Robotic functional alignment in knee arthroplasty minimizes impact on ankle alignment: Role of MPTA and LDFA preservation.

Abstract

Purpose: Alignment strategies in knee arthroplasty have a profound influence not only on knee biomechanics but also on the biomechanics of adjacent joints, particularly the ankle. Functional alignment (FA) represents a flexible alignment strategy aimed at achieving patient-specific balance. However, predefined boundaries are often applied to ensure mechanical stability, leading to adjustments in the medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA) while still respecting the patient's native alignment as much as possible. FA is a patient-specific strategy that seeks to respect the patient's preoperative phenotype or constitutional alignment while achieving a balanced and stable knee. The hypothesis is that FA strategies can protect the ankle from excessive biomechanical stress.

Methods: A retrospective cohort analysis was conducted on 300 consecutive patients who underwent robotic-assisted knee arthroplasty. Preoperative and 6-month post-operative radiographic evaluations measured key ankle parameters, tibial plafond inclination (TPI), talar inclination (TI) and Talar Tilt (TT). Statistical analyses evaluated the influence of alignment strategies on these parameters, with particular focus on whether MPTA and LDFA crossed the 90° threshold, indicating a shift from varus to valgus or vice versa.

Results: FA demonstrated smaller changes in ankle parameters compared to mechanical alignment (MA). In the FA group, mean changes were 1.8° for KTPA (standard deviation [SD] = 1.1°, p = 0.03), 2.4° for TPI (SD = 1.0°, p = 0.04), 2.1° for TI (SD = 1.3°, p = 0.05) and 1.7° for TT (SD = 0.9°, p = 0.04). The MA group showed greater deviations: 3.9° for KTPA (SD = 1.5°, p = 0.01), 5.2° for TPI (SD = 1.2°, p = 0.02), 4.8° for TI (SD = 1.4°, p = 0.03) and 3.6° for TT (SD = 1.1°, p = 0.04). Alterations in LDFA and MPTA exceeding 2° were significantly associated with worsening ankle alignment. Furthermore, FA, with its goal of maintaining Coronal Plane Alignment of the Knee (CPAK) classification, was associated with minimal modifications to ankle angles, suggesting potential biomechanical benefits as reported in the literature.

Conclusions: FA was associated with smaller changes in ankle alignment parameters, indicating its ability to better preserve native joint positioning. Future research should focus on longitudinal studies to confirm these benefits and further establish the FA strategy as a standard in knee arthroplasty, particularly its capacity to maintain CPAK classification alignment.

Level of evidence: Level III.