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Wear Flashcards

1
Q

Define wear?

A
  • IS A PROGRESSIVE LOSS OF BEARING SUBSTANCE from the material as a result of CHEMICAL OR MECHANICAL ACTION
  • chemical = CORROSION
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2
Q

Describe the 4 modes of wear?

A
  • mode 1- Generation of wear debris that occurs with MOTION between the 2 primary bearing surfaces
  • mode 2- primary bearing surface rubbing against a secondary surface in a manner not intended by designers
  • mode 3 - 2 primarily bearing surfaces with interposed 3RD body particles- cement
  • mode 4- 2bearing surfaces rubbing together e.g back side wear on a acetabular, fretting of morse taper, stem- cement fretting or femoral component impingement on rim of cup
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3
Q

What is the most common type of wear?

A
  • type 1 - occurs for most wear in a functioning hip
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4
Q

What are the mechanisms of wear?

A
  • ABRASIVE
  • ADHESION
  • FATIGUE
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5
Q

When does 2 body abrasive wear occur?

A
  • When a SOFT MATERIAL (UHMWPE0 comes into contact with a SIGNIFICANTLY HARDER MATERIAL ( METAL)
  • asperities of the harder material surface may plough into the softer surface->GROOVES / LOOSE WEAR DEBRIS
  • femoral head aperities 0.1um
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6
Q

When does 3rd body abrasive wear occur?

A
  • When EXTRANEOUS material e.g. METALLIC/ CERAMIC, BONE, DEBRIS enter INTERFACIAL region.
  • They may become embedded in the polymer and abrade the femoral head.
  • Raised edges abrade polymer at a greater rate. Single transverse stratch may increase wear factor by 10.
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7
Q

When does ADHESIVE wear occur?

A
  • When a junction is formed between the two opposing surfaces as they come into contact.
  • If the bond between the 2 materials stronger than the cohesive strength of the individual bearing material surface, fragments may be torn off the surface and adhere to the stronger material- UHWMPE adheres to metal, esp if dry-> shearing of UHMPE
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8
Q

What is FATIGUE WEAR? Can you name an example?

A
  • DELAMINATION, a form of failure that occurs in the structures subjected to dynamic and fluctuating stresses.
  • It is possible that failure can occur at a load e.g TKR when the joint is less conforming and the UHMPE is more highly stressed.
  • fatigue wear is more a problem in TKR as the joint is less conforming and the UHMWPE is more highly stressed
  • Repeated loading causes SUBSURFACE FATIGUE FAILURE at a depth of a few millimetres = as this in an area of MAX PRINCIPLE STRESS
  • no wear is not related to surface roughness
  • cracks appear when the endurance limit is exceeded
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9
Q

What is FATIGUE life?

A
  • This is the NUMBER OF CYCLES NEEDED TO CAUSE FAILURE AT A SPECIFIC STRESS LEVEL, taken from the S-n curve plot ( log graph)
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10
Q

What is MICRO DELAMINATION?

A
  • When the surface layer of UHMWPE breaks off , it produces large particles.
  • UHMPE asperities 1-10um ( 2 0rders higher than metal asperities).
  • These are plasticially deformed by loading, producing local stress concentrations above the yield stress of UHMPE -> failure by plastic deformation and rupture.
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11
Q

what factors can excerabate fatigue wear?

A
  • a subsurface layer of oxidation
  • subsurface faults
  • misaligned or unbalanced implants
  • thin UHMWPE
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12
Q

What are the type of wear?

A
  • Volumetric
  • Linear
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13
Q

What is VOLUMETRIC WEAR?

A
  • VOLUME OF MATERIAL DETACHED from the softer material as a result of WEAR - mm3/year
  • directly related to square of the radius of the head
  • creates a cyclinder
  • head size is most important factor in predicting particles generated.
  • measure by linear wear and square the radius of head
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14
Q

How can you measure volumetric wear in vitro?

A
  • pin on plate, rotating pin on disc or joint simulators
  • joint simulators mimic loading conditions in vivo - apply cycle 4Hz but with no rest periods. an vary temperature, and lubricant.
  • Problem to underestimate wear cf invivo 60 days to apply 10 million cycles with each million cycles = approx 1 year of clinical use
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15
Q

How can you measure volumetric wear in vivo?

A
  • direct examination of the explanted cup
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16
Q

What is LINEAR WEAR?

A
  • is the LOSS of HEIGHT of bearing surface mm/year
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17
Q

How can you measure liner wear in vivo?

A
  • Cup penetration measured initial and follow up X-ray- medial migration
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18
Q

Can you describe the law of wear?

A
  • Volume of material removed by wear
  • V= kLX
  • k- wear factor for the given materials incoorporating the hardness of the softer material
  • L- load
  • X- sliding distance
  • NB - a larger femoral head will have greater V as X is greater.
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19
Q

What factors affect wear?

A

Patient factors

1) WEIGHT- applied load
2) AGE and ACTIVITY - applied rate of load

Implant factors CROUCH LIPSS

a) COEFFICIENT OF FRICTION

b) ROUGHNESS - surface finish

c) TOUGHNESS - abrasive wear
d) HARDNESS- stratch resistance
e) SLIDING DISTANCE OF EACH CYCLE- diameter of femoral head
f) NO OF CYCLES

g) SURFACE DAMAGE

h) PRESENCE OF 3RD BODY WEAR- abrasive wear

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20
Q

Can you describe the wear in THR?

A
  • acetabulum- WEAR and CREEP- - ( CREEP is the viscoelastic property- time dependent irreversible plastic deformation in response to a constant load. the amount of creep depends on the APPLIED LOAD not on SLIDING MOVEMENTS BETWEEN SURFACES.
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21
Q

What is the direction of creep in an THR?

A
  • SUPEROMEDIALLY- as this is the direction of the compressive joint reaction force
  • 0.1mm for 1st milion cycles
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22
Q

What is the direction of WEAR in an THR?

A
  • SUPEROLATERAL- as this is PERPENDICULAR TO THE INSTANTEOUS AXIS OF ROTATION
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23
Q

What dominates the initial penetration rate in an THR? What is its rate?

A
  • Creep cf wear.
  • 0.1mm for first 1million cycles
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24
Q

What are the consequences of wear particles?

A
  • SYNOVITIS
  • ASEPTIC Osteolysis and LOOSENING
  • IMMUNE REACTION
  • INCREASED FRICTION OF THE JOINT
  • MISALIGNMENT OF THE JOINT
  • CATASTROPHIC FAILURE
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25
Q

How do wear particles exert their biological activity?

A
  • By Being PHAGOCYTOSED by MACROPHAGES WHICH STIMULATES THE RELEASE OF SOLUBLE PRO INFLAMMATORY CYTOKINES- IL6/IL1, TNF APLHA, AND PROSTAGLANDINS- PG E2
  • These mediators released near to bone cause osteolysis, aseptic loosening by stimulating osteoclasts.
  • macrophages directly can effect this by stimulating o2 free radicals and hydrogen peroxide
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26
Q

What size of wear particles are the biologically active?

A
  • 0.1-10 µm
  • 0.1-0.5 µm most potent!!
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27
Q

What has been quoted as the critical wear rate for osteolysis around the acetabular cup?

A
  • 140mm3/year
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28
Q

What are the criteria that affect osteolysis/ macrophage activation ?

A
  • Size of particles
  • Morphology of particles - irregular shaped are more active than spheres
  • Total no of wear particles
  • Volume of wear debris
  • Imune response to particles
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29
Q

With a 28mm head what are the wear rates; linear, volumetric, particle no and size for UHMPE vs metal Ceramic vs UHMPE Metal vs metal ceramic vs ceramic

A
  • UHMPE vs metal LW 150-200 um/pa,Vol 40-80mm3/pa, no 7x10 power 11 size 0.5-100 µm
  • ceramic vs UHMPE LW 75-100 um/pa. Vol 15-20mm3/pa, size 0.5-100 µm
  • METAL vs Metal LW 5-10 um/pa, vol 0.1-10 mm3/pa, no 4x10(12)- 2.5x10(14) size 0.05-0.5 µm
  • ceramic vs ceramic lw- negligible, VW 0.004mm3/pa, size 0.025 µm
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30
Q

What are the advantages of ceramic on ceramic bearing surfaces?

A
  • Low wear
  • Biocompatibility
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31
Q

What are the disadvantages of ceramic on ceramic bearing surfaces?

A
  • Risk head fracture
  • Abrasive wear
  • Edge loading
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32
Q

What are the advantages of metal on metal bearing surfaces?

A
  • Good long term clinical results
  • Ability to self polish
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33
Q

What are the disadvantages of metal on metal bearing surfaces?

A
  • Undetermined effects of elevated ions
  • Undetermined cancer risk potential
  • metal hypersensitivity
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34
Q

What are the advantages of cross linked UHMPE bearing surfaces?

A
  • Reduced wear
  • accomplished by perioxide chemisty, variable-dose ionzing radiation and electron-beam, irradiation
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35
Q

What are the disadvantages of bearing surfaces?

A
  • Particles biologically more active xs cross linked poly can lead to reduced mechanical properties short term clinical results
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36
Q

Where are UHMPE/metallic wear particles transported to ?

A

The liver, spleen and abdominal lymph nodes

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37
Q

What are the limitations of UHMPE as a bearing surface? Why?

A
  • Wear RESISTANCE UHMPE sterilised by gamma irradiation- 2-4mRads -> significant degradation oxidation during post irradiation ageing -> higher wear rates/delamination/ gross failure
  • By formation of free radicals BY GAMMA RADIATION. these free radicals react with O2 molecules->additional chain scissoring-> increasing CRYSTALLINITY, DECREASE FATIGUE STRENGTH, FRACTURE TOUGHNESS AND WEAR RESISTANCE
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38
Q

Where does the max oxidation occur in UHMPE?

A
  • 1-2mm below the surface at the SUBSURFACE WHITE BAND
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39
Q

Why is UHMWPE cross linked? How is this achieved?

A
  • To improve the oxidation and wear resistance
  • by Peroxide chemistry variable dose ionizing radiation electron beam irradation sterilisation with gamma radiation in O2 free environment including a vacuum/ inert gas ( argon/nitrogen) or use ethylene oxide or gas plasma
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40
Q

Does cross linked UHMPE produce particles

A
  • Yes, submicrometre and nanometre size large amount of these which have more functional biological activity - may lead to more osteolysis
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41
Q

How does surface roughness affect THR wear?

A
  • Damaged heads->higher Volumetric wear
  • higher total penetration rates
  • higher no of particles over prothesis lifetime
  • Damaged head generate->increased no small, biologically active particles <10 µm
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42
Q

How does thickness of UHMPE affect THR wear?

A
  • Thickness at least 8MM - as creep, stress and wear increase dramatically when thickness below this.
  • adequate uhmpe thickness obtained by a 40mm cup only by downsizing the femoral head to 22mm
43
Q

How does type of metal affect THR wear?

A
  • cobalt chrome excellent if cold worked as HARD, CORROSIVE RESISTANT AND RESISTANT TO FATIGUE
  • Stainless steel- cheap but EASILY SCRATCHED
  • Ti- POOR WEAR and HIGH COEFFICIENT OF FRICTION, sensitive to SURFACE FLAWS AND SCRATCHES- used for femoral components - uncemented/ tibial trays in tar
44
Q

How does Head size affect THR wear?

A
  • the LARGER HEAD SIZE > SLIDING DISTANCE AND VOLUMETRIC WEAR
  • vol wear debris = πr2P where P is the penetration , r radius and . p proportional to 1/r2 so larger femoral head less Penetration
45
Q

How does modularity affect THR wear?

A
  • increased wear in both uncemented and cemented metal back cups due to reduced UHMWPE THICKNESS and INCREASED PEAK STRESSES especially in incongruent and GAPS between liner and metal back
46
Q

How does back side occur?

A
  • Wear between from relative movement linear and shell.
  • worse if poor locking mechanism or screw holes with sharp unpolished margins.
  • UHMWPE may creep through holes - conduit for wear particles
47
Q

How does offset affect THR wear?

A
  • Decreased offset increases joint reaction forces and increase wear
48
Q

How does UHMWPE production affect THR wear?

A
  • Ram extrusion produces linear wear of 0.11mm/pa cf compression moulding 0.05mm/pa
49
Q

How does gamma sterilsation affect THR wear?

A
  • oxidation of the UHMWPE leads to crystallisation and reduction in fatigue strength
50
Q

What is tribiology?

A
  • as the science that deals with the interaction between surfaces in motion and consequences of the at interaction ie friction, lubrication and wear
51
Q

What is friciton?

A
  • As the resistance to sliding motion between 2 bodies in contact
52
Q

For dry friction three laws of friction apply?

A
  1. Fricitonal force F= coefficient of friction (µf) x applied load ( w)
  2. F is independent of the apparant area of contact or sliding speed (v)
  3. The knetic F is independent of V
53
Q

In the hip it is important to discuss frictional torque. What is frictional torque?

A
  • FT = frictional force ( F) x radius r= µf x W x r
54
Q

What are the projections on the surface of materials called?

A
  • Asperities
  • the taller and more numerous= the rougher the surface and greater the friction
55
Q

What is roughness?

A
  • Expressed as mean surface roughness or Ra
  • = the average height of the asperities
  • Ra for polishes exerter stem is 0.01-0.03 cf articular cartilage is 1-6ra
56
Q

What is the coefficient of friction for a normal knee/hip?

A
  • knee 0.005-0.02 µf
  • hip 0.01-0.04 µf
57
Q

What is the coefficient of friction for a metal on PE, metal on metal?

A
  • metal on PE = 0.02
  • metal on metal 0.8
58
Q

What is syovial fluid?

A
  • A dialysate of blood plasma
  • without clotting factors, erythtocytes or haemoglobin
  • clear, sometime yellow, viscous
  • contains Hyaluronate and plasma proteins
  • Behaviour is non newtonian- shear stress is not proportional to shear rate
  • Pseudo-plastic - undergoes shear thining ( viscosity decreases as shear rate increases)
  • Thixotrophic- undergoes shear thining with time when sheared at constant rate
59
Q

What is Rheology?

A
  • Science of deformation and flow of matter
60
Q

what is shear?

A
  • Rate of deformation of a fluid when subjected to a mechanical shearing stress
61
Q

What is shear stress?

A
  • Applied force per unit area needed to produce deformation in a fluid
62
Q

what is viscosity?

A
  • the measure of internal friction of fluid
  • this friciton becomes apparant when a layer of fluid is made to move in relation to another layer
  • the greater the friction the greater the amount of force required for this movement = shear
  • viscosity= shear stress/ shear rate
  • shear stress = force per unit area to produce a shear action dynes/cm2
  • shear rate is a measure of the change in speed at which the intermediate layers of fluid move with respect to each other measures in seconds-1, s-1
63
Q

What is newtonian fluid?

A
  • Fluid or dispersion whose rheoloigcal behaviour is decribed by newton’s law of viscosity
  • here the shear stress is poroportional to shear rate with the proportionality constant being viscosity
  • e.g. water, thin motor oils, synovial fluid in Ra ( enzymic degradation makes it a less effective lubricant
64
Q

What is non newtonian fluid?

A
  • When the shear rate is varied, shear stress doesn’t vary in the same proportion ( or even necessarily in the same direction)
  • the viscosity of such fluids therfore changes as the shear rate is varied
65
Q

What is shear thining/ pseudo-plastic?

A
  • describes a non -newtonian fluid whose viscosity of decreases as the applied shear rate increases
  • it is dervived from the alignment of the hyaluronic acid molecules as shear rate increasesa
66
Q

When happens in RA pt knees in regard to synovial fluid?

A
  • enzyme degradation of syonival fluid -> loss of non newtonian properties making the fluid a less effective lubricant
67
Q

What is Dilatant?

A
  • Non newtonian fluid whose viscosity increases as shear rate increases
  • = shear thickening
  • rarer than shear thining but is found in fluids containing high levels of deflocculated solids e.g. sand/water, clay surries
68
Q

What is plastic

A
  • Descibes a fluid that behaves as a solid under static conditions but once flow is induced with a force known as yield value the fluid may behave non- newtonian or Newtonian e.g tomato ketchup
69
Q

What is thixotropic?

A
  • Undergoes shear thining with time when sheared at a constant rate
  • ie viscosity decreases
70
Q

What is Rheopexy?

A
  • essentially opposite of thixotropic beahviour in that fluid’s viscosity increases with time as it is sheared at a constant rate
71
Q

What are the 2 main types of lubrication?

A
  • Fluid- filmed
  • boundary
72
Q

What is fluid filled lubircation?

A
  • Surfaces are separated by a fluid film, the minimum thickness of which must exceed the surface roughness of the bearing surface in order to prevent asperity contact
73
Q

What is boundary lubrication?

A
  • Contact bearing surfaces are separated by only a boundary lubricant of moelcular thickness which prevents excessive bearing friction / wear
74
Q

What does the biotribological preformance of a joint depends on the

A
  • Lamdba ratio
  • this is the ratio of fluid-film thickness to surface roughness
  • a ratio of 3 = fluid film lubricaton whilst 1 = boundary lubrication
75
Q

Name the different types of fluid film lubrication seen in synovial joints?

A
  • Hydrodynamic
  • Elastohydrodynamic
  • Micro elastohydrodynamic
  • Squeeze film
  • Weeping
  • Boosted
76
Q

Describe hydrodynamic lubrication?

A
  • Wedge of fluid becomes entrapped and pressurised
  • one suface rotates whilst the other slides
  • rapid speed but low loads
  • High viscosity
  • no contact between surfaces and so no wear
  • occurs during the high speed non -accelerating rotatory motion of the femur during swing phase of gait
77
Q

Describe elastohydrodynamic lubrication?

A
  • IN EHD deformation of the bearing surface serves to trap presssurised fluid and increase the surface area
  • increased SA-> increased shear rate => increased viscosity
  • increase capacity of fluid-film to carry load and decrease stress within the cartilage
78
Q

Describe micro-elastohydrodynamic lubrication?

A
  • assumes the asperities of articular cartilage are deformed under high loads
  • this smoothes out the bearing surface creates a film thickness of 0.5-1µm which is sufficient for fluid-film lubrication
79
Q

Describe squeeze film lubrication?

A
  • This occurs when bearing surfaces approach each other without relative sliding motion
  • because a viscous lubricant cannot instantaneously be squeezed out from the gap between 2 surfaces that are approaching each other, pressure is built up as a result of the visious resistance offered by the lubricant as it is being squeezed from the gap
  • the pressure is temporarily capable of supporting large loads before the fluid is squeezed out and surface contact occurs
  • squeeze film may occur during Heel strike
80
Q

Describe weeping lubrication?

A
  • Tears of lubricant fluid are generated from the cartilage by the compression of bearing surfaces
81
Q

Describe boosted lubrication?

A
  • assumes that under squeeze film conditions, water of synovial fluid is pressurized into the cartilage, leaving behind a more concentrated pool of hyaluronic acid-protein complex to lubricate the surfaces
82
Q

What lubrication occurs in prolonged standing?

A
  • Boosed ( FF)
  • Boundary lubrication
83
Q

What is present on the surface of articular cartilage to protect against abrasion/ reduce?

A
  • Monolayer of
  • glycoprotein - lubricin
  • Dipalmitoyl-phosphatidyl-choline ( phospholipid)
84
Q

what lubrication is present in heel strike?

A
  • Squeeze film
85
Q

what lubrication is present in stance?

A
  • Elastohydrodynamic lubrication and micro elastohydrodynamic lubrication
86
Q

what lubrication is present in toe off?

A
  • Weeping
  • Micro EHD + Elastohydrodynamic lubrication
  • Boundary
87
Q

what lubrication is present in swing phase?

A
  • Hydrodynamic lubrication
88
Q

What lubrication occurs in PE on Metal?

A
  • Boundary as fluid film is too thin
  • only large metal on metal articulation show FF lubrication
  • the effective radius determines the FF thickness
  • a large effective radius increases contact surface area and decreases interface stress
  • radial mismatch/clearance between femoral head and acetabular cup allow a large effective radius and so FF thickness
89
Q

What is wettability?

A
  • The relative affinity of a lubricant for another material
  • measured by angle of contact at the edge of a drop of lubricant applied to the surface of the material
  • ceramics have greater wettability cf metals due to hydrophilic
90
Q

What factors determine lubrication?

A
  • magnitude and direction of loading
  • geometry of bearing surfaces/ surface roughness
  • material properties of surfaces- wettability
  • velocity at which bearing operates
  • visocity of lubricant
91
Q

What is corrosion?

A
  • As unwanted dissolution of metal in solution resulted in its continued degradation
  • electrochemical deterioration f metal happens when positive metal ions are rejected from a reaction site ( anode) and electrons are allowed to flow to a protected site ( cathode)
92
Q

Why is insitu degradation of metal alloy implants is undesirable because?

A
  • the degradation process may decrease the structural integrity of the implant
  • the release of degradation products may elicit an adverse biological reaction in the host
93
Q

Name the electochemical processes of degradation?

A
  • Galvanic corrosion
  • Crevice Corrosion
  • Fretting corrosion
  • Pitting corrosion
94
Q

What is galvanic corrosion?

A
  • 2 dissmiliar metals are electrically coupled together
  • the difference in surface potential causes electron to flow between the 2 metals
  • the greater the diff in potential the more the driving force exists for this to occur
  • the more active allow becomes the anode and more noble metal the cathode
95
Q

What is crevice corrosion?

A
  • is the formation of a cavity or crevice where exchange in material from bulk solution is limited
  • this results in change in the local environment
  • the solution within the crevice will change leading to a decrease in the species required for oxygen reduction and a build up of aggresive species with a decrease in pH
  • tighter crevices reduce the amount of electrolyte that must be deoxygenated and acidified and wil thus cause a more rapid attack
  • after oxygen becomes depleted within the crevice the metal is oxidised and electrons migrate to areas outside the crevice where they are consumed in the reduction reaction
  • particularly damaging to passive films on metal implants
96
Q

What is fretting corrosion?

A
  • Synergistic combination of wear and crevice corrosion of 2 materials in contact
  • it results from micromotion between the 2, which disrupts the protective film of a metal
  • movement can be as little as 3-4mm and is dependent on the contact load and frequency of movement
97
Q

What is pitting corrosion?

A
  • Localised corrosion attack in which small pits or hles from
  • the pits ordinarily penetrate from the top of a horizontal surface downwards in a near-vertical direction
  • insidous form of corrosion often going undetected and with very little material loss until failure occurs
  • -> damage implant with substaintial release of metal ions
  • can occur especially if solution has a low pH and contains chloride ions
  • dissolution occurs within the pits, and oxygen reduction takes place on the adjacent surfaces
  • electrons flow between the 2 sites- anode = small area of active metal and cathode = large passive surface of the remaining metal
98
Q

What is stress ( fatigue) corrosion?

A
  • Metals that are repeatedly deformed and stressed in a corrosive environment show accelerated corrosion and fatigue damage
99
Q

What is intergranular corrosion?

A
  • Metals have granular structure with grains being the term for areas of continuous structure
  • grain boundary being the disordered areas between the grains
  • the grain is anodic and susceptible, wheras the grain boundary is cathodic and immune
  • alloys are more susceptible to intergranular corrosion than pure metals
100
Q

What is intragranular ( leaching) corrosion?

A
  • This occurs due to electrochemcial differences between the grains
101
Q

What is inclusion corossion?

A
  • This occurs due to inclusion of impurities, cold welding or metal transfer
  • e.g metal fragments in screwdriver
102
Q

What combination of metals is stable?

A
  • Colbalt- chromium and ti alloy if absence of movement
103
Q

What combination of metals is unstable?

A
  • stainless steel with either CoCr or Ti
  • with the steel being susceptible to attack

Basic Science (31 decks)

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