Wear Flashcards
Define wear?
- IS A PROGRESSIVE LOSS OF BEARING SUBSTANCE from the material as a result of CHEMICAL OR MECHANICAL ACTION
- chemical = CORROSION
Describe the 4 modes of wear?
- 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
What is the most common type of wear?
- type 1 - occurs for most wear in a functioning hip
What are the mechanisms of wear?
- ABRASIVE
- ADHESION
- FATIGUE
When does 2 body abrasive wear occur?
- 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
When does 3rd body abrasive wear occur?
- 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.
When does ADHESIVE wear occur?
- 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
What is FATIGUE WEAR? Can you name an example?
- 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
What is FATIGUE life?
- This is the NUMBER OF CYCLES NEEDED TO CAUSE FAILURE AT A SPECIFIC STRESS LEVEL, taken from the S-n curve plot ( log graph)
What is MICRO DELAMINATION?
- 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.
what factors can excerabate fatigue wear?
- a subsurface layer of oxidation
- subsurface faults
- misaligned or unbalanced implants
- thin UHMWPE
What are the type of wear?
- Volumetric
- Linear
What is VOLUMETRIC WEAR?
- 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
How can you measure volumetric wear in vitro?
- 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
How can you measure volumetric wear in vivo?
- direct examination of the explanted cup
What is LINEAR WEAR?
- is the LOSS of HEIGHT of bearing surface mm/year
How can you measure liner wear in vivo?
- Cup penetration measured initial and follow up X-ray- medial migration
Can you describe the law of wear?
- 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.
What factors affect wear?
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
Can you describe the wear in THR?
- 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.
What is the direction of creep in an THR?
- SUPEROMEDIALLY- as this is the direction of the compressive joint reaction force
- 0.1mm for 1st milion cycles
What is the direction of WEAR in an THR?
- SUPEROLATERAL- as this is PERPENDICULAR TO THE INSTANTEOUS AXIS OF ROTATION
What dominates the initial penetration rate in an THR? What is its rate?
- Creep cf wear.
- 0.1mm for first 1million cycles
What are the consequences of wear particles?
- SYNOVITIS
- ASEPTIC Osteolysis and LOOSENING
- IMMUNE REACTION
- INCREASED FRICTION OF THE JOINT
- MISALIGNMENT OF THE JOINT
- CATASTROPHIC FAILURE
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
What size of wear particles are the biologically active?
- 0.1-10 µm
- 0.1-0.5 µm most potent!!
What has been quoted as the critical wear rate for osteolysis around the acetabular cup?
- 140mm3/year
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
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
What are the advantages of ceramic on ceramic bearing surfaces?
- Low wear
- Biocompatibility
What are the disadvantages of ceramic on ceramic bearing surfaces?
- Risk head fracture
- Abrasive wear
- Edge loading
What are the advantages of metal on metal bearing surfaces?
- Good long term clinical results
- Ability to self polish
What are the disadvantages of metal on metal bearing surfaces?
- Undetermined effects of elevated ions
- Undetermined cancer risk potential
- metal hypersensitivity
What are the advantages of cross linked UHMPE bearing surfaces?
- Reduced wear
- accomplished by perioxide chemisty, variable-dose ionzing radiation and electron-beam, irradiation
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
Where are UHMPE/metallic wear particles transported to ?
The liver, spleen and abdominal lymph nodes
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
Where does the max oxidation occur in UHMPE?
- 1-2mm below the surface at the SUBSURFACE WHITE BAND
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
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
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