Phenotypes of the Knee and Limb: Rationale for Transitioning Toward Personalized Alignment in Total Knee Arthroplasty

For a long time, the alignment goal for total knee arthroplasty (TKA) was a neutrally aligned limb with orthogonal joint lines to the mechanical axes (mechanical alignment). This had mainly to do with the belief of a better long-term durability because of more equal load distribution in mechanically aligned TKA. Recognizing the variability in individual knee alignment and compromised functional outcomes in TKA, there is an increasing interest among knee surgeons for more personalized, more anatomical alignment methods. Among those newer alignment concepts, the kinematic alignment method is the most promising one. The goal of the kinematic alignment concept is to restore the prearthritic alignment of a patient. To achieve this goal, surgeons need to have a profound knowledge of the individual native anatomy of the knee and its variability. This chapter provides a basis for all alignment methods and a detailed overview of the current knowledge regarding the variability of lower limb alignment. First, a review of the literature will be presented. Following the discussion of the literature, the functional knee phenotype concept is introduced. Based on these phenotypes, the difference between three alignment goals of the most common alignment concepts (mechanical, anatomical, and restricted kinematic) and the native alignment will be presented and thoroughly discussed.