Osteoarthritis is a disease of synovial joints which is characterised by loss of articular cartilage and overgrowth of the underlying bone. Unlike rheumatoid arthritis, there is no pannus.

In the past osteoarthritis was considered to be a "degenerative joint disease", implying a passive process associated with old age. This is misleading because osteoarthritis is a multifactorial, active disease which usually begins in middle age.

Osteoarthritis is in fact a loosely defined group of diseases which may be triggered by factors such as:

• mechanical damage
• inflammation
• metabolic defects