Subscapularis tendon tearing

Lengthening of the subscapularis tendon as a sign of partial tearing in continuity.

These authors retrospectively identified 92 magnetic resonance arthrography studies of suspected rotator cuff lesions obtained 3 months before shoulder arthroscopy. The myotendinous junction was identified, and the subscapularis tendon and muscle lengths were measured. Findings on arthroscopy performed later were used as the diagnostic gold standard for tendon integrity and compared with the magnetic resonance data.

Arthroscopy showed an intact subscapularis tendon in 43 patients, tendinopathy in 21 patients, and a partial rupture in 28 patients. The mean subscapularis tendon lengths were 40 mm in cases of intact subscapularis musculotendinous units, 45 mm in cases of tendinopathy, and 53 mm in cases of partial tears, whereas the mean subscapularis muscle lengths were 105 mm, 94 mm, and 95 mm, respectively, in these groups.

Partial tears of the subscapularis tendon lead to muscle shortening by approximately 10% and elongation of the tendon by approximately 32%, which may be interpreted as muscle retraction and a tendon rupture in continuity. If the subscapularis tendon has an apparent length of greater than 60 mm, the probability of a tear is 98%.

Comment: The topic of failure in continuity of cuff tears is an interesting one that has been discussed previously in this blog (see this link).

It is of interest that this was not a study of patients suspected of having subscapularis lesions, but rather a retrospective look at shoulders with suspected cuff lesions having subsequent arthroscopy. It would have been of great interest to know the results of physical examination in these patients: Did they have increased external rotation? Did they have diminished strength of internal rotation? Was their palpable thinning of the subscapularis tendon?

The most common clinical situation in which questions arise regarding subscapularis integrity is after shoulder arthroplasty. In this situation, MRI has difficulty in visualizing the integrity of the repaired subscapularis so that physical exam rather than imaginng becomes the main diagnostic tool.

If the tendon is weak and or retracted, it can be reconstructed with a tendon allograft.

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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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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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