Someday soon, replacements may be available for severely damaged spinal discs. They're already being tested in rats.

Spinal discs are gel-filled sacs that sit between the vertebrae (bones) of the spinal column. Their main function is as a cushion or shock absorber. Sometimes, a weakness in a disc or trauma from adjoining vertebrae (often as the result of physical activity) ruptures the lining of the disc and allows the gelatinous material inside to poke through the outer fibrous covering. This is called a slipped, herniated or ruptured disc or a spinal hernia. It causes two problems. The first is terrible pain, which comes from pressure the bulging disc exerts on the surrounding spinal nerves. The second is that the disc is no longer performing its role as shock absorber as well as it used to.

Surgery is only performed on about 10% of all spinal hernias.

Initial treatment of a spinal hernia is almost always conservative. This starts with rest and may also include heat, physical therapy and medication. Surgery is only performed on about 10% of all spinal hernias. When surgery is performed, there are several types. Most of them try to relieve the pressure by creating more space for the disc to bulge in or by removing some of the bulging material. In extreme cases, the entire disc may be removed.

While most people with spinal hernias recover from their symptoms, the injured disc itself cannot be repaired, though it may recede back into its lining, much as toothpaste can sometimes recede back into its tube. Spinal discs don't have a blood supply, so healing in the conventional sense is out of the question.

For the worst cases, help may be on the way, thanks to the collaboration of a neurosurgeon and a biomedical engineer.

Previous research on replacement discs has mostly been performed in Europe. In October of 2004, the FDA approved a metal and plastic artificial disc for use in the US; this has seen limited use. To build a better disc, the researchers started with cells from human spinal discs that had been surgically removed. These cells were grown in culture and used to coat a bioengineered scaffold. The entire complex was then molded into a disc-like shape. The resulting artificial discs have been implanted as replacement discs in the spines of rats and have so far functioned well. They certainly sound softer than the metal and plastic versions.

Of course, a rat can't give a very detailed description about how its back is feeling. That will have to wait for clinical trials in humans, studies the researchers hope to begin soon.

The bioengineered spinal discs were created by Roger Härtl, a neurosurgeon at New York-Presbyterian Hospital/Weill Cornell Medical Center and assistant professor of neurological surgery at Weill Cornell Medical College, and Lawrence Bonassar, an associate professor in the departments of biomedical engineering and mechanical and aerospace engineering at Cornell University.