CPAK classification cannot be used to determine segmental coronal extra-articular knee deformity.

Abstract

While the coronal plane alignment of the knee (CPAK) classification serves as a useful guide in personalising total knee arthroplasty (TKA), the extent of its correlation with segmental coronal extra-articular knee deformities remains uncertain. This study aims to investigate the potential correlation between CPAK matrix groups and segmental coronal extra-articular deformities in prearthritic knees, shedding light on the relationship between these two factors that seems to be both essential to perform personalised TKA. A radiological assessment of 1240 nonarthritic knees was performed by evaluating lower limb measurements following the protocol established by Paley et al. Subsequently, all knees were classified into their respective CPAK matrix groups. In our quest to discern any correlation between the CPAK matrix groups and the presence of segmental coronal extra-articular knee deformities, nine potential coronal extra-articular deformity phenotype(CEDP)groupswere identified based on medial proximal tibial angle (MPTA) and mechanical lateral distal femoral angle (mLDFA). Neutral values for MPTA and mLDFAwere set at 90.0° ± 3.0°and then at 87.0° ± 2.0°. Each CPAK matrix group underwent detailed coronal morphology analysisand then, segmental coronal extra-articular deformities were assessed by comparing them with the CEDP groups. The study revealed a mean hip-knee-ankle angle (HKA) of 178.6° ± 4.4°,mLDFA of 86.9° ± 2.5°, MPTAof 85.4° ± 2.4°, arithmetic HKAof -1.4° ± 3.2°and joint line obliquityof 172.5° ± 3.7°. The varus CPAK groups (I/IV/VII) included 435 patients, the neutral groups (II/V/VIII) comprised 630 patientsand the valgus groups (III/VI/IX) had 175 patients. Notably, CPAK matrix groups were not distinctly associated with specific coronal extra-articular deformity phenotype (CEDP) groups. Particularly among the most common CPAK matrix groups (I/II/III/IV/V), there was a significant variation in segmental coronal extra-articular deformity patterns. Moreover, when neutral MPTA/mLDFA values were set at 87.0° ± 2.0°, the CPAK matrix groups exhibited even greater variability in coronal extra-articular deformities. The CPAK matrix groups do not exhibit a direct correlation with a specific extra-articular deformity pattern (CEDP), thus rendering them unsuitable for determining segmental coronal extra-articular knee deformities. Level III, retrospective diagnostic study.