Manual Stress and Gap Spacer Systematically Underestimate Intraoperative Joint Gaps Compared with a Tensor Device in Robotic-Assisted Total Knee Arthroplasty

Background: Robotic-assisted total knee arthroplasty (TKA) enables precise bone resection based on intraoperative ligament balance. However, commonly used methods for assessing ligament balance—manual varus/valgus stress and gap spacers—may lack accuracy, potentially affecting surgical outcomes. This study compared intraoperative joint gap measurements among 3 techniques: manual stress, gap spacers, and a novel tensor device. Methods: This prospective cohort study included 59 varus knees that underwent robotic-assisted TKA with a primary prosthesis. Intraoperative joint gaps were digitally measured in both medial and lateral compartments at full extension and 90° flexion using 3 methods: manual stress, gap spacers, and a tensor device applying 120 N force. Measurements were repeated for reliability, and statistical comparisons were made using one-way analysis of variance. Intramethod reproducibility was assessed using intraclass correlation coefficients (ICCs). Results: Joint gaps measured using gap spacers were significantly smaller than those obtained with the tensor device (p < 0.05), particularly in flexion. Manual stress yielded intermediate values. The tensor device consistently produced the largest joint gaps. ICCs were >0.8 for all methods, indicating high reproducibility, with the tensor showing the highest consistency. However, manual and spacer methods systematically underestimated joint gaps despite their reproducibility. Conclusions: Manual stress and gap spacer techniques underestimate intraoperative joint gaps, potentially leading to unintended ligament laxity in robotic-assisted TKA. The tensor device provides more accurate and reproducible gap assessment, supporting its use for improved intraoperative decision making. Level of Evidence: Level II Prospective cohort study. See Instructions for Authors for a complete description of levels of evidence.