Clinical Implications of Cpak Phenotype Transitions Following Robotic-Assisted Total Knee Arthroplasty in Varus-Predominant Asian Knees

Aims Robotic functional alignment (FA) in total knee arthroplasty (TKA) can modify the Coronal Plane Alignment of the Knee (CPAK) phenotype during gap balancing. In Western cohorts, phenotype is preserved in about two-thirds of knees and alteration is considered clinically inconsequential. However, in type I–predominant Asian populations, the frequency of FA-induced CPAK changes and their relationship with functional outcomes remain poorly characterized. This study aimed to quantify CPAK phenotype changes following robotic-assisted FA-TKA at a single institution and evaluate their association with one-year Knee Society Score (KSS) improvement. Methods From February 2019 to January 2022, 126 knees in 92 patients underwent robotic-assisted FA-TKA using a single implant design (cruciate-retaining, cementless). Standing full-length radiographs were obtained to calculate the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO), enabling preoperative and postoperative CPAK phenotype assignment. Preservation was defined as an unchanged CPAK type, whereas any shift constituted alteration. The primary outcome was KSS improvement from baseline to one year postoperatively. A retrospective linear regression model was used to assess the association between CPAK alteration (vs. preservation) and KSS improvement, adjusting for preoperative KSS, sex, and age. Results CPAK phenotype was altered in 116 of 126 knees (92.1%) and preserved in only ten (7.9%). Preoperatively, type I was predominant (65 knees; 51.6%); postoperatively, phenotypes V, IV, and VIII were most frequent. CPAK alteration was a significant negative predictor of KSS improvement (p = 0.033), whereas continuous changes in aHKA and JLO showed no significant association with KSS improvement. Conclusion In this Asian type I–predominant cohort undergoing robotic FA-TKA, CPAK phenotype alteration was not uncommon, and was associated with significantly less one-year KSS improvement compared to phenotype preservation. These findings suggest that FA-induced phenotype changes may carry clinical relevance in this population. Further research is warranted to establish phenotype-aware functional alignment targets and to explore the impact of intraoperative decision-making on long-term outcomes