Surgically Relevant Knee Phenotypes: The Modified Coronal Plane Alignment of the Knee System-A Deep Learning-Based Classification.

Abstract

The coronal plane alignment of the knee (CPAK) classification proposes nine knee phenotypes based on constitutional limb alignment and joint line obliquity. However, there is considerable confusion and variability in defining types, and its impact on surgical decision-making is unclear. A simple modified CPAK system is proposed to improve accuracy and facilitate decision-making. Herein, we applied a deep learning model to automate knee phenotyping and analyzed modified CPAK distributions in a large cohort. We included 972 patients (1,944 healthy and arthritic knees) who had full-limb radiographs from a large publicly available dataset. A previously validated deep learning model was used to measure the lateral distal femoral angle and the medial proximal tibial angle. Knee phenotypes were determined using a two-level classification framework designed to capture both overall limb alignment, as measured by arithmetic hip-knee-ankle, and femoral and tibial joint-line contribution, as measured by lateral distal femoral angle and medial proximal tibial angle. There were seven phenotypic patterns identified, with five accounting for 99.2% of knees. The most frequent phenotypes were varus knees with valgus femur/varus tibia (26.9%), followed by neutral knees (25.6%), and valgus knees with valgus femur/varus tibia (24.2%). Less common were valgus knees with valgus femur/valgus tibia (12.2%) and varus femur/varus tibia in varus knees (10.3%). Varus phenotypes were predominantly tibial-driven (99.2%), while valgus phenotypes were femoral-driven (98.9%). Among neutral knees, 95% demonstrated a valgus femur with varus tibia configuration, while perfectly neutral combinations accounted for fewer than 2% of the total cohort. The Modifed CPAK offers a simplified and precise framework for knee phenotyping. Knee alignment demostrates a consistent pattern, with varus primarly driven by the tibia and valgus by the femur. This system provides a surgically relevant assessment of the knee, supporting alignment-based surgical planning.