Disc nucleus replacement as an alternative to artificial disc replacement (Research article)

Traditionally, the standard of care for surgically treating low back pain from degenerative disc disease has been spine fusion surgery. Starting in 2004, an alternative to many types of spine fusion was introduced into the US—artificial disc replacement surgery, and many alternative forms of artificial discs are also in development and various stages of clinical trials. Now, there is another type of technology in development: disc nucleus replacement surgery. This type of surgery involves replacing just the inner portion of the disc, and leaving the outer layers of the disc intact, whereas artificial disc replacement surgery involves replacing the entire intervertebral disc.

Anatomy and disc nucleus replacement

The intervertebral discs that reside between each vertebra from the neck down into the lower back and tailbone act as shock absorbers between the vertebrae. The disc itself consists of a tough outer layer called the annulus, and soft inner material, called the nucleus. The soft nucleus absorbs the majority of the shock as the body moves, keeping the spine supple and supported. As one ages, both the annulus and the nucleus lose some of their cushioning ability, and a greater portion of the pressure is borne by the outside of the disc, the annulus.

Replacing the entire disc through artificial disc replacement surgery is an extensive and technically demanding operation which can involve removing the end plates (the cartilage between the vertebral bone and the disc), a large part of the outer portion of the disc, (the annulus), and the complete inner portion of the disc (the nucleus). Typically, the original disc is then replaced with an artificial disc: for example, a disc prosthesis comprised of a metal tray on the top and bottom, with a plastic piece on the inside to serve as a replacement nucleus. Artificial disc surgery is technically difficult for the surgeon(s), and involves some possible major risks to the patient (such as moving a number of large blood vessels in order to access the damaged disc).

An alternative to artificial discs currently in development in the US is called disc nucleus replacement. This approach replaces only the cushioning material within the disc nucleus, leaving the annulus (outer portion of the disc), cartilage, and related material intact. The goal of this alternative is to enable doctors to reestablish the original height of the disc and reestablish an even distribution of pressure across the vertebrae, without dismantling the vertebral structure. See Figure 1 and Figure 2. A variety of technologies to replace just the discs are currently being investigated in the laboratory, and some have been implanted in countries outside the US. Various materials are tested, each with specific qualities and construction that have the potential to mimic the qualities of the natural disc nucleus.

Potential benefits of disc nucleus replacement

One theoretical advantage to disc nucleus replacement is that it may be able to be used to prevent the development of more serious back injuries in the future. For example, it is hypothesized that a doctor could use disc nucleus replacement to prevent the collapse of a herniated disc, potentially sparing the patient progressively increasing pain. A prosthetic disc nucleus could also decrease the chances of additional disc herniation or recurrence of a disc herniation.

Another theoretical advantage of the nucleus replacement is that it can be inserted though the same approach as a diskectomy (from the back), and many spine surgeons have more experience with this type of approach. Most of the current nucleus replacement technologies allow a smaller incision than artificial disc replacement. In addition, the total disk replacement surgery requires an anterior incision over the abdomen, usually performed by a vascular surgeon or general surgeon working in conjunction with the spinal surgeon as this approach requires moving the large blood vessels in the front of the spine. Ideally, this would translate into less scar tissue for the patient and less risk involved by avoiding the large blood vessels.

In a case where additional surgery is needed, or where nucleus replacement has not alleviated the patient’s back pain or other symptoms, less scar tissue allows for much easier access to the same area. In addition, replacing only the disc nucleus retains artificial disc replacement as a potential option should problems continue. It is possible that disc nucleus replacement could potentially become a percutaneous out-patient procedure.

Replacement of just the nucleus of the disc is not appropriate for all cases, especially for patients with severe disc degeneration, where the disc has lost a lot of its natural height and the annulus has been affected. However, it is thought that replacing the disc nucleus could become an option earlier in the course of degenerative disc disease in order to minimize the need for the patient to eventually have a full artificial disc replacement or a spine fusion, which are both more extensive procedures.

Potential risks of disc nucleus replacement

Currently, none of the nucleus replacement technologies are approved for commercial use in the United States. In fact, at the time of this article, none of the disc nucleus technologies have been approved by the FDA for investigative trials in humans in the US.

Overall, all of the potential benefits of disc nucleus replacement are still theoretical and there is much more that needs to be learned about this procedure. Disc nucleus replacement is still in the early stages of investigation, and the technology needs to be proven clinically safe and effective before it will be available for patients.

By: John Sherman, MD
June 14, 2005