Increased medial component gap at 30° flexion is associated with subjective instability and worse patient-reported outcomes after image-free robot-assisted total knee arthroplasty.

Purpose: Instability after total knee arthroplasty (TKA), particularly in the mid-flexion range, is associated with reduced patient satisfaction and poorer functional outcomes. Image-free robot-assisted TKA (rTKA) enables quantitative intraoperative assessment and adjustment of soft-tissue balance and component position; however, it remains unclear which intraoperative component gap (CG) at each flexion angle and in each compartment is most strongly associated with postoperative outcomes and which component-positioning factors determine that gap. Methods: This prospective cohort study included 72 consecutive knees that underwent primary image-free rTKA using the Journey II Bi-Cruciate Stabilised knee system between 2021 and 2024. Intraoperative CG was measured at 0°, 30°, 60° and 105° of flexion using an independently tensioned medial-lateral ligament tensioner and was defined as the measured gap minus the thickness of the final insert used. Clinical assessment was performed preoperatively and at 1 year postoperatively using self-reported knee instability (SRKI) and the knee injury and osteoarthritis outcome score (KOOS). Associations of each CG with SRKI and KOOS total at 1 year were examined using regression analyses adjusted for age and body mass index (BMI). In addition, the CG associated with worse postoperative clinical outcomes was used as the dependent variable to analyse its associations with component-positioning factors. Results: Both medial and lateral CGs were significantly smaller at 30° and 60° of flexion than at 0° and 105°. Among all measured gaps, the medial CG at 30° of flexion (Med CG30°) showed the most consistent association with postoperative outcomes. An increase in Med CG30° was significantly associated with SRKI positivity at 1 year and with a lower KOOS total score. In multivariable analysis, Med CG30° was independently associated with femoral coronal, sagittal and rotational alignment, whereas no significant associations were found with tibial component-positioning factors. Conclusion: In image-free rTKA using the Journey II system, an increased medial CG at 30° of flexion was associated with postoperative subjective instability and worse patient-reported clinical outcomes. In soft-tissue balance assessment during TKA, attention should be paid not only to the extension and flexion gaps but also to the Med CG30°. Level of Evidence: Level II.