The influence of native Coronal Plane Alignment Knee phenotype on two-year outcomes of functionally aligned robot-assisted total knee arthroplasty

Abstract

Aims: Functional alignment (FA) has been proposed as an alternative alignment technique for total knee arthroplasty (TKA). Currently there is a paucity of literature on the outcomes of FA. The primary aim of this study was to assess two-year outcomes of patients undergoing FA-TKAs. The secondary aims were to assess the effect of native Coronal Plane Alignment of the Knee (CPAK) phenotypes on clinical outcomes, to describe the component positioning associated with FA-TKAs, and assess whether this results in a change to the native CPAK phenotype. Methods: Patients undergoing robotically assisted FA-TKAs were assessed over a two-year period. Native and postoperative CPAK phenotypes were calculated using CT scans. Two-year patient-reported outcome measures (PROMs) and the final component positions were recorded and subanalyzed according to the native CPAK type. The number of patients who changed CPAK phenotype following surgery, and range of motion, were assessed. Results: Out of 1,012 arthroplasties in 866 patients, 904 TKAs (89.3%) had a minimum of two years of follow-up. There was a clinically significant improvement in all PROMs, but there were no significant differences in the change of PROMs between the different CPAK phenotypes. The mean Forgotten Joint Score at two years improved from 17.3 (SD 17.6) preoperatively to 72.0 (SD 26.9). Postoperatively the mean arithmetic hip-knee-ankle angle was 1.9° varus (6.9° varus to 4.6° valgus; SD 2.6°), the tibia was in a mean of 4.3° varus (6.5° varus to 3.0° valgus; SD 1.7°), and the femur was in 2.3° valgus (3.1° varus to 7.9° valgus; SD 2.0°). Overall, 64.9% of patients (618/952) maintained the same CPAK phenotype as their native phenotype. There were no notable differences in PROM scores between those patients who maintained and those who changed phenotype. Conclusion: FA-TKA results in a significant improvement in clinical outcomes irrespective of native knee phenotype. One-third of patients change CPAK type following surgery.