BACKGROUND: This study aimed to compare extensibility changes in the rotator cuff musculotendinous unit after a surgically induced rotator cuff tear by correlating tendon retraction size, and fatty infiltration of the muscle and shear wave elastography (SWE) measures of rotator cuff muscle stiffness, using a rat model of chronic rotator cuff tear. METHODS: This study used Sprague-Dawley rats (n = 24). In the right shoulders, treated as chronic rotator cuff tear (cRCT) group, the supraspinatus and infraspinatus tendons were detached from the greater tuberosity. Then, an L-shaped resin was fixed firmly on the greater tuberosity with a 3-0 nylon suture to prevent cuff reattachment and scar tissue formation. In the left shoulders, treated as control group, rotator cuff tendon was not detached. The rats were euthanized at 1, 2, 4 and 8 weeks after the surgery. The resin was removed, and the length of the retraction of the supraspinatus tendon end from the greater tuberosity was measured. Thereafter, the whole rotator cuff muscle, scapula, and humerus were harvested from the shoulder of the cRCT and control groups, and were frozen immediately at -80°C. The specimens were thawed at room temperature. SWE measurement was performed initially, and the SSP muscle stiffness value was measured. Next, the medial edge of the scapula and supraspinatus tendon edge were fastened, and the tension based on the tensile test was applied in the mediolateral direction of the SSP muscle. The supraspinatus muscle specimens were processed from frozen sections and stained with Oil Red O. The ratio of fat area in the central third of the muscle was measured. The association between the supraspinatus muscle extensibility in the tensile test and other measurements was examined. RESULT: The rotator cuff maintained the tear without attachment to the humerus on all cRCT sides. The correlation values between extensibility and other measurements were as follows: SWE value - R = 0.573, P <
0.001
retraction length - R = 0.186, P = 0.384
and fatty infiltration ratio - R = 0.139, P = 0.518. Hence, the SWE value had the highest correlation. CONCLUSION: SWE can detect rotator cuff stiffness in the early stages of rotator cuff tear in this rat model. Further, SWE values have the strongest correlation with rotator cuff extensibility in the tensile test. Thus, SWE has the potential to be used in presurgical planning to predict difficulty due to decreased extensibility of the rotator cuff musculotendinous unit.