The shoulder is the most mobile joint in the body. The main shoulder joint is a ball-and socket joint, which allows a very wide range of movement. The joint is surrounded by a tough fibrous sleeve called the capsule, which helps to hold the joint together.
A group of four muscles and their tendons make up the rotator cuff, which controls movement and also helps to hold the joint together.
There's another smaller joint where the top of the shoulder blade meets the collarbone.
There are many causes of shoulder pain, but most cases will only affect a small area and are relatively short-lived.
Shoulder pain may also be part of a general condition such as rheumatoid arthritis or osteoarthritis.
Shoulder pain isn't always caused by a problem in the shoulder joint – problems in the neck can cause pain that's felt over the shoulder blade or in the upper outer arm.
Problems affecting the shoulder are:
For more information on these, please look at the related articles about 'Frozen Shoulder', 'Unstable Shoulder' and 'Rotator Cuff Injury'