“functional knee phenotype concept” and its impact on the controversy of proper frontal alignment in TKA

Background: While total knee arthroplasty (TKA) is the standard treatment for end-stage osteoarthritis, the optimal alignment strategy remains a subject of significant debate. Traditional mechanical alignment aims for a neutral axis, yet approximately 20% of patients report dissatisfaction, potentially due to alterations in native joint kinematics and the requirement for extensive soft tissue releases.

Objective: This article reviews contemporary TKA alignment concepts, including mechanical, anatomical, constitutional, and kinematic approaches, while introducing the functional knee phenotype concept (KPC) as a framework for individualized surgical planning.

Key Points: Mechanical alignment remains the clinical benchmark despite challenges to its necessity for implant longevity. Anatomical alignment attempts to recreate native joint line obliquity, while constitutional alignment allows for residual varus to avoid extensive releases. Kinematic alignment focuses on restoring pre-arthritic bony morphology and soft tissue tension. The KPC, derived from non-osteoarthritic population data, identifies significant variability in femoral mechanical angles (FMA) and tibial mechanical angles (TMA). Analysis reveals that a strictly neutral mechanical joint line (FMA 90°, TMA 90°) occurs in less than 4% of the population. The KPC categorizes patients into specific phenotypes based on limb, femoral, and tibial alignment, suggesting that a single alignment target may not accommodate the diverse anatomical variations observed clinically.

Conclusion: The functional knee phenotype concept provides a standardized, evidence-based classification for understanding native alignment variability. By clustering patients into specific phenotypic groups, surgeons may transition toward more personalized alignment strategies to potentially improve functional outcomes and restore natural knee kinematics.