Not ready for a knee replacement? You might be able to fix your cartilage instead.

An X-ray view of a knee joint. The joint is highlighted in bright yellow, with circles radiating out from it.
(Illustration by Janet Mac for The Washington Post)

New techniques include a pellet made of coral and a hydrogel that can be pressed around the bone to help eliminate pain.

By Andrew Zaleski

Alicia McHatton was in Kailua-Kona, on Hawaii’s big island, when she received the phone call that would change her life.

It was April 2023, and for the previous eight months, McHatton, a neonatal nurse at the University of California at Davis Medical Center in Sacramento, had lived with incessant pain in her left knee. The popping and clicking she felt, resulting from a torn piece of cartilage lodged behind her kneecap, made it difficult to move around. At just 39 years old, she didn’t want to have a full knee replacement. Over the phone, her doctor proposed something else: How would she feel about coral?

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“I am a huge outdoor freak and I thought the idea of putting coral in my knee sounded amazing,” recalls McHatton.

The coral is part of a product called Agili-C, approved in 2022 by the U.S. Food and Drug Administration and developed by CartiHeal, an Israeli company. It is made from a calcium carbonate derived from the exoskeletons of coral that acts as a scaffold in the body to help regenerate bone and tissue. After drilling into her left femur bone, doctors inserted a small, cylindrical piece of Agili-C. The results nine months later?

“Zero,” she says. “I have zero pain.”

Alternative surgeries

Many of Americans like McHatton deal with chronic knee pain. The culprit nearly always is injured cartilage — the elastic, almost Jell-O-like substance that protects the joints and bones of the human body. More than 30 million Americans suffer from osteoarthritis, a wear-and-tear condition that occurs when cartilage withers away, a defect especially prevalent in the knee. The last-resort treatment for those with bum knees is replacement surgery. According to the Academy of Orthopaedic Surgeons, more than 600,000 Americans get new knees made of metal every year. Indeed, total knee replacement is now the most common inpatient surgery for people over 45.

But why replace a knee if just the cartilage can be repaired instead? That line of thinking has led to new techniques flipping the script on how to mend troublesome knees.

“We’re not going to stop arthritis,” says Cassandra Lee, chief of the division of sports medicine at UC Davis Medical Center, as well as the orthopedic surgeon who operated on McHatton. “But can we push that knee replacement way down the road? That is, I think, the ultimate goal.”

Two surgeons stand above an operating table. One holds two medical tools and looks down, her face illuminated by light from the table.
Orthopedic surgeon Cassandra Lee performs a procedure using a scaffold of inorganic calcium to treat cartilage defects in knees. (University of California at Davis)

Cartilage has no blood supply and therefore cannot regrow naturally. It doesn’t contain nerves either, which means people with significant damage don’t know about it in the absence of accompanying symptoms such as aching joints, a hobbled gait, or discomfort that doesn’t fade after ingesting ibuprofen. Cartilage is also remarkably tough, often lasting for decades even with unnoticed damage.

“It can support the weight of a car on the area of a coin. It’s an insanely strong material,” says Benjamin Wiley, a chemistry professor at Duke University.

Its unique nature, however, makes it tricky to repair. Defects usually begin as small divots like potholes on a road. Various methods exist for trying to fix these gaps, but each has its problems. Microfracture involves drilling into the adjacent bone in the base of the divot to release blood and fat to create a clot, but the durability or longevity of the knee following this procedure is questionable. Another option involves taking cartilage from an undamaged part of the knee and using it to repair a cartilage gap, a sort of robbing-Peter-to-pay-Paul operation.

New ways to fix cartilage

Wiley and colleague Ken Gall, a professor of mechanical engineering and materials science at Duke, are instead trying to re-create cartilage in the lab. Over the last several years they’ve developed a hydrogel composed of polyvinyl alcohol, a polymer often used in contact lenses, and cellulose fibers. Tests in a compression machine, Wiley says, demonstrated that the product could support 1,100 pounds of force, simulating five years of use. The hydrogel, which is pressed into the end of the femur bone, is being used in a Phase 1 human trial in Latin America. Wiley and Gall hope to get the green light to begin human trials in the United States sometime next year.

The coral option was presented to McHatton after a failed surgery called MACI, the acronym for a matrix-induced autologous chondrocyte implantation, a procedure approved by the Food and Drug Administration in 2016. It involves removing cartilage cells from the knee, regrowing them in a lab, and embedding them on a membrane of collagen that is then implanted. At $40,000, the surgery is expensive for those without insurance. It’s also intensive: McHatton was lying in bed for a month because she couldn’t put any weight on her left knee. It’s also generally recommended that people avoid activities like running for almost a year afterward. In some cases, the implanted cartilage doesn’t take — which is what happened to McHatton.

The coral plug, by contrast, took a little more than an hour to put in and cost her $200 out of pocket (insurance covered the rest). Over time, the coral will be fully absorbed by her bone, and in its place will be a gooey substance composed of stem cells from her femur bone that acts similarly to natural cartilage. Full recovery is supposed to take two years, but McHatton says she’s already back to bike-riding, hiking and 12-hour shifts at the hospital.

A close-up of two gloved hands holding a tiny white, cylindrical object with small holes on its surface.
Lee holds a piece of Agili-C during an operation. (UC Davis)

“I’m just so happy to be where I’m at, going from being in pain every single day with every single step, to forgetting most days that this was an issue,” she says.

Robert Brophy, who heads the Orthopedic Clinical Research Center at the Washington University School of Medicine in St. Louis, says new options coming online to treat knee damage are exciting and show potential. But he also says more data is needed to demonstrate that something like a coral plug or a hydrogel should take the place of current methods of reconstituting or replacing damaged cartilage.

“We’re attempting to find new and different solutions to a problem that is still challenging and not perfectly solved,” he says. “Nobody has the holy grail yet. That doesn’t mean something in the pipeline isn’t potentially going to get there.”

Relief from knee pain

For some people, new technology has proved to be a sweet relief. Joe Barron has undergone 12 knee surgeries since 1987: seven on the right knee, and five on the left. The 54-year-old played high school football, and his job as a firefighter in the Lake Tahoe area in California — where he piles and burns forest debris to reduce the risk of wildfires — has been especially hard on his knees. When he started seeing Lee at UC Davis five years ago, nearly all the cartilage in both knees had worn away. A MACI surgery didn’t fill in all the gaps, and while a graft of donor cartilage from a cadaver alleviated some of his pain, Barron still had to wear cumbersome knee braces daily to attain full mobility.

“I didn’t know what there was for me, and I didn’t want to have knee replacement,” he says. “I’d kind of given up hope.”

After consulting with Lee, he opted for one of the newest tools, something called the MISHA Knee System. It was cleared by the FDA last year, and cost him about $1,300 out of pocket. Composed of metal and plastic, it looks like a shock absorber in a car and operates similarly, working to take pressure off the knee joint with each step. In December, he had it implanted on the inside of his right knee.

“It’s always been my worst knee,” says Barron. “It’s never been better since I’ve had this procedure.”

It worked so well for him that he opted to have the same device installed on his left knee in April. And now he’s back in the forest. “Most knee surgeries — I’d be months on end recovering,” he says. “To be back out in the woods, pile burning — it was pretty incredible to feel that.”

About this story

Editing by Bronwen Latimer. Copy editing by Anne Kenderdine. Photo editing by Haley Hamblin. Design and development by Audrey Valbuena. Design editing and art direction by Betty Chavarria. Project development by Evan Bretos and Hope Corrigan. Project editing by Marian Chia-Ming Liu.