I have several lumbar discs that are causing me problems. Eventually they will have to come out or I’ll need a spinal fusion. I’m anxious for the new central disc replacements to be perfected. That way, maybe I can escape the bad results my father had with his fusion operation. The surgeon told me this isn’t really ready for general use — too many complications. What kind of problems are they encountering?

Advanced technology now allows the surgeon to remove the damaged or degenerated nucleus pulposus and insert an implant in its place. Usually the entire disc is removed and replaced. But efforts to replace just the central nucleus of the disc are being investigated.

They are having some success but there are two commonly encountered complications: extrusion and subsidence. Extrusion can occur when a liquid gel is used to create the new nucleus. Once injected, the materials can shoot past or ooze out of the inner space for the nucleus.

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 extrusion 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.

The second potential problem mentioned (subsidence) refers to the fact that the new nucleus sinks down into the annulus (outer fibrous covering around the nucleus). Then the whole disc can sink into the endplates. Endplates are fibrous cartilage located between the disc and the vertebral bones. 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.

With each new type of material developed, there are individual problems that crop up. It’s possible that by the time a material is ready for market, stem cell research or gene therapy will have advanced enough to prevent disc problems from happening in the first place. Tissue reengineering may ultimately replace disc replacement technology.