Nucleus Arthroplasty Volume II

8/9/2019 Nucleus Arthroplasty Volume II

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NucleusArthroplasty

Volume II: Biomechanics &Development

Technology

in Spinal Care


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Table of Contents

This monograph series is a groundbreaking project in therapidly emerging field of non-fusion spinal surgery. Thefull range of nucleus replacement technologies is examined

with discussion on biomechanical and physiological proper-ties of the disc, detailed information on each cutting-edge

device technology, indications, and patient selection criteria.

Nucleus Arthroplasty Technology in Spinal Careis

published for the medical profession by Raymedica, LLC,

Minneapolis, MN 55431.

The views expressed in this series are those of the authors

and do not necessarily represent those of Raymedica, LLC.

ACKNOWLEDGEMENT

We, Raymedica, LLC, and the authors of this volume, wish

to acknowledge our debt of gratitude for the important

contribution of Steven J. Seme, Developmental Editor. His

guidance has added a great deal to the teaching value of

this volume.

Copyright 2006 and 2007 Raymedica, LLC. All rights

reserved. Printed in the U.S.A.

2 Introduction

4 Deputy Editorial Board

C H A P T E R 7 6 Biomechanics of the Degenerated Disc

C H A P T E R 8 11 Principles and Mechanical Requirements of Nucleus Implants

C H A P T E R 9 17 Kinematic Demands of Nucleus Arthroplasty Technology

C H A P T E R 1 0 24 Device Stiffness vs. Load-Sharing with Nucleus Arthroplasty Devices

C H A P T E R 1 1 34 Endplate Mechanics

C H A P T E R 1 2 41 Repair of the Anulus Fibrosus of the Lumbar DiscC H A P T E R 1 3 49 Integration of Nucleus Arthroplasty Technology into the Continuum of Care

IBC Conclusion

www.nucleusarthroplasty.com


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2

The first documented works describing the diagnosis and

treatment of the spine, spinal disorders, and spinal instabilitydate back to 1900-2500 B.C. Interestingly, the documents recom-

mended against the treatment of spinal cord injury. The develop-

ment of therapeutic treatments has a long history starting with

the cane, the first load-sharing device. Today, our efforts to

improve therapies to treat spine disease persist. We continue to

recognize problems, identify issues, and define variables in an

effort to better understand spinal degeneration and to develop

innovative solutions that utilize a wide array of materials and

technologies. Our field has had a rich history of advancements,

accomplishments, and inventiveness. We owe a great debt to the

pioneers who, armed with little more than a detailed knowledge

of anatomy, heralded in the era of spinal surgery. Their trials,

errors, innovations, and teachings have guided our efforts to

ultimately improve clinical outcomes.

Early on, it was recognized that the disc played a vital role in overall

spine health. With great effort and ingenuity, the unique anatomical,

biomechanical, and physiological properties of the disc were eluci-

dated and incorporated into elegant treatment algorithms. We now

have access to an almost overwhelming flow of information about

lumbar disc arthroplasty from countless sources. Central to the evo-

lution of therapies is a better appreciation of the complexities of thelumbar disc. By combining knowledge gleaned from anatomical

dissection, biochemical processes, and resultant physiology with a

disciplined foundation in biomechanics, we have created a fabric

of understanding never before enjoyed. Spine arthroplasty is now

an important and evolving area within the treatment of spinal

Reginald J. Davis, MD, FACSCHIEF OF NEUROSURGERY

Baltimore Neurosurgical Associates, PA

Baltimore, MD 21204

Federico P. Girardi, MDASSISTANT PROFESSOR

OF ORTHOPEDIC SURGERY

Hospital for Special Surgery

New York, NY 10021

Frank P. Cammisa, Jr., MD, FACSASSOCIATE PROFESSOR OF CLINICAL SURGERY


New York, NY 10021

William C. Hutton, DScPROFESSOR AND DIRECTOR OF

ORTHOPEDIC RESEARCH

Emory University

Atlanta, GA 30322

Introduction


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disorders. This sub-discipline represents the coalescence of manyareas of study focused on the development of new and exciting

solutions to address clinical problems.

These significant advances in our understanding of the spine rep-

resent a culmination of efforts occurring across many fronts. Our

increased understanding of the biological factors at work in disc

disease has been a driving force in the development and emer-

gence of new materials and delivery methods. The critical role

that advanced biocompatible alloys, polymers, and viscoelastic

hydrogels play in the innovation of disc arthroplasty technologies

cannot be over emphasized.

Technological advancements have played a vital role in supporting

and expanding our knowledge of motion preserving disc technolo-

gies. The latest imaging technologies allow a much more detailed

appreciation of pathological processes, such as disc degeneration,

and provide the ability to monitor the results of an intervention.

Computerized finite element analysis offers a risk-free environment

in which to test hypotheses and predict clinical impact. Biochemical

advancements yield an intimate understanding of the chemical envi-

ronment including chemical mediators and potential intervention

portals. This wealth of knowledge can be used to great advantage

when developing disc arthroplasty technologies.

Not to be overlooked, the socioeconomic challenges involved in the

development of new technologies, such as the Nucleus Arthroplasty

motion preservation system, have also become more apparent.

The all important variable of proper patient selection continues to

require constant reassessment and vigilance. Increasingly, third-party

payers control access to care and treatment choice to an alarming

degree. Such considerations can no longer be ignored in the quest

for ideal patient management methods.

This publication has been constructed to provide an overview of

the current biomechanical developments in Nucleus Arthroplastytechnology. Key elements include the Principles and Mechanical

requirements of Nucleus Devices, Kinematic Demands, Endplate

Mechanics, Device Stiffness vs. Load Sharing, and Anular Closure

Techniques. In addition, Volume II of this series will provide

insight into the potential market and the current players working

in the forefront of Nucleus Arthroplasty technology developmentactivities. This is an incredibly exciting field as technologies

focused on the repair and replacement of the diseased disc

nucleus will catapult us far beyond the treatment options we

have available today.

In conclusion, we can say that the spine arthroplasty specialist

of today is well prepared to deliver the most advanced solutions

to the clinical puzzle of disc disease with technologies based on

a rich tradition of innovation and compassion coupled with a

tremendous wealth of physiological knowledge and assessment

tools. As spine surgery evolves from mechanical solutions to

therapeutic solutions both surgeons and patients will benefit.

We hope you will find this series on Nucleus Arthroplasty

technology to be a valuable asset.

Reginald J. Davis, MD, FACS

Federico P. Girardi, MD

Frank P. Cammisa, Jr., MD, FACS

William C. Hutton, DSc

3


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Reginald J. Davis, MD, FACS

Dr. Davis is founder of Baltimore Neurosurgical Associates, chief

of Neurosurgery at the Greater Baltimore Medical Center, and a

faculty member at the Johns Hopkins School of Medicine and

the University of Maryland. He is a Fellow of the American

College of Surgeons and a Diplomate of the American Board of

Surgery. Dr. Davis received his medical degree from Johns

Hopkins University School of Medicine, Baltimore, Maryland.

He has broad experience in advanced procedures such as spinal

stabilization, intradiscal electrothermal therapy, and microendo-

scopic discectomy and has conducted physician training pro-

grams on these procedures. His professional affiliations include

the AANS-CNS Section on Disorders of the Spine, the American

Association of Neurological Surgeons, the Congress of

Neurological Surgeons, and the North American Spine Society.

Federico P. Girardi, MD

Dr. Girardi is assistant professor of orthopedic surgery, Weill

Medical College of Cornell University and is attending orthopedic

surgeon at the Hospital for Special Surgery, New York, New York.

He specializes in the treatment of spinal disorders including degen-

erative disc disease (DDD), spinal deformities, metabolic fractures,

and spinal tumors. Dr. Girardi received his medical degree from

the Universidad Nacional de Rosario, Rosario, Argentina.

He has performed extensive clinical research in the areas of mini-

mally invasive surgery, clinical outcomes, and spinal imaging. He

is also interested in basic research on bone, disc, and nerve tissue

regeneration and in the investigation of alternatives to spinal

fusion for the treatment of DDD. His professional affiliations

include the North American Spine Society, Scoliosis Research

Society, the European Spine Society, the International Society for

the Study of the Lumbar Spine, and the Spine Arthroplasty Society

Raymedica has selected Drs. Reginald J. Davis, MD, FACS, Federico P. Girardi, MD, Frank P. Cammisa, Jr., MD, FACS,and William C. Hutton, DSc to edit this series of monographs on Nucleus Arthroplasty technology, because of theispecial interest in this dynamic area of medicine. They are well respected for their clinical work and travel widely to speakand educate physicians. Drs. Davis, Girardi, and Cammisa are noted for their expertise in spine surgery and advanced

training in minimally invasive surgical techniques.

Deputy Editorial Board

4


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5

Frank P. Cammisa, Jr., MD, FACS

Dr. Cammisa is associate professor of clinical surgery, Weill

Medical College of Cornell University and is the Chief of Spinal

Surgical Service at The Hospital for Special Surgery in New York,

New York, where he also serves as an associate scientist in the

research division. Dr. Cammisa received his medical degree from

the College of Physicians and Surgeons at Columbia University,

New York, New York.

His clinical interests include non-fusion and motion preservation

technologies, minimally invasive, laparoscopic, and computer

assisted spinal surgery; microsurgery and athletic spinal injuries.

He is an active member of many spine societies, academic com-

mittees and editorial review boards. He has lectured widely and

published in numerous peer-reviewed journals and books.

William C. Hutton, DSc

Dr. Hutton is professor and director of orthopedic research,

Emory University in Atlanta, Georgia. He also attended

Universities in Glasgow, Birmingham, and London. Before coming

to Atlanta, he worked at educational institutes in London and

Adelaide, Australia. In Adelaide, he was professor of biomechanics

and chairman of the Department of Mechanical Engineering.

His major area of interest is biomechanics with a particular

focus on the spine. Dr. Hutton has published over 180 papers

in peer review journals. He has won many prizes for his work,

most recently (2004) the Russell S. Hibbs Award from the

Scoliosis Research Society. At present, he has a Research Career

Science Award from the Department of Veterans Affairs. He is

a member of the International Society for the Study of the

Lumbar Spine.


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6

Chapter 7 Biomechanics ofthe Degenerated Disc

Andrew A. Sama, MD

ASSISTANT PROFESSOR OF ORTHOPEDIC SURGERYWeill Medical College of Cornell University


New York, NY 10021

Federico P. Girardi, MDASSISTANT PROFESSOR

OF ORTHOPEDIC SURGERY


New York, NY 10021

INTRODUCTION

A s people age, the lumbar discs undergo a progressivealteration of chemical composition and biomechanicalproperties. This combination of chemical and biomechanical

changes can lead to back pain. Nucleus Arthroplasty inter-

ventions are emerging that intend to replace the nucleus of the

diseased lumbar disc for treatment of patients with discogenic

pain. Understanding the natural progression of disc degenera-

tion is paramount to under-

standing the function of such

devices. This chapter summa-

rizes what is generally under-

stood about the degenerative

cascade and the resulting

biomechanical changes.

NUCLEUS ARTHROPLASTY INTERVENTIONS ARE

EMERGING THAT INTEND TO REPLACE THE NUCLEUS

OF THE DISEASED LUMBAR DISC FOR TREATMENT OF

PATIENTS WITH DISCOGENIC PAIN.


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AGING AND LUMBAR DISC DEGENERATION

There is a complex arrangement of bones and cartilage that make

up each spinal motion segment. The intervertebral disc along with

the apophyseal joints allow for flexibility while also helping to

maintain the stability of the spine (Figure 1).1 The disc is some-

times seen as a cushion between vertebral bodies to help absorb

and distribute applied forces. Each disc is made up of an anulus,

a nucleus pulposus, and the cartilage endplates. The intricate

lamellar arrangement between the fibers of the anulus fibrosus

exteriorly, and the nucleus pulposus within, serve to absorb the

forces applied to the spine. In healthy discs, there is a harmonious

relationship between the components, such that compressive axial

forces produce an increase in hydrostatic pressure in the nucleus

pulposus, and this pressure is transmitted to the anulus where it

is absorbed as tensile stress.2 Thus, the anulus sees tensile stress as

well as compressive stress. The need to support this complex stress

explains, to some extent, the orientation of the fibers of the

anulus. The hydrophilic proteoglycans in the nucleus control the

diurnal influx and efflux of water and nutrients that keep the discs

healthy and give them their normal function. The osmotic and

hydrostatic properties of the disc are not static.3, 4 As people age,

the discs can begin to desiccate and become depleted of nutrients.Disc waste products accumulate, decreasing the concentration of

viable cells, resulting in changes in the biomechanical properties

of the disc.5

THE DEGENERATIVE CASCADE

As explained in Book IFundamentals, Chapter 1 of this series,

the result of the disc aging and degenerating is a net decrease in

the amount of aggregated proteoglycan and an increase in the

non-aggregated proteoglycans, which leads to lower osmotic

water binding capacity and loss of compressive resistance in the

lumbar disc. As forces are applied to these biologically altered

discs, there is a greater likelihood that damage will occur to the

anulus fibrosus creating a vicious cycle of degeneration.6, 7

Kirkaldy-Willis et al8 described lumbar disc degeneration as a

cascade that impacts the three joint complex. The first stage of

the cascade is known as the Dysfunctional Stage and corresponds

to the early onset of disc degeneration. The net result of the

Dysfunctional Stage is a tearing of the outer anulus fibrosus sec-

ondary to repetitive micro trauma. This is clinically manifested

7

Nucleus Anulus

IntervertebralDisc

Figure 1Major components of spinal complex.

THE RESULT OF THE DISC AGING AND DEGENERATING IS A NET DECREASE IN THE

AMOUNT OF AGGREGATED PROTEOGLYCAN AND AN INCREASE IN THE NON-AGGREGATED

PROTEOGLYCANS, WHICH LEADS TO LOWER OSMOTIC WATER BINDING CAPACITY AND

LOSS OF COMPRESSIVE RESISTANCE IN THE LUMBAR DISC.

THERE IS A COMPLEX ARRANGEMENT OF BONES AND

CARTILAGE THAT MAKE UP EACH SPINAL MOTION SEG-

MENT. THE INTERVERTEBRAL DISC ALONG WITH THE

APOPHYSEAL JOINTS ALLOW FOR FLEXIBILITY WHILE AL

HELPING TO MAINTAIN THE STABILITY OF THE SPINE.

ApophysealJoint


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8

as a mechanical low back pain that is episodic. During this stage

there is also a dehydration of the nucleus pulposus.8

The second stage is called the Instability Stage. This is represen-

tative of more significant damage to the disc secondary to a

delamination of the layers of the anulus fibrosus. Vertebral seg-

mental instability can occur resulting in further damage and loss

of proteoglycan composition in the nucleus pulposus.8

The third stage is the Stabilization Stage. This occurs when there

is resorption of the nucleus pulposus and worsening of interver-

tebral disc space collapse. This is the stage where osteophytes

form secondary to anular traction on vertebral endplates and

spinal stenosis may result.8

MRI CHARACTERISTICS OF

LUMBAR DISC DEGENERATION

The Kirkaldy-Willis stages described above can be radiographi-

cally characterized by multiple imaging techniques, but magnetic

resonance imaging (MRI) evaluation is the gold standard. Stage I

(Dysfunctional) is usually manifested on MRI imaging techniques

by the presence of a High Intensity Zone lesion of the posterior

anulus fibrosus, and an overall decrease signal intensity of the disc

on T2-weighted sequences in the sagittal plane (Figure 2a).9

Stage II (Instability) is manifested on MRI with further loss

of disc space height and progressive desiccation resulting in a

darker disc (Figure 2b). Disc herniations can present in Stage II

because a loss of anular integrity may result in herniation of the

nucleus pulposus.9

Stage III (Stabilization) is manifested on MRI by worsening of

disc space collapse and by the presence of osteophyte formation

coupled with soft tissue redundancy or hypertrophy, resulting

in central canal, subarticular lateral recess, or foraminal stenosis

(Figure 2c).9

Figure 2MRI images of degenerative cascade. (a) Dysfunction StageHigh Intensity Zone lesion of the posterior anulus fibrosus with decrease

signal intensity of the disc on T2-weighted image. (b) Instability StageDisc collapse with continued decrease in signal intensity.

(c) Stabilization StageContinued disc collapse, osteophyte formation, stenosis of neural structures.

a Dysfunction bInstability cStabilization

THE KIRKALDY-WILLIS STAGES DESCRIBED

CAN BE RADIOGRAPHICALLY CHARACTERIZED

BY MULTIPLE IMAGING TECHNIQUES BUT MRI

EVALUATION IS THE GOLD STANDARD.


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THE PAIN GENERATOR

The radiographic findings and histomorphology of disc degener-

ation are objective signs of the presence of a disease state that can

be measured, but these findings may not always correlate with the

patients symptoms.10 The pain generators in degenerative disc

disease (DDD) are multifactorial and can be chemically mediated

or mechanically induced. The loss of disc structure and biome-

chanical integrity results in alterations of the load-sharing prop-

erties of the lumbar disc and the vertebral bodies.3 The nerve

endings in the facet joints, spinal ligaments, and para-spinous

musculature are then stimulated and result in pain. The chemical

mediation of back pain occurs from the release of cytokines and

free radicals along with cellular debris from matrix degeneration.

This results in nocioceptive stimulation of nerve endings and

resultant back pain. These conclusions are based on Buckwalters

articles.5 As discs become progressively more degenerated, they

demonstrate an ingrowth of blood vessels and nerve fibers

beyond the outer anulus which may also contribute to the onset

of back pain. Normal lumbar discs do not typically have blood

vessels or nerve fibers inside of the outer anulus.

BIOMECHANICAL CHANGES

According to Horst et al, non-degenerated discs exhibit fluid-like

properties, whereas degenerated discs have properties that are

more like those of a solid. The normal nucleus pulposus behavessimilar to a viscous fluid: with degeneration, it shows an increase

in shear modulus and becomes stiffer and more elastic.11 As the

tissue transitions from fluid-like to more solid-like properties, there

is an associated decrease in hydrostatic pressurization. Hydrostatic

pressurization of the nucleus pulposus allows the intervertbral disc

to support large loads which may be several times total body

weight.16 The majority of these loads are typically carried through

the anterior column of the spine. However, as the nucleus pulposus

degenerates, a large proportion of load transmission is shifted to

the posterior elements and facet joints.13,17 This results in increased

facet loading and degeneration as well as an increase in back pain.

According to Buckwalter it appears that the degenerative process

affects the nucleus pulposus and cartilage endplate more sig-

nificantly than the anulus fibrosus with respect to changes in

material properties.5, 17

Studies have shown that there also are changes in the kinematics

of the discs as a result of varying degrees of degeneration. The

relationships between the soft tissue laxity and changes in kine-

matics of the lumbar spine were studied extensively by Mimura,

Fujiwara, Krismer, and Frei.12-15

Despite the fact that there weremixed results from the studies, an assessment of the general

trends suggests that segmental motion is increased and the

motion segment becomes more unstable in the earlier phases of

degeneration when the anulus becomes slack. As degeneration

continues to more advanced stages, a re-stabilization seems to

occur as disc space height is lost, the nucleus becomes more

fibrotic, and there is a net decrease in the flexibility.

Nucleus Arthroplasty technologies are emerging as an alternative

early surgical treatment for patients with degenerative disc dis-

ease. Removing the diseased disc nucleus and replacing it with a

nuclear replacement device could improve and possibly restore

the load bearing and kinematic properties of the degenerated

segment to a more physiologic level.19 Providing resistance to

compressive loads while reducing segment instability may help

ensure transmission of loads applied to the spine will be pre-

dominantly maintained in the anterior column, thereby unload-

ing the facet joints and decreasing pain.2 Also, by maintaining

segment height and anular stability, the stimulation of the free

nerve endings in the outer anulus may be minimized and there-

fore decrease discogenic back pain. Biomechanical studies evalu-

ating the kinematic restoration, axial load sharing, endplate

mechanics and anular repair with Nucleus Arthroplasty repair

are discussed in more detail in the following chapters.

PROVIDING RESISTANCE TO COMPRESSIVE LOADS

WHILE REDUCING SEGMENT INSTABILITY MAY HELP

ENSURE TRANSMISSION OF LOADS APPLIED TO THE

SPINE WILL BE PREDOMINANTLY MAINTAINED IN THE

ANTERIOR COLUMN, THEREBY UNLOADING THE

FACET JOINTS AND DECREASING PAIN.


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CONCLUSION

Taking into consideration the varying degrees of disc degeneration

and the biomechanical changes that occur, one of the challenges of

treatment becomes defining a point for surgical intervention.

Considering the important biomechanical role of the nucleus pul-posus, an attempt to restore or recreate the function of a healthy

nucleus appears to be a target point for intervention.18 Patients who

are found to have early or mid-stage degenerative disc disease, and

who show signs of progression of degeneration, are potential can-

didates for nucleus replacement. Removing the damaged nucleus

pulposus and replacing it with a Nucleus Arthroplasty device to

restore the biomechanical properties of the given motion segment

may be beneficial in breaking the degenerative cascade.

REFERENCES

1. White AA, Panjabi MM. Clinical Biomechanics of the Spine. Philadelphia:

Lippincott Williams & Wilkins; 1990.

2. Goins ML, Wimberley DW, Yuan PS, Fitzhenry LN, Vaccaro AR. Nucleus

pulposus replacement: an emerging technology. Spine J 2005 Nov-Dec;

5(6 Suppl):317S-24S.

3. Setton LA, Chen J. Mechanobiology of the intervertebral disc and relevance to

disc degeneration. J Bone Joint Surg Am 2006 Apr;88 Suppl 2:52-7.

4. Buckwalter JA, Mow VC, Bowden SD, Eyre DR, Weidenbaum M. Intervertebral

Disk Structure, Compostion, and Mechanical Function. In: Buckwalter JA,

Ainhorn TA, Simon SR, editors. Orthopaedic Basic Science Biology and

Biomechanics for the Musculoskeletal System. 2nd ed. Rosemont, IL:

American Academy of Orthopaedic Surgeons; 2000. p.548.

5. Buckwalter JA, Boden SD, Eyre DR, Mow VC, Weidenbaum M. IntervertebralDisk Aging, Degeneration, and Herniation. In: Buckwalter JA, Ainhorn TA,

Simon SR, editors. Orthopaedic Basic Science Biology and Biomechanics for

the Musculoskeletal System. 2nd ed. Rosemont, IL: American Academy of

Orthopaedic Surgeons; 2000. p.558.

6. Kurowski P, Kubo A. The relationship of degeneration of the intervertebral

disc to mechanical loading conditions on lumbar vertebrae. Spine 1986

Sep;11(7):726-31.

7. Rohlmann A, Zander T, Schmidt H, Wilke HJ, Bergmann G. Analysis of the

influence of disc degeneration on the mechanical behaviour of a lumbar

motion segment using the finite element method. J Biomech

2006;39(13):2484-90.

8. Kirkaldy-Willis WH, Wedge JH, Yong-Hing K, Reilly J. Pathology and patho-

genesis of lumbar spondylosis and stenosis. Spine 1978 Dec;3(4):319-28.

9. Benneker LM, Heini PF, Anderson SE, Alini M, Ito K. Correlation of radi-

ographic and MRI parameters to morphological and biochemical assessment

of intervertebral disc degeneration. Eur Spine J 2005 Feb;14(1):27-35.

10. Kjaer P, Albert H, Jensen TS, Leboeuf-Yde C, Bendix T, Wedderkopp N, et al.

Back pain, radiology and end plate changes by means of Modic. Ugeskr

Laeger 2006 Apr 24;168(17):1668,9; author reply 1669.

11. Horst M, Brinckmann P. 1980 Volvo award in biomechanics. Measurement of

the distribution of axial stress on the end plate of the vertebral body. Spine

1981 May-Jun;6(3):217-32.

12. Mimura M, Panjabi MM, Oxland TR, Crisco JJ, Yamamoto I, Vasavada A.

Disc degeneration affects the multidirectional flexibility of the lumbar spine.

Spine 1994 Jun 15;19(12):1371-80.

13. Fujiwara A, Lim TH, An HS, Tanaka N, Jeon CH, Andersson GB, et al. The

effect of disc degeneration and facet joint osteoarthritis on the segmental

flexibility of the lumbar spine. Spine 2000 Dec 1;25(23):3036-44.

14. Krismer M, Haid C, Behensky H, Kapfinger P, Landauer F, Rachbauer F.

Motion in lumbar functional spine units during side bending and axial

rotation moments depending on the degree of degeneration. Spine 2000

Aug 15;25(16):2020-7.

15. Frei H, Oxland TR, Rathonyi GC, Nolte LP. The effect of nucleotomy on

lumbar spine mechanics in compression and shear loading. Spine 2001

Oct 1;26(19):2080-9.

16. Johannessen W, Elliott DM. Effects of degeneration on the biphasic material

properties of human nucleus pulposus in confined compression. Spine 2005

Dec 15;30(24):E724-9.

17. Niosi CA, Oxland TR. Degenerative mechanics of the lumbar spine. Spine J

2004 Nov-Dec;4(6 Suppl):202S-8S.

18. Di Martino A, Vaccaro AR, Lee JY, Denaro V, Lim MR. Nucleus pulposus

replacement: basic science and indications for clinical use. Spine 2005 Aug

15;30(16 Suppl):S16-22.

19. Le Huec JC, Aunoble S, Basso Y, Tournier C, Yamada K. Biomechanical

Considerations for Total Lumbar Disk Replacement. In: Kim DH, Cammisa

FP, Fessler RG, editors. Dynamic Reconstruction of the Spine. Thieme

Medical Publishers; 2006. p.149.

CONSIDERING THE IMPORTANT BIOMECHANICAL ROLE OF THE NUCLEUS

PULPOSUS, AN ATTEMPT TO RESTORE OR RECREATE THE FUNCTION OF A

HEALTHY NUCLEUS APPEARS TO BE A TARGET POINT FOR INTERVENTION.


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11

Chapter 8 Principles and MechanicalRequirements of Nucleus Implant

Prof. Dr. Hans-Joachim Wilke

PROFESSSORInstitute of Orthopaedic Research and Biomechanics

University of Ulm

Ulm, Germany 89081

BASIC BIOMECHANICAL CONSIDERATIONS

N

on-fusion technologies in spinal surgery gain more and

more popularity. Constantly, new ideas are created and

turned into new products. Each idea has its own philosophy with

the principle goal to maintain the motion in the treated segment.

In contrast to total disc implants, nucleus implants are designed

to preserve as many spinal structures as possible. These implants

restore and maintain disc height and/or original mobility.

In many cases, disc degeneration or disc prolapse is treated just by

nucleotomy and decompression; this can produce a good out-

come due to decom-

pression of the nerve

roots. However, in some

patients this treatment isinsufficient and back

pain recurs after a while.

This may be explained

by the nucleotomy,

which causes a reduc-

tion in disc height proportional to 0.8 mm/g removed material.

Thus the average amount of removed nucleus material of 3 g leads

to a height loss of 2.4 mm, which can be associated with an

THE AVERAGE AMOUNT OF REMOVED NUCLEUS MATERIAL

OF 3 G LEADS TO A HEIGHT LOSS OF 2.4 MM, WHICH CANBE ASSOCIATED WITH AN INCREASE IN THE SEGMENTAL

RANGE OF MOTION OF ABOUT 20-30%, AND AN INCREASE

IN THE NEUTRAL ZONE OF UP TO 100%.


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12

increase in the segmental range of motion of about 20-30%, and

an increase in the neutral zone of up to 100%. This instability

can lead to more stress in the remaining anulus and facet joints

resulting in further degeneration. This height loss may also be

associated with bulging of the anular ring leading to an unphysi-

ological strain pattern in the anulus; this may affect the cells in

these structures. Bulging of the disc is about 1mm per 1 kN up

to 2.5 kN axial preload.1 It seems that this disc bulging is slightly

higher anteriorly than posteriorly.2 The data suggests that this

bulging may even increase further when the spine is bent. This

effect sometimes is compared with a rather flat car tire (this

problem is sometimes called flat-tire syndrome). Removing the

disc from one side creates a slight asymmetry due to the hole.

Finite element analysis suggests that the regions of the disc that

show the largest bulging also show the highest strain. This seems

to be the posterolateral region of the disc, where the strain is

exaggerated after removal of the nucleus.

VARIOUS TYPES OF NUCLEUS DEVICES

The advantage of a nucleus prosthesis as compared to a total

disc prosthesis is that the nucleus device generally allows the

preservation of the existing anatomical structures including

the anulus, vertebral endplates, and ligaments.

In theory, an optimum nucleus replacement should restore the

mobility and re-establish the intact disc height, thereby restoring

the nominal stresses and strains of the collagen fibres in the anu-

lus. This seems to be a superior situation as compared to leaving

the disc alone after a discectomy.

The origin of the idea to replace the nucleus goes back to the

1950s. The first idea was to fill the nuclear cavity with polymethyl

methacrylate (PMMA) or silicone.3,4 Since that time a variety of

solutions to replace the nucleus have been developed.5 This pres-

ent article tries to classify the different ideas into the three cate-

gories: mechanical nucleus devices, polymer implants, and tissue

engineered nucleus implants.

Nucleus Implants

Polymer Tissue EngineeredMechanical

Steelball, Fernstrm

Regain, Biomet Inc.

IPD, Dynamic Spine

knitted titanium, Buck

Natural

ScaffoldsSyntheticScaffolds

Pre-FormedIn situ

Formed

Hydrogel Polyurethane

with jacket w/o jacket

Silicone Hydrogel Other

Material

ThermoResponsive

PolymerDual Disc cylinders,

Ray and Corbin

PDN, Raymedica, LLC

SaluDisc, Spine Medica

Aquarelle, Stryker

NeuDisc,Replication Medical, Inc.

Newcleus, Zimmer Spine

DASCOR, DiscDynamics Inc.

SINUX ANR, DePuy

NuCore, Spine Wave

BioDisc, CryoLife

PNR, TranS1

DiscCell, Gentis

PMMA, Hamby andGlaser

Collagen Gels Hyaluronic Acid

Fleece of PLA Textiles

PDN-SOLO, Raymedica, LLC

PDR, TranS1

NUBAC, Pioneer Surgical

CL-Disc, Interpore Cross

Gelifex SP/ IP,SYNTHES Inc.

with balloon w/o balloon

HydroFlex, Raymedica, LLC Figure 1Classification chart of the different nucleus replacement devices.


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13

MECHANICAL IMPLANTS

Fernstrm implanted the first mechanical nucleus replacement

device in 1966. This implant consisted of a stainless steel ball

that was placed in the disc space.6 Clinically, it proved unsuc-

cessful because the ball subsided into the endplates and served

as a fusion device. Later, other mechanical devices were devel-

oped that were made of less stiff materials and incorporated a

larger contact area with the endplates. As an example, Regain

(Biomet, Inc.) is made of one piece of highly-polished, pyrolytic

carbon with a Youngs modulus similar to cortical bone.

NUBAC (Pioneer Surgical Technology) is a ball and socket

nucleus replacement device made of PEEK with tantalum mark-

ers. Other mechanical devices include the CL-Disc (Biomet,

Inc.) which is composed of solid, zirconia ceramic, incorporat-

ing a porous titantium keel; and the IPD (Dynamic Spine) that

consists of metallic springs and is fixed to the vertebrae.

The TranS1 PDR (Percutaneous Disc Reconstruction) can also

be considered as a mechanical device; this device allows a sup-

ported nucleus or mechanical nucleus replacement. It has all the

benefits of the PNR with an in situformed silicone absorber (as

described later), and a metal-on-metal central pivot that helps

share the load. The device is implanted using a TranS1 pre-sacral

axial approach to the lumbar spine. With the pre-sacral approach,

the integrity of the anulus and ligaments is maintained, as are any

future surgical options.

A new idea describes an implant made of knitted titanium fila-

ments Buck (Germany), which provides the physiological stiffness

and motion of a spinal segment and seems to have no dislocation

tendency. All these mechanical implants require suturing the anu-

lus after implantation.

POLYMER IMPLANTS

As the mechanical devices are often too stiff, many researchers have

experimented with various polymers to create softer nucleus

implants. These softer nucleus implants are used as either preformed

shapes or alternatively the shape is allowed to developin situ.

Preformed

Hydrogel

Hydrogel is the most preferred material for preformed nucleus

devices made of polymers. It has been demonstrated that PVA(non-ionic hydrogel) has a similar swelling pressure characteris-

tic of the natural nucleus. Due to their hydrophilic characteristic

the hydrogels can swell. This means that they can be implanted

in the dehydrated state through smaller channels in the anulus.

After implantation the hydrogel increases in size; this can reduce

the risk of expulsion.

In 1988, Charles Dean Ray, MD, FACS had the idea for a nucleus

implant that consisted of a doppel woven spiral of flexible, high-

tensile-strength polymeric fibers and tissue ingrowth-promoting,

polyglycolic acid filaments. The cylinder contained a viscous

hygroscopic semifluid. This resulted in the development of the

prosthetic disc nucleus (PDN) device marketed by Raymedica,

LLC. In the first stages of development, the PDN device consisted

of two parts, an anterior and posterior hydrogel pellet comprised

of hydrolyzed poly-acrylonitrile polymer (Hypan), each enclosed

in a woven polyethylene jacket. These were both oriented in a

transverse position within the disc cavity. Once implanted, the

hydrophilic hydrogel pellet absorbed fluid and increased in volume

until restricted by the confines of the now tight, polyethylene

jacket. The original intact disc height was restored after implanta-

tion of the device. However, an effect from the hydration could

not be measured.7 Because of a high expulsion rate, this two-part

device was modified into a single, but bigger, implant called

PDN-SOLO. The current version, the HydraFlex device, is a

softer, faster hydrating, more contoured device compared to the

PDN-SOLO, and its preformed shape fits the endplate surface

geometry better with the goal to minimize the risk of subsidence.

A hydrogel without a jacket was also used. The Aquarelle (1998,

Stryker Spine) is made of a semihydrated poly-vinyl-alcohol

(PVA) hydrogel allowing 80% water content. This provides vis-

coelastic properties similar to the nucleus. Unfortunately, clinical

trails have demonstrated a high expulsion rate.

In 2000, Replication Medical presented the NeuDisc; a modi-

fied hydrolyzed poly-acrylonitrile polymer (Aquacryl) rein-

forced by a Dacron mesh. This anisotropic implant is able to

absorb up to 90% of its weight. It takes 18 hours to reach full

size in laboratory testing.

Another hydrogel implant called SaluDisc (Spine Medica)

is made of Salubria. This material contains water in similar

proportions to human tissue.

THE FIRST MECHANICAL NUCLEUS IMPLANT

WAS SUGGESTED IN 1966 BY FERNSTRM WHO

IMPLANTED A STAINLESS STEEL BALL AS A

SPACER INTO THE DISC.


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14

Polyurethane

Polycarbonate urethane (PCU) was used for the Newcleus

(Zimmer Spine) in 2003. This consisted of a preformed spiral

which allowed implantation using a minimally invasive tech-

nique. The implant material was able to absorb water up to

35% of its own weight. Functionally, it acted as a spacer with

some shock-absorbing capabilities.

In situ formed

Manyin situformed nucleus prostheses have been tried out since

the first attempt by Nachemson in the early 1960s. The concept is

to inject a curable polymer into the nuclear space. The advantage

of anin situformed nucleus replacement is that it can be injected

non-invasively using a small needle. It is hoped that this type of

implant will be at a low risk for expulsion. In contrast to the pre-

formed implants, which are implanted into the cavity and can

increase disc height, the injectable polymers cannot be injected

with enough pressure to increase disc height.

It has yet to be proved that in situcured polymer can provide enough

mechanical strength to support the applied load plus have the

required mechanicalin vivofatigue life. One potential disadvantage

ofin situformed devices is toxicity due to unreacted monomers.

Therefore, forin situformed devices, the polymerization process

is critical to ensure long-term biocompatability.

Thermo responsive polymer

One product that is injected as anin situformed material is

DASCOR (Disc Dynamics). The material, a cool polyurethane

polymer (18 C), is injected under pressure into a polyurethaneballoon through an attached catheter. It takes a minimum of 12

to 15 minutes to solidifyin situ.

Another thermo-responsive polymer is poly (N-isopropylacrylamide)

or PNIPAAm, although specific details are not yet available.

A polymer-based hydrogel which is liquid at room temperature

is used for the Gelifex (SYNTHES). This implant looks like a

porous rubber ball after solidification at body temperature.

Silicone

A silicone-based artificial nucleus replacement SINUX ANR

(SiniTech AG, Depuy Spine) is made of a liquid polymethylsilox-

ane (PMSO) polymer. This is also injected into the void of the

disc and curesin situin approximately 15 minutes.

TranS1 PNR (Percutaneous Nucleus Replacement) is an in situ

formed nucleus replacement technology. The nucleotomy and

device implantation is completed using the trans-sacral axial

approach to the lumbar spine, preserving the anulus and liga-

ments. A hollow screw is threaded axially providing distraction

of the disc space. Silicone is injected through the screw to fill

the created cavity and help maintain motion.

Hydrogel

Severalin situcurable hydrogel implants are currently under

development. The Biodisc (CryoLife) is a protein hydrogel

device (PHD), which cures in 2 minutes after injection into the

disc space. The properties are supposedly similar to the human

nucleus and there is no exothermic reaction during the harden-

ing process. The material is similar to epoxy glue and bonds to

the anulus, which hopefully reduces the expulsion risk.

The NuCore Nucleus Device (Spine Wave) is based on a

hydrogel composed of synthetic silk-elastin copolymer. This

material has no measurable exothermic reaction.

Other Polymers

In 1959, Hamy and Glaser suggested injecting PMMA into the

disc. PMMA, which is commonly used as bone cement, cures with

a high exothermic reaction. This is a cheap procedure and was

often used clinically, particularly by neurosurgeons in Germany.

Anotherin situcured nucleus implant is the DiscCell (Gentis).

This is anin situpolymerising material that is injected into the

disc space. Little information is available at the present time.

Tissue Engineered Implants

Recently, it has been shown that seeding or reinserting cells inside

the intervertebral disc may preserve disc structures by slowing

down the degeneration processes.8,9 It was also hypothesized that the

inserted cells might restore the anulus and nucleus tissue. These cell

injection methods could not produce a restoration of disc height.

MANY IN SITUFORMED NUCLEUS PROSTHESES HAVE

BEEN TRIED OUT SINCE THE FIRST ATTEMPT BY

NACHEMSON IN THE EARLY 1960S. THE CONCEPT IS TO

INJECT A CURABLE POLYMER INTO THE NUCLEAR SPACE.


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15

The idea of a tissue-engineered nucleus implant is to seed cells

in a three-dimensional matrix. This matrix would serve as a scaf-

fold to produce a structure of mechanical stiffness and adequate

mechanical properties. The seeded cells could be intervertebral

disc cells or mesenchymal progentitor cells (MPCs).

Natural Scaffolds

Nucleus scaffolds can be produced from natural materials such

as collagen or hyaluronic acid. A recent study presented a three-

dimensional collagen-I matrix made of rat-tail collagen (ARS

Arthro, AG, Esslingen, Germany) that might be suitable to serve

as scaffold for cell seeding and eventual nucleus replacement.10

The problems of restoring disc height and implant expulsion

using a tissue-engineered nucleus have yet to be solved.11 Scaffolds

made of hyaluronic acid could be utilized, although the implant

might degrade after regeneration of the disc.

Natural material composites are currently being developed for

use as nucleus scaffolds. In any case, it is questionable whether

a natural scaffold would be sufficiently load-bearing.

Synthetic Scaffolds

Better load-bearing characteristics may be obtained with synthetic

materials, which may be fabricated of fleece or other combina-

tions of textiles. Because of a higher density and stiffness, they

might be introduced into the disc space and further compressed

to increase the stiffness, allowing more volume to be inserted.

Such approaches are currently under study at universities in

Dresden, Heidelberg and Ulm.

PRECLINICAL MECHANICAL AND BIOMECHANICAL

TESTING OF NUCLEUS DEVICES

Before being put into clinical practice, nucleus implants and the

surgical approaches for implantation should be tested. Some

of the new biological solutions may not be testable in all test

configurations. Nevertheless, the following principles are

suggested in order to compare the different nucleus devices.

Mechanical tests

Static tests

Prior to any implantation, mechanical tests on the implant mustbe carried out to ensure that the implant can sustain certain loads

and has sufficient fatigue life. For example, static loads are used to

determine the stiffness and the yield point of the implant.

Dynamic tests

Dynamic loads at a rate of 4 Hz or less are necessary to prove

that the implants are able to withstand at least 10,000,000 cycles

without collapsing, disconnecting, or deforming permanently.

They also may be used as wear tests to determine how much and

what kind of debris are produced under expectedin vivoloads

and motions.

These mechanical tests should be performed between two polyac-

etal or equivalent test blocks. The test blocks will eliminate the

effects of the variability of bone properties and morphology for the

fatigue tests. These tests should be performed in a defined testing

environment (e.g. in a 0.9% saline environmental bath at 37C).

Other tests such as hydrating tests may also be important with

implants made of hydrogel.

Biomechanical tests

Functional in vitro flexibility tests

In contrast to the pure mechanical tests, another category of tests

with the goal to determine thein situperformance is flexibility

tests. These are ideally performed using human cadaveric spine

specimens. These tests require mechanical testing machines that

allow loads that simulate the physiological motion of the specimens

with the implants in place.

TH E ID EA OF A T IS S U E- ENGINEERED NU C L EU S IM PL A NT IS TO S EED C EL L S

IN A TH REE- D IM ENS IONA L M A TRIX. TH IS M A TRIX W OU L D S ERVE A S A

S C A FFOL D TO PROD U C E A S TRU C TU RE OF M EC H A NIC A L S TIFFNES S A ND

A D EQU ATE M EC H A NIC A L PROPERTIES .


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16

They should be carried out in flexion/extension, lateral bending,

and axial rotation. It is recommended for standardization that

they are tested under pure moments without preload.12 Eventually,

tests under shear loading, compression, muscle forces, and other

representativein vivoloads should also be carried out. To decidethe best approach in a given clinical situation, in vitroevaluation

involving intact, nucletomized, or degenerated specimens with the

device implanted provide the most realistic option.

The parameters which should be determined are the range of

motion and the neutral zone in the different motion planes, shear

translations, and height changes. These parameters, however, do

not represent the full information about the kinematics. Therefore,

the center of rotation or helical axis could also be important infor-

mation about the load sharing between the different structures of

the spinal segment.

Additional biomechanical tests

A hard implant leads to a high stress concentration on the end-

plate (seen as Modic changes) which may lead to remodeling,

subsidence, or even to failure of the endplate, particularly with

poor bone quality. For this reason, other specific set-ups may be

required to determine the endplate deformations.

One of the goals of nucleus replacement is to re-establish the

physiological strain on the anulus, which is assumed to be one of

the prerequisites in order to maintain healthy tissue. This may beindirectly determined by comparing the bulging of the intact,

nucleotomized, treated disc.

In vitro test with cyclic loading

Depending on the type of device, expulsion or subsidence of the

implant may be a problem. In order to evaluate these types of

biomechanical failures the specimens should be subjected to

cyclic loading as well. Because of degradation of the cadaveric

tissue the testing time may be limited. Thus 100,000 cycles with

possibly exaggerated loads should at least be attempted.

Following this cyclic test or several times during the test secondary

stability tests should be performed as described above.12

Animal experiments

In addition to the tests described above, animal experiments may

be useful to evaluate the efficacy and reliability of nucleus

implants. However, the ability to design a scaled version for ani-

mal implantation, coupled with the validity of the available ani-

mal species, has potential limitations for extrapolating expected

human performance.

CONCLUSION

Nucleus replacement is an exciting technology and may be a

promising alternative to other non-fusion technologies. Many

different ideas are available or in the development stages. Besides

the mechanical challenges presented to the implant, the implant

has to re-establish the physiological biomechanics of a spinal

segment, plus remain in the disc space and not expulse or sub-

side through the endplate. However, the ultimate judge of the

implant is not the biomechanical data, but the clinical outcome.

REFERENCES

1. Stokes, I. A. (1988). Bulging of lumbar intervertebral discs: non-contacting

measurements of anatomical specimens. J Spinal Disord 1(3): 189-93.

2. Brinckmann, P. (1986). Injury of the anulus fibrosus and disc protrusions. An

in vitroinvestigation on human lumbar discs. Spine 11(2): 149-53.

3. Hamby, W. B. and H. T. Glaser (1959). Replacement of spinal intervertebral disc

with locally polymerizing methyl methacrylate: experimental study of effectsupon tissues and report of a small clinical series. J Neurosurg 16(3): 311-3.

4. Nachemson, A. (1962). Some mechanical properties of the lumbar interverte-

bral discs. Bull Hosp Joint Dis 23: 130-43.

5. Carl, A., E. Ledet, et al. (2004).New developments in nucleus pulposus

replacement technology. Spine J 4(6 Suppl): 325S-329S.

6. Fernstrm, U. (1966). Arthroplasty with intercorporal endoprothesis in

herniated disc and in painful disc. Acta Chir Scand Suppl 357: 154-9.

7. Wilke, H. J., S. Kavanagh, et al. (2001). Effect of a prosthetic disc nucleus on

the mobility and disc height of the L4-5 intervertebral disc postnucleotomy.

J Neurosurg 95(2 Suppl): 208-14.

8. Nishimura, K. and J. Mochida (1998). Percutaneous reinsertion of the nucleu

pulposus. An experimental study. Spine 23(14): 1531-8; discussion 1539.9. Okuma, M., J. Mochida, et al. (2000). Reinsertion of stimulated nucleus pul-

posus cells retards intervertebral disc degeneration: an in vitroand in vivo

experimental study. J Orthop Res 18(6): 988-97.

10. Neidlinger-Wilke, C., K. Wurtz, et al. (2005).A three-dimensional collagen

matrix as a suitable culture system for the comparison of cyclic strain and hydro-

static pressure effects on intervertebral disc cells.J Neurosurg Spine 2(4): 457-65

11. Wilke, H. J., F. Heuer, et al. (2006). Is a collagen scaffold for a tissue engi-

neered nucleus replacement capable of restoring disc height and stability in

an animal model? Eur Spine J 15 Suppl 3: S433-8.

12. Wilke, H.-J., K. Wenger, et al. (1998). Testing Criteria for Spinal Implants:

Recommendations for the Standardization ofIn VitroStability Testing of

Spinal Implants. European Spine Journal 7: 148-154.


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Chapter 9 Kinematic Demands ofNucleus ArthroplastyTechnolog

Denis J. DiAngelo, PhD

ASSOCIATE PROFESSORDepartment of Biomedical Engineering and Imaging

The University of Tennessee Health Science Center

Memphis, TN 38138

Brian P. Kelly, PhDASSISTANT PROFESSOR

Department of Biomedical Engineering and Imaging

The University of Tennessee Health Science Center

Memphis, TN 38138

17

KEY POINTS

The instantaneous axis of rotation (IAR) is an important factor

of spinal segment kinematics; however, there is no consensus on

where the IAR of the lumbar disc is during flexion/extension.

Alignment of device IAR and spinal segment IAR is important for

optimal performance of interbody motion preservation devices.

Previous testing of non-compliant interbody motion technolo-

gies demonstrated non-concentric IARs may lead to an over

constrained condition, resulting in failure of the device to

provide adequate motion restoration. Compliant nucleus replacement technologies do not have a pre-

scribed axis of rotation. A new test methodology is proposed for

evaluating the kinematic restorative effect for such devices.


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DEFINITIONS AND TERMINOLOGY

Kinematics:describes the movement between two rigid bodieswith no consideration to the forces involved. Movement of a

body can be described in Cartesian coordinates as having three

orthogonal translations and three rotations about each transla-

tional axis. The motion can be described in two-dimensions

(2D) or three dimensions (3D). For 2D motion, two translations

and one rotation are required. Additional kinematic parameters

can be calculated that describe motion and include the center of

rotation (CR) in 2D, the helical axis of motion (HAM) in 3D, or

their instantaneous components (ICR or IHAM).

Stiffness (inverse of flexibility):the ability of a structure todeform per unit displacement. The typical load-displacement

curve for spinal MSU displays a non-linear relationship having

two different regions: a low load region and a high load region

(Figure 1). As tissue is exposed to increasing displacements,

stiffness is greatly increased and small changes in displacement

induce large load responses.

Coordinate System:a reference system used to define the posi-tion and orientation of a body in space or relative to another body.

Motion Segment Unit (MSU):two adjacent vertebrae andinterconnecting disc and surrounding ligamentous tissues.

Intervertebral Disc:an inner nucleus pulposus core surroundedby an anulus fibrosus tissue.

NUCLEUS ARTHROPLASTY DESIGNS

Various types of nucleus arthroplasty (NA) devices exist which

can be categorized into three groups: Void fillers (NuCore, Spine

Wave; BioDisc, CryoLife), kinematically-constrained mechanical

devices (NUBAC, Pioneer; Regain, Biomet), and load sharing

devices (HydraFlex, Raymedica, LLC; NeuDisc, Replication

Medical; DASCOR, Disc Dynamics).1, 2, 3 Further, each device has

an associated surgical procedure for preparing the nucleus site

and placing the NA device that alters the physical properties of

the treated spinal level. Hence, in addition to studying the biome-

chanical properties and function of NA devices, one should also

understand the impact that the various surgical techniques have

on the stability and function of the treated disc. The influence of

these techniques include facet disruption, bony removal, the sur-

gical approach (anterior versus posterior), affect of an annular

incision (which depressurizes the nucleus), and the amount of

nucleus material removal: micro (associated with discectomies)

or complete. All of these incremental surgical alterations intro-

duce different degrees of instability to the spinal joint that must

be compensated for by the NA device itself.

HOW TO STUDY THE KINEMATICS

OF NUCLEUS ARTHROPLASTY

The nucleus pulposus is a pressurized gelatinous region consist-

ing of proteoglycans (glycosaminoglycans), loose Type II colla-gen fibrils, mineral salts, and water that is surrounded by an

anulus fibrosus structure. Together, the nucleus and anulus dis-

play nonlinear material properties that influence the biomechanics

of a spinal MSU. The goal of nucleus arthroplasty is to restore the

compliant function of the disc and spinal MSU and prevent over-

loading of the adjacent bony and soft tissue structures.

The design rationale for nucleus arthroplasty differs from that of

total disc arthroplasty. Total disc arthroplasty devices are non-

compliant devices designed to restore motion to the degenerative

disc. However, there is no consensus on where the IAR of the lum

bar disc is during flexion/extension activities, with various loca-

tions reported in the literature. Since most total disc replacement

devices (TDR) are mechanical joints, if the IAR of the device does

not coincide with that of the native spine an over constrained con-

dition within the MSU may occur, leading to overloading of adja-

cent structures or failure to provide adequate motion restoration.4

The goal of nucleus arthroplasty is to restore the compliancy of the

native intervertebral disc and recreate a flexible load bearing MSU

STIFFNESS

Displacement

Load

Low Load High Load

18

Figure 1Typical stiffness curve. The curve is generally nonlinear and has

two regions: a low load region and a high load region.

AS TISSUE IS EXPOSED TO INCREASING DISPLACEMENTS,STIFFNESS IS GREATLY INCREASED AND SMALL CHANGES

IN DISPLACEMENT INDUCE LARGE LOAD RESPONSES.


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19

system. A more flexible system serves to reduce the occurrence of

an over constrained, non-mobile condition. Moreover, if the IAR

of the MSU varies, the NA device may dynamically deform to

accommodate the positional changes. An improved method for

evaluating nucleus arthroplasty devices and their ability to restoresegment compliancy and motion is to impose a series of different

kinematic motion profiles and measure their reaction loads. The

different motion profiles force the implanted MSU to adjust to the

prescribed kinematic pattern since the axes of rotation would be

selected such that they are not co-centric with the disc center.

Thus, the reactive forces needed to follow the prescribed motion

represent how well the NA device performs. This chapter discusses

a new approach for studyingin vitro, the capacity of nucleus

arthroplasty to restore the kinematic of a spinal MSU in a human

cadaveric model. This new testing methodology is a paradigm shift

from the conventional displacement or load control methods.5

CURRENT BIOMECHANICAL TESTING METHODS

Limited biomechanical studies exist that evaluate NA devices.

Although the conventional testing method of applying a pure or

constant bending moment across the spinal construct and meas-

uring the motion response to that loading condition can be

done, there are significant limitations with this testing method-

ology. Physiologically, the spine is not loaded with a constant

bending moment, but rather experiences a moment distribution

that varies across all spinal levels as you go down the spine.

Although pure moment methods provide a standard approach

for comparing different lumbar spinal devices and may be

acceptable for testing fusion instrumentation,5 it is not well suited

for studying any type of spinal device that permits motion and/or

has a variable stiffness or modulus (i.e., is not a rigid metal struc-

ture). Alternatively, eccentric compressive load test methods havebeen used to study both fusion and non-fusion spinal instrumen-

tation.6 A compressive load is typically applied eccentric to the

long axis of the spine causing the spine to flex or extend under

a combined compressive load and bending moment. Using this

testing method, a more physiologic response in the rotational

involvement of each MSU occurs throughout the lumbar spine

(Figure 2).7, 8, 9, 10 However, even though the eccentric loading

method induces a physiologic rotational response across the

intact lumbar spine, this method may not have the sensitivity to

study the compliant properties and kinematic requirements of

NA devices or different disc conditions. With either load controlor displacement control methods limited information is available

about the loads acting on the disc and/or NA device, or the

amount of load sharing that occurs between the NA and adjacent

supporting structures as the MSU moves through a functional

range of motion.

NEW KINEMATIC TESTING PROTOCOL FOR

STUDYING COMPLIANT STRUCTURES

Human joints move under a state of minimum energy; they fol-

low the path of least resistance. A new testing protocol is pro-

posed that involves prescribing a known kinematic input to a

spinal MSU and measuring the capacity of the intact MSU to

accommodate the motion. The effects of changing the MSU

properties via surgery (nucleotomy) or placement of a nucleus

arthroplasty device, changes the effort or work required to move

the altered spine condition through a prescribed motion path

relative to the intact spine condition. The closer the loading

mechanics of the altered spine are to the intact spine condition,

the better the likelihood the device will restore the native prop-

erties. Further, the kinematic path can be a simple rotation about

a fixed point in space, or a coupled movement (displacement and

rotation) along a path.

Combined MSU Flexion/ExtensionRotation of the Lumbar Spine

0 5 10 15 20

L1-L2

L2-L3

L3-L4

L4-L5

L5-S1

MotionS

egmentUnitLevel

Range of Rotation (Degrees)

In Vitro

In Vivo (4 sources*)

Figure 2In VitroversusIn VivoMSU rotations of the lumbar spine. 6, 7, 8, 9


21/55

A custom-designed spine robot (Figure 3) was used that consisted

of four programmable degrees of freedom that can each be inde-

pendently operated under displacement control, force feedback

control, and combinations thereof.11 Using the spine robot, the

kinematic profile of an intact MSU can be programmed to follow

a specified path or to rotate about a fixed point in space (Figure

4). More advanced kinematic analyses are possible that map out

the motion response to a given multi-directional force profile.

A preliminary series of kinematic tests were performed on the

spine robot to study the flexion and extension mechanics of three

lumbar MSUs. Each specimen was tested in three different spine

conditions: the intact harvested condition, post-nucleotomy con-

dition, and post-implanted nucleus arthroplasty condition. The

hydrated HydraFlex device (Raymedica, LLC) was used for the

implanted condition. The orientation of each MSU in a neutral

alignment was measured on a radiograph (Figure 5A) and used

to establish the orientation of the MSU (Figure 5B) when mounted

in the spine robot (Figure 5C). The MSUs were tested under three

fixed points of rotation along the center line of the disc in the ante-

rior-posterior (A-P) direction: 1) the mid point of the disc (C), 2)

half way between the mid-point and anterior aspect of the disc

(A), and 3) half way between the mid-point and posterior aspect o

the disc (P) (Figure 5B). The MSUs were rotated about the desig-

nated fixed points of rotation until a target bending moment of

8Nm of flexionextension was reached or the shear or compres-

sive forces exceeded 400N. For all test conditions, MSU axial force

(+Fz net MSU tissue tension, -Fz net MSU tissue compres-sion), A-P shear force (+Fx net MSU posterior shear, -Fx net

MSU anterior shear), sagittal rotation (+y flexion, -y extension)

and sagittal bending moment were measured and compared using

a one-way ANOVA (P=0.05).

20

Figure 3Programmable

multi-axis

Spine Robot.

Figure 4Schematic of the

Spine Robot used

to move a verte-

bral body about a

fixed center of

rotation relative

to an adjacent

vertebral body.

Figure 5A) Radiograph showing neutral

alignment of lumbar spine MSU.

B) Maintenance of neutral align-

ment of MSU in mounting potsand location of fixed points of

rotation. C) Potted MSU specimen

mounted in Spine Robot.

A B C


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21

PRELIMINARY FINDINGS USING KINEMATIC

TESTING PROTOCOL

The mean values of the axial load, shear force, and MSU rotation

for the three different fixed axes of rotation conditions were cal-

culated and graphed (Figure 6). Using data for the harvestedspine condition, significant differences in the MSU rotation, axial

force, and shear force values occurred between the three different

points of rotation (P, C, A). MSU rotations were significantly dif-

ferent between all points of rotation in flexion and extension,

except between points C and P in flexion. The axial forces were

significantly different between P versus A and P versus C in flex-

ion. A similar trend occurred with the axial force values during

extension but the differences were not significant (likely due to

the small sample size). Shear forces were significantly different

between points P versus A and P versus C during flexion. In

extension the shear forces were significantly different between all

points of rotation (P versus A, P versus C, and A versus C). In

general, for the harvested intact MSU, when rotated in flexion

about the mid-point of the disc (location C), MSU posterior soft

tissue tension and resistance to posterior directed shear provided

the stabilization effect. When the point of rotation was shifted

posterior (location P), MSU anterior tissue compression and

resistance to anterior directed shear provided the stabilization

effect. Shifting the rotation point anterior (location A) had mini-

mal effect on tissue stabilization response, but decreased seg-

mental rotation compared to point C. The data demonstrates

that the point about which a single MSU is rotated has a signifi-

cant effect on the rotational range of motion as well as the soft

tissue stabilization response.

Compressive Force (Fz): Flexion

-350

-250

-150

-50

50

150

250

Harvested Nucleotomy Implant

Spine Conditions

For

ce A

CP

Compressive Force (Fz): Extension

-350

-250

-150

-50

50

150

250


Spine Conditions

For

ce

A

CP

Shear Force (Fx): Flexion

-250

-200-150

-100

-50

0

50

100


Spine Conditions

Force

A

C

P

Shear Force (Fx): Extension

-250

-200-150

-100

-50

0

50

100


Spine Conditions

A

C

P

-8

-3

2

7

12


Spine Conditions

Degrees

DegreesA

C

P

-8

-3

2

7

12


Spine Conditions

A

C

P

Rotation (0y) Flexion Rotation (0y) Extension

Figure 6Comparison of the three fixed points of rotations. Mean values of the axial compressive load, A-P shear force, and MSU rotation

during flexion and extension.


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Compressive Force (Fz): Flexion

-350

-250

-150

-50

50

150

250

A C P

Point of Rotation

Harvested

NucleotomyImplant

Compressive Force (Fz): Extension

-350

-250

-150

-50

50

150

250

A C P

Point of Rotation

Harvested

NucleotomyImplant

Shear Force (Fx): Flexion

-250

-200

-150

-100

-50

0

50

100

A C P

Point of Rotation

Harvested

Nucleotomy

Implant

Shear Force (Fx): Extension

-250

-200

-150

-100

-50

0

50

100

A C P

Point of Rotation

Harvested

Nucleotomy

Implant

-8

-6-4-202468

101214

A C P

Point of Rotation

Harvested

Nucleotomy

Implant

-8

-6-4-202468

101214

A C P

Point of Rotation

Harvested

Nucleotomy

ImplantDegrees

Degrees

Force

Force

For

ce

Force

Rotation (0y) Flexion Rotation (0y) Extension

22

When comparing between the different spine conditions (har-

vested, nucleotomy, and implanted) there was a trend in the data

that demonstrated the nucleotomy increased the MSU rotation,

altered the load response and appears to have more variation in

response (e.g. less stable). Following implantation of the

HydraFlex device, the MSU rotation returned to the harvested

condition and had less variation compared to the denucleated

condition (Figure 7). Alterations to the harvested MSU via the

nucleotomy or NA implant tended to reduce/lessen the A-P

shear forces, with the denucleated condition having a large varia-

tion. At posterior point P, the denucleated and implanted states

demonstrated less tissue compression. In extension, the implant

condition tended to reduce the compressive response of the

posterior elements at Point C and P.

In general, for all test conditions, the rotational range of motion

and the tissue loading response differed depending on the selected

kinematic axis of rotation (point C, P or A). Denucleating the

MSU led to more rotation and increased variation in the test data,

indicative of a less stable/predictable response. After implantation

of a hydrated HydraFlex device, variation was reduced and the

response profile trended more towards the intact state for all test

points. However, the small sample size and large variation noted

in the denucleated specimens may have limited significance from

occurring. Increasing the sample size should further confirm sig-

nificant differences between the spine conditions or points of

rotation, but the test method utilized demonstrates the impor-

tance of understanding the constraints of a test setup and how

results may vary as the constraints are changed.

Figure 7Comparison of the three different spine conditions at specific points of rotations. Mean values of the axial compressive load, A-P shear

force, and MSU rotation during flexion and extension.


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CONCLUSION

As new technologies evolve, the methods utilized to analyze

their biomechanical performance must evolve. The preliminary

data from this study demonstrated that utilizing a robot to

change the location of the MSU axis of rotation had an effect

on the kinematic response of the harvested, denucleated and

implanted MSU. Since thein vivoMSU axis of rotation is not

believed to be a single point of rotation and since compliant

nucleus replacement technologies do not have a prescribed axis

of rotation, evaluating the kinematic response at multiple loca-

tions of rotation may more effectively characterize the restora-

tive effect of these technologies compared to more traditional

test methods. Additional testing is being performed to furtherevaluate the utility of this method as well as evaluating the

restorative effect of the implant on a path that matches the nat-

ural motion of the MSU. Overall, this method looks very prom-

ising for thoroughly understanding the kinematic response of

Nucleus Arthroplasty technologies.

ACKNOWLEDGMENT

Elizabeth Sander and Nephi Zufelt for assistance with the bio-

mechanical tests and data processing.

REFERENCES

1. Davis R. J., and Girardi F. (eds.) 2006. Nucleus Arthroplasty in Spinal Care:

Book 1 Fundamentals. Minneapolis (MN): Raymedica, LLC.

2. Di Martino A., Vaccaro A., Lee J., et al. Nucleus pulposus replacement: basic

science and indications for clinical use. Spine. 2005;30 (suppl 16):16-22.

3. Klara P., Ray C. Artificial nucleus replacement: clinical experience. Spine.

2002;27:1374-77.

4. DiAngelo DJ, Foley KT, Morrow B, Kiehm KJ, Gilmour L, The Effects of Over-

Sizing of a Disc Prosthesis on Spine Biomechanics, Proceedings of the Sixth

Annual Spine Arthroplasty Society Meeting, 2006.

5. Goel VK, Wilder DG, Pope MH, and Edwards WT. Biomechanical testing of

the spine. Load controlled versus displacement-controlled analysis. Spine

1995;20:2354-7.

6. DiAngelo DJ, Scifert JL, Kitchel S, Cornwall GB, McVay BJ, Bioabsorbable

Anterior Lumbar Plate Fixation in Conjunction with Cage Assisted AnteriorInterbody Fusion, J Neurosurg. Nov;97(4 Suppl):447-55, 2002.

7. Dvorak J., Panjabi M.M., Chang D.G., et al. Functional radiographic diag-

nosis of the lumbar spine: flexion/extension and lateral bending. Spine 16

(5):562-71, 1989.

8. Pearcy M.J., Portek I., and Shepherd J.. Three-dimensional x-ray analysis of

normal movement in the lumbar spine. Spine 9 (3):294, 1984.

9. Froning E.C. and Frohman B. Motion of the lumbosacral spine after laminec-

tomy and spine fusion: Correlation of motion with the result. Journal of Bone

and Joint Surgery 50-A (5):897-918, 1968.

10. White A.A. and Panjabi M.M. Clinical biomechanics of the spine,

Philadelphia:J.B. Lippincott Co., 1990.

11. Kelly BP, DiAngelo DJ, Foley KT, Design Of A Multi-Axis Programmable

Spine Robot For The Study Of Multi-Body Spinal Biomechanics, Proceedingsof the 32th Annual Meeting of the Cervical Spine Research Society, Dec. 2004.

AS NEW TECHNOLOGIES EVOLVE, THE METHODS UTILIZED TO ANALYZE THEIR

BIOMECHANICAL PERFORMANCE MUST EVOLVE. THE PRELIMINARY DATA FROM THIS

STUDY DEMONSTRATED THAT UTILIZING A ROBOT TO CHANGE THE LOCATION OF

THE MSU AXIS OF ROTATION HAD AN EFFECT ON THE KINEMATIC RESPONSE OF

THE HARVESTED, DENUCLEATED AND IMPLANTED MSU.


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Chapter 10 Device Stiffness vs.Load-Sharing with NucleusArthroplastyDevices

Brian P. Beaubien, BME, MS

LEAD ENGINEERMidwest Orthopaedic Research Foundation

and the Gustilo Medical Education Center

Minneapolis, MN 55415

Andrew L. Freeman, BME, MSLEAD ENGINEER

Midwest Orthopaedic Research Foundation

and the Gustilo Medical Education Center

Minneapolis, MN 55415

KEY POINTS

The engineering term elastic modulus is used to characterize

the uni-axial stress-strain response of linear-elastic materials.

Biologic tissues and advanced polymeric biomaterials are typically

not linear-elastic and the stress-strain relationship is complex.

Physiologic interactions significantly impact the response char-

acteristics of devices made from advanced biomaterials.

Cadaver evaluation of polymeric devices demonstrated differ-

ences in the compressive load transfer through a device based on

bulk device properties (i.e. device modulus) as well as changes in

the level of constraint provided by physiologic tissue.

It is proposed that device design differences, as well as physiologic

interactions, require characterization of a Nucleus Arthroplasty

devices load-sharing properties in simulated use conditions,

which provides a more thorough characterization of performance

with increased clinical understanding.


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INTRODUCTION

The engineering term modulus is perhaps the most widelyused descriptor of the mechanical properties of a material.For the most basic conditions and materials, the modulus simply

relates the amount of stress and strain seen in a material and is

useful in predicting and describing the load-deformation behav-

ior. However, with biological tissues and advanced biomaterials

the stress-strain response is more complex and is dependent on

the surrounding environment. This chapter aims to highlight

important considerations in describing the stress-strain relation-

ship of tissues and biomaterials and to discuss the effects of these

properties, implant design and spine mechanics on thein vitro

performance of the native nucleus pulposus, anulus fibrosus and

Nucleus Arthroplasty (NA) devices.

MATERIAL PROPERTIES:

DEFINITIONS AND EXAMPLES

The modulus of elasticity (E)

The modulus of elasticity is used in Hookes Law (Robert

Hooke, 1635-1703) to describe a linear relationship between

stress and strain:

In this equation, stress (, force per unit area) and strain (, elon-

gation per unit length) are linearly related by the modulus of

elasticity (E), which is often called the elastic modulus or Youngs

Modulus (Thomas Young, 1773-1829). The elastic modulus is

measured in the linear region of the stress-strain curve and represents the slope of the curve. Each material generally has a unique

elastic modulus (Figure 1). In general, a low modulus equates to

a softer, more easily deformable material.

The elastic modulus provides a simple but powerful relationship

between the stress and recoverable strain in materials as diverse

as bone, rock and steel. However, this limited version of Hookes

Law accurately describes only tension and compression behavior

of linearly elastic materials in a uni-axial direction. It should be

noted that modulus may be direction-dependent or anisotropic.

This is important to consider for composite materials (e.g. car-

bon fiber implants, reinforced concrete) as well as biologic tissues

with internal structured arrangement (e.g. tendon, ligament).

Tangent modulus

For other materials, such as many used in NA devices, the rela-

tionship between stress and strain is nonlinear. In other words, at

small strains many polymers and biological tissues exhibit a rela-

tively low elastic modulus, whereas the same materials exhibit a

higher elastic modulus at higher strains (Figure 2). Two or more

Figure 1Diagram of a simple tension test (left) and typical stress-strain curves and elastic modulus values for materials

commonly encountered in orthopaedics.


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distinct regions may be observed, but the transition is generally

gradual, and the stress-strain relationship can thus be described

as nonlinear. For non-linear materials, the modulus is deter-

mined by the slope of a line tangent to the stress-strain curve at a

given point; this is called the tangent modulus. The tangent mod-ulus provides an intuitive measure of a materials non-linear

stress-strain response, analogous to the elastic modulus.

Viscoelasticity: Modulus dependence with time

For biological tissues and advanced polymeric biomaterials, the

stress-strain relationship is usually dependent upon the duration

of loading. When these materials are exposed to a given stress,

the resulting strain increases with time at a rate dependent upon

the material; this phenomenon is known as creep. Similarly, an

applied strain results in a steadily decreasing amount of stress

over time (Figure 3), and this stress relaxation may or may not

proceed at a rate proportional to creep.1 Materials exhibiting

time-dependent material properties are considered viscoelastic.

An important consideration in defining the elastic modulus

for a viscoelastic material is the rate and duration of loading. The

stress-strain-time relationship can be simplified from static tests

by describing the apparent modulus upon immediate loading or

deformation (instantaneous modulus) and after a long duration

of loading or deformation (equilibrium modulus). It is important

to recognize the various methods for calculating modulus and

the potential variation in outcomes with viscoelastic materials.

Fluid flow

For biologic tissue and some biomaterials (e.g. hydrogels), fluid

flow in and out of the material is important in determining the

time-dependent properties of the material. For example, the load

bearing capacity of the nucleus is attributed to its hydration

level.2 However, as hydration levels change as a result of mechan-

ical pressure, the permeability of the anulus and endplates allows

an outflow of water, altering the mechanical response of the

disc.3,4 Thus, the disc response to load is viscoelastic in nature.

Figure 2Stress-strain curve for a typical soft tissue or nonlinearly-elastic biomaterial.

Tangent lines for points A and B are indicated and the tangent modulus

calculation is shown for Point B (EB).

Figure 3Results for the stress-relaxation response of a porcine spinal ligament. An imme-

diate strain step was applied and held (top) and the resulting stress response was

observed (bottom). Stress-relaxation experiments are performed to explore the

stress-strain-time relationship.

FOR BIOLOGICAL TISSUES AND ADVANCED POLYMERIC

BIOMATERIALS, THE STRESS-STRAIN RELATIONSHIP IS

USUALLY DEPENDENT UPON THE DURATION OF LOADIN


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Over a 24 hour period the balance of mechanical and osmotic

pressure causes the disc volume and height to decrease such that

a ~20mm decrease in height is seen over the entire length of the

spine.5 In the healthy disc this change is balanced by swelling

during the discs nightly intake of water while the spine isunloaded.6 Factors governing the rate of fluid flow include

osmotic and physical pressure gradients, porosity, and the avail-

ability and viscosity of the surrounding fluid. While inherent

viscoelastic properties of a hydrogel or soft tissue and fluid-flow

appear similar in the discs response to loading, the respective

contributions of these variables to material properties are at least

partially independent.7

Radial deformations and the effect of constraint

The three-dimensional behavior of a material is only partiallydescribed by the uni-axial stress-strain and time dependency.

Equally important is the behavior of a material in a plane per-

pendicular to the applied force, or stated otherwise, the radial

direction. For the simple bar shown in Figure 1, the radial defor-

mation can be described as a proportion of the axial strain as

shown in the following relationship:

where is a constant known as Poissons ratio. Poissons ratio is

important not only in describing radial deformation, but also

the amount of volume change exhibited by a material for a given

load. The unit change in volume (V) of a cube of solid material

can be described using the following equation:

Note that for a Poissons ratio of 0.5, this change in volume is zero.

Such a material is considered incompressible. Water is the most

common example of a nearly incompressible material. Water can

easily undergo a shape change, but imposing a volume change is

nearly impossible if the water is constrained radially. Biological tis-

sues, rubber and other polymeric materials are examples of materi-

als that are often considered nearly incompressible with sufficient

radial constraint.

The apparent modulus of a water-containing material is depend-

ent upon the rigidity and permeability of its physical constraints.

If constrained radially in an undeformable and impermeable con-

tainer, such a material would behave in a relatively incompressible

manner and could thus withstand high loads with negligible

deformation. If the constraining container were made permeable,

then volume change of the same material would occur over time

as water seeped from the material. In a permeable container, the

material would appear to have a high modulus under rapid

dynamic loading, but would have a relatively low apparent modu-

lus after many hours of loading as the water content decreased

with time. Stress or time-dependent deformation of the con-

straining container would similarly result in decreased rigidity.

This phenomenon is illustrated by the varying tangent modulus

of a hydrogel material subjected to various constraints (Figure 4).

Thus, material modulus is dependent on both the constraint

rigidity and the constraint permeability for water-containing

materials such as cartilage, the nucleus pulposus, the anulus fibro-

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Nucleus Arthroplasty Volume II