Speaking the same language? A direct cartography between functional knee phenotypes and CPAK.

Area of Science:

• Orthopedic surgery
• Biomechanical analysis
• Knee alignment research

Background:

• Understanding knee alignment is crucial for total knee arthroplasty (TKA) outcomes.
• Existing classifications like the Coronal Plane Alignment of the Knee (CPAK) aim to standardize knee assessment.
• Functional knee phenotypes (FKP) offer a different perspective on native knee alignment variability.

Purpose of the Study:

• To analytically map functional knee phenotypes (FKP) to the Coronal Plane Alignment of the Knee (CPAK) classification.
• To evaluate CPAK's capacity to represent native coronal alignment in both non-osteoarthritic (NO) and osteoarthritic (OA) knees.
• To propose a simplified translational framework between FKP and CPAK systems.

Main Methods:

• Derived arithmetic hip-knee-ankle angle and joint line obliquity from mechanical angles for FKP to CPAK conversion.
• Constructed a theoretical cartography using mean values and angular intervals.
• Applied deterministic, interval propagation, Monte Carlo simulation, and least probable CPAK assignment strategies.
• Validated conversions using NO and OA knee cohorts and compared with CPAK cohort data.

Main Results:

• Each FKP corresponded to multiple CPAK types, indicating deterministic conversion underestimates variability.
• Stochastic conversion in OA knees showed no significant difference from the original distribution, suggesting probabilistic convergence.
• The HCT-9 framework identified zones of low and high ambiguity in alignment.
• Extreme FKP types were overrepresented in the rare scenario analysis.

Conclusions:

• The relationship between FKP and CPAK is inherently probabilistic.
• The HCT-9 framework provides a structured method for translating between FKP and CPAK.
• This framework supports a unified alignment language in total knee arthroplasty.