Comparative Evaluation of Sagittal Alignment in Total Knee Arthroplasty: Robot Sensor Versus Surgeon’s Eye and Influencing Factors

<b>Background</b>: Postoperative sagittal imbalance, including hyperextension and flexion contracture after total knee arthroplasty (TKA), adversely affects long-term outcomes. Conventional techniques depend on surgeons' visual estimation, which may vary with patient anatomy. This study compared the accuracy of sagittal alignment assessment between a robot-assisted system and surgeons' visual evaluation and analyzed the influence of body mass index (BMI) and anatomic factors on concordance. <b>Methods</b>: Sixty unilateral TKAs performed between October 2023 and May 2024 using the MAKO system were retrospectively reviewed. Sagittal mechanical axis angles were obtained from the robotic system (M group) and visually estimated by two blinded orthopedic surgeons from intraoperative lateral photographs (S group) for 9, 10, and 11 mm trial inserts. Final alignment was verified by C-arm radiographs. Inter-observer reliability was analyzed using Cohen's κ and ICC, and correlations with BMI, thigh circumference, and limb proportions were assessed. <b>Results</b>: In patients with overweight/obesity, the S group significantly underestimated sagittal alignment (<i>p</i> < 0.001), whereas the M group maintained consistent accuracy regardless of BMI. With thinner inserts (9 mm), hyperextension was often overestimated (mismatch 55%, <i>p</i> < 0.0001), and with thicker inserts (11 mm), flexion contracture was underestimated (mismatch 46.7%, <i>p</i> = 0.001). Inter-observer reliability was good (κ = 0.717, ICC = 0.816). <b>Conclusions</b>: Visual assessment may underestimate sagittal alignment, especially in patients with obesity or those with abundant soft tissue. The MAKO robotic system provided consistent, objective alignment evaluation regardless of body habitus. Robotic-assisted quantitative assessment enables more accurate and reproducible sagittal alignment, supporting safer and more reliable TKA outcomes.