(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.
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