In addition to the bones and joints, the shoulder contains a network of soft tissues, such as muscles, tendons, and ligaments. These soft tissues are prone to injury.
Muscles and Tendons of the Shoulder
The most frequently injured group of muscles and tendons within the shoulder is the rotator cuff. The rotator cuff originates at the scapula (shoulder blade) and attaches to the head of the humerus (arm bone). It stabilizes the glenohumeral joint by hugging the rounded humeral head to the concave glenoid cavity. The rotator cuff consists of 4 muscles and their tendons.
- The subscapularis is a large, triangular-shaped muscle located toward the front of the shoulder. It is responsible for rotating the head of the humerus internally (toward the body).
- The infraspinatus is a thick, triangular-shaped muscle located on the back of the shoulder blade. It is mainly responsible for providing the force needed for external rotation of the shoulder (away from the body).
- The teres minor lies below the infraspinatus at the back of the shoulder. It assists in adduction (movement toward the midline of the body) and extension (movement behind the body) of the shoulder.
- The supraspinatus is located directly on top of the shoulder and runs along the back. It is mainly responsible for helping to raise the arm away from the side. It is the smallest of the 4 rotator cuff muscles.
The rotator cuff, particularly the supraspinatus, is most at risk of injury, such as tearing, because the muscles and tendons are used in a wide range of motions and are often responsible for lifting heavy loads.
In addition to the rotator cuff there are a number of other shoulder muscles, including:
- The deltoid, a triangular-shaped muscle that covers the glenohumeral joint. The deltoid is responsible for a number of things, such as preventing joint dislocation when lifting heavy objects, helping raise the arm, and aiding other muscles in the chest.
- The triceps brachii, a muscle that assists in rotating the upper arm.
- The pectoralis minor, which begins in the upper ribs and fans out toward the shoulder area.
- The biceps brachii, a muscle located on top of the humerus. It rotates the forearm and flexes the elbow.
- The latissimus dorsi, a flat and rectangular muscle located on the back. It assists the arm in rotation and movement toward and away from the body.
The biceps brachii tendon, which connects the bicep muscle to bones in the shoulder and elbow, is most susceptible to injury. Generally, a person can recover from a bicep tendon injury without surgical treatment.
Ligaments, Labrum, and Bursae of the Shoulder
The ligaments, labrum, and bursae are fixed structures that reinforce and stabilize the shoulder.
Ligaments of the shoulder
Ligaments are fibrous bands of tissue that connect bone to bone. A network of ligaments stabilizes the shoulder. These ligaments include:
- Glenohumeral ligaments, which are 3 ligaments that reinforce the front of the shoulder’s glenohumeral joint. It spans from the edge of the glenoid cavity to the neck of the humerus (arm bone).
- Coracohumeral ligament, a strong and broad band that strengthens the upper aspect of the bicep brachii muscle.
- Transverse humeral ligament, which attaches to 2 different points at the top of the humerus. It creates an arched tunnel for the bicep tendon to pass under.
When one of these ligaments tears it can lead to shoulder separation or dislocation. When a joint in the shoulder separates or dislocates, a medical professional will usually have to manually return the bones to the correct alignment.
The labrum is a slippery, tough ring of cartilage that rims the glenoid cavity (shoulder socket). The labrum helps:
- Keep the head of the humerus in place
- Ensure smooth movement of the ball-and-socket joint
A shoulder’s labrum can be damaged or torn as a result of an acute injury, overuse, or as part of the aging process.
A bursa is a small, fluid-filled sac that reduces friction between bone and soft tissue. Bursae are located at various points around the body where muscles, tendons, and ligaments rub against bone during movement. The 2 primary bursae in the shoulder are the:
- Subscapular bursa, which is located between the glenohumeral joint (ball-and-socket joint) and the subscapularis muscle.
- Subacromial bursa, which is located directly under the acromion, a bony projection on top of the scapula (shoulder blade). The subacromial bursa helps the rotator cuff function.
A bursae can become inflamed from unusual pressure or overuse. The subacromial bursa, located above the glenohumeral joint and under the acromion, is the most susceptible to injury and is often the cause of shoulder impingement.