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The shoulder is one of the largest and most complex joints in the body. The shoulder joint is the main joint of the shoulder. It is a ball and socket joint that allows the arm to rotate in a circular fashion or to hinge out and up away from the body. The joint capsule is a soft tissue envelope that encircles the glenohumeral joint and attaches to the scapula, humerus, and head of the biceps. It is lined by a thin, smooth synovial membrane.

Important bones in the shoulder include:

  • The scapula or shoulder blade. A large triangular-shaped bone that lies in the upper back. 

  • The clavicle or collarbone. A long bone that serves as a strut between the shoulder blade and the sternum (breastbone). 

  • The humerus. A long bone in the upper arm located between the elbow joint and the shoulder. 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.

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 acromion is a bony projection off the scapula. The clavicle (collarbone) meets the acromion in the acromioclavicular joint.

  • The coracoid process is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula . Pointing laterally forward, it, together with the acromion, serves to stabilize the shoulder joint.


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