Validation of Three-Dimensional Magnetic Resonance Imaging-Based Volumetric Quantification for Fatty Infiltration in a Rabbit Model of Chronic Rotator Cuff Tears

<b>Backgrounds/Objectives:</b> Fatty infiltration (FI) of rotator cuff (RC) muscles is a critical prognostic factor after surgical repair. While the Goutallier-Fuchs grading system is widely used, its reproducibility is often debated. This study aimed to validate a previously reported three-dimensional (3D) magnetic resonance imaging (MRI)-based volumetric quantification method by comparing it with histologic findings in a chronic rotator cuff tear (RCT) rabbit model. <b>Methods:</b> Eighteen shoulders from nine rabbits were randomly assigned to three groups (<i>n</i> = 6 each): repair (A), chronic tear (B), and control (C). In groups A and B, a chronic RCT model was established by detaching the supraspinatus tendon, with group A receiving repair after six weeks. At 12 weeks after repair, 7.0T MRI was performed, and volumetric quantification of intra-muscular fat was performed using semi-automated 3D Slicer software. Histologic fat proportion was measured via Oil Red O staining and ImageJ analysis. <b>Results:</b> The muscle weight and MRI-based muscle volume were significantly lower in group B than group C (<i>p</i> < 0.05). The radiologically measured fat proportion was significantly higher in groups A (1.8 ± 0.8) and B (2.8 ± 0.7) compared to group C (0.5 ± 0.4, <i>p</i> < 0.001). Histologic analysis showed a corresponding pattern (3.0 ± 1.2%, 5.2 ± 1.0%, 1.7 ± 1.0% for groups A, B, and C, respectively; <i>p</i> < 0.001). A strong positive correlation was identified between the radiologic and histologic measurements of FI (r = 0.784, <i>p</i> < 0.001). <b>Conclusions:</b> Direct histologic comparison validates the reliability of 3D MRI-based volumetric quantification for evaluating FI of the RC muscle in a chronic RCT rabbit model. This objective approach may address the inherent limitations of the conventional qualitative grading system.