The patient was clearly in pain. Doctors were not sure how Johnny, a middle-aged male, had hurt his left knee, but they wanted to assess the damage. So they reached for an arthroscope, a video camera on a flexible, pencil-thin tube, and inserted it into the joint through a small incision.
Although arthroscopic surgery is fairly routine for diagnosing and treating knee injuries, Johnny's procedure was anything but. That's because Johnny is a monkey: an 11-year-old mandrill at the Pittsburgh Zoo. There's no precedent, says Freddie Fu of the University of Pittsburgh School of Medicine, the surgeon who performed the operation in June. I think this is maybe the first time anyone has ever 'scoped a monkey knee.
What Fu found is that Johnny had partially torn his anterior cruciate ligament (ACL), a band of tissue that stabilizes the knee, particularly as it rotates. A torn ACL is one of the most common major injuries in the sports world, says Fu, who guesses that he has mended some 5,000 ligaments during his 30 years in the orthopedic business. Such knee injuries are not limited to professional athletes, however: they can befall just about anyone who is physically active, including weekend warriors, high school students and--as Fu discovered--middle-aged mandrills. Sources in the orthopedic industry estimate that physicians reconstruct about 200,000 ACLs every year in the U.S. alone; worldwide the number of reconstructions may reach up to half a million.
But the problem doesnt end there. Tearing an ACL--even if the ligament subsequently gets repaired--can set the stage for osteoarthritis, a condition in which the entire joint begins to deteriorate. According to one Norwegian study, 100 percent of women who tear their ACL are going to have arthritis in that knee by 20 years out, says Tim Hewett, director of the Sports Medicine Biodynamics Center at Cincinnati Children's Hospital.
Even without an obvious injury to trigger it, osteoarthritis of the knee affects 6 percent of adults older than 30 and 12.5 percent of those older than 60. This is an extremely common disease, says David Felson, a rheumatologist who specializes in osteoarthritis at the Boston University School of Medicine. Everyone is at risk. And treatment often involves complete replacement of the knee joint with a prosthetic device that, unfortunately, can also wear out over time.
Given those statistics, bioengineers and orthopedic surgeons are working to come up with new methods and materials for repairing worn-out ligaments and other joint components biologically--so that people in their 40s, 50s and even 60s can delay or avoid replacing their knees with metal or ceramic prostheses. In the clinic and in the lab, these researchers are exploring promising approaches to relieve pain, restore motion and encourage damaged joints to heal themselves.
Anatomy of an Injury
Every joint in the body is subject to damage, but the knee is the best studied, perhaps because it is often injured and reasonably easy to diagnose, says Stefan Lohmander, an orthopedist who specializes in osteoarthritis at Lund University Hospital in Sweden. Knee injuries send a large portion of the population limping to the physician with complaints of pain and reduced mobility in that joint--problems that Lohmander notes can make getting around, going shopping, seeing friends and participating in life difficult.
The ACL is not the only structure in the knee that can cave under pressure. A few people snap the posterior cruciate ligament, which crosses the ACL in the center of the joint (hence the term cruciate, for cross-shaped). Another structure that is subject to wear and tear is the articular cartilage, which covers the ends of the thighbone (femur) and shinbone (tibia) like a thick coat of paint. This slippery material (think of the white stuff on the end of a chicken drumstick) not only decreases the friction between the moving parts of the joint--allowing the bones to slide over one another--but serves as a cushion to evenly distribute the load that is introduced with each step. Lose this cartilage, and bone will rub on bone. Additional shock absorption in the knee comes from the meniscus, a washerlike wedge of cartilage that rests between the femur and the tibia. This structure--really a pair of structures, one on the outer half of the knee, one on the inner half--can also be damaged or torn by injury. When the meniscus is damaged, the articular cartilage underneath it can begin to wear; this combination of injuries can pave the way for osteoarthritis, with its associated inflammation and pain.