trends in materials for spine surgery - elseviermrw.elsevier.com/bmt2/trends in materials for spine...

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

http://www.helpingbacks.com/spinalfractures.php

http://healthbase.wordpress.com/2007/06/13/spinal-fusion-surgery/

http://healthbase.wordpress.com/2007/06/13/spinal-fusion-surgery/

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.


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.


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.


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.


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


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.


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;


• 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


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.


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.


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


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.


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


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/

http://anulex.com/anulex_technology/inclose_us.asp

http://anulex.com/anulex_technology/inclose_us.asp

http://www.anulex.com/anulex_technology/xclose.asp


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.


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.


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

http://www.anulex.com/about_anulex/news_events.asp

http://www.hydrocision.com

http://www.intrinsic-therapeutics.com/barricaid_ard.shtml

http://www.intrinsic-therapeutics.com/barricaid_ard.shtml

trends in materials for spine surgery - elseviermrw.elsevier.com/bmt2/trends in materials for spine...

Documents