Beyond the CPAK dogma: robotic FA preserves CPAK better than MA but with equivalent proms at 5-year follow-up

Background

Mechanical alignment (MA) has long been the gold standard in total knee arthroplasty (TKA), but patient dissatisfaction has driven interest in more personalized alignment strategies. Functional alignment (FA) aims to restore a patient’s native joint line and balance flexion-extension gaps while minimizing soft tissue releases. This study compares the effects of robotic-assisted MA and FA on coronal plane alignment and clinical outcomes, using the Coronal Plane Alignment of the Knee (CPAK) classification as a reference.

Methods

This retrospective cohort study included 300 patients who underwent robotic-assisted TKA (RA-TKA) using the ROSA system. Patients were divided into MA (n = 150) and FA (n = 150) groups. Preoperative and postoperative CPAK classifications, coronal alignment parameters, and clinical outcomes—including the Forgotten Joint Score (FJS) and Knee Society Score (KSS)—were analyzed at five years postoperatively. Statistical analyses assessed the impact of CPAK class changes on functional outcomes.

Results

CPAK classification changed in 74.1% of MA cases and 46.1% of FA cases (p < 0.05), suggesting that FA better preserved native coronal alignment. LDFA varied significantly between groups (p = 0.005), while MPTA remained similar (p = 0.90). CPAK changes did not independently affect PROMs. In the MA group, LDFA increased significantly from 87.4° ± 2.2 to 89.7° ± 1.8 (p < 0.001), whereas in the FA group it remained stable (87.6° ± 2.1 to 87.9° ± 2.0;p  = 0.12). The original CPAK classification was maintained in 81.1% of FA cases compared to 43.4% in the MA group (p < 0.001). No significant differences in FJS or KSS were observed between groups at five years, with mean scores of 82.1 (MA) vs. 83.5 (FA) for FJS and 89.6 (MA) vs. 90.2 (FA) for KSS (p > 0.05). Both alignment strategies achieved comparable long-term clinical outcomes.

Conclusion

FA in RA-TKA preserves coronal alignment better than MA while achieving similar clinical outcomes at mid-term follow-up. FA showed a greater ability to maintain the patient’s native CPAK phenotype, potentially supporting a more physiological joint line orientation and soft tissue balance. CPAK classification changes did not negatively affect patient satisfaction or function, suggesting that both alignment strategies can yield favorable results. Further research should explore the role of sagittal alignment and patellofemoral biomechanics in optimizing TKA outcomes.