I had an ultrasound picture of my rotator cuff that showed something weird. The one muscle that was starting to show signs of wear and tear wasn't torn. It was the other (healthier looking) muscle next to it that went. Is there any explanation for this?
The rotator cuff is a group of four muscles and their tendons that surround the shoulder and give it both stability (keeps the shoulder in the socket) and mobility (allows the shoulder to rotate and move in so many directions.
The two most commonly injured or torn tendons are the infraspinatus and supraspinatus. Both of these muscles and tendons come from the back of the shoulder to attach in different places.
A recent study done at the Washington University at St. Louis Department of Orthopedic Surgery showed that the majority of degenerative rotator cuff tears seem to start where the two tendons meet.This might help explain why some people seem to have an infraspinatus tear but their supraspinatus tendon is the one that looks worn out and vice versa.
One of the current theories to explain why the tendons tear in this pattern is called the rotator crescent concept. The rotator crescent is a crescent-shaped area from the biceps tendon to the bottom border of the infraspinatus tendon. Along the edge of this area is a thick arch-shaped bundle of fibers called the rotator cable.
The cable protects the crescent from stress by its shape and design, which is much like a suspension bridge. With aging, the crescent loses blood supply and starts to thin out. The shoulder mechanics start to rely more and more on the cable. The area that starts to tear is right in the middle of that aging, thinning crescent.
That's just a theory right now. With more research and study, the full details of rotator cuff pathology will eventually come to light. Understanding the how and why of these tears (where they start and how they progress over time) will be helpful for surgeons. The goal is to direct prevention and treatment, especially guiding surgical strategies.
H. Mike Kim, MD, et al. Location and Initiation of Degenerative Rotator Cuff Tears. An Analysis of Three Hundred and Sixty Shoulders. In The Journal of Bone & Joint Surgery. May 2010. Vol. 92-A. No. 5. Pp. 1088-1096.
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