Background: Patient-specific knee alignment is regarded as a major target for improving total knee arthroplasty (TKA) outcomes. The Coronal Plane Alignment of the Knee (CPAK) classification has been proposed to capture the native patient-specific knee alignment, according to the Joint Line Obliquity (JLO) and arithmetic Hip Knee Ankle angle (aHKA), themselves based on the Lateral Distal Femoral Angle (LDFA) and Medial Proximal Tibial Angle (MPTA). This study aims to evaluate intra-operator, inter-operator, and test-retest reliability of the CPAK classification and associated angles in both knees with osteoarthritis (KOA) and TKA.

Methods: From our local arthroplasty registry, patients who sequentially underwent TKA on both knees within 18 months, with long-leg X-rays before and after each surgery between 2018 and 2023, were retrospectively selected. The contralateral knees before the 1st and 2nd TKA were used as test-retest for KOA and TKA knee, respectively. Four operators with increasing experience performed two measures of MPTA and LDFA for each image. The intra-operator, inter-operator, and test-retest reliability were assessed with Intraclass Correlation Coefficient (ICC(3,1)), Smallest Detectable Change (SDC), and Cohen's Kappa.

Results: The study included 34 patients. Angles showed good to excellent ICC apart from JLO in the KOA condition (moderate). Measures of LDFA on KOA and all TKA angles presented good to excellent SDC (< 3°), including test-retest conditions. MPTA, JLO, and aHKA on KOA showed moderate SDC (< 4.2°). CPAK classification was moderate to substantial for KOA (Kappa of 0.5 to 0.64) and substantial to almost perfect for TKA (Kappa of 0.69 to 0.81). Reliability increased with experience.

Conclusions: For experienced operators, CPAK classification and associated angles demonstrated levels of inter-rater reliability acceptable for clinical use in knees with TKA but at the limit of acceptability for knees with severe OA. In severe OA, one should interpret CPAK types cautiously, and angles may be preferable. MPTA in the knees with OA appeared as the main factor undermining reliability. Clarification on this angle may be needed to improve reliability, especially when using philosophies aiming at restoring the native alignment. Finally, test-retest reliability levels suggested that these measures are appropriate for longitudinal assessment.