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Perspectives on Modern Orthopaedics
Total Disk Arthroplasty
Eric L. Lin, MD
Jeffrey C. Wang, MD
Dr. Lin is Fellow, Spine Surgery,
Department of Orthopaedic Surgery,
Rush University Medical Center,
Chicago, IL. Dr. Wang is Chief,
Orthopaedic Spine Service and
Associate Professor of Orthopaedic and
Neurosurgery, UCLA Comprehensive
Spine Center, UCLA School of
Medicine, Santa Monica, CA.
None of the following authors or the
departments with which they are
affiliated has received anything of value
from or owns stock in a commercial
company or institution related directly or
indirectly to the subject of this article:
Dr. Lin and Dr. Wang.
Reprint Requests: Dr. Wang, UCLA
Comprehensive Spine Center, UCLA
School of Medicine, 1250 16th Street,
7th Floor, Santa Monica, CA 90404.
J Am Acad Orthop Surg 2006;14:705714
Copyright 2006 by the American
Academy of Orthopaedic Surgeons.
Volume 14, Number 13, December 2006
Abstract
Spinal fusion remains the gold standard for surgical management of
instability and mechanical low back or neck pain. However, even
in carefully selected patients, successful clinical results can be
difficult to achieve. Reasons for failure include pseudarthrosis and
adjacent spine segment disease. The theoretic advantages of
removing the painful disk while preserving motion have led to
increasing interest in total disk arthroplasty. Although disk
replacements have been implanted in Europe for decades, the
procedure is relatively new in the United States. Recently, two
artificial disks for symptomatic lumbar degenerative disk disease
have been approved by the US Food and Drug Administration;
several others are undergoing clinical trials. Short-term studies
demonstrate similar clinical improvements for both disk
replacements and fusion procedures at up to 2-year follow-up.
Issues requiring further research include optimal design
specifications, potential complications, and appropriate patient
selection. Consequently, the long-term benefit of total disk
arthroplasty over fusion for the treatment of axial low back or neck
pain remains to be determined.
A
xial spine pain is among the
more difficult treatment problems faced by spine surgeons. Several different modalities have been
developed in an attempt to help patients manage symptoms of mechanical back pain. Arthrodesis of
the spine is the gold standard for surgical treatment of low back pain.
However, the etiology of the specific pain generators involved can be
difficult to determine, and surgical
outcomes to date leave substantial
room for improvement.
The use of biologic growth factors
combined with bone grafting and
more rigid instrumentation has led
to an increase in spine fusion rates.
However, total disk arthroplasty
(TDA) has recently received signifi-
cant attention in the United States
as an alternative to spine fusion.
Etiology of Axial
Symptoms in the Spine
The etiology of mechanical back and
neck pain is not well understood.
Causes of the pain can be multifactorial, ranging from specific anatomic abnormalities to reasons that include psychiatric and social issues.
Anatomically, the functional spinal
unit consists of two vertebral bodies
and the intervertebral disk. Areas
with potential pain generators include the disk and facet joints. Arthritis of these articulations can lead
to spinal instability, abnormal or restricted motion, and progressive de705
Total Disk Arthroplasty
terioration, all of which can result in
significant pain. Moreover, research
in biology of the intervertebral disk
has shown that degenerative disk
disease itself can result in irritation
of pain fibers within the anulus fibrosus.
Radiographs and magnetic resonance imaging (MRI) aid the clinician in diagnosing degenerative disk
disease. Plain radiographs can demonstrate loss of disk space height, development of osteophytes, and end
plate sclerosis. Changes on MRI include loss of disk water content (diminished intensity on the T2weighted images), high-intensity
zone signal, and end plate irregularities. Clinically, provocative diskograms and facet joint injections can
be used to pinpoint the painful area
more accurately. However, because
these procedures are operator- and
patient-dependent, the consistency
of the results is highly variable.
History of Spine Fusion
Spinal arthrodesis for degenerative
disk disease is a controversial treatment modality for low back and
neck pain. Although fusion of an arthritic joint may reliably reduce pain
in the appendicular skeleton for
many patients, multiple factors can
adversely affect outcomes and lead
to suboptimal results in the spine.
Nevertheless, eliminating motion at
an arthritic functional spinal unit
has been the surgical answer for
many years to painful degeneration
of the spine, both to decrease symptoms and to prevent further instability. Successful patient outcomes
from lumbar spinal fusion range
from 60% to 85%.1-4 These studies
demonstrate the complexity of the
surgical approach to low back pain.
For example, pain relief can be incomplete even in patients with radiographic evidence of solid spine fusion. This suggests that fusion of the
arthritic functional spinal unit may
not be the answer and that the pain
could be from another source.
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Adjacent spinal disk segment disease is also a well-described entity
that can develop after cervical and
lumbar spine fusion.5-10 It is unclear
whether adjacent segment disease is
related to altered biomechanics, natural history of disk degeneration, a
genetic predisposition, or, more likely, a combination of several of these
factors. As well, harvesting autologous iliac crest bone graft for spinal
fusion can be associated with considerable donor site morbidity. However, in a landmark study, Fritzell et
al11 prospectively demonstrated that,
compared to nonsurgical modalities,
lumbar spine fusion significantly (P
= 0.0002) improved outcomes in patients with chronic low back pain.
Furthermore, better patient selection, improved spinal instrumentation, and biologics to improve bone
healing have accounted collectively
for increased rates of solid fusion. As
a result, all current US Food and
Drug Administration (FDA) investigational device exemption (IDE)
studies involving TDAs use spine fusion as the control treatment.
Theory of Disk
Arthroplasty
The intervertebral disk consists of
an outer anulus fibrosus and inner
nucleus pulposus. When healthy, the
intervertebral disk cushions the axial load of the spine during weight
bearing while acting as a joint during
spinal motion for translation and rotation. Over time and with continued stress, the spinal disks may degenerate and lose their inherent
mechanical properties, occasionally
resulting in abnormal motion and
pain.
TDAs were developed to replace
the diseased disk and to alleviate
pain and restore functional motion
at the level of the disk replacement.
The prostheses have been designed
using principles derived from total
hip and knee arthroplasties. The primary goal of TDAs is to remove the
pain generator while maintaining
disk height, ensuring spinal stability,
and preserving motion. In comparison with fusion, this approach
would potentially have two distinct
advantages. First, no fusion must occur; therefore, pseudarthrosis is removed as a potential complication,
allowing for earlier patient mobility.
Second, by preserving motion, the
TDA may decrease the incidence of
adjacent spine segment degeneration
by reducing stress at adjacent spine
levels.
In vitro biomechanical research
has shown normalization of adjacent
segment alterations after TDA
implantation in cadaveric specimens.12,13 In addition, several shortterm clinical studies have confirmed
that, not only do patients experience
symptom relief, but recovery time
also is comparable. Furthermore,
radiographic motion is maintained
with TDA compared with fusion.14,15 How this will affect intermediate and long-term outcome,
however, is unknown.
History and Current
Design Concepts
In the late 1950s, Fernstrom16 implanted the first disk prosthesis into
the cervical and lumbar spines of humans. The prosthesis consisted solely of a steel ball placed within the
anulus fibrosus after the nucleus
pulposus had been removed. The
theory was to maintain height and
motion. Predictably, after a short period of symptom relief, the prosthesis ultimately failed secondary to
subsidence of the implant within the
spine verebra. Since the introduction
of that prototype, more complex designed prostheses have been developed to replicate the mechanical
functions of a healthy spinal disk.
Disk arthroplasty can be classified as either nuclear replacement
and TDA; TDA is discussed below.
Current designs of both lumbar and
cervical disks are a “ball and socket”
or trough. Essentially, the functional characteristics of a prosthesis
Journal of the American Academy of Orthopaedic Surgeons
Eric L. Lin, MD, and Jeffrey C. Wang, MD
should include long-term endurance
(to last the lifetime of a patient),
composition of biologically compatible materials, and avoidance of premature disintegration.17
Table 1
Current Indications for Total Disk Arthroplasty18,19
Criteria
Inclusion
Indications
As with spinal fusion, the success of
TDA is highly dependent on patient
selection. This well-recognized fact
is reflected by the current inclusion
and exclusion criteria for enrollment
in the FDA IDE trials for a possible
TDA18,19 (Table1). Of particular importance is that the primary indication for lumbar TDA is isolated discogenic low back pain without
instability. This differs from the
cervical spine, where TDAs replace
fusion (when no instability is
present) after decompression for
radiculopathy/myelopathy. It is
hoped that, as experience is gained
with TDAs, the list of inclusion criteria will be modified to ensure safe
and effective implantation of these
prostheses.
With more widespread application of TDAs in the near future,
spine surgeons will continue to push
the currently accepted indications.
What will happen to the complication rate when strict guidelines are
not followed is unknown. Already,
however, reports of implanting
TDAs at degenerated juxtafusional
levels20 and revising failed TDAs
with another TDA21 have been published.
Surgical Technique
The surgical technique for both cervical and lumbar TDAs uses an anterior approach. In the cervical spine,
a standard Smith-Robinson approach
is used to gain access to the desired
disk space. Frequently in the lumbar
spine, a general or vascular surgeon
assists in accessing the diseased level through an anterior retroperitoneal dissection.
Once adequate exposure is
achieved, the intervertebral disk is
removed and the end plates of the
Volume 14, Number 13, December 2006
Exclusion
Cervical
Lumbar
Young age (18-65 yr)
Young age (18-60 yr)
Symptomatic 1- or 2-level
Subjective and objective
discogenic back pain
evidence of
radiculopathy/myelopathy (L3-S1), concordant with
radiographs and
with 1- to 3-level disk
diskograms
disease ± axial neck pain;
concordant with CT/MRI
Failure of >6 mo of
Failure of >6 wk of
conservative treatment
conservative treatment
AS, RA, OPLL, DISH
Central/lateral recess
stenosis
Insulin-dependent diabetes
Facet
arthropathy
mellitus
Cervical instability
Previous cervical fusion/
infection/fracture
Osteoporosis
Chronic corticosteroid use
Obesity
Pregnancy
Isolated axial neck pain
Spondylolysis/
spondylolisthesis
Radiculopathy secondary to
HNP
Scoliosis
Osteoporosis
Chronic corticosteroid use
Previous lumbar
fusion/infection/fracture
Obesity (>1 SD over ideal
body weight)
Pregnancy
AS = ankylosing spondylitis, CT = computed tomography, DISH = diffuse idiopathic
skeletal hyperostosis, HNP = herniated nucleus pulposus, MRI = magnetic resonance
imaging, OPLL = ossification of posterior longitudinal ligament, RA = rheumatoid
arthritis, SD = standard deviation
vertebral bodies are prepared. An important step is adequately releasing
the posterior anulus fibrosus to allow correct positioning and function
of the prosthesis. At this point, each
implant system varies in specific
technique; essentially, however, before implanting the final prosthesis,
the proper size and lordotic angle is
determined with anteroposterior/
lateral fluoroscopy to ensure proper
spinal implant placement.
Lumbar Disk
Arthroplasty
To date, the most experience and interest in lumbar TDA has been for
treatment of discogenic low back
pain. In Europe, thousands of prostheses have been implanted since
the mid-1980s. However, reports of
efficacy have been criticized for their
retrospective nature and lack of randomization with controls.
The first TDA in the lumbar
spine to be implanted in the United
States was the SB Charité III (DePuy
Spine, Raynham, MA) in March
2000, as part of a controlled randomized study. Other prostheses currently under investigation include the
ProDisc-L (Spine Solutions/Synthes,
Paoli, PA), Maverick (Medtronic Sofamor Danek, Memphis, TN), and
FlexiCore (Stryker Spine, Allendale,
NJ). Each design has specific differences with respect to material, bearing surface, number of articulations,
constraint, mobility of the center of
rotation, and fixation to bone22 (Table 2).
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Total Disk Arthroplasty
Table 2
Properties of Total Disk Arthroplasty Prostheses
Implant
Cervical
Bryan
Material
Bearing
Surface
Articulations
Constraint
COR
Fixation
Mobile
Milled
cavities/
bone
ingrowth
Screws
Titanium
polyurethane
Metal on
polymer
2
Unconstrained
Stainless
steel
CoCrMo
UHMWPE
CoCrMo
UHMWPE
Metal on
metal
Metal on
polymer
Metal on
polymer
1
Semiconstrained Mobile
1
Semiconstrained
Fixed
1
Semiconstrained
Fixed
CoCrMo
UHMWPE
Metal on
polymer
2
Unconstrained
Semiconstrained
Fixed
1
Semiconstrained
Fixed
FlexiCore
CoCrMo
Metal on
polymer
Metal on
metal
Metal on
metal
1
Maverick
CoCrMo
UHMWPE
CoCrMo
1
Fully
constrained
Fixed
Prestige
PCM
ProDisc-C
Lumbar
SB
Charité
III
ProDisc-L
Mobile
Bone
ingrowth
Keel/bone
ingrowth
Small
fins/bone
ingrowth
Keel/bone
ingrowth
Keel/bone
ingrowth
Small
fins/bone
ingrowth
COR = center of rotation, CoCrMo = cobalt-chromium-molybdenum alloy, UHMWPE = ultra-high molecular weight polyethylene
(Adapted with permission from Anderson PA, Rouleau JP: Intervertebral disc arthroplasty. Spine 2004;29:2779-2786.)
Figure 1
SB Charité III artificial disk (oblique view). A, anterior. B, lateral. C, CoCrMo alloy
end plates. D, fixation fins. E, polyethylene core. (Reproduced with permission from
Depuy Spine, Inc. Copyright Depuy Spine, Raynham, MA.)
The initial design of the SB
Charité III, developed by ButtnerJanz and Schellnack, consisted of
708
small bottlecap-like end plates with
a polyethylene core. Because of the
increased stress concentration over a
small contact surface area, subsidence into the vertebral bodies became a concern. The next generation
used thin lateral extensions to augment the surface area, but these succumbed to fatigue fracture.
The SB Charité III has end plates
manufactured
from
cobaltchromium-molybdenum (CoCrMo)
alloy with small fins projecting into
the vertebral bodies (Figure 1). To assist bony ingrowth, the outer layer is
porous-coated with plasma-sprayed
titanium and calcium phosphate
(TiCaP), which is available only outside the United States. An important
feature of the SB Charité III is the
sliding, unconstrained biconvex polyethylene core, which is designed to
allow for an instantaneous axis of rotation during flexion and extension,
more closely paralleling the natural
motion of the native disk (Figure 2).
Journal of the American Academy of Orthopaedic Surgeons
Eric L. Lin, MD, and Jeffrey C. Wang, MD
Figure 2
SB Charité III artificial disk implanted at L5-S1. Radiographs of flexion-extension in lateral (A and B) and anteroposterior (C)
views. (Reprinted with permission from McAfee PC, Fedder IL, Saiedy S, Shucosky EM, Cunningham BW: Experimental design
of total disk replacement-experience with a prospective randomized study of the SB Charite. Spine 2003;28:S153-S162.)
However, there are potential disadvantages to the SB Charité III disk.
Because it has two articulations, over
time it is theoretically more prone to
polyethylene wear and debris than
are single-articulation designs. In addition, as an unconstrained device,
the risk of polyethylene extrusion is
greater and potentially catastrophic.
Although the extent of complications is currently unknown, early
clinical reports suggest a low incidence. At 2-year follow-up, Guyer et
al23 reported similar significant (P <
0.001) improvements in Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) scores for both
100 patients with the SB Charité III
and 44 patients with an anterior fusion (BAK Interbody Fusion System,
Zimmer Spine, Warsaw, IN). Three
patients in the Charité group required additional posterior spinal fusion for persistent pain, but no cases
of dislodgement, significant subsidence, loosening, or infections were
reported.
The ProDisc I (Spine Solutions/
Synthes), developed by Marnay in
the 1980s, was designed in France
and implanted first in 1990. With an
average 8.7-year follow-up, Tropiano
et al24 reported 75% good to excelVolume 14, Number 13, December 2006
lent results in their series. Most recently, the ProDisc-L has achieved
FDA approval after undergoing modifications, including changing the
end plate metal from titanium to
CoCrMo and adding the polyethylene core bearing as a separate modular piece (Figure 3). In contrast with
the SB Charité III, end plate fixation
is accomplished with asingle, large,
porous-coated, midline sagittal keel.
The polyethylene core is snap-fit and
fixed to the inferior end plate, resulting in a semiconstrained design,
which hypothetically decreases the
risk of extrusion. On the other hand,
the fixed axis of rotation does not allow for coupled vertebral translation
with flexion and extension. Proponents claim that a degenerated disk
segment has an abnormally increased range of motion; thus, the
semiconstrained design permits stability during a more controlled arc of
motion and protects the facet joints
from shear. Opponents of this model argue that abnormal forces generated could shift to the bone–end
plate interface, causing loosening.25
Nevertheless, early results with
the ProDisc-L have been encouraging. Delamarter et al26 reported significant (P < 0.05) reduction in pain
Figure 3
ProDisc-L prosthesis (oblique view).
A, superior fixation keel. B, lateral.
C, polythylene core. D, anterior.
(Courtesy of Synthes, West Chester,
PA.)
(VAS) and disability (ODI) with
ProDisc-L at 6- and 12-week followups. By 6 months, however, the relative improvements were similar for
patients with the ProDisc-L and
those with anterior/posterior lumbar
spine fusion. In a separate study,
Tropiano et al27 showed similar improvements in functional scores but
an associated 9% complication rate
and 6% revision rate in 53 patients
treated with ProDisc-L (average
follow-up, 1.4-years).
The Maverick Artificial Disk
(Medtronic Sofamor Danek) was first
implanted in Europe in early 2002.
709
Total Disk Arthroplasty
Figure 4
Figure 5
Figure 6
FlexiCore implant (anterior view).
A, superior end plate. B, lateral fixation
fins. C, metal-on-metal articulation.
(Courtesy of Stryker Spine.)
Maverick Artificial Disk (lateral view).
A, superior fixation keel. B, metalon-metal articulation. C, anterior.
(Courtesy of Medtronic.)
The characteristic feature of this
prosthesis is its CoCrMo metal-onmetal bearing surface (Figure 4). Recent biomechanical and biomaterial
research on articulating surfaces has
shown that metal-on-metal bearings
generate significantly fewer wear
particles and stimulate less of an immune response than do metal-onpolyethylene bearings. The bottom
end plate includes a superiorly projecting dome that articulates with a
matching concavity from the superior end plate.
Like the ProDisc-L, the Maverick
uses large, porous-coated keels for
fixation and a semiconstrained design. However, with the intention of
more accurately reproducing normal
spinal motion, the fixed axis of rotation is slightly more posterior compared with that of the ProDisc-L.
Even so, the same theoretic advantages or disadvantages of a semiconstrained system apply to the Maverick. Recently, Mathews et al28
reported an average 36-point improvement in ODI scores in seven
patients with the Maverick Artificial Disk at 18 months postsurgery.
The FlexiCore Intervertebral Disc
(Stryker Spine) is the latest prosthesis to begin an FDA IDE clinical trial.
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This disk also has a metal-on-metal
(CoCrMo) articulating surface with
titanium porous coating for bony ingrowth (Figure 5). The unique feature
of the FlexiCore disk, however, is the
fully constrained design. To date, no
published data are available on the
FlexiCore prosthesis.
Cervical Disk
Arthroplasty
Treatment of isolated discogenic
neck pain with cervical fusion is
controversial. However, in patients
who have radiculopathy/myelopathy
and/or instability secondary to degenerated disks, anterior diskectomy
and fusion has been shown to have a
high rate of success. However, notable percentages of adjacent spinal segment disease after cervical fusion
have been reported.29 Swallowing difficulty secondary to anterior plate
fixation also has been described.30 In
an effort to avoid these complications, cervical spine TDAs have been
manufactured and implanted in humans internationally. In the United
States, FDA-sponsored IDE trials
have begun in the last year .
The
Bryan
Cervical
Disk
(Medtronic Sofamor Danek) is an
unconstrained, biarticulating, metalon-polyurethane prosthesis (Figure
6). The metal is titanium alloy with
porous-coated end plates. The Bryan
disk contains a polyurethane sheath
surrounding the nucleus; the sheath
Bryan Cervical Disk (oblique view).
A, titanium alloy superior end plate.
B, anterior flange. C, polyurethane
sheath. D, anterior. (Courtesy of
Medtronic.)
is filled with saline, which acts as
synovial fluid. Hypothetically, this
would aid in keeping any potential
wear debris within the cavity while
preventing soft-tissue ingrowth. In
vivo testing has confirmed satisfactory wear characteristics without
producing a significant inflammatory response.31 At 2-year clinical
follow-up, Goffin et al14 reported excellent, good, or fair outcomes in 44
of 49 patients (90%) implanted with
a single-level Bryan disk. Radiographically, a high percentage (93%)
demonstrated >2° of flexionextension (signifying motion) at the
implanted level (Figure 7). At the
same time, problems in maintaining
focal cervical lordosis have been described.32
The Prestige ST (Medtronic Sofamor Danek) is a modification of the
Prestige II (Figure 8), Prestige I, and
original Bristol-Cummins disk. (Its
articulation is also metal-on-metal
(stainless steel) but, in contrast with
other designs, the fixation is through
a screw-locking mechanism into the
vertebral body, similar to anterior
cervical plates. Following the relative success of its precursors, Porchet and Metcalf33 prospectively re-
Journal of the American Academy of Orthopaedic Surgeons
Eric L. Lin, MD, and Jeffrey C. Wang, MD
Figure 8
Figure 7
Prestige II prosthesis (lateral view). A,
metal-on-metal articulation. B, superior
fixation screw. C, anterior. (Courtesy
of Medtronic.)
Bryan Cervical Disk. A and B, Lateral flexion-extension cervical radiographs
demonstrate range-of-motion preservation at C5-C6. (Reproduced with permission
from Goffin J, Van Calenbergh F, van Loon J, et al: Intermediate follow-up after
treatment of degenerative disc disease with the Bryan cervical disc prosthesis:
single-level and bilevel. Spine 2003;28:2673-2678.)
ported similar improvements in
patient outcome measures with the
Prestige II device compared with fusion. Traynelis34 reported that the
Prestige disk successfully preserves
normal spinal range of motion
(follow-up, 2 years).
The Porous Coated Motion artificial cervical disk (Cervitech, Rockaway, NJ) is a minimally constrained
prosthesis with cobalt-chromium alloy end plates, TiCaP porous-coated
serrated surfaces, and a polyethylene
core affixed to the inferior end plate.
Short-term clinical results are encouraging. At 1-year follow-up of 53
patients, Pimenta et al35 showed significant improvements in VAS and
Neck Disability Index scores, with
97% of patients reporting excellent
or good results.
The ProDisc-C (Spine Solutions/
Synthes) is very similar in design to
its lumbar counterpart. Although in
vitro biomechanical analyses show
Volume 14, Number 13, December 2006
favorable results,36 no clinical outcomes are available to date.
Complications
van Ooij et al37 reported on 27 cases
of failed SB Charité prostheses. At an
average of 53 months after surgery,
adjacent level spinal disease, subsidence, and facet joint arthrosis were
the most common causes of failure.
Two patients experienced anterior
dislocation of the implant; overall,
11 patients required additional salvage surgery.
Low incidences of infection, vertebral body fracture, implant malposition, subsidence, mechanical failure, and paravertebral heterotopic
ossification also have been reported
by authors of the ongoing clinical
trials.23,26,27 Because follow-up is relatively short-term, it is likely that
most morbidities have been related
to technique and/or to surgical ap-
proach and not to the implant itself.
However, it is too early to fully determine all of the potential complications that may be associated with
TDAs.
Questions for the
Future
Many questions remain unanswered
regarding the safety and efficacy of
TDAs over the long term. For example, will TDAs be superior to spinal
fusion or simply be an option in a
subset of cases? Who will be the ideal patient? How many patients will
fit the current or future inclusion
and exclusion criteria? Can we reliably and accurately predict who will
have improved symptoms while
maintaining spinal stability and motion? How long can we expect these
prostheses to last before failing?
What will be the optimal design?
Will the property of shock absorption of a healthy disk, not addressed
by current TDAs, be an issue?
With variations in the current
prostheses (and with more designs
emerging), the clinical outcomes of
each will, in time, provide answers
to these questions. However, biologic treatment, including intradiscal
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Total Disk Arthroplasty
administration of growth factors
such as osteogenic protein-138 and/or
gene therapy, also may become
readily available to help regenerate
diseased disks.
Several biomechanical and shortterm clinical studies have demonstrated that TDAs preserve motion.
Huang et al39 retrospectively reported on 58 ProDisc implants at a mean
follow-up of 8.7 years; 66% of prostheses retained >2° of motion at the
implanted level. Although the incidence of adjacent segment disk degeneration in their series was 24%,
no patients required further surgery.
Furthermore, there seemed to be an
association between implant range
of motion and development of junctional degeneration.39 Will this hold
true in longer follow-ups? If so, will
it decrease the incidence of adjacent
spinal segment disease?
How reliably will surgeons be
able to accurately implant the prostheses and replicate physiologic motion? McAfee et al40 concluded that,
Additional Resources
DVD/video: Standing Room
Only 4: “Spine Surgery: Evolving
Applications and Techniques.” K.
Daniel Riew, MD, Editor. Includes several videos on disk
replacement. http://www4.aaos.
org/product/productpage.cfm?co
de=02815
Related clinical topics articles
available on Othopaedic Knowledge Online: “Lumbar Disk Herniation,” by Rick B. Delamarter,
MD: http://www5.aaos.org/oko/
spine / lumbar_disc_herniation /
pathophysiology/ pathophysiology.cfm
“L5-S1 Disk Degeneration,” by
D. Greg Anderson, MD, and Todd
Albert, MD: http://www5.aaos.
org/oko/ spine/ L5S1_degenerati
on/pathophysiology/pathophysio
logy.cfm
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at a minimum 2-year follow-up of
205 patients, accurate surgical placement of the SB Charité III significantly correlated with clinical outcomes including ODI, VAS, and
range of motion (P < 0.001 for all). As
surgeons who are at the “top of the
learning curve” in terms of experience, these authors were able to implant the Charité in an “ideal” position (defined as within 3 mm of ideal
in both planes) only 83% of the time.
It can be expected that, with more
general use by spine surgeons who
are less familiar with the procedure,
and who have performed fewer procedures, this percentage will be noticeably lower.
Will there be any adverse effects
from wear particles in the spine?
Aseptic loosening secondary to osteolysis and peri-implant immune responses are of particular concern. In
addition, the systemic effect of metal
ions from metal-on-metal articulations, including carcinogenicity, is
currently unknown. Loads sustained
by a single intervertebral disk during
normal activity, although not insignificant, are considerably less than
those of a hip or knee. As a result,
simulated wear cycling of TDAs predicts that prosthesis wear is relatively low in vivo and that the likelihood is minute of these small
amounts of debris causing a significant inflammatory response within
the epidural tissues.41
How will we deal with the complications and manage revision surgeries? One opinion is that a TDA
will not “burn any bridges,” that
failure is salvageable with fusion. Is
this the case? Notwithstanding the
potential devastation from implant
dislocation, difficulties with repeat
anterior surgical exposure of the
spine, subsidence, osteolysis, and
subsequent bone loss may be extremely challenging to repair.
Finally, if current and future research proves that TDA is superior
to fusion in a select group of patients, will TDA be more costeffective in the long run? Recently,
Singh et al42 reported that, by 2010,
an estimated $2.18 billion will be
spent on spinal arthroplasty procedures in the United States.
Summary
TDA is a novel and exciting technology for spine surgeons to consider as
a possible alternative to fusion. Potential advantages include motion
preservation, thereby preventing (or
at least postponing) adjacent segment disease; shorter recovery time;
and avoidance of fusion-related complications. Two lumbar designs have
been recently approved by the FDA
for commercial use; others will follow. Short-term clinical results appear to be promising, with acceptable complication rates, but should
continue to be critically analyzed in
peer-reviewed literature. Whether
hypothetic advantages will outweigh potential pitfalls has yet to be
determined. Only as reliable, prospective, long-term outcomes data
and cost-analysis research become
available will we understand the
true place of TDA in spine surgery.
References
Citation numbers printed in bold
type indicate references published
within the past 5 years.
Evidence-based Medicine: Level I
studies include references 11, 23, 24,
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