Coronal plane alignment of the knee (CPAK) phenotypes and their relationship with gap patterns in robotic-assisted total knee arthroplasty: a clinical trial

Abstract Background Achieving optimal soft-tissue balance remains a key challenge in total knee arthroplasty (TKA), even with robotic assistance. The coronal plane alignment of the knee (CPAK) classification provides a structured description of constitutional alignment phenotypes, yet its relationship with intraoperative gap behavior has not been fully defined. Methods A prospective series of 285 patients undergoing robotic-assisted TKA with the CORI system was analyzed. Knees were categorized according to the CPAK classification and intraoperative medial and lateral gaps were automatically recorded in extension (0°) and flexion (90°). Gap configurations were defined as balanced, varus, or valgus. Associations between CPAK phenotypes and gap types were assessed using χ 2 and logistic regression analyses, with CPAK IV serving as the reference phenotype. Results The CPAK I and CPAK II phenotypes showed a significantly higher likelihood of varus gaps in both extension (odds ratio, OR 5.41 and 3.38, respectively) and flexion (OR 1.58 and 2.20, respectively), whereas CPAK VII was protective in extension (OR 0.23). Even valgus-aligned phenotypes demonstrated varus-dominant behavior in flexion. These findings indicate that gap behavior is strongly influenced by CPAK phenotype rather than solely by surgical alignment. Conclusion Intraoperative soft-tissue balance during TKA reflects the underlying CPAK phenotype. Integrating CPAK classification into robotic planning enables anticipation of gap tendencies and facilitates phenotype-driven balancing, bridging constitutional anatomy with functional alignment. Graphic abstract