After I had a disc replacement in my lumbar spine, I found out they are making replacements just for the inner portion of the disc. That's probably what I really needed. How far are we from having that technology ready?

There's been a lot of talk about stem cell research. Many scientists are convinced that the results of aging-related degeneration of the spine can be altered with stem cells. For example, the breakdown of disc material between the vertebral bones could be repaired by regenerating discs with stem cells.

But until that's really available, other researchers continue to study alternative methods to replace the intervertebral discs. As you know, artificial disc replacement is now available. And just as you suspected, efforts are also underway to just replace the center portion of the disc called the nucleus pulposus.

Advanced technology now allows the surgeon to remove the damaged or degenerated nucleus pulposus and insert an implant in its place. The implant (prosthesis) is designed to bear the load through the spine at that level and prevent further collapse of the affected vertebral segments. The hope is that the remaining disc will be protected and remain strong over time.

Nucleus pulposus biotechnology has already gone through several generations of pulposus replacements. Replacement materials for the nucleus come in two basic types. There are mechanical metal or carbon devices that are inserted into the space left by removal of the pulposus. And there are injectable elastomers that fill up the nucleus pulposus cavity and harden after they have been squirted into place.

Studies so far have been encouraging. A large proportion of the small number of patients studied experienced pain reduction, decreased use of narcotic drugs for pain, and improved function. Complication rates have been low. And the discs have been shown to withstand 10 million cycles of fatigue when tested biomechanically using cadavers (vertebral segments preserved for study after death).

Most of the studies done so far have been completed by the companies that designed and developed the implants. Before these can be adopted for regular use, tests must advance from cadavers to animals to clinical trials with humans -- first by the disc replacement industry and then by outside, independent agents. The race is on to find a way to slow or reverse disc degeneration eliminating the need for nucleus (partial) or disc (complete) replacement.