By GRETCHEN REYNOLDS
Thursday, 17 December 2009
What Causes Early Arthritis in Knees?
By GRETCHEN REYNOLDS
Recently, Dr. Constance R. Chu, the Albert Ferguson associate professor of orthopedic surgery at the University of Pittsburgh and director of the Cartilage Restoration Center there, confirmed a theory, and found herself troubled by the results. It turned out that if you dropped a heavy weight onto parts of a cow’s knee joint from various heights, the joint was hurt. (While the parts of the joint were damaged, the cow itself was uninjured by the experiment; the knees came from a local abattoir.) When the weight hit the joint’s surface cartilage with great speed and force, the bone and cartilage fractured. No surprise there. But it is what happened in Dr. Chu’s experiment when the impact was more subtle — closer to, for instance, the perturbations inside a human knee when a ligament is torn — that concerned her. She found that with lighter impact, the various parts of the knee appeared, visually at least, to be fine.
But when Dr. Chu and her colleagues examined the cartilage cells just below the placid surface, they found carnage. “Many of the cells within the impact zone” — the area that had been directly thwacked by the weight — “were dead,” she said. They died instantly. More insidiously, other cartilage cells, those outside the injury site, began to die in the hours and days after the impact. “We saw an expanding zone of death,” Dr. Chu said. By the end of her group’s planned observation period, four days after the impact, cartilage cells well away from the original injury site were still dying.
The results are fascinating, in a gruesome sort of way. But why should escalating damage to cows’ cartilage matter to the average active human? Well, Dr. Chu says, this study, which was just published in the December issue of The American Journal of Sports Medicine, in conjunction with other researchers’ findings, may help to explain why, she said, “I’m seeing so many patients in their 20s and 30s with knee arthritis after joint injury.”
Human knees (as well as bovine ones) are remarkable instruments, able to bear large loads and pivot in multiple directions. But they also damage easily, as evidenced by the approximately 175,000 anterior-cruciate-ligament-reconstruction operations performed in the United States every year, a number that, by all estimates, has risen steeply in the past decade or so. (No agency tracks the procedures.) Many of these operations are being done now on teenagers, who rip an A.C.L. during a soccer or basketball game. (A.C.L. operations were relatively uncommon in young people before youth sports grew so popular.) Others are in men and women in their 20s and 30s who fall on the ski slopes, for instance.
What has been less remarked upon is the concomitant growth, Dr. Chu says, in cases of exceptionally early-onset arthritis. Once a disease associated primarily with people past retirement age (and still most prevalent in that age group), osteoarthritis, or degeneration of the cartilage in the knee, has been showing up in much younger people lately. “It’s not only in my practice,” Dr. Chu said. “Most orthopedic surgeons are seeing very young people with very old knees.”
An early and mass death of some of their cartilage cells may help to explain why, Dr. Chu says. If the results of her cow study can be extrapolated to human knees, then it’s possible that ripping an A.C.L. doesn’t damage just the A.C.L. The trauma from the incident affects the knee’s cartilage cells, too. These cells make up the tissue that coats the ends of the knee bones. Without this coating, the bones rub against each other. Pain and disability can follow.
It’s not yet fully clear, Dr. Chu says, what kills off the cartilage cells located away from the injury site. “The cells could have been injured by the initial impact, not recover and die,” she said. “They could be killed from exposure to noxious substances released by cells that have died. It could also be a combination of the two.” The result is a patch of cartilage in the knee that no longer functions well, if at all. “In a healthy knee, the cartilage is repairing itself all the time,” Dr. Chu said. “But if one loses too many cartilage cells, there can be too few cartilage cells to repair and maintain the cartilage in good condition.”
Unfortunately, the damage to the cartilage cells is invisible, Dr. Chu says. The dead or dying cells don’t show up on a typical M.R.I. scan. “The surface” of the cartilage “looks fine,” she says. The knee, in fact, after the A.C.L. reconstruction surgery, seems to have fully recovered. People return to full activity, including soccer games or skiing, “without realizing that their cartilage is weaker now,” and more prone to re-injury and disintegration, resulting in arthritis.
Not everyone who suffers an A.C.L. tear or other serious knee injury develops early arthritis, of course. “Right now, a good guess is that about 50 percent” will have clinical arthritis “within 5 to 10 years” after the injury, Dr. Chu said (meaning, for a 15-year-old, by the time he or she is 20 or 25, and for a 30-year-old, probably before he or she turns 40). “That’s quite a large number.”
Dr. Chu and many other researchers across the country are trying to develop methods to determine which people will develop arthritis after a knee injury and why. “Many labs are interested in this question,” she said. But for the moment, no one has had much success.
There also are no treatments yet available to slow or stop the deaths of cartilage cells in humans due to an injury. But, Dr. Chu says, one possible response is to “avoid overloading” the injured knee, to coddle the joint, perhaps a little more than might feel necessary. “You may want to alter your activities, even if the knee seems recovered and feels fine,” she said. “Maybe choose bicycling over marathon running,” she said. (For those who recall an earlier Phys Ed column about how running does not appear to harm knees, that finding applied to people who’d never suffered a major knee injury in the past.) The expectation, Dr. Chu says, is that the remaining, weakened cartilage cells will be “protected from a second assault that they might be able to resist when healthy, but are more vulnerable to after injury.” If you are careful with the knee, in other words, you probably lessen your chances of developing arthritis.
But that’s not necessarily a message active people want to hear. “I just had a teenage girl in here who’s only three months out” from an A.C.L. reconstruction, Dr. Chu said. “She felt great and wanted to know if she could go ski. It’s hard to tell someone like that that you just don’t know if she’ll ever have a completely healthy knee.”