Your shoulder is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle). The head of your upper arm bone fits into a rounded socket in your shoulder blade. This socket is called the glenoid. A combination of muscles and tendons keep your arm bone centered in your shoulder socket. These tissues are called the rotator cuff.
There are two joints in the shoulder, and both may be affected by arthritis. One joint is located where the clavicle meets the tip of the shoulder blade (acromion). This is called the acromioclavicular (AC) joint. Where the head of the humerus fits into the scapula is called the glenohumeral joint.
Also known as “wear-and-tear” arthritis, osteoarthritis is a condition that destroys the smooth outer covering (articular cartilage) of bone. As the cartilage wears away, it becomes frayed and rough, and the protective space between the bones decreases. During movement, the bones of the joint rub against each other, causing pain. Osteoarthritis commonly affects people over 50 years of age and is more common in the acromioclavicular joint than in the glenohumeral shoulder joint.
There are many risk factors for shoulder osteoarthritis, including age, genetics, sex, weight, joint infection, history of shoulder dislocation, and previous injury. Certain occupations, such as heavy construction or overhead sports, are also risk factors. The prevalence of shoulder osteoarthritis is increasing as the population ages.