1
Treatment of the Painful Motion Segment:
Cervical Arthroplasty
John B. Pracyk, M.D., Ph.D.
Vincent C. Traynelis, M.D.
Department of Neurosurgery
The University of Iowa
Iowa City, Iowa
Corresponding Author:
Vincent C. Traynelis, M.D.
The University of Iowa Hospitals and Clinics
Department of Neurosurgery
200 Hawkins Drive
Iowa City, Iowa 52242
Phone: 319-356-2774
Fax: 319-353-6605
Dr. Traynelis is a consultant for Medtronic Sofamor Danek
2
ABSTRACT
Study Design
A retrospective review of the literature.
Objective
This work serves as a comprehensive update of cervical arthroplasty.
Summary of Background Data
Cervical arthroplasty has developed as a means to preserve normal spinal
motion after an anterior cervical discectomy. Preserving motion may lead to an acute
improvement in patient outcome and may decrease the incidence of symptomatic
adjacent segment disease in the long term.
Methods
The literature concerning the outcomes following anterior cervical decompression
and fusion, the indications for cervical arthroplasty, the indications and contraindications
for arthroplasty, the surgical technique, and early outcome studies for those devices
currently in U.S. FDA IDE trials are reviewed
Results
The most data are available for the Prestige, Bryan, and ProDisc-C devices.
While these devices all preserve normal segmental motion, the articulations vary (metal
on metal, metal on polyurethane, and metal on ultra-high molecular weight
polyethylene). Wear testing indicates that these devices will have a long life once
implanted. Preliminary outcomes compare very favorably to anterior decompression
and arthrodesis.
3
Conclusions
Cervical arthroplasty is a promising new technology that may improve patient
outcome following anterior cervical decompression.
4
KEY POINTS
Although the outcomes following anterior cervical decompression and arthrodesis are
very good, improvement is possible.
Cervical arthrodesis appears to predispose patients to adjacent segment degeneration.
Cervical arthoplasty preserves normal segmental motion following decompression.
Cervical arthroplasty may improve patient outcome following anterior cervical
decompression
MINI ABSTRACT
The literature concerning the outcomes following anterior cervical decompression
and fusion, the indications and contraindications for arthroplasty, the surgical technique,
and early outcome sutdies for those devices currently in U.S. FDA IDE trials are
reviewed. Cervical arthroplasty has developed as a means to preserve normal spinal
motion after an anterior cervical discectomy. Preserving motion may lead to an acute
improvement in patient outcome and may decrease the incidence of symptomatic
adjacent segment disease in the long term.
5
The most data are available for the Prestige, Bryan, and ProDisc-C devices.
Preliminary outcomes compare very favorably to anterior decompression and
arthrodesis.
Cervical arthroplasty is a promising new technology that may improve patient
outcome following anterior cervical decompression.
KEY WORDS
Cervical arthodesis, cervical arthroplasty, cervical disc degeneration, cervical
discectomy, spinal motion
6
INTRODUCTION
Anterior cervical decompression and fusion, first described over 50 years ago,
has become the surgical procedure of choice for patients with symptomatic
degenerative cervical spondylosis which is refractory to non-operative therapy.1-3 This
operative technique allows the surgeon to safely eliminate the most common causes of
cervical neural compression, and the development of a solid arthrodesis provides longterm stability and halts the degenerative process at the treated segment. Cervical
spinal instrumentation led to improved outcomes in many arthrodesis patients in the
1980's. The subsequent 10 years witnessed further refinements in instrumentation, the
development of new interbody techniques, improvements in spinal navigation, and an
increasing focus on minimally invasive approaches and exposures. Despite all of these
changes, there remains a genuine disparity between current and ideal patient
outcomes. Historically, the primary goal has centered on the preservation and
restoration of neurological function. Currently, there is a tremendous interest in the
maintenance of the spine’s biomechanical properties through a new wave of
technologies that focus on the preservation of the functional motion segment.
THE GENESIS OF ARTHROPLASTY
Single-level fusion does not seem to significantly alter the global mobility of the
cervical spine, but motion is adversely affected when multilevel treatments are
necessary. Furthermore, while fusion is beneficial to the diseased level, it is probably
detrimental to the remaining motion segments. It has been demonstrated that cervical
arthrodesis increases motion at non-operated adjacent levels.4 Motion is not the only
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measured outcome that may be adversely affected. Several biomechanical studies in a
human cadaveric model have demonstrated increased intradiscal pressure recordings
in adjacent disc segments following fusion.5-7 Increased motion and elevated intradiscal
pressures cumulatively translate into increased stress on the adjacent non-operated
discs, which can accelerate the rate of disc degeneration.6,8-13 This hypothesis for
adjacent level degeneration is somewhat confounded, however, by several studies
which state that clinical symptoms correlate poorly with the degree of degenerative disc
changes observed radiographically.9,14,15
Hilibrand et al. provide perhaps the greatest insight into this problem.13 These
investigators studied 374 patients and found that symptomatic adjacent segment
disease occurred at a relatively constant rate of 2.9% during the decade following
surgical fusion. Although a portion of these patients probably had progression of their
disease due to the natural history of spondylosis, Goffin et al. showed that the rate of
radiographic adjacent segment disease following arthrodesis for traumatic cervical
injuries was 60% over five years.12 This key work indicates that the development of
adjacent segment disease is most likely related to the arthrodesis itself.
Clearly, there are growing concerns for the development of adjacent segment
disease following cervical arthrodesis, yet it is important to remember that
decompression of the neural elements, not the fusion, remains as the objective for
anterior cervical surgery. The search for a comprehensive solution that provides the
benefits of neural decompression without the drawback of placing adjacent motion
segments at risk has been the driving force behind cervical arthroplasty.16 Preserving
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motion at the operated level while simultaneously providing biomechanical stability and
global neck mobility are the hallmarks of cervical arthroplasty.
With the growing interest in cervical arthroplasty now arriving in the U.S., this
review will examine the current role of this technology. Patient selection, indications,
contraindications, and device development will be discussed. A detailed analysis of the
Prestige, Bryan, and ProDisc-C arthroplasty devices will also be provided.
Complications and lessons learned from other total joint prostheses will be reviewed.
This will include a discussion of tribology, biomechanics, material science, and the role
of the inflammatory response in prosthetic failure.
PATIENT SELECTION
A comprehensive discussion of the indications and contraindications of total
cervical arthroplasty is not possible since this is a relatively new treatment. In general,
these patients should have normal cervical spinal alignment and mobility along with one
of the following pathologic entities: 1) radiculopathy caused by disc herniation (soft or
hard), 2) radiculopathy caused by foraminal osteophytes, 3) myelopathy caused by disc
herniation (soft and hard) producing spinal cord compression, or 4) any combination of
the above entities (Table 1). Essentially, any patient with normal or near-normal
segmental motion and anterior pathology may be a candidate for reconstructive
arthroplasty. Surgical candidates should have experienced a failure in medical
management, including activity modification, physical therapy, nonsteroidal antiinflammatory medication, and possibly selective nerve root blocks.
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Arthroplasty is contraindicated in the setting of significant segmental or global
deformity. The outcomes of patients with isolated neck pain alone (without evidence of
radiculopathy or myelopathy) who have been treated with arthroplasty have not been
fully delineated. Patients without pre-existing motion cannot be expected to regain
mobility by implanting a total prosthetic disc replacement. Moreover, patients with
radiographic instability will not be served well by a reconstruction with an artificial disc
replacement. These two patient populations are better treated with arthrodesis. A
recent history of infection or osteomyelitis would preclude the use of a prosthetic disc
device. Other relative contraindications include rheumatoid arthritis, renal failure,
osteoporosis, cancer, and preoperative corticosteroid medication.17
PRESTIGE CERVICAL DISC
History of the Prestige Cervical Disc
The Prestige device has it origins with Mr. Brian Cummins, who attempted to
address the shortcomings of cervical arthrodesis some 16 years ago when he began to
develop an artificial cervical disc in collaboration with the Department of Medical
Engineering at Frenchay Hospital, Bristol, U.K. in 1989.18 His pioneering efforts in the
development of a metal-on-metal artificial cervical disc laid the foundation upon which
the current generation Prestige was built. After two years of multiple modifications, the
original ball-and-socket Cummins design had been modified and finally approved for
human implantation.18 Between 1991 and 1996, 22 joints were implanted in 20 patients.
Eighteen patients were reexamined in 1996, and all but 2 had preserved motion at the
implanted level. Although there were some minor setbacks, with the removal of one
10
device due to a manufacturing error and some screw breakage and minor backout, no
implant failed. Patients with radiculopathy improved, and those with myelopathy either
improved or stabilized.
Prestige I Cervical Disc
The second generation of the Cummins cervical disc was the Prestige I
developed in 1998 (Medtronic Sofamor Danek, Memphis, TN).19 This stainless steel,
metal-on-metal, semiconstrained device consisted of a hemispherical cup on the lower
component which articulated with a dome on the superior component. The major
design change was the conversion of the socket portion of the articulation to a trough
design. This allowed for physiological anterior-posterior translation to be coupled with
flexion/extension motion as it is in the normal human condition. The screws by which
the device was fixed to the vertebrae were standardized, and a locking cap was added
to prevent backout. The prosthetic disc was produced in accordance with the strict
manufacturing standards necessary for all spinal implants.
Prestige II Cervical Disc
The Prestige II (Medtronic Sofamor Danek, Memphis, TN) was designed in 1999.
A key improvement over the Prestige I was a more anatomic endplate design. The
hemispherical cup was replaced with an ellipsoidal saucer, along with a roughened
endplate surface to promote bony ingrowth for long-term stability. This allowed for a
more physiological motion with a reduced profile and less friction to minimize debris
generation. The design of the two components of the joint allows the upper vertebral
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component to passively find its own axis of rotation as determined by the facet joints
and coupled motions of the adjacent vertebrae.
Prestige ST Cervical Disc
The Prestige ST became available in 2002. The major change between the
Prestige II and the Prestige ST was a 2 mm reduction in the height of each anterior
flange. The Prestige ST is the device currently being evaluated in a U.S. FDA
Investigational Device Evaluation study. It is constructed of stainless steel and consists
of two articulating components attached to the cervical vertebrae with screws (Figure 1).
The ball-and-trough design of the Prestige ST provides relatively unconstrained motion
comparable to that of a normal cervical spinal segment (Figure 2). The angulation
between the base of the components and the anterior portion matches the normal
anatomy of the cervical vertebrae. The anterior face is 2.5 mm thick, which is
comparable to the thickness of the majority of anterior cervical plates. The surfaces of
the device contacting the endplates are grit-blasted to promote bone osteointegration.
The Prestige ST comes in a variety of sizes. There are implants of 12 mm depth that
will allow for interspace heights of 6, 7, and 8 mm. Larger patients may require the 14
mm depth, and these implants have interspace heights of 7, 8, and 9 mm. The width of
all devices is comparable to clinically successful anterior cervical plate systems at a
consistent 17.8 mm. The specific instrumentation required to implant the Prestige ST
includes interspace sizing trials. There is also an anterior reamer and an implant holder
which also serves as drill, tap, and screw guide.
12
Implantation of the Prestige ST Cervical Disc
The patient is positioned supine on the operating table, and all pressure points
are padded. The surgical site is sterilely prepared. The anterior cervical spine is
exposed through a standard approach via a fascial dissection. The disc is removed and
all osteophytes resected. It is important to be certain that the decompression is
meticulous. Particular attention should be directed toward completely removing
osteophytes arising from the uncinate processes. The decompression should be
bilateral.
The anterior inferior lip of the rostral vertebral body is resected with a 45 degree
rongeur if necessary. Any remaining anterior vertebral column osteophytes should be
removed with a rongeur or bur. The vertebral endplates are denuded of cartilage and
burred until there are two parallel surfaces of subchondral bone. This may be
accomplished with any of a variety of high-speed burs. Many surgeons partially or
completely prepare the endplates for placement of the Prestige device prior to the
decompression because doing so often increases the corridor through which the spinal
canal is accessed.
Prestige ST sizing trials are placed into the interspace to determine the
appropriate size of the prosthesis. The trial device (Figure 3A) should fit snugly. If it is
too tight, the ligaments which surround the segment may become overly taut, thereby
restricting motion. The trial device is inserted in the proper orientation. If it does not fit
precisely, more endplate preparation is necessary. Once the interspace preparation is
completed, the sizing trial is inserted and the anterior vertebral reamer is placed over
the handle of the trial device (Figures 3B & 3C). This reamer is rotated back and forth
13
to plane the anterior surface of the vertebral bodies so that the anterior face of each of
the components of the Prestige total disc replacement lies flush against the spine. A
final check of the interspace and anterior vertebral surfaces is performed by sweeping
the anterior face profile trial medially and laterally (Figure 3D). Any bony irregularities
detected during this maneuver should be addressed.
A Prestige device matching the interspace trial device is attached to the implant
holder and inserted into the interspace (Figure 3E). A sleeve is placed into one of the
implant holder drill ports, and a hole is drilled into the vertebral body (Figure 3F). The
sleeve is removed, and the hole is tapped and a screw placed. This is repeated until all
screws are engaged. The implant holder is then detached from the Prestige implant.
Two locking cap screws are engaged and tightened to prevent backout of the bone
screws. It is optimal to place the device using fluoroscopy. If the surgeon chooses not
to use intraoperative imaging, a lateral cervical radiograph should be obtained at this
point to assure that the device is properly implanted (Figure 4). The wound is irrigated
thoroughly and closed in the usual fashion. Postoperative immobilization is not
necessary, and the patient may resume all activities as soon as they feel able to do so.
Outcomes for Prestige I Cervical Disc
The Prestige I was evaluated prospectively in a cohort of 17 patients, each of
whom required a surgical intervention in a segment adjacent to a previous fusion. The
objective of the study was assessment of the safety and stability of the device once
implanted as well as the preservation of motion in the cervical spine after
decompression and arthroplasty.20 The patients were assessed for 4 years following
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placement of the
device.21
Throughout the study, the Prestige I effectively maintained
motion. The mean sagittal rotation (flexion/extension) was 7.5 degrees. At 24 months,
all joints were mobile; there was an average of 6.5 degrees of flexion/extension motion.
AP translation was also preserved. There was no backout of the screws, and none of
the joints failed. Radiographic analysis at 48 months revealed that 12 patients had a
mean sagittal plane rotation of 5.7 degrees and a mean translation of 0.83 mm. These
results are consistent with the data from an earlier biomechanical study that
emphasizes the maintenance of normal motion at all segments of the spine with the
placement of the Prestige cervical disc compared with the decreased motion of adjacent
segments following surgical arthrodesis.4
The Neck Disability Index (NDI) of the Visual Analog Scales (VAS) for neck and
arm pain is a 10-question assessment instrument that measures cervical pain and
disability associated with the activities of daily living.20 Higher scores on the NDI reflect
increased neck pain and greater disability. NDI assessments were performed
preoperatively and at regular intervals (6, 12, 24, 36, and 48 months) postoperatively.
Fourteen patients were evaluated at the end of 48 months, and they demonstrated a
30% improvement in their NDI scores.
The SF-36 questionnaire is an instrument that measures a patient's general
health status, with breakdown into mental and physical component scores. Similarly, it
was administered preoperatively and at regular intervals postoperatively. The SF-36
demonstrated positive results during the duration of arthroplasty implantation.
Two screws broke without clinical consequence, and no screws backed out. Two
implants were removed. In one case the amount of bone removal was excessive,
15
producing overloading of the facets which led to a chronic pain syndrome. No
significant tissue reaction was noted, and the implant showed minimal wear of the
articular surfaces. The second removal was required to treat an adjacent level preexisting condition. This implant, removed more than 3 years after the original surgery,
had minimal wear, and there was no evidence of a host inflammatory response.
In summary, the Prestige I study demonstrated that over time all devices
maintained motion at the treated levels along with the reestablishment of disc height.
The procedure was deemed safe, and the implant proved stable, with no dislocation of
the components. The success of this pilot study permitted further device evolution and
paved the way for application of the Prestige device on a more widespread scale.
Wigfield et al. assessed the motion of the segments adjacent to levels
reconstructed with either the Prestige I or an autologous bone graft.22 The adjacent
segments were assessed with an MRI scan preoperatively and scored in terms of the
degree of degenerative change present. Preoperative sagittal angular motion was also
measured in these segments. There was no difference in these parameters between
the two treatment groups. Flexion/extension motion was measured again at 6 and 12
months following surgery. A slight reduction in the motion at the adjacent segments
was noted in the patients who received the Prestige I. Overall cervical motion was
maintained in these individuals; however, this was because of the mobility preserved at
the operative level. In contrast, arthrodesis resulted in a significant increase in sagittal
plane rotation at the adjacent levels 1 year following the fusion surgery.
16
Outcomes for Prestige II Cervical Disc
The Prestige II was important in that it was the first cervical artificial disc to be
compared with fusion in a prospective randomized trial comprised of patients with
primary disease.20 This study randomized patients in a 1:1 fashion to compare
decompression and placement of the Prestige II device to decompression and
uninstrumented arthrodesis with autograft. Patients with symptomatic single-level
disease who had no prior cervical surgery were candidates for the study. Twenty-seven
patients were randomized to the Prestige II group and 28 to the arthrodesis (control)
group. Cohort demographics were comparable. The 2-year data demonstrate that
most outcome measurements favor arthroplasty over arthrodesis with regard to the NDI,
VAS, and the mental and physical component scores of the SF-36.20 It is important to
acknowledge that these differences only constitute a trend and are not statistically
significant. More importantly, motion analysis demonstrated maintenance of motion in
the arthroplasty patients, whereas the fusion patients displayed no significant motion. If
the Prestige II disc alleviates pain and symptoms comparable to a fusion while
simultaneously maintaining radiographic motion across the segments operated in every
patient, then its utility is validated. Additional long-term follow-up is certainly warranted
because the observation period is to too short to draw any conclusions about the
presumed benefit of the preserved motion segment. Moreover, further trials are
necessary with larger sample sizes and greater statistical power to confirm these
preliminary results.
17
Prestige LP
The Prestige LP (Figure 5) is the latest generation in the Prestige family of
cervical discs. Several features differentiate this device from its predecessors. The
Prestige LP is manufactured from a unique titanium ceramic composite material which
is highly durable and image-friendly on CT and MRI scans. A porous titanium plasma
spray coating on the endplate surface facilitates bone ingrowth and long-term fixation.
The surgical technique is similar to an anterior cervical discectomy and fusion (ACDF).
Instead of screw fixation like earlier versions of the Prestige disc, initial fixation is
achieved through a series of four rails, two on each component, which engage the
vertebral bodies (Figure 6A). After decompression, the endplates are burred to make
them paralllel. A rasp may be used to help with this step (Figure 6B). A trialing device
is inserted to verify the correct size of the implant (Figure 6C). A rail cutting guide is
inserted into the intersapce, and two small holes are drilled into each vertebral body. A
rail punch is used to connect these holes with the interspace (Figures 6D through 6G).
The Prestige LP is then gently tamped into place (Figure 6H).
BRYAN CERVICAL DISC
History of the Bryan Cervical Disc
In the early 1990’s, neurosurgeon Vincent Bryan conceived and developed the
Bryan Cervical Disc. Pre-clinical trials were conducted in a variety of rigorous models,
including bench top mechanical testing, cadaveric experiments, and a unique
chimpanzee survival study. After this extensive research and development, the first
18
Bryan Cervical Disc was implanted by Dr. Jan Goffin in Leuven, Belgium, in January
2000. Approximately 6,000 patients have since been treated with the Bryan disc.
Description of the Bryan Cervical Disc
The Bryan Cervical Disc prosthesis (Medtronic Sofamor Danek, Memphis, TN) is
a cervical disc replacement designed to allow for motion similar to the normal cervical
spine functional unit.23 This device consists of two titanium alloy shells with a
polyurethane nucleus (Figure 7). The bone implant interface of each shell has an
applied porous coating to facilitate ingrowth of bone and promote long-term stability.
The nucleus is surrounded with a polyurethane sheath to establish a closed articulation
environment. Sterile saline is injected into this sheath and functions as an initial
lubricant. Titanium alloy seal plugs allow for retention of the saline lubricant.24 Anterior
stops on the shells prevent posterior migration of the device and also serve as a method
by which the device can be inserted and removed, if necessary. It is currently
manufactured in five diameters: 14, 15, 16, 17, and 18 mm.
Implantation of the Bryan Cervical Disc
The patient is positioned supine on the operating table, and all pressure points
are padded. The surgical site is sterilely prepared. The anterior cervical spine is
exposed through a standard approach via a fascial dissection. The disc is removed and
all osteophytes resected. It is important that the decompression is meticulous.
Particular attention should be directed toward completely removing osteophytes arising
from the uncinate processes. The decompression should be bilateral.
19
After discectomy, a gravitational referencing system is used to determine the
exact center of the disc space.23 The desired location is determined by identifying a
point within the surgical site using a system of levels and protractors positioned from the
internal anatomic features of the disc space (Figure 8A). After locating the center of the
disc space, a milling fixture is positioned appropriately and affixed to the vertebral
bodies with bone anchors (Figure 8B). The milling fixture precisely controls the
locations of the powered cutting instruments that prepare the vertebral body endplates
for placement of the prosthesis (Figure 8C). The milled concavity of the vertebral
endplates exactly matches the geometry of the implant’s convex outer surface,
capturing the rim of each shell inside a ridge of bone. This precision fit provides
immediate anteroposterior, lateral, and torsional stability.
Outcomes for the Bryan Cervical Disc
Goffin et al. provide an initial preliminary report on a prospective, concurrently
enrolled, multicenter trial of the Bryan Cervical Disc prosthesis for the treatment of
single-level degenerative disc disease.25 Patients with symptomatic radiculopathy
and/or myelopathy were studied. The primary endpoints for this study were the relief of
preoperative symptoms (as assessed by the Cervical Spine Research Society and SF36 questionnaire instruments) as well as the relief of objective neurological signs as
determined by the examining physician. The results were entered into a database and
were scored and categorized. There were no controls in the study, but enrolled patients
were compared to historical controls identified through a literature review of reported
results for anterior cervical discectomy and arthrodesis. Based upon a statistical
20
analysis of the relevant articles, a success rate of 85% was defined a priori as the
historical average for ACDF successes. Study patients were measured against this
benchmark. Preliminary analysis was performed on 60 patients at 6 months and 30
patients at 1 year (from a total of 97 cumulative implantations). Fifty-two of the 60
patients (86%) examined at 1 year and 27 of the 30 patients (90%) examined at 6
months were classified as clinical successes; thus, the Bryan Cervical Disc outcomes
exceeded the historically defined success rate of 85% at both follow-up intervals.
The authors also performed radiographic analyses of the Bryan prostheses.
Anteroposterior movement above a 2 mm detection threshold defined device migration.
Evidence of AP migration was established in one patient and suspected in another. No
patient demonstrated migration greater than 3 mm. Postoperative Cobb angles for
flexion/extension of the functional spinal unit at the implant level demonstrated motion of
all implanted devices at both the 6- and 12-month reporting intervals. Admittedly, the
relief of symptoms may actually be a function of the extent of decompression, including
osteophyte resection, and not related to the performance of the disc prosthesis.
Nonetheless, symptom alleviation and functional improvement continued to occur
throughout the 1-year follow-up period. The utility of segmental motion-preserving
devices will not be determined through short-term analysis. An objective evaluation
over a longer interval, such as 5 years, will be far more informative. Ideally, a 10- to 15year longitudinal analysis is more realistic.
To this end, Goffin et al. have subsequently provided an intermediate follow-up at
the 1- and 2-year intervals.23 At the time of these reporting intervals, both single- and
bi-level (dual) patients were available for evaluation. In the single-level study, 90% of
21
patients evaluated at the 2-year follow-up, 86% of patients evaluated at the 1-year
follow-up, and 90% of patients evaluated at the 6-month follow-up were classified as
clinical successes. In the bi-level study, 96% of patients evaluated at 1 year and 82% of
patients evaluated at the 6-month interval were deemed a clinical success.
Complications identified through CT scans included paravertebral ossification. It
is important to note, however, that this observation was radiographically visible only on
high-resolution spiral CT scans and was not associated with clinical symptoms or
outcomes. The material composition of the Bryan Cervical Disc allows for this type of
high-resolution imaging, whereas most other cervical arthroplasty devices do not; thus,
the phenomenon may not be identifiable with devices made from other materials.
Ossification has been observed in other orthopedic applications, including spinal fusions
and total joints, and is the subject of ongoing research and debate. Fortunately,
heterotopic ossification appears to be prevented with the use of perioperative NSAID
(Tortolani PJ, Heller JG, Park AE, Tong F, Goffin J. Computed tomography (CT)
assessment of paravertebral bone after total cervical disc replacement: temporal
relationships and the effects of NSAIDs. Presented at the Cervical Spine Research
Society, Scottsdale, Arizona, December 2003).
Radiographic surveillance showed that the Bryan Cervical Disc preserves
segmental motion. No device migration was reported. In summary, this intermediate
progress report extends the reporting intervals and enrollment numbers of this cohort of
European patients receiving the Bryan device. The long-term data from the 5-year
follow-up interval for these patients is eagerly awaited.
22
A smaller nonrandomized, prospective cohort study based in London, Ontario,
Canada, independently confirms the favorable clinical and radiographic outcomes of the
Bryan cervical prosthesis.17 Thirty Bryan devices were implanted in 26 patients (4 bilevels) and then followed for a period of up to 2 years. Results demonstrate that
patients suffering from either hard or soft disc disease, myelopathy, or radiculopathy
had improved NDI and SF-36 scores following Bryan cervical arthroplasty; however,
unlike the European series, no comparisons to historical controls are made. A unique
feature of this study involves a subset of patients with at least 6 months of follow-up that
were evaluated using motion analysis software (Medical Metrics, Inc., Houston, TX).
This program precisely quantitates intervertebral motion radiographically. The mean
sagittal range of motion (ROM) at the treated disc space was 7.8 degrees on follow-up
assessments. This did not differ significantly from the 10.1 degrees measured
preoperatively. Most importantly, no significant changes in ROM were encountered at
individual segments either adjacent or distant to the operative site. Global cervical
sagittal motion distributed across all levels (C2-C7) increased moderately in the late
follow-up evaluations. These results quantitatively demonstrate the functional motionpreserving capabilities of the Bryan device.
In both the European and Canadian experience with the Bryan device, there has
been a predominant number of patients with radiculopathy (93% and 77%,
respectively). The role of cervical arthroplasty in the management of cervical
stenosis/myelopathy was examined exclusively over the short to intermediate term (1831 months) in a series of 11 patients from Australia.26 The clinical outcome measures
reflected significant improvements in subjective neck and arm pain, postoperative NDI,
23
and Nurick grade. Two complications were reported. One patient had heterotopic
ossification across the disc segment, and another experienced myelopathic symptom
progression due to edema. Three patients experienced worsening of their preoperative
radiographic alignment. On balance, these authors regard the Bryan device as a
satisfactory management option for the treatment of cervical myelopathy with low
morbidity and good intermediate term results.
Bryan ACCEL Cervical Disc
The Bryan Accel Cervical Disc Instrumentation is a second-generation system
that provides a simplified surgical alternative for the implantation of the Bryan Cervical
Disc. The design objectives of Bryan Accel duplicate the accuracy and reproducibility of
the Bryan Cervical Disc System. Alignment instruments and milling blocks incorporate
a simple referencing system that replaces the table-mounted construct. These
instruments are secured to the vertebral bodies and provide a framework for proper
distraction and decompression (Figure 9A). A measuring device uses the anterior and
posterior margins of the vertebral endplate to determine the proper prosthesis size.
Trials are used to verify adequate disc space preparation (Figure 9B). The endplates
are then prepared using the same precision milling instruments as the original Bryan
system, providing the same immediate stability for the Bryan Cervical Disc prosthesis
(Figures 9C and 9D).
PRODISC-C CERVICAL DISC
History of the ProDisc-C Cervical Disc
24
The ProDisc-C was developed on the same design principles as the ProDisc
lumbar disc prosthesis, integrating the knowledge and practical experience gained after
more than 6,000 implantations of the lumbar prosthesis since 1990. The implant
materials, ball-and-socket design, and fixation features are similar to that of the lumbar
device. The first implantations of the ProDisc-C were performed by the two clinician
developers, Dr. Thierry Marnay (Montpellier, France) and Dr. Rudolf Bertagnoli
(Straubing, Germany) in December 2002. A U.S. FDA multicenter, prospective,
randomized clinical trial was initiated in August 2003 to compare the ProDisc-C to
ACDF.
Description of the ProDisc-C Cervical Disc
The ProDisc-C cervical disc (Figure 10) (Synthes Spine, Paoli, PA) is a metal-onpolyethylene articulating device similar in design to the ProDisc lumbar disc prosthesis.
The modular implant consists of two cobalt-chrome-molybdenum endplates and an
ultra-high molecular weight polyethylene (UHMWPE) inlay. The endplates are initially
secured to the vertebral bodies with central keels and have a plasma-sprayed titanium
coating for long-term fixation stability. The UHMWPE inlay is pre-assembled into the
inferior endplate, and the implant is inserted en bloc as a one-piece device. The
ProDisc-C ROM allows for physiological restoration of the spinal segment: ±10 in
flexion/extension, ±10 in lateral bending, and no limitation in axial rotation. There are
five anatomical footprint sizes and four heights (5-8 mm) to accommodate all anatomical
variations.
25
Implantation of the ProDisc-C Cervical Disc
A transverse skin incision, avascular tissue plane dissection, and exposure of the
anterior vertebral bodies are accomplished in the traditional manner. The midline of the
operative level is determined using fluoroscopic control, and the retainer screws are
inserted into the anterior cortex both cephalad and caudal to the treatment level. The
screws should be positioned parallel to the endplates, ensuring that the spacing allows
for the height of the implant keel. The screw length is maximized without perforating the
posterior cortex. The vertebral body retainer is then assembled onto the screws and
locked in place. The retainer construct acts as an external fixator providing stabilization
of the vertebral bodies for endplate preparation and implant insertion (Figure 11A). A
complete discectomy and decompression are performed in the traditional manner. The
cartilaginous endplates must be removed, but care must be taken to avoid damage to
the bony endplate. The posterior longitudinal ligament should be preserved unless its
resection is required for adequate neural decompression or remobilization of the spinal
segment. Following discectomy, the wound is irrigated to remove any particulate
material. Distraction and height restoration are accomplished using an intervertebral
distractor; the vertebral body retainer is locked to maintain distraction throughout the
procedure.
The remainder of the procedure is performed in three main steps: trial, chisel,
and implant insertion. Appropriate sizing is accomplished using a system of trial
implants. The trial implant is inserted into the prepared disc space under fluoroscopic
control (Figure 11B). A properly-sized implant should cover a majority of the endplate to
decrease the risk of subsidence. Care must be taken not to overdistract or use
26
oversized trials because this may lead to tension upon the root, facet irritation, and loss
of motion. Once the optimal trial is selected and properly positioned, the channels for
the implant keel are cut precisely in a two-step chiseling process. An adjustable stop
prevents posterior migration of the trial and chisel during impaction (Figure 11C).
The ProDisc-C implant is then assembled onto the implant inserter and inserted
as a one-piece construct. After the implant has been aligned with the keel cut, it is
impacted into position using fluoroscopic guidance. The insertion device is removed
and the final position of the implant confirmed (Figure 11D). Bone wax can be used to
address any bleeding at the implant insertion site and retainer screw removal sites.
Of note, the technique must be modified for bi-level implantation in that the
retainer screws must be inserted off midline to prevent unnecessary weakening of the
vertebral bodies. The vertebral body retainer can be used on each level independently,
or it can be elongated to span both levels. If it appears that one level will be more
challenging than the other, then the most difficult level should be performed first and
thoroughly completed prior to initiating the discectomy at the second level.
Outcomes of the ProDisc-C Cervical Disc
The in vitro biomechanics of the ProDisc-C have been examined in an
established cadaveric model.27 Traditionally, pure-moment protocols are used to
evaluate spinal fixation devices. These are less useful for evaluating spinal arthroplasty
where fusion does not take place. Alternatively, these authors report on a
programmable testing apparatus that replicates physiological flexion/extension, lateral
bending, and axial rotation. Using this programmable apparatus, the ProDisc-C implant
27
maintained the biomechanical integrity of the cervical spine. Experimental arthrodesis
reduced motion at the surgical site and increased motion at immediate adjacent
segments. This compensatory adjacent segment phenomenon was not seen with the
ProDisc-C device.
Preliminary results have been reported from a prospective, nonrandomized trial
of 52 patients (67 treated levels) for the European clinical experience with the ProDiscC device (Bertagnoli R. Indications for full prosthetic disc arthroplasty in the cervical
spine. A study of clinical outcomes of different indications with ProDisc-C total disc
replacement. Presented at the 11th International Meeting on Advanced Spine
Techniques, Bermuda, July 2004). At the 1-year follow-up interval, an overall reduction
in pain intensity (VAS mean score decrease from 7.8 to 4.3) as well as a decrease in
pain frequency for both the arm (8.8 to 5.1) and the neck (7.4 to 3.7) were noted.
Although no radiographic data are reported and no comparisons are made to historical
controls, the patients reported that they were completely satisfied (57%) or generally
satisfied (43%) with the procedure. Ongoing follow-up is continuing with this group of
patients in order to obtain long-term data.
A prospective randomized clinical trial comparing the ProDisc-C to ACDF is
currently under way in the United States. Patient randomization is performed using a
1:1 ratio of ProDisc-C recipients to ACDF control recipients in the treatment of
symptomatic cervical disc disease at only one level between C3 and C7. To date, 30
patients have been enrolled in the FDA study, and the early 6- and 12-month data are
now available (Delamarter RB, Pradham BB. Indications for cervical spine prostheses.
Early experience with the ProDisc-C in the U.S.A. Presented at the Innovative
28
Technologies in Spine Surgery Meeting, Cabos, Mexico, July 2004). Clinical outcomes
revealed significant improvements in VAS and Oswestry scores for both the ProDisc-C
and ACDF groups. For the ProDisc-C group, average VAS and Oswestry scores
decreased from preoperative values of 7.5 and 29.0 to 1.2 and 7.3, respectively. The
VAS and Oswestry scores also decreased for the fusion group from 6.6 to 0.6 and from
23.0 to 10.0, respectively. There was no significant difference in outcomes scores
between the two treatment groups. There was an increase in mean flexion/extension
motion compared to preoperative assessments for the arthroplasty group. The fusion
group exhibited a mean decrease in motion. At six months, the average motion was 2.1
to 6 degrees for arthroplasty patients, in contrast to the 0 to 1 degree for fusion patients.
Analogously, lateral bending motion increased for the ProDisc-C patients, ranging from
3.5 to 8.5 degrees, and decreased for the fusion patients, ranging from 0 to 1 degree.
No device-related complications were reported.
COMPLICATIONS OF CERVICAL ARTHROPLASTY
The human intervertebral disc serves two biomechanical roles: load transmission
and maintenance of a controlled ROM for the neck.28 Cervical arthroplasty offers the
advantage of preserving the cervical ROM, but at what cost? Clearly, there are lessons
to be learned from the extensive experience already encountered with large total joint
arthroplasties such as the knee and hip. Fundamental to the principle of arthroplasty
are the corollary concepts of repetitive stress and the generation of wear-related debris.
These topics are novel to most spine surgeons, but as we transition into this new era of
spinal arthroplasty, they certainly deserve consideration.
29
Tribology is the study of friction, lubrication, and the wear of interacting surfaces
in relative motion. It is not surprising that this is a primary focus for the developers of
joint prostheses.28 The success of any cervical prosthetic device is predicated upon the
effective utilization of tribology to produce designs that do not wear out or produce
excessive amounts of wear-related debris. The best material for minimizing wear is the
alloy cobalt-chromium-molybdenum.29 Many designs incorporate two different materials
articulating with each other. In a metal-on-polymer pairing, the polymer wears
preferentially, whereas in a metal-on-ceramic pair, the metal wears to a greater extent.
In the metal-on-metal designs, the amount of metallic debris generated is several orders
of magnitude higher than a metal-on-polymer articulation, but the metal generates a
lesser inflammatory response compared to polymer debris.30
As one might predict, wear debris represents a nidus that stimulates the host to
generate macrophages and multinucleated giant cells that phagocytize the debris and
set up a regional inflammatory response. Granuloma formation can ensue, and a Type
IV delayed hypersensitivity response may be triggered.30 An inflammatory zone that
encompasses spinal instrumentation undergoing wear has already been demonstrated
clinically and experimentally.31 Proinflammatory cytokines like tumor necrosis factor
alpha, metalloproteinase-1, interleukin-1, interleukin-6, and prostaglandin E2 are
upregulated and produce osteoclastic bone resorption.32 This cascade may lead to
osteolysis, loosening of the prosthetic device, and ultimately its dislodgement and
failure.33 In contrast to large joint arthroplasty, a loose cervical prosthesis represents a
situation of a higher magnitude that has the potential to create serious morbidity.
30
EXPLANT ANALYSIS
One way to reconcile theoretical concerns for cervical arthroplasty failures
against reality is to examine the actual material response to the biological environment
and the host’s response to the device following prosthesis retrieval.34 A total of 11
Bryan devices and 3 Prestige devices have been explanted. Six of the Bryan devices
and 2 of the Prestige devices were analyzed along with surrounding periprosthetic
tissue specimens. Histological analysis revealed no metallic debris in the periprosthetic
tissue samples and no findings indicative of host response to the device; however, foci
of polymeric debris were discovered and phagocytosis of the debris by inflammatory
cells was indeed demonstrated.34 No osteoclastic resorption or osteolysis was shown in
the explanted Bryan devices. As expected, both Prestige devices exhibited metallic
debris in the periprosthetic tissue samples. Neither patient exhibited osteoclastic
resorption or osteolysis. Wear-related analysis of all devices allowed for direct
comparisons of genuine in vivo wear to simulator-produced wear. Interestingly, actual
wear of both the Bryan and Prestige devices was significantly less than predicated by
the simulators by a magnitude of 5- to 10-fold. It therefore appears that simulation
testing involves either harsher loading conditions or a significantly increased number of
repetitive cycles than the actual in vivo condition.
31
HETEROTOPIC OSSIFICATION
Early in the experience with large joint arthroplasties, the development of
heterotopic ossification was noted. This exuberant periprosthetic bone formation can
bridge across a joint and render it immobile, thus defeating the purpose of the motionpreserving approaches. Although in traditional cervical spine arthrodesis it is a frequent
objective to specifically avoid NSAID because they adversely affect bone fusion, a twoweek course of NSAID has routinely been employed postoperatively to reduce the
incidence of heterotopic, paravertebral ossification with arthroplasty devices. 16
CONCLUSION
Cervical arthroplasty holds the promise of preserving normal motion at an
affected level following discectomy while striving to concomitantly prevent
compensatory motion, increased intradiscal pressures, and stress at adjacent motion
segments. Taken together, the conservation of global motion across the entire cervical
spine will hopefully decrease the incidence of adjacent segment degeneration. Clinical
trials currently under way will chronicle the success of the Prestige, Bryan, and ProDiscC prosthetic devices to physically reconstruct the cervical spine by providing a solid
axial load-bearing support while at the same time preserving its normal physiologic
motion. The ultimate objective of these efforts is improved patient outcomes.
Researchers, engineers, and surgeons must work together to make this goal a reality.
In doing so, our knowledge of degenerative spinal disease will certainly increase.
32
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Cummins BH, Robertson JT, Gill SS. Surgical experience with an implanted
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38
TABLE 1
Indications for Cervical Arthroplasty

Radiculopathy caused by disc herniation

Radiculopathy caused by foraminal osteophytes

Myelopathy caused by disc herniation

Any combination of the above entities
39
FIGURES
Figure 1
Prestige ST. This total joint replacement device consists of two
articulating components that are attached to the adjacent vertebral bodies
by four bone screws. A locking cap screw prevents backout of the bone
screws.
Figure 2
Ball-and-trough articulation of the Prestige ST
Figure 3
A
Trial sizing device for a 6 mm x 12 mm implant
B
Anterior vertebral reamer
C
Anterior reamer over size trial device
D
Interspace anterior face device for a 6 mm x 12 mm implant
E
Prestige ST mounted in the implant holder
F
Prestige ST mounted in the implant holder with drill engaged
Figure 4
Flexion (A) and extension (B) lateral cervical radiographs of a patient with
a Prestige implant
Figure 5
Prestige LP
Figure 6
A
Burring endplates
B
Rasp in interspace
C
Sizing trial in interspace
D
Prestige LP railcutter
E
Initial rail holes in vertebral bodies
F
Prestige LP rail punch
G
Completed rail holes in vertebral bodies
40
H
Implant in place
Figure 7
The Bryan Cervical Disc prosthesis
Figure 8
A
The Bryan gravitational referencing system
B
The Bryan milling fixture
C
The Bryan powered cutting instruments
A
Bryan Accel distractor and decompression device
B
Bryan Accel trial in position
C
Bryan Accel milling instrument
D
Insertion of Bryan disc using Accel instrumentation
Figure 9
Figure 10
The ProDisc-C Cervical Disc prosthesis
Figure 11
A
Distraction and height restoration by the ProDisc-C vertebral
distractor
B
The ProDisc-C trial implant
C
Chiseling the ProDisc-C keel cut guide
D
The completed ProDisc-C implant