Douglas E. Padgett, MD, is chief of the Adult Reconstruction and Joint Replacement Division and an associate clinical investigator at the Hospital for Special Surgery in New York City.
A hip replacement operation can be a godsend for the thousands of people each year who have been living in pain from arthritis or other hip problems. Key to the long-term success of the operation is the choice of which type of replacement hip is used, since materials have different advantages for different types of people… and unfortunately, as Douglas E. Padgett, MD, chief of the Adult Reconstruction and Joint Replacement Division and an associate clinical investigator at the Hospital for Special Surgery in New York told me, none of the choices are perfect. “What’s best is human cartilage on human cartilage — everything else is second best,” he said. Given that, what factors influence which kind of joint is used on you? And, what are the risks involved with each option? I spoke with Dr. Padgett about it…
Just like a natural hip, an artificial hip joint is composed of a ball (highly polished metal or ceramic) and a socket (a durable cup of metal, plastic or ceramic, sometimes with an outer metal shell). However, the bones of natural hip joints are sheathed with cartilage that enables them to glide smoothly, whereas in artificial ones the friction is literally hundreds of times greater. As a result, they may wear out in 15 to 20 years depending on how active the patient is and which materials are used. Of greater concern is what’s known as “wear debris” — particles that get released when the surfaces rub together, causing inflammation, scar tissue and degradation of surrounding tissue. This can result in the need for additional surgery or even a “redo” hip replacement.
Since all materials have their pros and cons, it makes sense to know as much as you can about the different kinds of implants so you can participate with your surgeon in deciding which is right for you. Factors that affect the decision include age, gender, activity level and the extent of damage to your natural hip.
Dr. Padgett reviewed with me the options currently available. There are advantages and disadvantages to each type of bearing. “I use all of them for different reasons, for different types of patients,” he said.
Metal or ceramic ball on polyethylene liner bearings. In this type of coupling, a polyethylene cup is mated with either metallic (cobalt chromium) or a ceramic (alumina or zirconia) head. “Polyethylene is a medical-grade plastic, much like the material used to make cutting boards,” explained Dr. Padgett. “Early versions of polyethylene bearings were highly susceptible to wear, but recent technology has improved its durability. Polyethylene bearings are well tolerated in the body and they almost never fracture.” Unfortunately, with wear these bearings erode and they may potentially shed polyethylene debris. The resulting inflammatory response around the bone may lead to resorption (osteolysis) or destruction of bone… and may be associated with implant loosening or even bone fracture.
Dr. Padgett says: “I use metal or ceramic on polyethylene bearings for older adults in their 70s and 80s, whose activities are somewhat less demanding — this type of coupling should last them for the rest of their lives.” For younger, more active people, including those who engage in high-impact sports, he uses metal on polyethylene, noting “the newer polyethylene is more forgiving and less likely to lead to catastrophic failure.”
Metal on metal bearings. Made of cobalt chromium, metal bearings are “highly polished surfaces associated with low friction and low wear rates,” said Dr. Padgett. However, he cautions, there may be problems associated with metal bearings. “At the end of the day, it’s still metal in your body, even though new technology has helped reduce the problem by decreasing impurities and improving the hardness and smoothness of the metal bearings.”
Also, he noted that some individuals develop a hypersensitivity to the metal, with an abnormal reaction leading to pain, loosening of the joint or implant failure. “There have been several reported cases of metal hypersensitivity leading to bone resorption and soft tissue injury,” he said, adding that for anyone, leaving metal in the body for a long time may lead to other problems, including long-term concerns about kidney or liver toxicity that may cause disease or even be possibly carcinogenic.
Active people are at greatest risk for having their hip replacement wear out, with some data showing greater metal ion release with greater activity. However, this type of implant has potential for long-term function. Dr. Padgett uses these implants on young males, but not patients with known kidney or liver disease and not females of childbearing age, due to concerns that the ions have the potential to penetrate the placental barrier and affect a fetus.
Ceramic bearings. Ceramics are composed of metal and non-metal compounds. “Ceramic surfaces are smooth and wettable, and will hold lubrication — factors which dramatically reduce wear rates for these bearings,” said Dr. Padgett.
However, as with the other bearings, ceramics have some inherent problems — most significant is that they are brittle, and therefore vulnerable to chipping, cracking and breaking. When this became apparent after numerous adverse incidents in the 80s and 90s, surgeons abandoned ceramics in favor of other bearings, Dr. Padgett told me, adding that “improvements in the materials have made this rare, but not unheard of, today.”
Lately there have been reports that some ceramic hips squeak audibly as patients go about their lives. Possible causes include a lack of lubrication of the joint by the body or the deposition of metal debris on the ceramic surface. Orthopedists have had a mixed response to this — some are unconcerned, but Dr. Padgett calls it worrisome. “If you are squeaking I would be concerned that you either have non-lubricated surfaces rubbing against one another, or some abnormality of the ceramic surface which could cause some damage to the bearing,” he said.
Dr. Padgett says: “I use a ceramic on ceramic hip for patients who remain active but aren’t involved in repetitive high-impact activities.” “Patients in their 50s who want to remain actively engaged in less rigorous, low-impact sports such as golf are typically a good fit for this type of implant.”
Unfortunately, all bearings (metal, ceramic, polyethylene) have the potential to cause wear debris and elicit an inflammatory response. Scientists continue to work on developing new materials for artificial hips that promise longer wear and fewer potential problems. Possibilities include different types of metallurgy, such as diamond. “Highly compressed carbon would be very hard, very smooth and shatterproof,” explained Dr. Padgett. For more information on hip replacement, go to the American Academy of Orthopaedic Surgeons Web site at www.aaos.org… or the American Association of Hip and Knee Surgeons at www.aahks.org.