When the glenohumeral joint (shoulder joint) has abnormal motion resulting in symptoms or damage, it is said to have instability. Some problems of the shoulder such as impingement or bursitis may actually be the result of instability.
Impingement and Rotator Cuff Tears
Impingement syndrome can range from simple bursitis to a tearing of the rotator cuff. It involves a pinching of the shoulder bursa and a portion of the rotator cuff between the acromion bone and the head of the humerus. This pinching may also involve the long head of the biceps tendon.
A rotator cuff tear is a tear or separation of the tendons that make up the cuff of tissue that surrounds the shoulder bone and is responsible for many of its primary movements.
The bones of the shoulder joint, like all other joints in the body are covered by hyaline cartilage. This is a remarkable substance which provides for a nearly frictionless surface for many decades. When the cartilage gets damaged, either by trauma, infection, or just long-term wear and tear, osteoarthritis results. There are other types of arthritis, but osteoarthritis is simply the result of the cartilage surface wearing out.