Anterior Infraspinatus Tendon Lesions and Selective Superior Muscle Belly Atrophy: A Case Series Introducing a Novel Ultrasound-Based Approach for Precise Rotator Cuff Tear Localization.

INTRODUCTION: Accurate ultrasound (US) localization of full-thickness rotator cuff tears remains challenging when lesions extend from the supraspinatus to the anterior infraspinatus tendon, due to anatomical overlap at the greater tuberosity. We hypothesized that selective hypotrophy of the superior infraspinatus muscle belly represents an early imaging marker of combined tendon involvement. METHODS: Five adult patients (mean age 53.6 ± 19.1 years) with recent traumatic shoulder pain and full-thickness tears (Snyder C 1-2; Patte I-II) underwent US evaluation, combining axial and longitudinal tendon scans with comparison of infraspinatus muscle trophism with the contralateral side. Hypotrophy was graded by contour, echogenicity, and cross-sectional area; strain elastography was used to calculate strain ratios. Paired comparisons used both a paired Student's t-test and an exact two-sided Wilcoxon signed-rank test. RESULTS: US showed involvement of both the supraspinatus and anterior infraspinatus tendons in all cases, indicating that standard imaging may underestimate tendon involvement and misclassify these as isolated supraspinatus tears. Selective superior infraspinatus hypotrophy was consistently observed, with mean scores of 1.6 ± 0.55 (profile), 1.0 ± 0.71 (echogenicity), and 1.8 ± 0.45 (area). Strain ratios were lower in the hypotrophic than in the contralateral muscle in all five patients (t-test p = 0.00055; Wilcoxon p = 0.0625). Interobserver agreement was excellent (κ = 1.00; intraclass correlation coefficient = 0.90). Magnetic resonance imaging, available in three cases, confirmed our findings. CONCLUSIONS: Superior infraspinatus hypotrophy is a reproducible US marker of anterior infraspinatus tendon involvement and may improve precise rotator cuff tear localization.