Lorne Gale Thesis Lubrication

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Biotribological Assessment for

Artificial Synovial Joints: The Roleof Boundary Lubrication

Doctorate of Philosophy

in

Biomedical Engineering

by

LORNE RGALE,BE(Mech, Hons)

Thesis submitted for the degree of Doctor of Philosophy,

School of Engineering Systems,

Institute of Health and Biomedical Innovation (IHBI),

Queensland University of Technology, Brisbane

2007


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Abstract

Biotribology, the study of lubrication, wear and friction within the body, has

become a topic of high importance in recent times as we continue to encounter

debilitating diseases and trauma that destroy function of the joints. A highly

successful surgical procedure to replace the joint with an artificial equivalent

alleviates dysfunction and pain. However, the wear of the bearing surfaces in

prosthetic joints is a significant clinical problem and more patients are surviving

longer than the life expectancy of the joint replacement. Revision surgery is

associated with increased morbidity and mortality and has a far less successful

outcome than primary joint replacement. As such, it is essential to ensure that

everything possible is done to limit the rate of revision surgery. Past experience

indicates that the survival rate of the implant will be influenced by many

parameters, of primary importance, the material properties of the implant, the

composition of the synovial fluid and the method of lubrication. In prosthetic

joints, effective boundary lubrication is known to take place. The interaction of the

boundary lubricant and the bearing material is of utmost importance. The identity

of the vital active ingredient within synovial fluid (SF) to which we owe the nearfrictionless performance of our articulating joints has been the quest of researchers

for many years. Once identified, tribo tests can determine what materials and more

importantly what surfaces this fraction of SF can function most optimally with.

Surface-Active Phospholipids (SAPL) have been implicated as the bodys natural

load bearing lubricant. Studies in this thesis are the first to fully characterise the

adsorbed SAPL detected on the surface of retrieved prostheses and the first to

verify the presence of SAPL on knee prostheses.

Rinsings from the bearing surfaces of both hip and knee prostheses removed from

revision operations were analysed using High Performance Liquid

Chromatography (HPLC) to determine the presence and profile of SAPL. Several

common prosthetic materials along with a novel biomaterial were investigated to

determine their tribological interaction with various SAPLs. A pin-on-flat

tribometer was used to make comparative friction measurements between the

various tribo-pairs. A novel material, Pyrolytic Carbon (PyC) was screened as a


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potential candidate as a load bearing prosthetic material. Friction measurements

were also performed on explanted prostheses.

SAPL was detected on all retrieved implant bearing surfaces. As a result of the

study eight different species of phosphatidylcholines were identified. The relative

concentrations of each species were also determined indicating that the unsaturated

species are dominant. Initial tribo tests employed a saturated phosphatidylcholine

(SPC) and the subsequent tests adopted the addition of the newly identified major

constituents of SAPL, unsaturated phosphatidylcholine (USPC), as the test

lubricant. All tribo tests showed a dramatic reduction in friction when synthetic

SAPL was used as the lubricant under boundary lubrication conditions. Some tribo-

pairs showed more of an affinity to SAPL than others. PyC performed superior to

the other prosthetic materials. Friction measurements with explanted prostheses

verified the presence and performance of SAPL.

SAPL, in particular phosphatidylcholine, plays an essential role in the lubrication

of prosthetic joints. Of particular interest was the ability of SAPLs to reduce

friction and ultimately wear of the bearing materials. The identification and

knowledge of the lubricating constituents of SF is invaluable for not only the future

development of artificial joints but also in developing effective cures for several

disease processes where lubrication may play a role. The tribological interaction of

the various tribo-pairs and SAPL is extremely favourable in the context of reducing

friction at the bearing interface. PyC is highly recommended as a future candidate

material for use in load bearing prosthetic joints considering its impressive

tribological performance.

Keywords

SAPL, orthopaedics, biotribology, boundary lubrication, prosthetics, total joint

replacement, PC, USPC, PyC, Pyrolytic Carbon, surfactant, synovial fluid, SF,

arthritis, joint disease, cartilage, artificial joints, BL.


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List of Abbreviations

SAPL - Surface Active Phospholipid

TJR - Total Joint Replacement

PC - PhosphatdiylCholine

USPC - Unsaturated Phosphatdiylcholine

SPC - Saturated Phosphatdiylcholine

BL - Boundary Lubrication

HPLC - High Performance Liquid Chromatography

SF - Synovial Fluid

GAG - Glycosaminoglycan

HA - Hyaluronic Acid

DPPC - Dipalmitoyl Phosphatidylcholine

DLPC - Dilinoleoyl Phosphatidylcholine

PLPC - Palmitoyl Linoleoyl Phosphatidylcholine

POPC - Palmitoyl Oleoyl Phosphatidylcholine

DOPC - Dioleoyl Phosphatidylcholine

SLPC - Stearoyl Linoleoyl Phosphatidylcholine

PSPC - Palmitoyl Stearoyl Phosphatidylcholine

OSPC - Oleoyl Stearoyl Phosphatidylcholine

BSA - Bovine Serum Albumin

- Coefficient of friction

OA - Osteo Arthritis

RA - Rheumatoid Arthritis

PyC - Pyrolytic Carbon

LGP - Lubricating Glyco ProteinLTI carbon - Low Temperature Isotropic carbon

GCS - Glucosamine & Chondroitin Sulfate

NSAIDS - Non-Steroidal Anti-Inflammatory Drugs

VS - Visco Supplementation


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Glossary of Terms

adsorption: A mechanism of attaching to a surface by chemical or

physical bonding.amphipathic: One end of a molecule has an affinity for the phase in which

it is in, the other end is repelled by the same phase.

arthrocentesis: The surgical puncture and aspiration of a joint.

arthrology: That part of anatomy which treats of joints.

asperities: roughness of a surface , highest points.

boundary lubrication: Lubrication where there is solid-to-solid contact of the

sliding surfaces.

chondrocyte: The cellular component of the cartilage matrix

contact angle: The angle subtended at the edge of a droplet at the triple

point.

colloid: Microscopic particles suspended in some sort of liquidmedium.

cytokine: Mediator of inflammation.

detritus: A mass of substances worn off from solid bodies by

attrition, and reduced to small portions.

esterified: A chemical reaction in which two chemicals (typically an

alcohol and an acid) form an ester as the reaction product.

glycosaminoglycan: Any of a class of polysaccharides derived from hexosamine

that form mucins when complexed with proteins: formerly

called mucopolysaccharide.

HPLC: HPLC is used to separate components of a mixture by using

a variety of chemical interactions between the substancebeing analysed and the chromatography column.

hyaluronic acid: A mucopolysaccharide serving as a viscous medium in the

tissues of the body and as a lubricant in joints: a GAG.

hyaluronidase: An enzyme that catalyses the breakdown of hyaluronic acid

in the body, thereby increasing tissue permeability to fluids.

Hydrodynamic Lubrication where the sliding surfaces are separated by

lubrication: a wedge of fluid.

hydrophilic: Highly compatible with the aqueous phase, or water-

loving.

hydrophobic: Repels the aqueous phase, or water-hating .

lamellar body: Layered structure (A storage form for surfactant).lipid: Any of a group of organic compounds, including the fats,

oils, waxes, sterols, and triglycerides, that are insoluble in

water but soluble in nonpolar organic solvents, are oily to

the touch, and together with carbohydrates and proteins

constitute the principal structural material of living cells.

lipase: Any of a class of enzymes that break down lipids.

lubricin: A glycoprotein implicated as the boundary lubricant for the

synovial joint.

lyophilic: Characteristic of a material that readily forms a colloidal

suspension.


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mucin: Any of a class of glycoproteins found in saliva, gastric juice,

etc., that form viscous solutions and act as lubricants or

protectants on external and internal surfaces of the body.

non-Newtonian: As the rate of shear increases, viscosity decreases; as the

rate of shear decreases, viscosity increases.

osteoarthritis: A degenerative joint disease where the initiating event canbe joint trauma, acute or repetitive.

proteolipid Any of a class of lipid-soluble proteins.

proteoglycan: Any of various mucopolysaccharides that are bound to

protein chains in covalent complexes and occur in the

extracellular matrix of connective tissue.

rheumatoid arthritis: A degenerative disease which is predominantly

inflammatory in nature and origin.

Ringer's solution: An aqueous solution of the chlorides of sodium, potassium,

and calcium that is used topically as a physiological saline.

surfactant: A substance that can effectively modify the surface energy

of an interface.synoviocyte: The cellular component of the synovial membrane.

synovium: Another term for the synovial membrane.

tribology: The science and technology of surfaces that are in contact

and move in relation to each other.

tribo-pair: The name given to the two materials that slide over each

other in a lubricated system.

triple point: The point where solid, liquid and air all meet.

trypsin: An enzyme capable of breaking down proteins .

turbostratic: A type of crystalline structure where the basal planes have

slipped sideways relative to each other, causing the spacing

between planes to be greater than ideal.

zwitterion: A molecule in which each end carries a different charge

(positive or negative)to produce a charge dipole.


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List of Publications and Manuscripts

Refereed Publications

[1] GALE, L.R., R. COLLER, D.J. HARGREAVES, B.A. HILLS, and R.

CRAWFORD (2007): 'The role of SAPL as a boundary lubricant in prosthetic

joints', Tribology International, 40(4), pp. 601-606

[2] GALE, L.R., Y. CHEN, B.A. HILLS, and R.W. CRAWFORD (2006):

'Boundary lubrication of joints: Characterisation of Surface-Active Phospholipids

found on retrieved implants',Acta Orthopaedica, 78(3), pp. 309-314

[3] GALE, L.R., R.W. CRAWFORD, D.J. HARGREAVES and J. KLAWITTER

(2006): 'Boundary lubrication of Pyrolytic Carbon with Surface Active

Phospholipids: Tribological Assessment for Artificial Joints',Acta Orthopaedica,

Under Revision, pp.

[4] L.R. GALE , GUDIMETLA, P., Y. CHEN, R. CRAWFORD and D.J.

HARGREAVES (2007): ' Tribological Testing of Saturated and Unsaturated

Surface Active Phospholipids: Implications for artificial joints', Proceedings of the

Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine,

Submitted, pp.

Refereed Conference Publications

[1] GALE, L.R., Y. CHEN, B.A. HILLS, and R.W. CRAWFORD (2005):

'Boundary Lubrication of Synovial Joints: Characterisation of the Lubricant', 12th

International Conference on Biomedical Engineering ICBME 2005. Singapore,

2005, pp. 3A4-14.

[2] GALE, L.R., B.A. HILLS, R.W. CRAWFORD, and J. KLAWITTER (2005):

'Tribological Evaluation of Pyrolytic Carbon and Surface Active Phospholipid for

Artificial Joints', 12th International Conference on Biomedical Engineering

ICBME 2005. Singapore, 2005, pp. 3A4-12.


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Table of Contents

Abstract ......................................................................................................................... iii

List of Abbreviations.......................................................................................................v

Glossary of Terms ........................................................................................................ vii

List of Publications and Manuscripts .............................................................................ix

Table of Contents ...........................................................................................................xi

Statement of Original Authorship .................................................................................xv

Acknowledgements ......................................................................................................xvi

List of Figures ............................................................................................................ xvii

List of Tables.................................................................................................................xx

Chapter 1 Introduction 1

1.1 Description of Scientific Problems Investigated.......................................................1

1.2 Overall Objectives of the Study ................................................................................3

1.3 Specific Aims of the study ........................................................................................4

1.4 Account of Scientific Contribution Linking the Scientific Papers............................5

Chapter 2 Literature Review - Biotribology 7

2.1 Tribological studies of the performance of natural synovial joints.........................10

2.1.1 Overview - Lubrication of the Diarthrodial Joint: Search for the

Lubricating Factor.......................................................................................................12

2.2 Tribological aspects of prostheses...........................................................................16

2.3 Summary .................................................................................................................18

Chapter 3 Literature Review Anatomy & Physiology of Diathrodial Joints 19

3.1 Anatomy of the Synovial Joint................................................................................19

3.2 Articular (Hyaline) Cartilage...................................................................................21

3.2.1 The Articular Surface.........................................................................................22

3.2.2 Articular Cartilage Matrix .................................................................................23

3.2.3 Response to load ................................................................................................27

3.3 Synovial Membrane ................................................................................................28

3.3.1 The Synoviocytes...............................................................................................29

3.3.2 Removal of Substances from the Joint Space....................................................30


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3.3.3 The Synovium in Disease ................................................................................. 30

3.4 Synovial Fluid......................................................................................................... 30

3.4.1 Synovial Fluid Composition ............................................................................. 31

3.4.2 Production of Synovial Fluid ............................................................................ 34

3.4.3 Functions of Synovial Fluid.............................................................................. 35

3.4.4 Rheology of Synovial Fluid .............................................................................. 36

3.4.5 Synovial Fluid Lipids........................................................................................ 37

3.4.6 Synovial Fluid in Disease ................................................................................. 38

3.5 Osteoarthritis........................................................................................................... 38

3.5.1 Treatments of Osteoarthritis.............................................................................. 41

3.6 Total Artificial Joint Replacement.......................................................................... 43

3.6.1 Artificial joint failure ........................................................................................ 45

3.6.2 Biomaterials ...................................................................................................... 46

3.6.2.1 Pyrolytic Carbon ..................................................................................... 47

3.7 Summary................................................................................................................. 53

Chapter 4 Literature Review Lubrication of Joints 55

4.1 Physical Science of Lubrication, Friction and Wear .............................................. 59

4.1.1 Fluid-Film Lubrication...................................................................................... 65

4.1.2 Boundary Lubrication ....................................................................................... 68

4.1.3 Mixed Lubrication............................................................................................. 71

4.1.4 Wear .................................................................................................................. 71

4.2 Natural Joint Lubrication: A Review...................................................................... 73

4.2.1 Experimental techniques and apparatus used to determine the lubrication

of joints ...................................................................................................................... 75

4.2.2 Fluid-Film Models ............................................................................................ 80

4.2.2.1 Hydrodynamic Lubrication ..................................................................... 80

4.2.2.2 Weeping Lubrication............................................................................... 81

4.2.2.3 Elastohydrodynamic Lubrication ............................................................ 83

4.2.3 Mixed Lubrication Models................................................................................ 84

4.2.3.1 Osmotic Lubrication................................................................................ 86

4.2.3.2 Squeeze-Film Lubrication ....................................................................... 86

4.2.3.3 Boosted lubrication ................................................................................. 884.2.4 Boundary Lubrication Models .......................................................................... 89


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4.2.5 The Search for the Boundary Lubricant ............................................................90

4.2.5.1 Enzyme Studies........................................................................................93

4.2.5.2 Lubricating Glycoprotein.........................................................................94

4.2.5.3 Lipids .......................................................................................................95

4.2.5.4 Surface-active Phospholipid ....................................................................97

4.3 Artificial Joint Lubrication: A Review....................................................................98

4.3.1 Fluid Film Models ...........................................................................................101

4.3.2 Boundary Lubrication Models .........................................................................102

4.3.3 Mixed Lubrication Models ..............................................................................103

4.3.4 Tribological Studies for total joint replacements.............................................104

4.4 Summary ...............................................................................................................107

Chapter 5 Literature Review Boundary Lubrication for Artificial Joints 109

5.1 Surface Chemistry .................................................................................................111

5.1.1 Surfaces and Surface Energy ...........................................................................111

5.1.2 Hydrophobic vs Hydrophilic ...........................................................................112

5.1.3 Surface Tension and its Measurement .............................................................113

5.1.4 The Young Equation ........................................................................................113

5.1.5 The Contact Angle ...........................................................................................115

5.1.6 Surfactants .......................................................................................................115

5.1.7 Electrical Charge..............................................................................................116

5.1.8 Adsorption .......................................................................................................116

5.2 Tribochemistry ......................................................................................................117

5.3 Surfactants for Boundary Lubrication ...................................................................118

5.3.1 Lubrication via Surfactants ..............................................................................119

5.3.2 Biological Surfactants......................................................................................120

5.3.3 Types of Lipids ................................................................................................122

5.3.4 Phospholipid Analysis .....................................................................................126

5.3.5 Adsorption in Biology .....................................................................................126

5.3.6 Biological Surfactants and Lubrication ...........................................................127

5.4 Boundary Lubrication via SAPL...........................................................................128

5.4.1 SAPL and Wear in artificial joints...................................................................130

5.5 Summary ...............................................................................................................131


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Chapter 6 Scientific Paper I - The Role of SAPL as a Boundary Lubricant in

Prosthetic Joints 133

Chapter 7 Scientific Paper II Boundary lubrication of Pyrolytic Carbon with

Surface Active Phospholipids: Tribological Assessment for

Artificial Joints 141

Chapter 8 Scientific Paper III Boundary lubrication of joints:

Characterisation of Surface-Active Phospholipids found on

retrieved implants 149

Chapter 9 Scientific Paper IV - Tribological Testing of Saturated and

Unsaturated Surface Active Phospholipids: Implications for

artificial joints 157

Chapter 10 General Discussion 165

10.1 Conclusions......................................................................................................... 173

10.2 Future Work........................................................................................................ 174

References 177


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Statement of Original Authorship

The work contained in this thesis has not been previously submitted for a

degree or diploma at any other tertiary education institution. To the best of my

knowledge this report contains no material previously published or written by

another person except where due reference is made.

Signed:

Date: 24th

Sept 2007


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Acknowledgements

I would like to acknowledge my supervisors Professor Doug Hargreaves, Professor

Ross Crawford and Professor Brian Hills. Doug, thank you for your faith and trust

in me. Ross, thank you for your financial support and exposure to the orthopaedic

world. Brian, rest in peace.

A special thank you to my fellow scholars. Your guidance, help and support has

been highly valued.

A big thank you to my friends and family that actually understood and appreciated

what I was doing. Your interest and enthusiasm provided the essential motivation

required along the way.

Most importantly I am indebted to my wife Michelle for her endless patience and

support throughout. Without her love, care and encouragement the journey would

have never been possible.

My daughter Portia has been the best part of this journey, providing me with

continual entertainment and love that only a child knows how.


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List of Figures

Figure 3.1.Diagram of the structure of the knee. Source: (MowHayes)

Figure 3.2:Schematic of a proteoglycan molecule and schematic of a proteoglycan

aggregate (BaderLee)

Figure 3.3:Schematic of the Hypothetical Layers of Articular Cartilage

(BlackHastings)

Figure 3.4. Various forms of PyC indicating surface finish. (Left to Right) As-

deposited, machined and polished.

Figure 4.1. Amontons' Laws of Friction. Source: (Shi)

Figure 4.2. Range of coefficients of kinetic friction reported in the literature for the

mammalian joint are depicted over a physiological range of sliding velocities and

compared with the a modified classical Stribeck diagram.Source: (Hills)

Figure 4.3.Lubrication regimes Source: (Dowson,Wrightet al April)

Figure 4.4.Molecular Structure of Common Solid Lubricants (a) graphite, and (b)

molybdenum disulfide. (Erdemir 2001)

Figure 4.5:Hydrodynamic Lubrication; Diagram showing the formation of the

pressure generated due to a wedge of fluid that separates the moving bearingsurfaces. (Dinnar)

Figure 4.6: Weeping or Hydrostatic Lubrication;a) as load is applied fluid

flows towards the rubbing surfaces at high pressure, carrying the load with minimal

friction. b) when the load is removed the cartilage expands, drawing in synovial

fluid. (Adapted from (McCutchen))

Figure 4.7: Elastohydrodynamic Lubrication; The top surface in this diagram is

deformable, providing a larger fluid film when load is applied.(Adapted from

(Dowson,Wrightet al April))

Figure 4.8: Microelastohydrodynamic Lubrication;Diagram showing the

deformation of the asperities on the surface of the cartilage under load decreasing

the risk of contact and allowing maintenance of a thinner fluid-film. (Adapted from

(DowsonJin))

Figure 4.9.Mixed lubrication showing that one lubrication regime does not answer

the operating conditions in the joint but a combination of mechanisms. Source:

(PanjabiWhite)


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Figure 4.10: Mixed Lubrication;When the fluid film fails, friction is prevented

by boundary lubrication. (Adapted from (Dowson,Wrightet al April))

Figure 4.11: Squeeze Film Lubrication;The arrows indicate the movement of

fluid away from the load-bearing region leaving an enriched film of synovial fluid

between the surfaces. (Adapted from (Hou,Mowet al March))

Figure 4.12: Boosted Lubrication;a) path of the fluid flow into the cartilage

surfaces while loaded. b) schematic diagram of the pools of enriched synovial fluid

formed on the surface of the cartilage. (Adapted from (McCutchen)

Figure 4.13: Boundary Lubrication; The surface layer prevents the articulating

surfaces from coming into contact.(Adapted from (WrightDowson February))

Figure 4.14.Dependence of the efficiency on the friction coefficient in natural and

artificial joints. Source: (Gavrjushenko)

Figure 4.15.Geometric configurations of various tribometers. Source:

(Dumbleton)

Figure 5.1.The triple point in cross section. Depicting the balance of forces at the

edge of a droplet where the liquid, solid and air all meet to subtend a contact angle

().

Figure 5.2. General structure of phosphoglycerides, emphasizing their amphipathic

nature (Schwarz). Various groups for X are given in Figure 5.3.

Figure 5.3.Various polar head groups for the general phosphoglyceride depicted in

Figure 5.2. Source: (http://en.wikipedia.org/wiki/Membrane_lipids)

Figure 5.4.A molecular model for the adsorption of phospholipid zwitterions to a

negatively charged surface in which cations in the plane of the phosphate ions pull

those ions together, thus enhancing close packing of both polar and non polar

moieties and imparting coherency. (Hills)

Figures within Submitted Papers

Chapter 6

Figure 1: Phospholipid model. This model shows the basic mechanism of

phospholipid adsorption to the cartilage surface, rendering it more hydrophobic,

and how interspersed cations pull the phosphate molecules together enabling high

cohesion (adapted from Hills, 2000).


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Figure 2: Oligolamellar structure of a common solid lubricant graphite.

Figure 3:Hounsfield test rig set-up showing horizontal position and custom made

attachments.

Figure 4:Schematic of Hounsfield set-up: (1) Hounsfield control panel and drive;

(2) plate head; (3) UHMWPE pin in pin holder; (4) stainless steel plate; (5) heating

resistors and thermocouple; (6) temperature control unit; and (7) force transducer.

Figure 5:Coeff. of friction at ambient UHMW PE/SS.

Figure 6:Coeff. of friction at 371 UHMW PE/SS.

Figure 7:Coeff. of friction at ambient UHMW PE/PyC.

Chapter 7

Figure 1:Schematic of pin-on-flat tribometer: 1 - control panel and drive, 2 - plate

head, 3 - UHMWPE pin in pin holder, 4 - stainless steel plate, 5 - heating resistors

and thermocouple, 6 - temperature control unit, 7 - force transducer.

Figure 2: Coefficient of friction for the material combinations under the three

lubrication conditions. Dark columns represent dry conditions, light grey columns

represent saline lubrication and light columns represent DPPC lubrication.

Chapter 8

Figure 1: Average proportions of PCs (%). Total average PC profile of the 40

implants analysed (all components included). Error bars represent standard

deviation.

Chapter 9

Figure 1:Friction force exhibited by different surfactants (0.2% concentration)

Figure 2:Effect of Concentration on the Friction Force in USPCs

Figure 3: Comparison of the Behaviour of different combinations of surfactant

species.


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List of Tables

Table 4.1.Boundary lubricants within SF suitable for tribo tests. Source: (Brown

& Clarke 2006)

Tables within Submitted Papers

Chapter 7

Table 1: Surface roughness of Samples (Ra roughness average)

Table 2:Adsorption of DPPC to PyC

Chapter 8

Table 1: Profile of phosphatidylcholine species detected on retrieved implants: (a)

Polymer components (b) Metallic components


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Chapter 1: Introduction

1

Chapter 1

Introduction

This thesis seeks to contribute to solving the problems of inadequate artificial joint

design from a tribological perspective. Artificial joints have a limited life time and

this has been traced to wear related issues. By understanding the methods of

lubrication in joints, in particular boundary lubrication which is the dominant

regime in artificial joints, engineers will be better suited to developing longer

lasting implants.

1.1Description of Scientific Problems InvestigatedThe Bone & Joint Decade (2000-2010) has been dedicated to improving the

quality of life for the millions of sufferers of bone and joint disorders. This thesis

represents a contribution to this ongoing effort. The major offender of bone and

joint disorders is arthritis. Osteoarthritis (OA) has emerged to be one of the most

serious and costly health problems encountered in the last century. Simply OA is a

massive problem.OA accounts for more than half of all chronic conditions in the

elderly and statistics show that 85% of the population will suffer from OA in their

lifetime (American Academy of Orthopaedic Surgeons 2002). Any light that can be

shed on this debilitating disease will be most beneficial to the world at large.

Total joint replacement (TJR) offers a partial solution to this problem but has

problems of its own.At best it is a temporary solution to a much larger problem.

Hip and knee replacement surgery has become a common procedure in recent years

as a method of eliminating pain and discomfort and to improve joint functionality

for patients with end stage arthritis in their lower extremities (Scmalzried &

Callaghan 1999). Although a very successful operation, TJR does not offer the

same performance as the natural joint and suffers from a limited lifetime.

Previously, joint replacement was reserved for the elderly. However, due to the

success of the procedure it is increasingly used in younger individuals. This,


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combined with an ageing population, has resulted in an increase in the incidence of

primary joint replacement. The rate of revision surgery is also increasing. Revision

surgery is associated with increased morbidity and mortality and has a far less

successful outcome than primary joint replacement. As such, it is essential to

ensure that everything possible is done to limit the rate of revision surgery

(Australian Orthopaedic Association 2002). Because we are living to older ages we

are essentially outliving the lifetime of current joint replacement designs. Research

shows that current implants are surviving to the 15 year mark (Charnley 1982;

Donnelly 1997; Kobayashi 1997) but much beyond that is questionable.

The main problem with TJRs is that they fail.

Past experience indicates that the survival rate of the implant will be influenced by

the micro and macro geometry as well as the material properties of the implant

(Donnelly 1997; Kobayashi 1997; Sumner 1998; Simmons, Shaker et al. 2001).

The reasons for failure of current hip and knee joint replacements are, in order of

proportion; loosening, dislocation and wear (Australian Orthopaedic Association

2002). Dislocation is beyond the scope of this thesis but may lie with a better

education of surgeons and the use of computer guided surgery. With dislocation

aside the other two modes of failure, loosening & wear, account for more than half

of the revision procedures and may be related. Loosening may be caused by the

osteolysis of the bone around the implant which is thought to be due to the bodys

response to foreign wear particles produced in the artificial joint. The loss of

material at the bearing surface not only causes the implant to be worn away but it is

this very wear debris which may cause failure in TJR due to loosening. Essentially

many TJRs are failing because of a tribological problem. Any reduction in wearwill be beneficial to the lifetime of the implant. Wear is reduced by lubrication.

There are considerable political, economic, social and technical reasons to improve

the wear performance of biomaterials used in joint replacements.

The tribology of TJRs is not well understood. Boundary lubrication is the last

defence in lubrication engineering. The conditions suited to boundary lubrication

are low relative surface velocities, like reciprocating motion and high loads which

are conditions matched by the human joint. Boundary lubrication can only occur if


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there is in fact a boundary lubricant present; if not, a dry bearing will exist and

direct contact will occur leading to high wear. The human synovial joint is not a

dry bearing by any means, even in a diseased, arthritic state. In fact the joint is

filled with synovial fluid, a liquid made mostly of water. It is obvious that this is

natures provision for a lubricant, yet it is known in lubrication engineering that

water is a poor lubricant. Even more so, boundary lubrication dictates the

requirement of some surface binding substance that can adsorb to the bearing

surfaces and provide a protecting film. So what else is in synovial fluid that can be

utilised as a lubricant? Synovial fluid also contains small amounts of other

substances. As yet no consolidation has arisen as to the component of synovial

fluid which provides the effective boundary lubrication that is known to exist. The

problem with artificial joints is that they rely upon boundary lubrication; however,

the boundary lubricant has not yet been completely identified. In order to improve

the lubrication of artificial joints the lubricating component of synovial fluid must

first be completely identified and understood.

In summary, TJR is currently not an entirely sufficient solution to OA. Better

artificial joint design should be instituted which means understanding how joints

are lubricated and designing to suit using materials that complement the boundary

lubricant.

1.2Overall Objectives of the StudyThe overall objective of this research included understanding and defining the

tribological aspects of the artificial synovial joint and in particular, the importance

of the role of boundary lubrication. The identification of the boundary lubricant in

synovial joints was essential to this objective. This knowledge may then be used to

increase our understanding of the relationship between the boundary lubricant and

prosthetic materials, both common and novel. Extending our understanding of

artificial synovial joints and their tribological nature may indeed provide an insight

to the crippling disease, osteoarthritis, and, at the very least, provide a better basis

for joint replacement design.


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1.3Specific Aims of the studySpecifically, the aims of the thesis were to:

1) Determine the tribological performance of common prosthetic materialslubricated in vitroby Surface Active Phospholipid (SAPL), which has been

implicated as the boundary lubricant in the joint.

2) Determine the tribological performance of a novel load-bearing prostheticmaterial: Pyrolytic Carbon (PyC).

3) Show evidence of SAPL on the surface of retrieved knee implants.

4) Use High Performance Liquid Chromatography (HPLC) to identify thecomposition of SAPL found on the surface of retrieved hip and knee

implants.

5)

Provide evidence of boundary lubrication in TJR by measuring thefrictional performance of retrieved implants exvivo.

6) Use the profile of SAPL identified via HPLC to develop a syntheticboundary lubricant suitable for laboratory testing.

7) Determine the tribological performance of the synthesised test lubricant andcommon prosthetic materials.

8) Provide recommendations for the future of artificial joint design.

9) Provide recommendations for the implementation of a standardised testlubricant suitable for in vitro laboratory testing for the purpose of

evaluating artificial implants.

Aims 2-7 make up the original features of this thesis and to the best of the authors

knowledge have not been investigated else where.


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1.4Account of Scientific Contribution Linking theScientific Papers

This thesis begins with a literature review of biotribology at large. Chapter 2 is an

overview of the science of biotribology with a focus on the lubrication within the

human body. Subsequent chapters 3-5 give an increasing in detail review to the

area of biotribology of interest. Namely, Chapter 3 reviews the structure and

function of the human diarthrodial joint, its failure due to OA and the current

remedy of Total Joint Replacement and its failure. Chapter 4 is an overview of the

lubrication of human joints. Chapter 5 is an in depth look at boundary lubrication.

Chapters 6-9 are scientific papers that have been written by the thesis author

reporting on research into the lubrication of artificial joint materials.

Chapter 6 is the first scientific paper published and was an initial study to

determine the tribological interaction of a synthetic SAPL, dipamitoyl

phosphatidylcholine (DPPC) and a novel load bearing prosthetic material, PyC.This study was performed to screen a new candidate material for hip and knee

implants with respect to producing a low friction bearing surface and to further

support the notion that effective boundary lubrication exists between SAPL and

prosthetic materials.

Chapter 7 is the second scientific paper submitted for publication and is a more

advanced study that extends the previous work with PyC given the encouraging

results produced in the preliminary study (Chapter 6). This study was a tribological

assay of several types of PyC lubricated with DPPC. In addition a goniometer was

used to determine the interaction of the SAPL with the PyC surface. Adsorption

tests were performed to establish the tenacity of DPPC to the PyC surface. This

study revealed the frictional performance of many forms of PyC and confirmed the

ability of SAPL to act as a boundary lubricant on artificial surfaces.

Previous work by our research group (Purbach, Hills et al. 2002) established the

presence of SAPL on the surface of retrieved hip implants. Ongoing data collection


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6

from the implant retrieval studies generated sufficient data so that a further report

could be published. This report, the third scientific paper of the thesis forms

Chapter 8. This study fully characterised the SAPL found on the surface of

retrieved hip and knee implants by HPLC. This knowledge would allow for a more

accurate definition of the boundary lubricant present on the surface of artificial

joints and support for the boundary lubrication of artificial joints. This study

produced a profile of the constituents of SAPL that revealed that DPPC was not in

fact the dominant portion suggesting that future friction testing of artificial joint

materials should utilise a lubricant similar to the profile of SAPL detected on the

artificial joint surface.

Considering the information obtained in the study described in Chapter 8 it

followed that the next study should test again the common and novel artificial joint

materials for their frictional performance using a synthetic copy of the determined

SAPL profile. The aim of this study was to compare the work done previously

using only DPPC as the lubricant to the results achieved using the more accurate

definition of the boundary lubricant. This paper forms Chapter 9 of the thesis.

The four scientific papers (Chapters 6-9) are the thesis authors original

contribution to this field of research.

The thesis concludes with a general discussion of the outcomes and a summary of

the four papers presented for examination. Nature has provided an effective

boundary lubricant in the form of SAPL as found on retrieved implant surfaces.

Boundary lubrication is instrumental to the reduction of friction between prostheticjoint materials. Future artificial joint design should incorporate these parameters in

order to improve the currently unsatisfactory lifetime of TJRs. Novel materials,

such as PyC, can interact favourably with SAPL and suggest a tribologically

satisfactory biomaterial suitable for future artificial implants. Better artificial joint

design may be achievable by means of material selection and surface modification

that can capitalise on the nature of the lubricant present. This thesis also promotes

the use of a standardised lubricant for in vitro testing that is similar to what is

found to lubricate artificial joints.


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Chapter 2

Literature Review - Biotribology

As the thesis author has a background in Mechanical Engineering the following

literature review is broad in order to provide a sound background to the

biotribology field. Biotribology (BT) is a very large field of research indeed. This

thesis will focus on the application of this science, biotribology, to human artificial

synovial joints. This chapter will outline the general topic of BT and the

subsequent literature review chapters will cover further detail of the thesis topic.

The literature review, Chapters 2-5, includes a discussion of the joints themselves,

the fluid within the joint, the disease osteoarthritis (OA) and the current solution of

artificial joint replacement, the bearing materials, a review of the various modes of

lubrication and a focus on boundary lubrication and SAPLs,

BT is a relatively new term introduced in the early 1970s to describe a group ofsciences that converge on one single topic: the study of friction, wear and

lubrication within biology. In consideration that the invention of this term and the

creation of this field of research would not have occurred if it had not been for the

increasing incidence of OA it is essential that a review be made of the natural

synovial joint itself. This will form part of Chapter 3 which will also include a

review of the degenerative joint disease, OA, the current remedy TJR and the

failure of these replacement joints.

Originally BT was applied to natural synovial joints in an effort to understand the

joints mechanical functions and the mechanisms of failure with the hope of

reducing the effects of OA. Physicians, physiologists, biochemists,

rheumatologists, biologists, rheologists and engineers joined forces in an effort to

understand how the natural joint was lubricated and, more importantly, how the

joint was being compromised by OA. This will form part of Chapter 4. OA has not

been cured and the best remedy to alleviate joint dysfunction has been the

invention of TJRs. Artificial joints are prone to failure and the science of BT has


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Chapter 2: Literature Review - Biotribology

8

now turned its focus to the artificial joint in an attempt to improve the lifetime of

the prosthesis. The remainder of Chapter 4 discusses the lubrication of artificial

joints.

It is well known that effective boundary lubrication occurs in artificial joints.

Chapter 5 is an in depth look at boundary lubrication.

Biotribo1ogy is a challenging, multidisciplinary field of research, involving

biology, orthopaedics, biomechanics, biomaterials science and tribology.

Tribology, an area of engineering, is the science that studies the lubrication of

interacting surfaces in relative motion. Friction is the resistance to relative sliding

or rolling motion of the surfaces. Overcoming friction dissipates energy and causes

wear of the surfaces. Lubrication provides an effective means of reducing friction

and wear by separating contacting solids with a thin layer of material of low shear

strength. The purpose of tribological research of prosthetic joints is to minimize

friction and wear of the implant, and thereby to increase the lifetime of the joint

(Calonius 2002). So, tribology plays a major role in the effective treatment of one

of the most common medical conditions known in the western world (Unsworth

1991).

Biotribology is natures way of turning a science into an art, so amazing is the

ability of biology to lubricate its mechanical functions. Engineers may be the only

ones that truly appreciate and marvel at natures eloquent yet complex methods of

lubrication. BT applies the principles of lubrication engineering in an attempt to

understand how the body lubricates its articulating bearings: the synovial joints. Amechanical bearing analysis is not a complete analysis of the human joint, as it is a

biological bearing where biochemistry and surface chemistry play a very important

role, potentially far more important than the mechanical part (Dowson 1990). This

requires engineers to call upon the expertise of other disciplines to understand

human joint lubrication and to design suitable replacements. As will be seen by the

diversity of this literature review, biotribology requires skills from the engineer that

far extend beyond traditional engineering principles.


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Novel expressions such as biolubrication have been introduced by a research

group (Benz, Chen et al. 2005) to describe the dynamic properties of very thin

aqueous films between two biological surfaces in relative motion or for water

flowing through pores or between two stationary surfaces; but more generally this

expression also covers related phenomena such as the adhesion, friction,

deformations, damage, and wear of the surfaces or the lubricating fluid. These

nanoscale phenomena ultimately determine the way biofluids flow through narrow

pores or effectively lubricate a joint.

BTs large focus is to increase the lifetime of artificial joints. Historically, new

bearing materials were 'tried out' in patients (Charnley 1966; Fisher 2000). The

consequence of incorrect design and material selection and subsequent failure now

means that there are extensive pre-clinical requirements for evaluation of materials

prior to implantation (Fisher 2000). This is by no means a simple problem.

Environmentally (biochemical) conditions in the body are harsh, biomechanical

requirements are complex and variable, and the biological response to wear

particles is largely unknown and dependent on the genetic profile of the recipient

(Fisher 2000). Biotribology is a highly multidisciplinary subject crossing

engineering materials and physical science, biological science and medicine.

Predicting the mechanical and tribological performance of bearing surfaces, and

understanding the biological responses to wear debris and the resulting potential

clinical outcomes, remains a substantial scientific and technological challenge. Key

factors limiting the successful development of improved products are our limited

understanding of biotribological science, the capability and capacity to simulate in

vivo conditions in the laboratory in pre-clinical tests, and a lack of fundamentalunderstanding of the complex and heterogenous biological reactions and

biocompatibility of wear debris in the body (Fisher 2000).

Tribology is itself an inter-disciplinary subject, being concerned with ". . . the

science and technology of interacting surfaces in relative motion and the practices

related thereto" (Dowson & Wright 1973). It embraces studies of lubrication,

friction, wear, tribochemistry, rheology and surface chemistry each of which calls


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upon contributions from chemists, physicists, mathematicians, engineers, materials

scientists, and tribologists.

At a 1970 Conference on Rheology in Medicine and Pharmacy, Dr G. W. Scott

Blair, with some justification, referred to "our somewhat precocious sister-science

of tribology" (Ferguson & Nuki 1973). He commented on the interesting link and

that paper was seen as a small attempt to encourage the courtship between the

disciplines.

Dowson & Wright introduced the term "bio-tribology" to mean those aspects of

tribology concerned with biological systems (Dowson & Wright 1973). There is a

growing interest in the relevance of tribology to biological systems, with some of

the main areas of activity being grouped together as follows:

1. The abrasive wear characteristics of human dental tissues.2. Fluid transport in the body.3. Locomotion of micro-organisms4. The motion and lubrication by plasma of red blood cells in narrow

capillaries.

5. The action of saliva.6. Tribological studies of the performance of natural synovial joints.7. Tribological aspects of prostheses

This thesis will explore the areas concerned with joints.

2.1Tribological studies of the performance of naturalsynovial joints

It is true that the largest and most successful activity in the field of biotribology has

been the study of human joints.

The human joint is a remarkable bearing. It has a low coefficient of friction (0.002)

(Jones 1934; Charnley 1959; Hills & Crawford 2003) and it is expected to survive

the dynamic loading associated with the normal activities of life for at least 70

years. The bearing material (articular cartilage) is elastic and porous. It has an


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11

initial thickness of a few millimeters and, like conventional plain bearing materials,

it is mounted on a hard backing (bone). The lubricant is synovial fluid; a highly

non-Newtonian fluid which is contained within the joint space by the synovial

membrane. It consists of a dialysate of blood plasma with varying amounts of

protein/mucopolysaccharide (hyaluronic acid complex).

Theoretical and experimental studies have suggested that the joint experiences most

of the lubrication modes familiar to tribologists; hydrodynamic,

elastohydrodynamic, mixed, and boundary, together with a unique squeeze film

characteristic. The range of loading experienced by the joint is considerable and

peak loads of more than ten times body weight can be anticipated in some

activities.

In spite of the remarkable characteristics of healthy human joints, many show signs

of wear and general distress during their working life. Osteoarthritis is a process in

which the articular cartilage is roughened and worn away, with consequent

discomfort and loss of mobility. There appear to be certain similarities between the

wear of some engineering bearings and the development of osteoarthritis, and it is

for this reason that physicians, surgeons, biochemists, rheologists and engineers

have joined forces for an attack on the problem.

If an engineering bearing shows signs of distress it can often be cured by

improving the lubricant. This suggests that it might be possible to influence the rate

of development of osteoarthritis by the introduction of synthetic lubricants. The

main problem is that even if the synthetic lubricant could be introduced andretained within the joint space, we do not have an adequate understanding of the

normal lubrication mechanism to enable us to write a specification for the synthetic

material. In addition, and maybe more importantly, is that consolidation is still

lacking as for the identification of the lubricant in the natural joint.

A large part of BT has focused on the identification of the lubricant within the joint

that facilitates such an engineering feat.


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2.1.1Overview - Lubrication of the Diarthrodial Joint: Search forthe Lubricating Factor

The diarthrodial or synovial joint allows relative motion of the bones. The bone

ends meet within a fibrous enclosure termed the joint capsule. The joint cavity is

filled with a pale yellow, viscous fluid known as synovial fluid. The lubricating

factor that allows the relative motion of the bones is believed to be contained

within the synovial fluid. Daily use of the synovial joints of the lower limbs - the

hips, knees and ankles, involves large ranges of relative motion in multiple

directions experiencing loads often as high as six times the body weight during a

normal walking cycle (Mow & Mak 1987). These loads must be sustained by the

biological bearings, the synovial joints, with characteristics of friction, wear and

lubrication that are the envy of modern engineering science. Cartilage rubbing on

cartilage has extremely low coefficients of friction (), in the range of 0.003- 0.024

(Jones 1934; Charnley 1959; Linn 1968) this is much lower than the values attained

using any synthetic bearing materials in equivalent situations, the best of which is

Teflon (PTFE) rubbing Teflon which gives a value for of 0.04.

The mechanics and biochemistry of synovial joint lubrication has been the subject

of detailed investigation since the early 1900s. Interest in the biomechanics of the

joint is widespread, extending into the fields of medicine and veterinary science

due to the high incidence of osteoarthritis (OA) or degenerative joint disease (DJD)

in todays society and the equine industry. Current literature implicates a direct

mechanical cause in the initiation of OA (Lane & Buckwalter 1993; Felson &

Radin 1994), the most commonly affected joints being those that bear load or those

that are likely to be subjected to acute injury (Meachim & Brooke 1984; Wyn-

Jones 1986; Felson 1990; Panush 1990). Deterioration of these joints causes great

pain and loss of mobility for the sufferer. Two of the major aspects in maintaining

joint mobility is lubrication (Cooke, Dowson et al. 1978) and the general

maintenance of a good load bearing surface (Freeman & Meachim 1974). It may be

that the development of OA follows the compromise of the lubricating system of

the synovial joint. What system could act within the joint to enable the exceptional

properties of friction, lubrication and wear seen at the bearing surface?


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The major theories of joint lubrication are based upon one or a combination of two

main mechanisms: boundary lubrication mechanism, where there is solid-to-solid

contact; or, a fluid-film mechanism, where the two sliding surfaces are separated by

a fluid-film or wedge of liquid which keeps them from touching. Fluid-film

mechanisms typically reach much lower coefficients of friction than boundary

mechanisms (Williams 2005) but require velocities roughly an order of magnitude

higher than typical joint sliding rates in order to maintain the wedge that separates

the two surfaces. Below this velocity, the two surfaces touch and boundary

lubrication is all that remains to facilitate motion.

Initially, the lubrication qualities of synovial fluid were thought to relate to its

characteristic viscosity, a characteristic imparted by the hyaluronic acid component

of the synovial fluid (Ogston & Stanier 1953). It was believed that the greater the

viscosity of the fluid, the better the lubricity. Hyaluronic acid is a high-molecular-

weight polysaccharide which is present in high concentrations in synovial fluid.

Apart from being responsible for the viscosity of synovial fluid, it is also very

slippery. However, it fails to lubricate under any significant load (McCutchen

1967; Linn & Radin 1968; Radin, Swann et al. 1970; Radin, Paul et al. 1970).

Further studies using hyaluronidase (McCutchen 1966) added to synovial fluid

revealed substantially reduced viscosity following hyaluronic acid destruction, but

the lubricating abilities of the synovial fluid were unchanged. Conversely, tryptic

digestion (Wilkins 1968) left viscosity unchanged but severely compromised the

lubricity of synovial fluid. These studies demonstrated two things:

(1) That the lubricating abilities of synovial fluid are independent of itsviscosity and hence, the hyaluronic acid component.

(2) That the lubricating component of the synovial fluid is somehow

associated with a protein.

These experiments were the beginning of the search for the ingredient of synovial

fluid that has the high-load-bearing capabilities necessary for lubrication of the

lower extremity joints.


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Studies have also been performed to demonstrate the lubricating advantage of

synovial fluid over saline using a system of fresh cartilage rubbing on glass (Jones

1934; Charnley 1959). It was found that synovial fluid had little advantage over

saline in terms of lubricating ability, at least over a short period of time. Over a

longer period, saline ceased to lubricate and the synovial fluid was clearly superior.

The fact that saline lubricated at all was significant. It indicated that there may be a

lubricant attached to the cartilage surface which required replenishing from the

synovial fluid.

Following a rigorous experimental protocol, it was demonstrated that the

lubricating abilities of synovial fluid were completely recovered in the protein

fraction of synovial fluid as opposed to the hyaluronate fraction (Radin, Swann et

al. 1970). Further refinement of the gross protein fraction showed that the

lubricating ability was located in a glycoprotein fraction that could be separated

from the bulk of the synovial proteins. Since serum proteins did not possess similar

lubricating abilities, the glycoprotein was considered unique and termed a

Lubricating Glycoprotein (LGP) or Lubricin (Swann 1978; Swann, Hendren et al.

1981; Swann, Slayter et al. 1981). Numerous characterisation tests were carried out

in an attempt to identify the glycoprotein; however, only 8790.8% has been fully

characterised (Swann, Slayter et al. 1981; Swann, Bloch et al. 1984). Another

interesting fact is that the molecular weight (220,500) varied with the analyses used

to identify the protein (McCutchen 1966). The remaining unidentified 9.213%

has been shown to be lipidic in nature (Schwarz & Hills 1998) and has been

heralded as the active boundary lubricant in SF and labelled Surface Active

Phospholipid (SAPL). Studies demonstrating that LGP could adsorb or otherwisebind to the articular cartilage surface have also been performed. These showed that

14% of the LGP molecule could actually adsorb to the cartilage surface (Swann,

Hendren et al. 1981). It would seem rather coincidental that these amounts were

very nearly the same suggesting that LGP (Lubricin) may in fact be a carrier for the

active lubricating ingredient (SAPL) , rather than the lubricant per se (Schwarz &

Hills 1998).


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Other published works, for example (Tsukamoto, Yamamoto et al. 1983), have

shown that the coefficient of friction does not correlate with the concentration of

the common proteins, globulin and albumin, in synovial fluid. Also there is no

correlation between the coefficient of friction and the concentration of hyaluronic

acid. This means that the lubricating properties of synovial fluid depends upon

other substances (Gavrjushenko 1993). Attention has therefore turned to lipids

which are widely distributed in the body. The lubricating ability of lipids is

attractive because lipids have good solubility in SF and the supply is practically

inexhaustible.

Almost all studies concerning lubrication of the joint have ignored the lipids,

despite the oily nature of the cartilage surface and the presence of lipids in the

synovial fluid in concentrations comparable to that of the polysaccharides and

proteins. One exception to this is the study by (Little, Freeman et al. 1969) who

found that rinsing the cartilage surface with a lipid solvent increased friction, i.e.

the value of by 500%. However, the work involving lipids and their role in joint

lubrication appeared to cease at this point. At least a decade later, Hills reignited

interest in the field by suggesting that the surfactant identified in the lung may have

lubricating abilities in many other locations in the body. More recently interest has

grown in the area with several groups researching the role of lipids in lubrication

(Gavrjushenko 1993; Williams III, Powell et al. 1993; Craig & LaBerge 1994;

Higaki, Murakami et al. 1997; Saikko & Ahlroos 1997; Stachowiak & Podsiadlo

1997; Pickard, Fisher et al. 1998; Ethell, Hodgson et al. 1999; Bell, Tipper et al.

2001; Nitzan 2001; Kawano, Miura et al. 2003; Gale, Chen et al. 2006; Gale,

Coller et al. 2007).

Considering the work of Little et al (1969) in light of the work of Radin, Swann

and their co-workers raises the possibility that a form of lipid is involved in the

lubrication of the articular surface and that the glycoprotein is simply the carrier for

the highly insoluble lubricating component. This was confirmed by work done by

(Schwarz & Hills 1998).


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The equivalent lubrication system used in industry, i.e. solid to solid rubbing, uses

a monolayer of surfactant. Surfactants readily adsorb to surfaces rendering

hydrophilic surfaces more hydrophobic. Interestingly, the articular surface is

hydrophobic, a property readily demonstrated by measuring the contact angle

occurring when a droplet of saline is placed upon the cartilage surface. If a

component of synovial fluid were a surfactant, a surfactant might actually be

responsible for the lubricity under load of synovial fluid and the articular surface.

Indeed, a major portion of the lipid component of synovial fluid is phospholipid

(Rabinowitz, Gregg et al. 1984). Phospholipids are well known for their surface-

activity and have the capability of readily rendering a surface hydrophobic.

Phospholipids also provide values for equivalent to the very low values obtained

by rubbing cartilage on cartilage (Hills 2000). Moreover, phospholipids are also

recognised as possessing substantial load bearing abilities.

2.2Tribological aspects of prosthesesWhen prostheses are introduced into the human body and relative motion of the

components are involved, the rheological and tribological features of the material,

the prosthesis and the bodys fluids become important. Examples include the wear

of some heart valves, fretting corrosion of plates and screws, and the friction and

wear characteristics of human joints.

The hip joint has received the most attention and it has been estimated that there

are over one million operations each year the world over (Bowsher & Shelton

2001). A variety of materials and designs have been witnessed in the developments

which have led to the present position. There are three major forms of material

combinations; metal-on-metal, ceramic-on-ceramic and metal-on-plastic. The first

is tribologically undesirable under most circumstances and engineering bearings in

which contact and sliding occur usually consist of a soft bearing material and a

relatively hard metal. However, in the environment of the body, corrosion might

readily occur if dissimilar metals are used. Ceramic-on-ceramic has had varied

success and seem to exhibit similar tribological failings as the other combinations

(Jazrawi, Kummer et al. 1998). The metal-on-plastic arrangement is currently most


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17

popular, the favourite materials being stainless steel and ultra high molecular

weight polyethylene. In the early stages of development of this form of prosthesis,

the low friction plastic polytetrafluoroethylene (PTFE) or commonly called Teflon

was employed, but the results were disastrous, owing to the high rate of wear of the

softer material. The present combination of materials provides some confidence in

the long-term future of the prosthesis but beyond the 15 year mark is still in

question. It is worth noting that although wear-rate is probably the most significant

tribological feature of artificial joints, friction is fundamentally important to wear.

In engineering a reduction in friction in a bearing will nearly always reduce wear

and guarantee a longer life for the bearing but there are exceptions to the rule, at

least in the body, as mentioned above in regards to the use of PTFE. The short- and

long- term reactions of body tissue to wear particles is also important.

The knee joint probably suffers more from osteoarthritis than the hip, and yet the

development of the knee joint may be still some way behind the development of

the hip joint. The knee lacks the basic stability associated with the hip, the

geometry and motion is more complicated, and the stress levels generally higher.

Hinge joints have been used successfully, but the motion is in many ways an

unsatisfactory substitute for the natural condition, and there are a number of

medical objections to the arrangement. Present designs are more in the form of

replacement linings of the natural bearing materials. In prosthetics, the combination

of metal and Ultra High Molecular Weight Polyethylene (UHMWPE) seem to be

favoured at the present time.

The number of biological subjects in which the science of tribology has made acontribution to the overall understanding of the problem is extensive and

expanding. Many of the examples are concerned with the common ground between

the sciences of rheology (Ferguson & Nuki 1973), tribology (Nakano, Momozono

et al. 2000) and surface chemistry (Benz, Chen et al. 2005).


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2.3SummaryThe main aim of biotribology is to understand natures treatment of tribology and

use this knowledge to design prosthetic joints with the aim to develop joint

couplings that minimise wear and friction in order to improve the long-term

performance of these prostheses. The lubrication of joints is complex as is the role

of the lubricating factors in synovial fluid and both will receive further discussion

in the subsequent chapters. Three substances have been implicated as the

indigenous lubricating portion of synovial fluid: Hyaluronic acid, Lubricin and

Phospholipids. It has been shown that HA fails to lubricate under any significant

load. Lubricin is a protein and never before has a protein been shown to lubricate.

Lipids in particular phospholipids are known for their lubricating abilities.

Interestingly a portion of Lubricin has been identified as phospholipidic in nature

suggesting that phospholipids are indeed the lubricating fraction of synovial fluid.

Hence, the next stage of research into the lubricating component of the joint

environment would seem to be one of testing for the presence of phospholipids at

the bearing surface. Further evidence supporting a role for phospholipid in the

lubrication of the joint would also open a new avenue of research in thedevelopment of an effective artificial synovial fluid for use in both the natural and

artificial joint.


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Chapter 3: Literature Review - Anatomy & Physiology of Diathrodial Joints

19

Chapter 3

Literature Review Anatomy &

Physiology of Diathrodial Joints

This chapter includes a discussion of the joints themselves, the fluid within the

joint, the disease osteoarthritis and the current remedy, artificial joint replacement.

It will review the materials used in TJRs and discuss the failure of artificial joints.

It should be noted that in an examination of the literature on synovial joints farmore information is available on the natural joint than its replacement. It is an

essential step in engineering to have a sound understanding of the original and the

reasons for failure of the original before designing a replacement.

3.1Anatomy of the Synovial JointDiarthrodial or synovial joints are found at the articulations of the long bones of the

skeleton (hip, knee, shoulder, fingers etc.). Diarthroses refer to the degree of

movement allowed (function) by the joint: freely movable articulations as opposed

to either amphiarthroses (slightly movable articulations) or synarthroses

(immovable articulations). Synovial refers to the structure of the joint: articular

surfaces covered with hyaline cartilage, connected by ligaments and lined by a

synovial membrane to create a joint cavity filled with synovial fluid (Gray 1918).

Synovial joints allow movement between articulating bones. The loads experienced

by synovial joints are complex and variable, exceeding 100 million cycles within a

lifetime without failure. During walking, for example, joints experience high

loading (five to six times body weight) at low surface velocities during heel strike

and toe off and very low loads at maximum surface velocity during swing phase

(Unsworth 1978). This calls for incredible load bearing capacity combined with an

extremely effective lubrication system.


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The knee is a diarthrodial or synovial joint and the discussion here will detail the

knee even though the proposed study will also include the hip. Essentially, the

following biotribological review applies equally to the majority of synovial joints

including the hip.

The knee consists of three articulations in one: two condyloid joints (Figure 3.1),

one between each condyle of the femur and the corresponding meniscus and

condyles of the tibia (tibiofemoral joint); and a third between the patella and the

femur (patellofemoral joint). (Figure 3.1)

Figure 3.1. Diagram of the structure of the knee. Source: (Mow & Hayes 1997)


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The articular capsule surrounds the joint, forming the joint cavity, and is a thin but

strong, fibrous membrane. The inner layer of the capsule is the synovial membrane

(Gray 1918). The menisci or semilunar fibrocartilages are found on the articular

surface of the tibia and improve articulation with the condyles of the femur,

enlarging the joint contact area, hence aiding articular cartilage in load transmission

and distribution. When removed, stress in the subchondral bone can be up to 5.2

times higher than when the menisci are present (Fukuda, Takai et al. 2000). A layer

of articular cartilage 1.5 to 3.5 mm thick (Bader & Lee 2000) lines the femoral,

tibial, and patella articulating surfaces.

3.2Articular (Hyaline) CartilageArticular cartilage covers the articulating bone ends of synovial joints forming a

bearing surface that enables the surfaces to resist compression, transmit and

distribute loads and maintain low frictional resistance and wear. Freeman et al

(Freeman, Swanson et al. 1975) stated that the function of articular cartilage is: to

reduce the stresses present in the articulating bone when load is applied to the joint,

to protect the bones from abrasive wear, and to reduce the friction in the joint.

However, some authors, including Fukuda et al claim that the stress reducing role

of cartilage is only minimal (Fukuda, Takai et al. 2000).

This bearing material has a thickness of between 1 and 5mm, depending upon both

species and location of the joint. It is a porous elastic material with a surface

topography determined largely by the underlying structure of collagen fibres

(Kuettner, Aydelotte et al. 1991). The absence of blood vessels, nerve fibres and

lymphatics as well as basement membranes on either side of the tissue makes adult

articular cartilage unique among connective tissues (Huber, Trattnig et al. 2000). It

is dependent on the diffusion of molecules into the synovial fluid from the well

vascularized synovial membrane for nutrition and the pumping action generated by

the repetitive loading of the joint is essential for sufficient nutrition.

The combination of articular cartilage and synovial fluid provide an almost

friction-free articulation. The biomechanical properties of articular cartilage depend


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upon the structure of the extracellular matrix, which is composed of collagen fibres

and a well-hydrated ground substance made up of proteoglycans, glycoproteins,

traces of phospholipids and elastin (Kuettner, Aydelotte et al. 1991; Nixon,

Bottomley et al. 1991). The important functional properties of cartilage which

include stiffness, durability and distribution of load, depend on the extracellular

matrix.

3.2.1The Articular SurfaceThe articular surface is a thin membrane-like coating of the cartilage matrix. In

electron micrographs it presents as an amorphous, electron-dense layer which at

high magnification shows a particulate and filamentous appearance (Ghadially,

Lalonde et al. 1983) which coats a layer of collagen fibrils. Morphological studies

by Hills have provided visible evidence of oligolamellar phospholipid on the AC

surface (Hills 1989; Hills 1990). Guerra et al reported that the surface of normal

articular cartilage is covered by a discontinuous, mono/multilayered pseudo-

membrane and seems to consist of phospholipids, glycosaminoglycans and proteins

(Guerra, Frizziero et al. 1996). They suggested this membrane-like structure might

have a protecting role in preventing direct contact between the articular cartilage

and toxic agents present in the synovial fluid and/or exert a lubricating effect

within the articular joint. Ballantine also found the same and concluded that a layer

of phospholipids was present on the surface of articular cartilage and that the layer

could clearly be viewed in SEM and OM (Ballantine & Stachowiak 2002). They

suggested that the lipid layer acts as a boundary lubricant and is critically important

to the proper functioning of synovial joints

The surface roughness of the cartilage will be considered briefly from the point of

view of its relevance to lubrication: To the naked eye, the surface appears

glistening, smooth and free from noticeable unevenness, irregularities and

roughness. However, the issue of cartilage surface roughness has been the source of

considerable controversy. Most, if not all observations made with the scanning

electron microscope, whether on cartilage itself (e.g. (Walker, Dowson et al. 1968))

or on cast replicas (e.g. (Dowson, Longfield et al. 1968) show some surface


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irregularities. Ghadially (1983) explains the presence of surface asperities as

artifacts of tissue preparation; however, there are a number of earlier studies which

describe surface roughness. Davies et al (1962) suggests that the surface is very

smooth, with irregularities in the range of 0.02m. Dowson et al (1968) and Jones

et al (Jones & Walker 1968) found much greater roughness, which increased with

age. The values ranged from about 1m for foetal cartilage to about 2.7 m for

adult cartilage. Further study by Sayles et al reported average roughness between 1

and 6m (Sayles, Thomas et al. 1979). Today, it is accepted that asperities between

2-6m are commonly present (Dowson 1990).

3.2.2Articular Cartilage MatrixCartilage is an anisotropic material consisting of cells (chondrocytes) embedded in

an extracellular matrix consisting of water (60 to 80% by weight), proteoglycans

(PGs), collagen, and some glycoproteins (non-collagenous proteins). The

organisation of these constituents varies with depth from the surface within the

same joint and between joints (Broom 1988). The fact that cartilage is aneural,

avascular, and has slow cell turnover rates means it has minimal reparative abilities

(Caplan, Elyaderani et al. 1997). Even when cartilage appears to repair an

osteochondral defect, the repair tissue often has a significantly lower aggregate

modulus and Poisson's ratio and a higher permeability than the surrounding

cartilage. These changes in properties indicate that cartilage repair tissue may not

be hyaline in nature and, therefore, inadequate for long term function in the joint

(Hale, Rudert et al. 1993).

Chondrocytes

Cartilage is maintained by the chondrocytes, the cellular component of the

cartilage, which account for less than 10% of the total volume of the cartilage.

They are responsible for the production of matrix material, hence for growth and

repair of cartilage tissue. Chondrocytes synthesise all of the basic molecular

components of the extracellular cartilage matrix and maintain the tissue via a

balance between anabolism and catabolism of the appropriate molecules. The

surface layers rely on the synovial fluid and matrix water exchange for chondrocyte


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metabolism and waste exchange. As the joint ages, the population of chondrocytes

depletes and the mitotic activity of the remaining cells decrease. (Ghadially 1978)

Proteoglycans

PGs are large, electronegative macromolecules found within articular cartilage.

These are embedded within the fibrillar network (Figure 3.2) and give articular

cartilage the ability to undergo reversible deformation. They consist of monomers

formed by a protein core with a large number of glycosaminoglycans (GAGs)

attached in a bottlebrush fashion (Figure 3.2). These GAG side chains are long,

unbranched carbohydrates and are present within the joint mainly as chondroitin

sulphate and keratin sulphate. Proteoglycans are space-filling within the tissue and

show specific interactions with the extracellular cartilage matrix (Comper &

Laurent 1978; Greenwald, Moy et al. 1978; Neame & Barry 1994).

The GAG side chains are linear polymers composed of repeating dimers

(disaccharides). The disaccharide unit contains one or two negatively charged

groups which, when formed into chains of 50-70 dimeric units as is found in a

proteoglycan molecule, represent strongly repelling chains that extend stiffly from

the protein core (Nixon, Bottomley et al. 1991). As highly negatively charged

macromolecules, proteoglycans attract water creating a hydration sheet that gives

the cartilage stiffness and compressibility. Upon loading, the proteoglycan

aggregates are compressed and water is expelled from the tissue thus increasing the

negative charge density which in turn increases the resistance of water flow until

equilibrium between loading forces and swelling pressure is reached. Removal of

the load allows the water to return, with nutrients, and the proteoglycan monomersswell to their former volume. Swelling is restricted to about 20% of its maximum

by the collagen fibrillar network (Maroudas & Bullough 1968; Maroudas 1976;

Maroudas & Venn 1977; Muller, Pita et al. 1989; Nixon 1991). Damage to the

collagen network restraining the proteoglycan hydration shell allows the cartilage

to swell with water. This is one of the early recognised changes in degenerative

joint disease or osteoarthritis.


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On one end of the PG is a small hyaluronic acid (HA) binding region. This allows a

large number of

Lorne Gale Thesis Lubrication

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