The Coronal Plane Alignment of the Knee (CPAK) classification system has emerged as a popular tool for phenotyping knee alignment in coronal plane studies — especially in total knee arthroplasty (TKA) planning. However, for surgeons undertaking deformity correction (osteotomies) or assessing extra-articular malalignment, the question arises whether CPAK phenotypes reliably reflect the presence or nature of segmental extra-articular deformities (bone origin rather than articular). This paper investigates that relationship.
Study design & methods at a glance
- A retrospective radiographic study of 1,240 non-arthritic knees.
- Knees were classified into CPAK matrix groups (varus/neutral/valgus × joint-line apex orientation) and also categorised into nine segmental coronal extra-articular deformity phenotype (CEDP) groups based on medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) values.
- The primary aim: assess whether CPAK phenotype correlates with segmental coronal extra-articular deformity patterns.
Key findings
- Mean HKA was ~178.6° ± 4.4°, MPTA 85.4° ± 2.4°, LDFA 86.9° ± 2.5° in the cohort.
- No discrete correlation was found between CPAK matrix groups and specific segmental deformity patterns (CEDP). In other words, within a given CPAK group, there was substantial variation in MPTA/LDFA patterns, meaning extra-articular deformities differed widely despite similar CPAK types.
- Conclusion: CPAK classification cannot reliably predict segmental coronal extra-articular knee deformities, so surgeons should not depend solely on CPAK for planning osteotomies or deformity corrections.
Clinical implications for deformity correction surgeons
- While CPAK remains useful for phenotyping in TKA or general alignment studies, it is insufficient when planning corrective osteotomies that require precise understanding of deformity origin (femoral vs tibial) and segmental magnitude.
- Pre-operative deformity analysis must include direct measurement of MPTA, LDFA, joint line convergence angle (JLCA), and full-length radiographs — CPAK alone is not enough.
- Surgeons performing joint-preserving osteotomies should be aware that two limbs with the same CPAK type may have entirely different bone deformity profiles, and thus require different surgical strategies.
- The paper strengthens the case for detailed deformity analysis (e.g., using methods outlined by David Paley) rather than relying only on alignment/phenotype classification systems.
Take-home messages
- Don’t let CPAK classification replace segmental deformity analysis when planning osteotomies or correction of extra-articular malalignment.
- Use CPAK as one tool among many — but always verify origin, magnitude, and nature of deformity (tibial vs femoral vs combined).
- Tailor the osteotomy strategy (site, wedge, hinge, correction) based on precise bone measurements rather than phenotype alone.
Link to full paper: CPAK classification cannot be used to determine segmental coronal extra-articular knee deformity
Sources used in report overview:
- https://www.researchgate.net/publication/379990857_CPAK_classification_cannot_be_used_to_determine_segmental_coronal_extra-articular_knee_deformity?_tp=eyJjb250ZXh0Ijp7InBhZ2UiOiJzY2llbnRpZmljQ29udHJpYnV0aW9ucyIsInByZXZpb3VzUGFnZSI6bnVsbH19
- https://pubmed.ncbi.nlm.nih.gov/38643399/
- https://esskajournals.onlinelibrary.wiley.com/doi/full/10.1002/ksa.12168
