Biomechanics critical in understanding different types of rotator cuff tear

One investigator says he does not see rotator cuff tears becoming less frequent.

HOLLYWOOD, Fla. — Understanding, and perhaps solving, the frequency with which shoulder pathologies occur could be a matter of proper classification and a closer look at the biomechanical processes behind the injuries, according to one physician.

Christian Gerber, MD, from the Department of Orthopaedics, University of Zurich, presented his take on the issue of rotator cuff tears at the 29th Annual Meeting of the Arthroscopy Association of North America, here.

Economics

Gerber opened by underlining the impact of rotator cuff tears, pointing out that the economical impact is sizeable.

“What we know is that in the United States of America, 8.8 million people seek medical advice every year for shoulder pain,” Gerber said. “If we transfer that to a little country like Switzerland, this is 240,000 consultations for a population of 7 million.”

“If you have a shoulder problem — shoulder pain, a partial tear, a tear — your sick leave in the United States of America will be longer than if you have back pain,” he added. “If we are looking at the tendons we repair, the cost per tendon overall is $55,000. That includes sick leave, medical care and rehabilitation.”

Difficult to spot

In further understanding different tears, Gerber said it is critical to understand how each type of tear can have a different cause or form of development.

“There is a classification which is the basis for all classifications — articular-side tears, intratendinous tears, and bursal tears,” he said. “If there are three different types of tear, it may be that these three types develop differently.”

Gerber said that impingement of the undersurface of the acromion exclusively creates a bursal surface lesion — never an articular-side or intratendinous lesion — leading him to believe it is true that bursal-side tears are attritional in nature.

On the subject of intratendinous tears, Gerber noted that the misconception of their frequency, or lack thereof, is rooted in how difficult they can be to spot.

“We all think that [intratendinous supraspinatus tears] are probably not very frequent,” he said. “The reason is we miss them every time. We do not see them in arthroscopy because the outer surface looks okay and the inner surface looks okay, but if we look at the data we see that it is the most frequent tear.”

He added, “We have no clue or concept how these frequent tears arise. I think that if we look at how the supraspinatus tendon functions, it is inevitable that you have several different layers which move against each other. If for any reason — be it inflammatory or traumatic — these layers get stuck to each other and you develop lamination.”

The importance of the acromion

Gerber said he has found that the lateral extension of the acromion in patients with a cuff tear is significantly higher than in patients who do not have a cuff tear, partial or otherwise.

“We now have three different studies that show an increased acromion index or large lateral acromion is prevalent in patients with rotator cuff tears,” he said, adding that a large acromion has also been associated with higher repair failure rates.

The biomechanical reasoning behind this finding, Gerber said, indicates that with a large acromion the vector of the deltoid is pointed upward — as opposed to a short acromion, where the vector of the deltoid is pointing medially. This means that with a shorter acromion, the deltoid works more appropriately with the supraspinatus — as opposed to being an antagonistic force.

“I think that this is something we should look at,” Gerber said. “If the lateral extension plays a role, maybe the anterior extension also plays a role, as it prevents the deltoid from wrapping around the humeral head and compressing it.”

The first 20° to 30°

Gerber also noted that the “big difference” in the shoulder is within the first 20° to 30° of elevation.

“If you have a long acromion, between 0° and 20°, the force of the anterior deltoid is substantially higher and the stabilizing ratio is substantially worse than if you have a short acromion,” he said. “If you look at the deltoid, you see that the large acromion will automatically lead to a high moment for the shoulder. That means the force exerted by a single contraction is relatively high.”

“Together, this means that … if you have a large acromion between 0° and 30° of elevation, any time you want to stabilize the arm in space — just hold it in space — you need double the load for the supraspinatus than if you have a short acromion,” he added.

“I believe that the most frequent rotator cuff tears — partial tears — are caused by repetitive movement with the arm elevated between 10· and 30·,” Gerber said. “I believe that their development is favored by a genetically large extension of the acromion, be it anterior or lateral.”

Gerber also noted that he does not think rotator cuff tears will become any less frequent, as for a number of people simply holding a computer mouse could exert a significant amount of tension on the supraspinatus. – by Robert Press

Reference:

• Gerber C. Partial rotator cuff tears. International Guest Speaker presentation at the 29th Annual Meeting of the Arthroscopy Association of North America. May 20-23, 2010. Hollywood, Florida.

• Christian Gerber, MD, can be reached at christian.gerber@balgrist.ch