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 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°).

Keywords: A direct referencing method; Accuracy; Availability; Medial tibial plateau; Medial unicompartmental knee arthroplasty; Posterior tibial slope.

Fig. 1

Evaluation of the height of the anterior/posterior osteophyte and expansion of anterior/posterior subchondral bone depression (attrition) on the medial second quarter of the MTP using sagittal MRI. A Osteophyte height is classified into three categories (N: none, L: low, H: high) relative to a line t. B Expansion of the depression is classified into three categories (N: none, M: moderate, S: severe) based on location. ap: anterior most prominent aspect of the MTP, pp: posterior most prominent aspect of the MTP, t: a line tangent to the ap and pp

Fig. 2

Measurement of the angles on sagittal knee MRI. A The preoperative bony slope of the MTP was defined as a line tangent to the most prominent aspects of the anterior and posterior cortices of the MTP (line m), and the cartilage slope of the MTP was defined as a line tangent to the most prominent aspects of the anterior and posterior cartilage remnants (line l). B The frontal view of the knee MRI, in which the MTP is divided into four parts. C On the medial second quarter of the MTP, the angle between lines m and l is measured. ap: anterior most prominent aspect of the MTP pp: posterior most prominent aspect of the MTP

Fig. 3

Measurement of the angles on preoperative, intraoperative, and postoperative lateral knee radiography. Preoperative bony PTS (α) and postoperative implant PTS (β) on a lateral radiograph were measured according to a method previously reported (A, B). On the intraoperative lateral knee radiograph, an angle (γ) between the native bony MTP (m₃) and the cartilage MTP (l₃) was measured (C)

Fig. 4

Percentage of the knee classified according to the categories in the total (n = 73), Grade 2 (n = 13), Grade 3 (n = 46) and Grade 4 (n = 14). A height of marginal osteophytes, which is classified into three categories (N: none, L: low, H: high). Only one case in Grade 4 had a “high” posterior osteophyte among patients undergoing medial UKA in Study I. B Expansion of subchondral bone depression, which was classified into three categories (N: none, M: moderate, S: severe). There were no cases in which the anterior or posterior subchondral depression expanded over the most prominent aspect of the anterior or posterior cortices, respectively

Fig. 5

Box plots showing angles between the native bony MTP (line m in Fig. 2) and the cartilage MTP (line l in Fig. 2) in the total (n = 72), Grade 2 (n = 13), Grade 3 (n = 46) and Grade 4 (n = 13) of Study I. There was no significant difference between the mean angles of Grades 2, 3 and 4

Fig. 6

Findings in Study II (n = 36). A Box plots showing angles between the bony MTP and the cartilage MTP on preoperative sagittal MRI and intraoperative lateral knee radiographs. The absolute value of the mean angle on the intraoperative radiograph was significantly smaller than that on the MRI. B A scatter diagram of the angles on the preoperative sagittal MRI and the intraoperative lateral knee radiograph. There was a week but significant correlation between them (r = 0.38, p < 0.05). In many cases, the angle on the intraoperative radiograph was smaller than that on the preoperative MRI. C Box plots showing preoperative boney PTS and postoperative implant PTS. D A box plot showing the difference between pre- and postoperative PTS. E. A scatter diagram of the angles of pre- and postoperative PTS. A strong correlation was noted (r = 0.84, p < 0.001)

Fig. 7

Case presentation. A Preoperative knee radiographs with Grade 4 OA. B Preoperative MRI. A red line: a slice line on the medial second quarter of the MTP. *: anterior and posterior “low” osteophyte. ↑: anterior and posterior “mild” depression on the MTP. C A hook probe. D An intraoperative lateral knee radiograph with the probe on the medial second quarter. E A photograph showing how to set the extramedullary cutting guide in this method. g: A gauge inserted into the slot of the tibial cut block. p: the probe. F Postoperative lateral knee radiograph. The native bony PTS is recreated