Dr. Thomas Hoerner, wearing a surgical mask and gown, slipped a metal prosthetic piece over the end of his female patient's thigh bone during a knee-replacement procedure last month. It fit nicely.

"A woman's femur tends to be narrower," he said, pointing out how this new implant, made specifically for women, fits the bone without sticking out at the edges. A traditional implant, used in men and women, would have been slightly wider for the same length, he said, maybe even a little wider than her bone.

Doctors have known for years that women's knees are different from men's. Women's knees are narrower, they attach at a slightly different angle, and they are more prone to injury.

Now the business world is catching on. As scientists learn more about the biomechanics of women's bodies, companies are using the information to market everything from women's mountain bikes to women's knee braces.

The new artificial knee made for women, released by the medical device company Zimmer in August, was quickly adopted as a standard by many local orthopedic surgeons.

Knee-replacement surgeries are a last-resort treatment for debilitating arthritis pain. Most of the major implant makers sell the same implants for men and women, and surgeons use the smaller sizes for women. The Zimmer implant has slightly different proportions that, according to the company, match a women's anatomy better.

"It's hard to argue intuitively that a component that fits a patient's knee better is not better," said Hoerner, an Andover orthopedic surgeon who uses Zimmer's new knee on his female patients.

Yet the data to support that is not in yet. It will be years before women have walked around on the new implants long enough to tell if they're really better.

"I'm not totally sold on it yet," said Dr. Mahlon Bradley, an orthopedic surgeon who runs High Performance Sports in Peabody.

There's a lot of marketing targeted at women right now, he said, and some of it is just spin.

For example, in his opinion the knee brace made for women fails to solve the main problem women have with knee braces: that the brace actually hinders the way a woman's hamstring fires and, as a result, makes her knee more vulnerable to injury. He doesn't use knee braces on women at all.

On the other hand, he said, shoe companies are on the verge of making cleats for women that would absorb some of the force between the turf and the foot, potentially preventing knees from blowing out.

As far as the female knee implant, it does seem intuitively like a good idea, Bradley said. But at the same time, women have had great results with regular knee implants, just as men do, so there's no reason to think women need a special fit.

"They're not struggling with these knees," Bradley said. "They're doing very well with these knees."

That may be because competitors' products aren't much different.

Dr. Joseph McCarthy, vice chairman of the orthopedic surgery department at Massachusetts General Hospital, helped design the Triathlon artificial knee for Stryker, Zimmer's largest competitor. Though Stryker doesn't market the Triathlon as a knee made specifically for women, the Stryker design is based on the same data as Zimmer's. McCarthy was one of the doctors who did the study.

It's not surprising, then, that Zimmer's new knee looks a lot like the Stryker knee already on the market, McCarthy said.

"They're very similar," he said. "If you look at the dimensions, they're within a millimeter of the Triathlon."

Dr. Ira Evans, a surgeon who specializes in knees at Sports Medicine North in Peabody, uses the new Zimmer female implant, but he doesn't make any guarantees that women are going to notice a difference.

"There are no outcome studies out there yet," Evans said. "That's the real issue. ... The outcomes really aren't out there yet to say, 'No question, the (Zimmer) Gender Solutions High-Flex knee without a doubt works better than the standard high-flex design.'"

Which knee a patient gets is not a decision they usually make themselves. Typically, it's the surgeon who decides which type of implant to use.

Eleanore Harrison, 68, of Atkinson, N.H., was the patient whose knee was replaced last month with the gender-specific implant by Hoerner at Caritas Holy Family Hospital.

She didn't even know she was getting a special implant made for women until a day or two before surgery. She was just desperate for the surgeon to do something to relieve her pain.

"Stairs are not my friends," she said. "Actually I can get around, like if I have to go to Wal-Mart or something I can, but it hurts. ... I just go in, do what I can and get out, just so I can get off my knee."

Because of the surgery, she probably won't be able to go shopping with her daughters the day after Thanksgiving, she said, but she's hopeful that once she fully recovers, she'll be able to linger in stores to her heart's content without pain.

Knee replacements have come a long way in the past 20 to 30 years, Hoerner said. In the beginning, manufacturers experimented with a variety of designs and concepts. Now if you look at the knee implants sold by Zimmer, Stryker, DePuy, Smith & Nephew and other major manufacturers, they're all fairly similar.

"What it tells you is there was kind of a right answer," Hoerner said.

The new developments now, like the gender-specific knee, are just incremental changes and tweaks of existing designs, he said.

"The results are very good historically," Hoerner said. "Knee replacements are very durable. ... In general, it's a very good surgery."

A female problem

Boys and girls have about the same rate of knee injuries until they reach puberty.

Then around age 12, girls develop a vulnerability to knee injury that far surpasses that of boys.

Athletic girls are two to eight times more likely than their male counterparts to injure their anterior cruciate ligament (ACL), the band that keeps the shin bone from popping out in front of the knee.

Then, as they move into adulthood, women are more likely than men to suffer arthritis in their knees. About 70 percent of knee-replacement surgery patients are women.

Why is there such a dramatic difference?

First, there is basic anatomy. Women tend to have wider hips, causing their thigh muscles to meet their knees at a slightly wider angle than men's do.

Women's quadriceps tend to be stronger on the outside than the inside, which can pull the kneecap outward, especially if the woman has flat or turned-in feet, said Dr. Mahlon Bradley, an orthopedic surgeon at High Performance Sports in Peabody.

Hormones also seem to play a role. Studies have shown that women are more likely to hurt their knees during the ovulation phase of their menstrual cycle, when estrogen levels are high.

"Estrogen tends to cause ligaments to be slightly more lax," said Timothy DiFrancesco, a physical therapist and athletic trainer at Peak Performance Physical Therapy in Swampscott.

Also, for reasons that are still mysterious, women's hamstrings tend to fire a little late when they do cutting motions. Hamstrings stabilize the knee, so without their support, the knee is vulnerable to injury.

And lastly, when it comes to older women and arthritis, the higher surgery numbers could simply be because women tend to live longer than men, and doctors have increased the upper age limits on knee surgery in recent years.

Prevention

Just because knee replacements are getting better doesn't mean that you want to end up in the operating room. You can't avoid all knee problems, but you can reduce your risk. Try these techniques from the National Institute of Arthritis and Musculoskeletal and Skin Diseases for preventing knee injury:

r Warm up your knees before exercising or playing sports. Walk or ride a stationary bicycle; then do stretches. Stretch the muscles in the front of the thigh (quadriceps) and the back of the thigh (hamstrings). This reduces tension on the tendons and relieves pressure on the knee.

r Do exercises that strengthen the leg muscles, such as walking up stairs or hills, riding a stationary bicycle or functional strength training (under the supervision of a professional).

r Avoid sudden changes in the intensity of exercise. Increase the force or duration of your workout gradually.

r Wear shoes that fit properly and are in good condition. Good sneakers will help maintain balance and leg alignment as you walk or run. Knee problems can be caused by flat feet or over-pronated feet (feet that roll inward). Special shoe inserts (orthotics) can often reduce some of these problems.

* Maintain a healthy weight to reduce stress on the knee. Obesity increases the risk of osteoarthritis of the knee.

r If you are prone to knee pain, you should choose gentle exercises such as swimming, other aquatic activities, or walking and avoid jarring exercises such as jogging or high-impact aerobics.

r Don't ignore the pain. See a doctor or physical therapist if you have persistent knee pain.

Got knee pain? There are a lot of things your doctor may try before surgery:

r Resting the joint

* Orthotics (shoe inserts)

r Physical therapy

r Knee brace

* Anti-inflammatory pain medication

r Injections of hyaluronate, a component of joint fluid. Such injections can sometimes jump-start the knee into making its own joint fluid again.