Spinal discs are the cushions between our vertebrae, carrying the load of the body. They're one thing you don't want to go wrong—acute and debilitating pain can result if they are damaged. And while many people recover from a slipped or herniated disc without surgery, the treatment options are fairly bleak for those who don't: The offending disc must be removed and the adjoining vertebrae fused together, or an artificial disc made of metal or plastic can be implanted in the space.
A new study has the potential to add a third, more appealing, option to the table. This week in the journal Proceedings of the National Academy of Sciences, researchers revealed their creation of a biological replica of a spinal disc.
A normal spinal disc is composed of an inner and outer ring (called the nucleus pulposus and the annulus fibrosus). The inner layer, or nucleus, is softer than the outer annulus. Lawrence Bonassar, an associate professor of biomechanical engineering at Cornell University and an author of the paper, says the two layers work a little like a tire filled with air. If pressure is applied to the interior air, it creates tension in the outer tire wall; in the same ways, pressure applied to the nucleus of a spinal disc creates tension in the annulus. The tension in the outer layer is what bears the load of the body throughout the day, as a tire's exterior bears the weight of a car.
The research group used the same structure to design the biological model. They used two different kinds of gel to make the model, with a material called alginate for the nucleus and tougher collagen for the annulus. "The idea is that there needs to be some mechanical integrity at the start," Bonnasar says.
Implanted in the respective layers were cells from the nucleus pulposus and annulus fibrosus, which the researchers hoped would grow within the material. (In this study, the cells came from sheep.) Bonnassar and his group let the models develop in the lab for two weeks, then implanted the lab-created discs in rats, replacing a normal disc. Six months later, the scientists found that the implants were working well. The cells had grown within the gel and developed into a working biological spinal disc, eliminating the need for the gel setup.
Robert Mauck, an assistant professor of bioengineering at the University of Pennsylvania, says that creating a disc with the ability to mature within the body is a remarkable success. "This paper... has shown that biological disc replacement is possible," he says. Biological disc replacement has long been a goal for scientists within the field. Though metal and plastic implants were approved for use in humans only as recently as 2004, the use of artificial materials has always had its disadvantages. Most importantly, metal and plastic components cannot fix themselves when damaged. So if they wear out or break, the patient may need additional surgeries. Biological discs have the potential to recover from small amounts of wear or damage. "It's not the one-way street that metal and plastic is," Mauck says.
Bonassar says that the first human trials of this technology could be five to 10 years out. That might seem like a long time (and some new technologies, especially in medicine, seem perpetually five years away). But for Bonassar, the pace of discovery has been amazing. "For us, this was a pipe dream five to 10 years ago," he says.
The key challenge to create a human version is scaling up the size of the implant. The discs used in the rats are just 4 millimeters in diameter. A human disc, by contrast, can have a diameter of around 3 or 4 centimeters—100 times as large. That won't be easy, Mauck says. "But everything starts at this level."
As Bonassar's team moves on to a human version, they'll also be using human cells to seed the gel rather than sheep cells. They can't predict whether our cells will respond differently or work as well. But, Bonassar says, using human cells could reduce the chance of the body rejecting the biological disc.
Though back pain is a very common ailment, the underlying causes are often vastly more complicated. The treatment would likely be available first to those with the most damaged discs, and, unfortunately, the amount of damage isn't always proportional to the amount of pain a patient is in. But for those eligible for treatment, this study has the potential to create a more compatible, longer lasting implant, offering relief from a chronic malady.