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Spinal Disk Arthroplasty
Mark S. Gerber, MD
Robert M. Galler, DO
Stephen M. Papadopoulos, MD
Division of Neurological Surgery, Barrow
Neurological Institute, St. Joseph's Hospital and Medical Center,
Phoenix, Arizona
The cervical and lumbar portions of the spine are frequently fused
to treat instability and degenerative disease of the spine.
As a result of alterations in the natural biomechanics of the
spine, such patients have a high risk of developing progressive
spondylotic disease at levels adjacent to the levels fused.
To decrease the risk of further degeneration, ongoing research has
sought to create a functional prosthetic disk that preserves motion
at the diseased level thereby resolving adjacent level arthrosis
physiologically. This article reviews the history and
development of the artificial disk and its status in human
trials.
Key Words: artificial disk,
adjacent segment disease, spinal arthroplasty
During the last 50 years, the treatment of symptomatic degenerative
spinal disease has evolved significantly. Despite advances in
techniques for cervical and lumbar surgery, opinions about the
overall long-term success of these operations continue to
differ. Hilibrand et al.[12] found that 19% of patients had
adjacent segment disease 10 years after undergoing anterior
cervical fusion. Hilibrand et al.[12,13] reported a 3% per
year rate of symptomatic disk degeneration at levels adjacent to
cervical fusion. Others have found that 7 to 15% of patients need a
reoperation at adjacent levels.[1,5,10,15] Considering the
overall excellent fusion rates and outcomes associated with
anterior cervical fusion, adjacent segment disease is likely to be
an ongoing problem for patients and surgeons.
Lumbar fusion effectively relieves pain by
eliminating motion due to segmental instability, by restoring disk
and foraminal height, and by halting the progression of
degeneration at the treated level. However, evidence suggests
that these procedures also may accelerate the development of
adjacent segment disease. In one series, 50 to 60% of the
patients had persistent back pain and 20 to 30% had recurrent
radiculopathy 10 years after undergoing lumbar diskectomy.
From a biomechanical perspective, adjacent
level disease is a relatively easy concept to understand.
After the cervical or lumbar vertebra fuse, the fused vertebral
bodies create a longer lever arm. The load forces once shared
across the fused bodies now concentrate on either end of the fusion
mass, thereby accelerating the degeneration of these joints.
DiAngelo et al.[6] have shown that fusion increases local motion in
adjacent segments as well as global cervical motion.
The idea of joint replacement for the
treatment of degenerative disease revolutionized the practice of
orthopedic surgery. It is possible that disk arthroplasty may
similarly affect the treatment of spondylotic disease of the
spine. This initiative has motivated a significant portion of
the impetus to develop artificial disks.
History of Disk
Replacement
In 1966 Fernstrom[8] published his experience
with 199 intracorporal endoprostheses implanted into 125 patients
and followed for 4 to 7 years. The prosthesis was a
corrosion-resistant stainless steel ball placed in the center of an
evacuated disk space. Prosthetics of different sizes were
implanted in both the cervical and lumbar spine. An anterior
approach was used for the cervical implantations, and a laminectomy
was used to access the lumbar spine. During the follow-up
period, subsidence occurred in 88% of the patients leading to the
abandonment of the technique.
At the First International Symposium on the
Artificial Disk held in Berlin in 1989, only a few artificial
nucleus designs were presented. Since then research and
development of artificial disks have blossomed. Overseas, the
Conformité Europeene has already approved several replacement
disks. Consequently, most of the clinical data and experience
come from these centers experimenting and developing this
technology.
Prosthetic
Design
The functions of the disk are to prevent
collapse of the vertebral bodies, to maintain motion, to insure
stability, and to reduce pain. Using technology gained from
orthopedic joint replacements, numerous strategies have been
devised to create a successful prosthetic disk. Elastomers,
viscous fluids, fluid-filled chambers, and articulating components
have all been entertained. Strength, durability, and
biomechanical and biochemical compatibility must be
considered. Furthermore, ease of implantation and the
immediate postoperative and long-term stability of the device will
affect the utility of a replacement disk. Wear can cause
debris to accumulate within the artificial joint space, causing the
joint to fail prematurely and disrupting the interface between
materials such as metal-polymer, metal-metal, and metal-bone.
Joint stability also is greatly affected by the ability of the
joint to fuse with adjacent bony surfaces. Based on this
information, prosthetics disks can be divided into two major
categories: nucleus replacements and total disk
replacements.
Figure 1. Photographs demonstrating the (A) PDN, (B) LINK SB
Charité III disc,
(C) Prodisc, and (D) Acroflex Disc. From Traynelis VC: Spinal
arthroplasty.
Neurosurgical Focus 13(2):Article 10, 2002. With permission from
Journal of Neurosurgery.
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Nucleus
Replacement
Nucleus replacements are designed for use
when the major feature of the degenerative process involves the
nucleus pulposus but has spared the annulus and supporting
structures. The Prosthetic Disc Nucleus (PDN, Raymedica,
Bloomington, MN) is an implant with a hydrogel core wrapped in a
woven polyethylene cover (Fig. 1). A dehydrated spacer, it
expands as it absorbs water after implantation. The size of
the implant varies. It can be placed through an anterior or
posterior approach. For proper balance it should be used in
pairs. Outcomes of a cadaveric study of the biomechanics of
this implant were favorable. The cadaveric model, however,
failed to permit full hydration of the device.19 Since 1996,
423 patients have received the PDN since 1996 with a surgical
success rate of 90%. Clinical results have been
encouraging. The main problem, the 10% migration rate, has
led to a series of modifications to the procedure.[14]
Outcomes are not yet available for two other
nucleus replacements. Aquarelle Stryker (Howmedica,
Rutherford, NJ) is a hydrogel material based on polyvinyl alcohol,
which is hydrated before implantation. This material can be
implanted through a tapered cannula placed in an annulotomy via a
posterior or lateral approach. The Prosthetic Intervertebral
Nucleus (Raymedica, Bloomington, MN) is a polyurethane material
instilled into a balloon that is inserted into a disk space.
Once placed into the disk space, the chamber is filled with the
material, which then cures in situ.
Total Disk
Replacement
In contrast to nucleus replacement, total
disk replacement addresses degenerative processes throughout the
disk and associated structures. It represents a major
reconstructive procedure. These prosthetics are designed to
restore the normal movement of the diseased motion segment.
In the lumbar spine, the Link SB Charité III (Waldemar Link
GmbH and Co., Hamburg, Germany), Acroflex-100 prosthesis (DePuy
AcroMed, Raynham, MA), and Prodisc (Aesculap AG and Co.,
Tuttlingen, Germany) are the most widely implanted total-disk
replacements (Fig. 1). The Cummins/Bristol disk (Medtronic
Sofamor Danek, Memphis, TN) and the Bryan Cervical Disc Prosthesis
(Medtronic Sofamor Danek, Memphis, TN) are under investigation for
use in the cervical spine.
Figure 2. (A) Lateral and (B) anterior views of the Bristol
disc. From Traynelis VC: Spinal arthroplasty.
Neurosurgical Focus 13(2):Article 10, 2002.
With permission from Journal of Neurosurgery.
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Cervical
Arthroplasty
The Cummins/Bristol disk is a two-piece
ball-and-socket device that is secured to the anterior vertebral
bodies with screws (Fig. 2). Clinical outcomes were good in
26 patients followed for 2.4 years. Unfortunately, the
stainless steel devices, which were manufactured at a local
foundry, may have suffered from a production error. There
were five cases of hardware failure, one of which required surgical
revision. Wigfield et al.[17,18] have shown that the
implantation of an artificial disk seems to decrease adjacent level
motion compared with fusion.
Figure 3. Photograph of the Bryan cervical disc.
From Traynelis VC: Spinal arthroplasty.
Neurosurgical Focus 13(2):Article 10, 2002.
With permission from Journal of Neurosurgery.
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In Europe the Bryan Cervical Disc (Medtronic
Sofamor Danek, Memphis, TN; Fig. 3) prosthesis has been implanted
in 97 patients, and results have been promising. At the time
of writing, 30 patients have been followed for 1 year, and 67
patients have been followed for 6 months. The clinical
success rate was 90% and 86%, respectively, based on Odom’s
criteria of excellent, good, or fair (Table 1). Long-term
follow up is needed to determine whether the disks remain
functional and what effect they have on adjacent levels.[9]
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Lumbar
Arthroplasty
In 1984 Drs. Karin Büttner-Janz and Kurt
Schellnack began to work with the Waldemar Link GmbH & Company
(Hamburg, Germany) to develop a replacement lumbar disk.
After several design iterations, the final version of the disk, the
LINK SB Charité III, is currently available in
Europe.[2,3,11] The device is composed of a ultrahigh
molecular weight core sandwiched between two cobalt-chromium alloy
endplates. More than 3000 of these disks (all generations)
have been implanted throughout Europe.[20] In 1994 a
multicenter retrospective review of the early clinical results with
the LINK SB Charité lumbar artificial disk was
published.[11] Compared to their preoperative condition,
pain, mobility, walking distance, and weakness improved
significantly in 93 patients. The complication rate, as a
result of disk migration or dislocation and device failure, was
6.5%. Cinotti et al.[4] and Zeegers et al.[21] also reported
their experiences with the LINK SB Charité III disk.
Forty-six patients were studied a mean of 3.2 years after
implantation: 63% reported satisfactory results. The success
rate was 69% in patients who underwent isolated disk replacement
and 77% in patients who had undergone previous back surgery.
Two patients had the prosthesis removed. Seven patients
underwent posterolateral fusion without removal of the device.
Enker et al.[7] reported their experience
with the Acroflex artificial lumbar disk. This prosthesis
consists of two titanium endplates vulcanized to a polyolefin
rubber core (Fig. 1). The disk was implanted in six patients
via a midline transabdominal approach, four of whom reported
satisfactory results. In one patient, the rubber core
fractured and a revision was necessary.
The ProDisk, developed by Dr. Thierry Marnay,
consists of two CrCoMo alloy endplates covered with titanium
plasmopore to enhance osteointegration (Fig. 1). The inferior
articulation is a monoconvex surface that slides into a concave
upper rim. This design permits the implant to be inserted
with significantly less distraction of the disk space than the LINK
SB Charité III disk. Because movement is across two
surfaces, the motion is semiconstrained. The clinical
outcomes associated with the ProDisk have been presented at several
meetings, but published data are limited.[16]
Risks of
Artificial Disk Replacement
A major concern of artificial disk
replacement is that of implant migration. In the cervical
spine, disk migration can have grave consequences whereas more
leeway exists in the lumbar spine. Because most neurosurgeons
are familiar with the anterior approach to the cervical spine, the
first prosthesis to be approved by the Food and Drug Administration
(FDA) will likely be for use in the cervical spine.
Furthermore, cervical spondylosis is often attacked through an
anterior approach. It should be possible to implant an
artificial disk in the lumbar spine through an anterior or
retroperitoneal approach. A posterior approach, performed
similar to a posterior lumbar interbody fusion, also may be
possible. From a technical perspective, advancing and
anchoring a disk from a posterior approach will be difficult if the
artificial disk replacements for the lumbar spine remain in their
current configuration.
Longevity and choice of materials are other
issues that need to be addressed. Most of the present
artificial disk replacements have been modified in some
fashion. In fact, some artificial disk replacements have
failed. Automated test jigs permit relatively straightforward
testing of artificial disk replacements for fatigue and other
biomechanical features.
Status in the
United States
The hurdles to FDA approval for these devices
are numerous. Obvious challenges include the use of
biocompatible materials. Given the present data on many
metals and other composite materials already used in joint
prosthesis, this bar should not be too difficult to overcome.
Other issues include the perceived patient risk-to-benefit
ratio. In particular, artificial disk replacement must be
shown to be safe and effective, especially when many alternatives
for the treatment of degenerative disk disease already exist.
Furthermore, the environment in the United States, highlighted by
the litigations over pedicle screws and breast augmentation,
mandates that these devices be tested rigorously before they are
marketed. Of great concern is the risk of disk migration and
the potential for devastating neurologic injury.
At present in the United States several
artificial disks are undergoing testing in humans. The LINK
SB Charite III disk and the Prodisc have already been implanted at
several centers. The Investigational Device Exemption Study
for the Charite III has recently been completed and is still in
progress for the ProDisk. The Bryan Cervical Prosthetic Disk
is to begin testing in the United States in the near future.
Discussion
Anterior cervical diskectomies are among the
most common spinal procedure performed by neurosurgeons. Each
year more than 35,000 Americans undergo diskectomy and
fusion. Most patients do very well after this
procedure. Complication rates are low, and the associated
symptoms tend to improve significantly. Lumbar fusion also is
used widely, but its rate of clinical success is more variable.
Considering that most neurosurgeons are
familiar and comfortable with the anterior cervical approach,
anterocervical disk replacement is likely to be the first location
that will enjoy the benefit of this new technique. In
contrast, most lumbar procedures are performed with the patient in
a prone position. Placing an artificial disk into the lumbar
spine from this position would be exceedingly difficult and would
likely require a lateral approach. Otherwise, retroperitoneal
or anterior approaches, which tend to be associated with relatively
high rates of morbidity and mortality, will need to be used.
Given the increased interest in minimally
invasive spinal surgery, it will be interesting to see how these
two strategies compete with one another. Surgeons know that
the paraspinous muscles confer a significant amount of strength and
stability and that stripping these muscles from the spinous
processes increases the loads on the anterior longitudinal ligament
and facet joints. Minimally invasive techniques can be
combined with spinal arthroplasty to minimize procedure-related
morbidity while maximally preserving the functional integrity of
the spine.
The treatment of spinal disease with
arthroplasty continues to evolve in a way that recalls the
different methods of joint replacement developed in orthopedic
surgery. The optimal treatment of spinal spondylosis will
likely be reconstituting the physiological state of motion rather
than fusing and immobilizing the joints involved.
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