Of all the parts of the human body, the little discs between the bones in the spine represent one of the more remarkable feats of nature’s design.

Consisting of tough, rubbery rings of collagen with jellylike centers, they compress with every step we take. They twist. They flex. Over a lifetime of wear and tear, they replenish their supportive matrix of collagen despite having no internal blood supply.

“It is really a marvel of engineering,” said Robert L. Mauck, a biomedical engineer at the University of Pennsylvania.

But nature’s design, though rugged, does not last forever. With age, nearly everyone’s spinal discs degenerate to a degree. Lower-back pain was the world’s leading cause of disability in 2017, said the Institute for Health Metrics and Evaluation at the University of Washington. In severe cases, patients have vertebrae fused together, but they lose flexibility and one-third come back for repeat surgery.

A team at Penn has published a promising study of what they hope is a better option: replacement discs made from a combination of synthetic materials and living cells.

So far, the researchers have implanted their discs only in rats and goats, but they appear to behave much like the real thing, said Penn Medicine orthopedic surgeon Harvey E. Smith, the clinical leader. “It’s a living structure,” Smith said.

It has taken more than a dozen years for the team to get to this point, and a lot of varied expertise. In addition to engineers and orthopedists, the group includes veterinarian Thomas P. Schaer, a research director at Penn’s New Bolton Center, who joins Smith in implanting the experimental discs in the necks of goats.

Some of their funding came from the U.S. Department of Veterans Affairs because members of the military suffer especially high rates of disc degeneration and back pain, Mauck said.

Bus and truck drivers also may be at higher risk of disc degeneration, as are cigarette smokers — likely because smoking harms blood vessels, and therefore discs.

When a disc ruptures or collapses, pain can arise in several ways, though the cause is not always clear. Nearby nerves can be compressed, causing pain to shoot down the leg — a condition called sciatica. If the failed disc is in the neck, similar pain can travel down the arm.

Less clear is how disc failure causes pain in the back itself, said Smith, an associate professor at Penn’s Perelman School of Medicine. Some of the pain may be a warning signal: the body’s attempt to limit motion so that an injured back can heal. Inflammatory chemicals and swelling can play a role.

In some cases of disc rupture or herniation, back pain lasts for years. But in others, it goes away after a few months, though their MRIs may look similar to those patients who remain in great pain. And plenty of older people with degenerating discs never experience the pain at all.

“I could have someone who’s 60, and they have bone on bone, and they don’t have any pain,” Smith said.

For those who suffer disc-related pain, one option is fusion, in which the surgeon removes the damaged disc and inserts a piece of bone in between the vertebrae. That decompresses the nerves but the joint can no longer move, so there may be more stress on the vertebrae above and below it.

Another option is a replacement disc made of metal and plastic. Those allow more motion than spinal fusion, but they wear out, eventually shedding small particles of debris — which can trigger immune reactions. And unlike other joints that surgeons replace with metal-and-plastic devices, such as the hip, the spine is a less forgiving environment, said Lawrence J. Bonassar, a Cornell University biomedical engineering professor. The fluid-filled hip joint is better able to clear out some of the bits of plastic and metal. “In the spine, there’s no place for all these particles to go,” he said.

That is why the Penn team uses biodegradable materials to make its replacement discs: a sugarlike synthetic gel to mimic the collagen-based jelly inside a natural disc, and layers of a high-tech polymer to mimic the stiffer type of collagen that makes up the outer ring of a real disc. The polymer is made through a process called electrospinning, using electricity to draw out threads of fiber from a nozzle.

Also, as in nature’s design, the Penn team’s replacement discs are “seeded” with living stem cells, which produce collagen over time to replace the synthetic materials as they degrade. “In the long run, it’s the cells we’re putting in that are going to maintain this living tissue,” Mauck said.

The results have impressed other researchers working on the same challenge. “Clearly, there is more work to do, but obviously these results are incredibly encouraging,” said Bonassar, whose team has reported success with implanting a different type of replacement disc in dogs.

The Penn team’s discs in goat necks performed well for the two months that their progress was followed. The animals retained their normal range of motion and the discs resisted compression much like natural discs, the authors reported.

Next steps include implanting discs into more goats, which will be followed for two years, and implanting discs in the lower back of rabbits.

It will be years before Smith can implant replacement discs into his human patients. If it works, people would achieve relief from pain while maintaining flexibility. And maybe the surgeon and his colleagues will get to the core of why some people suffer pain and others do not.

“Back pain is sort of like the common cold,” Smith said. “Everyone has it. We know how to treat it. But we’re still trying to get down to the root causes. In terms of the fundamental science, we have a long way to go.”