The Shoulder and Rotator Cuff

Today, let’s talk about the rotator cuff… one of the most common injuries and surgeries. I will explain the anatomy, function, and common causes of dysfunction. Friday’s post will discuss treatment options available as well as a few exercises. I apologize ahead of time, today’s post is going to seem very technical as the shoulder is complex in its structure and function.

Anatomy

The rotator cuff is actually a group a muscles and tendons that act to stabilize the shoulder and allow for its extensive range of motion. Most people think the rotator cuff is one structure, but it is more complex than that. Of the seven shoulder muscles, four of them make up the rotator cuff. The four muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis muscles.

Supraspinatus Muscle

• Moves the arm away from the body (Abducts the humerus)

• Nerve supply is given by the Suprascapular nerve (C5)

• Produces the first 15 degrees of motion

• Most commonly involved in a rotator cuff tear

Infraspinatus Muscle

• Allows us to scratch the back of your head (Externally rotates the humerus)

• Nerve Supply is given by the Suprascapular nerve (C5-C6)

• Covers the back of your shoulder blade deep below the skin and close to the bone

Teres Minor Muscle

• Produces the movement that allows us to wash our hair (Externally rotates the humerus)

• Nerve supply is given by the Axillary nerve (C5)

• Small, narrow muscle on the back of your shoulder blade

Subscapularis Muscle

• Getting something out of your back pocket (Internally rotates the humerus)

• Nerve supply is given by the Upper and lower subscapular nerves (C5-C-6)

• Large triangular shaped muscle that lies deep in your armpit

• Strongest, largest, and most used of the rotator cuff muscles

• Attaches to the front not the back of the upper arm, unlike the other three

The four tendons of these muscles come together to form the rotator cuff tendon. These tendinous insertions along with the articular capsule, the coracohumeral ligament, and the glenohumeral ligament complex, blend together before they connect to the humerus. The infraspinatus and teres minor fuse near their musculotendinous junctions (closer to the muscle bellies), while the supraspinatus and subscapularis tendons join as a heath that surrounds the biceps tendon near the front of the shoulder. Take a look at the pictures below to get a better understanding of what was just said.

Function

These muscles are important in shoulder movement and stability. They arise from the shoulder blade (scapula) and connect to the head of the humerus, forming a cuff at the shoulder joint. They hold the head of the humerus in the small and shallow glenoid fossa of the shoulder blade (scapula). The shoulder joint (glenohumeral joint) has been described as a golf ball (head of the humerus) sitting on a golf tee (glenoid fossa).

During abduction of the arm, moving it outward and away from the body, the rotator cuff compresses the glenohumeral joint, an action known as concavity compression, in order to allow the large deltoid muscle to further elevate the arm. Basically, without the rotator cuff, the humeral head would ride up partially out of the glenoid fossa, lessening the efficiency of the deltoid muscle. The forward and backward (anterior and posterior) directions of the glenoid fossa are more susceptible to shear force movements as the glenoid fossa is not as deep relative to the up and down (superior and inferior) directions.

Aside from stabilizing the glenohumeral joint and controlling humeral head translation, these muscles also perform multiple functions including moving the arm away from the body (abduction), allowing us to reach into our back pocket (internal rotation), and allowing us to scratch the back of our head (external rotation of the shoulder). The infraspinatus and subscapularis have significant roles in the Y in YMCA movement (scaption), generating forces that are two to three times greater than the force produced by the supraspinatus muscle. The front portion of the supraspinatus tendon is submitted to a significantly greater load and stress, and performs its main functional role.

Dysfunctions

A rotator cuff injury can happen suddenly, such as falling on your outstretched arm or it can develop slowly, resulting from repetitive motions or age-related degeneration.

Tendinopathy

This is pain in and around the tendons. Tendinitis and tendinosis are variations. Rotator cuff tendinitis is considered the mildest form of rotator cuff injury. It can develop from:

• Age-related degeneration

• Overuse

• Repetitive motion

• Trauma

Tear

The tendons at the ends of the rotator cuff muscles can become torn, leading to pain and restricted movement of the arm. A torn rotator cuff can occur following trauma to the shoulder or it can occur through the “wear and tear” on tendons, most commonly the supraspinatus tendon found under the acromion (see picture). Partial tears of the rotator cuff tendons mean the tendon is damaged or frayed but isn’t torn away from the bone. Full-thickness tears are when the tendon is completely torn from the bone, chronic degeneration is usually the reason.

Rotator cuff injuries are commonly associated with motions that require repeated overhead motions or forceful pulling motions. Athletes who experience such injuries are baseball and softball pitchers, quarterbacks, firefighters, cheerleaders, weightlifters (especially power lifters), rugby players, volleyball players, swimmers, boxers, kayakers, tennis players, and drummers.

Impingement

This occurs when the top of the shoulder (the acromion) rubs against the tendon and the bursa and irritates the rotator cuff. Between 44 and 66 percent of all shoulder pain is thought to come from subacromial impingement syndrome, which is the most common shoulder disorder.

Bursitis

Bursitis is when the bursa (small sac filled with fluid that protects your rotator cuff) gets irritated. This can happen when you repeat the same motion over and over again, like throwing a baseball or lifting something over your head. It is also can be caused by an infection.

Bone Spurs

These can form when rotator cuff tendons rub on the shoulder bones. Bone spurs don’t always cause a rotator cuff injury, however if too much friction is occurring, fraying is possible.

As you can see the shoulder is a complex joint, with many working parts. It is amazing how strong and supportive this joint has to be throughout our day. It amazes me that there aren’t more shoulder injuries due to the complexity of the joint and the surrounding rotator cuff. Stay tuned for Friday’s post going over treatment options for the dysfunctions discussed today. Disclaimer: I am in no way diagnosing anyone, if you have persistent pain you need to call your local physical therapist and schedule an appointment.