Summary

The significant change in posterior tibial slope experienced by patients undergoing medial unicompartmental knee arthroplasty does not affect the medial proximal tibial angle.

Abstract

Background

Recommendations regarding the surgical modification of the posterior tibial slope (PTS) in medial unicompartmental knee arthroplasty (UKA) vary among knee system manufacturers. However, most systems recommend a specific target PTS. Given its high variability among patients undergoing UKA, this often results in a significant change of patients’ PTS. However, it remains unclear whether this change in PTS further impacts the coronal alignment of patients' knees, specifically the medial proximal tibial angle (MPTA).

Purpose

The purpose of this study was to report on the change in the MPTA and its potential correlation with the change of PTS in patients undergoing conventional medial UKA.

Methods

Pre- and postoperative radiographs of 96 patients undergoing conventional medial UKA using the Oxford Partial Knee System (Zimmer Biomet, Warsaw, USA), for which a postoperative PTS of 7 degrees is recommended, were analyzed. Pre- and postoperative MPTA and PTS were measured on anterior-posterior and lateral radiographs by two observers, according to the methods described by Petersen & Engh (MPTA) and Dejour & Bonnin (PTS), and reported. The differences in pre- to postoperative MPTA and PTS, as well as the difference between the mean postoperative PTS and the target value of 7 degrees, were tested for statistical significance using two-sided and one-sided Student's t-tests. The Pearson correlation coefficient was used to assess the correlation between changes in MPTA and changes in PTS.

Results

With a mean preoperative MPTA of 85.39° (SD 2.34°) and a mean postoperative MPTA of 84.12° (SD 2.55), UKA resulted in an average change in MPTA of -1.28° (SD 2.55; p < 0.001). The mean PTS was reported as 9.27° (SD 3.41°) preoperatively and 7.25° (SD 2.23°) postoperatively, with a mean change of -2.02° (SD 3.84°; p < 0.001). Overall, 71.7% of cases had a postoperative PTS within ± 2 degrees of 7 degrees, and there was no statistically significant difference between the mean postoperative PTS and the target value of 7 degrees (p = 0.797). The Pearson correlation coefficient revealed a no correlation (r = 0.05) between the change in MPTA and the change in PTS.