Your joints are involved in almost every activity you do. Simple movements such as walking, bending, and turning require the use of your hip and knee joints. Normally, all parts of these joints work together and the joint moves easily and without pain. But when the joint becomes diseased or injured, the resulting pain can severely limit your ability to move and work.

Understanding the Causes of Joint Pain

What is a joint ?

A joint is formed by the ends of 2 or more bones that are connected by thick bands of tissue called ligaments. For example, the knee joint is formed by the lower leg bone, called the tibia or shinbone, and the thighbone, called the femur. The hip joint is a ball-and-socket joint, formed by the ball, or femoral head, at the upper end of the thighbone, and the rounded socket, or acetabulum, in the pelvis.

The ends of the bone in a joint are covered with a smooth, soft material called cartilage. Normal cartilage allows nearly friction-less movement. The rest of the surfaces of the joint are covered by a thin, smooth tissue lining called the synovium. The synovium produces fluid that acts as a lubricant to reduce friction and wear in the joint.

Common Causes of Joint Pain

One of the most common causes of joint pain is arthritis. The most common types of arthritis are:

• Osteoarthritis (OA)— sometimes called degenerative arthritis because it is a "wearing out" condition involving the breakdown of cartilage in the joints. When cartilage wears away, the bones rub against each other, causing pain and stiffness. OA usually occurs in people aged 50 years and older, and frequently in individuals with a family history of osteoarthritis.

• Rheumatoid Arthritis (RA)— produces chemical changes in the synovium that cause it to become thickened and inflamed. In turn, the synovial fluid destroys cartilage. The end result is cartilage loss, pain, and stiffness. RA affects women about 3 times more often than men, and may affect other organs of the body.
  

• Post-traumatic Arthritis— may develop after an injury to the joint in which the bone and cartilage do not heal properly. The joint is no longer smooth and these irregularities lead to more wear on the joint surfaces.
  
  
• Avascular Necrosis— can result when bone is deprived of its normal blood supply. Without proper nutrition from the blood, the bone's structure weakens and may collapse and damage the cartilage. The condition often occurs after long-term treat-ment with cortisone or after organ transplantation.

Joint pain can also be caused by deformity or direct injury to the joint. In some cases, joint pain is made worse by the fact that a person will avoid using a painful joint, weakening the muscles and making the joint even more difficult to move.