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Objectives and questions: Recommendations regarding the surgical modification of the medial posterior tibial slope (mPTS) in medial unicompartmental knee arthroplasty (UKA) vary. Most knee systems recommend a specific target mPTS. Given the high preoperative variability among patients undergoing UKA, this often results in a significant change of the patient’s mPTS. However, it remains unclear whether this change in mPTS impacts the coronal alignment, specifically the medial proximal tibial angle (MPTA).

Material and methods: Pre- and postoperative radiographs of 96 patients undergoing conventional medial UKA using a knee system for which a postoperative mPTS of 7° is recommended, were analyzed. Pre- and postoperative mPTS and MPTA were measured on anterior-posterior and lateral radiographs by two observers and reported. Their difference as well as the difference between the mean postoperative mPTS and the target value of 7°, were tested for statistical significance using two-sided and one-sided Student’s t-tests. For further analyses cases were grouped regarding their pre- to postoperative mPTS change into cases with < 3° and cases with ≥ 3° mPTS change. The Pearson correlation coefficient was used to assess the correlation between changes in mPTS and changes in MPTA.

Results: The mean mPTS 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 mPTS within ± 2° of 7° without statistically significant difference between the mean postoperative mPTS and the target value of 7° (p = 0.797). 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). However, Pearson correlation coefficient revealed only a very weak correlation between the change in mPTS and the change in MPTA in all cases (r = -0.05), those with < 3° mPTS change (r = 0.13), and those with ≥ 3° mPTS change (r = -0.12).

Discussion and conclusions: Targeted mPTS modification can be achieved and implies a significant change from preoperative mPTS values in patients undergoing UKA. However, the change in mPTS does not affect the change in MPTA.