A New Knee, Talked Up to Women, Must Walk the Walk

U.S.News & World Report—July 3, 2007

Manufacturers of everything from running shoes to deodorants design products specifically for women. One of the latest entries to the market: an artificial knee created for—and heavily advertised to— females. Without a track record, though, doctors say it's too soon to tell whether the Gender Knee truly represents a giant leap for womankind or whether it merely gives its manufacturer, Zimmer Holdings Inc., a leg up in marketing.

Clearly, men and women are different. In the case of the knees, according to Zimmer, that means three things. First, the patella, or kneecap, is thinner in women than in men. Also, women's wider hips create a different angle between the pelvis and knee, which can mean the kneecap gets pulled to the side when the quadriceps contract. And the end of the thighbone is different—it's typically narrower in men.

Those differences may contribute to the fact that knee replacements in women aren't as successful—when measured by reported pain and the rates of "revision," or do-over corrective surgery—as those in men.

The Zimmer Gender Knee certainly fits into a market niche. More than 14,000 of the new joints were implanted in the first quarter of 2007, during which Zimmer launched a direct-to-consumer marketing campaign that included TV ads. Some 300,000 knee replacements occur annually.

But does the new (and more expensive) replacement actually serve women better than what was already available? "In theory, yes, but the evidence isn't there," says Kimberly Templeton, an orthopedic surgeon and a spokesperson for the American Academy of Orthopaedic Surgeons. The new implant is a modified version of Zimmer's other implants, so the FDA approved the device without requiring the company to demonstrate clinical superiority. Sheryl Conley, Zimmer's chief marketing officer, says seven studies now underway will look at things like patient satisfaction and range of motion. Preliminary data will be available in a year or so, she says. The most meaningful results, like whether the knee requires fewer revisions, will take even more time.

Research might eventually show that Zimmer's tinkering has created a better replacement knee. Or, it might not. Anatomical differences aside, Templeton says, replacement knees may underperform in women at least in part because females tend to delay surgery—sometimes until they're housebound by disability. "We know that how people do afterwards depends on how they're doing before," she says.

In addition, it's not clear that the manufacturer's specialized design will translate to less pain, says Steven Haas, M.D., an orthopedic surgeon and chief of the knee service at Hospital for Special Surgery in New York. For example, making the replacement kneecap thinner by one millimeter won't necessarily make a noticeable difference to recipients. "There's no data to show the problems are caused by what they're fixing," he says.

Dr. Haas adds that having a correctly fitted device is clearly important, but that other companies have also modified their smaller-size knees—which are almost exclusively implanted in females—to account for gender differences in anatomy. "Women have not been ignored in knee design," he says. "They are the majority of knee replacement recipients," especially in the smaller sizes. (Dr. Haas has consulted with Smith & Nephew, a rival to Zimmer.)

More important, he says, is to find a skillful surgeon. "At the end of the day, if I sat you down with all these implants, you couldn't tell them apart. The difference between them is relatively minor compared to the technique of the surgeon putting them in," he says. Templeton agrees. "You don't want to force a surgeon to use an implant they're not used to."