The most movable joint in the body, the shoulder is also one of the most
potentially unstable joints.

As a result, it is the site of many common problems. They include sprains, strains, dislocations, separations, tendinitis, bursitis, torn rotator cuffs, frozen shoulder, fractures, and arthritis.

According to the Centers for Disease Control and Prevention, nearly 1.5 million people in the United States visited an emergency room in 2006 for shoulder problems.

Sourced from:
National Institute of Arthritis and Musculoskeletal and Skin Disease (NIAMS)
National Institutes of Health