Correlation Between Radiographic and MRI Posterior Tibial Slope Measurement on a Pediatric Population

<b>Background/Objectives:</b> Posterior tibial slope (PTS) is an established risk factor for anterior cruciate ligament (ACL) injury in adults. In pediatric population, this relation is less established, and the PTS measurement is not clearly defined. To determine the agreement between X-ray (XR) and magnetic resonance imaging (MRI) PTS measurements and to establish an MRI cutoff corresponding to the standard radiographic ≥12° definition of "high slope". <b>Methods:</b> In this retrospective study, 108 adolescent knees with ACL rupture underwent paired XR and MRI evaluation by two reviewers. Agreement was assessed with Pearson and Spearman correlation, intraclass correlation coefficient (ICC), Bland-Altman analysis, and Deming regression. Diagnostic performance of MRI thresholds was compared with XR ≥ 12° as reference. <b>Results:</b> Mean PTS was higher on XR (10.2 ± 3.1°) than on MRI (8.4 ± 2.8°), with a systematic bias of +1.8° revealed by Bland-Altman analysis. These two measurements showed strong positive correlation (r = 0.602, <i>p</i> < 0.001) and moderate concordance (ICC = 0.506, 95% CI, 0.186-0.696, <i>p</i> = 0.0015). Individual differences ranged up to ±5° between modalities. Using XR ≥ 12° as reference for "high slope," ROC analysis identified an optimal MRI cutoff of 8.8° with excellent diagnostic accuracy (AUC = 0.841, 95% CI, 0.760-0.922). <b>Conclusions:</b> Radiographic measurements systematically overestimate PTS relative to MRI. Numeric thresholds are not interchangeable between modalities. An MRI cutoff of approximately 9° corresponds to the radiographic ≥12° definition of high slope and may serve as a pragmatic reference for interpreting MRI-based measurements in pediatric patients, requiring further validation.