Replace or Regenerate Degenerated Disc Material: The Race is On
Stem cell research continues to make the front-page news from time to time. Many scientists are convinced that the results of aging-related degeneration of the spine can be altered. 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. In this report, efforts to just replace the center portion of the disc called the nucleus pulposus are the focus.

Intervertebral discs separate the vertebrae (bones of the spine). The discs are made of connective tissue. Connective tissue is the material that holds the living cells of the body together. Most connective tissue is made of fibers of a material called collagen. These fibers help the disc withstand tension and pressure.

A disc is made of two parts. The center, called the nucleus, is spongy. It provides most of the disc's ability to absorb shock. The nucleus is held in place by the annulus, a series of strong ligament rings surrounding it. Ligaments are connective tissues that attach bones to other bones.

Healthy discs work like shock absorbers to cushion the spine. They protect the spine against the daily pull of gravity. They also protect it during strenuous activities that put strong force on the spine, such as jumping, running, and lifting.

Artificial disc replacements are already on the market and have been in use in the United States since 2004. These replace the entire disc unit (both the nucleus and the annulus). But there's a need for less invasive treatment. That's where just replacing the center portion (the nucleus pulposus) comes in. 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.

It's these biodegradable elastomer type nucleus pulposus replacements that are the subject of this article. Surgeons from the University of Pittsburgh present an update on what's happening in the world of nucleus pulposus replacement technology. The elastomer type provide a more natural substance that mimics the body's own disc material. It provides a scaffold or frame for the body to build (rebuild) its own nucleus.

It sounds like a simple procedure but there are many, many factors to take into consideration. That's when we get a much better appreciation for all that the native disc actually accomplishes on a day-to-day basis. For example, the disc must be able to handle compression, bending, rotation, and vibrations -- thousands, if not millions, of these forces over time.

And like natural discs, the replacement materials must perform all these functions while still preventing degeneration of the rest of the disc. The materials must not transfer load to the annulus (outer covering). When injected, the materials must not shoot past or ooze out of the inner space for the nucleus.

This complication is called extrusion. The tiny hole made in order to inject the material into the disc center leaves an opening for a quick exit of the same material. The hole must be closed to keep this from happening. Finding the right material to make the patch, seal the hole, or develop an effective suture to close the open edges is part of today's research efforts.

Another potential problem is subsidence -- the new nucleus sinks down into the annulus (outer covering). Researchers are tinkering with the shape and size of the implant to help stop this problem. By measuring the stress or load placed on the endplates, they have been able to see higher stress at the center of the endplate. This information has guided them in reshaping the implant.

Right now scientists are still working with various liquids that flow smoothly and then self-cure (harden after injection) into one piece. This process is called polymerization. The process requires careful temperature control to move the liquid at body temperature and shift from a liquid to a gel without clumping or swelling while still conforming to the shape and size of each patient's disc space.

Different materials are being tested. For example, silk and elastin proteins are being tested as an implantable liquid that cross-links with collagen within 90 seconds of injection. The advantage of that feature is that it restores the disc strength and biomechanics right away. The next test will be to see if it can hold up under the many, many fatigue cycles placed on it by the average adult.

In other laboratories, a different approach is being investigated. Preshrunk forms are inserted into the disc center and rehydrated as the implant sucks up nearby fluid, swelling to fill the cavity. The fully hydrated implant transfers load from the disc to the vertebral endplates. The endplates are fibrous cartilage structures between the disc and the vertebral bone.

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 (expulsion and subsidence) 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.
Steven Leckie and James Kang, MD. Recent Advances in Nucleus Pulposus Replacement Technology. In Current Orthopaedic Practice. May/June 2009. Vol. 20. No. 3. Pp. 222-226.