Sensitivity to measurement errors in the CPAK classification may affect the accuracy of coronal knee alignment classification

PURPOSE: The coronal plane alignment of the knee (CPAK) classification is based on fixed cut-off values derived from the mechanical lateral distal femoral angle (LDFA) and the medial proximal tibial angle (MPTA). The present study evaluated the sensitivity of the CPAK classification to clinically plausible measurement error and examined its impact on population-level category distributions. METHODS: A deterministic sensitivity analysis was conducted using four data sets: osteoarthritic knees (population A), the original CPAK healthy cohort (population B), and Turkish (population C) and Korean (population D) cohorts. CPAK distributions derived from original measurements were compared with those obtained after introducing fixed systematic measurement errors of 0.41° for LDFA and 0.71° for MPTA. Furthermore, a structured perturbation analysis was conducted by systematically varying the error magnitude from -4° to 4° to evaluate classification performance across a broader error spectrum. RESULTS: Clinically plausible measurement error resulted in substantial changes in CPAK distributions across all populations. The mean absolute percentage changes were 36.2% for population A, 43.95% for population B, 89.94% for population C and 45.18% for population D. The most pronounced effects were observed at classification thresholds, where minor angular deviations frequently led to category reassignment. CONCLUSIONS: The CPAK classification system demonstrates marked sensitivity to minor errors in LDFA and MPTA measurements. These findings suggest that differences in category distributions may partly reflect measurement-related variability rather than genuine anatomical variation. Consequently, the application of strict categorical cut-off values in alignment strategies for total knee arthroplasty could introduce clinically significant inaccuracies. LEVEL OF EVIDENCE: N/A.