Attaching the rotator cuff to bone, how does nature do it?

The attachment of a tendon to bone is called an enthesis. Here is a picture of the cuff insertion to bone in an animal a presented at the most recent Orthopaedic Research Society meeting.

We can see the zones that are so critical to managing the bending and twisting loads that are applied to the junction of flexible tendon to inflexible bone. At upper right we can see the wavy tendon fibers. At lower left we can see the solid bone. Between, stained in green, is fibrocartilage - more flexible than bone, less flexible than tendon. Nearer the bone, the fibrocartilage is calcified and nearer the tendon the fibrocartilage transitions to tendon fibers. 

This arrangement is similar to that of a modern electrical plug (see below), which has to manage the mechanical transition between the flexible wire and the rigid body of a laptop. As in the case of the normal cuff enthesis, this is accomplished by a transition zone from more flexible on the right to less flexible on the left.

When this progressive transition is lacking, the attachment is at risk for failure at the junction of the flexible to the stiff. This is, of course, where rotator cuff defects occur.

A couple of lessons may be derived from this observation:

(1) Maintaining shoulder flexibility through gentle stretching may help reduce the risk of rotator cuff failure.

(2) Surgical repairs of the rotator cuff do not, of themselves, restore this transition. 

Rather, the re-establishment of the transition zone is accomplished by progressive remodeling over time. Until the transition zone is re-established, we can suspect that the cuff repair is vulnerable to failure.

Some of the readers may remember the old-style electrical plug without the transition zone. It's easy to guess where the failure occurred.

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