Degenerated coracoacromial ligament in shoulders with rotator cuff tears shows higher elastic modulus: Measurement with scanning acoustic microscopy

Purpose: The purpose of this study was to determine the elasticity of the coracoacromial ligament in shoulders with and without rotator cuff tears. Methods: The coracoacromial ligaments from 20 cadaveric shoulders (average patient age 79.5 years; 8 men, 12 women) were divided into six portions - three portions (acromial, central, and coracoid) in two layers (superficial and deep). A total of 120 samples were studied. First, the samples were classified by the collagen fiber orientation into three degeneration patterns: wavy, straight, irregular. For each pattern, the tissue sound speed, which shows a positive correlation with elasticity, was measured with scanning acoustic microscopy. Next, the samples were divided into three groups: 60 samples from shoulders with rotator cuff tears (RCT group), 30 samples from shoulders with an intact rotator cuff and a subacromial spur (spur group), and 30 samples from shoulders with an intact rotator cuff without a subacromial spur (control group). All shoulders with rotator cuff tears had subacromial spurs. The tissue sound speed and the histological findings were compared among the groups. Results: The sound speeds in the wavy, straight, and irregular patterns were 1592 ± 17.2 m/s (mean ± SD), 1626 ± 28.0 m/s, and 1607 ± 29.8 m/s, respectively (P < 0.0001). The sound speed in the straight pattern was higher than that in the wavy pattern (P < 0.0001), and that in the irregular pattern was lower than that in the straight pattern (P = 0.0023). The RCT group and the spur group had more straight patterns (P = 0.0002) and fewer wavy patterns (P < 0.0001) than did the control group. Significant differences in the sound speed were observed between the groups (P < 0.0001): 1596 ± 19.1 m/s in the control group, 1630 ± 31.5 m/s in the spur group, 1612 ± 28.6 m/s in the RCT group. Conclusions: The coracoacromial ligament in shoulders with rotator cuff tears shows higher elastic modulus than in age-matched normal shoulders due to degeneration of the ligament.