Hip Pain and Joint Replacement

The hip is a ball-and-socket joint where the head of the femur articulates with the cuplike acetabulum of the pelvic bone.

The term “arthritis” literally means inflammation of a joint, but is generally used to describe any condition in which there is damage to the cartilage. Inflammation, if present, is in the synovium. The proportion of cartilage damage and synovial inflammation varies with the type and stage of arthritis. Usually the pain early on is due to inflammation. In the later stages, when the cartilage is worn away, most of the pain comes from the mechanical friction of raw bones rubbing on each other.

Osteoarthritis mainly damages the joint cartilage, but there is often some inflammation as well. It usually affects only one or two major joints (usually in the legs). It does not affect the internal organs. The cause of hip osteoarthritis is not known. It is thought to be simply a process of “wear and tear” in most cases. Some conditions may predispose the hip to osteoarthritis, for example, a previous fracture that involved the joint. Growth abnormalities of the hip (such as a shallow socket) may lead to premature arthritis. Some childhood hip problems later cause hip arthritis (for example, a type of childhood hip fracture known as a Slipped Epiphysis; also Legg-Perthe's Disease). In osteoarthritis of the hip the cartilage cushion is either thinner than normal (leaving bare spots on the bone), or completely absent. Bare bone on the head of the femur grinding against the bone of the pelvic socket causes mechanical pain. Fragments of cartilage floating in the joint may cause inflammation in the joint lining, and this is a second source of pain. X-rays show the “joint space” to be narrowed and irregular in outline. There is no blood test for osteoarthritis.

Rheumatoid Arthritis (R.A.) starts in the synovium and is mainly “inflammatory”. The cause is not known. It eventually destroys the joint cartilage. Bone next to the cartilage is also damaged; it becomes very soft (frequently making the use of an uncemented implant impossible). R.A. affects multiple joints simultaneously. It also affects internal organs. Another form of hip arthritis that is mainly “inflammatory” is Lupus. There are other more rare forms of arthritis that are also mainly “inflammatory”. They are basically similar to R.A.. X-ray changes in R.A. are essentially similar to osteoarthritis plus a loss of bone density.

Osteonecrosis is another serious cause of hip pain. It is not “arthritis”. It is a condition in which part of the femoral head dies. This dead bone can not stand up to the stresses of walking, the femoral head collapses, and becomes irregular in shape. The joint then becomes more painful. The most common causes of osteonecrosis are excessive alcohol use and excessive use of cortisone-containing medications. In most cases the cause is “idiopathic”, i.e. unknown.