Intervertebral disc replacements have been under design for over three
decades but are now receiving increased attention. This is largely due to
several devices that have received Investigational Device Exemption status
from the United States Food and Drug Administration, resulting in clinical
trials in the United States. There is tremendous early enthusiasm for the
concept of disc replacement and motion preservation as an alternative to
arthrodesis, with the hope that adjacent segment degenerative changes can be
averted. Skeptics hold the position that adjacent segment changes are due more
to the individual's genetic predisposition and normal aging than to the
mechanical changes resulting from a spinal arthrodesis. In addition, there are
potential long-term consequences to disc replacement that remain unknown.
These issues include failure, wear debris, and the effects of stress-risers as
patients age and become osteopenic. The two basic categories of disc
replacements are nuclear replacements and total disc replacements. The
critical issues pertaining to total disc replacements were recently explored
in a symposium at the 2003 Annual Meeting of the American Orthopaedic
Association in Charleston, South Carolina, and are summarized in this
review.
The vast majority of patients with degenerative disc disease are managed
with nonoperative methods, including analgesic and anti-inflammatory
medication, muscle strengthening, weight control, and aerobic training.
Management with injection of corticosteroids may also be attempted. With these
modalities, the results are generally good. Surgical management has been
reserved for the patients in whom these treatment options have failed.
Numerous authors have reviewed the complications and failures of spinal
arthrodesis in the low
back1,2.
The purpose of this discussion is not to list and comment on all of the
difficulties with an invasive approach but to focus on the difficulties that
may be alleviated or ameliorated by a stable disc arthroplasty.
The first major problem with spinal arthrodesis is the lack of
predictability of pain relief. Various authors have documented a wide range of
efficacy for this operation. Historically, surgeons have pointed to
psychosocial factors, a lack of precision in the diagnosis, and a lack of
definition of the proper indications for spinal arthrodesis among other
factors that may lead to lack of pain relief. As surgeons have recognized
these factors and have tried to narrow the pool of patients with degenerative
disc disease, the results of arthrodesis have improved.
A second problem with lumbar arthrodesis for the treatment of degenerative
disc disease is loss of motion. By definition, the arthrodesis is considered a
success if motion is terminated; however, with respect to the kinematics of
the lumbar spine, most surgeons would agree that the permanent elimination of
motion is worse than if the kinematics were normal.
A third problem directly related to spinal arthrodesis is the prevalence of
failure of fusion or pseudarthrosis. If internal fixation is used, the
assessment of fusion radiographically is further complicated. In addition, a
pseudarthrosis may be present with or without clinical symptoms. If the
patient is symptomatic from lack of healing of the fusion, is it because the
lack of fusion has allowed direct axial compression of a diseased disc with
motion or is it because the failure of fusion allows for abnormal translation,
or angulation, that creates pain?
A fourth important difficulty with lumbar arthrodesis in the treatment of
degenerative disc disease is the lack of precision of the sagittal balance.
The surgeon may make radiographs with the patient in the standing position
preoperatively to determine current sagittal balance; however, it is not
possible to know what the normal sagittal balance would be if the patient were
asymptomatic. Most surgeons try to maintain or increase lordosis with internal
fixation and arthrodesis, but this approximation is only a guess as to what
the asymptomatic sagittal balance might be. Furthermore, an arthrodesis allows
for only one static lordotic angle and does not permit the dynamic changes in
lordosis that occur in different positions (e.g., sitting compared with
standing).
A fifth major problem is adjacent segment
disease3,4.
Various authors have reported on disc herniation, facet hypertrophy, facet
disruption, degenerative spondylolisthesis, ischemic spondylolisthesis, and
gross disc incompetence adjacent to the site of a lumbar
arthrodesis3,4.
The vast majority of these reported cases affect segments directly adjacent to
the site of the arthrodesis, although there have been reports of such cases in
a segment that is separated from the site of the previous arthrodesis by a
normal segment. The time-period for the degeneration is highly variable.
Obviously, a patient who has disc degeneration at one level has a
predisposition to this disease at other levels, and segmental fusion only
increases the forces that are seen at other levels of the lumbar spine. This
situation may hasten a breakdown of a level that was predisposed to this type
of difficulty even if there were normal lumbar kinematics elsewhere. Other
studies have suggested that the remaining mobile segment can compensate for
the loss of motion at a fused segment, and they do not support a mechanical
basis for adjacent segment disease.
Donor-site complications are also an important source of disability. Pain
at the level of the abductor mechanism origin on the posterior aspect of the
ilium has been reported by many authors. Sensitivity of the incision after
long-term follow-up is also a common finding. The advent of osteoinductive
bone-graft substitutes, such as bone morphogenetic proteins, may eliminate
this drawback of arthrodesis.
At this point, six important areas of concern with regard to the results of
spinal arthrodesis have been identified
(Table I). These areas are
important because they are problems for which disc replacement may be relevant
as an intervention.
In the broad area of concern about lack of pain relief, the prospective,
randomized studies for the Food and Drug Administration will determine whether
disc replacement is at least equivalent to lumbar arthrodesis with respect to
pain relief. In this area, many of the study participants are hopeful and the
early results are quite encouraging (as they often are). Only with two, five,
ten, and twenty years of follow-up will we know for sure.
Ideally, disc replacement will allow motion so that stability may be
maintained with some degree of nonpainful movement allowed. While this motion
may not be physiologic, the kinematics of the lumbar spine in a patient with a
successful result at the time of the two-year follow-up will be improved
compared with the so-called successful result after arthrodesis. Despite the
desire for motion preservation, the cases of some patients who have had
spontaneous fusion after disc replacement have been reported and, even in the
patients with a successful result, the long-term motion achieved is likely to
be less than physiologic.
The third problem with arthrodesis, i.e., the lack of healing or
pseudarthrosis, may be resolved in several ways. First, because disc
replacement allows motion between the vertebrae in the healing phase of
osseous ingrowth, the stress at the bone-implant interface is necessarily
diminished compared with an arthrodesis, which needs to have relative rigidity
maintained between the two vertebrae. Second, a disc replacement does not rely
on healing of bone to be successful.
The issue of sagittal balance and its abnormalities may be ameliorated by
disc replacement in that, with the patient in the standing position, the disc
angle may be free to assume the appropriate position for that particular
patient in an asymptomatic condition. Lateral radiographs made in normal
patients while they were standing have shown a broad range of lumbar lordosis
in aggregate and a broad range of disc angulation for each disc between L1 and
the sacrum. Disc replacement might indeed allow patients to return to their
own asymptomatic natural sagittal balance.
The issue of adjacent segment disease will be closely watched in long-term
follow-up studies of disc replacement. The current hypothesis is that, with
some motion at the recreated disc, the abnormal forces at adjacent levels will
be diminished. Only long-term follow-up of the prospective, randomized cohort
will confirm this belief.
Obviously, the issues of donor-site complications, stripping of the lumbar
paraspinal muscles, and injury to the neural elements by direct invasion
should fare very well in the patients undergoing disc replacement compared
with those undergoing arthrodesis.
For all of the above reasons and for other reasons not considered, disc
replacement may offer an improved quality of life for patients with
degenerative disc disease. The question of disc replacement for more
substantial instability patterns, for ischemic or degenerative
spondylolisthesis, or for other spinal disease entities is speculative at this
time. Prostheses with differing constraints, ranges of motion, and fixation
patterns will undoubtedly evolve.
The surgical treatment of disabling lumbar disc disease with a mobile
implant has been an alternative to arthrodesis in Europe since the early
1980s, with more than 4000 SB Charité III prostheses (Waldemar Link,
Hamburg, Germany) (Fig. 1) and
more than 2000 ProDisc prostheses (Spine Solutions/Synthes, New York, NY)
(Fig. 2) implanted there since
1987. Artificial disc replacement is a relatively new phenomenon in the United
States, and it was not until October 2000 that the first Link SB
Charité disc was implanted in this country and not until October 2001
that the first ProDisc was implanted in the United States
(Table II).
By the middle of 2003, only two lumbar artificial disc designs had
completed the lengthy clinical process for an Investigational Device Exemption
from the Food and Drug Administration. The Link Spine Group completed patient
enrollment for the SB Charité III disc in December 2001. Spine
Solutions/Synthes completed patient enrollment for the ProDisc single-level
arm and will be near completion of enrollment in their double-level arm.
According to the protocols of the Food and Drug Administration, only when the
last patient has completed a twenty-four-month evaluation can the application
for approval be submitted. The Maverick disc (Medtronic Sofamor Danek,
Minneapolis, Minnesota) (Fig.
3), although implanted in more than 100 patients in Europe, has
not yet been approved for clinical trials in the United States. Additional
animal studies to evaluate metal wear debris were ordered by the Food and Drug
Administration before human clinical trials would be considered.
Clinical trials for the Acroflex disc (Johnson and Johnson DePuy Acro-Med,
Raynham, Massachusetts) were cancelled by the manufacturer, and this device is
not currently in the pipeline for further human testing. Several other designs
for lumbar disc arthroplasty are under development, but no other human testing
in the United States for these next models has begun yet.
Link SB Charité III
The Link SB Charité III device is marketed as an unconstrained
three-part anatomic disc replacement. The current model, a refinement of the
original design by Dr. Karin Büttner-Janz and Dr. Kurt Schellnack, has
been implanted since 1987. It uses a sliding polyethylene middle core to mimic
the movement of the center of the natural nucleus posteriorly in lumbar
flexion and anteriorly in extension. The metal-alloy end plates are
hydroxyapatite-coated in Europe, but they are uncoated in the United States.
Osseous attachment of the end plates is by teeth embedded into the subchondral
bone. Containment of the free-floating core is by implant geometry and
soft-tissue tension.
In vitro cadaver studies have demonstrated ranges of motion equivalent to
those of intact human discs. In vivo ranges of motion of the spinal segment
six months after implantation were demonstrated in baboon spines in a study by
McAfee et al.5.
Radiographically, similar clinical ranges of motion have been demonstrated in
patients who received Charité
prostheses6,7.
Biologic ingrowth of hydroxyapatite-coated Link SB Charité III end
plates was demonstrated histomorphometrically by McAfee et
al.5.
Büttner-Janz demonstrated that no wear debris particles were generated
after cyclical testing equivalent to eleven years of clinical use. No
polyethylene particles and no inflammatory reaction could be identified
histologically in a biopsy done eighty-one months after implantation.
Several clinical series have been reported in Europe. Cinotti et
al.6 reported that
63% of forty-six patients had a satisfactory result, although nine patients
required a reoperation (eight were performed because of continued pain and
one, because of implant dislocation). In a study of 105 patients, Lemaire et
al.7 reported that
79% had a good outcome at a mean duration of follow-up of fifty-one months. In
a study of fifty patients who were followed for two years, Zeegers et
al.8 reported that
70% had a good result, but twelve patients had a total of twenty-four
reoperations, none of which were described as a result of device failure.
In the Investigational Device Exemption study for the United States Food
and Drug Administration, 267 Link SB Charité III discs were implanted
during both the "teaching" and experimental phases of the study.
Artificial discs were implanted at either L4-5 or L5-S1 and were randomized
2:1 to an anterior lumbar interbody arthrodesis with BAK cages and autograft.
Preliminary results from one of our centers were recently published and
involved fifty-six patients, twenty-two of whom had had twelve months of
follow-up9. The
patients who received the artificial disc had substantial improvements in pain
scores, as measured with a visual analog scale, and in the Oswestry disability
scores at the six-week follow-up, and the improvement was maintained during
the twelve-month follow-up.
ProDisc II
ProDisc is a semiconstrained modular system that is assembled in vivo. Two
metal-alloy end plates with midline keels are impacted into the surgically
evacuated disc space, and then an ultra-high molecular weight
polyethylenedomed component is snap-fit into the inferior end plate. Assembly
is done inside the disc space with the space internally distracted. It is
thought that the semiconstrained design of the polyethylene dome articulating
with a concavity on the underside of the superior metal end plate may protect
the facets from excessive shear forces.
ProDisc was developed by Dr. Thierry Marnay, a French spine surgeon, in the
late 1980s. Between 1990 and 1993, Marnay implanted the ProDisc (an earlier
model with double keels) into sixty-four patients. No further implantations
were done, and he elected to study this group over time. Ninety-five percent
of these patients were available for seven to eleven years of follow-up. All
implants were intact and functioning, and 93% of the patients were satisfied
with the result. Two-level implants were doing as well as single-level
implants10.
More than 2000 ProDisc devices have been implanted in Europe since 1999.
Unfortunately, very little data have been published. However, two recent
reports have indicated good results with these devices. In an unpublished
study by Bertagnoli and Kumar, 134 patients were followed for three to
twenty-four months. The authors reported excellent results in 90.8% of the
patients. The study by Mayer et al. involved only thirty-four patients, and
the mean duration of follow-up was only 5.8
months11. Their
preliminary results indicated improvements in the pain scores on the visual
analog scale as well as the Oswestry scores and a high rate of patient
satisfaction.
A multicenter Food and Drug Administration study began in the United States
in October 2001. ProDisc was randomized 2:1 to a 360° anterior-posterior
arthrodesis with pedicle screws and posterolateral arthrodesis with iliac
crest autograft. Surgery could involve the L3-4 disc as well as L4-5 and L5-S1
levels, but two-level surgery had to be at contiguous levels. Multicenter
clinical results are being tabulated for the Food and Drug Administration
study, but analysis of the prospective randomized data for the first sixty-two
patients to reach the six-month follow-up evaluation at the single busiest
study site found a shorter operating time, less blood loss, shorter
hospitalization, and improved patient satisfaction and Oswestry scores in the
patients managed with the ProDisc. These encouraging results were maintained
in the first twenty-two patients at the one-year follow-up examination.
Additionally, a return to walking without assistance and light recreational
activity were accomplished faster in the ProDisc group.
Maverick
Maverick is a two-piece metal-on-metal design. It uses a smaller and more
posteriorly placed rotational center, which is thought to help unload the
facet joints. Two large keels are used for rotational control, similar to the
ProDisc, and the cobalt-chromium-alloy components are coated with
hydroxyapatite. A unique four-in-one guide instrument maintains intraoperative
lordosis, measures depth and height distraction, and guides chiseling during
insertion. By February 2003, slightly more than 120 Maverick discs had been
implanted in Europe. No clinical data were available from those studies at the
time of writing. No Maverick discs have yet been implanted in the United
States as part of a formal study.
Wear debris toxicity has been studied in metal-on-metal hip replacements.
Because of the differences in joint stresses as well as the synovial
environment, it may be difficult to extrapolate from those studies to the
lumbar intervertebral disc space. Accordingly, at the request of the Food and
Drug Administration, Medtronic Sofamor Danek is performing an animal study
pertaining to metal-on-metal wear debris in artificial discs.
Acroflex
The Acroflex disc was developed as a constrained single-piece implant. A
Bion rubber middle portion was bonded to sintered titanium-alloy metal end
plates. The Acroflex disc was acquired by Johnson and Johnson, and further
clinical investigation was ultimately terminated. Information on clinical
experience is not publicly available.
In a study by Cunningham et
al.12, an Acroflex
disc was implanted and an autologous iliac crest arthrodesis was performed at
two noncontiguous lumbar segments through a transperitoneal approach in
baboons. At the twelve-month follow-up examination, the range of motion of the
functional spinal unit at the Acroflex levels was not significantly different
from that at the fused levels, predominantly because of longitudinal osseous
bridging. Heterotopic ossification across the disc space and questions about
material integrity presumably led to the cessation of clinical studies with
this specific implant. Anatomic and histologic studies demonstrated impressive
osteointegration at the implant-bone interface.
Short-term follow-up studies in the United States have indicated that
results after artificial disc implantation compare favorably with those after
arthrodesis. Intermediate-term results in studies from Europe have offered
encouragement that the implants are standing up to routine use and that wear
does not appear to be a major factor. The complications associated with the
operative technique to implant these devices are likely to be similar to those
encountered with anterior lumbar interbody arthrodesis. In the literature
review, no reports of frank device failure such as fracture of the end plates
or collapse of a polyethylene core were found. Device subsidence is a
potential complication, and this was noted with the early designs of the Link
SB Charité disc, which resulted in redesign of the device to have
greater end-plate coverage. Also, it is recommended that patients with
osteoporosis not receive total disc replacements.
Longer-term clinical studies may help to resolve questions about the ideal
position of the center of rotation of the artificial disc, whether
unconstrained or semiconstrained implants affect the facet joints differently,
and whether transitional level breakdown is lessened with mobile disc
replacements rather than arthrodesis.
For five decades, spine surgeons have refined the techniques of anterior
decompression of the cervical spinal cord and nerve roots such that today
these methods are the mainstay of operative treatment for cervical disc
herniations and spondylosis. As the methods and safety of the decompression
have improved, so too have the grafting and internal fixation options. Today,
a number of devices are available to choose from when instrumentation is
deemed appropriate. There are equally as many bone-grafting options, ranging
from traditional tricortical iliac crest autograft to diaphyseal and dense
cancellous allografts to synthetic cages or spacers filled with either
autograft or even rhBMP (recombinant human bone morphogenetic protein). When
asked, most surgeons volunteer that an anterior cervical discectomy and
arthrodesis is among the most reliable and successful procedures that they
have to offer a patient.
If anterior decompression and arthrodesis is such a great option, why do we
find ourselves actively pursuing the clinical investigation of artificial
cervical discs? The answer lies not with issues of neurologic decompression
but with the intent to preserve motion. As we have become much more facile at
performing arthrodeses, we are also becoming more aware of the long-term cost
to our patients as we trade motion for neurologic benefit and pain relief.
Thus, the spine surgery community finds itself investigating techniques that
preserve the kinematics of the functional spinal unit after successful relief
of symptomatic spinal cord or nerve root compression.
The principal issue is that of disc degeneration of the adjacent segment
following anterior arthrodesis. There is evidence to suggest that a fusion has
an independent adverse influence above and beyond the natural history of
cervical spondylosis. Hilibrand et
al.13 reviewed 374
patients at two to twenty-one years following anterior cervical decompression
and arthrodesis. They reported an incidence of reoperation at an adjacent disc
level of 2.9% per year per level fused. Goffin et
al.14 investigated
the radiographic evidence of new or progressive adjacent segment spondylosis
five years or more after an anterior arthrodesis. Ninety-two percent of their
180 patients showed evidence of such progression, and this observation was
independent of age, gender, use of an anterior plate, or diagnosis leading to
the index procedure. The authors also noted a correlation between these
radiographic findings and clinical deterioration. They concluded that there
was an independent effect of the fusion itself on the natural history of the
adjacent segments. Corroborative evidence for this conclusion may be found in
the observations reported by others. Birney and
Hanley15 reported
such adjacent segment changes in two of eight children undergoing arthrodesis
for the treatment of traumatic injuries of the subaxial cervical spine.
McGrory and
Klassen16 observed
adjacent disc-space degeneration among 66% of children and adolescents
undergoing a posterior arthrodesis for the treatment of cervical trauma.
Thus, the role of a cervical disc prosthesis is to alter the natural
history of the motion segments adjacent to the operative level or levels after
a successful neurologic decompression. Additional benefits for the patient may
include the avoidance of issues related to graft or fixation failure,
elimination of the need for postoperative immobilization, and an earlier
potential return to normal activities. The indications for the surgery are not
intended to differ; one still reserves surgery for the treatment of the
symptoms and signs of spinal cord or nerve root compression that is refractory
to reasonable medical management. These technologies are not intended to be a
panacea for discogenic axial neck pain.
At present, several devices are in various stages of clinical use in
different countries around the world, and two of them are currently undergoing
Investigational Device Exemption studies in the United States. These two
devices vary considerably in design and materials and in the way that they
have arrived at similar points in the human clinical evaluations. The older of
the two concepts was derived from the prototype prosthesis devised by Cummins
et al. for insertion at degenerative motion segments adjacent to the site of
previous multilevel cervical
arthrodeses17
(Fig. 4). The stainless-steel
(metal-on-metal) ball-in-socket device was implanted in twenty-two cervical
motion segments in twenty patients in an attempt to preserve motion adjacent
to multisegmental arthrodeses or Klippel-Feil lesions as a salvage procedure,
and it was, in fact, observed to preserve motion for more than a decade
following surgery. The original design and the crude way in which it was
manufactured have since been replaced with a ball-in-groove concept, which is
better adapted to the regional kinematics and anatomy and is now produced with
state-of-the-art manufacturing practices.
A distant relative of the original device designed by Cummins et al., the
Prestige cervical prosthesis (Medtronic Sofamor Danek, Memphis, Tennessee)
(Fig. 4) has undergone two
clinical investigations in Europe. Fifteen patients had the device implanted
in an end-stage study, much like the original Cummins group. At the two-year
follow-up evaluation, the mean range of motion (and standard deviation) at the
operative level had been maintained (7.5° ± 4.6° of motion
preoperatively compared with 6.5° ± 3.8° postoperatively) and
the scores on the neck disability index, visual analog pain scale, and Short
Form-36 (SF-36) had noteworthy
improvements18. A
randomized, prospective, so-called primary indication study of isolated
single-level disc abnormality is nearing completion in Europe and Australia.
In the study, sixty patients were randomized either to implantation of the
device or to a control group managed with an anterior discectomy and
arthrodesis. As one might expect in this treatment group, the preliminary
results have suggested a more striking change in patient symptoms and function
than those in the so-called end-stage, or salvage indication, study. As
previously mentioned, a larger-scale randomized, prospective Investigational
Device Exemption study in which patients are being enrolled for the treatment
of single-level disc herniations with radiculopathy or myelopathy is underway
in the United States. The patients in the control group are randomized (1:1)
to an anterior discectomy and arthrodesis with an allograft and an anterior
fixation device. No data from this study were available at the time of
writing.
The Bryan cervical disc prosthesis (Medtronic Sofamor Danek) has arrived at
a chronologically similar point in its clinical investigation, but it has come
to this point by an altogether different path. Engineered from the ground up,
the device is intended to provide the full range of coupled motions within a
subaxial cervical motion segment, as well as offer a load-dampening effect
similar to the native disc. The device does so by means of its design with a
uniquely shaped polycarbonate polyurethane core situated between two
porous-coated titanium shells (Fig.
5). The device underwent extensive testing of the materials and
design in vitro and later with in vivo primate and goat studies prior to
initiating the one-level prospective European human clinical trial in January
2000. To date, approximately 100 patients are being followed for a minimum of
two years in this historically controlled trial. A two-level disc replacement
cohort in which nearly forty patients have been enrolled is also being
followed. Data from the one-level study have been reported to show
preservation of a mean (and standard deviation) of 9° ± 5° of
motion among the patients who were seen at the two-year follow-up evaluation.
Noteworthy improvements in SF-36 subscale scores have been achieved and
maintained postoperatively. When compared with either historical controls from
a meta-analysis of studies in the literature or with control data from the
Investigational Device Exemption studies for other cervical arthrodesis
products, the clinical outcomes for the Bryan device are similar. On the basis
of these data, the device has received clearance for marketing in major
countries, with the exception of the United States and Japan. As mentioned, an
Investigational Device Exemption study, with a randomized prospective design
and the same control group procedure as that used in the Investigational
Device Exemption study of the Prestige device, is underway in the United
States. The inclusion criteria include single-level disc herniations with
radiculopathy and/or myelopathy with minimal spondylosis and no substantial
adjacent-level degeneration. As yet, no data are available from the study
underway in the United States, and follow-up continues for the one-level and
two-level studies in Europe.
Will these devices be proven safe and effective as alternatives to
anterior arthrodesis following decompression surgery?
The data from studies outside the United States, although small in size,
have suggested that both devices can maintain motion and permit analogous
clinical results in the short-term. But as we have learned from extensive
clinical experience with hip and knee arthroplasty, data from five and ten
years of follow-up are the critical discriminators.
Are such devices durable and must we concern ourselves with
particulate wear debris, as we have in total joint replacements elsewhere in
the body?
Cervical spine motion simulators were devised to test the Bryan device
(Medtronic Sofamor Danek) under physiologic loads at 37°C in bovine serum.
At ten million cycles, or a theoretical ten years of human activity, 2% wear
of the polymer core was noted. Edge contact of the titanium shells due to wear
of the core has not been observed at less than thirty-five million cycles. A
Prestige device (Medtronic Sofamor Danek) retrieved at 3.25 years
postoperatively from a patient in the trial showed negligible wear on the
device and minimal debris in the periprosthetic tissue samples. Two Bryan
devices have been subjected to analysis upon retrieval at eight and ten months
postoperatively when the patients had a reoperation because of persistent
nerve-root compression. No periprosthetic particulates were observed among the
tissue samples obtained. Time will tell whether the theoretical differences
between a metal-on-metal and a metal-polymer-metal design will amount to more
than an intellectual debate. What can be said is that the amount of debris
shed in vitro is orders of magnitude less than that reported for total hip
replacements, which undoubtedly speaks to the reduced contact forces and range
of motion in cervical spine applications. One is also wise to consider that
the cervical intervertebral space into which these devices are inserted is not
a synovial space, which may favorably affect the biological importance of any
particulates.
Are multilevel applications possible with either device?
There are instruments for multilevel insertions of the Bryan device, with
two and three-level studies underway outside the United States. When three or
more motion segments are causing spinal cord compression, especially in the
presence of congenital cervical stenosis, one might have to consider whether
another approach, such as a laminaplasty, might be more appropriate. In its
presently available design, the Prestige prosthesis is best suited to
single-level applications.
What are the postoperative imaging capabilities after artificial
cervical disc replacement?
The Bryan disc does not preclude any form of imaging since its titanium
shells produce very little artifact on computed tomography or magnetic
resonance imaging. The stainless-steel version of the Prestige disc, as it is
currently provided for the United States Investigational Device Exemption
study, generates substantial artifact that limits the quality of imaging of
the cervical spine at the operative level and possibly the immediate adjacent
segments. When the Prestige disc is offered in titanium, the imaging
characteristics will be better.
What salvage options are available in the event that a device
requires removal?
The early in vivo primate trials of the Bryan disc were mandated to be
survival studies. Thus, each animal had explantation of the device and
conversion to an anterior cervical arthrodesis. Among the two known retrievals
of the Bryan disc in humans to date, one patient had a successful
reimplantation of the device and the other had conversion to an arthrodesis.
The degree of bone ingrowth into the shell surfaces has not precluded
successful extraction of the device. The single Prestige device that has been
retrieved was readily removed, and the level was converted to an
arthrodesis.
What are the essential differences between artificial disc
replacements in the cervical and lumbar spine?
This is a multifaceted question. First, one can readily appreciate the
differences in kinematics and sustained loads in the two regions. Thus, the
design envelope has widely different boundaries, which could influence both
design and material options. Surgical access to the anterior cervical spine is
widely used at present, with few complications associated with the exposure
itself. This is certainly not the case with anterior lumbar exposures and the
substantial amount of vessel mobilization that may be required, let alone the
issue of retrograde ejaculation in males. The exposure issues become even more
complex in the event of revision surgery or salvage surgery. Although both
types of devices have the potential to displace into the spinal canal and
cause catastrophic neurologic consequences, anterior displacement in the
cervical region is associated with the risk of esophageal injury and/or airway
obstruction, whereas anterior displacement in the lumbar region threatens the
aorta and the vena cava. Lateral displacement is of little practical
consequence for lumbar devices, but it may result in vertebral artery injuries
in the cervical spine. However, perhaps the most important distinction at this
time is with regard to the surgical indications. Proponents of lumbar disc
replacement have stated that the procedure is primarily indicated for the
treatment of discogenic axial pain. The indication for cervical disc
replacement remains the treatment of refractory cervical radiculopathy or
myelopathy.
The Bryan and Prestige disc concepts differ in numerous ways beyond their
historical development. One design attempts to take advantage of the latest in
polymer science, while the other is a simple metal-on-metal design. One is
less complex and rather intuitive in its insertion technique, while the other
takes cues from total joint preparation techniques. One design found its way
from "the back of a napkin" into pilot studies in humans, while
the other underwent extensive engineering and preclinical testing before human
trials were initiated. How then are spine surgeons to sort out the strengths
and weaknesses? The answer lies in rigorous analysis of the data to come from
the large-scale randomized trials currently underway. As we judge their
success or failure, we must bear in mind their intended benefits: preservation
of motion, early return to function, and elimination of the adverse influence
of fusion on adjacent motion segments. The current devices cannot restore
motion to an immobile spondylotic segment, and they are not likely to help to
relieve pain arising from arthritic facet joints. Yet, with adherence to
rational selection criteria, there may be reason for optimism with regard to
these evolving technologies.
The discussion about intervertebral disc replacement is reminiscent of the
discussions concerning lumbar arthrodesis over the last two decades. In April
1993, one of the authors (E.N.H. Jr.) wrote an editorial for the Bulletin
of the American Academy of Orthopaedic
Surgeons19.
He noted that the criteria for lumbar arthrodesis were ill defined and anyone
with persistent back pain might be subjected to this surgery. He stated that
"this expansion in indications has opened up an almost endless supply of
patients who are potential candidates for this procedure." Likewise, the
published information on lumbar arthrodesis has been incomplete and ambiguous.
"Unfortunately, scientific reports contain precious little objective
information to substantiate such approaches, particularly in the difficult
patient population on which these operations are so often performed. To some,
this is viewed as the `Red Badge of Courage' operation, while others maintain
that `someone has to try to help these patients' or `if I don't do it, someone
else will.'"
An ongoing discussion in the debates about lumbar arthrodesis has been the
definition of what is success and what is failure, e.g., Is complete or almost
complete relief of pain and return to full function and employment necessary
for a successful outcome, or is some decrease in pain and the use of pain
medication alone enough? What is an acceptable rate of complications for these
primarily early elected procedures? What about repeat procedures? Can they be
justified? What about the cost?
The description of intervertebral disc replacement is eerily similar to
that of lumbar arthrodesis but with an odd twist. Arthrodesis is currently
held up as the "gold standard" against which disc replacement is
compared. In no way can arthrodesis be considered a gold standard. At best,
the mediocre results of spinal arthrodesis for the treatment of back pain
should be considered a weak comparison. Attempting to make the case that the
results of disc replacement are similar to those reported for spinal
arthrodesis to justify marketing and implantation of these devices is suspect.
A review of the extensive literature on spinal arthrodesis for the treatment
of back pain indicated that "some improvement occurs as a result of
operative treatment in about 75% of patients, but major or complete relief of
pain and recovery of function are seen in 50% or
less."20
Early series of disc replacements appear to have had similar success
rates.
Although disc replacement surgery is treated by the lay press as mature
technology merely awaiting approval by the Food and Drug Administration in the
United States, this basis is meager at
best21. We know of
no satisfactory scientific literature on disc replacement in the cervical
spine. In the meager literature on disc replacement in the lumbar spine,
success rates of approximately 70% with poorly defined outcome criteria have
been reported. Perhaps, if the outcome criteria are more appropriately
defined, the success rates will be substantially lower. Published reoperation
rates have ranged from 5% to >20% with complications reported in >10% of
the
patients6,8,9,22.
So what do we know about the complex problem of discogenic neck or back
pain, and what might be the indications for surgery? The available scientific
evidence indicates that (1) the outer portion of the anulus fibrosus is pain
sensitive, (2) accurate diagnosis of the pain source is difficult, (3)
multiple compounding psychosocial factors are present in patients with chronic
spinal pain, (4) magnetic resonance imaging and discography are of
questionable reliability in the diagnosis, and (5) published surgical results
of arthrodesis or disc replacement are mediocre at best. Is this kind of
surgery worth it?
So what might be the criteria for defining long-term "success"
of implanted intervertebral disc replacement devices? The recommended criteria
for success of the procedure should be the following: (1) long-term (more than
ten-year) clinical outcomes that are equivalent to or better than those after
arthrodesis, (2) demonstrated preservation of motion at the operative level,
(3) less "adjacent segment" disease compared with that after
arthrodesis, (4) cost-effectiveness, (5) predictable, safe implantation by
spinal surgeons, (6) manageable wear and wear-related problems, (7) the
ability for safe or effective revision or salvage, and (8) control of
misuse.
We must be careful not to fall into faulty logic when considering the
rationale for and justification of disc replacements. One key assumption yet
to be proven is that, since spinal arthrodeses are not uniformly successful
for relieving axial pain, disc replacements may be better. A second key
assumption is that preservation of motion will eliminate or substantially
diminish the prevalence of adjacent segment disease. This, too, is a flawed
assumption since no published study has conclusively proven that arthrodeses
increase the risk of adjacent segment problems. In fact, the literature
contradicts that assumption. The study of cervical arthrodesis by Hilibrand et
al.13, which was
mentioned earlier in support of disc replacement, actually showed that the
incidence of degeneration at levels adjacent to longer arthrodeses was
decreased compared with that at levels adjacent to short arthrodeses; this
finding supports the contention that adjacent segment degeneration is not
caused by stiff segments but rather by genetic predisposition. To further
support the genetic predisposition theory, studies in twins have also shown
that mechanical exposures and physical labor are not as strong determinants of
disc degeneration as genetic factors are. Finally, one of the most common
reasons used to justify disc replacements is the high morbidity associated
with arthrodeses because of nonunion, graft donor-site complications, and slow
postoperative recovery. Most of the clinical trials of disc replacement are
attempting to show that the clinical outcomes after disc replacement are
superior to those after combined anterior and posterior arthrodesis or
posterior arthrodesis with instrumentation. But would the early clinical
outcomes after disc replacement be superior to those after percutaneous
interbody arthrodesis with use of an injectable BMP formulation? Perhaps the
minimally invasive arthrodesis of the future would be a more appropriate
control group, not the more morbid arthrodesis procedures of the past.
Other remaining issues that need to be addressed in intervertebral disc
replacement to make it a viable surgical option include the role of single
compared with multiple-level surgery, the issues of osteoporosis in patients
being considered for the procedure and in patients in whom it develops years
after they have undergone the procedure, and disease affecting other parts of
the three-joint spinal complex (the facets). Additionally, long-term issues
related to the development of spinal stenosis at persistent mobile segments,
operative scar complications, implant subsidence and extrusion, osteolysis,
and potential acute and late vascular problems need to be addressed. Revision
surgery will be a necessary part of intervertebral disc replacement regardless
of surgical training and technique. What will be the roles of repeat
replacements, prosthesis removal or retention, and attempted arthrodesis at
the site of a failed implant?
A real-time survey of the 133 members of the American Orthopaedic
Association in attendance at this symposium was done before and after these
presentations. The surgeons were asked what treatment they would prefer for
themselves if they had unrelenting low-back pain from a single degenerative
disc at L5-S1 that was affecting the activities of daily living. Seven percent
of them stated that they would choose disc replacement; 12%, anterior spinal
arthrodesis; 35%, posterior spinal arthrodesis; and 47%, chronic pain
management. In a similar question with regard to cervical radiculopathy, only
7% chose the option of cervical disc replacement. The primary concerns of the
audience regarding this technology were long-term durability of the implants
(59%), overuse (30%), and surgical complications (9%). In addition, 70% of the
audience believed that this technology would be very popular for a while and
then fall out of favor as long-term results become known.
While intervertebral disc replacement offers promising technology, more
information is needed to further define its potential role now and in the
future. One must remember that 80% of the population will have low-back pain
at some point during their lifetime. Moreover, it is estimated that the number
of spinal arthrodeses performed annually in the United States has steadily
increased to more than 300,000 per year. These impressive statistics must be
considered in the context of the lack of improvement in the rates of
successful healing or clinical outcomes after arthrodesis that have been
reported in the last twenty-five years, despite the advent of magnetic
resonance imaging, pedicle screws, cages, and other technological advances.
Will disc replacements deliver any better results for this ubiquitous human
condition of back and neck pain? Rapid adoption of immature technology with
inadequate clinical information may lead to yet another unfulfilling fad in
the treatment of back disorders. We need to avoid repeating the many
experiences of the twentieth century in the diagnosis and treatment of back
pain. As Richard Deyo stated in an article published in 1991 in The New
England Journal of Medicine: "The history of medical care for low
back pain—one of the most common causes of morbidity and absenteeism in
the United States— involves serial fashions in diagnosis and
therapy."23
With proper scientific input, rigorous clinical trials, controlled marketing
and distribution, and post-marketing surveillance, intervertebral disc
replacement may develop into a reasonable surgical option for some
appropriately selected patients and may avoid becoming a dangerous fad
remembered only for historical interest.
Note: The authors acknowledge the assistance of Donna D.
Ohnmeiss, PhD, in the preparation of the lumbar disc replacement portion of
this manuscript.
Frymoyer JW, Hanley E, Howe J,
Kuhlmann D, Matteri R. Disc excision and spine fusion in the management of
lumbar disc disease. A minimum ten-year followup.
Spine.1978; 3:
1-6.31
1978
[PubMed][CrossRef]
Lehmann TR, Spratt KF, Tozzi JE,
Weinstein JN, Reinarz SJ, el-Khoury GY, Colby H. Long-term follow-up of
lower lumbar fusion patients. Spine.1987;12:
97-104.1297
1987
[PubMed][CrossRef]
Brunet JA, Wiley JJ. Acquired
spondylolysis after spinal fusion. J Bone Joint Surg
Br.1984; 66:
720-4.66720
1984
Lee CK. Accelerated degeneration
of the segment adjacent to a lumbar fusion. Spine.1988; 13:
375-7.13375
1988
[PubMed][CrossRef]
McAfee PC, Cunningham BW, Orbegoso
CM, Sefter JC, Dmitriev AE, Fedder IL. Analysis of porous ingrowth in
intervertebral disc prostheses: a nonhuman primate model.
Spine.2003; 28:
332-40.28332
2003
[PubMed]
Cinotti G, David T, Postacchini
F. Results of disc prosthesis after a minimum follow-up period of 2 years.
Spine.1996;21:
995-1000.21995
1996
[PubMed][CrossRef]
Lemaire JP, Skalli W, Lavaste F,
Templier A, Mendes F, Diop A, Sauty V, Laloux E. Intervertebral disc
prosthesis. Results and prospects for the year 2000. Clin
Orthop.1997; 337:
64-76.33764
1997
[CrossRef]
Zeegers WS, Bohnen LM, Laaper M,
Verhaegen MJ. Artificial disc replacement with the modular type SB Charite
III: 2-year results in 50 prospectively studied patients. Eur Spine
J.1999;8:
210-7.8210
1999
[CrossRef]
Hochschuler SH, Ohnmeiss DD, Guyer
RD, Blumenthal SL. Artificial disc: preliminary results of a prospective
study in the United States. Eur Spine J.2002;11(Suppl 2):
S106-10.11S106
2002
[PubMed]
Marnay T. Lumbar disc
replacement, 7-10 year results with ProDisc. Trans Spinal
Arthroplasty Soc.2001.
2001
Mayer HM, Wiechert K, Korge A, Qose
I. Minimally invasive total disc replacement: surgical technique and
preliminary clinical results. Eur Spine J.2002;11(Suppl 2):
S124-30.11S124
2002
[PubMed]
Cunningham BW, Lowery GL, Serhan HA,
Dmitriev AE, Orbegoso CM, McAfee PC, Fraser RD, Ross RE, Kulkarni SS.
Total disc replacement arthroplasty using the AcroFlex lumbar disc: a
non-human primate model. Eur Spine J.2002; 11(Suppl 2):
S115-23.11S115
2002
[PubMed][CrossRef]
Hilibrand AS, Carlson GD, Palumbo MA,
Jones PK, Bohlman HH. Radiculopathy and myelopathy at segments adjacent to
the site of a previous anterior cervical arthrodesis. J Bone Joint
Surg Am.1999;81:
519-28.81519
1999
Goffin J, Geusens E, Vantomme N.
Long-term follow-up after interbody fusion of the cervical spine. J
Spinal Disord Tech.2003; in press.
2003
Birney TJ, Hanley EN Jr.
Traumatic cervical spine injuries in childhood and adolescence.
Spine.1989;14:
1277-82.141277
1989
[PubMed][CrossRef]
McGrory BJ, Klassen RA.
Arthrodesis of the cervical spine for fractures and dislocations in children
and adolescents. A long-term follow-up study. J Bone Joint Surg
Am.1994; 76:
1606-16.761606
1994
Cummins BH, Robertson JT, Gill
SS. Surgical experience with an implanted artificial cervical joint.
J Neurosurg.1998;88:
943-8.88943
1998
[PubMed][CrossRef]
Wigfield CC, Gill SS, Nelson RJ,
Metcalf NH, Robertson JT. The new Frenchay artificial cervical joint:
results from a two-year pilot study. Spine.2002;27:
2446-52.272446
2002
[PubMed][CrossRef]
Hanley EN Jr. Lumbar spine
fusion: matching expectations and outcomes. AAOS Bull.41:
6-7,1993.416
1993
Hanley EN Jr, David SM. Lumbar
arthrodesis for the treatment of back pain. J Bone Joint Surg
Am.1999;81:
716-30.81716
1999
Neergaard L. Artificial discs
advance in U.S. Charlotte Observer.2003
May 20;Sect A: 3.3
2003
Griffith SL, Shelokov AP,
Buttner-Janz K, Lemaire JP, Zeegers WS. A multicenter retrospective study
of the clinical results of the LINK SB Charite intervertebral prosthesis. The
initial European experience. Spine.1994;
19: 1842-9.191842
1994
[PubMed][CrossRef]
Deyo RA. Fads in the treatment of
low back pain. N Engl J Med.1991;325:
1039-40.3251039
1991
[PubMed][CrossRef]