Association Between Achieved Postoperative Posterior Tibial Slope and Flexion–Extension Mechanics in Robotic-Assisted Total Knee Arthroplasty

Introduction: Posterior tibial slope (PTS) influences sagittal knee mechanics, affecting flexion–extension, ligament tension, and implant stability in total knee arthroplasty (TKA). Robotic-assisted (RA) TKA with functional alignment (FA) allows precise control of PTS, but the clinical relevance of small variations in achieved postoperative PTS remains unclear. This study evaluated whether achieved postoperative PTS is associated with differences in knee mechanics and patient-reported outcomes after RA-TKA with a FA workflow. Methods: In this single-center observational study, consecutive patients undergoing primary RA-TKA with FA were included. Patients were stratified according to achieved postoperative PTS into a near-3° group (2.5–3.5°) and a predominantly lower-PTS comparator group. Clinical outcomes included flexion, extension, and range of motion (ROM). Patient-reported outcomes were assessed using KOOS, KOOS-JR, Forgotten Joint Score (FJS), and PASS thresholds. Statistical comparisons were adjusted for false discovery rate, with multivariable regression analyses performed. Results: A total of 100 patients were analyzed. Mean postoperative PTS was 3.00° ± 0.25° in the near-3° group and 1.82° ± 1.10° in the predominantly lower-PTS comparator group. The predominantly lower-PTS comparator group showed greater ROM (129.5° vs. 122.2°), mainly due to increased hyperextension (−1.7° vs. 0.8°; adjusted p = 0.04). A moderately achieved postoperative PTS was associated with less terminal hyperextension. Patient-reported outcomes were largely comparable between groups. Multivariable analyses confirmed an association between increasing achieved postoperative PTS and reduced terminal hyperextension, whereas no significant associations with global patient-reported outcomes remained after FDR correction. Conclusions: In this implant-specific robotic functional-alignment cohort, a moderately achieved postoperative PTS was associated with less terminal hyperextension, without clinically meaningful differences in global patient-reported outcomes or PASS achievement.