Arthritis is the word used to describe pain swelling and stiffness in a joint or joints. A joint is where two or more bones meet, for example at the knee as seen in the diagram. The ends of the bones within a joint are covered with cartilage. This is a smooth thick layer that allows the bones to move against one another. Thick fluid surrounds the joint and this is held together by a strong joint capsule and ligaments.

Osteoarthritis starts with a roughening of the layer of smooth cartilage, the growth of tiny bits of extra bone (osteophytes)and an increase in the amount of fluid in the joint causing the capsule and ligaments to stretch. This may mean the joint eventually loses its defined or recognisable shape.

In the early phases there may not be much pain or trouble from this condition. However as osteoarthritis progresses, it can cause pain, swelling, stiffness (loss of movement) and weakness, resulting in some functional limitations.

The knee joint is commonly affected by osteoarthritis.