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JULY / AUGUST 2005

AROUND HARVARD

This article originally appeared in the June 2005 Harvard Men’s Health Watch and is provided courtesy of Harvard Health Publications.

The best hip

People are clamoring for the ceramic-on-ceramic hip that Jack Nicklaus received, but they’re expensive.

As a golfer, Jack Nicklaus was admired for his long, beautifully controlled drives and steady putting. Now he’s becoming well known for another reason — his hip replacement. The 65-year-old golfer is a pitchman for the Stryker Corporation, a leading maker of hip and knee replacements. Six years ago, Nicklaus received a Stryker ceramic-on-ceramic hip replacement at New England Baptist Hospital in Boston. Orthopedic surgeons have been inundated with requests for them ever since.

The tried and true
But what’s a ceramic-on-ceramic hip replacement? Your hip is a ball-and-socket joint. The ball is the bulbous top, or head, of the thighbone (femur). The socket is a small concave area of the pelvis called the acetabulum (pronounced as-eh-TAB-you-lum). Muscles and tissue enclosing the joint hold them together.

In hip replacement, the surgeon removes the head and neck of the femur and substitutes a synthetic replacement for those parts; the surgeon also attaches an artificial socket, or cup, to the pelvic bone in place of the acetabulum (see illustration). So when Nicklaus’s hip replacement is called ceramic-on-ceramic, that means a ceramic ball has taken the place of the head of the femur and a ceramic-lined cup has taken the place of the acetabulum.

Hip replacement

The traditional replacement consists of a hard metal (usually a cobalt-chromium alloy) ball that fits into a hard plastic (polyethylene, to be precise) cup. The major drawback has been wear on the plastic cup. When tiny pieces of the plastic flake off, that provokes the production of cytokines, enzymes, and other factors that weaken bone tissue. As a result, the artificial hip pulls away from the bone, so it has to be replaced.

Orthopedic surgeons euphemistically refer to these second surgeries as revisions. No one wants to go through hip replacement surgery twice. For the surgeon, there’s also less bone tissue to work with the second time around, so the hip replacement is less secure. The risk of infection increases.

The replacement manufacturers have taken several steps to make the hips last longer. About 10 years ago, they started sterilizing the implants in inert or oxygen-free containers. And about five years ago, they started to make the implants with “cross-linked” polyethylene, which is supposed to hold up better. Cross-linking refers to carbon bonds in the material.
There’s a difference of opinion among surgeons about the cross-linked polyethylene. Those who like it say the cross-linking improves the durability of plastic. Skeptics are waiting for clinical data.

Other choices
Alternatives to the metal-on-plastic design include ceramic-on-ceramic and metal-on-metal versions. Ceramic hips were developed in Europe about 30 years ago. Initial enthusiasm flagged after the brittleness of the material seemed to make the hips more likely to crack. Several surgeons reported higher-than-normal failure rates. But after some tinkering with the design, improvements in the ceramic material itself, and a marketing push, ceramic implants have made a comeback — so much so that some surgeons now regard them as the gold standard for hip replacement.

The ceramic used in replacement hips contains some aluminum. It has a very smooth finish and seems to wear well, though some surgeons still believe it’s prone to chipping. Other types of ceramics have been tried, but they haven’t worked out as well. Four years ago, the high fracture rate of ceramic implants containing zirconia resulted in the manufacturers pulling them off the market in the United States.

The metal-on-metal hips have low wear rates. But some studies have found that patients may be more likely to have pain with them. Other research has found that the metal-on-metal hips seem to cause increases in blood levels of chromium, presumably because small amounts of the metal rub off and enter the bloodstream. Whether that poses a health risk is unknown, though.

Which is the best?
Most surgeons believe the ceramic-on-ceramic and metal-on-metal implants are more durable than the conventional implant, even if the cross-linked polyethylene is an improvement. Perhaps the main drawback is cost. For example, a ceramic-on-ceramic implant is twice as expensive as a conventional metal-on-plastic implant. So the conservative approach has been to reserve the longer-lasting, more expensive implants for younger and more active patients and to stick with the conventional design for older and less active people.

But where do we draw the line between young and old these days? At age 65, an American woman now has, on average, another 19 years ahead of her, and an American man, another 16 years. Many older people are quite active. All of these factors must be considered. Certainly age alone can’t be used to decide which type of hip replacement is right for someone.