Regional Variability of Normalized Signal Intensity in the Native Anterior Cruciate Ligament: A Quantitative MRI Study

Aims: This retrospective clinical MRI study aimed to evaluate regional differences in normalized signal intensity (SI) of the native anterior cruciate ligament (ACL) and to assess the influence of demographic and clinical factors on SI distribution. Methods: MRI scans obtained on a 3T scanner from 84 patients were analyzed. Regions of interest were defined in the proximal, middle, and distal portions of the native ACL, with normalization to the posterior cruciate ligament (PCL) tibial insertion. SI was expressed as the anterior–posterior native cruciate ligament ratio (APRn). A linear mixed-effects model with a random intercept for the patient was used to account for repeated measurements, and pairwise regional comparisons were performed with Tukey adjustment. Statistical significance was set at p < 0.05. Results: A significant regional effect was observed (p = 0.0001), with the distal portion demonstrating the highest SI, followed by the proximal and middle regions. Compared with the distal region, both the middle (β = −0.557, p = 0.0001) and proximal (β = −0.568, p = 0.0002) portions showed significantly lower SI values. The ACL signal was approximately 1.65–2.45 times higher than that of the PCL. Post hoc Tukey comparisons confirmed significant differences between all anatomical regions. The log-transformed model demonstrated improved fit (marginal R2 = 0.24; conditional R2 = 0.26). Inter- and intra-observer reproducibility demonstrated excellent agreement (ICC = 0.88–0.91). Conclusions: Native ACL signal intensity demonstrates a location-specific pattern, with the distal region showing the highest values. These findings provide reference data for regional native ACL signal distribution in a clinical cohort.