Influence of total knee arthroplasty on coronal plane phenotypes and associated clinical outcomes: a CPAK-based comparison between mechanical and functional alignment.

Mechanical alignment (MA) in total knee arthroplasty (TKA) pursues a neutral mechanical axis but may alter constitutional coronal morphology and require soft-tissue release. Functional alignment (FA), by contrast, is a soft-tissue-respecting strategy that achieves balance through individualized adjustment of implant position and bone resection, with little or no ligament release. The coronal plane alignment of the knee (CPAK) classification, based on arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO), categorizes coronal knee phenotypes into nine types and may help evaluate how alignment philosophy influences phenotype and outcomes. This study compared MA and FA with respect to CPAK classification, phenotype transition, and patient-reported outcomes (PROMs) after TKA. We retrospectively reviewed 271 primary TKAs, including 138 MA and 133 FA knees. Preoperative and postoperative CPAK classifications were determined on full-length standing radiographs. CPAK distribution, phenotype preservation, and transition patterns were compared between groups. PROMs were assessed using the Oxford Knee Score (OKS), Forgotten Joint Score-12 (FJS-12), and Visual Analog Scale (VAS). Exploratory subgroup analyses were performed separately according to postoperative CPAK phenotype change status and alignment philosophy. Type I was the most common preoperative CPAK phenotype in both groups (MA, 63.0%; FA, 45.9%). Postoperatively, type IV predominated in MA (37.7%), whereas type V predominated in FA (42.1%). CPAK phenotype remained unchanged in 18.8% of MA knees and 22.6% of FA knees. FA achieved a higher OKS than MA (43.0 vs. 41.0, p<0.001), while FJS-12 and VAS were comparable. In exploratory subgroup analyses, higher postoperative OKS associated with FA was observed among patients younger than 65 years and among knees with postoperative CPAK phenotype change. Compared with conventional manual MA-TKA, robot-assisted FA-TKA was associated with different postoperative CPAK transition patterns and higher postoperative OKS. These findings suggest that alignment philosophy influences postoperative coronal phenotype patterns, although the clinical implications of specific phenotype transitions require further investigation.