Bursitis
The shoulder is made up of three bones: the scapula (shoulder blade), the humerus (upper arm) and the clavicle (collarbone). A number of important ligaments surround the shoulder joint. Ligaments are connective-tissue structures that link bones together. The shoulder's joint capsule is formed by a group of ligaments connecting the humerus to the glenoid (the socket of the shoulder joint) on the scapula. The next layer around the joint capsule consists of muscles and tendons. The tendons of four muscles form the rotator cuff: the supraspinatus, the infraspinatus, the teres minor and the subscapularis. Tendons attach to bone, while muscles are able to move bones by pulling on them through these tendons.
Several bursae (fluid-filled sacs) are found within the shoulder joint, located at points where friction can occur during movement. Movement of tendons or muscles needs to take place with as little friction as possible. A bursa is filled with a thin layer of lubricating fluid and functions rather like a small roller bearing, or a cushion. Dozens of these bursae are found throughout the body. They can become inflamed, for example through overload, resulting in a very painful condition. Bursitis is an inflammation of a bursa located at the point where a tendon attaches to bone, allowing the structures on either side of the bursa to move against each other with minimal friction. Inflammation of a bursa can develop within a few days (acute) or follow a very slow (chronic) course.