Anatomy
Your shoulder is a ball-and-socket joint made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle).
The head of your upper arm bone fits into a rounded socket in your shoulder blade. This socket is called the glenoid. Surrounding the outside edge of the glenoid is a rim of strong, fibrous tissue called the labrum. The labrum helps to deepen the socket and stabilize the shoulder joint. It also serves as an attachment point for many of the ligaments of the shoulder, as well as one of the tendons from the biceps muscle in the arm.
Strong connective tissue, called the shoulder capsule, is the ligament system of the shoulder and keeps the head of the upper arm bone centered in the glenoid socket. This tissue covers the shoulder joint and attaches the upper end of the arm bone to the shoulder blade.
Your shoulder also relies on strong tendons and muscles to keep your shoulder stable. Some of these muscles are called the rotator cuff. The rotator cuff is made up of four muscles that come together as tendons to form a covering or cuff of tissue around the head of the humerus.
The biceps muscle in the upper arm has two tendons that attach it to the shoulder blade. The long head attaches to the top of the shoulder socket (glenoid). The short head attaches to a bump on the shoulder blade called the coracoid process. These attachments help to center the humeral head in the glenoid socket.
In addition to the ligaments and rotator cuff, muscles in the upper back play an important role in keeping the shoulder stable. These muscles include the trapezius, levator scapulae, rhomboids, and serratus anterior, and they are referred to as the scapular stabilizers. They control the scapula and clavicle bones — called the shoulder girdle — which functions as the foundation for the shoulder joint.
Cause
When athletes throw repeatedly at high speed, significant stresses are placed on the anatomical structures that keep the humeral head centered in the glenoid socket.
Of the five phases that make up the pitching motion, the late cocking and follow-through phases place the greatest forces on the shoulder.
- Late-cocking phase. In order to generate maximum pitch speed, the thrower must bring the arm and hand up and behind the body during the late cocking phase. This arm position of extreme external rotation helps the thrower put speed on the ball, however, it also forces the head of the humerus forward which places significant stress on the ligaments in the front of the shoulder. Over time, the ligaments loosen, resulting in greater external rotation and greater pitching speed, but less shoulder stability.
- Follow-through phase. During acceleration, the arm rapidly rotates internally. Once the ball is released, follow-through begins and the ligaments and rotator cuff tendons at the back of the shoulder must handle significant stresses to decelerate the arm and control the humeral head.
When one structure — such as the ligament system — becomes weakened due to repetitive stresses, other structures must handle the overload. As a result, a wide range of shoulder injuries can occur in the throwing athlete.
The rotator cuff and labrum are the shoulder structures most vulnerable to throwing injuries.
Common Throwing Injuries In the Shoulder
SLAP Tears (Superior Labrum Anterior to Posterior)
In a SLAP injury, the top (superior) part of the labrum is injured. This top area is also where the long head of the biceps tendon attaches to the labrum. A SLAP tear occurs both in front (anterior) and in back (posterior) of this attachment point.
Typical symptoms are a catching or locking sensation, and pain with certain shoulder movements. Pain deep within the shoulder or with certain arm positions is also common.
Bicep Tendinitis and Tendon Tears
Repetitive throwing can inflame and irritate the upper biceps tendon. This is called biceps tendinitis. Pain in the front of the shoulder and weakness are common symptoms of biceps tendinitis.
Occasionally, the damage to the tendon caused by tendinitis can result in a tear. A torn biceps tendon may cause a sudden, sharp pain in the upper arm. Some people will hear a popping or snapping noise when the tendon tears.
Rotator Cuff Tendinitis and Tears
When a muscle or tendon is overworked, it can become inflamed. The rotator cuff is frequently irritated in throwers, resulting in tendinitis.
Early symptoms include pain that radiates from the front of the shoulder to the side of the arm. Pain may be present during throwing, other activities, and at rest. As the problem progresses, pain may occur at night, and the athlete may experience a loss of strength and motion.
Rotator cuff tears often begin by fraying. As the damage worsens, the tendon can tear. When one or more of the rotator cuff tendons is torn, the tendon no longer fully attaches to the head of the humerus. Most tears in throwing athletes occur in the supraspinatus tendon.
Problems with the rotator cuff often lead to shoulder bursitis. There is a lubricating sac called a bursa between the rotator cuff and the bone on top of your shoulder (acromion). The bursa allows the rotator cuff tendons to glide freely when you move your arm. When the rotator cuff tendons are injured or damaged, this bursa can also become inflamed and painful.
Internal Impingement
During the cocking phase of an overhand throw, the rotator cuff tendons at the back of the shoulder can get pinched between the humeral head and the glenoid. This is called internal impingement and may result in a partial tearing of the rotator cuff tendon. Internal impingement may also damage the labrum, causing part of it to peel off from the glenoid.
Internal impingement may be due to some looseness in the structures at the front of the joint, as well as tightness in the back of the shoulder.
Instability
Shoulder instability occurs when the head of the humerus slips out of the shoulder socket (dislocation). When the shoulder is loose and moves out of place repeatedly, it is called chronic shoulder instability.
In throwers, instability develops gradually over years from repetitive throwing that stretches the ligaments and creates increased laxity (looseness). If the rotator cuff structures are not able to control the laxity, then the shoulder will slip slightly off-center (subluxation) during the throwing motion.
Pain and loss of throwing velocity will be the initial symptoms, rather than a sensation of the shoulder “slipping out of place.” Occasionally, the thrower may feel the arm “go dead.” A common term for instability many years ago was “dead arm syndrome.”
Glenohumeral Internal Rotation Deficit (GIRD)
As mentioned above, the extreme external rotation required to throw at high speeds typically causes the ligaments at the front of the shoulder to stretch and loosen. A natural and common result is that the soft tissues in the back of the shoulder tighten, leading to loss of internal rotation.
This loss of internal rotation puts throwers at greater risk for labral and rotator cuff tears.
Scapular Rotation Dysfunction (SICK Scapula)
Proper movement and rotation of the scapula over the chest wall is important during the throwing motion. The scapula (shoulder blade) connects to only one other bone: the clavicle. As a result, the scapula relies on several muscles in the upper back to keep it in position to support healthy shoulder movement.
During throwing, repetitive use of scapular muscles creates changes in the muscles that affect the position of the scapula and increase the risk of shoulder injury.
Scapular rotation dysfunction is characterized by drooping of the affected shoulder. The most common symptom is pain at the front of the shoulder, near the collarbone.
In many throwing athletes with SICK scapula, the chest muscles tighten in response to changes in the upper back muscles. Lifting weights and chest strengthening exercises can aggravate this condition.
Adapted with permission from https://orthoinfo.aaos.org