Targeting ACL injuries
by Nancy Fliesler
Martha Murray, MD, may have found a better fix for an injury suffered by some 100,000 Americans each year: tears to the anterior cruciate ligament, or ACL.
The ACL is one of the four main ligaments of the knee, giving the joint mechanical stability, and is notorious for not healing. Injuries typically occur while playing sports that involve jumping and pivoting, like soccer or basketball. Among teenage girls, who are increasingly playing competitive sports, ACL injuries are epidemic, with statistics showing that they are five times more likely than boys to tear the ligament (for some theories on why this is, see sidebar).
The current technique for treating ACL tears is surgical reconstruction, which involves complete removal of the torn ligament and replacement with a graft of tendon taken from elsewhere in the body. This painful operation allows patients to return to sports—after significant rehabilitation—but it can't fully restore knee function.
"Reconstruction has been able to restore stability of the knee," says Murray, an orthopaedic surgeon at Children's Hospital Boston. "But it doesn't restore the normal mechanics of the knee."
The ACL is an exquisitely complex ligament made up of 17 separate rope-like fascicles (bundles of fibers) organized into two main bundles that form a fan shape. A replacement graft will only restore one of the main bundles, so while the graft may give the knee the same or better strength, it doesn't distribute mechanical forces optimally.
This may explain why people with ACL injuries frequently develop osteoarthritis of the knee, or breakdown of cartilage in their knee joints, at an early age. One study found that 78 percent of patients had developed knee arthritis 14 years after their ACL tear, whether they had reconstruction or not.
"It worries me that my young patients' risk for arthritis is so high," says Murray. "If you're only 14, getting arthritis 14 years later is a big problem."
Before becoming a surgeon, Murray trained as a materials scientist, studying the physical, chemical and mechanical properties of materials; her undergraduate degree was in mechanical engineering. While in graduate school, Murray had a friend who tore his ACL, and she wondered why it couldn't be stitched back together, allowing him to keep his own ACL.
Until the 1970s, surgeons did try sewing the ligament ends back together. But even the best sutures consistently failed—in fact, the five-year failure rate was 90 percent. And so the operation, known as primary repair, fell out of favor.
As a PhD candidate 15 years ago, Murray wanted to do her thesis on ACL healing, but at the time, engineering programs had little interest in biomedicine. "I couldn't find a faculty advisor for the project," Murray says. "So I went to medical school."
In the medical world, there was interest, but no one believed it was possible to repair a torn ACL. "I remember scrubbing in as a resident and asking the surgeons this question," she says. "The universal answer was, 'primary repair doesn't work.'"
But Murray persisted. "I like to fix things," she says.
Knowing that ligaments should, in theory, heal easily—they're made up of fibroblast cells, which are workhorses in the body and easy to grow—she decided to do a "failure analysis," just as engineers do on collapsed buildings.
She and colleagues first examined what happens in a torn ACL at the microscopic level. To Murray's surprise, they found that the ligament tries valiantly to heal itself—cells migrate to the wound, growth factors are secreted and blood vessels appear to nourish the new tissue. But the ligament ends never join.
What was missing, Murray realized, was something to bridge the gap. In most torn ligaments, a blood clot forms and acts as a temporary scaffold or bridge. Cells migrate onto this bridge and begin to fuse the ligament ends together. But the ACL is located inside the knee joint, which is bathed in a fluid that dissolves the clot and washes the bridge away before it can form. Most other ligaments, like the knee's medial collateral ligament, are outside the joint and don't have this problem.
"Even though ACL cells are happy to participate in the repair process, there's no place for them to do it," Murray says. "Our big finding was recognizing that the cells are fine—they just need a bridge that they like."
Her team searched for a material that could fill in the gap and allow natural healing to occur. They first tried a collagen sponge, often used for other tissue repairs, but it didn't work. "Cells didn't like crawling into it," Murray says. A colleague suggested hydrogel, a material used for treating external wounds, ulcers and burns. It worked much better than the sponge, but it didn't do enough to stimulate wound healing. Adding growth factors helped a little, but one growth factor alone wasn't enough, and getting multiple growth factors into the gel was complicated and expensive.
"As a surgeon, I wanted a simple and safe solution," Murray says.
It was Murray's husband, Mike, a geneticist, who came up with the idea that eventually worked: using blood platelets for ACL repair.
"We were driving home from work together one day," says Murray, "and I was moaning about how I needed a lot of growth factors, but didn't know how to deliver them. My husband said, 'You should think about platelets.' The body's already optimized them for healing. They're growth-factor factories, and they're readily available."
Murray and colleagues isolated platelet-rich plasma from drawn blood, but unfortunately, it didn't stiffen enough to make a stable bridge. So the team went back and combined it with a collagen hydrogel, creating a firmer material that couldn't easily be dissolved by joint fluid.
In animal studies, they implanted the resulting gel into a torn ACL. Cells soon migrated into it, regenerated ligament tissue and made a permanent bridge, mending the tear. Murray will soon publish findings that show good healing, appropriate biomechanical function and a 50 percent return in strength six weeks after ACL injury. "No one's ever shown these tissues to heal," she says. "No one's ever seen fusion of the tissue edges."
With funding from the NIH, the Center for Minimally Invasive Technology and the National Football League, Murray hopes to develop a way to accomplish the ACL fix arthroscopically—via two small incisions, a camera to view the tear and a "gun" to squirt in the gel. Her group is also working on various enhancements to the gel in an effort to speed the healing process—since, for many young athletes, the sooner they can back to sports, the better.
In one set of laboratory experiments, published in the journal Molecular Therapeutics in August 2004, the team used gene therapy to try to stimulate natural repair mechanisms. Before implanting the gel, they "seeded" it with a virus carrying a growth-factor gene. The experiments showed that cells from a severed end of the ACL crawled into the gel, picked up the virus and its gene, and began making the growth factor. Other ACL cells responded by migrating into the gel and depositing collagen, one component of the healing response. The next step will be to take the technique into a living animal model and see if actual tissue fusion takes place.
In the future, Murray hopes to extend her regeneration technique to human patients and to other injuries like meniscus and rotator-cuff tears. But the true "holy grail" of orthopaedics is cartilage regeneration in anyone with joints severely damaged by osteoarthritis—not just those with ACL injuries.
Cartilage is a tissue with no blood supply and limited capacity to repair itself, and no one has figured out how to heal it. So Murray decided to do another failure analysis—this time of cartilage—and salvaged tissue from patients with arthritis undergoing total knee replacement.
Her team made a discovery much like that in the ACL. "The cartilage cells proliferated like crazy in the Petri dish," she says. "Even in bad osteoarthritis, cartilage has active cells."
As with the ACL, the first step is to find a suitable scaffolding material. Murray envisions another gel-like substance that could be placed arthroscopically. It would coat the pitted surface of broken-down cartilage to recreate the original smooth, nearly friction-free surface—like filling in potholes in a road.
"The cells are trying to find structure, but they just don't have it," Murray says. "They need a thing to move into; a place to live."
To learn more about supporting orthopaedic research at
Children's Hospital Boston,
contact Donna Richardson in the Children's Hospital Trust at (617) 355-2061 or firstname.lastname@example.org.