Intraoperative reference points on the proximal tibia in image-free robotic-assisted total knee arthroplasty should be determined by preoperative posterior tibial slope.

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The purpose of this study was to investigate the accuracy of the tibial cut in sagittal plane and intraoperative optimal reference points on the proximal tibia for achieving the targeted posterior tibial slope (PTS) in image-free, robotic-assisted total knee arthroplasty (TKA). A mechanical tibial axis determined by intraoperative reference points would affect the measurement of the PTS and thereby postoperative PTS in image-free robotic assist TKA. Fifty-eight patients (70 knees) who underwent primary image-free robotic-assisted TKA were included. Pre- and postoperative PTS were evaluated using whole-leg computed tomography images, which were analysed with three-dimensional planning software. Change in PTS (ΔPTS) was calculated by subtracting the target PTS from postoperative PTS. The proximal tibial axis ratio was defined as the proportion of distance between the anterior border of the anterior cruciate ligament (ACL) footprint and the tibial axis on the proximal joint surface to the anteroposterior width of the ACL footprint. The mean ΔPTS was -0.4 ± 2.0 °. Eight outliers (11.4%; |ΔPTS| >3°) were identified. The proximal tibial axis ratio was -13.2 ± 19.9% and showed a significant negative correlation with preoperative PTS and ΔPTS (r=-0.87 and -0.29, p < 0.001 and p = 0.01, respectively). The tibial axis passed through the anterior border of the ACL footprint when preoperative PTS was 9.6 °. These results indicated that a larger preoperative PTS was associated with a more anterior tibial axis on the proximal joint surface. Preoperative PTS significantly correlated with ΔPTS (r = 0.34 and p = 0.004). In image-free robotic-assisted TKA, when the preoperative PTS is >9.6 °, positioning the proximal tibial reference point anterior to the anterior border of the ACL footprint is recommended. III.1.