A Direct Referencing Method of the Tibial Plateau for the Posterior Tibial Slope in Medial Unicompartmental Knee Arthroplasty

Abstract Purpose There is no consensus on intraoperative references for the posterior tibial slope (PTS) in medial unicompartmental knee arthroplasty (UKA). An arthroscopic hook probe placed on the medial second quarter of the medial tibial plateau (MTP) in an anteroposterior direction may be used as a direct anatomical reference for the PTS. The purpose of this study is to investigate availability and accuracy of this method. Methods Marginal osteophyte formation and subchondral depression of the MTP, and angles between the bony MTP and the cartilage MTP were retrospectively evaluated using preoperative sagittal MRI of 73 knees undergoing medial UKA. In another 36 knees, intraoperative lateral knee radiographs with the probe placed on the MTP were prospectively taken in addition to the preoperative MRI. Then, angles between the bony MTP and the probe axis and angles between the preoperative bony MTP and the postoperative implant MTP were measured. Results Among 73 knees, one knee with Grade 4 osteoarthritis had a posterior osteophyte higher than the most prominent point of the cartilage MTP. No subchondral depression affected the direct reference of the MTP. Mean angle between the bony MTP and the cartilage MTP was − 0.8°±0.7° (-2.6°-1.0°, n = 72), excluding the one knee with “high” osteophyte. Mean angle between the bony MTP and the probe axis on the intraoperative radiograph was − 0.6°±0.4° (-1.7-0.0, n = 36). Mean angle between the pre- and the postoperative MTP was − 0.5°±1.5° (-2.9°-1.8°). Root mean square (RMS) error of these two PTS angles was 1.6° with this method. Conclusion Cartilage remnants, osteophyte formation and subchondral bone depression does not affect the direct referencing method in almost all knees for which medial UKA is indicated. When the posterior “high” osteophyte of the MTP is noted on preoperative radiograph, preoperative MRI or CT scan is recommended to confirm no “high” osteophyte on the medial second quarter. Accuracy of this method seems equal to that of the robotic-assisted surgery (the RMS error in previous reports, 1.6°-1.9°).