Axial Plane Alignment of the Knee (APAK): A Bony-Based Framework for Describing Flexion-Plane Knee Phenotypes and Its Association With Intraoperative Flexion-Gap Patterns.

BACKGROUND: A structured classification describing knee alignment in flexion would be valuable in total knee arthroplasty (TKA). This study introduces the axial plane alignment of the knee (APAK) classification, which integrates distal femoral rotation and tibial coronal alignment at 90° of flexion, and evaluates its association with intraoperative flexion-gap patterns.

METHODS: Radiological analysis of 500 healthy and 500 osteoarthritic knees defined APAK using the arithmetic hip-knee-ankle angle in flexion (aHKA-F, varus/neutral/valgus) and joint-line obliquity in flexion (JLO-F, apex distal/neutral/apex proximal). A cohort of 180 robot-assisted TKAs was used to examine the association between APAK phenotypes with intraoperative flexion-gap patterns and compare medial-lateral differences at 90° between mechanical alignment (MA) and functional alignment (FA) across APAK phenotypes.

RESULTS: Both healthy and osteoarthritic cohorts demonstrated a mild varus alignment at 90° of flexion, with comparable aHKA-F values (-1.7 ± 4.2° and -1.8 ± 3.9°, respectively). The APAK phenotype distributions were similar between healthy and osteoarthritic populations, with Types I and II most common (healthy: 35.6 and 41.4%; osteoarthritic: 45.0 and 37.0%), whereas Types VII to IX were rare. The APAK and the coronal plane alignment of the knee (CPAK) phenotypes were moderately correlated, but not predictive. There was a significant association between APAK phenotype and intraoperative flexion laxity patterns (Pearson Chi-square = 83.321, P < 0.001), with a moderate-to-strong effect size (Cramer's V = 0.481). The FA significantly reduced medial-lateral gap differences at 90° compared with MA across major APAK types (Type I: 3.12 ± 1.7 to 0.49 ± 1.1 mm; Type II: 1.31 ± 1.6 to -0.17 ± 1.0 mm). Postoperative phenotypes converged primarily to APAK Types IV (62.2%) and V (33.3%).

CONCLUSIONS: The APAK provides a structured framework for describing knee alignment at 90° of flexion. With consistent phenotype distributions across healthy and osteoarthritic populations and significant association with intraoperative flexion-gap patterns, APAK may serve as a complementary tool for flexion-plane phenotyping and research standardization in TKA.