Break your arm, and it will probably heal—but lose a big chunk of your chin, and that’s a different story. Now, researchers have made a significant advance toward replacing large pieces of bone in humans by growing a pig’s own cells into bone in the lab and then using it to restore a piece of missing jaw.
“Being able to translate into a large, functional animal model is an important step for this technology,” says Scott Hollister, a biomedical engineer at the University of Michigan, Ann Arbor, who was not involved in the study. But he says the new technique, now inching toward testing in humans, will still have to prove its advantages over other experimental bone regeneration approaches.
The face is an especially tricky testing ground for bone reconstruction. It takes precision to match our subtle features and symmetry, and bones in the jaw must be strong enough to withstand the force of chewing. Today, surgeons often repair facial damage by grafting a piece of bone from somewhere else, such as the patient’s pelvis or rib. But that procedure, called an autograft, creates a whole new bone injury that can be painful and slow to heal. The extracted bone is also tough to carve into the desired shape, and surgeons sometimes struggle to find a piece big enough to replace what’s missing.
Instead, researchers have been experimenting with ways to grow new facial bone, many of them based on synthetic scaffolds—porous structures shaped like the missing region that can be grafted onto existing bone to encourage the body to rebuild. Some experiments have implanted these empty scaffolds on their own and let new bone grow into them. Others have loaded scaffolds up with growth factors or cells that can transform into bone, or both. Several such techniques have made it to large animal testing—a key step in proving they’re worth trying in humans. But these approaches have involved adding precursor cells or growth factors to the scaffold right before it’s implanted and allowing the bone to develop inside the body.
Growing new bone in the scaffold for weeks before implanting it could have advantages, says Gordana Vunjak-Novakovic, a biomedical engineer at Columbia University and senior author on the new paper. It would give growth in the scaffold a kind of head start, so that the synthesis of bone matrix is more likely to keep up with the body’s natural process of degrading bone. And if cells are already part of a growing tissue, rather than a smattering of precursor cells recently injected into the scaffold, she says, they may be less likely to disperse out of the scaffold (and go to waste) after they’re grafted.
In the new study, Vunjak-Novakovic and her colleagues tested that idea in 14 pigs with facial bone damage. They cut a hammer-shaped chunk roughly 6 centimeters long out of the back of each pig’s jawbone—the vertical piece that extends up toward the ear and bears the most weight when an animal chomps down. To replace those chunks, the team first crafted scaffolds out of cow leg bone by removing all the living cells to leave behind a porous matrix of minerals and proteins. These were carved with a tiny computer-guided pin that shaved away material to match 3D images of each pig’s jaw, created from a computed tomography scan.
Next, the researchers sucked some fat from the pigs’ backs and isolated precursor cells that can become bone, known as mesenchymal stem cells. Forcing these cells—and culture medium to nourish them—into the tiny spaces of the scaffold is no easy task. By simply soaking the scaffold in the medium, “you’d get something that looks like M&M candy,” Vunjak-Novakovic says: Healthy bone cells might coat the outside, but they wouldn't penetrate to the center. So the team designed a silicon chamber cut to fit snugly around each scaffold so that the injected medium would be forced inside it instead of diverted around it. The cells were permeated with culture medium as they grew in this bioreactor for 3 weeks.
Six pigs received a personalized graft seeded with their own cells. To test the specific benefit of these cells, six other pigs received a custom-carved scaffold with no cells growing inside. The remaining two were left to heal on their own. Over 6 months, pigs in all three groups regrew a portion of the missing jaw, but regrowth was more complete in the seeded grafts, the researchers report online today in Science Translational Medicine. In tests of mechanical strength, only these grafts could withstand the same stress as the original bone, and only they reached and maintained the height of the original bone. The unseeded scaffolds, meanwhile, actually shrank slightly over time, which suggests the body was degrading bone matrix faster than it could be formed, Vunjak-Novakovic says. “This is convincing evidence that you do need provisional bone to get new bone.”
The use of a bioreactor to grow bone before grafting holds definite advantages, says Dietmar Hutmacher, a biomedical engineer at Queensland University of Technology in Brisbane, Australia, who was not involved in the study. But there is still debate over the precise recipe for a bone-stimulating scaffold. He suspects, for example, that adding growth factors known as bone morphogenetic proteins—a step this group avoided out of concerns for uncontrolled bone growth—would make for a stronger and more complete bone.
The group will also need to show that the technique is an improvement over other approaches, including the currently used autografts, Hollister notes. “There will be lots of questions whether, for example, growing [bone] in a bioreactor gives you an advantage.” Hollister and his collaborators have managed to regrow pig facial bone by adding precursor cells to a scaffold right when it’s implanted, with no cultivation ahead of time.
Vunjak-Novakovic, who founded a company called EpiBone in Brooklyn, New York, to commercialize the technology, is now planning to gather more evidence that the procedure is safe to try in humans. She hopes it will be in human trials in roughly 3 years.