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.

The bones of the shoulder. Reproduced with permission from J Bernstein, ed: Musculoskeletal Medicine. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2003.

The ligaments of the shoulder. Reproduced with permission from J Bernstein, ed: Musculoskeletal Medicine. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2003.

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.

This illustration shows the biceps tendons and the four muscles and their tendons that form the rotator cuff and stabilize the shoulder joint. Reproduced and adapted with permission from The Body Almanac. (c) American Academy of Orthopaedic Surgeons, 2003.

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.

Muscles in the upper back help to keep the shoulder stable, particularly during overhead motions, like throwing. (Note: this illustration has been drawn in such a way to show the many layers of muscle in the back.) Reproduced with permission from JF Sarwark, ed: Essentials of Musculoskeletal Care, ed 4. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2010.

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.

The phases of pitching a baseball. Reproduced and adapted with permission from Poss R (ed): Orthopaedic Knowledge Update 3. Rosemont, IL. American Academy of Orthopaedic Surgeons, 1990, pp 293-302.

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.

(Left) The labrum helps to deepen the shoulder socket.

(Right) This cross-section view of the shoulder socket shows a typical SLAP tear.

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.

(Left) The biceps tendon helps to keep the head of the humerus centered in the glenoid socket. (Right) Tendinitis causes the tendon to become red and swollen.

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.

Rotator cuff tendon tears in throwers most often occur within the tendon. In some cases, the tendon can tear away from where it attaches to the humerus.

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.

Left untreated, throwing injuries in the shoulder can become complicated conditions.

In recent years, there has been a great deal of attention on preventing throwing injuries of the shoulder.

Proper conditioning, technique, and recovery time can help to prevent throwing injuries. Throwers should strive to maintain good shoulder girdle function with proper stretches and upper back and torso strengthening.

In the case of younger athletes, pitching guidelines regarding number of pitches per game and week, as well as type of pitches thrown, have been developed to protect children from injury.