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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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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 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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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.
1.2Overall 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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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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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 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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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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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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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
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