Wear Flashcards
(103 cards)
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
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
3
Q
What is the most common type of wear?
A
- type 1 - occurs for most wear in a functioning hip
4
Q
What are the mechanisms of wear?
A
- ABRASIVE
- ADHESION
- FATIGUE
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
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.
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
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
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)
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.
11
Q
what factors can excerabate fatigue wear?
A
- a subsurface layer of oxidation
- subsurface faults
- misaligned or unbalanced implants
- thin UHMWPE
12
Q
What are the type of wear?
A
- Volumetric
- Linear
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
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
15
Q
How can you measure volumetric wear in vivo?
A
- direct examination of the explanted cup
16
Q
What is LINEAR WEAR?
A
- is the LOSS of HEIGHT of bearing surface mm/year
17
Q
How can you measure liner wear in vivo?
A
- Cup penetration measured initial and follow up X-ray- medial migration
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.
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
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.
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
22
Q
What is the direction of WEAR in an THR?
A
- SUPEROLATERAL- as this is PERPENDICULAR TO THE INSTANTEOUS AXIS OF ROTATION
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
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
25
How do wear particles exert their biological activity?
* 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
26
What size of wear particles are the biologically active?
* **0.1-10 µm**
* **0.1-0.5 µm most potent!!**
27
What has been quoted as the critical wear rate for osteolysis around the acetabular cup?
* **140mm3/year**
28
What are the criteria that affect osteolysis/ macrophage activation ?
* **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**
29
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
* 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
30
What are the advantages of ceramic on ceramic bearing surfaces?
* Low wear
* Biocompatibility
31
What are the disadvantages of ceramic on ceramic bearing surfaces?
* Risk head fracture
* Abrasive wear
* Edge loading
32
What are the advantages of metal on metal bearing surfaces?
* Good long term clinical results
* Ability to self polish
33
What are the disadvantages of metal on metal bearing surfaces?
* Undetermined effects of elevated ions
* Undetermined cancer risk potential
* metal hypersensitivity
34
What are the advantages of cross linked UHMPE bearing surfaces?
* Reduced wear
* accomplished by perioxide chemisty, variable-dose ionzing radiation and electron-beam, irradiation
35
What are the disadvantages of bearing surfaces?
* Particles biologically more active xs cross linked poly can lead to reduced mechanical properties short term clinical results
36
Where are UHMPE/metallic wear particles transported to ?
The liver, spleen and abdominal lymph nodes
37
What are the limitations of UHMPE as a bearing surface? Why?
* 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
38
Where does the max oxidation occur in UHMPE?
* 1-2mm below the surface at the **SUBSURFACE WHITE BAND**
39
Why is UHMWPE cross linked? How is this achieved?
* 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
40
Does cross linked UHMPE produce particles
* Yes, submicrometre and nanometre size large amount of these which have more functional biological activity - may lead to more osteolysis
41
How does surface roughness affect THR wear?
* 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
42
How does thickness of UHMPE affect THR wear?
* 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
How does type of metal affect THR wear?
* 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
How does Head size affect THR wear?
* 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
How does modularity affect THR wear?
* 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
How does back side occur?
* 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
How does offset affect THR wear?
* Decreased offset increases joint reaction forces and increase wear
48
How does UHMWPE production affect THR wear?
* Ram extrusion produces linear wear of 0.11mm/pa cf **compression moulding 0.05mm/pa**
49
How does gamma sterilsation affect THR wear?
* oxidation of the UHMWPE leads to crystallisation and reduction in fatigue strength
50
What is tribiology?
* **as the science that deals with the interaction between surfaces in motion and consequences of the at interaction ie friction, lubrication and wear**
51
What is friciton?
* **As the resistance to sliding motion between 2 bodies in contact**
52
For dry friction three laws of friction apply?
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
In the hip it is important to discuss frictional torque. What is frictional torque?
* FT = frictional force ( F) x radius r= µf x W x r
54
What are the projections on the surface of materials called?
* Asperities
* the taller and more numerous= the rougher the surface and greater the friction
55
What is roughness?
* 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
What is the coefficient of friction for a normal knee/hip?
* knee 0.005-0.02 µf
* hip 0.01-0.04 µf
57
What is the coefficient of friction for a metal on PE, metal on metal?
* metal on PE = 0.02
* metal on metal 0.8
58
What is syovial fluid?
* **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
What is Rheology?
* Science of deformation and flow of matter
60
what is shear?
* Rate of deformation of a fluid when subjected to a mechanical shearing stress
61
What is shear stress?
* Applied force per unit area needed to produce deformation in a fluid
62
what is viscosity?
* 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
What is newtonian fluid?
* 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
What is non newtonian fluid?
* 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
What is shear thining/ pseudo-plastic?
* 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
When happens in RA pt knees in regard to synovial fluid?
* enzyme degradation of syonival fluid -\> loss of non newtonian properties making the fluid a less effective lubricant
67
What is Dilatant?
* **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
What is plastic
* 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
What is thixotropic?
* Undergoes shear thining with time when sheared at a constant rate
* ie viscosity decreases
70
What is Rheopexy?
* essentially opposite of thixotropic beahviour in that fluid's **viscosity increases with time as it is sheared at a constant rate**
71
What are the 2 main types of lubrication?
* **Fluid- filmed**
* **boundary**
72
What is fluid filled lubircation?
* **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
What is boundary lubrication?
* **Contact bearing surfaces are separated by only a boundary lubricant of moelcular thickness which prevents excessive bearing friction / wear**
74
What does the biotribological preformance of a joint depends on the
* 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
Name the different types of fluid film lubrication seen in synovial joints?
* **Hydrodynamic**
* **Elastohydrodynamic**
* **Micro elastohydrodynamic**
* **Squeeze film**
* **Weeping**
* **Boosted**
76
Describe hydrodynamic lubrication?
* 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
Describe elastohydrodynamic lubrication?
* 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
Describe micro-elastohydrodynamic lubrication?
* 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
Describe squeeze film lubrication?
* 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
Describe weeping lubrication?
* **Tears of lubricant fluid** are generated from the cartilage by the **compression of bearing surfaces**
81
Describe boosted lubrication?
* 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
What lubrication occurs in prolonged standing?
* **Boosed ( FF)**
* **Boundary lubrication**
83
What is present on the surface of articular cartilage to protect against abrasion/ reduce?
* Monolayer of
* glycoprotein - **lubricin**
* **Dipalmitoyl-phosphatidyl-choline** ( phospholipid)
84
what lubrication is present in heel strike?
* **Squeeze film**
85
what lubrication is present in stance?
* **Elastohydrodynamic** lubrication and **micro elastohydrodynamic** lubrication
86
what lubrication is present in toe off?
* **Weeping**
* **Micro EHD** + **Elastohydrodynamic** lubrication
* **Boundary**
87
what lubrication is present in swing phase?
* **Hydrodynamic** lubrication
88
What lubrication occurs in PE on Metal?
* 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
What is wettability?
* 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
What factors determine lubrication?
* 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
What is corrosion?
* **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
Why is insitu degradation of metal alloy implants is undesirable because?
* 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
Name the electochemical processes of degradation?
* **Galvanic** corrosion
* **Crevice** Corrosion
* **Fretting** corrosion
* **Pitting** corrosion
94
What is galvanic corrosion?
* **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
What is crevice corrosion?
* **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
What is fretting corrosion?
* **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
What is pitting corrosion?
* 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
What is stress ( fatigue) corrosion?
* Metals that are repeatedly deformed and stressed in a corrosive environment show accelerated corrosion and fatigue damage
99
What is intergranular corrosion?
* 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
What is intragranular ( leaching) corrosion?
* This occurs due to electrochemcial differences between the grains
101
What is inclusion corossion?
* This occurs **due to inclusion of impurities, cold welding or metal transfer**
* e.g metal fragments in screwdriver
102
What combination of metals is stable?
* Colbalt- chromium and ti alloy if absence of movement
103
What combination of metals is unstable?
* **stainless steel** with either **CoCr or Ti**
* with the **steel being susceptible to attack**
