Squishy, doughnut-shaped disks can make the difference between a pain-free, active lifestyle or years of back discomfort. When the disks that normally cushion each vertebra in the spine start to degenerate, due to aging or injury, nerves can be pinched and movement impeded. But degenerating disks may soon be replaceable with bioengineered disk implants grown in the laboratory. A research team has implanted living, biologically based disks into rats' spines and found that they allow for as much movement as native, healthy disks.
"This is, in my opinion, in a whole different league than tissues that have been engineered before," says University of Pennsylvania orthopedic bioengineer Robert Mauck, who was not involved in the study. "This is essentially opening the door for replacement of a tissue that's central to humans walking."
The current course of treatment for degenerative disks includes painkillers, physical therapy, and steroid injections to ease inflammation. As a last resort, patients can undergo surgery that fuses together two vertebrae, removing the need for a disk between them but also limiting the flexibility of the back. Within the past 5 years, artificial disks have also become an option. Current disk implants are made of metal or plastic, however, and have limitations. They don't provide a full range of spinal movement, and they can wear out as they rub against vertebrae.
Hoping to eliminate those pitfalls, Lawrence Bonassar, a biomedical engineer at Cornell University, and his team created an artificial scaffold shaped like a disk, with collagen on the outside to provide structural stability and a gel in the center. Then they added two types of living disk cells taken from a rat's spine: one from the outer edges of a disk, which they added to the collagen, and another type found in the center of disks, which they seeded into the gel. For 2 weeks, they let the cells grow around the scaffold, creating a living disk and taking over both parts of the artificial scaffold. Then they surgically replaced a spinal disk in a rat's tail with the new implant.
The bioengineered disk, the researchers found, provided as much cushioning space between spinal vertebrae as a typical disk does. Moreover, cells from the implant didn't just populate the space within the scaffold—they started growing outward into the rest of the spine, as the cells in a normal disk do.
"This is the first indication that this type of tissue can be made outside the body and placed back into the body with some level of function," Bonassar says. "Not only are the cells making new tissue, but they're integrating into the surrounding tissue," he says. "We actually saw collagen fibers run between the new disk and neighboring vertebrae."
Over the 6-month study period, the implanted disk showed no signs of wear, the team reports today in the Proceedings of the National Academy of Sciences. In fact, the disk began to function even better, in terms of the amount of cushioning it provided between vertebrae, as it filled out with living cells and became integrated into the spine. The researchers confirmed that the implant allowed the spine to bear as much weight and move as freely as a normal disk.
Although the bioengineered disks haven't made it to human testing yet, Mauck says the new study is a step in that direction.
Translating the engineering from a rat to a human is no small feat, however. "The next question is one of scale. We've seen that this technology can perform remarkably, but the rat tail is quite small," Mauck says. "Expanding the technology up to a human-size disk is the next step."
In addition, the research was performed in mice lacking complete immune systems. A challenge with any human implant is learning how to prevent the immune system from attacking the new organ. However, no blood vessels innervate spinal disks, and this reduces the chances of the immune system attacking the implants, Bonassar says. Further research will have to analyze how humans react to the materials and cells used in the disks.
One way to get around these issues, Bonassar says, is to craft the disk out of the patient's own cells. "Theoretically, we could, in the future, take a person's own cells, or their own stem cells, and grow them into a personalized disk for that person."