Computed tomography versus long-leg radiography for CPAK-based coronal alignment assessment in total knee arthroplasty: a prospective evaluation

Abstract Background Accurate assessment of coronal alignment is essential for total knee arthroplasty (TKA) planning. The Coronal Plane Alignment of the Knee (CPAK) classification integrates mechanical alignment and joint line obliquity into nine phenotypes, but its reliability depends on measurement accuracy. This study aimed to compare the accuracy and reliability of CPAK classification and coronal alignment parameters obtained from computed tomography (CT) and long-leg standing radiographs. Methods A prospective comparative study was conducted on 100 patients undergoing primary TKA for degenerative arthritis. Each patient underwent standardized long-leg standing radiographs and full-limb CT scans using MAKO robotic planning software. Measurements included the hip–knee–ankle (HKA) angle, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and joint line obliquity (JLO). CPAK classification was determined from HKA and JLO values. Two independent observers recorded all parameters. Inter-modality differences were analyzed using paired t-tests, and reliability was assessed using intraclass correlation coefficients (ICC) and Cohen’s kappa. Results A total of 100 patients were analyzed, with complete datasets for 84–86 knees. CT consistently produced higher values for HKA, LDFA, and MPTA compared with radiographs (p < 0.001), while JLO and CPAK classifications showed no significant difference. Both modalities demonstrated excellent interobserver reliability (ICC > 0.88) and near-perfect CPAK agreement (κ = 0.86–0.88). These results indicate that CT offers greater precision for coronal alignment Level of Evidence Level II – Prospective comparative study.