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					Total Disc Replacement — St. Mary's Experience
Vikram Parmer MD, James Zucherman MD, Ken Hsu MD, Paul Fuchs MD

   I.    Functions of the intervertebral disk
            a.    Mobility
            b.    Weight Bearing
            c.    Provide stability
   II.   What happens to the diseased intervertebral disk?
 III.    Limitations of current treatments for the diseased intervertebral disk
  IV.    Goals of intervertebral disk replacement
   V.    Contraindications
  VI.    Designs
 VII.    Components of the modern disk replacement
VIII.    Prospective randomized double blind trial of the ProDisc intervertebral disk prosthesis versus
         interbody fusion
  IX.    Inclusion and exclusion criteria
   X.    Surgical Technique
  XI.    Results
 XII.    Complications
XIII.    Future Goals


It is classically taught that the lumbar spine is designed to provide axial rigidity to the lower trunk, to
sustain axial compression loads exerted from the trunk and upper limbs and to permit movements between
the trunk and pelvis. A functional spinal motion segment has three articulations — the intervertebral disk
and the two posterior facet joints. The intervertebral disk is responsible for transmission and stabilizing a
combination of compressive, torsional and bending forces subjected to the trunk of the body. The
hydrostatic pressures in the inner gel like nucleus pulposus resist compressive and bending forces while
torsional forces are mainly resisted by the outer annular fibers. The facets joints provide a posterior
articulation between adjacent vertebrae and limit mobility of the motion segment. There is a close
relationship between the intervertebral disk and the posterior facet joints. Facet arthrosis and degeneration
do not occur without the presence of adjacent disk degeneration.


During the degenerative process, changes occur in the nucleus and annulus. In the nucleus, chemical
changes decrease proteoglycan content, thereby impairing its ability to bind water and bear axial loads. This
results in abnormal loading of the motion segment with the annular fibers being subjected to excessive
stresses. Deterioration of the annulus results. This allows stimulation of the pain fibers in the annulus
leading to back pain symptoms. The combination of degenerative disk changes and accompanying facet
deterioration are hallmarks of the degenerative cascade. In addition to axial back pain, end stage
consequences of the degenerative cascade may include symptomatic spinal stenosis, degenerative
spondylolisthesis and degenerative scoliosis.


Current surgical treatments for degenerative spinal problems do not restore normal spinal function.
Diskectomy and laminectomy violate the three joint complex and often increase segmental instability. The
instability may require fusion. Fusion itself is not without problems. Fusion procedures have been shown to
accelerate the degenerative process and lead to adjacent instability and spinal stenosis. Other problems with
fusion include chronic bone donor site pain and the need for further procedures such as implant removal.


In reviewing the literature, spinal fusion has varied success rates. A 65% to 93% success rate has been
described in the current literature. Even when one attains "successful" lumbosacral arthrodesis or fusion,
there are still long-term sequelae that must be considered. Additionally, there are inherent drawbacks.
Kostuik in Orthopaedic Clinics of North America 1998, estimated that approximately 30% of patients who
underwent fusion had long-term sequelae. He did state, though, that fusion was successful in the majority
of patients. The main issue with spinal fusion is adjacent segment degeneration. After spinal fusion, motion
segments are removed from function, and as a result spinal levels adjacent to the fusion have additional
stress placed upon them. Premature disc degeneration, instability, and premature arthritis are all potential
issues at the adjacent segments after a fusion. Authors such ask C.K. Lee in Spine in 1988 and T.R. Lehman
in Spine in 1987 have confirmed the phenomenon of adjacent segment degeneration. Additionally, Hsu and
Zucherman at the NASS meeting in 1988 presented their experience. They showed that the addition of rigid
instrumentation commonly used during fusion procedures can lead to even further stress upon the adjacent

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segments. There are still further concerns, such as pseudarthrosis, graft site pain, stress shielding, disuse
osteoporosis, and instrument failure in association with fusion procedures.


Because of concerns with fusion procedures, there is a significant amount of interest in total disc
replacement (TDR). Proponents of TDR state that total disc replacement would potentially relieve symptoms,
prevent long-term issues associated with fusion, and also remove the diseased disc, commonly known as
the "pain generator." In addition, other major goal of disk replacement surgery includes maintaining and
restoring normal segmental spinal motion. It would restore the disk space height and therefore restore the
segmental lordosis. It would allow indirect decompression of foraminal stenosis and protect adjacent levels
from iatrogenically accelerated degeneration.


Fernstrom is credited with the first disc replacement in 1966 (See Fig. 1 and 2). The disc replacement
consisted of essentially a ball bearing that was placed between the vertebral bodies in the spine. This failed
because of subsidence into the vertebral body over time, as well as loosening and expulsion. Since that
time, other physicians have invented their own form of disc replacement. While the literature is somewhat
sparse, Lee, Ray, Steffee, Buttner-Jans (Charite) (Fig. 3), and Marnay (ProDisc) (Fig. 4) have documented
their individual experiences with their respective disc replacements. These authors have proposed disc
replacement as a substitution for fusion in order to treat back pain and at the same time preserve vertebral
motion.


Today, both in Europe and the United States, there has been great success with total joint arthroplasty,
specifically in the knee and hip (Fig. 5 and 6), and to a somewhat lesser extent in the shoulder. Prior to total
hip arthroplasty, for example, a patient had to deal with decreased function as well as long-term pain. Their
options for treatment were essentially limited. Some patients were told just to "deal with their pain." Others
tried bracing, pain medicines, therapy, and activity modification. Unfortunately, they had to refrain from
activities they enjoyed. Fusion was an option, and it provided significant pain relief, but at the expense of
motion. Additionally, these patients underwent premature wear and tear of the surrounding joints, most
notably the contralateral hip, the knees, the feet, and the spine. At this time, total joint arthroplasty of
knees and hips has surpassed fusion as the standard of care. The reason for this is that the long-term
results are excellent. The patients sustain significant functional improvement. Additionally, the indications
for this procedure are straightforward. On the other hand, spinal fusion is still widely performed for many
etiologies. Very commonly in the United States, spinal fusion is performed for discogenic back pain as well
as instability.


From a clinical standpoint general indications for disk replacement surgery include: (1) patients with back
and leg pain not responsive to appropriate nonsurgical treatment, (2) symptomatic 1 or 2 level disk changes
associated with collapse, (3) symptomatic early stage disk changes identified by MRI and/or diskogram in
the absence of facet arthritis, (4) and patients without prior history of back surgery at the affected level.


Major contraindications include facet joint arthrosis, spinal instability, altered posterior elements secondary
to prior surgery (such as laminectomy, and facetectomy), infection and metabolic bone diseases
(osteoporosis).


Kostuik in Orthopaedic Clinics of North America reviewed his prerequisites and criteria for biomechanically
and functionally successful disc replacement. First, disc replacements would need to be durable. As disc
replacements would be placed in patients in the age range of 30 to 50 years of age, they should be able to
last 40 years. He estimated that during this time period, a motion segment would undergo 85 million cycles,
and as a result testing should be performed at 1 million cycles. Also, the materials must be compatible, and
we must be conscientious of the volume of wear particles. It is known from the total joint literature that the
body responds to wear particles and osteolysis is the result. Osteolysis is a major concern because of bone
loss and loosening of implants. Additionally, there are corrosion issues. Implants with dissimilar metals can
potentially cause corrosion and premature wear of the implant. The implant must also have a long fatigue
life so that fracture and breakage do not occur. Geometry of the disc placement prosthesis needs to be such
that neurovascular structures are not impinged upon or put at risk. There also needs to be the ability to
implant the disc replacement at contiguous levels. The kinematics of the disc replacement should ideally
match those of the healthy disc. Numerous studies have been performed, which have determined the range
of motion of the native discs, and the disc replacements should emulate these ranges of motion. The disc
replacements need to have dynamics which are equal to the inherent disc as well. The disc has a natural
stiffness about it. The disc replacement should duplicate the native disc stiffness in order to transmit


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physiologic stresses. Without the transmission of physiologic stresses, there could be bone resorption, which
would lead to implant loosening, or with too much stress there would be bone deposition. Bone deposition
could cause decreased range of motion of the prosthesis, or impinge upon neurovascular structures. Finally,
bone fixation is critical. The implant has to have immediate and long-term bone fixation to maintain its
function. The disc replacement needs to be fail-safe so it can resist catastrophic failure and maintain its
integrity in the case of a major trauma to the spine.


There have been numerous prototypes and models invented.


The Fernstrom, as stated, was essentially a ball bearing, but failed because of settling, loosening, and
expulsion. (Fig. 1 and 2)


The Lee disc replacement was a polymer combination in order to replicate the consistencies of the natural
disc. There was a central portion of the disc which was a soft polymer in order to replicate the soft nucleus
pulposus of the inherent disc. On the outer portion of the disc replacement, the polymer had a moderate
hardness in order to emulate the annulus fibrosus. The endplates had a significant hardness to them so that
they would be more similar to the bony endplates in the native spine. (Fig. 7)


Froning developed a disc replacement that had implant anchoring pins and an inflatable bladder, which could
be inflated to desired pressures. (Fig. 8)


Edland had a disc replacement that looked similar to a tire with spokes. This disc replacement could be
folded and then inserted into the disc space. (Fig. 9)


Khvisyuk developed a disc replacement with metal endplates and spikes for anchoring. There was a silicone
cushion between the two endplates, and it was "pinned" into place. (Fig. 10)


Kuntz developed a disc replacement that resembled a "clothespin" and had a central hinge to allow for
flexion and extension. (Fig. 11)


Patil developed a disc replacement that resembled a box with springs. Unfortunately, the main limitation
was scar formation, which filled in the spaces and inhibited the spring action. (Fig. 12)


The Steffee disc had metal endplates with porous coating to allow for bone ingrowth as well as an
elasteromeric cushion between the two endplates. (Fig. 13) The early versions of this disc replacement
failed because of separation of the elasteromeric cushion from the endplates. Additionally, there were
concerns about toxicity of the intervening material.


The Downey disc replacement looked similar to a top with anchoring pins as well as an elasteromeric wafer,
which was bonded to and over the plates. (Fig. 14)


Ray had a disc replacement that was slightly different than the previous ones. (Fig. 15) The native annulus
was left intact, but the nucleus was replaced. These were "bags" that were semipermeable with a core made
of hyaluronic acid, which imbibed water and allowed the disc replacement to swell. These devices were
inserted posteriorly after disc removal. Problems experienced with this disc replacement included expulsion
of the prosthesis.


Kostuik invented a disc replacement that was all metal. (Fig. 16 and 17) The endplates were made of cobalt-
chrome, and the springs were made of titanium. There was a posterior hinge as well as screws that allowed
the replacement to be secured for early fixation.


Buttner-Jans invented the Charite disc replacement. (Fig. 18, 19, and 20) This disc replacement has metal
endplates with spikes, as well as a high molecular weight polyethylene spacer. These endplates are porous
coated for bone ingrowth. The polyethylene insert does not lock into place, and there is motion on both the
superior portion of the polyethylene and the inferior portion of the polyethylene on the respective metal
endplates.


There is a paucity in the literature today concerning total disc replacement. The Charite has the most data.

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The Charite has gone through three generations, and it dates back to 1984. There have been device failures
with earlier designs, but no device failures with the most recent model. The Charite comes in three sizes,
and still maintains the cobalt-chromium endplates and the ultra high molecular weight polyethylene sliding
core. It is inserted through an anterior transabdominal approach. There are three sizes of the polyethylene
core, which allow for customization of disc height. During implantation, it is anchored to the bony endplates
in the vertebral bodies and inserted with special intervertebral spreader tools.


Several authors have reviewed their experiences of the Charite device. Buttner-Jans in 1988 reviewed his
initial clinical experience of 62 patients, and he submitted that there were 83% very satisfactory or "better-
than-before" operation results. T. David in the European Spine Journal in 1993 reviewed his results with the
Charite in 22 patients. He had a minimum of a one-year follow-up and 65% excellent or good results. He
concluded that the disc replacement procedure had a very narrow application, and artificial disc
replacement, in his opinion, was not a routine procedure. Griffith et al in Spine in 1994 had the first multi-
center, multi-surgeon, retrospective review of the Charite. There were 93 patients in which the Charite 3
device was inserted. The most common diagnosis was degenerative disc disease in 52%, and the most
common level implanted was the L4-5 level. The follow-up was 11.5 +/- 8.4 months. He reported a
significant functional and pain improvement, although there was no work status change in the patients
evaluated. There was a 6.5% incidence of migration, subsidence, and dislocation. He concluded that
prospective randomized studies were needed to compare artificial disc replacement versus fusion
procedures. Cinotti et al in Spine in 1996 also performed a one-surgeon retrospective review of 46 Charite 3
devices. He had a minimum of a two-year follow-up. The diagnoses were approximately 50% degenerative
disc disease and 50% failed disc excision. They had a 69% satisfactory result for one-level procedures, and
a 40% satisfactory result with two-level procedures. He determined that central and posterior placement of
the prosthesis allowed more motion postoperatively. Additionally, they determined that patients who were
allowed to undergo early exercise (versus three months of corset wear) had significant improvement in
range of motion at the implanted level. They hypothesized that their poorer results in two-level procedures
were because of the following: this device was difficult (in their experience) to implant at two levels because
of excessive distraction that had to be performed at the second level; additionally, at the second level, a
smaller device needed to be inserted, and this device needed to be inserted more anteriorly, very likely
contributing to decreased range of motion at that second level. They concluded that in their hands, success
was less than seen in fusion cases. The poor outcome was possibly due to their poor patient selection. The
Charite in their opinion was not suitable for two levels, and they also determined that prospective
randomized studies were needed to evaluate disc replacement versus fusion procedures.


The ProDisc total disk prosthesis utilizes total joint replacement knowledge in its design. The goal for the
prosthesis is to restore anatomy and motion, and at the same time relieve pain and improve outcome. The
ProDisc has a modular design with a superior and inferior cobalt-chrome endplate. (Fig. 21) The surfaces are
porous (titanium plasma spray) to allow for bone ingrowth. Additionally, the inferior endplate allows for a
polyethylene insert to be snapped into place utilizing a locking mechanism. (Fig. 22) The superior and
inferior endplates have keels and two pegs on them, which allow for initial stability. (Fig. 23) Long-term
fixation is afforded by bony ingrowth into the porous coating on the superior and inferior endplates. The
ProDisc implant offers 6° or 11° of lordosis, and has heights of 10, 12, or 14 mm. Also, the design of the
ProDisc is modeled closely after the morphology of the inherent disc. (Fig. 24) The ProDisc allows 13° of
flexion and 7° of extension, closely following the normal range of motion of 10° of flexion and 5° of
extension documented in the literature. Lateral bending is slightly more than the native disc. The ProDisc
allows 10° of right and left lateral bending, compared to the inherent motion segment with left and right
lateral bending of 5°. The axial rotation of the ProDisc is unconstrained as compared to the inherent motion
segment, which allows 3°. The potential concern in this case is that excess stress will be placed upon the
posterior elements. This could possibly cause premature stress upon the articular cartilage and resulting
arthritis. This is speculative, though. There has been mechanical testing of the ProDisc to assess the "creep"
of the ultra high molecular weight polyethylene, and to assess motion of the implant into the vertebral
bodies. Also, the implant's response to shear stresses and wear of the ultra high molecular weight
polyethylene has been evaluated. The ProDisc representatives, through their research, report that the
ProDisc has a minimum of 22% less creep that the Charite device. Also, the ProDisc has less progressive
increase in creep deformation at increasing load than the Charite. The ProDisc has a minimum 10% less
permanent deformation than the Charite as well. The ProDisc inlay shows no loss of the snap-in function of
the polyethylene. Migration of the implant starts at 12 KN without any evidence of tilting. The ProDisc
results in a smoother pressure transition at the implant rim because of the radius of the implant. Compared
to the Charite, the ProDisc shows fixation strength in shear without sudden failure. In mechanical testing,
dislocation starts at 450 newtons of shear force and 0.3 mm displacement without any evidence of tilting.

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Wear fatigue testing shows the ProDisc to have a 30% less wear factor compared to total knee and total hip
replacements. The ProDisc model shows minimal ultra high molecular weight polyethylene (UHMWP) cold
flow. Finally, the ProDisc in biomechanical testing shows no signs of fatigue, breakage, or delamination
under microscopic evaluation.


The earlier ProDisc results have been evaluated by their inventor, Dr. Marnay. He had an outside
organization perform a prospective review of 61 of his cases. 95% were available for follow-up at seven to
eleven years. 33% of these procedures were performed at two levels. At follow-up, all of the prostheses
were intact and functioning, and there was no evidence of subsidence or migration. Additionally, there were
no revisions or removals, and there were 92.7% of the patients who were satisfied or entirely satisfied. They
concluded that there was no difference in results in one- or two-level implantations, and there were no
device-related safety issues. Weichert et al reported a 6 month followup of 16 ProDisc patients where the
mean Visual analog scale (VAS) score improved from 7.0 to 1.9 and the Oswestry Pain Score improving
from 23.3 to 10.2. Bertagnoli and Kumar published results of a 108 patient series with a followup period of
3-24 months. They reported that 91% of the patients have excellent outcome, 8% with a good outcome and
1% with a fair outcome. The obvious drawback was that the specific criteria used to make the classification
system were not clearly defined.


The St. Mary's Spine Center (San Francisco, California) is involved in a multi-center prospective randomized
controlled clinical trial, evaluating the ProDisc total disc replacement. The study consists of 510 cases. 340
cases utilize the ProDisc, and these will be compared to 170 fusion cases. The follow-up will be a minimum
of 24 months, and the goal is to assess the safety and effectiveness of the disc replacement. The inclusion
criteria include: (1) Back and/or leg pain and radiographs (CT,MRI, plain films or myelograms) showing any
one of the following: (i) instability (> 3mm translation, ,> 5 mm angulation), (ii) decreased disk height >
2mm, (iii) scarring/thickening of the annulus fibrosis, (iv) herniated nucleus pulposus, (v) vacuum
phenomenon, (2) age range 19 to 59 years old, (3) failed 6 months conservative treatment, (4) Oswestry
low back pain disability questionnaire score at least 20 out of 50 and a (5) signed informed consent.


Exclusion criteria include: (1) greater than 2 levels of degenerative disk disease, (2) known allergy to
titanium, polyethylene, cobalt, chromium and molybdenum, (3) prior fusion at any lumbar level, (4)
clinically compromised vertebral bodies at affected levels secondary to current or past trauma , (5)
radiographically documented evidence of facet joint disease, (6) lytic spondylolisthesis or spinal stenosis, (7)
degenerative spondylolisthesis greater than grade one, (7) back or leg pain of unknown etiology, (8)
osteoporosis, (9) Paget's disease, (10) morbid obesity (Body Mass Index > 40, weight > 100 pounds over
ideal body weight, (11) pregnant or plan pregnancy within 3 years, (12) taking medications that interfere
with bone/soft tissue healing, (13) presence of rheumatoid arthritis or autoimmune disease, (14) active
malignancy.


Spinal fusion was chosen as the randomized control in the study. A circumferential fusion was performed in
all the patients randomized to receive a fusion. The fusion was either an anterior lumbar interbody fusion
(ALIF) or a posterior lumbar interbody fusion (PLIF) using a commercially available femoral ring allograft
along with a posterior lateral fusion with autogenous iliac crest autograft. Pedicle screw instrumentation
using the Ti alloy CD system from Medtronic Sofamor Danek was placed in each patient receiving a fusion.


The surgical technique for the disk replacement in all patients consisted of 8 reproducible steps. All cases
were performed with the patient in a supine position on a standard radiolucent operative table. The surgical
approach was an anterior retroperitoneal approach for each patient. Once the appropriate disk level was
exposed, a complete diskectomy was performed including removal of the cartilage from the superior and
inferior endplates and removal of the posterior longitudinal ligament. Next trial implants were inserted to
determine the size of the prosthesis. After, trialing for the size of the final component, a chisel was used
under fluoroscopic guidance to prepare the position of the keel of the superior and inferior components.
Next, the two plate components, pre-assembled on a back table were inserted in the determined position.
Under fluoroscopic guidance, the surgical goal was to place the prosthesis in the midline in the frontal plane
and as posterior as possible in the sagittal plane without entering the spinal canal. Distraction was next
performed with a distractor instrument and the polyethylene inlay component was inserted and locked into
the inferior plate component. Finally, the insertion instruments were removed and water tight closure was
performed. The patients were ambulated with the assistance of a physical therapist on post operative day
one. Post-operative follow up data will be recorded at 6 weeks and then at 3,6,9,12,18, and 24 months.




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Outcome measures recorded during the study include four patient self-assessments: (1) Oswestry Low Back
Pain disability questionnaire, (2) SF-36 Health Survey, (3) Overall Pain on a 10 cm Visual Analog Scale, (4)
Satisfaction on a 10 cm Visual Analog Scale. In addition, the primary investigator performed a physical and
neurological examination checking for range of motion, bone graft donor site, root tension signs, reflexes,
muscle strength, and sensory deficits. Radiographic examination consisting of standing AP and lateral films,
flexion/extension films and lateral bending films were also obtained. The above outcome measures were
recorded pre-operatively and during each post-operative visit.


The series at St Mary's Spine center currently consists of 29 patients who have received 40 intervertebral
disk arthroplasties. In addition, 12 patients were randomized to fusion. Three patients who received a total
of 4 disk arthroplasties have 18 month follow-up. Five patients who received 5 disk arthroplasties have 12
to 18 month follow up. Seven patients who received 11 disk arthroplasties have 6 to 12 month follow up and
13 patients who received 19 disk replacements have 3 to 6 month follow-up. One patient has currently been
followed for two months. For the 29 patients, the average pre-op Oswestry pain score was 25.2 and the
average score at the most recent follow-up was 19.8, a 22 percent decrease. In patients with 18 month
follow up, the average Oswestry score has fallen from 29.6 (range 29-31) to 19.3(range 14-24), a 35
percent decrease. In the 12-18 month group, the average score fell from 31.8 (range 24-39) to 22.4 (range
11 to 30), a 30 percent decrease. In the 6 to 12 month group, the average score has dropped from 26
(range 20 to 31) to 22 (range 16-31), a 15 percent decrease. In the three to six month follow up group, the
average Oswestry score fell from 21 (range 21-40) to 17 (range 0 to 36) , a 20 percent decrease. As would
be expected, the further out from the replacement procedure the patients get, the greater the decrease in
the Oswestry score. This allows for the patients' surgical sites to heal and for the patients to complete the
appropriate post-op rehabilitation regimen.


The pre-op range of motion average at the operated levels in the group followed for at least 18 months
included 8.25 degrees on the flexion/extension and 10.75 degrees on the lateral bending. The range of
motion at the most recent follow-up averaged 8 degrees on the flexion/extension views and averaged 6.5
degrees on the lateral bending views. In the group with 12-18 month follow up, the pre-op and most recent
post-op visit range of motions on flexion/extension views were 6.8 and 6.2 degrees respectively. On the
lateral bending views, the pre-op and most recent post-op visit range of motions was 4 and 2 degrees
respectively. In the group with 6-12 month follow-up, the pre-op and post-op range of motions on the
flexion/extension views were 4.2 and 4.2 degrees respectively. On the lateral bending views, the pre-op and
post-op range of motions was 3.0 and 4.2 degrees respectively. In the group with 3-6 month follow up, the
pre-op and post-op range of motions on the flexion/extension views averaged 5.2 and 7 degrees
respectively while on the lateral bending views, the pre-op and most recent post-op visit wee 2.2 and 3.3
degrees respectively. The above values suggest that range of motion was maintained at the operated levels.
The long term effect of preserving the range of motion in preventing the degenerative cascade requires
further follow up. Of the 40 cases 16 disk replacements involved the L4-L5 level. The pre-op range of
motions of flexion/extension and lateral bending were 9.2 and 4.8 degrees respectively. The post-op range
of motion in flexion/extension and lateral bending averaged 7.14 and 5 degrees respectively. The other 24
replacements were at the L5-S1 level. The pre-op range of motions for flexion/extension and lateral bending
were 5.25 and 2.7 degrees while the post-op range of motions were 5.4 and 3.3 degrees respectively.


Further review of the radiographs revealed no patient who received an arthroplasty had implant migration
and subsidence (> 3mm). No patient had extensive radiolucency along the implant/bone interface (> 25%
of the interface's length for each endplate). There has been no loss of disk height in any operated disk
arthroplasty and no arthroplasty has evidence of bony fusion.


Commonly associated post-operative complications associated with lumbar disk replacement procedures
include: (1) abdominal wall hematomas, (2) vascular injury, (3) dural tears, (4) Nerve injury, (5)
Retrograde ejaculation in males, (6) Migration of the prosthesis. In our series of 40 disk replacements, there
has been one incident of a common iliac artery tear discovered intra-operatively and repaired. The patient's
post-operative course proceeded without incident. There have been no re-operations in either the disk
arthroplasty group or the fusion group.


Total disk replacement is essentially in its infancy. The early studies of Marnay and the ProDisc are
encouraging. There is no doubt further randomized prospective trials are needed to evaluate total disc
replacement versus fusion as a viable option in degenerative lumbar disk disease. The advantage of this
study is that in addition to being a prospective, randomized double blind trial, outcomes are being measured


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using standard assessments such as the Oswestry low back pain questionnaire and visual analog scale. As
spine surgeons learn more about total disk replacement, it is imperative that strict indications are
developed.




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