Poster 108: The Deep Region of the Supraspinatus Is Architecturally Distinct

OBJECTIVES: The supraspinatus (SSP) tendon is the most frequently involved in rotator cuff tears, a common orthopedic condition. Previous anatomical studies have described distinct anterior and posterior regions of the SSP muscle. The purpose of this study is to determine if the deep region of the supraspinatus muscle has significantly different architectural properties from the other regions. This could have implications on the mechanical and functional nature of the deep supraspinatus muscle. METHODS: Architecture measurements were collected from 25 cadaver SSPs (Figure 1). There were 13 SSPs without a tear, 3 with partial tears, and 9 with full tears. The anterior to posterior dimension of the humeral head (APHH), musculotendinous junction length (MTJL), muscle mass and length were recorded. The muscle regions were demarcated as deep, [superficial] anterior, and [superficial] posterior. Raw and normalized fiber length, pennation angle, sarcomere length and number were analyzed for each region. Measurements were compared using repeated measures ANOVAs. The intact SSPs were analyzed first, and then a sub-analysis was performed based on presence of tear. RESULTS: In 13 SSPs without tears (Figure 2), the deep region showed a lower mean pennation angle of 3.3±1.0° than the posterior region (11.0±3.9°, p<0.0001), which had a higher angle than the anterior region (7.6±2.6°, p=0.0005). Normalized fiber lengths in the deep region were 21.1% (p=0.0052) and 34.5% (p<0.0001) shorter than posterior and anterior, respectively. Deep region sarcomeres measured 3.4±0.2 µm compared to 3.1±0.2 µm (posterior, p=0.0012) and 3.2±0.2 µm (anterior, p=0.0390). Sarcomere number also decreased in the deep region by 21.2% (p=0.0056) and 34.2% (p<0.0001) compared to posterior and anterior regions, respectively. After including partial (n=3) and full (n=9) tears, all architecture measurements remained different in the deep region versus anterior or posterior. Finally, to help visualize the deep region of the SSP, diffusion tensor imaging with tractography was performed on a healthy volunteer’s MRI (Figure 3) to depict the tissue microstructure and muscle fiber orientation/length. CONCLUSIONS: The deep supraspinatus has lower pennation angles, shorter fiber lengths, and fewer but longer sarcomeres. These structural differences suggest a functionally unique ‘submuscle’ within the supraspinatus.