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Three years ago, Kim Bertin and a handful of fellow orthopedic surgeons marveled about how great it would be if women didn't have to trade a bad knee for a man's knee.

The LDS Hospital surgeon and his colleagues commiserated over how the fit of standard artificial knees was sometimes off by a fraction of an inch in women, just enough to cause discomfort or tightness after surgery.

The musing led to the new Gender Solutions High-Flex Knee, which Bertin helped design. On Tuesday night, surgical teams in Salt Lake City and Philadelphia performed the first implantations of the new artificial knee, tailored specifically to replicate women's narrower, thinner anatomy.

Bertin implanted one into 78-year-old Rosemarie Brinkerhoff at LDS Hospital, and was celebrating Wednesday at a Salt Lake City news conference.

Until now, ''we've really been putting men's shoes on women's feet," Bertin said.

Brinkerhoff isn't up and running just yet, but she is eager to see how her new knee handles stairs.

"I've been going up and down stairs with a crutch," she said Wednesday. "I've been dealing with knee pain for several years, and then it just blew up in December."

She is expected to return to her Lovelle, Wyo., home today.

Rather than changing size, the new knee changes shape to better fit women. That distinction grew out of common imperfections identified by Bertin and seven other surgeons - including Aaron Hofmann, a professor of orthopedic surgery at the University of Utah.

A biomedical engineer then created a three-dimensional map of the female knee using computer tomography imaging. This imaging illuminated three distinct differences between men's and women's knees:

* Men's knees are boxy like a rectangle, while women's knees are narrower and shaped more like a trapezoid.

* Women have a flatter bone at the front of their lower thighbone. Conventional implants are thicker than the damaged bone they are replacing in women, leaving some patients to feel extra bulk and discomfort.

* Women's legs are aligned at a different angle from the pelvis, which leads the patella - the kneecap - to track at a different angle along the groove at the lower end of the thighbone.

Engineers at Indiana-based Zimmer, the world's largest manufacturer of artificial knees and joints , took the data and built the Gender Solutions knee. The FDA approved it in late April.

Surgeons say the knee will improve recovery time and comfort. It will also simplify surgery, creating less need for surgeons to compromise on the operating table, said Robert Booth, chief of orthopedic surgery at Pennsylvania Hospital and principal developer of the implant.

"Women are not little men," he said during a separate teleconference from Philadelphia. "It's not adequate to just make smaller parts for them. You have to make different parts for them. For the first time, instead of making the women fit the parts, we have parts that will fit the women."

That's not to say women with traditional implants have been cheated, Hoffman said. The implant is a new, gender-specific generation of Zimmer's NexGen artificial knee.

"Patients are happy with what they have," Hofmann said. "We just want better."

The innovation is the latest in a series of improvements to artificial knees since they were first introduced in 1970. Surgeons used to have only three sizes to choose from; now they have eight. They used to put the same shape in left and right knees; now knees are designed for each side.

Booth predicts about 60 percent of female knee-replacement patients and 10 to 15 percent of male patients will be candidates for the new knee.

Women account for two-thirds of the 400,000 U.S. knee replacements done annually. That marks a significant shift from 20 years ago, when women accounted for about half of all knee replacements. Zimmer officials attribute the increase to the rise in obesity and osteoarthritis, as well as women's tendency to delay treatment.

The American Academy of Orthopaedic Surgeons anticipates the demand for total knee replacement will skyrocket to 3.5 million during the next 25 years.

''It's a clever marketing stratagem,'' said Bruce Nudell, an analyst with Sanford C. Bernstein & Co. in New York. The product's success will hinge on advertising that targets consumers directly, he said.

''To date, there have been no published results demonstrating any functional difference between that design and a conventional design,'' he said. ''We'll see whether they can steal some market share with it.''

Zimmer officials say they will track and publish outcome data on Gender Solutions patients.

Zimmer captures about 30 percent of sales for artificial joints compared with about 25 percent each for Johnson & Johnson and Stryker Corp. and about 10 percent each for Biomet Inc. and Smith & Nephew PLC, Nudell said.

Zimmer shares rose $1.03, or 1.7 percent, to $62.52 at 12:36 p.m. in New York Stock Exchange composite trading. The stock fell 8.8 percent this year through yesterday.


Bloomberg News Service contributed to this report.