Intervertebral Disk Replacement - Keivan Anbarani's Electronic ...

2010
Intervertebral Disk
Replacement
Material 495 Final Report
Keivan Anbarani, Emma Amiralaei, Dorothy Ching
4/21/2010

Table of Contents
Introduction..............................................................................................................................................................1
Spinal Fusion..........................................................................................................................................................3
History.......................................................................................................................................................................3
Mechanics of Spine................................................................................................................................................4
Background.............................................................................................................................................................4
Natural Loading ....................................................................................................................................................6
Case Study I: Finite Element Analysis .........................................................................................................7
Case Study II: Human Cadaver Analysis.....................................................................................................9
Materials .....................................................................................................................................................................9
Metal-­‐on-­‐Metal ...................................................................................................................................................10
Metal-­‐on-­‐Polymer .............................................................................................................................................11
Ceramic-­‐on-­‐Polymer........................................................................................................................................12
Ceramic-­‐on-­‐Ceramic ........................................................................................................................................13
Clinical Total Disc Implants ..........................................................................................................................14
Complications and Success Rates...............................................................................................................15
Future Directions................................................................................................................................................17
ICORD (International Collaboration on Repair and Discoveries) ................................................17
Conclusion...............................................................................................................................................................18
References...............................................................................................................................................................20
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Lists of Figures and Tables
Figure 1. Examples of Disc Problems ...............................................................................................................2
Figure 2. Causes of SCI............................................................................................................................................3
Figure 3. Intervertebral Disc................................................................................................................................5
Figure 4. Intervertebral Disc under Compression......................................................................................5
Figure 5. Human Spine ...........................................................................................................................................6
Figure 6. Spinal Cord Movement........................................................................................................................6
Figure 7. Natural Loading of the Spine............................................................................................................7
Figure 8. Constructed 3D model of disc with applied force of 400N..................................................8
Figure 9. Stress Analysis with intact Disc and artificial Disc .................................................................8
Figure 10. Displacement analysis of intact Disc and artificial Disc.....................................................8
Figure 11. Comparison of Displacement angle for intact Discs and implanted Discs.................9
Figure 12. a) Cervicore and b) Prestige Disc..............................................................................................11
Figure 13. a) PCM and b) Bryan and c) Mobi-­‐C Disc...............................................................................12
Table 1. Spinal Cord Injuries for each Province in 2000-­‐2001.............................................................2
Table 2. International Statistics for SCI...........................................................................................................2
Table 3. Material Properties.................................................................................................................................7
Table 4. Goals for Total Disc Replacement..................................................................................................10
Table 5. Artificial Discs Statistics Exhibit ....................................................................................................13
Table 6. Comparison of Disc Design Types.................................................................................................15
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Introduction
Intervertebral discs provide flexibility to the spine and transmit loads from body weight
and muscle activity. The discs consist of three highly specialized structures, the endplates,
the annulus fibrosus and the nucleus pulposus. The two cartilaginous endplates form the
inferior and superior interface between the disc and the adjacent vertebrae, therefore
enclosing the disc axially. The annulus fibrosus is made up of several lamellae consisting of
parallel collagen fibers interspersed by elastin fibers. Surrounded by the annulus fibrosus
is the nucleus pulposus, the gelatinous core, which consists of randomly organized collagen
fibers, radially arranged elastin fibers and a highly hydrated aggrecancontaining gel. The
highly hydrated proteoglycans in the nucleus pulposus are essential to maintaining the
osmotic pressure and therefore have a major effect on the load bearing properties of the
disc.
Disc degeneration is possibly caused by three factors: mechanical loading, genetic
pre disposition and nutritional effects and aging (G. Paesold, et al.). Injuries applied to these
discs result in a variety of consequences ranging from normal motor and sensory function
to a complete loss of motor and sensory function. Spinal cord injuries are divided into two
main categories: traumatic and non-­‐traumatic. Traumatic spinal cord injuries (TSCI) are
due to a traumatic impact applied to the spinal cord, such as automobile accidents or sports
injuries. The second category of non-­‐traumatic spinal cord injuries (NTSCI) is due to any
damages to the spinal cord not caused by trauma such as genetic disorders and aging.
Additional examples of NTSCI also include vascular malformations, neoplastics, and
degenerative diseases (Figure 1).
1

Figure 1. Examples of Disc Problems
Source: Spine Universe 2010
Table 1 below shows the number of spinal cord injuries in Canada between the years 2004-­‐
2005. Table 2 shows an international statistics to compare the top six countries with the
highest number of SCI.
Table 1. Spinal Cord Injuries for each Province in 2000-2001
PROVINCE TOTAL ADMISSIONS - SCI
Alberta 199
British Columbia 190
Manitoba 50
New Brunswick 53
Newfoundland 14
Northwest Territory 3
Nova Scotia 14
Ontario 488
Prince Edward Island 2
Quebec 316
Saskatchewan 53
Territories 3
Table 2. International Statistics for SCI
Country/Population (millions) Injuries/annum and ratio Population estimated living
(millions)
with SCI
USA (260) 10000/(40) 250,000
Canada (30) 843/ (27) 30,000
UK (59) 700/ (12) 35,000
Australia (17) 241/ (13.2) 10,000
Japan (125) 2665/ (21.3) N/A
2

Source: Rick Hansen Spinal Cord Injury Registry, 2001
The following pie chart demonstrates the causes of traumatic spinal cord injuries that have
occurred in the USA.
Figure 2. Causes of SCI
Source: fscip.org, 2010
Spinal Fusion
Over the last decades, anterior cervical discectomy and fusion (ACDF) have been
considered to be the most established and highest standard of treatment for degenerative
disc disease in the spine. ACDF involves a surgical procedure to treat nerve root or spinal
cord compression by removing the ruptured disc and nerve roots of the spine (mainly in
the cervical region) in order to relieve pressure on the nerve roots or on the spinal cord
(Spine Universe, 2010). This procedure is carried out from the front of the neck through a
small incision, hence the name anterior. During the surgery, soft tissues of the neck are
separated and the disc is removed. However, if the disc space is not sufficient to maintain a
normal height, a procedure, using pieces of bone material from a patient or
artificial/synthetic materials to replace the missing space, called bone grafting may be
used. Bone grafting is an extremely complex and significantly risky procedure that can also
result in failures to heal properly. In the long term, spinal fusion of the segment may also
lead to progressive degeneration of the adjacent vertebrae. Hence, introduction of TDR
procedures appear to be a promising clinical procedure for the treatment of SCI.
History
The first total disc replacement (TDR) created and implanted was Fernstram’s steel ball in
1966. Eight patients had 13 corrosion-­‐resistant, stainless steel ball-­‐shaped prostheses
implanted. Reitz et al. also reported implants of the same type on 32 patients, however
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which both studies lacked long-­‐term follow up and the implants abandoned due to
problems such as subsidence of the device and segmental hypermobility (A.T. Villavicencio,
MD, et al, 2007). More extensive TDR implant types included the first elastic disc implant
(Fassio’s elastic disc replacement), and Lee and Associate’s intervertebral disc spacer (C.M.
Bono, et al., 2004). The first Food and Drug Administration (FDA) approved disc
replacement was approved in 2004; developed by Drs. Karin Buttner-­‐Janz and Kurt
Schellnack in 1988, the Charite disc composed of cobalt chromium outside and
polyethylene in the core (K. Buettner-­‐Janz et al., 1988).
After implanting a Charite disc, patients were observed to have motion between 0 and 21
degrees while bending forward and backward. This is one of the most distinct features of
the Charite disc, because unlike spinal disc fusion, the Charite disc allows the patients to
preserve some spinal cord motion. This type of disc replacement also does not require any
bone graft. The FDA has approved the Charite disc for use in treating pain associated with
degenerative disc disease. The device was approved for use at one level in the lumbar spine
(from L4-­‐S1) for patients who have had no relief from low back pain after at least six
months of non-­‐surgical treatment (Spine Universe, 2010). The focus of this report is to
highlight the development and evolution of one of the most promising fields in biomedical
and biomaterials necessary for the treatment of spinal cord injuries and pain.
Mechanics of Spine
Background
The human spine is made out of 25 different vertebrae bones separated by
intervertebral disc segments (Figure 5). As mentioned previously, these discs are
comprised of an outer annulus fibrosus and inner nucleus pulposus (80% by volume of
water). Unlike the muscles and organs of the human body, these discs have no blood
supply. Therefore, in order for nucleus pulposus to receive necessary nutrition and carry
out wastes, it relies solely on mechanical means and the flow of water through small pores.
4

Figure 3. Intervertebral Disc
Source: Spine Universe, 2010
When standing up or sitting, the spine is loaded in which the nucleus pulposus is
compressed creating a pressure inside that will force the water out (Figure 5). Similarly, no
load on the spine exists when lying down there and the water will flow back in. Overtime,
as the body ages, more water leaves the disc than enters-­‐ causing spinal disc degeneration
and increased lower back pain among older people.
Figure 4. Intervertebral Disc under Compression
Source: ICORD, 2010
The vertebrae bone is divided into three different sections: cervical, thoracic and
lumbar (Figure 7). Each section is named using the first initial of the section name followed
by a sequential numbering system starting from the top at number one and increasing to
the number of vertebrae of that section. The last bone after L5 is sacrum and is usually
referred as S1. Due to a higher load and stress concentration on the lower back area,
biomechanical analysis will focus on the lumbar vertebrae (discs L1 to L5). This higher
level of stress concentration is a result of the force applied by the majority of upper body
weight. During spinal cord bending, flexion and extension, creates the biggest moment arm
at the lumbar vertebrae (Figure 6).
5

Figure 5. Human Spine
Figure 6. Spinal Cord Movement
The goals of disc replacement, like many other prostheses, should reproduce the normal
motion of the cervical spine while retaining the normal biomechanical properties of the
intervertebral disc. The limiting nature of man-­‐made materials must require existing
biomaterials to represent a compromise of material strength and function.
Natural Loading
The natural load on a spine varies from person to person as it depends on body
weight and shape. In order to simplify the problem for the purpose of this report, analysis
will be conducted under the assumption that an average person weighs about 80 kg with
70% of the body weight located in the upper body. Upper body generates a concentrated
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force in the middle of the vertebrae when a person is standing straight and a concentrated
force on the side when bending (Figure 8).
Case Study I: Finite Element Analysis
Figure 7. Natural Loading of the Spine
A study was done by a group of people in Malaysia using Finite Element Analysis
simulating L3-­‐L4 disc arthroplasty. The 3D model was constructed from human
tomography image database and the disc was replaced with an artificial intervertebral disc.
Table 3 contains the material properties that were used in this study.
Table 3. Material Properties
After constructing the models and setting proper material properties, a force of 400
N was then applied at the top of L3 to simulate the natural loading of the spine (Figure 9).
Both models were simulated to perform extension, flexion and lateral bending and the
stress concentration and displacement that were caused by the above mentioned
movements were then compared as shown in Figures 10 and 11.
7

Figure 8. Constructed 3D model of disc with applied force of 400N
Figure 9. Stress Analysis with intact Disc and artificial Disc
Figure 10. Displacement analysis of intact Disc and artificial Disc
8

It is interesting to note that the there is a maximum stress concentration of 15 MPa
on the artificial disc, versus an almost zero stress on the intact disc. Additionally, Figure 11
illustrates that more displacement can be achieved with an artificial disc versus the intact
intervertebral disc. In comparison to the traditional spinal fusion methods where the spine
is fixed and limits movement, using TDR not only restores the normal movement of the
spine but actually increases it by 4 degrees.
Case Study II: Human Cadaver Analysis
The second case study chosen for this report was done by Sun-­‐Kon et al. where an
artificial intervertebral disc is implanted into a human cadaver. However, in this study, all
the discs from L1 to S1 were replaced and similarly a 400 N constant force was applied to
simulate normal loading on the spine. Extension, flexion, lateral bending and axial rotation
were then performed on the spine and similarly, results indicated that having an artificial
disc allows for more flexibility of the spine versus the limited spinal fusion method.
Figure 11. Comparison of Displacement angle for intact Discs and implanted Discs
Materials
Key features of artificial disc design are center of rotation, short-­‐ and long-­‐term stability,
and material interfaces. Material choice should also be determined by the needs of both the
articulating surface and the interface between prosthesis and vertebral body. Hallab et al,
identified several criteria important for optimizing materials selection in TDR-­‐
preservation of kinematics and biomechanics, preservation of intervertebral space,
biocompatibility, revisability, and life expectancy of the materials used (Table 4).
Significant knowledge gained from the development of hip and knee replacements have
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een used as a foundation for the manufacture of arthroplasty prostheses. Improvements
in material and designs specifications are slowly leading R&D toward the ideal total disc
replacement. Artificial disc designs may involve metal-­‐on-­‐metal, metal-­‐on-­‐polymer (poly),
ceramic-­‐on-­‐polymer, or ceramic-­‐on-­‐ceramic articular interfaces.
Three most commonly used metal materials in TDR include titanium alloys, cobalt alloys,
and stainless steels. Titanium alloys typically possess the highest corrosion resistance and
tissue compatibility, thus making it an attractive material for porous coatings of end plates.
In addition, titanium alloys for higher-­‐quality for imaging technology such as MRI and CT
scans. Cobalt alloys, demonstrate good wear resistance that make them useful for
articulation with polymer surfaces. Stainless steel exhibits good ductility but poor
corrosion resistance. Polymers include ultra high molecular weight polyethylene
(UHMWPE) or polyurethane which provides the needed flexibility. UHMWPE is useful for
providing low-­‐friction surfaces but can raise concerns with wear debris
Table 4. Goals for Total Disc Replacement
Remove all disc “pain generators”
Restore spine kinematics
Longevity > 40 years
Safe implantation
Metal-on-Metal
This type of articulating surface is most commonly used for any prostheses; usually
produced from titanium, stainless steel or chromium. Metals provide the necessary
strength, ductility, and toughness needed for load bearing. However, metal components can
often wear, corrode, and fracture; therefore, metal alloys are used to create a balance
between metals. Wear debris from metal components have been found to increase serum
and urine levels of heavy metals (A.T. Villavicencio, MD, et al, 2007).
Metal-­‐on-­‐metal devices include the Cummins Design, the Bristol Disc, the Prestige Disc, and
the Cervicore system. The Cummins Design was the original metal-­‐on-­‐metal design for TDR
which used a type-­‐316 stainless steel in a ball-­‐and-­‐socket articulation to allow low
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apparent translation. Titanium screws located anteriorly in the vertebral body are screwed
to the bone for affixation. Additional stability is achieved through compression of lower
and upper vertebral bodies against ridges in the metal prosthesis.
The second generation of all metal prosthesis was the Bristol Disc-­‐ a modification of the
Cummins design, through a ball-­‐and-­‐trough articulation to allow for more physiologic
translation at the level of replacement. The Prestige Disc also using stainless steel, has a
similar design but with a lower profile and improved instrumentation for easier
implantation (Figure 12a). Metal implants may also reduce osteolysis, however, not yet
observed in TDR. Other metals such as titanium-­‐on-­‐titanium have historically been a poor
bearing surface. However, promising metal designs have looked, these devices are also
usually very large and bulky; the majority not getting much further than the patent
application approval (G.M. McCullen et al., 2003) Other metal-­‐on-­‐metal discs include the
Kineflex|C Disc, and the Cervicore Disc, both of which currently have unavailable clinical
experience at the present time (Figure 12b).
Metal-on-Polymer
Figure 12. a) Cervicore and b) Prestige Disc
Source: Spine-Health, 2010
The combination of metals and polymer are the most commonly used materials for
artificial disc design. Polymers have a higher wear rate but lower stiffness and may offer
some degree of shock absorption. In addition, materials such as elastomers (silicone,
polyurethane, polyethylene) possess similar mechanical qualities than that of metals. With
a lower modulus of elasticity, it is easier to replicate disc dynamics. Although this type of
design may lessen the degree of heavy wear debris, metal-­‐on-­‐polymer designs also show a
higher inflammatory response as well as a wear rate twice as high than that of an all metal
design (A.T. Villavicencio, MD, et al, 2007).
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The Prodisc-­‐C prosthesis uses a cobalt-­‐chromium-­‐molybdenum alloy combination for end
plates and UHMWPE as the central material. The endplates are coated with titanium
plasmapore for tissue compatibility and bone ingrowth with the locking core of UHMWPE
to provide the ball-­‐and-­‐socket articulation. The porous coated motion, or PCM device,
approved for clinical trains in the U.S. in 2004 uses a large-­‐radius UHMWPE core with
cobalt-­‐chrome endplates to promote bone ingrowth (Figure 13a). The endplates have
serrated surfaces with a titanium-­‐calcium phosphate bony ingrowth coating to enhance
postoperative stability with an additional two layers of pure titanium plasma sprayed onto
the back of the endplates, followed by an electrochemically applied calcium phosphate
hydroxyapatite layer. Two modified types of this design exist: a low-­‐profile PCM disc for
patients in whom the posterior longitudinal ligament (PLL) is preserved, and a fixed PCM
disc with screws when the PLL is removed. A rectangular shaped design is used to
maximize support in the lateral areas. The Bryan Disc (Figure 13b) is another metal-­‐on-­‐
polymer disc that consists of a low friction elastic polymer nucleus and two anatomically
shaped titanium alloy shells equipped with anterior rigid wings to prevent posterior
migration. A flexible membrane encompasses the disc that contains saline to diminish any
friction and wear. A porous titanium coating is also used on bone-­‐contacting surfaces of
the shells that contribute to long-­‐term stability. Other discs include the Mobi-­‐C Disc (Figure
13c) and the Secure-­‐C Disc in which has entered a clinical phase study expecting to involve
almost 400 participants, including 100 nonrandomized training cases (A.T. Villavicencio,
MD, et al, 2007).
Ceramic-on-Polymer
Figure 13. a) PCM and b) Bryan and c) Mobi-C Disc
Source: Spine-Health, 2010
In TDR, bioactive ceramics are used on surfaces to coat UHMWPE polymers to enhance
viscoelasticity. An example is the 3-­‐DF, a three-­‐dimensional fabric woven by polymer
12

fibers coated with ceramics, such as the Kanada one-­‐rod SR system. This type of disc
implant has undergone years of experimental observation work in which excellent bone
ingrowth was observed and thus, a potential hope for clinical breakthrough with further
necessary refinements in terms of design and surgical strategy (Y.Kotani, et al).
Ceramic-on-Ceramic
Ceramics have become a recent alternative for use in artificial disc design. These materials
can provide excellent wear resistance, but require proper manufacturing techniques that
may otherwise result in fractures if defective (A.T. Villavicencio, MD, et al, 2007). The
Cerpass, is an original ceramic-­‐on-­‐ceramic disc, currently waiting for FDA approval for
investigation. Its potential for TDR is the material and design’s ability to provide better
durability and eliminate potential problems of wear debris. Another ceramic-­‐on-­‐ceramic
prosthesis is the Catalina, not currently available in the United States, and the Cervidisc.
Table 5 below summarizes the above mentioned designs and many others that incorporate
many other material composites currently undergoing research or pending clinical phase
trials.
Table 5. Artificial Discs Statistics Exhibit
Company Product Description Regulatory Status
Abott Spine ISD Elastomer core in woven
cover; replaces anterior
Aesculap Active L
longitudinal ligament
CoCr on poly, semi mobile
bearing
IDE began 2007
Amedica Altia Silicon nitride ceramic IDE began 2009
AxioMed Freedom Elastomer/Ti endplates IDE began 2008
Biomet Min T Ceramic on endplate, Ti keel
fixed to vertebra
Trials in Australia
Cervitech PCM-­‐V, PCM-­‐TI, PCM-­‐EF CoCr or Ti on poly, broad U.S. pivotal clinical trial
radius
enrolment complete
DePuy Charite CoCr
socket
on poly, ball and FDA cleared
DePuy Discover Ti on poly IDE began 2006
Disc Motion TrueDisc-­‐C, TrueDisc-­‐L Ball and socket
Globus i) Secure-­‐C; ii) Alliance i) Metal on poly/semi
LDR Spine i) Mobi-­‐C; ii) Mobidisc
mobile bearing; ii) CoCr on
poly
Ti-­‐HA coated CoCr on poly,
mobile core
Medtronic i) Prestige ST; ii) Prestige i) Stainless steel metal-­‐on-­‐ FDA cleared
13
i) IDE enrolment complete;
ii) in development
i) FDA cleared; ii) IDE
enrolment complete

LP metal, and ii) ceramic-­‐on-­‐
creamic; ball and trough
Medtronic Bryan Ti on polyurethane, ball and
socket
Nexgen Spine Physio-­‐C CoCr with Ti porous coated
endplates + polycarbonate
polyurethane core
14
IDE ongoing; recommended
for FDA clearance
NuVasive CerPass Ceramic-­‐on-­‐ceramic IDE began 2008
Orthofix Advent Ti on polyurethane, ball and
socket
IDE began 2008
Pioneer Surgical NuNec HA-­‐coated PEEK on PEEK 1
semi-­‐constrained
trough hybrid
ball and
st implant2008, IDE began
2008
Rainer CAdisc-­‐C Variable modulus elastomer Undergoing development
Scient’x DiscoCerv Ceramic-­‐on-­‐ceramic IDE began 2008
SeaSpine Catalina CoCr w/Peek endplates IDE began 2008
Spinal Kinetics M6 Polymeric nucleus, poly Feasibility study underway
Spinal Motion i) Kineflex C; ii) Kinelfex-­‐L
fiber annulus, Ti endplates
CoCr on CoCr with CoCr
mobile core
IDE enrolment complete
Stryker i) Cervicore; ii) FlexiCore i) CoCr on CoCr saddle; ii) IDE began 2006; PMA
CoCr on CoCr ball and application submission
socket
anticipated 2009
Synthes ProDisc-­‐C CoCr
socket
on poly, ball and FDA cleared
Theken Disc eDisc Elastomer/Ti
with electronics
endplates, Undergoing development
Clinical Total Disc Implants
To date, an estimated 5000 patients have had the Charite disc implanted since its
development in 1988 with the vast majority inserted in Europe (United Kingdom, France,
Germany, and the Netherlands). Clinical results have been published totalling nearly 300
patients who have undergone 1-­‐3 years of relatively brief observation and current study is
ongoing in the follow-­‐up phase with outcomes to be completed within the next couple of
years.
Between 1990 and 1993, 64 patients were implanted with the ProDisc. By 1999, 95% of
these patients were available for follow-­‐up stating all implants were reported to be “intact
and functioning”. No removals, revision, failures, or subsidence were reported for this
design. Approximately 93% of the patients were reported to be “satisfied” with the
outcome of the implant. Ongoing study for this device will continue for the next couple
years (P.M. Klara, 2002).

Recently published reports reviewed the works of the Bryan disc, implanted into a total of
97 patients. Data was available for only 60 patients after a 6 month and 1 year follow-­‐up. 7
of the 11 reported failures have been secondary to incomplete neurologic decompression.
In addition, an unexpectedly high rate of heterotopic ossification (16 of the 97 patients)
was also observed. Of these 16 patients, 10 patients reported less than 2 degrees of motion
at the operated level (A.T. Villavicencio, MD, et al, 2007). The Cummins design also
reported similar results: 3 of 14 implanted discs were fused by 7–12.7 years follow-­‐up after
implantation. Table 6 below shows a comparison between the different types of disc
designs and the total amount implanted as of December 2004.
Table 6. Comparison of Disc Design Types
Bearing Surface
Material
Metal-on-Metal Prestige, Cervicore,
Kineflex|C, Secure-­‐
C
Disc Type Advantages Disadvantages Total
Implanted
(worldwide)
As of Dec. 2004
Common; balance
undesirable features
15
Heavy metal wear
debris; corrode;
fracture
Prestige: 500
Metal-on-Ceramic Cervidisc Cervidisc: 52
Metal-on-Polymer Bryan, Pro-­‐Disc C,
Porous Coated
Motion Device
(PCM), Disc,
Mobidisc
Polymer
Composite
Ceramic-on-
Polymer
Ceramic-on-
Ceramic
Common; promotes
high friction
NeoDisc High wear rate;
shock absorption
Cerpass Excellent wear
resistance
Inflammatory
response; wear rate
2x metal-­‐on-­‐metal
Low stiffness
Critical
manufacturing
processes; fracture
Bryan: 7000
Pro-­‐Disc C: 800
PCM: 300
Complications and Success Rates
Even if an excellent surgery has taken place for implantation by a good surgeon, long or
short term complications are bound to occur. Common complications of TDR are listed
below:
• Problems with anesthesia
• Thrombophlebitis (blood clots)
• Infections

• Nerve damage or paralysis
• Problems with the implant
• Ongoing pain
• allergic reaction to the implant materials
• bladder problems
• death
• pain or discomfort
• slow movement of the intestines
• spinal cord or nerve damage
• spinal fluid leakage
• the need for additional surgery
• tears of the dura (a layer of tissue covering the spinal cord)
• incision problems
The complications due to the implant are because of its biocompatibility and dynamic
loading that is placed on the implant. The following list demonstrates common
complication causes due to the implant:
• shifting and dislocation;
• implant failure over time from wear; artificial discs are estimated to last 15-­‐20
years;
• neurological compression with neurologic symptoms can occur due to loss of
normal disc height.
In order to decrease the occurrence of long term complications, patients must rehabilitate
after surgery. Surgeons prescribe patients physical therapy within one to two weeks after
surgery has taken place in order to improve flexibility, strength, and endurance. Gentle
stretching exercises for the back are also commonly prescribed along with a series of
strengthening exercises such as treadmill walking, swimming, or stationary biking on a
regular basis to help tone and control the muscles for back stability. For the last 17 years,
many implants have been functioning properly invivo where laboratory testing has also
16

demonstrated remarkable durability over time (assuming an average of 125,000 significant
bends each year or approximately 340 per day). As with any implant however, patient
activity levels and life style can have an impact on the final and long term results (Charite
Disc, 2009).
Clinical data from 1-­‐ and 2-­‐ year follow-­‐ups for TDR have reported that artificial disc
devices have continued to maintain physiological segmental motion at 24 months after
implantation. At last follow-­‐up approximately 90% of those with disc implants were mobile
and had statistically significant improvements as assessed by the Neck Disability Index, the
Neck Pain Score, and other physical component scores (Jaramillo-­‐de la Torre, J.J., et al.,
2008). The investigational group also showed improved neurological success, improved
clinical outcomes, and a reduced rate of secondary surgeries compared to that of ACDF.
Other studies have reported significant improvements in pain and functional outcome in
patients treated with TDR prostheses at 12-­‐18 months and 4 years of follow-­‐up with
reservation of motion and without development of further spinal degeneration (Robertson,
J.T., et al., 2004).
Future Directions
Slowly, since its development, the artificial disc is becoming a reality. It has been shown
that motion preservation after TDR will decrease the incidence of further disc
degeneration. However, longer-­‐term follow-­‐up studies are needed to continue to assess
further issues regarding the disc implants. Far from becoming a “routine” procedure, TDR
will be optimized within a defined, narrow clinical window. As experience is increased,
additional potential indications may emerge such as neck pain, deformity correction, or
revision fusion (C. Mehren et al., 2005). The continuing research in cervical spine
biomechanics, biomaterial science, and surgical technique gives potential hope in the
future for alternative prosthesis with improved designs for TDR.
ICORD (International Collaboration on Repair and Discoveries)
ICORD research brings together anything from cellular to community level research to
address questions that concern the promotion of improved functional outcomes and
17

quality of life for those with SCI. The organization’s research spans the continuum from
basic, preclinical discovery, to human-­‐based discovery, to acute clinical interventions, to
chronic care and rehabilitation, as well as to community integration and participation.
Although TDR has not yet fully entered the realms of the organization’s research, current
studies can form a solid foundation for potential research.
Current studies encompass many factors critical in the framework of the spinal cord
system that include the heart, blood vessels, nervous system, and even sensory functions of
the human body. By analyzing the control of the heart and blood vessels in individuals who
have sustained SCI, researchers hope to be able to develop and improve treatments for the
management of cardiovascular complications. Diagnosis and understanding of cervical SCI
is also an ongoing study in order to examine and better understand the different nerve
fibres in the spinal cord and the extent of spinal cord damage. This may allow for the
developments of new techniques, routine assessments in individuals with SCI to optimize
treatment methods. Other studies include physical rehabilitation devices that incorporate
virtual reality training exercises and games, combining electrical stimulation in order to
improve hand function, as well as rehabilitation of upper limb function after spine injury.
Conclusion
Artificial spinal disc replacements have been one of the most attractive alternate methods
for management of disc degeneration and various types of spinal cord injury. Unlike spinal
cord fusion, spinal disc replacement does not limit movement of the patient implantation
nor pose any further health complications. Theoretic advantages of TDR include
preservation of normal motion and biomechanics in the spine and reduction of segment
degeneration. Additional potential advantages include faster return to normal activity and
elimination of the need for bone graft.
TDR implants compose of a wide variety of materials that range from metals to polymers,
ceramics as well as a combination to benefit from the advantages of each material.
However, the success of disc implantation ultimately depends on the patients’ compatibility
of the artificial discs as well as the degree of rehabilitation post surgery. As experience
increases, additional potential indications may emerge further such as neck pain, deformity
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correction, or even revision of previous fusion. Continued future studies, within the
biomechanics and biomaterial science of the spine must clearly stratify the degree of
degenerative change, not just within the disc but also within the entire joint system to
include the ligaments, adjacent discs, and cord itself.
19

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