Fig. 2 – Arthritic shoulder
Arthritis occurs when the layer of cartilage on the bones on either side of the joint wears out or is destroyed. The cartilage is normally approximately 3 mm thick and provides a cushion as the bones move against each other. When looking at an x-ray a space is seen between the bony surfaces (fig 1), this space represents the cartilage (which is not seen on a x-ray) and is seen to be wearing when this space decreases (Fig 2).
There are 2 main types of arthritis; osteoarthritis (wear and tear) and inflammatory arthritis (e.g. rheumatoid arthritis).
A. Primary – Normal wear and tear with aging (when there is no apparent cause).
i). Trauma – a previous fracture or dislocation of the shoulder; these may damage the cartilage and lead to rough surfaces moving against each other.
ii). Infection – infection may destroy the cartilage
iii). Inherent deformities of the joint – irregular surfaces lead to abnormal wear to the cartilage.
iv). Tear of the rotator cuff tendons – a massive chronic tear of the rotator cuff leads to the ball riding up and not staying against the socket or glenoid. This leads to abnormal wear as the surfaces are no longer concentric.
The most common type is rheumatoid arthritis where destruction of the cartilage is due to an autoimmune reaction i.e. bodies own antibodies destroy the cartilage.