Hip Replacement: An Option for Arthritis, cont.
The Wear Debris Problem
The process of bone loss and loosening is known as wear debris osteolysis. The severity of the osteolysis depends on the amount of wear
debris a joint and its bearing surface produce.
The amount of wear debris is linked to several factors, including the forces across the joint, the amount of its use, and the materials used to make the joint surface.
The weight of the patient and his or her level of activity (jogging versus walking) determine the force across the joint. The amount of use of the joint is measured most easily by how many steps the patient takes and by how long the patient lives after the joint is replaced.
For these reasons, most orthopedic surgeons discourage joint replacement in heavy, active patients who are young. Finally, the materials used to make the bearing surface of the joint can play a large role in the amount of wear debris. The first joint replacements used metal balls on Teflon, but they were abandoned because of a high amount of wear debris. Ultrahigh molecular weight polyethylene, strong white plastic, has been used most frequently and has the longest track record. More than 90 percent of metal-on-polyethylene joints last 10 years or more; however, some do fail earlier. Patients who have more than 15 to 20 years of life expectancy run greater risk of difficult revisions.
Advances in public health and medicine have created a healthier baby boom generation. Its members’ increased life expectancy and their desire for a more active lifestyle also place greater demand on the technology of joint replacements.
Minimizing wear debris
Recent advances in the field of joint replacement have focused on trying to minimize the wear debris problem. New surface materials include advanced cross linked polyethylene, ceramic heads and cups, and metalon- metal bearing surfaces.
At this time, ceramic surfaces have the lowest amount of measurable wear and are popular in Europe. The brittle nature of ceramics has raised concern in the United States. Though the fracture rate of ceramic heads is low, it is an immediate problem that requires a revision operation to correct.
Metal-on-metal bearings have been used more commonly and have low wear rates. Increased metal ions in the blood and urine of patients with metal bearings raise concern, but have not been shown to cause any measurable clinical problem.
Newer plastics known as highly cross-linked polyethylenes have been developed that show improvements in laboratory testing compared to the old standard plastic. The new plastic is processed with radiation to form more bonds between the long chains of molecules in the plastic. At least three different companies have three different highly cross-linked plastics on the market. Usually, the plastic bearing surfaces are less costly.
The decision as to which bearing surface to use is controversial; there is no consensus within the scientific or orthopedic communities. With any new technology in joint replacement, it will take 10 to 15 years to demonstrate any measurable improvement. Clinical research is the only way to measure the results from the choices available to the surgeon.
After Surgery
Joint replacement surgery can relieve pain and help patients get back to enjoying normal, everyday activities. The vast majority of individuals who undergo hip replacement surgery experience a dramatic reduction of hip pain and significant improvement in ability to perform the common activities of daily living. However, hip replacement surgery will not enable patients to do more than they could before their hip problems developed.
Following surgery, patients will be advised to avoid certain activities for the rest of their life, such as jogging and other high impact sports.