Understanding the phenotype of the arthritic knee using the Coronal Plane Alignment of the Knee classification in the...

Optimal coronal alignment is critical for successful total knee arthroplasty (TKA). The Coronal Plane Alignment of the Knee (CPAK) classification defines nine phenotypes based on the arithmetic hip-knee-ankle angle (aHKA) and joint-line obliquity (JLO). This study aimed to describe the distribution of CPAK in arthritic Indian knees and to assess bilateral CPAK symmetry.

Patients scheduled for TKA between 2018 and 2025 underwent standardised long limb radiographs. The medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) were measured, and the aHKA and JLO were calculated to assign CPAK types. A total of 4033 arthritic knees were analyzed. Bilateral concordance in 1979 patients was evaluated using cross-tabulation and Cohen's Kappa.

Of 2050 subjects (63% female), 4033 knees were included. CPAK1 was the most frequent phenotype (36.1%), followed by CPAK4 (26.4%) and CPAK2 (12.4%), together comprising 74.9% of knees. Varus phenotypes (CPAK1, 4, 7) accounted for 67.7% of the population, while CPAK7, 8, and 9 accounted for 7.5%. Only 46.5% of subjects demonstrated identical CPAK types in both knees, with CPAK1 being the most common (48.0%).

In Indian arthritic knees, among patients seeking TKA, CPAK1 and CPAK4 predominate, with a strong varus bias. Bilateral CPAK symmetry is present in fewer than half of patients, suggesting that relying on the contralateral limb as a constitutional template should be approached with caution.