Shoulder osteoarthritis develops slowly over time usually not producing symptoms until middle age. Symptoms include stiffness, weakness, swelling, and chronic pain felt deep in the joint. Often people will avoid shoulder movements in an attempt to lessen arthritic pain, which can lead to a tightening or stiffening of the soft tissue parts of the joint, including the joint capsule, resulting in a painful restriction of movement and further weakness.

One main process involved in arthritis is degradation and thinning of the cartilage that lines the bone surfaces in the joint. Over compression of cartilage during habitual low-level loading is a common cause of shoulder arthritis. 

Bone spurs most often develop as a response to excess pull or compression on the bone either directly or by muscle. In an attempt at strengthening, the bone abnormally grows more bone into the tendon or ligament where it attaches making them more brittle and vulnerable to tearing or rupturing. The most common site for a shoulder bone spur is in the ligament connecting your collar bone and shoulder blade called the coracoclavicular ligament.