The hip joint is composed of a ball portion called the femoral head, and the upper end of the thighbone (femur), which fits into a socket portion of the pelvis called the acetabulum. In a normal hip joint, about 4/5 of the femoral head is covered by the acetabulum, providing a stable hip joint.
A stable hip joint, working in a coordinated manner with the surrounding muscles, allows us to move our legs back and forth and from side to side freely.

The surface of a normal joint is covered by smooth layer called cartilage.
Cartilage, which is free of nerves and blood vessels, consist of about 80% water, function to absorb shocks to the joint, resulting in smooth movement.

In addition, the joint is covered by a sac like envelope called the articular capsule, inside which a membrane called the synovium secretes joint fluid for lubrication and nutritional support.
The normal functioning of cartilages and joint fluid allows us to move our joints with no pain.

What are possible diseases of the hip joint?

In 1962, Dr. John Charnley, a British surgeon, first devised an artificial hip joint in which polyethylene was introduced as a bearing surface (articular surface), based on the idea of “reducing pain and restoring motion,” successfully lengthening the usage life of artificial hip joints.
This allowed total hip replacement to become a common treatment.
Now, about 50 years later, polyethylene is still used as a material for the bearing surfaces of artificial hip joints because of its superior material properties.

Replacement of the affected joint with an artificial joint made of metal, ceramic, polyethylene, or other materials eliminates pain and improves ambulation. This procedure is called artificial joint replacement surgery and, in Japan, approximately 120,000 patients receive the procedure every year, mainly for the hip joint and knee joint.

What are some problems associated with artificial hip joints?

Artificial hip joint replacement does not eliminate all problems. When artificial hip joints are used for a long period, their articular joint may wear, leading to potential loosening.

Loosening may require revision surgery every 10 to 20 years, depending on the patient, and as the patients become older, the revision surgery become more difficult.

About bone resorption and loosening

“Wear debris” from materials is generated in artificial hip joints when the liner and femoral head ball move in contact. When the human body (cells) recognizes wear debris as foreign matter, it responds to induce “bone resorption,” a process in which bone around the artificial hip joint is resorbed. As a result, a gap is formed around the artificial hip joint that was previously securely fixed.
This is called “loosening.”

It goes without saying that examination of materials for artificial hip joints has been conducted in order to reduce wear debris, including the use of metal materials resistant to wear. However, it has been reported that such measures result in the occurrence of problems with other soft tissues or bones, and even a change of materials has not resolved the occurrence of all problems.