The rotator cuff (so named because it helps the shoulder to rotate) comprises the tendons of four muscles—the supraspinatus, infraspinatus, teres minor, and subscapularis—which surround the shoulder blade (scapula), wrap around the upper arm bone (humerus), and create a “cuff” that sucks the humeral head into the shoulder socket (glenoid fossa). It is one of the most important structures influencing the health of the shoulder because its individual muscles not only move the shoulder in specific directions, but because, as a unit, it acts as the major stabilizer of this very mobile joint.
Rotator cuff repair has become big business. From 1996 through 2006, the number of arthroscopic rotator cuff surgeries increased 600 percent, while the number of traditional open repair surgeries increased only 34 percent. Researchers speculate that surgeons are becoming more aggressive in treating rotator cuff tears, especially as outpatient surgery becomes more commonly available. Tears are only part of the picture, and other common rotator cuff problems include tendinitis, bursitis, trigger points, and everyday muscle aches, pains, and strains.
Rotator cuff anatomy
So why is the rotator cuff so vulnerable? Unlike the hip, the shoulder relies primarily on those four small muscles and their converging tendons to hold the humerus in place. In fact, the head of the humerus is larger than the glenoid fossa, so by its very structure it slides around much more in and over its socket than does the head of the femur. The head of the femur is also stabilized by the ligamentum teres femoris (also called the round ligament), which ties the femoral head into the acetabulum and provides another layer of protection for the hip that the shoulder lacks. Thus, some scientists consider the glenohumeral joint to be a muscular joint because its stability relies on muscle action.
The muscles of the rotator cuff are pictured in Figures 1 and 2. Three are located on the back (posterior) surface of the scapula and one on the anterior (front) surface that slides over the ribs:
-The supraspinatus fills the top part of the scapula, above the spine, and it abducts the shoulder.
-The infraspinatus fills the triangular portion below the spine, and it externally rotates the shoulder.
-The teres minor attaches on the lateral border near the axilla (armpit), and it externally rotates the shoulder.
-The subscapularis coats the entire front surface, closest to the ribs, and it internally rotates the shoulder.
The shoulder girdle itself comprises four major joints, three skeletal and one functional:
-The glenohumeral (GH) joint, the synovial ball-and-socket joint where the head of the humerus articulates with the glenoid fossa of the scapula.
-The acromioclavicular (AC) joint, the synovial plane joint where the scapula articulates with the clavicle (collarbone).
-The sternoclavicular (SC) joint, the synovial saddle joint where the clavicle articulates with the sternum (breastbone), which is the skeletal attachment of the shoulder girdle to the rest of the body; and
-The scapulocostal (ScC) joint, the functional joint formed by the anterior surface of the scapula sliding over the posterior surface of the ribcage.
These four joints function in concert to make the arm the most mobile limb in the human body, and because the shoulder girdle allows for incredible degrees of movement like flexion, abduction, extension, and rotation, it is prone to overuse and injury. Improper joint mechanics compound the problem; for example, if your shoulder is internally rotated and you attempt to abduct it more than 90º, you’ll end up pinching the supraspinatus tendon because the head of the humerus will crash into the acromion process due to the structure of the joint. The supraspinatus tendon is one of the most frequently torn tendons because it runs underneath the acromion process to attach to the humerus. When it is continually compressed by poor joint position, its already limited circulation is further degraded and its ability to repair becomes badly compromised. Hello, surgery.
But when the four rotator cuff muscles act together, the glenohumeral joint is in a much better position to perform movements in a stable, agile manner. One 2000 study found that the rotator cuff and biceps activated prior to movement, and Cook and Jones (2006) state that the “rotator cuff functions at a level we can’t consciously control,” citing grip as an important factor to activate the rotator cuff as a shoulder stabilizer.
Risk Factors for Injury and Recovery
Unfortunately, time is not on our side. The deterioration of the tendons appears to be inevitable as we age. Most people over the age of 50 already have damage to their rotator cuffs, and studies have found that more than half of people over age 65 likely have rotator cuff tears. As a 2010 study of 683 Japanese mountain villagers found, many of these tears are asymptomatic, so we may not even know that our shoulder structure is compromised.
Other risk factors for rotator cuff injury include having a high body-mass index, engaging in heavy labor, and being weak in shoulder abduction and external rotation.
Even when surgery is performed, if the rotator cuff muscles—especially the infraspinatus—are weak, the rotator cuff may not regain full function after the procedure. Gladstone and colleagues found that for patients whose infraspinatus and supraspinatus were weak and who had fatty infiltration of those tissues, flexion improved postsurgery but external rotation did not. Those patients were also more prone to reinjuring their rotator cuffs.
While I have not had shoulder surgery, I am squarely in the dysfunctional shoulder camp. Come back Friday for my favorite ways to rebalance the rotator cuff!
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