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.