Interobserver reliability of Coronal Plane Alignment of the Knee (CPAK) phenotype classifcation
File(s)
Published version
Author(s)
Wynell-Mayow, W
Hall, TAG
Singh, A
Arkel, RJ Van
Jones, GG
Type
Journal Article
Abstract
Aims
Coronal Plane Alignment of the Knee (CPAK) phenotyping is gaining momentum in research and clinical practice to understand individualized knee alignments and predict knee balance in total knee arthroplasty (TKA). The nine CPAK classes are based on joint line obliquity (JLO) and arithmetic hip-knee-ankle angle (aHKA), which are calculated using the medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA). This study aims to assess CPAK classification reproducibility, and analyze what level of angular error is associated with CPAK misclassification.
Methods
Two readers labelled 75 long-leg radiographs (LLRs) from the Osteoarthritis Initiative database for analyses of CPAK inter-reader reproducibility. A single reader then labelled and classified phenotypes for an aggregate total of 1,128 LLRs. Finally, Monte Carlo simulations were run based on 1,128-patient phenotype distribution and the inter-reader reproducibility statistics to understand how CPAK agreement rates were affected by the reproducibility of MPTA and LDFA measurements.
Results
There was excellent reproducibility in MPTA and LDFA measurements (mean absolute error: 0.41°/0.71°; and intraclass correlation coefficient: 0.96°/0.91°, respectively). These small angular deviations led to one-in-five disagreement in CPAK classification (20.0%; 95% CI 10.9% to 29.1%). An aHKA mean absolute error of < 0.1°, which is potentially unattainable, would be required to reduce inter-reader CPAK disagreement to below 95%.
Conclusion
CPAK phenotyping from long-leg radiographs may result in clinically significant rates of misclassification. CT imaging may improve reliability, particularly in cases where aHKA and JLO are near to discriminatory values.
Coronal Plane Alignment of the Knee (CPAK) phenotyping is gaining momentum in research and clinical practice to understand individualized knee alignments and predict knee balance in total knee arthroplasty (TKA). The nine CPAK classes are based on joint line obliquity (JLO) and arithmetic hip-knee-ankle angle (aHKA), which are calculated using the medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA). This study aims to assess CPAK classification reproducibility, and analyze what level of angular error is associated with CPAK misclassification.
Methods
Two readers labelled 75 long-leg radiographs (LLRs) from the Osteoarthritis Initiative database for analyses of CPAK inter-reader reproducibility. A single reader then labelled and classified phenotypes for an aggregate total of 1,128 LLRs. Finally, Monte Carlo simulations were run based on 1,128-patient phenotype distribution and the inter-reader reproducibility statistics to understand how CPAK agreement rates were affected by the reproducibility of MPTA and LDFA measurements.
Results
There was excellent reproducibility in MPTA and LDFA measurements (mean absolute error: 0.41°/0.71°; and intraclass correlation coefficient: 0.96°/0.91°, respectively). These small angular deviations led to one-in-five disagreement in CPAK classification (20.0%; 95% CI 10.9% to 29.1%). An aHKA mean absolute error of < 0.1°, which is potentially unattainable, would be required to reduce inter-reader CPAK disagreement to below 95%.
Conclusion
CPAK phenotyping from long-leg radiographs may result in clinically significant rates of misclassification. CT imaging may improve reliability, particularly in cases where aHKA and JLO are near to discriminatory values.
Date Issued
2025-11-01
Date Acceptance
2025-11-01
Citation
Bone & Joint Open, 2025, 6 (11), pp.1425-1435
ISSN
2633-1462
Publisher
The British Editorial Society of Bone & Joint Surgery
Start Page
1425
End Page
1435
Journal / Book Title
Bone & Joint Open
Volume
6
Issue
11
Copyright Statement
© 2025 Wynell-Mayow et al. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (CC BY-NC-ND 4.0) licence, which permits the copying and redistribution of the work only, and provided the original author and source are credited. See https://creativecommons.org/licenses/by-nc-nd/4.0/
Identifier
10.1302/2633-1462
Subjects
ARTHROPLASTY
CLASSIFICATION
IMPACT
JOINT LINE OBLIQUITY
Life Sciences & Biomedicine
LOWER-LIMB
Orthopedics
Science & Technology
Publication Status
Published
Date Publish Online
2025-11-11