Aims
The Coronal Plane Alignment of the Knee (CPAK) classification provides a standardized framework using arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO) to define coronal knee alignment. Most cohorts show disproportionate upper row clustering in only 2 types, and the system doesn’t clearly define how a phenotype should be recreated through the femur and tibia. This study evaluated a phenotype restoration and execution framework, CPAK-RX. The aims of the study were to determine; (1) would knees recenter around a central phenotype to improve the discriminatory utility of the classification, and (2) would the matrix provide clearer femoral and tibial segment specific phenotype targets for surgical execution.
Methods
Weight-bearing long leg radiographs of 455 arthritic knees were evaluated for LDFA and MPTA. The original CPAK (aHKA 0+/-20 and JLO 180+/-30) and CPAK-RX (aHKA 0+/-20 and actual JLO (aJLO) 0+/-30) distributions were compared descriptively. An execution analysis classified the femur and tibia separately using 900 +/-10 thresholds.
Results
The original CPAK classification showed marked apex distal clustering, with 78.2% of knees in types 1 to 3, and only 13.4% in type 5. CPAK-RX redistributed the cohort toward the apex-neutral row, reducing the apex distal row to 31.9%, and increasing type 5 to 40.0%. CPAK-RX provided a more even distribution across physiologic types 1 to 6. Mean matrix distance from type 5 decreased from 1.26 to 0.80 and mean absolute deviation from equal distribution decreased from 10.67% to 7.86%. CPAK-RX also produced clearer femoral and tibial execution patterns.
Conclusion
The CPAK-RX provides a clinically relevant classification system to differentiate the morphological variances in coronal knee alignment. Additionally, it provides surgeons with distinct femoral and tibial coronal alignment boundaries when aiming to recreate a knee phenotype.
