The exact cause of OA is still unknown. The "wear and tear" on bones and joints associated with leading an active life was once thought to be the most important cause of OA.

However, recent research suggests that the cause of OA is not that simple. Inflammation of the joint happens when pieces of cartilage break off and become lodged in the gel-like liquid (known as synovial fluid) that provides a cushion between the joints. The inflammation is caused by enzymes that are released from the lining of the joint. These enzymes break down the joint further, resulting in a vicious cycle of chronic, mild joint inflammation and cartilage loss.