Alignment Phenotype Restoration After Total Knee Replacement: A CPAK Classification Analysis

Background: Total knee replacement (TKR) is a well-established procedure for end-stage knee osteoarthritis, traditionally guided by mechanical alignment principles aiming for a neutral limb axis. However, growing evidence indicates significant variability in native knee alignment, and the Coronal Plane Alignment of the Knee (CPAK) classification has emerged as a useful framework to describe this diversity and support personalized alignment strategies.

Objectives: To evaluate restoration of native CPAK phenotype following TKR and to assess changes in joint line obliquity and coronal alignment parameters.

Methods: This prospective observational study included 50 patients with primary osteoarthritis undergoing TKR. Standardized long-leg standing radiographs were obtained pre- and post-operatively. Hip–knee–ankle (HKA) angle, mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle (MPTA), and joint line obliquity (JLO) were measured, and CPAK classification was applied to determine phenotype restoration and alignment changes.

Results: Varus phenotypes predominated pre-operatively (72%), with Type V being most frequent. Restoration of the original CPAK phenotype occurred in 40% of patients, while 30% shifted to adjacent phenotypes. Neutralization to Type V was observed in 20%, and valgus over-correction occurred in 10%. Significant improvements were observed in all radiographic parameters, including reduction of mean HKA from 7.59° ± 1.94° to 0.61° ± 1.07° and JLO from 5.84° ± 1.43° to 2.01° ± 1.08° (p < 0.001).

Conclusion: TKR effectively corrects coronal deformity but frequently alters native knee phenotype when performed using mechanical alignment principles. CPAK classification provides valuable insight into alignment changes and may facilitate personalized surgical planning. Future TKR strategies should emphasize restoration of constitutional alignment to optimize biomechanical and functional outcomes