Your shoulder joint is a type of ball and socket joint. The ball is the end of your humerus (upper arm bone) called your humeral head and the socket is part of your shoulder blade (scapula) called your glenoid. This type of joint allows your shoulder to be very flexible and allows you to have a great range of movement with your shoulder and arm.
Your shoulder joint is lined with smooth glistening articular cartilage, which allows painfree smooth movement.
Osteoarthritis is the most common type of shoulder arthritis. Also called wear-and-tear arthritis or degenerative joint disease, the cartilage is progressively worn away. As the protective cartilage surface of the joint is worn away by shoulder arthritis, bare bone is exposed within the shoulder.
The other common type of shoulder arthritis is rheumatoid arthritis. Rheumatoid arthritis is a condition that causes inflammation of the lining of the joints. This inflammation can, over time, invade and destroy the cartilage and bone.