Monday, May 20, 2013

Effect of immobilization on the tendon - bone interface

The Effect of Immobilization on the Native and Repaired Tendon-to-Bone Interface

To set the stage for our review of this article, here's a picture of a device marketed as a 'shoulder immobilizer'.

This is a carefully done study the goal of which was to examine the effects of immobilization on a rat tendon-bone junction. The rat knee joint was immobilized at 90 degrees using an external skeletal fixator, the exact stiffness of which is not specified.   The tendon-bone insertion site was evaluated after immobilization with use of histologic, radiographic, and biomechanical analyses. Immobilization led to a significant decrease in the load to failure and stiffness compared with the native tendon at both two and four weeks. The authors also note that immobilized repaired tendons had better mechanical properties than immobilized intact tendons at one month (but were less strong than native, non-immobilized tendon). The protocol did not include non-immobilized repaired tendons.

Comment: The effects of rigid immobilization on the intact tendon-bone complex were significant. It is difficult to know the mechanism for this effect: is it the lack of motion or a change in loading or both? While the authors caution: "Surgeons who manage patients with immobilization should be aware of the changes at the bone-tendon complex," it is unusual for surgeons to immobilize joints rigidly with a spanning external fixator as was done here. We suggest that 'immoblization' is not a defined state, but rather a relative concept. A sling, as often used after a rotator cuff repair, or a brace, often used after a knee ligament repair, does not completely immobilize the joint nor does it protect the tendon-bone complex from loading. 

There can be no question that the mechanical environment of connective tissue can have profound effects on its material and structural properties as well as on the healing and remodeling of surgical repair. A better understanding of these effects will help inform the way we manage conditions affecting the attachment of tendon to bone.

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