Impact of preoperative and constitutional alignment on soft tissue stiffness in robot-assisted total knee arthroplasty.

Abstract

Background: Although robot-assisted TKA improves alignment accuracy, the understanding of soft tissue stiffness remains limited. This study aimed to investigate the impact of preoperative and constitutional alignment on the stiffness of knee compartments during robot-assisted TKA.

Methods: We included 151 patients who underwent primary robot-assisted TKA between May 2021 and May 2022. A digital joint-tensioning device was used intraoperatively to apply stepwise increasing tension (30-90 N) to the medial and lateral knee compartments. The device measured corresponding gap changes at 0°, 10°, and 90° of flexion. Linear regression was used to analyze the relationship between gap changes and applied tension, and the regression slope (K value) was used to compare stiffness between compartments. Preoperative factors, including hip-knee-ankle angle and Coronal Plane Alignment of the Knee (CPAK) subtypes, were assessed for their influence on stiffness.

Results: There were significant differences in stiffness (K values) between medial and lateral compartments, particularly at higher flexion. The medial compartment generally showed greater stiffness. The medial-to-lateral stiffness ratio increased with greater varus alignment. Significant differences in K_M/K_L ratios were found among CPAK subtypes at 0° and 10° flexion.

Conclusion: This study introduced gap-tension regression for assessing soft tissue stiffness in robot-assisted TKA and showed that stiffness is influenced by preoperative and constitutional alignment. Varus alignment was associated with higher medial-to-lateral stiffness, and CPAK subtypes showed distinct stiffness patterns. These findings may help optimize soft tissue balancing and improve outcomes in TKA.