Coronal plane alignment of the knee classification system does not reliably predict tibiofemoral joint gaps in arthritic knees undergoing total knee arthroplasty

Purpose: Achieving optimal mediolateral balance in extension and flexion is critical for improving outcomes in total knee arthroplasty (TKA). Knee classification tools, like coronal plane alignment of the knee (CPAK), do not directly consider the soft tissue profile of the joint. This study evaluated the variation in tibiofemoral gap measurements across CPAK phenotypes and examined the relationships between preoperative tibiofemoral joint gaps and the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO), important for TKA planning and execution. 

Methods: A retrospective analysis of 433 knees from the Joint Dynamics Registry computed tomography (CT) database was performed. Patients received preoperative long-leg CT scans and extension and flexion distracted radiographs. The CT scans were segmented and landmarked to produce three-dimensional bone models and derive anatomical measurements, including aHKA and JLO. The models were registered to the two distracted radiographs, and an osteophyte correction algorithm was applied to calculate the medial and lateral joint gaps in extension and flexion. Composite gap measurements (mean and difference) were also determined. Pearson's correlation and multivariate regression assessed the relationships between joint gaps and aHKA and JLO. ANOVA compared joint gaps across CPAK groups. 

Results: Small but statistically significant differences in joint gap measurements were observed between CPAK groups I and III, and II and III. Weak univariate correlations were observed between aHKA and joint gaps (r ≤ |0.32 |), with fewer statistically significant relationships for JLO (r ≤ |0.15 |). Multivariate regression explained only 10.2% and 1.4% of aHKA and JLO variance, respectively. 

Conclusion: While useful for describing coronal alignment, CPAK displayed limited predictive capability for preoperative tibiofemoral joint gaps in TKA patients. Direct assessment of joint gaps remains crucial for surgical planning. Future research should focus on integrating joint gap measurements with bony morphology in preoperative planning workflows to improve TKA personalisation. 

Level of Evidence: Level II.