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Objectives and questions: The primary aim was to assess the association between medial posterior tibial slope (MPTS) and medial knee osteoarthritis (OA), hypothesizing higher MPTS values in OA knees. The secondary aim was to evaluate the prevalence of increased MPTS (≥12°), hypothesizing a higher prevalence in the OA group compared with the non-OA group.

Material and methods: Patients were retrospectively identified from an institutional database. The OA group comprised patients with isolated medial knee OA (Kellgren–Lawrence [K/L] grade ≥3), while a non-OA group (K/L grade <1) served as controls. Patients with prior knee surgery, fracture, tumor, or radiographs unsuitable for MPTS measurement were excluded. Groups were frequency-matched for age, sex, and body mass index (BMI). MPTS and medial proximal tibial angle (MPTA) were measured by two raters. Between-group comparisons, multivariable logistic regression, and receiver operating characteristic (ROC) analysis were performed (p < 0.05).

Results: A total of 562 patients (304 OA, 258 non-OA) were included. Groups did not differ in age (59.8 ± 7.7 vs. 59.6 ± 5.1 years), sex (50.0% vs. 56.6% female), BMI (28.5 ± 3.7 vs. 27.9 ± 3.5 kg/m²), or MPTA (86.6° ± 3.6° vs. 87.2° ± 3.4°) (all p > 0.050). Mean MPTS was higher in the OA group (7.8° ± 3.5° vs. 6.6° ± 2.4°; p < 0.001), and MPTS ≥12° was more frequent in OA knees (13.2% vs. 1.9%; p < 0.001). Each 1° increase in MPTS was associated with higher odds of medial OA (OR 1.14, CI 1.07–1.20), while MPTS ≥12° increased the odds of medial OA 7.9-fold (OR 7.9, CI 3.07–20.49). ROC analysis showed modest discriminative ability for medial OA (AUC = 0.59; optimal cutoff, 8.6°).

Discussion and conclusions: Increased MPTS was significantly associated with medial knee OA. These findings highlight sagittal tibial geometry as a potentially relevant morphological factor and support further longitudinal investigation.