Artificial Cervical Discs

Fig. 1: Prodisc-C™ cervical
artificial disc
(larger view)

Fig. 2: PCM™ (Porous Coated Motion) Cervical Disk Replacement
(larger view)

Fig. 3: Bryan™ Cervical
Disk Replacement
(larger view)

Fig. 4: Medtronic Sofamor Danek Prestige™ Cervical Disk Replacement
(larger view)

Artificial cervical disc technologies are being developed in an effort to treat symptomatic degenerative disc disease more effectively. The main goal of this type of technology is to maintain spinal motion following anterior discectomy, to reduce the incidence of degeneration of adjacent disc levels of the spine (adjacent-segment disease), and to permit more rapid return to normal activity.

Theoretically, the artificial disc is designed to take the place of the real intervertebral disc and be placed between two vertebral bodies where the disc has been surgically removed in order to decompress the spinal cord or nerve root in the neck. Ideally the artificial disc acts like a normal disc, providing motion while acting as a shock absorber in the spine (unlike a fusion, which eliminates both motion and shock absorption in the fused segment of the spine).

There are a variety of artificial discs that are in various stages of the process of seeking approval by the United States Food and Drug Administration (FDA) through clinical trials.All technologies share similar goals of replacing the original disc, but differ in their designs and materials.

Types of cervical artificial discs
At the time of this article, there are a number of artificial cervical discs currently in development in the US.

• One general artificial disc design comprises metal endplates with an intervening low friction polymer (plastic) to allow for motion between the polished metal surface and the polymer. Currently two different polymers are in clinical evaluation:
  

  • Metal and Polyethylene devices: These are the same materials that are used for total hip and knee arthroplasties, and have been shown to have no ill effects on the body in long term studies. Examples of this type of technology include:

    • Prodisc-C™ by Synthes Spine Solutions (see figure 1), currently in a U.S. clinical trials.

    • PCM cervical artificial disc by Cervitech (see figure 2), not yet in U.S. clinical trials.
      

  • Metal and Polyurethane device: This comprises application of a softer polymer intended to provide not only motion, but shock absorption closer to the human cervical disc:

    • Bryan™ Cervical Disc Prosthesis by Medtronic Sofamor Danek (see figure 3), currently in a U.S. clinical trial.
      
      

• Another disc replacement design uses a metal-on-metal joint to mimic normal motion at the cervical disc space. Examples of this type of technology include:
  

  • Prestige™ cervical disc by Medtronic Sofamor Danek (see figure 4), currently in U.S. clinical trials.

As indicated above, several of these designs are currently involved in clinical trials, which means that a prospective, randomized study is being conducted. A patient who is determined by a physician to be a surgical candidate, and who fits the strict enrollment criteria, may enroll in such a trial. However, the patient is randomized to receive either a traditional surgery (such as an anterior cervical discectomy and fusion with a cadaveric bone graft and plate) or an artificial disc replacement. In such studies, neither the patient nor the surgeon has any influence over which procedure the patient is assigned. The randomization process minimizes bias and helps ensure accurate evaluation of the device in comparison to the currently acceptable surgical treatment.

The outcomes of these trials will be collected and analyzed by the FDA over the next couple years. Outside the United States, the predecessor of the Prestige™ was first used in Bristol, England over ten years ago. Following a successful European clinical trial, the Bryan™ device was approved for use outside the U.S. in 2002. Similar disc replacement concepts have been used in the lumbar spine for more than 10 years in Europe and have been studied in clinical trials over the past two years in the United States. The strengths and limitations of all these technologies will be better understood with the passage of time and further study.

• Article written By:
• Jeffrey A. Goldstein, MD
• Mark Mikles, MD

April 4, 2007

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