Background: Long-standing tears of the rotator cuff can lead to substantial and perhaps irreversible changes in the affected rotator cuff muscles. We developed a chronic rotator cuff tear in a canine model to investigate and quantify the time-related changes in passive mechanics, volume, and fat of the infraspinatus muscle. We hypothesized that infraspinatus muscle stiffness would increase, volume would decrease, and fat content would increase at twelve weeks following tendon detachment.

Methods: The right infraspinatus tendon of eight adult mongrel dogs were surgically detached from the proximal part of the humerus. The uninvolved left shoulder served as a control. Muscle volume changes were quantified with use of magnetic resonance imaging. At twelve weeks, the passive mechanical properties of the chronically detached and control muscles were determined intraoperatively with use of a custom-designed device. Intramuscular fat was evaluated histologically at the time that the animals were killed.

Results: After twelve weeks of detachment, the stiffness was significantly increased in the detached infraspinatus muscles relative to that in the controls (p < 0.0001). Magnetic resonance image analysis demonstrated that the detached muscle volumes decreased by an average of 32% in the first six weeks and remained constant thereafter. Intramuscular fat increased significantly in the detached muscles and to a greater extent in the lateral regions (p < 0.05).

Conclusions: The chronically detached muscle is not merely a smaller version of the original muscle but, rather, a different muscle. The detached muscle becomes stiffer, and the passive loads required to repair it can become excessive. A significant reduction in muscle volume occurs within days to weeks following tendon detachment (p < 0.0001). The nonuniformity of changes in muscle fat suggests that fat content should be used cautiously as an indicator of muscle quality.

Clinical Relevance: Clinically, chronic large rotator cuff tears are observed to have a qualitatively shorter and stiffer muscle-tendon unit than normal. We developed a chronic rotator cuff model to quantitatively investigate changes in the detached infraspinatus muscle. The passive mechanical properties of a chronically torn rotator cuff muscle-tendon unit may be a useful predictor of repairability and clinical outcome.