The Mark Coventry Award: Does Matching the Native Coronal Plane Alignment of the Knee (CPAK) Improve Outcomes in Primary Total Knee Arthroplasty?

Abstract

Background: The Coronal Plane Alignment of the Knee (CPAK) classification system was developed to assess personalized alignment strategies, such as kinematic alignment, in total knee arthroplasty (TKA). However, CPAK has not been studied with regard to patient-reported outcomes measures (PROMs). This study evaluated whether incidentally matching a patient's native preoperative CPAK classification with TKA implant position meaningfully impacted postoperative PROMs.

Methods: A retrospective review of 2,427 primary TKAs was performed. Knees were classified using the CPAK classification on standardized preoperative and postoperative short-leg radiographs using a 5-degree adjustment for the lateral distal femoral angle based on available literature. Surgeries were performed using adjusted mechanical alignment strategies with a three-degree boundary. Patients who had a matching preoperative and postoperative CPAK were compared to all other combinations. Modern PROMs and clinically important differences were analyzed utilizing univariate and multivariate analyses.

Results: There were 94.5% of native knees classified as CPAK-I to III, like published CPAK distributions. The distribution of postoperative CPAK classification was significantly different, with 25.6% of TKAs classified as CPAK-I to III (P < 0.001) and only 11.5% (N = 266) of postoperative CPAK matched the native preoperative classification. The CPAK was not associated with preoperative (P ≥ 0.208) or postoperative PROMs (P ≥ 0.085), except CPAK-I had significantly higher preoperative pain with level walking compared to CPAK-III only (P = 0.027). Patients who had matching preoperative and postoperative CPAK classes demonstrated no difference in PROMs at a median of 24 months of follow-up (P ≥ 0.143). Statistical power was ≥ 93.9%.

Conclusion: Study results demonstrate that matching a patient's native knee coronal alignment classified by CPAK was not predictive of PROMs. This supports prior research that suggests TKA outcomes are multifactorial and related to complex interactions between implant position in three dimensions as well as soft-tissue balance and kinematics.