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Painful arthritis of the knee is on the rise — as is the number of middle-aged people who refuse to let the condition interfere with their favorite sports or exercise. Active people in their 40s and 50s are challenging doctors to provide treatments that not only keep them walking but keep them running and jumping as well.

Joints rely on slippery caps of cartilage that allow bones to glide past each other with a minimum of friction. "It's the smoothest material known to man," says Dr. Andrew Spitzer, director of the joint replacement program at the Cedars-Sinai Orthopaedic Center in Los Angeles. But with age, that cartilage wears away. "As it becomes damaged, the bones essentially grind against each other," Spitzer says.

Osteoarthritis of the knee is happening more and more in the 40-to-60 age range, doctors say; even people in their late 30s are seeking medical attention. In a 2007 study, scientists at Exponent Inc., a scientific consulting firm, projected that the demand for replacement knee joints would more than sextuple by 2030, with 3.48 million people in need of a new knee.

Spitzer suggests several explanations for the surge. For one, many people are keeping fit. All that physical activity is good, but it also adds to stress on the joints and risk of injury. In particular, young athletes who require knee surgery are more susceptible to osteoarthritis as they age.

On the flip side, many people are obese. The increased weight they carry adds to pressure on the knee.

It's important to see a doctor early if you experience lasting pain, says Dr. Brian Feeley, an orthopedic surgeon at UC San Francisco. Often people wait to come in until the arthritis has progressed beyond easy treatments, he says. There is no cure for osteoarthritis; once the cartilage is gone, the body is unable to regrow it. However, newer treatments attempt to replace lost tissue. Cartilage restoration is most appropriate for people younger than 40, Feeley says, who have plenty of cells left to fill in the gaps. For active middle-aged people, there still are many ways to manage or treat pain.

"The hard part is that patients have an expectation to remain active.... What do you do with the 45-year-old who still wants to play tennis and run?" Feeley says. "We need to be able to give people a variety of different treatment options."

In a review article published by the Journal of the American Academy of Orthopaedic Surgeons, Feeley and colleagues describe several kinds of treatment, ranging from pain medication to surgery. The goal, doctors say, is to offer a variety of options and put off knee replacement surgery for as long as possible.

Among the newer options are injections that help lubricate the joint and cartilage restoration. Those join many others:

• Physical therapy and strength training are first-line treatments. "By strengthening the muscles around the joint, you take the pressure off the joint itself," says Dr. Heather Gillespie, a sports medicine physician at UCLA.

• If you're overweight, try losing a few pounds. When you walk, your knee bears the pressure of five times your body weight, Gillespie says, so losing 5 pounds will take 25 pounds off your knee.

• Doctors recommend low-impact exercises, such as swimming or biking, to minimize pressure on the joint. If you must run, you might try running less or at a lower intensity, Feeley says.

• Painkillers and anti-inflammatories are a good option for early arthritis, Feeley says, if they relieve your pain and allow you to stay active.

• Glucosamine is a natural cartilage component and nutritional supplement thought to improve joint health, Gillespie says. According to a 2008 review published by the nonprofit Cochrane Collaboration, some studies suggested it's beneficial, but the highest-quality research found it made no difference.

• Sometimes doctors inject the knee with corticosteroids to reduce inflammation and pain. However, doctors are cautious about the treatment because of potential side effects. In a 2009 study in the journal Arthroscopy, researchers found that corticosteroids can be toxic to cartilage, particularly when combined with the lidocaine used to numb the injection site.

• Viscosupplementation is another injected treatment that is gaining in popularity. "It's an oil change for the knee," Gillespie says. Doctors inject hyaluronic acid, a natural component of the joint, to provide extra lubrication and shock absorption. In a 2006 Cochrane review of 76 studies, the authors found that viscosupplementation diminished pain, particularly five to 13 weeks after the injection. Overall, the effects were comparable to corticosteroid injections or other anti-inflammatories, the authors reported. According to a 2004 review in the Canadian Family Physician, viscosupplementation is slower to act but longer lasting than corticosteroids.

• Arthroscopy is a common treatment in which surgeons remove part of a torn or damaged meniscus, a component of the knee's shock-absorption tissues. It does not directly address the lost cartilage, and the effectiveness of this option is controversial. According to a 2003 study in the Journal of Bone and Joint Surgery, fewer than half of patients with osteoarthritis reported less pain two years after the operation. Moreover, 15% needed a new knee within a year of surgery. And in a 2002 study of veterans, published in the New England Journal of Medicine, arthroscopy for osteoarthritis was no more effective than sham surgery. However, Feeley and co-authors suggest that it may help certain younger patients, such as those who have a meniscal tear but normal knee alignment.

• If only one part of the knee is arthritic, a knee brace can shift your weight so it rests on the healthy part, relieving pressure and pain. Braces relieved pain for 78% of patients in a 2006 study in the Journal of Arthroplasty.

• For people who find a brace helpful but don't want to wear one, a surgery called osteotomy works similarly. Surgeons realign the bone by removing a wedge of bone or adding a bone graft just below or above the knee. This shifts weight to the healthy part of the knee. Osteotomy is best for younger patients, because in those older than 65 there is a good chance all the cartilage is deteriorating, Feeley and co-authors write. Osteotomy can delay a knee replacement — sometimes for decades — but eventually you'll need a new joint. In multiple studies, researchers have found that 2% to 25% of people with osteotomy need a new knee within 10 years, and 10% to 34% require replacement within 15 years.

• The typical last resort is a knee replacement. Surgeons try to avoid this treatment in young people because the new joints only last for approximately 15 years, Feeley says, and "the more active you are, the more likely they will wear out," Switching out a mechanical joint is a more complex surgery than getting the first replacement, so doctors try to limit this treatment to older people unlikely to need multiple replacements. In addition, knee replacement limits activity in many of recipients, and surgeons advise switching to lower-impact exercises. Researchers found that, although 94% of patients participated in sports at some point before the surgery, only 34% did so afterward, according to a 2005 study in the Annals of Rheumatic Diseases.

• Cartilage restoration is appropriate for young people with early arthritis, when plenty of healthy tissue is still left. For small cartilage losses, surgeons use a technique called microfracture. They drill tiny holes in the bone, and the area naturally fills up with cartilage-like scar tissue. For larger gaps, surgeons can pull out a bit of healthy cartilage and send it off to a lab where it grows into a lot of healthy cartilage. Then they re-implant the new tissue. Not every orthopedic surgeon is experienced in these techniques, Feeley says, so you may need to search for a doctor who can provide these treatments.