Artificial cervical discs
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.
- Flexicore™ cervical disc replacement
by SpineCore (see figure
5),
not yet 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.
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