Motion Preservation Technologies in Development
The U.S. market for spinal implants is predicted to
double from $2.4 billion in 2004 to $5 billion in 2009.
This dramatic growth will be fueled in part by the development
of artificial lumbar discs, cervical discs, dynamic
stabilization systems and nuclear disc prostheses.(i)
These innovative motion preservation devices for the
treatment of back pain are emerging as viable, dynamic
alternatives to the estimated 400,000 fusion procedures
performed each year in the United States.(ii)
“These new devices are not panaceas for all back
pain,” cautions Noam Y. Stadlan, M.D., a founding
member of the Chicago Institute of Neurosurgery and
Neuroresearch (CINN) Institute for Spine Care. “Some
patients will be candidates for the new devices. Others
will be better served with older technology.”
Lumbar Artificial Discs
Results
from the Pivotal Charitò Disc Study
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In late October of 2004, the U.S. Food and Drug Administration
approved a lumbar disc replacement device that some
believe has the potential to revolutionize spine surgery.(iii)
The Charité™ artificial disc (DePuy Spine,
Raynham, MA) had been commercially available outside
the United States for nearly two decades(iv) before
the FDA deemed its third-generation design safe and
effective for American populations with single-level
lumbar Degenerative Disc Disease (DDD).
“I believe the Charité artificial disc
shows tremendous promise,” says Noam Y. Stadlan,
M.D., associate medical director of the CINN program
in the North Shore suburbs and an investigator for the
artificial disc’s FDA-sanctioned Investigational
Device Exemption (IDE) study. “There are a number
of others being tested in Europe, but the Charité
is the only replacement spinal disc currently approved
by the FDA.”
The concept of an artificial disc is not new. In fact,
numerous designs have been brought to the table over
the past 35 years, but few have actually been produced.
The driving goal has been to create a dynamic alternative
to arthrodesis—surgical immobilization—which
remains the gold standard for surgical treatment of
lumbar DDD. There are a number of surgical approaches
to arthrodesis, the most common of which are Anterior
Lumbar Interbody Fusion (ALIF), Posterior Lumbar Interbody
Fusion (PLIF), and Transforaminal Lumbar Interbody Fusion
(TLIF). The inherent problem with fusion—through
any means—is that it eliminates the normal motion
of lumbar segments. This might make it a logical choice
in cases where intervertebral motion itself is the root
cause of pain, but even in these applications, fusion
does not provide a cure for DDD; it merely reduces the
associated pain. In addition to masking the initial
problem, fusion may lead to problems in other parts
of the spine over time. The segments adjacent to the
fused level can be subjected to increased motion and
stress, resulting in an initiation and/or acceleration
of their own degenerative process.(v)
In contrast to fusion, the goal of lumbar artificial
discs is to provide pain relief through the restoration
and maintenance of normal physiological motion. They
dynamically stabilize the diseased segment in a manner
that does not subject adjacent segments to abnormal
loads and motions. In doing so, an effective artificial
disc restores intervertebral space height, lordosis
and the instantaneous axis of rotation. Normal motion
and function returns and is maintained over time.vi
So far, the Charité device has demonstrated the
highest propensity for accomplishing these objectives.
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| Charitòš Artificial Disc |
The unconstrained design of the Charité artificial
disc includes two metallic (CoCrMo) endplates with anterior
and posterior “teeth.” Between the endplates
is a free-floating, Ultra-High Molecular-Weight Polyethylene
(UHMWPE) sliding core. Operating similarly to the mobile
knee bearing of contemporary knee implants, the Charité’s
mobile core eases the load of facet structures to replicate
a normal range of motion, both posteriorly in flexion
and anteriorly in lumbar extension. The device is also
designed to be forgiving in case it is positioned slightly
off-center.
In March of 2000, enrollment for the Charité
device’s FDA IDE multicenter study opened to patients
with single-level L4-5 or L5-S1 symptomatic DDD who
had failed six months of nonoperative treatment. The
CINN medical group—one of only 17 trial sites across
the nation—implanted more than 80 artificial discs
over the course of the three-year clinical trial. The
study aimed to compare the safety and effectiveness
of the Charité artificial disc to that of anterior
fusion with BAK cage devices—hollow, titanium threaded
cylinders that foster bone growth between upper and
lower vertebrae. Fusion with the BAK cage was implanted
in a control group of 99 subjects; the remaining 205
received the Charité artificial disc.
Clinical and radiographic evaluation of each patient
occurred at six weeks and three, six, 12 and 24 months
following surgery. Before the operation and at these
intervals, all patients responded to Oswestry Disability
Index (ODI), Visual Analog Scale (VAS), and Short Form-36
questionnaires. Based on this criteria, the two procedures
were found to result in similar adverse event profiles.
But in terms of patient ratio success, the Charité
(57%) emerged as numerically superior to the BAK cage
(46%).(vii) Charité disc-treated patients presented
greater improvement in ODI scores at all test points,
though at times the margins were quite narrow.(viii)
VAS scores of the Charité were also slightly
superior, and subsequent meta-analysis of lumbar fusion
literature further revealed that the Charité
device’s clinical outcomes were equivalent to or
better than those achieved by 360-degree or stand-alone
interbody fusion.(ix)
Proven to restore and maintain segmental motion at
two years postoperatively, the Charité device
has been cleared for marketing in the U.S. by the FDA.
Nevertheless, further follow-up evaluation beyond two
years is needed to corroborate the long-term results
demonstrated by Charité in Europe.(x) Until then,
physicians of the Chicago Institute of Neurosurgery
and Neuroresearch Medical Group (CINN) will be very
cautious about applying the device. This is especially
true of Kenneth S. Heiferman, M.D., associate director
of the CINN Institute for Spine Care at Elmhurst Memorial
Hospital. “Several of the clinical criteria of
the IDE study (VAS score; ODI score) showed that the
Charité disc was only as good as fusion, not
better,” he says, referring to the lack of statistical
significance achieved at the 24 months endpoint. “Additional
studies are necessary to understand the long-term outcomes
and the disc’s impact on adjacent level disease.”
Dr. Heiferman’s colleague, CINN neurosurgeon Harel
Deutsch, M.D., adds, “Patient satisfaction appears
greater with the artificial disc, and they get better
faster.” This has certainly been true in Europe,
where several studies have reported the percentage of
Charité patients returning to work at 67% - 89%.(xi)
Such studies continually build the case for artificial
discs over fusion, demonstrating that with the Charité
device, patients can maintain flexibility, experience
improvements in pain and function, leave the hospital
sooner and be more satisfied with the procedure.(xii)
“The artificial disc will not replace fusion altogether,”
Dr. Deutsch predicts. “It will, however, augment
what we can offer.”
Cervical Artificial Discs
As first generation lumbar devices are commercialized
in the U.S. market, there is speculation that artificial
disc replacements will emerge as the most successful
product class in recent medical device history. Within
this class, many feel that cervical applications may
prove even more significant than those of lumbar.(xiii)
European outcomes are compelling. In April of 2005,
U.K.-based medical device manufacturer Pearsalls Limited
announced very promising early clinical results for
their artificial cervical disc device. The Pearsalls
Cervical Disc, also known as the neodisc, was implanted
in nine patients at a Birmingham hospital. At six months,
the first four patients were cured of their original
symptoms and showed significant improvement in all scores
measured. At time of publication, the remaining five
patients are reportedly progressing extremely well and
are highly satisfied with their results. Pearsalls is
planning to apply to the FDA to begin clinical trials
in the U.S., contingent on the successful completion
of the study.(xiv)
“We may get involved in a cervical artificial
disc study in the near future,” speculates Dr.
Stadlan. “It will challenge cervical disc fusion,
which is considered the gold standard for one-level
treatment.” Motion-restoring cervical disc replacement
(arthroplasty), however, may become the next gold standard,
according to the early clinical and biomechanical results
of a recent Canadian study. The prospective cohort study
involved the implantation of the Bryan Cervical Disc
Prosthesis (Medtronic Sofamor Danek, Memphis, TN) in
26 patients for one- or two-level treatment of cervical
DDD. Motion was successfully preserved in the treated
spinal segments for up to 24 months postsurgery.(xv)
“If the long-term benefit of cervical artificial
discs is as good as their short-term benefit, they will
be promising new devices,” says Dr. Stadlan.
Long-term outcome of a device similar to the Bryan
does in fact support the prospective longevity of its
benefits. In a pilot study, the Prestige I cervical
disc system (Medtronic Sofamor Danek, Memphis, TN) has
demonstrated significant effectiveness at 48 months.
At this point, collected data from the study’s
questionnaires indicated improvement in all aspects
of patient function and quality of life, with no evidence
of adverse events such as adjacent-segment disease.(xvi)
The next generation of this device, the Prestige II,
is currently the subject of the first prospective randomized
trial in which cervical arthroplasty is compared directly
with fusion. Preliminary results indicate that the Prestige
II disc is potentially a viable alternative to fusion
for primary cervical disc disease; however, further
clinical studies with larger sample sizes will be required
to show statistical equivalence.(xvii)
Dynamic Stabilization Devices
Artificial discs are promising, but of course, they
are not for everyone. Patients with isthmic spondylolisthesis,
for example, may not be the best candidates for artificial
discs. Also, the fact that disc replacement can involve
two operations—a posterior nerve root decompression
and an anterior disc replacement as in cases of radiculopathy—may
be prohibitive for some populations.(xviii) In these
and other instances, a dynamic stabilization device
may provide a more appropriate treatment option.
Dynamic stabilization devices, like artificial discs,
are built on the concept of soft stabilization. Intended
for treatment of lower back and leg pain, they unload
the disc and permit near normal motion, making it possible
for dynamic stabilization devices to allow the degenerative
disc to repair itself or facilitate the reparative potential
of gene therapy.(xix) One such device has emerged as
particularly encouraging in this regard.
Results of a European in vitro study of the Dynesys
Dynamic Stabilization Spinal System (Zimmer Spine, Minneapolis,
MN) were published in 2003. The researchers suggested
that the device could provide substantial stability
in case of degenerative spinal pathologies and could
therefore be considered as a motion-preserving alternative
to fusion surgery.(xx) Dynesys has since been cleared
in the United States for use as an adjunct to fusion,
and is currently in an investigational device study
for a non-fusion application.(xxi)
Three parts to the Dynesys system are designed to stabilize
the affected joints while preserving much of the spinal
anatomy. Titanium alloy pedicle screws anchor the Dynesys
device in the pedicles. Polyethylene cords set limits
on bending movements. Polyurethane external spacers
hold the segments in a normal anatomical position. A
dynamic push-pull relationship between the spacer and
the cord stabilizes the affected joints and naturally
positions the vertebrae while leaving the joints intact.
The Dynesys stabilizing device is implanted on both
sides of the affected vertebrae, which can be accomplished
through a minimally invasive posterior approach. Unlike
other pedicle screw systems, Dynesys is implanted with
its screws lateral to the facets, so that flexible bands
may be threaded through vertically. “The Dynesys
device is fixed to the vertebral body,” explains
Dr. Stadlan, “and instead of attaching two screws
with a metal rod, we use elastic bands to allow for
more movement.”
As a new and very unique implant device, the Dynesys
system has undergone extensive biomechanical testing.
Each of its components was tested separately for strength
and durability by manufacturer Zimmer Spine. They were
also tested together in screw-cord and screw-cord-spacer
constructs. The system exhibited initial and long-term
rigidity in assembly tests and effectively impacted
spinal instability during in vitro kinematic tests.
According to presentation materials distributed by Zimmer,
these biomechanical tests as well as subsequent animal
and cadaver tests “demonstrate that the Dynesys
Spinal System has the capacity to survive and work.”(xxii)
An FDA clinical study is currently examining this likelihood
in more detail.
A total of 20 institutions throughout the United States
are participating in an FDA clinical study to assess
the safety and effectiveness of the Dynesys System.(xxiii)
Like the Charité device, Dynesys shows potential
but more proven outcomes are needed before it can be
fully endorsed. “While this device seems like a
good idea, the question becomes, ‘what problem
will it really solve?’” says Dr. Stadlan.
“Does it treat pain from Degenerative Disc Disease
or Facet Degeneration effectively, or are these problems
better treated with other approaches?” If the successful
treatment of over 8,000 patients in Europe is any indication,
the FDA clinical study can be expected to provide strong
evidence of the Dynesys system’s efficacy, marking
a major step forward in treating lower back and leg
pain.(xxiv)
Disc Nucleus Replacement
An altogether different approach to preserving motion
is the intriguing—though largely unproven—concept
of disc nucleus replacement. Disc nucleus replacement
technology uses minimally invasive surgical techniques
to replace only the inner portion of the spinal disc
(the nucleus) with a prosthesis, preserving the function
of the surrounding tissue. The prosthesis can be made
of metal, ceramic, hydrogel, elastic coils or various
injectable materials, which, once implanted inside the
disc, work to mimic normal disc movement. ECRI, a nonprofit
health services research agency, predicts that disc
nucleus replacement is likely to be used more extensively
than total disc replacement because of its less-invasive
approach. If and when disc nucleus replacement receives
FDA approval, it may be offered at a lower cost than
both full disc replacement and traditional fusion procedures,
with fewer contraindications than artificial discs.(xxv)
The safety and effectiveness of several Prosthetic
Disc Nucleus (PDN) device designs are currently under
investigation. “Medtronic is working on disc nucleus
replacements, but there are a few problems,” says
Dr. Deutsch. “The devices tend to pop out.”
It’s true that migration issues have plagued PDN
devices since their first clinical trials in 1996, but
a 2002 study in China may have found the key to curbing
such incidences in the treatment of lumbar disc herniation:
single device implantation. Implanting the devices as
pairs is thought to have caused previous complications.
The Chinese study regulated one device per patient,
and at six months, evaluations revealed no incidence
of device migration, failure or dislocation. Additionally,
the study noted relief of pain intensity, and improvements
in walking distance, lumbar mobility, neurologic weakness,
Oswestry and Prolo scores, and intervertebral disc height.(xxvi)
Long-term results are needed to validate the full effectiveness
of this and other disc nucleus prostheses. “The
technology is coming,” says Dr. Deutsch, “it
just isn’t quite at the level it needs to be yet.”
Conclusion
“We are at the dawn of an age where we have viable
alternatives to fusion—we can now replace diseased
joints and maintain motion,” says Dr. Deutsch.
Lumbar and cervical artificial discs are being proven
more and more effective as the U.S. FDA corroborates
successful results in Europe and abroad. Dynamic stabilization
devices, and the more radical concept of disc nucleus
replacement are beginning to merge the stability of
traditional fusion treatments with the natural spinal
mobility offered by soft stabilization technology.
“Motion preservation technologies show promise,”
agrees Dr. Heiferman. That said, each patient must be
individually assessed to determine the best course of
treatment.”
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