Multi-Planar Expansion of the Coronal Plane Alignment of the Knee Classification? A Computed Tomographic Study Indicates No Significant Correlation With Alignment Parameters in Other Planes.

Background: The Coronal Plane Alignment of the Knee (CPAK) classification categorizes knee phenotypes based on constitutional limb alignment (arithmetic hip-knee-ankle angle or aHKA) and joint line obliquity (JLO). This study aimed to determine if sagittal and rotational knee alignments vary among CPAK types in order to establish whether this classification should be expanded beyond coronal plane assessment.
Methods: Coronal, sagittal, and rotational alignment measurements were made and CPAK types were calculated from computed tomographic data of 437 patients (509 knees) who underwent robotic-assisted total knee arthroplasty (TKA). Differences in femoral, tibial, and tibio-femoral angular measurements were compared across CPAK types, and correlations were made to aHKA and JLO. Nonparametric and linear regression tests were used to analyze between-type differences.
Results: There were no differences in tibial slope or femoral rotational measures across CPAK phenotypes. However, CPAK Type III knees had a greater tibio-femoral rotation mean difference than CPAK Type I, II, IV, and V knees (P < .05). We also found increased femoral flexion in Type I knees when compared to Type VI knees (P = .01). The aHKA had a weak correlation with femoral flexion angle, and JLO had a weak correlation with femoral posterior condylar axis to tibial antero-posterior axis angle.
Conclusion: Few clinically important differences in sagittal and rotational alignments were found between CPAK types, indicating that CPAK phenotype has little correlation to 3-dimensional alignment characteristics. Need for an expansion of the CPAK classification beyond coronal plane alignment is not supported from these results.
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