As any sports fan knows, a torn anterior cruciate ligament — or ACL — is a common and dreaded injury, sidelining thousands of athletes every season. Several countries planning to participate in the 2023 Women’s World Cup lost stars to torn ACLs.
But now a surprising new study may upend conventional treatment for the injury.
Almost all knee experts considered torn ACLs irreparable, with surgical reconstruction usually required to keep the knee functional. But the new research, which focused on 80 injured knees, found that many torn ACLs can apparently knit themselves back together and heal naturally — reducing the need for expensive and often painful surgery.
In the study published this month in the British Journal of Sports Medicine, 90 percent of 80 ruptured ACLs studied showed signs of healing and repair on scans about three months later. The patients had followed a newly developed protocol of bracing and physical therapy.
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The study “is fascinating,” said Lyle J. Micheli, a professor of orthopedic surgery at Harvard Medical School and director emeritus of the division of sports medicine at Boston Children’s Hospital. He has treated and studied injured knees but was not part of the study. “It has simply been assumed that in most cases after ACL injury, spontaneous healing in adults was not possible.”
The finding has sparked heated debate among sports medicine physicians, researchers, therapists and, especially, orthopedic surgeons, who conduct thousands of ACL surgeries annually. The finding that in many cases such surgeries may not be needed could redefine future treatment of ACL injuries and suggests that the knee is more resilient than previously believed.
Why ACL tears are so common
“I was 18” and had just torn an ACL playing soccer, said Stephanie Filbay, a senior research fellow at the University of Melbourne, who led the new study. “I still remember the surgeon saying, ‘Do you ever want to play sports again?’” and, if so, reconstruction of the ligament would be needed. She had the surgery.
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In 2018, the Centers for Disease Control and Prevention reported that 283,810 ACL surgeries took place between 2002 and 2014, with the numbers increasing 22 percent over that time. The total is almost certainly much higher now. Each of these surgeries costs, on average, $24,707, according to a 2019 study in Arthroscopy.
The ACL diagonally connects the femur (thigh bone) to the tibia (shin bone) inside the knee joint. It’s one of the knee’s most vital and fragile support tissues, and is easily overstretched. Hit the knee from the side, turn sharply, or leap and land knock-kneed and the ACL often pops, ripping down the middle or shearing away from the bone.
Athletes aren’t the only ones injured. People tear ACLs stumbling off a curb, over a puppy or sneezing too hard, as a friend of mine found out.
Hints that the ACL can heal
For decades, it was thought a torn ACL wouldn’t heal, because it lacks blood flow. Standard treatment, especially for active people, has been removing and replacing the flayed ACL with a ligament cobbled together from tissues harvested from elsewhere in the body or a corpse.
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Recovery afterward is slow and re-injuries are frequent. People often lose strength and balance in the affected leg because of lingering muscle atrophy and the severing of nerves during surgery. In many cases, the wounded knee becomes arthritic within a decade.
But there have been hints that ACLs sometimes defy medical orthodoxy and get better. Anatomical studies in the 1990s showed the ligament had blood flow. And surgeons occasionally saw unexpected, spontaneous healing when they opened or examined knees.
“From the clinic, we know healing sometimes occurs,” said Richard Frobell, an associate professor of orthopedics at Lund University in Sweden, who has conducted large-scale European studies of ACLs.
“But we did not know how frequent it was.”
Bracing and physical therapy to mend a torn ACL
Her reconstructed knee never quite the same, Stephanie Filbay began wondering a few years ago about the necessity of ACL surgery. So she and colleagues first reanalyzed results from a study overseen by Frobell of 120 ACL tears, some of which had been surgically repaired and some not.
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Two years after the injury, MRI scans suggested that about 30 percent of the ACLs in people who’d skipped surgery were healing.
In hopes of improving those odds, Filbay and her collaborators, particularly Mervyn Cross, an orthopedic surgeon and his physician son Tom Cross, developed a specialized, new bracing protocol, designed to keep the injured knee locked in a 90-degree angle for a month, so the ligament’s tattered ends could be held close together to facilitate healing.
While braced, patients completed extensive physical therapy exercises, such as repeatedly clenching their thigh muscles, to reduce atrophy.
Therapists relaxed the brace’s angle every few weeks and removed it after 12 weeks. The injured knees were scanned at the start and end of the three months.
Reviewing scans from the first 80 patients for the new study, Filbay and her colleagues noted healing — which they defined as MRI signals indicating tissue was bridging the torn edges — in 90 percent of the knees.
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None were fully healed, according to a standard, 4-point scale of ACL health, which scores a total tear of the tissue as a zero. But 50 percent now qualified as grade 1 injuries, showing some entwining of the torn ends, 40 percent achieved grade 2, and 10 percent were at grade 3, representing a nearly intact ACL.
The higher the score, the more satisfied people felt with their knee’s function and stability.
“I am 13 months post injury” from a torn ACL, said Meike van Haeringen, a Danish medical student in Melbourne who has torn both ACLs playing field hockey and is now part of the research team studying ACL healing.
In 2018, surgeons reconstructed her right knee, but more recently, her left knee was treated with the new bracing protocol.
“At this point, I can do everything I want to do with both legs,” she said, including playing elite field hockey again. But while her right leg, with its surgically constructed ACL, “feels like it’s at about 90 to 95 percent” after five years, her left leg, in which the ACL seems to be healing, “is at 95 to 100 percent already, after 13 months.”
Skepticism from the surgical community
Some experts scoff, though. “It does not seem plausible” that ACLs heal themselves, said Timothy Hewett, a professor of orthopedic surgery at the Marshall University Joan C. Edwards School of Medicine, who’s co-written hundreds of peer-reviewed articles, many about ACL injuries.
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The MRI scans that appear to show the raw ends of the torn ACLs rejoining are probably picking up scarring or being misread, Hewett said. “The term ‘healing’ should not be used by the authors in this context,” he said.
At a recent international conference of surgeons and knee researchers in Boston, Hewett continued, attendees discussed — and to some extent, dismissed — the new study. “One world-renowned researcher could be easily overheard stating, ‘That is B.S.,’” he said.
But other scientists are enthusiastic. “It’s a groundbreaking study,” said Daniel Belavy, a researcher at the Hochschule for Gesundheit (University of Health Sciences) in Bochum, Germany, who co-wrote a 2022 review of outcomes after ACL reconstruction or physical therapy. He wasn’t involved in the new research.
The scans and functional improvements in people’s knees indicate “the ACL can heal after a tear,” he said.
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The study and controversy echo what happened “with tears of the meniscus of the knee,” Micheli said. “Early on, the tendency was to either trim or repair meniscal tears. Subsequent studies suggested that there is a capacity for healing in many of these cases,” and surgery might be unnecessary or counterproductive.
Many questions remain about the new study and the possibility of ACL healing, though, including whether mended ACLs will be as sturdy as uninjured ligaments, and why some knees in Filbay’s study showed no signs of spontaneous repair.
Filbay hopes to mount a large randomized, controlled trial soon, comparing results after the bracing protocol with those after ACL surgery or no treatment. She’s also talking with international researchers and therapists, including in the United States, about using the bracing protocol in their practice and contributing data about results.
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Meanwhile, she’s been tracking outcomes from an additional 280 ACL patients who’ve completed the bracing. So far, their knees show comparable gains to those from the first 80, she said. Some of the patients are now years out from their injury and reportedly feel their unreconstructed ACL is almost as good as new.
On the other hand, some have opted for reconstructive surgery, because their knees wound up feeling unstable or sore over the long-term.
“That’s always an option,” Filbay said. “There’s no statute of limitations on surgery. You can decide to do it months or years later.”
That possibility could help make the new study’s effects on ACL treatment immediate and substantial, Belavy said. “If it was a friend or a loved one” who’d just torn an ACL, he said, “I’d say to them, ‘Hey, I’d personally wait to see what happens, consider this bracing, but I wouldn’t rush to surgery.’”
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