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Human Anatomy

Picture of the Shoulder

The shoulder is one of the largest and most complex joints in the body. The shoulder joint is formed where the humerus (upper arm bone) fits into the scapula (shoulder blade), like a ball and socket. Other important bones in the shoulder include:

The acromion is a bony projection off the scapula.
The clavicle (collarbone) meets the acromion in the acromioclavicular joint.
The coracoid process is a hook-like bony projection from the scapula.

The shoulder has several other important structures:

The rotator cuff is a collection of muscles and tendons that surround the shoulder, giving it support and allowing a wide range of motion.
The bursa is a small sac of fluid that cushions and protects the tendons of the rotator cuff.
A cuff of cartilage called the labrum forms a cup for the ball-like head of the humerus to fit into.

The humerus fits relatively loosely into the shoulder joint. This gives the shoulder a wide range of motion, but also makes it vulnerable to injury.

Shoulder Conditions

Frozen shoulder: Inflammation develops in the shoulder that causes pain and stiffness. As a frozen shoulder progresses, movement in the shoulder can be severely limited.
Osteoarthritis: The common "wear-and-tear" arthritis that occurs with aging. The shoulder is less often affected by osteoarthritis than the knee.
Rheumatoid arthritis: A form of arthritis in which the immune system attacks the joints, causing inflammation and pain. Rheumatoid arthritis can affect any joint, including the shoulder.
Gout: A form of arthritis in which crystals form in the joints, causing inflammation and pain. The shoulder is an uncommon location for gout.
Rotator cuff tear: A tear in one of the muscles or tendons surrounding the top of the humerus. A rotator cuff tear may be a sudden injury, or result from steady overuse.
Shoulder impingement: The acromion (edge of the scapula) presses on the rotator cuff as the arm is lifted. If inflammation or an injury in the rotator cuff is present, this impingement causes pain.
Shoulder dislocation: The humerus or one of the other bones in the shoulder slips out of position. Raising the arm causes pain and a "popping" sensation if the shoulder is dislocated.
Shoulder tendonitis: Inflammation of one of the tendons in the shoulder's rotator cuff.
Shoulder bursitis: Inflammation of the bursa, the small sac of fluid that rests over the rotator cuff tendons. Pain with overhead activities or pressure on the upper, outer arm are symptoms.
Labral tear: An accident or overuse can cause a tear in the labrum, the cuff of cartilage that overlies the head of the humerus. Most labral tears heal without requiring surgery.

Shoulder Tests

Magnetic resonance imaging (MRI scan): An MRI scanner uses a high-powered magnet and a computer to create high-resolution images of the shoulder and surrounding structures.
Computed tomography (CT scan): A CT scanner takes multiple X-rays, and a computer creates detailed images of the shoulder.
Shoulder X-ray: A plain X-ray film of the shoulder may show dislocation, osteoarthritis or a fracture of the humerus. X-ray films cannot diagnose muscle or tendon injuries.

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