Background: Time dependent, quantitative studies of muscle atrophy or passive muscle mechanics following chronic muscle detachment have not been previously reported. We developed a chronic tear of the rotator cuff tendon in a canine model to investigate and quantify the time related changes in the passive mechanics, volume, and fat of the infraspinatus muscle. We hypothesize that infraspinatus muscle stiffness will increase, volume will decrease, and fat content will increase at 12-weeks following tendon detachment.
Methods: The right infraspinatus tendons of eight adult mongrel dogs were surgically detached from the proximal humerus. The non-operated left shoulder served as a control. Muscle volume changes were quantified using MRI scans. At 12 weeks the passive mechanical properties of the chronically detached and control muscles were determined intraoperatively using a custom device. Intramuscular fat was evaluated histologically at sacrifice.
Results: After 12 weeks of detachment, the stiffness and modulus were significantly increased in the detached infraspinatus muscles relative to controls. MRI analysis demonstrated that the detached muscle volumes decreased by 33 percent in the first 6 weeks and remained constant thereafter. Intramuscular fat increased significantly in the detached muscles, and to a greater extent in the lateral regions.
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. The non-uniformity of muscle fat changes suggests that fat content should be used cautiously as an indicator of muscle quality.
Clinical Relevance: Clinically, chronic, large rotator cuff tendon tears are observed to have a qualitatively shorter and stiffer muscle-tendon unit than normal. We have 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 reparability and clinical outcome.
