Comparable clinical outcomes but different postoperative Dynamic Alignment of the Knee (DyAK) patterns between cruciate-retaining and posterior-stabilized inserts in functionally aligned robotic-assisted total knee arthroplasty

The influence of polyethylene insert design during robotic-assisted total knee arthroplasty (TKA) performed according to functional alignment (FA) principles remains poorly understood. This study compared cruciate-retaining (CR) and posterior-stabilized (PS) inserts with respect to implant positioning, clinical outcomes, implant survivorship and postoperative dynamic knee behaviour using the Dynamic Alignment of the Knee (DyAK) classification.

A retrospective comparative study was performed on 284 primary functionally aligned robotic-assisted TKAs. The cohort comprised 148 CR and 136 PS fixed-bearing cemented implants with a minimum follow-up of 3 years (median 44.5 months). Clinical outcomes included Knee Society Score (KSS) and Forgotten Joint Score (FJS). Intraoperative robotic implant positioning, bone resections, polyethylene thickness, DyAK classification, complications and Kaplan-Meier implant survivorship were compared between groups.

The CR group demonstrated greater preoperative varus alignment (median robotic hip-knee-ankle angle [rHKA] 5° vs. 3°, p = 0.01). Compared with PS implants, CR inserts were implanted with greater tibial varus (2° vs. 1°, p < 0.0001), greater posterior tibial slope (3° vs. 1°, p < 0.0001) and slightly greater femoral external rotation (p = 0.009). Polyethylene inserts >9 mm were required more frequently in the PS group (39.0% vs. 15.5%, p < 0.0001). At final follow-up, KSS knee (p = 0.56), KSS function (p = 0.62) and FJS (p = 0.06) were comparable between groups. No statistically significant difference in survivorship was detected for aseptic loosening (0.68% vs. 0.74%, p = 0.95) and all-cause revision (2.03% vs. 1.47%, p = 0.72). The DyAK classification changed significantly more frequently in the PS group than in the CR group (69.9% vs. 52.7%, p = 0.003), despite similar postoperative clinical outcomes.

CR and PS polyethylene inserts showed no significant differences in final clinical scores or implant survivorship at a minimum follow-up of 3 years but demonstrated different patterns of postoperative dynamic alignment, suggesting that the DyAK classification may provide complementary information beyond conventional outcome measures.

Level III.