Do your knees hurt? If so, you've got a lot of company and soon will have more.
The knee is the largest joint in the body, the one most often injured and usually the first to deteriorate with age.
More than half of Americans say they live with chronic or recurrent pain, according to a 2005 survey conducted by ABC News, USA Today and the Stanford University Medical Center. Knee pain (cited by 12 percent of respondents) was second only to back pain (25 percent) as a source of discomfort.
Arthritis is the most common cause of disability, according to the Centers for Disease Control and Prevention. About 22 percent of Americans over the age of 18 suffer from it.
Osteoarthritis (degenerative joint disease) is the most common form of arthritis, and osteoarthritis of the knee is most common form of osteoarthritis.
The elderly and athletes who have suffered knee injuries are the most likely to suffer from osteoarthritis. Seniors are as likely to suffer from it as from cardiovascular disease. Seniors who suffered knee injuries earlier in their lives are especially likely to develop osteoarthritis.
The knee is the junction of three bones: the femur (thigh bone), the tibia (shin bone) and the patella (kneecap). The bones are covered with articular cartilage, an elastic material which helps absorb shock and allows the knee joint to move smoothly. The bones are separated by two pads of connective tissue called menisci, which serve as shock absorbers.
The hamstring muscles in the back of the thigh help to bend the knee. The four quadriceps muscles in the front of the thigh help to straighten it from a bent position.
The quadriceps tendon connects the quadriceps muscle to the kneecap and provides the power to straighten the knee.
The media collateral ligament provides stability to the inner part of the knee. The anterior cruciate ligament limits rotation and forward movement of the shin bone. The posterior cruciate ligament limits backward movement of the shin bone.
As we age, articular cartilage -- the protective cushion between bones -- deteriorates. As the cartilage thins, bones grow thicker. When the articular cartilage is gone, bones rub together and wear away. Normal activity becomes painful and difficult.
How fast our knees wear out, and how much they deteriorate, is determined chiefly by our habits.
"The majority of knee replacements are done with people who are obese," said Nicholas Sotereanos, director of the Center for Restorative Joint Surgery at Allegheny General Hospital. "The rates [of knee replacement surgery] in Japan and South Korea are exponentially lower than in America. It's a direct correlation to body weight.
"If you weigh 200 pounds, you are putting 1,000 pounds [of pressure] per square inch whenever you go upstairs," Dr. Sotereanos said. "When you weigh 300 pounds, you are putting 1,500 pounds of pressure. The knees were not designed to take that stress. They will fail."
More than 350,000 knee replacement surgeries are performed in the U.S. each year. Originally, doctors considered the procedure appropriate only for patients ages 60 to 75. It was thought that younger, more active patients would put more stress on the artificial joint, requiring it to be replaced in 10 or 20 years. Patients older than 75 were thought to be too frail to endure the procedure.
The age is moving down, partly because the knees of the obese wear out sooner, and partly because artificial joints and the surgical techniques to implant them are getting better. Only 10 percent of artificial joints today are likely to require replacement after 10 years, only 20 percent after 20 years.
Another reason is that recent research indicates the results of knee replacement surgery are better when it is conducted before the deterioration of the knee is far along.
"One problem with waiting too long for knee replacement is that bone as well as cartilage may be worn away, making the procedure more difficult," Johns Hopkins University says on its website. "Increased age may also be correlated with an increased risk of surgical complications."
"A study of 222 people with osteoarthritis who underwent knee or hip replacement reported that those with poorer function at the time of the surgery also had worse function two years after the surgery, compared with people with less pain and better function before surgery," the site says.
The average age for knee replacements today is 58, Dr. Sotereanos said.
Though it provides mobility and pain relief to hundreds of thousands of people, knee replacement surgery is expensive and painful, and recovery from it is time consuming. So researchers such as Constance Chu, director of the Cartilage Restoration Center at UPMC, are searching hard for ways to detect deterioration early, to slow it down and to improve function in the knee and other joints.
One of the nation's most prominent researchers -- she has received more grant money from the National Institutes of Health than any other orthopaedic surgeon -- Dr. Chu got into research in part because "I saw 38-year-olds with knees worse than the 70-year-olds I was doing knee replacements on."
Athletes and others who suffer knee injuries are likely to develop osteoarthritis long before they're eligible for Social Security.
Each year about 175,000 people -- mostly athletes ages 15 to 25 -- suffer ACL tears that are surgically repaired.
"After an ACL injury, nobody's cartilage is normal," Dr. Chu said. "Half of the people who tear ACLs will have osteoarthritis within 10 years."
"If you've had an ACL reconstruction, you are at a two or three times higher rate of needing a knee replacement," Dr. Sotereanos said. "We can never quite reconstruct the ACL as it was originally designed."
About half of the athletes who tear an ACL also damage a meniscus.
"The meniscus helps to distribute the load on the knee," Dr. Sotereanos said. "Once it is torn, it never functions quite the same."
An athlete knows when he or she tears an ACL (mostly she; women are nearly eight times more likely to tear an ACL). But repeated small blows to the knee which don't appear to impair function and to which the athlete pays little attention can add up to big trouble years later, Dr. Chu said.
"Knee injuries resemble concussions in that the effects can last longer than they appear," she said. "Even if the knee feels fine, you shouldn't go full-out just yet. The way the joint operates, you will not know whether you have caused additional injury until 10 or 20 years later."
"If you get a running back who had an ACL injury and a meniscus injury at age 17, he will do well until around to the age 40," Dr. Sotereanos said. "At age 40, your knee is going to look like it's 65."
The ImPACT test, developed in Pittsburgh, has taken the guesswork out of determining whether an athlete has recovered fully from a concussion. At the Cartilage Restoration Center, Dr. Chu and her associates are developing means -- chiefly through the application of imaging technologies such as optical coherence tomography -- to look beneath the skin to determine the extent of injury and the pace of recovery.
OCT, a diagnostic tool originally created to map nerve tissue in the eye, makes it possible for Dr. Chu to find microscopic cartilage damage that isn't detectable by conventional means. An OCT scan is about 100 times more detailed than an MRI.
Dr. Chu and her team also are researching ways other than replacement surgery to relieve pain and restore function in damaged and aging knees. Cartilage has a very limited capacity for regrowth, but it has some. Two promising methods of stimulating regrowth are localized gene therapy and stem cell treatments.
In localized gene therapy, stem cells from the damaged area are removed, genetically altered in a lab and then reinjected in the knee. When an external trigger is received, these genetically altered cartilage stem cells stimulate the growth of new cartilage cells.
With one injection into the joint, "we're giving the cells within the joint the capacity to make a medicine that will improve the health of the joint," Dr. Chu said.
Stem cell therapy is based on the fact that stem cells are the only cells in the body that can morph into other types of specialized cells. Stem cells are harvested from a patient's bone marrow. When reinjected into the damaged joint, they appear to transform into chondrocytes, the cells that go on to produce new cartilage.
Both procedures are still experimental, though a few doctors around the country offer stem cell treatments (they're not covered by insurance), and Dr. Chu thinks she may be no more than a year away from human trials of localized gene therapy.
Gene therapy is more promising, Dr. Chu said and is "unproven" in humans, she stressed.
Gene therapy "is on the right track," Dr. Sotereanos agrees. But he thinks stem cells offer more promise in the near term.
"Genes make proteins in cells," he said. "Rather than turn them on and off, [we have] a better understanding of how to get stem cells to turn into cartilage than to get genes in cartilage to produce more cartilage cells."
He worries that once the therapeutic genes have been turned on, it might not be possible to turn them off precisely when the physician wants to, raising the risk of cancer, Dr. Sotereanos said.
"That needs to be understood in very precise detail," he said. "We're years away from a gene therapy modality. But the future is bright."
Before the future arrives, there's a lot we can do besides losing weight to diminish the likelihood we'll need knee replacement surgery.
After weight management, exercise is most important, Dr. Chu said. Strengthening the quadriceps and hamstrings will help relieve stress on the knees.
But the effects of exercise depend upon the stability of the joint, she cautioned. In a stable knee, exercise improves the condition of the joint. In an unstable knee, exercise accelerates cartilage loss and the onset of arthritis.
"If arthritis is significant, exercise will increase the problem," Dr. Sotereanos said.
So you should have your knees thoroughly checked out before you resume exercise after a knee injury, or -- if you're elderly -- begin an exercise program.
High-impact exercises, such as running, wear down cartilage. So those with healthy knees who want to keep them that way should mix in low-impact exercises such as swimming and cycling, Dr. Chu said.
"Osteoarthritis is a decades-long process," she said. "The choices an individual makes on a day-to-day basis have a cumulative effect."
Jack Kelly: email@example.com or 412-263-1476