Shoulder Anatomy

The shoulder consists of a ball rotating on a small socket like a golf ball on a tee. This allows for significant range of motion. The ball-and-socket shoulder joint is known as the glenohumeral joint. The humeral head, or ball, rotates on the glenoid, commonly known as the socket, with the help of rotator cuff muscles. Four small muscles attach to the humeral head and are primarily responsible for the stability of this mobile joint. These “rotator cuff muscles” originate at the shoulder blade and become tendons when they cross the joint and attach to the humeral head. The tendon in the front of the joint is called the subscapularis tendon, and the tendon on top is the supraspinatus tendon; two tendons in the back of the shoulder are the infraspinatus and teres minor tendons.

Between the rotator cuff tendons and the roof of the shoulder, or the acromion, lies a sac of tissue known as the “subacromial bursa.” This sac acts as a cushion between the bone and the tendon. Within the shoulder joint, another soft tissue structure called the “labrum” sits on the glenoid and deepens the socket to provide added stability to the humeral head. The labrum is similar to an “O-ring.” Additionally, the long head of the biceps attaches to the top of the labrum or O-ring, at the 12 o’clock position. This is one of two tendons that make up the biceps muscle. A soft tissue structure called the capsule contains many ligaments that provide stability to the glenohumeral joint. Another smaller joint above the shoulder is called the “acromioclavicular” joint. This is where the collarbone, or clavicle, attaches to the roof of the shoulder, or acromion, with capsule and ligaments.

Shoulder Pathologies

Shoulder problems can arise across many age groups. Among the young, active population, overuse tendinitis, or bursitis, is common. This can occur when the shoulder mechanics are altered. When the subacromial bursa sac becomes inflamed, this is known as bursitis. It is referred to as “shoulder impingement” when the bursa sac gets pinched between the humeral head and the acromion with overhead motion.

For the older population, osteoarthritis is more common. Osteoarthritis results from years of wear and tear on the joint surfaces or remote trauma resulting in a loss of smooth, slippery surface on the ball or socket. Osteoarthritis can also involve the glenohumeral joint or the smaller acromioclavicular joint. Osteoarthritis frequently results in pain and gradual loss of motion.

Sometimes, an injury occurs that results in disruption of the anatomic structures, such as rotator cuff tendon(s), labrum, or biceps tendon. This is often associated with a sudden increase in pain and/or weakness. If the rotator cuff or the labrum is injured, the shoulder motion mechanics are usually altered. When the labrum is injured, some patients report their shoulder is “loose” or unstable. The glenohumeral joint may also become dislocated and is often associated with trauma. Similarly, the smaller acromioclavicular joint can also become dislocated; this is known as “separated shoulder.”

Another common shoulder pathology involves a sudden “pop” in the shoulder that results in a bulged muscle (a “Popeye muscle”) in the biceps. This is a complete tear of the long head of the biceps in the shoulder joint and is often is associated with rotator cuff abnormality and/or tears.

Treatment Options

Depending on the pathology, physical therapy may be beneficial in restoring the normal shoulder mechanics. This may involve stretching and/or strengthening the shoulder blade muscles and rotator cuff muscles. Other common non-operative treatments used in conjunction with therapy are ice, nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroid injections. These treatments are intended to decrease inflammation around the shoulder to facilitate restoration of joint mechanics, particularly when used with therapy.

Shoulder Surgeries

Shoulder surgery consists of arthroscopic and open surgeries. Shoulder arthroscopy involves using multiple small incisions to look inside the shoulder joint with a camera. Instruments are used during the surgery to deal with the various pathologies, such as rotator cuff tears, labral tears, or bursitis. Open shoulder surgery is often utilized for shoulder stabilization and recurrent dislocations, certain rotator cuff repairs, and shoulder replacements.

Shoulder Replacement

An "anatomic" shoulder replacement involves replacing the worn-out surfaces of the ball and socket by replacing the humeral head with a metal ball and stem and placing a plastic socket-bearing surface on the glenoid, or socket.



A "reverse" shoulder replacement is reserved for patients with irreparable rotator cuff tears – with and without arthritis – as well as failed previous replacements or severe fractures. The “reverse” shoulder replacement involves placing the ball where the socket is normally located in the glenoid. The socket of the implant is then placed in the humerus. This design of the implant allows the shoulder to regain function and motion without the assistance of the rotator cuff muscles. This procedure is typically limited to older patients.