Functional alignment in total knee arthroplasty is an umbrella term—A call for better definition and reporting quality!

Abstract

PURPOSE: Functional alignment (FA) aims to achieve a symmetric, rectangular extension gap and a rectangular or trapezoidal flexion gap, positioning the components in a manner that reduces the compromises to the soft tissue envelope. Because the FA surgeon can accomplish this goal with a multitude of adjustments to the components' position or soft tissue balance, the purpose of the present study was to analyze the possible paths for achieving FA. METHODS: Ten knees undergoing robotic total knee arthroplasty (TKA) were analyzed. Based on the intraoperatively acquired data, a macro was created to perform a post hoc analysis of the 10 possible paths to align a TKA functionally: (1) starting from mechanical alignment (MA) or kinematic alignment (KA), (2) preserving either the position of the femoral or tibial component and (3) with equal, 1 mm gaps or with increased 90° lateral flexion laxity. RESULTS: Ten different knee phenotypes were analyzed (Var-Neu-Neu 2x; Neu-Neu-Val; Val-Val-Val; Neu-Val-Neu; Var-Neu-Var 3x; Var-Var-Var; Var-Var-Val). On average, 3.5 different paths resulted in FA for each TKA (range: 0-8). Two TKAs (Val-Val-Val; Var-Var-Val) could not be functionally aligned using any of the ten evaluated paths. One TKA could be functionally aligned using eight different paths. CONCLUSION: Functionally aligning a TKA can be achieved through multiple adjustments, resulting in various implant positions and soft tissue balances. To better understand the different combinations behind FA, a more detailed nomenclature is needed, including which initial alignment (MA or KA) was utilized, which component's position (femoral or tibial) was preserved, and whether a rectangular or trapezoidal flexion space was targeted. LEVEL OF EVIDENCE: Level IV, therapeutic study.