How Functional Alignment TKA alters constitutional phenotypes: a CPAK-based analysis. The novel concept of “Mechanical-functional alignment”

Total knee arthroplasty (TKA) traditionally aims to restore the mechanical axis; however, up to 20% of patients remain dissatisfied postoperatively. Functional alignment (FA) has emerged as an alternative strategy that seeks to reproduce each patient’s native joint anatomy and soft-tissue balance. Also recently, the Coronal Plane Alignment of the Knee (CPAK) classification enables the evaluation of constitutional knee phenotypes and postoperative alignment outcomes. The study assesses the changes in CPAK classification following robot-assisted functional alignment TKA and analyzes variations in key radiographic parameters before and after surgery. This retrospective observational cohort study included 106 patients (106 knees) who underwent ROSA®-assisted TKA (Zimmer Biomet, USA) between January 2024 and May 2025. Pre- and postoperative full-length standing radiographs were analyzed to determine the mechanical hip–knee–ankle angle (mHKA), lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), arithmetic HKA (aHKA), and joint line obliquity (JLO). CPAK classification was assigned according to the method of MacDessi et al. Statistical analyses included the McNemar–Bowker test for class distribution changes and paired t-tests with Bonferroni correction for radiographic parameters. Following surgery, 90 of 106 knees (84.9%) changed CPAK class, with a significant redistribution toward classes IV–V (χ 2 = 60.07, df = 8, p = 4.5 × 10 -10 ). Preoperatively, class I predominated (42.5%), while postoperatively classes IV (38.7%) and V (33.0%) were most frequent. Significant postoperative modifications were observed in LDFA, MPTA, aHKA, and JLO (all p < 0.05 after correction). No demographic or preoperative alignment variable predicted CPAK stability. Robot-assisted functional alignment TKA, within current intraoperative boundaries, produces a consistent shift toward mechanically neutral CPAK classes, reflecting a “mechanical-functional” alignment rather than full restoration of native phenotypes. These findings underscore the influence of procedural constraints on achieving true functional alignment and highlight the need for further prospective research integrating radiographic, intraoperative, and functional outcomes.