Sunday, September 22, 2013

Tendon healing and remodeling - the effects of mechanical loading

The Role ofMechanical Loading in Tendon Development,Maintenance, Injury, and Repair

This is an important article because it points to the fact that the mechanical environment of the repaired tendon has a major effect on the quality of the healing.

The authors point to the complex, zonal interface of tendon to bone is characterized by the "integration of a tendon’s collagen fibers transitioning through a fibrocartilaginous region into the mineralized bone. The differences in material properties of the soft and hard tissue lead to high stress concentrations at this site, contributing to injury. In an effort to improve tendon-to-bone healing, the application of static or cyclic loading at the insertion site may be necessary to restore the zonal phenotype."

So it is worthwhile asking "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.

The authors of this review conclude that "Tendons are dynamic tissues composed of a cell population capable of responding to mechanical cues by altering the extracellular matrix. While it is known that loading and tension play a large role in overall tendon function, it is still necessary to determine the most suitable methods of incorporating these findings toward improving tendon repair. How does a tendon naturally heal and when is the most effective phase to implement repair procedures and/or intervention? When is the optimal time for incorporating loading regimens in patient rehabilitation protocols? How much and how often should loading regimens be implemented in a clinical setting? Should it be based on type of injury and/or location? How can loading parameters be incorporated when creating tissue-engineered constructs that are best primed for in vivo tendon repair?"

When we see all these questions without answers we can understand the current uncertainty about the management of the shoulder after a rotator cuff repair.

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