial angle (MPTA), lateral distal femoral angle, mechanical hip–knee–ankle angle, arithmetic hip–knee–ankle angle, and femur flexion angle were automatically calculated by MAKO planning software based on predefined anatomical landmarks and reference axes. Correlation and multivariate regression analyses were performed to assess the relationships between PTS and other coronal, sagittal alignment parameters. Results: The cohort showed an overall varus tendency, with a mean mechanical HKA angle of −8.3° ± 5.21°, while coronal alignment itself was not used as an exclusion criterion. PTS demonstrated substantial variability among individuals as well as between the medial and lateral sides within the same knee. The distribution of PTS values showed a wide range, with MPTS ranging from −4.8° to 24.5° and LPTS ranging from −1.4° to 17.4°. The MPTS was significantly greater than the LPTS (7.85° ± 4.72° vs. 6.33° ± 4.04°, p < 0.001). No statistically significant association was observed between MPTS and MPTA. Multivariate linear regression demonstrated that LPTS was the strongest positive predictor of MPTS (β = 0.365, p < 0.001), while height (β = −0.169, p = 0.006) and femoral flexion angle (β = −0.195, p < 0.001) were significant negative predictors. Conclusions: In Asian patients with knee OA, PTS showed substantial inter-individual variability and significant medial–lateral asymmetry on 3D CT analysis. These findings suggest that a fixed PTS target may not fully reflect patient-specific native morphology, and further outcome-based studies are needed to define the optimal compartment-specific PTS reference for TKA planning.