First comes the pop; then, the pain.
There is little doubt when an anterior cruciate ligament tears, but its inability to mend remains a puzzle, one that Martha Murray is trying to solve.
The ACL forms a fan-shaped bridge vertically spanning the knee’s interior and linking the long leg bones above and below the joint. Multiple crisscrossed fibers help stabilize the knee, fine-tuning its motion from within. Twisting and turning—the mainstays of slopestyle skiers, pickup basketball players, and tennis pros—depend on the smooth operation of this ligament.
Left alone, a torn ACL cannot knit itself back together, a fact that Murray, an HMS associate professor of orthopedic surgery at Boston Children’s Hospital, found compelling and curious. After all, ligaments outside the knee can heal. But unlike those neighbors, the ACL is bathed in synovial fluid. The lubricant helps the knee move smoothly, but it also dissolves the blood clots and scar tissue that a ruptured ACL would depend upon to rejoin its two torn ends.
Since the 1970s, surgical reconstruction of the ACL has been the standard of care for such traumas. Instead of sewing the pieces back together, surgeons insert a graft—perhaps part of a hamstring or a patellar tendon—to reconnect the back of the femur with the front of the tibia. After rehabilitation, patients resume activity, including sports at the Olympic level.
Most people recover well, but not the ones who worry Murray.
The peak age for young women to tear their ACLs is from 15 to 19 years old. But researchers say as many as 30 percent of female soccer players under age 20 who return to the game after an ACL reconstruction will tear their grafts, compared with 2 percent of adults.
People of all ages face another problem: After an ACL tear, the risk of developing osteoarthritis in the same knee within 15 years is as high as 75 percent. No one knows why.
Murray believes helping the ACL heal is the answer. After two decades of animal research, she has developed a collagen scaffold that helps unite the two stumps of a torn ACL. Infused with ligament proteins and filled with blood, the tubular scaffold is inserted between the torn ends, stabilizing a biologic bridge where healing can take place. She hopes to soon gain approval for a clinical trial in patients.
“My grandmother used to say, ‘Everything is mendable,’ ” says Murray. “I’m just trying to mess with Mother Nature as little as possible, and provide only what is missing—a stable bridge—and then let the knee work as its own bioreactor.”