Anatomy

The shoulder joint is formed by the articulation of the head of humerus (ball of the arm bone) with the glenoid cavity (or fossa) of the scapula (shoulder blade).

Like most synovial joints, the articulating surfaces are covered with hyaline cartilage. The head of the humerus is much larger than the glenoid socket. This gives the joint a wide range of movement , but also makes it inherently prone to instability. 

Causes of shoulder joint damage:

Osteoarthritis: This is an age-related wear and tear type of arthritis. It usually occurs in people 50 years of age and older and often in individuals with a family history of arthritis. The cartilage cushioning the bones of the hip wears away. The bones then rub against each other, causing hip pain and stiffness.

Rheumatoid arthritis: This is an autoimmune disease in which the synovial membrane becomes inflamed and thickened. This chronic inflammation can damage the cartilage, leading to pain and stiffness. Rheumatoid arthritis is the most common type of a group of disorders termed inflammatory arthritis.

Posttraumatic arthritis: This can follow a serious hip injury or fracture. The cartilage may become damaged and lead to hip pain and stiffness over time.

Types of shoulder replacement:

• Shoulder hemiarthroplasty: In this procedure, the surgeon will remove the damaged/broken head of humerus (ball of the arm bone) and replace it with a metallic spherical prosthesis. This can be done only when only ball is damaged and socket is healthy.
• Total shoulder replacement: This surgery is indicated when both the sides of the shoulder joint-the ball and the socket(glenoid) are damaged as a result of arthritis. In this procedure, a special plastic polymer is implanted on as a new socket and the humeral head is replaced with new metallic spherical implant fitted in the bone.
• Reverse shoulder replacement: This surgery is reserved for special situations where in addition to damage of ball and socket of shoulder, there is worn out rotator cuff (important muscle responsible for over-head movements and stability). It is called reverse because, to support the biomechanics- the ball is fitted in the natural socket(glenoid) and the socket on the natural position of ball(humeral head).