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Biotribological Assessment for Artificial Synovial Joints: The Role of Boundary Lubrication Doctorate of Philosophy in Biomedical Engineering by LORNE R GALE, 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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  • Biotribological Assessment for

    Artificial Synovial Joints: The Role

    of Boundary Lubrication

    Doctorate of Philosophy

    in

    Biomedical Engineering

    by

    LORNE R GALE, 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

  • ii

  • iii

    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 near

    frictionless 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

  • iv

    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.

  • v

    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 Protein

    LTI carbon - Low Temperature Isotropic carbon

    GCS - Glucosamine & Chondroitin Sulfate

    NSAIDS - Non-Steroidal Anti-Inflammatory Drugs

    VS - Visco Supplementation

  • vi

  • vii

    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 liquid

    medium.

    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 substance

    being 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.

  • viii

    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 can

    be 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.

  • ix

    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.

  • x

  • xi

    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

  • xii

    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 ................................................................................. 88

    4.2.4 Boundary Lubrication Models .......................................................................... 89

  • xiii

    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

  • xiv

    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

  • xv

    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

  • xvi

    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.

  • xvii

    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 bearing

    surfaces. (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)

  • xviii

    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).

  • xix

    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.

  • xx

    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

  • 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.

    1.1 Description of Scientific Problems Investigated

    The 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,

  • Chapter 1: Introduction

    2

    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 wear

    will 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

  • Chapter 1: Introduction

    3

    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.

    1.2 Overall Objectives of the Study

    The 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.

  • Chapter 1: Introduction

    4

    1.3 Specific Aims of the study

    Specifically, the aims of the thesis were to:

    1) Determine the tribological performance of common prosthetic materials

    lubricated in vitro by 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 prosthetic

    material: Pyrolytic Carbon (PyC).

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

    4) Use High Performance Liquid Chromatography (HPLC) to identify the

    composition of SAPL found on the surface of retrieved hip and knee

    implants.

    5) Provide evidence of boundary lubrication in TJR by measuring the

    frictional performance of retrieved implants ex vivo.

    6) Use the profile of SAPL identified via HPLC to develop a synthetic

    boundary lubricant suitable for laboratory testing.

    7) Determine the tribological performance of the synthesised test lubricant and

    common prosthetic materials.

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

    9) Provide recommendations for the implementation of a standardised test

    lubricant 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.

  • Chapter 1: Introduction

    5

    1.4 Account of Scientific Contribution Linking the

    Scientific 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

  • Chapter 1: Introduction

    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 prosthetic

    joint 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.

  • 7

    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 of

    sciences 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

  • 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. A

    mechanical 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.

  • Chapter 2: Literature Review - Biotribology

    9

    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 fundamental

    understanding 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

  • Chapter 2: Literature Review - Biotribology

    10

    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-organisms

    4. 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.

    2.1 Tribological studies of the performance of natural

    synovial 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

  • Chapter 2: Literature Review - Biotribology

    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 and

    retained 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.

  • Chapter 2: Literature Review - Biotribology

    12

    2.1.1 Overview - Lubrication of the Diarthrodial Joint: Search for

    the 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?

  • Chapter 2: Literature Review - Biotribology

    13

    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 its

    viscosity 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.

  • Chapter 2: Literature Review - Biotribology

    14

    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 otherwise

    bind 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).

  • Chapter 2: Literature Review - Biotribology

    15

    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).

  • Chapter 2: Literature Review - Biotribology

    16

    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.

    2.2 Tribological aspects of prostheses

    When 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

  • Chapter 2: Literature Review - Biotribology

    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 a

    contribution 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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    2.3 Summary

    The 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 the

    development 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

    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 far

    more 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.

    3.1 Anatomy of the Synovial Joint

    Diarthrodial 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.

    3.2 Articular (Hyaline) Cartilage

    Articular 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.

    3.2.1 The Articular Surface

    The 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).

    3.2.2 Articular Cartilage Matrix

    Cartilage 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 monomers

    swell 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 PGs to interact with a single HA chain forming PG aggregates

    (Figure 3.2). The PG-HA complex is stabilized by link proteins. (Muir 1980)

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

    proteoglycan aggregate (Bader & Lee 2000)

    Collagen

    Collagen fibres are responsible for the structural element of the matrix, forming a

    three dimensional fibrillar network of rope-like molecular aggregates which is

    arranged with specific orientation in the various zones related to function of the

    bearing surfaces. Trapped in this fibre network are macromolecules formed by

    proteoglycan aggregates (fig. 3.2). These consist of proteoglycan sub-units attached

    to hyaluronic acid molecules. Collagen provides the cartilage with tensile stiffness

    and strength. The property of compressive stiffness is supplied by the