1. Osteoarthritis is common in older people over the age of 50 years and affects more often the acromioclavicular than the gleno-humeral joint. It results from the disruption of the cartilage tissue covering the bone extremities (acromion and clavicle), reducing the intra-articular space of the joint so that during the shoulder movement the interfacing bones touch each other, provoking pain.
2. Rheumatoid Arthritis (RA) is a chronic, autoimmune disease that involves several joints of the body simultaneously and symmetrically. It is the result of the immune system not recognising and attacking its own tissue, producing chronic inflammation with degenerative consequences for the joint. Over time RA affects the synovium, which is a thin membrane filled with fluid that lubricates the joint facilitating the movement of the humerus head. Consequently, the synovium becomes inflamed, swells, degenerates and elicits pain and shoulder stiffness.
3. Post-Traumatic Arthritis is a form of degenerative osteoarthritis that develops following an injury, such as a fracture or dislocation of the shoulder. Injuries occur in sports, motor vehicle accidents, falls or any other source of physical trauma. With damage to the cartilage and/or bone the relationship of the joint structures changes, creating a friction among its parts, which over time results in wear and tear of the joint. This rubbing is exacerbated with intensive sport or when practicing other physical activities. Excessive body weight is another factor accelerating posttraumatic arthritis due to the pressure posed on the joint.
4. Rotator Cuff Tear Arthropathy: Shoulder osteoarthritis can develop following a tear of the rotator cuff tendons. When the damaged rotator cuff is unable to stabilise the humeral head and hold it within the glenoid socket, the friction of the humeral head against the acromion will damage the surface of both bones. Over time this functional alteration sets the development of shoulder joint arthritis. When combined, a rotator cuff tear and advanced arthritis result into severe pain and inability of the patient to lift the arm.
5. Avascular Necrosis: When blood supply to the head of the humerus is impaired, the cells of the affected bone area will die, causing an avascular necrosis. This is an ongoing condition, which over time leads to a severe destruction of the shoulder joint. Initially, avascular necrosis only involves the humeral head, which eventually collapses damaging the articular cartilage and the glenoid socket. This alteration progresses into full arthritis. Avascular necrosis can be the consequence of steroid overdose, alcohol abuse, sickle cell disease and a traumatic fracture of the shoulder. When specific causes are not known, this condition is defined as idiopathic avascular necrosis.
6. Overuse The main aetiology for the development of osteoarthritis of the shoulder joint is the overuse of the shoulder in ageing individuals and active sportsmen/women. Weightlifters, other athletes or workers who repeatedly lift heavy objects with overhead movement are vulnerable to develop in particular acromio-clavicular (AC) joint arthritis.