Defining Distinct Stress Curve Morphologies for Coronal Plane Alignment of the Knee (CPAK) Phenotypes using an Imageless Navigation Robotic Platform in Total Knee Arthroplasty.

The coronal plane alignment of the knee (CPAK) classification system divides coronal knee anatomy into nine phenotypes, suggesting different soft tissue balancing is needed for optimal outcomes. We investigated the interplay between CPAK phenotypes and gap stress curves throughout the knee's range of motion, aiming to understand their impact on total knee arthroplasty (TKA) balancing. There were 1,112 TKA cases from two imageless robotic assisted navigation systems using posterior stabilized implants that were classified into CPAK phenotypes. Medial and lateral initial gap values were measured throughout the knee flexion-extension arc, gap curve morphologies were generated, and medio-lateral (ML) gap balance was calculated for each phenotype. The most common phenotypes were included in this study, CPAK I to VI. Each phenotype exhibited a distinct gap curve morphology. Type I maintained the largest ML gap difference (-3.6 to -2.1), with the medial compartment tightest in extension. Type II showed relative laxity in the lateral compartment compared to the medial gap (-1.0 to -1.9), with the medial compartment tightening through flexion. Type III had a looser medial and tighter lateral compartment in extension that inverts to a tighter medial compartment in deep flexion (2.1 to -0.8), while Type IV showed a decreasing compartment difference with increased flexion (-3.7 to 0.6). Type V had fluctuating tightness (-0.6 to 1.8), and Type VI had the medial compartment tightening more with flexion (0.6 to 1.8). The distinct stress curves and ML gap behavior provide a "fingerprint" for each corresponding CPAK phenotype. Investigating these morphologies can help determine the best phenotype-specific treatments, including alignment strategy, implant selection, and gap balance, for optimal functional and patient outcomes.

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