trends in materials for spine surgery - elseviermrw.elsevier.com/bmt2/trends in materials for spine...
TRANSCRIPT
6.610. Trends in Materials for Spine SurgeryM Marcolongo, S Sarkar, and N Ganesh, Drexel University, Philadelphia, PA, USA
ã 2011 Published by Elsevier Ltd.
6.610.1. Introduction 1286.610.2. Anatomy and Physiology of the Spine 1286.610.2.1. The Vertebral Column 1286.610.2.2. The Intervertebral Disc 1296.610.2.3. The Nucleus Pulposus 1296.610.2.4. The Annulus Fibrosus 1296.610.2.5. Intervertebral Disc Degeneration 1306.610.3. Spinal Fusion 1316.610.3.1. Metals for Spinal Cages and Interbody Spacers 1326.610.3.2. Polymers and Composites for Fusion Cages 1326.610.3.3. Wires, Tapes, and Cables in Spinal Fusion 1336.610.3.4. Pedicle Screws, Plates, Bands, and Rods for Fusion and Dynamic Stabilization 1346.610.3.5. Bioresorbable Materials for Spinal Fusion Implants 1346.610.4. Total Disc Arthroplasty 1356.610.4.1. Material Requirements 1366.610.4.1.1. Implant endplate 1366.610.4.1.2. Bone implant interface 1366.610.4.1.3. Bearing surfaces 1366.610.4.1.4. General material specifications 1366.610.4.2. Discontinued Artificial Discs 1376.610.4.2.1. Acroflex 1376.610.4.3. Currently Available Devices 1376.610.4.3.1. Charite artificial disc 1376.610.4.3.2. ProDisc 1386.610.4.3.3. Maverick 1396.610.4.3.4. Flexicore 1406.610.4.4. Future Trends 1406.610.5. Annulus Repair 1406.610.5.1. Procedure 1406.610.5.2. Mechanical Role 1406.610.5.3. Reported Treatment Methods 1416.610.5.3.1. Suture techniques 1416.610.5.3.2. Sealant and bulking techniques 1416.610.5.4. Current Technologies 1416.610.5.4.1. Xclose™ tissue repair system 1416.610.5.5. Barrier Technologies 1426.610.5.5.1. Inclose™ surgical mesh system 1426.610.5.5.2. BarricaidW prosthesis 1436.610.5.5.3. SpineJet XLW 1436.610.5.6. Material Requirements 1436.610.5.7. Future Trends 1446.610.6. Concluding Remarks 144References 144
GlossaryDiscogenic back pain Discogenic back pain is also known
as lumbar disc pain. The most frequent symptoms of
discogenic back pain are lower back pain and spasms.
Fretting corrosion Fretting corrosion refers to corrosion
damage at the asperities of contact surfaces. This damage is
induced under load and in the presence of repeated relative
surface motion and wear.
Hemangiomas An abnormal proliferation of blood vessels
that may occur in any vascularized tissue.
Laminectomy Spine surgery that removea the portion of the
bone arch on the dorsal surface of a vertebra.
127
128 Orthopedic Surgery – Spinal Treatment
Osteopenia A condition in which bone mineral density is
lower than normal, but not low enough to be classified as
osteoporosis.
Pseudo elasticity A reversible response to an applied stress,
caused by a phase transformation between the austenitic and
martensitic phases of a crystal.
Vertebroplasty and kyphoplasty Medical spinal procedures
in which a material is injected through a small hole in the
skin (percutaneously) into a fractured vertebra and hardens;
the goal of the procedure is to relieve the pain caused by
osteoporotic compression fractures.
AbbreviationsAF Annulus fibrosus
GAG Glycosaminoglycan
IVD Intervertebral disc
NP Nucleus pulposus
PG Proteoglycan
TDA Total disc arthroplasty
6.610.1. Introduction
The spine has two main purposes: (1) it must provide a strong
mobile central axis to support the skeleton and (2) it must also
provide protection for the delicate nerves that travel from the
brain to the periphery. A balance between stability and flexi-
bility are essential to achieve this dual purpose. Aging, injury,
disease, and genetic conditions can disrupt this balance result-
ing in back pain with more than 60% of the population experi-
encing back pain throughout a lifetime.1 Low back pain alone
accounts for 2% of all US physician office visits.2 Pain can be
generated in the spine from damage to several anatomical
elements including the vertebral bodies, intervertebral discs
(IVDs), facet joints, dura of the nerve roots, ligaments, facia,
and muscles.3 Stabilization of the spine has been the major
focus of surgical interventions to alleviate back pain.4
Interventional techniques have evolved from the earliest use
of metal screws and plates to correct scoliosis adapted from
peripheral extremity repair in the late 1950s.5 Internal fixation
of the spine using pedical screws was first introduced in 1959 by
Boucher in Canada.6 In the 1960s, the advent of bony fusion
techniques greatly increased the success of these procedures and
currently, advancements are still beingmade onmaterials choice
and structural design of spinal internal fixation devices.7 The
IVD has also been a major focus in spinal procedures with the
first surgeries on disc protrusion reported in 1934 by Mixter and
Barr.6 Current techniques in treating discogenic back pain
include removal of disc material and replacement with pros-
thetic devices8; however, advances in knowledge of the IVD
ultrastructure have led to recent investigations into the repair
of individual IVD components, that is, nucleus replacements9
and annulus repair technologies (see Chapter 6.613, Nucleus
Replacement).10 Recent advances in minimally invasive surgery
have alsomade a large impact on spine surgery and the potential
for novel therapies.
Most notable is the development of the vertebroplasty and
kyphoplasty procedures developed in the late 1980s for the
percutaneous treatment of vertebral fractures (see Chapter
6.611, Injectable Bone Cements for Spinal Column Augmen-
tation: Materials for Kyphoplasty/Vertebroplasty).11–13 The
development of materials for spinal procedures has been
greatly influenced by advances in technology that permit the
visualization of spinal structures, innovations of novel surgical
procedures, and a better understanding of the biology and
structure of the native tissue. Early materials were adapted
from procedures designed for the repair of bones and joints
in the extremities; however, as more is understood about the
unique structure and mechanical needs of the spine, novel
materials are being developed for use in spine specific proce-
dures. Tissue engineering and drug delivery have also moti-
vated the development of new materials with porous and
regenerative surfaces. This chapter outlines the current materi-
als utilized in the major surgical interventions (spinal fusion,
total disc replacement, and annulus repair) and spine pathol-
ogies as well as the future direction of material development.
6.610.2. Anatomy and Physiology of the Spine
6.610.2.1. The Vertebral Column
The human vertebral column or spine functions to transfer
loads and bending moments of the head, trunk, and any
external loads to the pelvis, allows sufficient physiological
movement and flexibility of the upper body, and protects the
spinal cord from dangers due to motion and trauma.14 It also
serves as protection to other vital internal organs and as a base
of attachment for upper-body ligaments, tendons, and mus-
cles. These functions help explain its form and structure.
The spine consists of 24 individual vertebrae, the sacrum,
and the coccyx, and spans from the skull to the pelvis. The
vertebrae are stacked on top of each other and are grouped into
five distinct regions – there are 7 cervical vertebrae, 12 thoracic,
5 lumbar, 5 fused sacral, and 3 fused vertebrae that make up
the coccyx (Figure 1). From posterior perspective, the spine
appears to be straight, but lateral view shows four normal
curves that mechanically serve to give the spinal column
increased flexibility and shock-absorption ability.
The vertebral body is the primary weight bearing area, and
adjacent vertebrae are connected by an IVD (Figure 2). The ver-
tebral foramen is the large hole in the center of the vertebra that
is surrounded by lamina, forming the spinal canal. The spinal
cord runs through this opening and is protected by it. The
spinous process protrudes from the central posterior region of
the vertebra and acts as a connection point for ligaments. There
Figure 3 Vertebral compression fracture. Reproduced fromStryker-Interventional. 2008. Compression Fractures of the Spine.http://www.helpingbacks.com/spinalfractures.php.
Lateral (side)spinal column
Cervical Cervical
Thoracic
Lumbar Lumbar
Sacrum Sacrum
Coccyx Coccyx
Thoracic
Posterior (back)spinal column
Figure 1 Regions of the human spine with respect to the vertebrae andcorresponding intervertebral discs (http://healthbase.wordpress.com/2007/06/13/spinal-fusion-surgery/).
Lumbar vertebrae
Transverseprocess Vertebral
body
Pedicle
Axial(overhead
view)
Lateral(sideview)
Vertebralbody
Spinousprocess
Spinousprocess
Lamina
Figure 2 The lumbar vertebrae. Reproduced from Toida, T.;Kakinuma, N.; Toyoda, H.; Imanari, T. Anal. Sci. 1994, 10, 537–541.
Trends in Materials for Spine Surgery 129
are pairs of transverse processes which are orthogonal to the
spinous process and provide attachment for the back muscles.
There are also four facet joints on each vertebra that are found in
pairs (one superior and one inferior), interlocking with adjacent
vertebrae to provide stability to the spine. When osteoporosis
occurs in the spine, vertebral collapse, or compression fractures,
may occur (Figure 3.). Vertebral compression fractures may also
be caused by hemangiomas, multiple myeloma, and osteolytic
metastases.15 Multiple fractures lead to a stooped or hunched
posture, chronic pain, and loss of height.
6.610.2.2. The Intervertebral Disc
The IVD separates adjacent vertebrae and acts as a shock
absorber, dissipating energy under loading. The disc is the
largest avascular tissue in the human body requiring nutrients
to enter and waste to be removed primarily through diffusion
and fluid flow.14 The disc structure is quite complex combining
three distinct regions: the nucleus pulposus (NP), an inner
gelatinous core; the annulus fibrosus (AF), a surrounding
ordered fibrous structure; and the bony endplates that are
partly cartilaginous and partly bony and serve as an interface
between the disc and the vertebral bodies (Figure 4). The
cellular content of the disc is very low, less than �0.25% of
tissue volume.16 The extracellular disc matrix plays the main
role in determining the mechanical properties of the disc.
6.610.2.3. The Nucleus Pulposus
The central NP region of the IVD is a gelatinous structure that is
mostly water (77% of wet weight) but also contains a large
constituent of proteoglycans (PGs) (14% of wet weight) which
are composed of charged glycosaminoglycan (GAG) chains
covalently attached to a protein core. Randomly organized
collagen fibers are also present in the NP (about 4% of wet
weight) and associate with the PGs to form a loose network.17
Collagen fibers within the NP provide tensile strength while
PGs create a large osmotic swelling pressure and draw water
into the tissue (Figure 5). Charged groups on the GAG chains
of the PGs carry with them mobile counter ions such as Naþ
which draw water into the tissue because of the osmotic imbal-
ance generating a hydrostatic pressure within the NP.18
6.610.2.4. The Annulus Fibrosus
The AF of the IVD is a structure that serves to contain the
gelatinous NP. The chemical composition of the AF comprises
65–90% wet weight of water, 50–70% dry weight of collagen,
10–20% dry weight of PGs, and noncollagenous proteins such
as elastin.19 The structure of the AF is laminate in nature, con-
sisting of a minimum of 15 (posterior) and 25 (lateral) concen-
tric layers. The layers are made up of type I collagen fibers which
alternate in angles from 30� at the peripheral AF to 44� at the
central AF with reference to the transverse plane of the disc
(Figure 6). The concentric rings of the AF are referred to as
lamellae, and the spaces between them as interlamellar septae.19
Link protein
Aggrecan
Hyaluronan
Proteoglycanswelling
pressure H2O
CollagentensionCollagen
fibril
Figure 5 Constituents of the nucleus pulposus and subsequentdevelopment of hydrostatic pressure. Reproduced fromKiani, C.; Liwen, C.;Yao Jiong, W. U.; Yee, A. J.; Burton, B. Y. Cell Res. 2002, 12, 19–32.
Nucleuspulposus
Annulusfibrosus
7−10mm
Lamellae ofannulus fibrosus
Cartilaginousend-plateVertebral
body
Nerveroot
Nucleus pulposus
Annulus fibrosus
4 cm
Vertebral endplate
Figure 4 The intervertebral disc (IVD) location between adjacent vertebrae and the individual structures and average dimensions of the IVD.Reproduced from Raj, P. P.; Fipp, A. Pain Practice 2008, 8, 18.
C
P
20
a
40
T
Figure 6 Load transfer in the intervertebral disc (IVD) and lamellarstructure of the annulus fibrosus. When a compressive load (C) is placedon the IVD, a hydrostatic pressure (P) is generated in the NP and tensilestress (T) on the annulus. Lamella of the annulus has oblique collagenfibers in alternating directions. On average, there are 20 collagen lamellaein the annulus with approximately 40 collagen bundles per lamella.Reproduced from Adams, M.; Roughley, P. Spine 2006, 31, 2151.
130 Orthopedic Surgery – Spinal Treatment
Spinal compression generates a hydrostatic pressure in the
NP that places the annulus into tension thereby resisting the
applied load (Figure 6).20 The AF has a very highly organized
structure which results in complex anisotropic behavior with
loading. The tensile, compressive, and shear properties of the
AF differ in the axial, radial, and circumferential directions.
The AF can be further divided into the inner and outer AF on
the basis of the structural differences, the inner AF being sub-
jected to higher hydrostatic pressures of theNP than the outer AF
which is subject to tensile forces.
6.610.2.5. Intervertebral Disc Degeneration
The IVD of the spine is subject to degenerative changes induced
by normal aging as well as injury, loading, or genetically
induced accelerated disc degeneration.21 The exact mechanism
of IVD degeneration is not well understood; however, possible
degenerative cascades have been proposed which lead to even-
tual changes in disc mechanical properties (Figure 7). Initial
changes in the IVD are thought to be brought about by changes
in the nutrient flow to and waste product flow out of the
nucleus. Calcification of the cartilaginous endplates, the
major route of nutrient supply to the disc, leads to harsh
cellular environments, subsequently reducing matrix synthesis
and increasing enzyme production further reducing PG and
collagen content of the disc, in particular in the NP.21,23,24 The
loss of PGs in the NP has major effects on the load-bearing
behavior of the disc, causing the osmotic pressure of the disc to
fall and reducing the disc water content disrupting the NP’s
ability to behave hydrostatically under load. Loading can then
Matrixdegradation
Noc
icep
tors
Nociceptors
Fissures
Tran
spor
t
Endplate sclerosisEndplate capillariesStress
matrix damage
Cytokines
Lactate
OxygenNutrients
Load
Load
Proteases
Nucleus
Annulus
Vertebra
Stress
Disc cell
Endplate sclerosisEndplate capillaries
Stress
A
Distance
Degenerated
Old
P
Young
Figure 7 Cascade of events in intervertebral disc (IVD) degeneration22 and changes in mechanical properties of the IVD with degeneration(Normal disc to degenerated disc from top to bottom). Reproduced from Adams, M. A.; Dolan, P.; McNally, D. S. Matrix Biol. 2009, 28, 384–389.
Herniation of the lumbar disc Signal changes in vertebral end plates
Degeneration of the lumbar disc
L5
L5
L5
L5
L5
(a)
(c)
(b) (d)
L5
L5
L5
Annular fissures with high-intensity signals
Figure 8 MRI and schematic images of intervertebral disc disruptions. (a) Herniation of the lumbar disc with extrusion of disc material found in 1–18%of asymptomatic subjects. (b) Degeneration of the lumbar disc increases with age and is found in 25–70% of asymptomatic subjects. (c) Signalchanges in the vertebral endplates, found in 10% of asymptomatic subjects. (d) Disc with annular fissures seen in 14–33% of asymptomaticsubjects. Despite high prevalence of these damage modes in healthy persons, these findings are often associated with serious back pain and areoften treated with spinal fusion or disc replacement strategies. Adapted from Carragee, E. J. N. Engl. J. Med. 2005, 352, 1891–1898.
Trends in Materials for Spine Surgery 131
lead to inappropriate stress concentrations along the endplate
and in the annulus with progressive changes as disc degenera-
tion proceeds (Figure 7).
Changes in stress concentration can lead to damage of
the AF as well as the endplates with stress concentrations
being associated with discogenic pain.17 Several manifesta-
tions of disc degeneration and damage can be visualized
with MRI and are represented in Figure 8. Although these
alterations in the IVD are prevalent in the asymptomatic
population, disc disruptions have been associated with
back pain and are targets for surgical interventions such as
spinal fusion, total disc replacement, nucleus replacement,
and annulus repair.25
6.610.3. Spinal Fusion
Spinal fusion and stabilization utilizes a wide range of devices
such as screws, wires, artificial ligaments, and vertebral cages.26
The instrumentation used in fusion surgery is designed to
stabilize the spine during fusion and promote osseous
fusion.7 Material choice for these devices is therefore tar-
geted toward the promotion of bone ingrowth, and imme-
diate mechanical stabilization of the spine. Postoperative
imaging and analysis of surgical success is also of impor-
tance in material choice for spinal fusion devices.27 Early
spinal fusion devices were based on metals commonly uti-
lized in other orthopedic surgeries such as in hip implants.
132 Orthopedic Surgery – Spinal Treatment
Metals are still widely used in spine fusion implants today;
however, advances have been made in implant design and
material development in order to address the unique needs
of the spine including maintenance of spine flexibility and
to avoid the occurrence of adjacent area damage.
6.610.3.1. Metals for Spinal Cages and Interbody Spacers
The removal of the IVD in order to address discogenic back
pain and disc herniation will lead to the loss of disc height and
lead to spinal instability. Cage structures have been developed
to assist in implanting allo- and autologous bone graft inter-
body spacers. These cages enhance stabilization of the spinal
segment and prevent postoperative collapse.
The BAK™ interbody fusion system is a cage structure made
of porous Ti–6Al–4V shell with a cavity for the placement of
bone grafts (Figure 9).28 This device is implanted centrally or
Figure 9 The BAK cervical spinal fixation device. A threaded hollowtitanium implant that allows bone grafts to be implanted within the shellto facilitate fusion. Ends are capped with UHMWPE to minimize adhesion.Reproduced from Martz, E. O.; Goel, V. K.; Pope, M. H.; Park, J. B.J. Biomed. Mater. Res. 1997, 38, 267–288.
Table 1 Material properties of common metals in spinal implants
Property 316L SS (cold worked) Wrought Co
ASTM designationa F 138 (grade 2) F 562 (coldDensity (g cm�3)b 7.9 9.2Elastic modulus (GPa)a 193 220–234b
Yield strength (MPa)c 690 1586Tensile strength (MPa)c 860 1793Elongation (%)c 12 8Fatigue strength (MPa) 310 500(107 cycles)a (F55 cold worked) (cold worke
aFrom Black.30
bFrom Park and Lakes.31
cASTM F 138, ASTM F 562, ASTM F 136, and ASTM F 67.
Source: ASTM. Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1992; Park, J. B.;
Biomaterials in Research and Practice; Churchill Livingstone: New York, 1988; Martz, E. O.;
bilaterally between the vertebral bodies.28,29 The ends of this
device are capped with ultrahigh molecular weight polyethyl-
ene (UHMWPE) plugs in order to contain the bone grafts and
to minimize adhesion to surrounding nerves and blood ves-
sels. The strength of the titanium material allows for the load
bearing function of the spine and protection of the graft mate-
rial while the implant design which incoroporates threading
to stabilize the implant, and a hollow center to allow for bone
delivery, achieves the other functions of this device. Metals,
however, are subject to fatigue failure if the graft does not
fuse; also the high modulus of metals can lead to stress shield-
ing and eventual failure of the device to integrate into its
surroundings.26 See Table 1 for some common metals used
in spine implants and their material properties.
Cobalt chrome has also been developed into a porous
metal sponge for use in spinal surgery.32 The porous structure
was designed to match the structure of cancellous bone and the
implant could be loaded axially up to 28.5MPa. Problems
arose, however, with metal ion release because of the high
surface area imparted by the porous structure. Released metal
ions may increase pain at the implant site and cause loosening
of the implant and local bone resorption at the implant
site.33,34 Decreased pH levels at the implant site can be brought
about by the inflammatory response and infection and may
also have a deleterious effect on metallic implants causing
fretting corrosion.33,34
6.610.3.2. Polymers and Composites for Fusion Cages
In order to address some of the drawbacks of metal fusion
cages, composite materials have been developed and utilized
in spinal cages. A cage made from a composite of poly ether
ketone ether ketone ketone (PEKEKK) with pyrolytic carbon
fibers interspersed within the polymer matrix was developed.35
The material was molded to incorporate teeth located superi-
orly and inferiorly in order to resist expulsion (Figure 10).
Material properties were modulated to better mimic those of
bone in order to better support physiological loads as the
graft material slowly integrates into the surgical site and in
case the graft does not fuse. The cage was designed to have a
Young’s modulus of �17GPa, close to that of cortical bone.35
This matched modulus decreases the likelihood of stress
NiCrMo Ti–6Al–4V Pure Ti (Grade 4)
worked and aged) F 136 F 674.5 4.51105 100795 483–655860 55010 15520 240
d) (forged, annealed) (as fabricated)
Lakes, R. S. Biomaterials: An Introduction; Springer: Berlin, 2007; Black, J. Orthopaedic
Goel, V. K.; Pope, M. H.; Park, J. B. J. Biomed. Mater. Res. 1997, 38, 267–288.
Trends in Materials for Spine Surgery 133
shielding. An additional benefit of the nonmetallic implant is
the ability to visualize the postoperative implant area because
of the radiolucent property of the material.
Polyaryletherketones (PAEKs) are in the family of high
temperature thermoplastic polymers and have been widely
used for spinal cage implants after their introduction in the
1990s as an improvement over metal devices by Acromed
(Cleveland, OH, now DePuy Spine, Raynham, MA).36 PAEKs
consist of an aromatic backbone molecular chain intercon-
nected by ketone and ether functional groups. Specifically, poly
(aryl-ether-ether-ketone), PEEK, and poly(aryl-ether-ketone-
ether-ketone-ketone), PEKEKK, have been used in orthopedic
and spinal implants. The chemical structure of PAEKs allows
stability at high temperatures, exceeding 300 �C,37 resistance tochemical and radiation damage,38 compatibility with reinfor-
cing agents such as glass and carbon fibers, and greater strength
than evenmetals.36Mechanical andmaterial properties for some
PEEK formulations in comparison to other commonly used
polymer materials are given in Table 2.36 These materials are
also radiolucent making them a good alternative to metallic
spine biomaterials.
PEEK and PEKEKK were first used in the development of the
Brantigan cage with 68% by weight carbon fiber reinforcement
in a 2-year clinical trial beginning in 1989.39 Clinical results
were good with 100% interbody fusion identified. Interbody
Table 2 Typical average physical properties of PEEK and CFR-PEEK strumethacrylate (PMMA)
Property (ISO) Selected lavibio PEEK biomaterials (OPTIMALTI)
Unfilled(OPTIMALTI)
30% (w/w) chopped carbon fiberreinforced (LTICA30)
Polymer type SemicrystallineMolecular weight(106 gmol�1)
0.08–0.12 0.08–0.12
Poisson’s ratio 0.36 0.40Specific gravity 1.3 1.4Flexural modulus(GPa)
4 20
Tensile strength(MPa)
93 170
Tensile elongation(%)
30–40 1–2
Degree ofcrystallinity (%)
30–35 30–35
Testing conducted at 23 �C.
Figure 10 Carbon fiber composite cage made from Ultrapek.Reproduced from Martz, E. O.; Goel, V. K.; Pope, M. H.; Park, J. B.J. Biomed. Mater. Res. 1997, 38, 267–288.
fusion could be identified because of the radiolucency of the
PAEK materials which continued to be a major reason for the
use of this material (Figure 11).36,40
In addition to carbon reinforced PEEK, neat PEEK is also
utilized in the development of spinal cages and implants.41–43
Studies have also been conducted to investigate the improve-
ment of PEEK cages to incorporate accelerated fusion perfor-
mance by adding bioactive materials such as hydroxylapatite or
rhBMP-2.41,43,44 Wear and fracture can be potential concerns
with the use of PEEK as a load-bearing implant material; how-
ever, these concerns are not unique to PEEK. Although debris
from wear has been noted in biopsies from carbon fiber PEEK
cages, there has been no evidence of an inflammatory reaction
making PAEK type materials promising in this application.45
PEEK materials have also been investigated for other spine
stabilization devices such as posterior dynamic stabilization
devices46 and total disc replacements.36
6.610.3.3. Wires, Tapes, and Cables in Spinal Fusion
Fusion of single or multiple spinal segments is also accom-
plished through the use of wires, tapes, and cables to secure
adjacent bone segments. The most commonly used wire mate-
rial is 316L stainless steel (Table 1).26 Stainless steel is rela-
tively strong, ductile, and relatively inexpensive. Stainless steel
316L is especially preferred because of its resistance to corro-
sion as a result of low carbon content (0.03% maximum)
which prevents sensitization of the stainless steel. Nevertheless,
even stainless steel 316L is susceptible to localized corrosion
associated with a loss of the protective chromium oxide surface
layer.47 A concern in the use of metal wires for fixation is the
possibility of neurological damage as well as concerns that the
wire may cut through the bone if the bone is soft, which is
especially the case in children.26 As an alternative, polymer
tapes have been introduced such as nylon and woven strands
of polyethylene (Mersiline) for use in procedures on scoliotic
children48,49
ctural composite biomaterials, compared with UHMWPE and polymethyl
UHMWPE PMMA
68% (v/v) continuous carbon fiberreinforced (Endolign)
Semicrystalline Amorphous0.08–0.12 2–6 0.1–0.8
0.38 0.46 0.351.6 0.932–0.945 1.180–1.246135 0.8–1.6 1.5–4.1
>2000 39–48 24–49
1 350–525 1–2
30–35 39–75 Noncrystalline
134 Orthopedic Surgery – Spinal Treatment
6.610.3.4. Pedicle Screws, Plates, Bands, and Rods forFusion and Dynamic Stabilization
Screws, plates, and rods are often used to augment spinal
fusion. Screws placed in the pedicles or vertebral bodies act as
anchor points for bands, rods and plates working together to
restore spinal stability. The screw–bone interface is especially
influential in the success of fixations. Surface treatments of
metal implants of stainless steel50 or titanium51 have been
utilized to increase fatigue life of the implants and promote
Table 3 Dynamic stabilization devices
Category and trade name Key features
Pedicle screws and artificial ligamentsDynesys (Dynamic Neutralization System for theSpine); Zimmer Spine, Warsaw, Ind (18)
Semirigid artificialpolycarbonaturetplaced under ten
Graf ligament; Surgicraft, Redditch, England (19) Nonelastic polyestprevent rotationforaminal narrow
IsoBar; Scient’x USA, Maitland, FL Includes a mobileM-Brace; Applied Spine Technologies, New Haven,Conn
Implanted by using
Stabilimax NZ; Applied Spine Technologies Utilizes a dual spriDynamic Soft Stabilization System (20) Elliptical metal coilInter-spinour process decompression systemWallis ligament; Abbott Spine, Austin, Tex (21) Interspinous proce
processes with DX STOP; St Francis Medical Technologies, Concord,NH (22)
Titanium device inflexion; can be inis thus useful in
Diam; Medtronic So famor Danek, Memphis, Tenn H-shaped siliconeCoflex; Paradigm Spine, New York, NY Posteriorelement replacement systems
U-shaped device th
Total Facet Arthroplasty System; Archus Orthopedics,Redmond, Wash
Consists of a sphevertebral body
Total Posterior System; Impliant, Princeton, NJ Posterior elementssimilar to pedicle
Note: Some of the devices listed may not yet have been approved by the FDA for clinical us
Source: Zucherman, J. F.; Hsu, K. Y.; Hartjen, C. A.; et al. Spine 2005, 30, 1351; Senegas, J.;
Mot. 1988, 74, 15; Sengupta, D. K. The Lumbar Spine; Lippincott, Williams & Wilkins: Phila
Dubois, G.; Schwarzenbach, O. Eur. Spine J. 2002, 11, 170–178; Senegas, J. Eur. Spine J.
(a) (b)
Figure 11 (a) Brantigan spine fusion cage and (b) lateral radiograph ofa Brantigan cage with a solid fusion. (Note that the Brantigan cage hastantalum microspheres for visualization on radiographs.) Reproducedfrom Kurtz, S. M.; Devine, J. N. Biomaterials 2007, 28, 4845–4869.
better integration. Stainless steel fatigue strength is enhanced
by the implantation of nitrogen ions by inducing the forma-
tion of extremely hard nitrides at the surface of the material
while maintaining the bulk properties of the material.26 Tita-
nium can be modified by the introduction of bioactive titanate
layers of, for example, sodium titanate, calcium titanate, or
titanium oxide which induce apatite formation on the implant
surfaces and have been demonstrated to enhance osteoinduc-
tivity as well as osteoconductivity for improved implant
integration.51
Dynamic stabilization may be an alternative to fusion in
some patients and may provide an improvement over fusion
by altering load bearing and controlling abnormalmotionwhile
maintaining some flexibility. Dynamic stabilization devices
incorporate a flexible ligament like component or other spring
or mobile joint to allow motion in the spine while controlling
movement such as flexion and extension. The diverse function-
ality of these devices often requires the use of both flexible
polymeric materials and more rigid materials such as metals
(Table 3).7
6.610.3.5. Bioresorbable Materials for Spinal FusionImplants
Another key element in the successful stabilization of the spine
in fusion surgeries is the restoration of the normal curvature of
the spine. If the material system is too rigid, it becomes difficult
to bend and twist to provide the appropriate curvature.
ligament system composed of titanium alloy pedicle screws andhane spacers connected by polyster cords (artificial ligaments) that aresioner ligament looped around pedicle screws and placed under tension towhile allowing some flexion; if tension is too high, hyperlordosis,ing, and nerve root impingement may resultjoint within metal rodsa minimally invasive technique
ng mechanismconnecting adjacent pedicle screws
ss distraction device composed of a spacer held between spinousacron tapetwo pieces, placed between adjacent spinous processes to hold spine inserted by using a minimally invasive approach with a local anesthetic andelderly patients with degenerative spinal stenosisdevice held in place by a mesh band and sutureat controls flexion and extension
re that slides along a curved plate anchored by pegs passing into the
are removed and a plastic device is implanted and anchored with devicesscrews
e.
Etchevers, J. P.; Vital, J. M.; Baulny, D.; Grenier, F. Rev. Chir. Orthop. Reparatrice Appar.
delphia, PA, 2004; Vol. 1, pp 373–383; Graf, H. Rachis 1992, 4, 123–137; Stoll, T. M.;
2002, 11, 164–169.
Trends in Materials for Spine Surgery 135
Inducing an abnormal curvature may lead to stress induced
osteopenia of the vertebral bodies and spinal degeneration in
adjacent spinal levels.26 This is, however, in contrast to the
needs of a rigid system in order to promote the solid fusion
process. Therefore, the use of a system whose rigidity decreases
with time is desirable. Bioresorbable materials provide promise
for such a system. Bioresorbable cages have stiffnesses compa-
rable to those of bone, are radiolucent, and resorb over time.52
These cages provide the additional benefits of eliminating risks
associated with permanent implants such as implant removal
on revision surgery. In addition, degradation of the polymer
may allow for the incorporation of a drug release system into
the implanted device.
The most commonly used biodegradable materials are
polyesters derived from poly(a-hydroxy acids) such as poly
(lactic acid) (PLA) and poly(glycolic acid) (PGA), and their
isomers and copolymers. The chemical backbones of thesemate-
rials are hydrolytically unstable, and the chains will degrade
when placed in aqueous environments such as the body. PLA
and PLGA degrade to lactic acid and glycolic acid respec-
tively and are natural, biocompatible metabolic compounds.
The strength and degradation rate of these polymers can be
modified by the fabrication of copolymer systems. PGAdegrades
within months and is therefore used only in small amounts in
copolymer systems, while PLA is more widely used in spinal
implants. Chemical and physical properties of some commonly
used PLA and PLA isomers are summarized in Table 4.
The polymer degradation kinetics and mechanical proper-
ties can be influenced by the polymer system utilized; however,
the implant design and geometry itself has a large influence on
the rates and types of degradation experienced by the implant.
PLA devices degrade faster when they are less porous, less
permeable, and have a bulkier design and wall thicknesses.52
Sterilization of the implant can also have drastic effects on the
physical and mechanical properties of the polymer. Many con-
ventional sterilization techniques such as steam sterilization
can cause melting of the polymer system and therefore cannot
be utilized. Ionizing radiation is an alternative sterilization
Table 4 Chemical and physical characteristics of PLLA and PLDLLA
PLLA(literature)
PLLA (ownuse)
70/30PLDLLA(literature)
70/30PLDLLA(own use)
Mn
(gmol�1)240 500 140 250
Mw
(gmol�1)395 500 253 300
D (Mw/Mn) 1.64 1.81Intr. visc.(dl g�1)
2.68 1.57
Inh. visc.(dl g�1)
2.42 1.45
Crystallinity(%)
13.0 Amorphous Amorphous
Tg (�C) 60–65 64–67 55–60 60–62
Tm (�C) 173–178 180 Amorphous AmorphousDegr. time(min)
>24 36–48 12–18 >12
Reproduced from Wuisman, P.; Smit, T. H. Eur. Spine J. 2006, 15, 133–148.
technique; however, this technique can also induce damage
in the polymers causing changes in biocompatibility and bio-
mechanical characteristics. Treatment with ethylene oxide or
plasma sterilization provides better alternatives and shows
limited effect on molecular weight and tensile strength of
PLA-based materials.53,54 Biocompatibility of PLA and related
polymers has generally been demonstrated to be good with
the absence of significant toxicity.52 A local reduction in pH
imparted by the degradation products, however, may be
responsible for some adverse tissue reactions such as osteolysis
and bone resorption around the implant.55
Bioresorbable PLA plates have performed well in compari-
son to titanium plates in controlled clinical studies. In an
original study by Nabhan et al., bioresorbable plates made of
L-lactic and D,L-lactic acid in the ratio of 80:20 in the INION S-1
Biodegradable Anterior Cervical Fusion System were investi-
gated in a prospective randomized controlled study with 40
patients.56 The bioresorbable plates in this study were designed
to hydrolyze over the course of 2 years. Plates maintained 90%
of their initial strength after 6months, then continued
to decrease slowly to 70% after 9months, and continued to
absorb after 2 years. The fusion incidence and rates of fusion
were found to be comparable to those of titanium plates but
with several added benefits associated with the use of a biode-
gradable polymer. Advantages of the PLA plates included
the radiolucency of the plates which allowed visualization
of the graft material giving reassurance that the plates had not
migrated. The plates were also somewhat flexible and were able
to be bent to match the vertebral contours of the patients.
Gradual resportion of the plates may allow the fusion to grad-
ually assume the role of structural support and enhance fusion
rates while reducing stress shielding.56
6.610.4. Total Disc Arthroplasty
The first mode of treatment proposed for the treatment of the
majority of patients who suffer from disc degeneration is non-
operative care. On the basis of carefully considered guidelines,
surgical interventions are recommended in the event that
conservative remedies do not address the clinical needs of
the patient. Arthrodesis or spinal fusion is the traditionally
accepted surgical intervention technique advocated in cases
of isolated disc degeneration in the cervical or lumbar spine.
The procedure involves the removal of a motion segment
through the use of bone grafts and sometimes through internal
fixation. Indications for the procedure are instability of a
motion segment caused by traumatic injury or degeneration.57
A successful fusion procedure eliminates motion that causes
pain and offers the ability to restore intervertebral height and
alignment.58
However, spinal fusion has its limitations, some of which
include the following41:
• No significant improvement seen in the success rate of
fusion procedures despite the advances made in terms
of surgical techniques and instruments;
• Morbidities associated with bone graft harvests during
the procedure;
• Accelerated adjacent level disc degeneration;
136 Orthopedic Surgery – Spinal Treatment
• Long postoperative recuperation times;
• Increased costs of fusion procedures.
Disc arthroplasty is a surgical procedure that has been getting
a lot of attention in the spine community. The procedure aims to
potentially eliminate a painful disc while preserving/restoring
motion.58 The goals of arthroplasty include successful pain relief
and functional recovery, acceptable levels of morbidity asso-
ciated with the procedure (equal to or less than those seen in
fusion), shorter postoperative recuperation times, and easily
implantable surgical techniques.41 The biomechanical objectives
of disc arthroplasty are the restoration of disc function to relieve
abnormal stress or strain caused by degeneration. More impor-
tantly, it is essential that the procedure does not cause any
detrimental biomechanical effects on surrounding structures
within the motion segment and on adjacent levels.41
6.610.4.1. Material Requirements
In order to understand the material requirements, it is neces-
sary to understand the biomechanics of disc arthroplasty.
The following are some of the biomechanical parameters that
might be considered while designing a total disc replacement
implant59:
• Restoration of mobility while avoiding segmental instability;
• Restoration of correct spinal alignment;
• Protection of adjacent structures from overloading and
resultant accelerated degeneration;
• Stability and wear properties of the device.
The material requirements for a TDA prosthesis are dis-
cussed according to each component of the artificial disc to
be designed, along with some examples where possible.
6.610.4.1.1. Implant endplateThe vertebral endplate is subject to repetitive physiological
loads, and therefore, a material chosen for this component
needs to be durable enough to withstand such types of
loads. Other aspects to be considered include the reactivity
of the material, its modulus of elasticity, ultimate tensile
strength, imaging characteristics, ease of manufacture, and
cost.57 Commonly used materials for endplates are stainless
steel, cobalt–chromium alloys, and titanium alloys.58
Stainless steel implants are inexpensive to fabricate, show
relatively low corrosion, and have high moduli of elasticity.
Cobalt chromium alloys have properties in between those of
steel and titanium alloys. Depending on the alloy, processing
of the end product can be tuned to a wide range of strengths
and ductility, thus making these alloys versatile in their appli-
cability as implants. However, such implants are more ex-
pensive to manufacture.58 Currently used prostheses have
endplates made out of these types of alloys.
Titanium alloys are attractive as implant materials owing to
their high biocompatibility. They are also more corrosion
resistant than the other two types of alloys. Further, they
yield fewer debris particles as detected by imaging with mag-
netic resonance. Their mechanical properties are closer to those
of cancellous bone, but they have higher susceptibility to wear
debris generation. Pure titanium implants have not been stud-
ied clinically, but titanium coatings are being used currently.
6.610.4.1.2. Bone implant interfaceAttachment of an implant to the endplate can be achieved
through the use of screws, serrations, or fins. These structures
help insert the device, prevent early displacement, and facili-
tate long term stability. Long term attachment of the device can
be facilitated by the presence of microstructured features on the
surface of the implant in order to encourage bony ingrowth.
Other options for inducing bony ingrowth include titanium
spraying or meshes, calcium sulfate coatings, and hydroxyapa-
tite coatings.58
6.610.4.1.3. Bearing surfacesSpecial attention is to be paid while selecting materials for the
bearing surfaces of the implant. The bearing surface usually deter-
mines the long term stability of a well-fixed TDA implant. The
articulation of these surfaces must allow low friction motion,
resist permanent deformation, and show good wear resistance
(seeChapter 6.614,Wear: Total IntervertebralDisc Prostheses).
Typically, surgical intervention to correct disc degeneration
is performed on patients between the ages of 35 and 50 years.
In order to reduce incidence of revision surgeries, the estimated
life of a prosthesis is proposed to be over 40 years. Taking into
consideration the number and amplitude of load cycles a
lumbar disc experiences in a year, it can be assumed that the
average adult bends 125 000 (extension and flexion move-
ments) annually and takes about 2million steps (gait cycles),
the device is expected to endure roughly 85million cycles of
loading during its lifetime without significant degeneration.60
It is recommended that the device be tested for a 100million
cycles of loading. This hypothesis is made after taking into
account the implant’s capacity to resist fatigue, and immuno-
logic response to wear particles including their toxicity.
UHMWPE is used as a bearing material in several implants
owing to its low creep properties. Even though polyethylene
shows wear debris in its use with other joints, animal studies of
metal-on-polyethylene disc implants have not shown signifi-
cant evidence of wear particles.61,62
6.610.4.1.4. General material specificationsMetals are considered to be good candidates for a total disc
replacement device owing to their inherent high fatigue
strength. Also, metal alloys have been shown to possess good
biocompatibility.57 On the other hand, nonmetallic materials
such as polymers and elastomers are also good candidates
because of the similarities between their mechanical properties
and those of native disc tissue. Over a short period of time, the
lower elastic moduli of nonmetals help replicate disc behavior
adequately. However, in terms of a long term device, such
materials tend to pose a challenge to develop. Overall, the
need of the hour is to develop materials that demonstrate
both biomechanical applicability and biocompatibility while
being user friendly in a surgical environment.63
In order tomeet the needs for an artificial disc, combinations
of both metallic and nonmetallic components have been
researched. In most cases, these take the form of metal–
polymer–metal sandwiches, wherein the metal portion is used
to improve fixation using screws/spikes or porous coatings to
promote tissue ingrowth. The polymer component of the fixed
implant can provide the flexibility seen in native discs.57
Trends in Materials for Spine Surgery 137
6.610.4.2. Discontinued Artificial Discs
6.610.4.2.1. AcroflexThe Acroflex artificial disc was developed in the 1970s by
Arthur Stefee. The device represents a leap in the concepts
and design of artificial discs.39,62 The design of the disc com-
prises two porous metal end plates coated with titanium and
fused to a hyperplastic polymer disc which is made up of a
hexane based polyolefin rubber.62,64 The design of the device is
such that the vertebral bone is intended to grow into the
titanium plates to ensure long term fixation, while the polymer
disc is intended for restoration of flexibility and motion to the
functional spinal unit.39
The Acroflex device had significant biomechanical advan-
tages. It helped in the preservation of disc height and allowed
semiconstrained motion in all modes including compression.58
Over the years, there have been three generations of the
Acroflex disc developed. Each generation of the device success-
fully passed the mechanical testing protocols to which they
were subjected, only to fail in the short term when implanted
in a series of limited human clinical trials.10,39
6.610.4.2.1.1. First generation
The first generation of Acroflex was composed of CoCr alloy
and polyethylene as the polymeric core cemented together and
implanted through an anterior approach.39 The disc was tested
in cadaveric spines; however, concerns were raised regarding
the bone–cement interface, and the device idea was subse-
quently abandoned.62
A cementless design was developed in the 1980s using
titanium end plates with a double layer of sintered 250 mmbeads to enhance long term fixation. In this version of the disc,
the polymeric core was made out of a polyolefin rubber known
as Hexsyn, manufactured by Goodyear for fatigue resistant
biomedical applications.39
The device was subjected to preliminary mechanical testing,
and despite some incidences of local material failure, the tests
were judged to be successful. The next step was animal testing;
however, the company was never able to obtain permission to
carry out animal studies. Instead, on the basis of in vitro studies
to study biocompatibility, a decision was made to proceed to
human trials directly.39
The device was implanted in six patients and follow-
up studies were conducted. However, in 1990 the FDA
raised concerns of the use of Hexsyn, which contained
2-mercaptobenzothiazole, a potential carcinogenic.57 The disc
was eventually removed from the market.
Figure 12 Second generation Acroflex implant with radiographic images poMoore, R. J.; Vernon-Roberts, B.; Fraser, R. D. Eur. Spine J. 2008, 17, 1131–
6.610.4.2.1.2. Second generation
There is very little information available in the literature about
the second generation of the Acroflex device (Figure 12). In
this iteration of the design, the polymeric core was composed
of a silicone elastomer and clinical trials were performed in
eight patients. Data for this generation of studies are not avail-
able because of the controversies surrounding the use of the
silicone gel in breast implants. Class action lawsuits had been
filed against the manufacturer of the silicone (Dow Corning),
making the developers of the disc hesitate to publish their
findings.62
6.610.4.2.1.3. Third generation
The third generation of the Acroflex disc once again used a
hexane based polyolefin rubber core. The endplate design was
modified to reduce the profile to facilitate easier implantation.
The endplates were made from a Ti–6Al–4V ELI alloy, coated
with porous beads made of pure Ti.10 Mechanical testing was
performed on the device and concluded to be satisfactory.
Debris testing was not undertaken as the disc had no articulat-
ing surfaces.65
Animal studies conducted on 20 baboons showed signifi-
cant evidence of osteointegration into the porous coating after
6months. There was one case of failure and histological reac-
tion because of the presence of wear particles in one animal.66
Pilot human trials were conducted with two groups of
patients, with subtle design differences in the endplate of the
device. Both sets of trials did not yield significant results to
conclude the success of the device.10,66
Eventually, even after three decades of changes to the design
and materials used in the Acroflex artificial disc, the device did
not achieve significant clinical success. The design concept of
using a one piece nonarticulating artificial disc is very attrac-
tive, as it would mimic natural disc function very well. How-
ever, this is hindered by the complex loading environment
seen in the spine. The device was successful in terms of achiev-
ing rapid osteointegration. The biggest hurdle to the success of
the Acroflex artificial disc though was the designers’ inability to
find a suitable polymeric core material to meet the required
needs for an artificial disc.62
6.610.4.3. Currently Available Devices
6.610.4.3.1. Charite artificial discThe Charite Artificial Disc (marketed by DePuy Spine), unlike
the Acroflex was designed with the goal of replicating the
kinematics of the disc as opposed to the motion. The device
st implantation. Reproduced from Melrose, J.; Smith, S. M.; Little, C. B.;1148.
138 Orthopedic Surgery – Spinal Treatment
was designed by Kurt Shellnack M.D., and Karin Buttner-
Janz M.D., Ph.D, who were affiliated at the time to the
Charite Center for Musculoskeletal Surgery at the University
of Berlin.67 The basic design of the device incorporates two
metallic end plates made of cobalt-chromium alloy57 which
are attached to adjacent vertebral bodies, and articulate against
a central polymeric core made up of UHMWPE.67
The UHMWPE core has two domed (convex) surfaces
that help to articulate against the concave metallic endplates.
The hypothesis is that in conditions of full flexion or tension,
the core is free to translate or rotate when the polyethylene rim
of the core contacts the metal rims of the endplates. The device
is implanted through an anterior approach.67 The device pro-
vides unconstrained motion during rotation, and semi con-
strained motion during flexion, extension, lateral bending, and
translation, to some extent.58
The Charite Aritficial disc has undergone four iterations in
design since its inception and offers the largest and longest
clinical trial of any existing artificial disc.57,62 The rationale for
choosing an UHMWPE core was because of the outstanding
friction and fatigue properties of the material.
6.610.4.3.1.1. SB Charite I and II
Only retrospective clinical data are available for the SB Charite
I and II, the first two generations of the device. The first two
generations were made up of stainless steel endplates, which
had protruding teeth for better fixation.62 In the SB I disc, the
end plates were circular and shaped like bottle caps, while in
the SB II they were expanded by providing lateral wings to
enhance contact with the vertebral end plates.62 The SB I and
II discs were implanted in 15 and 22 patients respectively and
follow-up data obtained after a period of 17.5 years. Follow-up
studies revealed a success rate of 87% (13/15) for the SB I disc,
and 68% (15/22) for the SB II discs (Figure 13). The success was
defined in terms of the absence of long term complications.68
The inventors performed mechanical testing on the devices
and found that the height of the UHMWPE core was reduced
SB I SB II
Figure 13 SB Charite I and II.
SB III
Figure 14 SB Charite III, and radiographic image of the implanted device.
by 10% during testing. They claimed that there was minimal
wear and debris formation posttesting67; however, it has been
seen that their claims are consistent with testing in a single
direction, which produces far less wear particles than testing in
multiple directions.
6.610.4.3.1.2. SB Charite III
Waldemar Link GmbH and Co. was the company that com-
mercialized the Charite artificial disc, resulting in the third
generation SB Charite III (Figure 14). Major changes were
made to the endplate material, which was now constructed
out of a proprietary CoCr alloy called VACUCAST (0.25% C,
28–20% Cr, 5.5–6.5% Mo, max 0.5% Ni, max. 0.5% Fe,
0.4–1% Si, 1% Mn, and the remaining 57.1–69% Co). The
back of the alloy was ‘satin’ finished by corundum blasting,
and the device was fitted with six sharp teeth to achieve fixa-
tion. The UHMWPE core design remained unchanged.62
There have been a number of clinical studies undertaken to
document the performance of the SB Charite III. In summary,
most of these studies have demonstrated that the Charite arti-
ficial disc has the potential to preserve motion and survive long
term implantation in the body with good to excellent out-
comes. However, cohort studies undertaken have shown varia-
bility in the complication and success rates of the device, which
raises concerns regarding the repeatability and reproducibility
of the procedure.
6.610.4.3.2. ProDiscThe ProDisc (Synthes Spine, Paoli, PA) also consists of metallic
endplate with an UHMWPE core. Unlike the Charite, the Pro-
Disc is attached firmly to the inferior end plate through a
locking mechanism. The convex surface of the polymer core
articulates against the superior end plate. There have been two
generations of the ProDisc, ProDisc I and II, but only the
ProDisc II has been commercialized.69
The ProDisc has undergone the second largest clinical trial
of any artificial disc after the Charite. The ProDisc I had metal-
lic endplates made of Ti alloy, whose back surfaces were
plasma sprayed with a titanium coating to promote bone
growth.70 Follow-up reports for ProDisc I showed some proce-
dure related complications. Minimal wear of the UHMWPE
core was detected.70
Clinical studies on the ProDisc II have only been short term
(17–31months), with reports of positive outcomes. It is to be
noted that the studies for ProDisc are more scientifically
sound than the studies for the Chartite disc, which has only
been retrospectively studied in Europe. Studies on the ProDisc
Figure 15 Components of the ProDisc. From left to right: superior endplate, polyethylene inlay, and inferior endplate. Source: SynthesProduct Literature.
Figure 16 Front and side views of the ProDisc-L artificial disc. Source: Synthes Product Literature.
Figure 17 The Maverick artificial disc. Reproduced from Mathews, H.; Lehuec, J.; Friesem, T.; Zdeblick, T.; Eisermann, L. Spine J. 2004, 4, 268–275.
Trends in Materials for Spine Surgery 139
II however, are prospective and employ validated outcome
measures such as the Oswestry Disability Index, which can
be compared with alternative spine procedures (Figures 15
and 16).62
6.610.4.3.3. MaverickThe Maverick total disc replacement device was developed in
2001 by Medtronic Sofamor Danek (Figure 17). Initial proto-
types were designed using alumina ceramic, but the commer-
cially available device now features a two piece metal-on-metal
design, which comprises superior and inferior endplates
made of CoCr alloy. Bearing surfaces are made of CoCr alloy
based on ultra low wear rates obtained in hip replacements.
The endplates have been treated with hydroxyapatite to pro-
mote bone growth.62
Clinical trials of the Maverick disc began in 2002. Two-year
follow-up results have been reported so far. Clinical success
was reported in 75% of the patients. No revisions or device
related complications were reported in this study. Mechanical
testing on the disc showed good shock loading, and showed no
mechanical damage after 10million loading cycles. However,
wear debris was produced, but the test methods were not
published in the literature. On the basis of this, the life span
of the device was predicted to be around 31.5 years in vivo, but
this claim cannot be validated until further studies are
conducted.71,72
140 Orthopedic Surgery – Spinal Treatment
To sum, the Maverick disc is a good alternative to metal-
polymer-metal disc replacements that are currently available.
Two-year follow-up studies and biomechanical testing have
been reported. Metal-on-metal articulation produces less wear
thanmetal-on-UHMWPE prostheses; however, it also produces
metallic wear debris. This can be a cause for concern. Only
further studies can tell if the Maverick disc will be clinically
successful.
6.610.4.3.4. FlexicoreThe Flexicore disc manufactured by Stryker Spine is the most
constrained of lumbar discs and consists of four CoCr
components: a superior and an inferior baseplate, a spherical
head that attaches to the superior endplate, and a shield
(Figure 18).73
The device is capable of supporting tensile loading because
of the presence of the spherical head. It also contains mechan-
ical stops in order to limit flexion/extension and lateral bend-
ing. The device endplates are coated with titanium plasma
spray coating to promote bone growth. Mechanical testing
has been conducted in tension, compression, and shear. The
studies showed that the device is mechanically sound and has
strength limits excessive to the load limits for the IVD. Wear
testing showed that minimal or no wear damage was seen.73
The disc is undergoing a clinical trial at present, but the
results are not expected for several years. The success of the
Flexicore disc can only be determined when more clinical data
become available.
6.610.4.4. Future Trends
The field of total disc replacement is a rapidly evolving area of
new technologies and techniques, and great strides have been
made in the development of spinal implants that are able to
replicate and preservemotion in the IVD. The arena of total disc
replacement has seen many leaps in terms of achieving short
term benefits such as patient satisfaction and speed of recovery.
New implants are being designed, developed, and tested.
The near future might hold answers regarding the long term
benefits of using total disc replacements, especially the goal of
reducing the incidence of adjacent segment degeneration.
More comprehensive follow-up studies will reveal the future
for these implants.
From a materials standpoint, there have been different
design concepts explored. The use of all-metal implants and
that of metal–polymer implants have been investigated. At this
juncture, it is hard to say which system works better. Each type
of design has its own inherent advantages and disadvantages;
for example, the main issue with metal–polymer implants is
the wear of the polymer core, which can cause early failure of
Figure 18 Components of the Flexicore artificial disc. Reproduced from Val
the implant. The all metal implants show excellent wear resis-
tance; however, the generation of wear debris is an issue of
concern. The consequences of these wear particles and their
corrosion solubility products are unknown at present. There
might also be concerns regarding the maintenance of bony
attachment with the degradation of bone tissue with age and
osteoporosis with respect to these devices.
Nevertheless, the future holds a lot of promise for this
field, and as more data are published on the available and
developing implants, changes in design and materials can be
conceptualized and implemented to meet the objective of
achieving a successful implant that will meet all the needs
of patients who require total disc replacement.
6.610.5. Annulus Repair
6.610.5.1. Procedure
The repair of the AF has seemed to be an intuitive idea for
many researchers, but given the limitations posed by the cur-
rent microscopic and endoscopic surgical techniques, this has
not been attempted much. Advanced technologies are being
developed to close annulus defects. These could also enhance
the use of intradiscal prostheses which are contained by the AF.
Efforts are also underway to develop devices that will support
the annulus after a discectomy procedure.
6.610.5.2. Mechanical Role
The AF of the IVD is a structure that serves to contain the
gelatinous central portion of the disc, the NP. The chemical
composition of the AF comprises 65–90% water, 50–70% dry
weight of collagen, 10–20% dry weight of PGs, and noncolla-
genous proteins such as elastin.19
The structure of the AF is laminate in nature, consisting of a
minimum of 15 (posterior) and 25 (lateral) concentric layers.
The layers are made up of type I collagen fibers which alternate
in angles from 28� at the peripheral AF to 44� at the central AFwith reference to the transverse plane of the disc. The concen-
tric rings of the AF are referred to as lamellae, and the spaces
between them as interlamellar septae.19
The AF has a very highly organized structure which results
in complex anisotropic behavior. The tensile, compressive, and
shear properties of the AF differ in the axial, radial, and circum-
ferential directions. The AF can be further divided into the
inner and outer AF on the basis of the structural differences,
the inner AF being subjected to higher hydrostatic pressures of
the NP than the outer AF which is subject to tensile forces.
Age related degradation of annulus tissue manifests in the
form of annular tears and fissures, and can ultimately lead to
devit, A.; Errico, T. Spine J. 2004, 4, 276–288.
Trends in Materials for Spine Surgery 141
disc herniation.74 Circumferential delamination and the for-
mation of radial fissures can cause the extrusion of the NP
material which is contained by the annulus.13 This is referred
to as disc herniation. Herniation can be defined as the loca-
lized displacement of disc material beyond the limits of the
intervertebral disc space.75 When this happens from the poste-
rior region of the annulus, the release of chemical inflamma-
tory signals can result in severe pain in the back.13,76
Degeneration of the annulus results in a transfer of load
from the NP to the posterior portion of the annulus.
To compensate for the added stress, the annulus tends to
broaden, which often causes significant protrusion of the
disc. This manifests in the form of circumferential disc bulging
or focal herniation. Tissue remodeling in the posterior annulus
is often accompanied by calcification and other changes that
pose a challenge to the treatment of this issue.77
In the past, surgical intervention to reverse disc degenera-
tion has focused on re-establishing disc height and geometry,
while minimizing the flexibility of the disc. But recently, tech-
niques that seek to retain or restore disc motion are being
investigated in a bid to avoid the elimination of spinal segmen-
tal motion. Some of these include the replacement of the ‘total
disc’ (i.e., the NP and a significant portion of the annulus) with
a prosthetic implant. An alternative to this approach is to
replace the NP while retaining a major part of the annulus.78
Studies have shown that an incision through the annulus
can alter the mechanics of the involved disc. Hampton et al.
showed that incisions made in the posterior annulus showed
limited healing potential and the presence of a persistent defect
could form a pathway for the leakage of nuclear fluid into
surrounding perineural tissue.76 This view was reinforced by
other researchers who investigated annulotomy techniques in
animal models. They found that a box type or window annu-
lotomy technique produces a significantly weaker healing
response than a slit or cruciate incision.79,80
On the basis of the results reported, it can be inferred that
the AF has a limited capacity for regeneration after an annu-
lotomy. On the basis of the type of annulotomy performed, the
healing process leads to the formation of a thin layer of inferior
fibrous tissue. The poor regenerative capacity may be due to the
fact that the external repairs do not meet the needs of fiber
recruitment to withstand the tensile forces applied to the annu-
lus.81,82 In order for the annulus to shift between axial loading
and circumferential tension, it is necessary that the nucleus
volume remains elastic, deformable, and contained. When
the annulus suffers damage from an annulotomy or degrada-
tion, its ability to retain the nucleus is lowered.19 In light of
the above discussion, we can say that preservation of as much
annular tissue as possible should be the goal during surgical
interventions to treat disc degeneration.
6.610.5.3. Reported Treatment Methods
6.610.5.3.1. Suture techniquesThere have been reports of closing annular defects with sutures
after a microdiscectomy. The first instance of suture usage to
close annular defects was reported by Yasargil in 1977, wherein
he used 7-0 sutures to close a defect made to remove nucleus
material. He reported that this method could help prevent
adhesions in the annulus, but did not suggest any mechanism
for the same. He tested his method on 105 patients, and
recorded no reherniations, neurological impairment, or post-
operative radiculopathy in them.83
Lehmann et al. have reported suture related techniques to
close annular defects by using 4-0 sutures to close the flaps of
the posterior longitudinal ligament, peridural membrane, and
outer fibers of the annulus. In a study conducted in 152
patients, a larger percentage of patients who received sutures
had less postsurgery pain compared to those who did not
receive them; however, the result was not statistically signifi-
cant. The investigators did not report on the incidence of
reoperation or reherniation.84
Cauthen et al. have studied suturing of the annulus with a
focus on reducing the rate of reoperations. He studied 254
patients and reported that there was a 21% chance of recurrent
disc herniation when the patient did not receive sutures at
2 years. The use of one suture to perform microsurgical sutur-
ing showed a decreased recurrence rate of 10% while the use
of more than one suture brought it down to as low as 5%.85
6.610.5.3.2. Sealant and bulking techniquesBiocompatible tissue adhesives have been used inmany areas of
the body containing soft tissue, for example, fascial hernias or
cardiovascular procedures.86,87 Such procedures, however, have
not been extended to the AF because of the undesirable effects of
adhesion that might occur with neural tissue in the vicinity.
Instead, ‘sealing’ biomaterials, which are polymeric solutions
that cure in situ, have been injected into the annulus.88 In
concept, these bulking hydrogels are intended to form a sealant
which will take on the complex, irregular shape of the AF defect,
and may bond strongly to the neighboring tissue around the
defect. There have been no clinical reports of such a technique
being applied to the annulus; instead, these types of biomater-
ials seem more applicable to nucleus replacement.89,90
6.610.5.4. Current Technologies
6.610.5.4.1. Xclose™ tissue repair systemThe Xclose™ Tissue Repair System marketed by Anulex Tech-
nologies, Inc. is an FDA approved technology, and is recom-
mended for reapproximation of soft tissue such as the AF after
a discectomy procedure (Figure 19). The system consists of two
tension band devices and one knot pusher. The device works
by placing tension bands at the annulotomy site and securing
them with tension bands.78
6.610.5.4.1.1. Material
All implanted components of the device are made up of poly-
ethylene terephthalate.78,91
6.610.5.4.1.2. Procedure
All the components of the kit are provided in sterile conditions
and preloaded on disposable delivery instruments. A tension
band consists of a pair of nonabsorbable suture loops each of
which is attached to a T-anchor. A third loop of suture with a
pretied knot is then used to connect the two loops together,
while facilitating the tightening of the tension band, which
helps to draw the construct together and hence closes the defect
in the tissue.78
Cadaveric experiments have been performed to study the use
of the Xclose™ system. The results were presented at the annual
142 Orthopedic Surgery – Spinal Treatment
meeting of the Spine Arthroplasty Society in Berlin. The ability of
the system to remain in place after cyclic loading of the repaired
tissue was analyzed. A partial laminectomy was performed to
make a posterior annular incision. A limited amount of nucleus
was removed (�0.2 cm3), and the incision was repaired using
the Xclose™ system.Nine spinal units were tested in this study.92
Cyclic fatigue loading was performed, consisting of 20K
cycles of flexion–extension, 15K cycles of lateral bending,
and 5K cycles of axial rotation. Calibrated digital macropho-
tography was used to assess the system’s ability to keep the
repaired tissue in place, and the knot slippage was measured to
check for loosening of the suture knots.78
The study showed that all annulotomy defects remained
closed and intact after 40K cycles of loading. Post cyclic load-
ing, the loosening in the system was minimal and knot slip-
page was measured to be 0.8� 0.7mm. Also, the system did
not migrate out of the annulus or pull out of the tissue.92
On the basis of the results of the study, we can infer that the
Xclose™ system is a simple and effective method to close annu-
lar defects incurred during a discectomy. The materials used do
Figure 20 Inclose™ Surgical Mesh System and the configuration of the devinclose_us.asp).
Figure 19 Schematic of the Xclose™ Tissue Repair System.Reproduced fromhttp://www.anulex.com/anulex_technology/xclose.asp.
not detrimentally affect the repaired tissue, and the system has
shown stability upon cyclic loading. So far, the Xclose™ system
has been implanted in over 6000 patients in the United States.93
The Xclose™ system is undergoing a Phase IV randomized
clinical trial at present, but no clinical data are currently avail-
able. In 2009, the company also launched a PostMarket Clinical
study in which 750 patients from 34 facilities around the United
States were enrolled.93 On the basis of data from the clinical
trials, more can be said about the materials that are used to
construct the device and their role in the bodypost implantation.
Until data from more long term studies in vivo are made avail-
able, the final understanding of this approach cannot be
formulated.
6.610.5.5. Barrier Technologies
6.610.5.5.1. Inclose™ surgical mesh systemThe Inclose™ Surgical Mesh System is marketed by Anulex
Technologies (Minnetonka, Minnesota) (Figure 20). The
device is designed to act as a barrier and provide a scaffold
for the repair of soft tissue such as the AF.94 The surgical mesh
implant is made up of a monofilament polymeric braid which
is preloaded on a disposable delivery device.
6.610.5.5.1.1. Materials
The braided mesh is biocompatible, expandable, and com-
posed of polyethylene terephthalate,91 a commonly used poly-
mer in medical devices.75
6.610.5.5.1.2. Procedure
The mesh is a low profile device which enables it to be inserted
into the annulus in a minimally invasive manner. The device is
inserted using a preloaded delivery device supplied with the
kit. The system is secured in place using nonabsorbable surgi-
cal suture anchors.
The cylindrical device is mounted on a delivery instrument
which allows it to extend in a circumferential direction to form
a barrier by latching the cylinder ends together. The mesh
device can be inserted into any tissue aperture, such as an
annular defect, and once deployed into position, it can be
tethered using sutures.75
ice post implantation (http://anulex.com/anulex_technology/
Trends in Materials for Spine Surgery 143
6.610.5.5.1.3. Studies on the device
In vitro biomechanics studies were conducted using the device,
and demonstrated that implantation of the Inclose™ system did
not alter segmental spinal biomechanics. The study also showed
that the implant position remained intact despite application of
complex cyclic loading.95 In theory, a stable spinal motion seg-
mentmaybe able to reduce risks of herniation in an adjacent disc
by avoiding stress redistribution. Also, this study suggests that the
repair of the annulus using such a device might indirectly avoid
the need of nucleus material removal which is often a solution
employed by surgeons to mitigate the effects of disc herniations.
Animal studies on intradiscal implants are faced with com-
plications of downsizing the device in order to accommodate
smaller tissue areas. In general, no animal models exist that can
accurately represent the exact nature of disc space characteristics
as seen in humans and correlate to human clinical use. Despite
these limitations, animal models were designed to study the
surgical implantation of the Inclose™ device and histological
response to the PET mesh.
Peppelman et al. studied Inclose™ implantation in a Nubian
goat model and showed that a lateral approach could be used to
place the implant safely in the proximity of the annulus. When
positioned and affixed properly, the implant remained intact
over the course of 12weeks, as seen using radiography. Also, the
device was found to be incorporated histologically without any
detrimental effects on the surrounding tissues.96
The device was studied after implantation in cadaveric and
animal models. On the basis of a report submitted at the Spine
Arthroplasty Society in Berlin in 2007, the studies demonstrated
that the presence of wear particulates wasminimal. Also, motion
of the implantwas negligible and it hadnodetrimental effects on
spinal flexibility. Early findings showed that there were no recur-
rent herniations when the device was implanted.97
Early clinical use of the Inclose™ mesh device has shown
promising results. Around 10 patients were implanted with the
device after an endoscopic L4-L5 or L5-S1 discectomy. The
implant was inserted with minimal disruption to the annulus,
with access openings between 3 and 8mm. There was inade-
quate length of follow up, because of which long term effects of
the implant cannot be predicted yet. However, it was noted
that the best technical results were obtained in patients with
focal, protruded, and/or extruded disc herniations.98
6.610.5.5.2. BarricaidW prosthesisThe Barricaid® Prosthesis device is currently not clinically
available in the United States. It is a device aimed at recon-
struction of the annulus in the region of disc herniation. The
Barricaid® implant works by forming a strong and flexible wall
between the AF and NP, where it is implanted.
The salient features of the prosthesis include its ability to
create a mechanical barrier that closes the annulus defect, while
allowing more nucleus material to be retained in the disc.
According to the manufacturers, the device can be implanted
quickly, simply, and securely (Intrinsic Therapeutics).104
6.610.5.5.2.1. Material
The Barricaid® Prosthesis consists of a woven mesh made of
polytetrafluoroethylene (PTFE) supported by a nitinol frame.
The frame is made up of a nickel–titanium alloy base, which
anchors the annulus and reinforces it while closing the
defect.104 The frame functions as a shape memory alloy, and
exhibits pseudo elasticity (Intrinsic Therapeutics).
6.610.5.5.2.2. Procedure
The Barricaid® Prosthesis is implanted into the annulus
through a small incision (5� 10mm defect) made on the
posterior side, as part of a standard limited discectomy, just
prior to wound closure (Intrinsic Therapeutics). The device
covers a major portion of the medialateral distance in the
posterior AF, and expands cephalad–caudad postinsertion.75
6.610.5.5.2.3. Studies on the device
In vitro biomechanical studies have been performed to evaluate
the Barricaid® device, and they have demonstrated that the rate
of recurrent herniations is decreased.99 The ability of the device
to withstand intradiscal pressure was demonstrated in a study by
subjecting the device to complex applied loads, and measuring
the intradiscal pressures.100 The clinical use of the device was
also tested, and the study demonstrated that compared to
patients who were not implanted with the prosthesis, patients
who received the device after a standard discectomywere seen to
experience restoration in disc height lost because of the proce-
dure.101 However, the evaluation of the long term effects of the
device in a clinical setting will require longer follow-up studies.
6.610.5.5.3. SpineJet XLW
The SpineJet HydroSurgery system is a novel technology that
harnesses the power of water in medical applications such as
surgery (Figure 21). The system comprises a controlled hair-
thin supersonic stream of water that can be used as an effective
cutting, ablation, and collection device. The advantage of such
a system is that it has the power of laser technology and radio-
frequency devices, but avoids the damage to tissue that such
technologies cause.102 The SpineJet XL is a percutaneous
hydrodiscectomy device that can be used in the minimally
invasive surgical intervention of intervertebral disc degenera-
tion. The device aspirates tissue as it is being used, and can be
used to remove cartilage and nucleus material.
The device is designed with four cutting surfaces. Annular
thinning is accomplished using a windshield wiper motion to
scrape the heel and toe of the device along the inner surface of
the annulus. Lateral cutting sides are used to scrape cartilage
off the endplates to increase the surface area for interbody
lumbar fusion.102
The device is mainly used for the removal of NP, in order to
perform spinal fusion. The system’s use of cold fluid prevents
thermal damage to tissue. The design of the device is such that
it enables the removal of precise amounts of nucleus tissue
without annular puncture or endplate damage.102
The device underwent a trial on 13 patients in 2008–2009.103
Initial results suggested that percutaneous hydrodiscectomy
could be a good alternative for the treatment of lumbar degener-
ative disc disease. However, more clinical data are required to
properly assess the device before more widespread use.
6.610.5.6. Material Requirements
On the basis of the discussion of current materials/technolo-
gies, we can say that closure of an annulus defect using suture
Venturisuctioneffect
Saline
Waste
In flowtube
Salinestream
Evacuationtube
Figure 21 Schematic of the SpineJet XL hydrodiscectomy system and the complete setup.
144 Orthopedic Surgery – Spinal Treatment
approximations would require materials that are nonabsorb-
able, given the nature of an annular defect. With respect to the
barrier type of technologies that have been developed so far,
the material in question should be able to support the annulus
appropriately on the basis of the biomechanical environments
it is subject to, without the device getting damaged or causing
damage to the annulus; that is, the material has to have
mechanical properties matching those of the annulus. Any
motion of the device due to functioning of the annulus should
not generate any wear particles that might bring about adverse
effects in the tissue. Further, considering that such types of
techniques are limited by the method of implantation of the
device, it is also required that the material in question has
properties that enable molding it in a manner that facilitates
easy and efficient implantation into the AF.
6.610.5.7. Future Trends
The field of annulus repair is still in its infancy, with data still
being collected on implantation of devices that are currently
available in the market. However, preserving the integrity of
the annulus is an important consideration while repairing the
intervertebral disc, as it is the only means of gaining access to
the NP. On the basis of the discussion presented above, it is
intuitive to try to preserve as much of the annulus as possible as
the annulus is responsible for the mechanical properties of the
disc. As more follow-up data become available, investigators
can look to modifying the currently available technologies to
refine their applicability, as well as lay the foundation for the
design and development of newer devices that can be
implanted minimally invasively, which will help retain the
properties of the annulus while allowing access to the NP.
6.610.6. Concluding Remarks
Early spinal implants were based on metals because of the
utility of metals in other orthopedic devices. The complex load-
ing nature and function of the spine however necessitated the
development of novel materials and techniques in surgical
spine interventions. Composite materials and polymers have
been utilized in spinal implants as an improvement overmetals
which do not have matching mechanical properties to the
spinal anatomy and are subject to wear. A better understanding
of spine anatomy, physiology, and pathophysiology has also
led to the development of new materials with several materials
being developed specifically for application in the spine. Failure
of earlier devices has especially expanded the breadth of mate-
rials utilized in spinal interventions. Adjacent level disc degen-
eration with spinal fusion, expulsion of implant materials
through surgical sites, and subsidence of implants into the
endplates have driven the development of new surgical techni-
ques and provided the impetus for the development of novel
spine materials. This is especially illustrated by the develop-
ment of in situ curing hydrogel and nonhydrogel nucleus repla-
cements where materials were designed such that they could be
delivered with minimal damage to the AF, a major way for
nucleus replacement as well as the development of sealants
for the AF itself. Another drive in materials for spine implants
is the utilization of tissue engineering approaches. Tissue scaf-
folds that support nucleus and AF cell growth are under investi-
gation; however, major limitations in nutrient supply to the
implant after implantation must be addressed before tissue
engineering approaches for the IVD can be realized.
References
1. Atlas, S. J.; Deyo, R. A. J. Gen. Intern. Med. 2001, 16, 120–131.2. Martin, B. I.; Deyo, R. A.; Mirza, S. K.; et al. JAMA 2008, 299, 656–664.3. Kuslich, S. D.; Ulstrom, C. L.; Michael, C. J. Orthop. Clin. North Am. 1991, 22,
181–187.4. Benzel, E. C. Biomechanics of Spine Stabilization. American Association of
Neurological Surgeons: Rolling Meadows, IL, 2001.5. Knoeller, S. M.; Seifried, C. Spine (Phila Pa 1976) 2000, 25, 2838–2843.6. Klenerman, L. The Evolution of Orthopaedic Surgery. RSM Press: Oxon, UK, 2002.7. Rutherford, E.; Tarplett, L.; Davies, E.; Harley, J.; King, L. Radiographics 2007,
27, 1737.8. Sakalkale, D. P.; Bhagia, S. A.; Slipman, C. W. Pain Physician 2003, 6, 195–198.9. Coric, D.; Mummaneni, P. V. J. Neurosurg. Spine 2008, 8, 115–120.
Trends in Materials for Spine Surgery 145
10. Melrose, J.; Smith, S. M.; Little, C. B.; Moore, R. J.; Vernon-Roberts, B.;Fraser, R. D. Eur. Spine J. 2008, 17, 1131–1148.
11. Deramond, H.; Depriester, C.; Galibert, P.; Le Gars, D. Radiol. Clin. North Am.1998, 36, 533–546.
12. Galibert, P.; Deramond, H.; Rosat, P.; Le Gars, D. Neurochirurgie 1987, 33, 166.13. Thongtrangan, I.; Le, H.; Park, J.; Kim, D. H. Neurosurg. Focus 2004, 16, E13.14. White, A. A.; Panjabi, M. M. Clinical Biomechanics of the Spine; Lippincott:
Philadelphia, PA, 1990.15. Lieberman, I. H.; Togawa, D.; Kayanja, M. M. Spine J. 2005, 5, 305S–316S.16. Errington, R. J.; Puustjarvi, K.; White, I. R. F.; Roberts, S.; Urban, J. P. G. J. Anat.
1998, 192, 369–378.17. Raj, P. P.; Fipp, A. Pain Pract. 2008, 8, 18.18. Kiani, C.; Liwen, C.; Yao Jiong, W. U.; Yee, A. J.; Burton, B. Y. Cell Res. 2002,
12, 19–32.19. Bron, J.; Helder, M.; Meisel, H.-J.; Van Royen, B.; Smit, T. Eur. Spine J. 2009,
18, 301–313.20. Adams, M.; Roughley, P. Spine 2006, 31, 2151.21. Roughley, P. J. Spine (Phila Pa 1976) 2004, 29, 2691–2699.22. Masuda, K.; Lotz, J. C. Tissue Eng. B Rev. 2010, 16, 147–158.23. Le Maitre, C.; Pockert, A.; Buttle, D.; Freemont, A.; Hoyland, J. Biochem. Soc.
Trans. 2007, 35, 652–655.24. Urban, J. P. G.; Smith, S.; Fairbank, J. C. T. Spine 2004, 29, 2700.25. Carragee, E. J. N. Engl. J. Med. 2005, 352, 1891–1898.26. Martz, E. O.; Goel, V. K.; Pope, M. H.; Park, J. B. J. Biomed. Mater. Res. 1997, 38,
267–288.27. Vaccaro, A. R.; Singh, K.; Haid, R.; et al. Spine J. 2003, 3, 227–237.28. Kuslich, S. D.; Bagby, G. The BAK interbody fusion system: Early clinical results
of treatment for chronic low back pain. In 8th NASS Annual Meeting. San Diego,USA, 1993; pp 175–176.
29. Matge, G. Acta Neurochir. 1998, 140, 1–8.30. Black, J. Orthopaedic Biomaterials in Research and Practice. Churchill
Livingstone: New York, 1988.31. Park, J. B.; Lakes, R. S. Biomaterials: An Introduction; Springer: Berlin, 2007.32. Waisbrod, H. Arch. Orthop. Trauma Surg. 1988, 107, 222–225.33. Merritt, K.; Brown, S. A. Biological Effects of Corrosion Products from Metals.
ASTM International: Philadelphia, PA, 1985; p 195.34. Merritt, K.; Brown, S. A. Clin. Orthop. Relat. Res. 1996, S233–S243.35. McMillin, C. R.; Brantigan, J. W. Soc. Adv. Mater. Process Eng. 1993, 582–590.36. Kurtz, S. M.; Devine, J. N. Biomaterials 2007, 28, 4845–4869.37. Hay, J. N.; Kemmish, D. J. Polymer 1987, 28, 2047–2051.38. Katoozian, H.; Davy, D. T.; Arshi, A.; Saadati, U.Med. Eng. Phys. 2001, 23, 505–511.39. Brantigan, J. W.; Steffee, A. D. Spine (Phila Pa 1976) 1993, 18, 2106–2107.40. McMillin, C. R. Soc. Adv. Mater. Process Eng. 1993, 591–598.41. Cho, D.-Y.; Lee, W.-Y.; Sheu, P.-C. Surg. Neurol. 2004, 62, 378–385.42. Ferguson, S. J.; Visser, J. M. A.; Polikeit, A. Eur. Spine J. 2006, 15, 149–156.43. Toth, J. M.; Wang, M.; Estes, B. T.; Scifert, J. L.; Seim, H. B., III; Turner, A. S.
Biomaterials 2006, 27, 324–334.44. Mastronardi, L.; Ducati, A.; Ferrante, L. Acta Neurochir. 2006, 148, 307–312.45. Togawa, D.; Bauer, T. W.; Brantigan, J. W.; Lowery, G. L. Spine 2001, 26, 2744.46. Senegas, J. Eur. Spine J. 2002, 11, 164–169.47. Lemons, J. E.; Lucas, L. C. J. Arthroplasty 1986, 1, 143–147.48. Gaines, R. W., Jr.; Abernathie, D. L. Spine 1986, 11, 907.49. O’Brien, J. P.; Stephens, M. M.; Prickett, C. F.; Wilcox, A.; Evans, J. H. Clin.
Orthop. Relat. Res. 1986, 203, 168.50. Liu, Y.; Njus, G. O.; Bahr, P. A.; Geng, P. O. Spine 1990, 15, 311.51. Kokubo, T.; Yamaguchi, S. Materials 2009, 3, 48–63.52. Wuisman, P.; Smit, T. H. Eur. Spine J. 2006, 15, 133–148.53. Gogolewski, S.; Mainil-Varlet, P.; Dillon, J. G. J. Biomed. Mater. Res. 1996, 32,
227–235.54. Nuutinen, J.-P.; Clerc, C.; Virta, T.; Tulamo, R. M.; Tormala, P. J. Biomater. Sci.
Polym. Ed. 2002, 13, 1325–1336.55. Jun, S.; Harold, A. J. Appl. Biomater. 1993, 4, 13–27.56. Nabhan, A. M. D.; Ishak, B. C. M.; Steimer, O. M. D.; et al. J. Spinal Disord. Tech.
2009, 22, 155–161.57. Bao, Q.; Mccullen, G.; Higham, P.; Dumbleton, J.; Yuan, H. Biomaterials 1996,
17, 1157–1167.58. White, A.; Lawrence, J.; Grauer, J. In Spinal Reconstruction: Clinical Examples of
Applied Basic Science, Biomechanics and Engineering, Informa Healthcare:New York, 2007; p 263.
59. Galbusera, F.; Bellini, C.; Zweig, T.; et al. Eur. Spine J. 2008, 17, 1635–1650.60. Hedman, T. P.; Kostuik, J. P.; Fernie, G. R.; Heller, W. G. Spine 1991, 16, S256.61. Cunningham, B.; Dmitriev, A.; Hu, N.; McAfee, P. Spine 2003, 28, S118.62. Kurtz, S. M.; Edidin, A. A. Spine Technology Handbook. Academic Press:
New York, 2006.
63. Edeland, H. J. Biomed. Eng. 1985, 7, 57.64. Szpalski, M.; Gunzburg, R.; Mayer, M. Eur. Spine J. 2002, 11, 65–84.65. Serhan, H. Personal Communication, 2005; DePuy Spine, Raynham, MA.66. Cunningham, B.; Lowery, G.; Serhan, H.; et al. Eur. Spine J. 2002, 11, 115–123.67. Buttner-Janz, K.; Hochschuler, S.; McAfee, P. The Artificial Disc. Springer: Berlin,
2003.68. Putzier, M.; Funk, J.; Schneider, S.; et al. Eur. Spine J. 2006, 15, 183–195.69. Delamarter, R.; Bae, H.; Pradhan, B. Orthop. Clin. North Am. 2005, 36, 301–313.70. Tropiano, P.; Huang, R.; Girardi, F.; Cammisa, F., Jr.; Marnay, T. J. Bone Joint
Surg. 2005, 87, 490.71. Lehuec, J.; Kiaer, T.; Friesem, T.; Mathews, H.; Liu, M.; Eisermann, L. Spine
2003, 28, 346.72. Mathews, H.; Lehuec, J.; Friesem, T.; Zdeblick, T.; Eisermann, L. Spine J. 2004,
4, 268–275.73. Valdevit, A.; Errico, T. Spine J. 2004, 4, 276–288.74. Hickey, D.; Hukins, D. Spine 1980, 5, 106.75. Kenneth Yonemura, J. S.; Peppelman, W., Jr.; Griffith, S.; Davis, R,.;
Cauthen, J. C., III. In Spinal Reconstruction: Clinical Examples ofApplied Basic Science, Biomechanics and Engineering, 1st edn.; Kai-UweLewandrowski, M. J. Y. E., Kalfas, I., Park, P., Mclain, R. F., Trantolo, D. J., Eds.;Informa Healthcare: New York, 2007.
76. Hampton, D.; Laros, G.; Mccarron, R.; Franks, D. Spine 1989, 14, 398–401.77. Adams, M.; Mcnally, D.; Dolan, P. J. Bone Joint Surg. Br. 1996, 78,
965–972.78. Griffith, S., Davis, R., Hutton, W., Eds.; In Repair of the Anulus Fibrosus of the
Lumbar Disc; Raymedica, LLC: USA, 2007.79. Ahlgren, B.; Lui, W.; Herkowitz, H.; Panjabi, M. Spine 2000, 25, 2165.80. Ethier, D.; Cain, J.; Yaszemski, M.; et al. Spine 1994, 19, 2071.81. Guerin, H.; Elliott, D. J. Orthop. Res. 2007, 25, 508–516.82. Iatridis, J. C. P.; Weidenbaum, M. M. D.; Setton, L. A. P.; Mow, V. C. P.
Spine 1996, 21(10), 1174–1184.83. Yasargil, M. Adv. Neurosurg. 1977, 4, 81.84. Lehmann, T. R.; Titus, M. K. Refinements in technique for open lumbar
discectomy. In Proceedings of the International Society for the Study of theLumbar Spine (ISSLS), June 1997.
85. Cauthen, J. Spinal Arthoplasty: A New Era in Spine Care; Quality Medical:St. Louis, MO, 2005; pp 157–177.
86. Basu, S.; Marini, C.; Bauman, F.; et al. Ann. Thorac. Surg. 1995, 60, 1255.87. Miyano, G.; Yamataka, A.; Kato, Y.; et al. J. Pediatr. Surg. 2004, 39,
1867–1870.88. Haldimann, D. System for repairing inter-vertebral discs. US Patent App.
10/207,285, 2002.89. Bao, Q.; Yuan, H. Spine 2002, 27, 1245.90. Carl, A.; Ledet, E.; Yuan, H.; Sharan, A. Spine J. 2004, 4, 325–329.91. Wilke, H.-J. P.; Neef, P. M. D.; Caimi, M. M. D.; Hoogland, T. M. D.;
Claes, L. E. P. Spine 1999, 24, 755–762.92. Bourgeault, C.; Beaubien, B. P.; Freeman, A. L.; Bentley, I.; Houfburg, R.;
Griffith, S. L. In Biomechanical Assessment of Anulus Fibrosus Repair withSuturetethered Tissue Anchors. Proceedings of the 2007 Annual Meeting of theSpine Arthroplasty Society (SAS7), Berlin, Germany, May 2007.
93. Anulex Technologies, Inc. Completes Patient Enrollment Phase of Post MarketStudy and Announces the Treatment of over 6000 Patients with the Xclose™ TissueRepair System, 2009. http://www.anulex.com/about_anulex/news_events.asp.
94. Anulex Technologies, Inc. Anulex Technologies Receives FDA 510(k) Clearancefor the Inclose™ Surgical Mesh System. 2005 [Online].
95. Cunningham, B. C.; Hu, N. B.; Beatson, H.; McAfee, P. In Fifth Global Symposiumon Motion Preservation Technology; Author, C. D., Ed.; Spine ArthroplastySociety: New York, 2005.
96. Peppelman, W. C.; Kraus, D. R.; Sherman, J.; Yonemura, K.; Griffith, S. L.;Cauthen, J. In Feasibility Results of a Novel Annular Repair Device in a GoatModel. World Spine III, Rio de Janeiro, Brazil, 2005.
97. Guillermo Bajares, A. P.; Diaz, Manuel; et al. Spine Arthroplasty Soc. 2007, 7.98. Bajaras, G.; Perez-Olivia, A. In Fifth Global Symposium on Motion Preservation
Technology; Spine Arthroplasty Society (SAS): New York, 2005.99. Gorensek, M.; Vilendecic, M.; Eustacchio, S.; et al. Spine J. 2006, 6, 144.100. Yeh, O.; Chow, S.; Small, M.; Einhorn, J.; Lambrecht, G. Spine J. 2006, 6, 146–147.101. Emir Kamaric, O. Y.; Velagic, A.; Einhorn, J.; Osman, G.; Vilandecic, M.;
Lambrecht, G. Spine J. 2005, 5, S178–S179.102. HYDROCISION, The SpineJet System. 2010. http://www.hydrocision.com.103. Han, H. J.; Kim, W. K.; Park, C. K.; Oh, S. H.; Lee, D. G.; Doh, E. S. Kor. J. Spine
2009, 6, 187–191.104. Intrinsic Therapeutics, Inc. The BarricaidW Prosthesis: Optimizing Outcomes for
Discectomy Patients. Available at http://www.intrinsic-therapeutics.com/barricaid_ard.shtml