A direct referencing method of the tibial plateau for the posterior tibial slope in medial unicompartmental knee arthroplasty

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 the 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. The mean angle between the bony MTP and the cartilage MTP was −0.8° ± 0.7° (−2.6°–1.0°, n = 72), excluding one knee with a “high” osteophyte. The 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). The mean angle between the pre- and postoperative MTP was −0.5° ± 1.5° (−2.9°–1.8°). The 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 do 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 radiography, preoperative MRI or CT scan is recommended to confirm no “high” osteophyte on the medial second quarter. The accuracy of this method seems equal to that of robotic-assisted surgery (the RMS error in previous reports, 1.6°–1.9°).