But Fu argues that the current approach only does half the job, because the human ACL is actually made of two bundles: one that stabilizes the knee as it bends; the other that stabilizes the knee as it rotates. To restore normal anatomy--and range of motion--Fu says surgeons need to connect A to A and B to B.
Convincing his colleagues, however, has been an uphill battle--in part because a double-bundle replacement takes longer to perform. Fifteen years ago I would do an ACL in 30 minutes: bang, bang, bang, no problem, Fu notes. But looking back, I see that what I did before this double-bundle procedure was quite crude.
Odd as it might seem, Fu's curiosity about the function of the human knee is what led him to operate on Johnny the mandrill. Fu was probing the monkey's knee for insight into the anatomy and mobility of animal ACLs, information that he hopes will help surgeons see that different parts of the ligament serve different purposes--and that all parts need to be restored for the joint to function properly. Johnny's ACL, it turns out, has three bundles, which give his knee an even greater ability to rotate--a feature that Fu thinks might come in handy when the monkey is reaching for a tree branch to get to some ripe fruit or running from a bigger monkey.
Enter the Bioreactor
An ACL, once torn, cannot spontaneously mend itself--in part, Murray says, because there's no structure for the cells in the stumps to crawl into to build a new ligament. So some researchers are trying to come up with materials that can bridge that gap, allowing the ACL to regrow. Murray is betting on the blood cells called platelets. She has produced a platelet-rich gel that she squeezes into the space that remains when a torn ACL is sewn back together. The mixture forms a clot, which promotes the formation of scar tissue--the same way platelets help to heal a cut in the skin. In experiments with pigs--published in the Journal of Orthopaedic Research in January 2007--Murray reports robust healing: at four weeks the repair has regained 40 to 50 percent of the strength of an intact ACL. Preliminary results for later in the healing process show blood vessels growing into the new tissue and cells dividing and producing collagen--one of the main components of a ligament.
The results are particularly encouraging, considering the patients. The pigs feel good enough within a couple days that theyre up walking around and standing on their repairs, Murray says. Yet despite the lack of couch time and crutches, the pigs ligaments appear to heal. That makes us hopeful that when we get to clinical trials, where we have more control over the patients, the results will be even better, she says. Although with some of my teenage patients, Im not sure how much control I really have.
Greg Altman, president and CEO of Serica Technologies in Medford, Mass., is taking a similar approach, co-opting the body's natural repair capabilities to mend a wrecked knee. In studies with goats, instead of sewing the loose ends back together, Altman and his colleagues remove the damaged ACL entirely and replace it with a scaffold made of silk. This material is strong enough to support the growth of a new ligament, and the researchers have treated the fiber so that the body degrades it over time, leaving a strong new ACL in its place. The results of Altman's studies--presented at the annual meeting of the American Orthopaedic Society for Sports Medicine in July 2007--look good: the ACL regrows, the repaired joint is stable and, after 12 months, the researchers see no signs of arthritis. We observed a very calm, intact joint, says Altman, who is initiating a clinical study in Europe to test the safety and effectiveness of the grafts in patients.
Best of all, the silk graft needs no special storage or refrigeration, so a surgeon should be able to size a patient's joint and literally pull a replacement off the shelf. Avoiding the need for harvesting grafts from the patient's own knee was an important consideration in the development of the silk scaffold, says Altman, who tore his own ACL as an offensive lineman during his senior year at Tufts University and received a graft from his patellar tendon to take its place. My ACL was pretty good, he recalls, but my patellar tendon was killing me. The experience drove Altman to enroll in graduate school--initially so that he could play a fifth year of football but also so he could help other people recover better from ACL injury.