Marlene Freed knows what's coming. For 10 minutes, she has been lying on her back, with ice chilling the swollen tissue around a seven-inch scar on her right leg - the spot where doctors inserted her new titanium knee.
The 68-year-old Olney, Md., resident is laughing and chatting. But when physical therapist Chris Gnip climbs onto the table and starts to bend her knee, pushing it carefully but relentlessly toward her chest, Freed clams up.
She closes her eyes and starts to breathe deeply. Her hand clutches the plastic cushion on the office table. The more Gnip pushes, the more the pain registers on Freed's face.
"OK," she mutters, and Gnip holds the knee in place.
"Five, four, three, two, one," he counts down. And finally lets go of the knee.
Freed has plenty of company in rehab these days. In 2003, the last year for which figures are available, doctors replaced more than 400,000 knees in the United States, two-thirds of them in women, according to the National Center for Health Statistics.
One reason for the continuing increase is that more active baby boomers are having knees replaced - and doing it earlier than their predecessors. Women are having more successful replacements than they used to - and some people credit a generation of new artificial knees designed just for them.
That happy knowledge does not necessarily make it easier for Freed, who had her left knee replaced in June and the right one replaced in January.
For an hour, Gnip puts her through her paces: leg raises, knee bends, quadriceps stretches.
"This is another fun one," Freed says, rolling her eyes and sliding her foot down the wall, bending the knee on her own while supporting it with her left foot. By this time she is breathing hard and is pink in the face.
"You're doing well," Gnip says.
Freed just chuckles. "You start out and you don't think it'll ever change," she says.
For Freed and others who wind up with replacement surgery, arthritis, injuries or the wear and tear of life have caused their knee cartilage to degenerate.
When the tissue that cushions the bones in the knee wears out, the upper and lower leg bones begin to grind together, causing pain and swelling.
When patients no longer can tolerate the pain, and other treatments such as cortisone shots and arthroscopic surgery have failed, knee replacements are options.
Freed decided to get the first one when she traveled to Turkey and met two women her age who had had double replacements. "Then," she said, "when I realized how much I could do with this new knee, I decided to get a matching one."
Among people 65 and older, the U.S. Centers for Disease Control and Prevention estimates that the rate of knee replacement increased eightfold between 1979 and 2002.
A 2006 study by the American Academy of Orthopaedic Surgeons predicts an astonishing 673 percent increase in the number of knee replacements during the next 23 years, with 3.5 million artificial knees implanted annually by the year 2030.
Obesity and a generation of baby boomers keen to stay active are the two main reasons for the most recent increase, explains Annie Hayashi, media relations manager for the surgeons' group.
Additionally, more people are getting their new knees earlier in life, says Dr. Kamala Littleton, the director of the Orthopedic Program for Women at Mercy Medical Center in Baltimore.
"We would wait, in effect putting Band-Aids on the problem, until they were 65," Littleton said. "Surgeons were hesitant to perform surgery on younger people, trying to avoid more complicated revision surgery when the implants wore out.
"Now we're seeing 86 percent of knee implants still functioning well at 15 years. Essentially by waiting, we were making this person live a life of someone 20 years older. You don't want somebody who's 52 hobbling around, needing a cane, who can't play with their children or grandchildren, who can't enjoy activities such as golf or bowling or even just walking."
The knee replacement operation, first performed in 1968, sounds as much like furniture repair as it does medicine.
The bits of bone and cartilage, damaged by injury or arthritis or both, are cut away. A shiny new metal saucer, shaped like the bottom of the femur, is inserted into the thighbone.
A shiny new saucer, shaped like the top of the tibia, is inserted into the tibia bone. A high-tech plastic insert is sandwiched between them, and a disc of plastic is glued on the back of the kneecap to make sure everything slides together easily. Viola - a new knee.
The surgery typically takes two hours and, in 2003, the last year for which figures were available, cost about $30,000 in 2003. That includes three to five days in the hospital afterward, according to the AAOS.
The latest developments involve implants designed for and marketed to women. They're designed more like a woman's real knee, with angles and bone shapes somewhat different from men's.
Gender-specific knees are "a great idea," says Mercy's Littleton. "It allows us to customize more to each patient. Although data is not available to show it to be superior, it gives us the ability to better replicate a woman's anatomy."
Women's thigh bones are slimmer than men's, and if the new knee is cut slimmer, then, "You'll have less over-stuffing of the joint, with more range of motion," Littleton says. "When you're trying to replicate a person's anatomy, it gives you the option of replicating it better."
This is still a controversial notion - some surgeons reject gender-specific implants altogether. Others are intrigued by the idea but haven't tried it.
Women are more likely to have knee problems than men, for a combination of reasons, says Dr. Kimberly Templeton, chief of orthopedic surgery at the Kansas City VA Medical Center. They tend to develop osteoarthritis more often because of sports injuries, obesity and living longer. Recent studies also show that as estrogen decreases in aging women, cartilage breaks down more easily.
Moreover, studies have long shown that women have fared worse than men after knee replacements, Templeton says.
"Maybe the implants weren't fitting as well - maybe it's because the cut of the bones, the size and the orientation are different in men than in women. But the other factor is that women get knee replacements later, and if you're more debilitated to start with, your outcome won't be as good," she says,
Her solution: Give women knee replacements earlier in life.
All sides agree that once the surgery is done, the key to recovery is physical therapy. Many patients are pulled to their feet to try a cane or walker on the day of the operation.
Two weeks after the operation, patients should be able to bend their new knee to a 90-degree angle, so they can go downstairs and sit down by themselves, said Megan Greco, clinic director at Physiotherapy Associates.
How bad is the pain?
"It does hurt," Greco says. "They absolutely hate it when we say, 'OK, let's push it.' And you have to do the exercises at home. We can tell when you're not."
But with eight to 10 weeks of therapy, most patients do well - and are in less pain than they were before the operation.