Bearing surfaces THR
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Bearing Surfaces in Total Hip Arthroplasty
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HistoryInterpositional arthroplasty
1912- Sir Robert Jones- Gold foil
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Mould arthroplasty 1923-Smith Peterson
Restore congruous articular surfacesBleeding cancellous bone of femoral head
and acetabulum Metaplasia of fibrin clot to fibrocartilage
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Glass
Pyrex – viscalloid - celluloid derivative
Bakelite
Fragility and foreign body reaction
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1937-Venable and Stuck- Vitallium- results encouraging
Aufranc
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Judet brothers- heat curved acrylic femoral head prosthesis-fragmentation of acrylic with wear- severe tissue reaction-bone destruction
Thompson and Moore – metallic endoprosthesis with medullary stems- erosion of bone on pelvic side
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Metal on metal Urist, Ring and McKee Farrar
Friction metal wear
High incidence of loosening and pain.
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Sir John Charnley -Low friction torque arthroplasty-Surgical alteration of hip biomechanics-Lubrication-Material design-Operating room environment-PMMA
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1 st Charnley prosthesis Moore prosthesis stainless steel femoral
component
Thin polytetrafluroethylene shell for acetabulum
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Tribology Surfaces interacting under an applied load
and in relative motion
Study of: -Friction -Lubrication -Wear
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Wear Types of wear:
1.Adhesive wear2.Abrasive wear3.Third body wear4.Fatigue wear
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Adhesive wear Adhesion during contact of opposing bearing
surfacesSliding breaks these contactsStrength of adhesion exceeds strength of
materialParticles are pulled from the material
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Abrasive wear
Hard projection on one surface cuts into the opposing surface
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Third body wear
Hard particles such as bone or PMMA if trapped between bearing surfaces cause abrasive damage
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Fatigue wear
Repetitive loading of the bearings during articulation
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Wear modes
Conditions under which the prosthesis was functioning when the wear occurred
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Mode 1
Motion of 2 primary bearing surfaces against each other
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Mode 2
Primary bearing surface moving against a secondary surface that was not intended to come into contact with the first
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Mode 3
Contaminant particles directly abrade one or both of the primary bearing surfaces
Third body abrasion or wear
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Mode 4 2 secondary surfaces rubbing together
Backside wear
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Wear debris causes osteolysis Compromise fixation
Complicate revision procedure.
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Wear threshold value
0.1 mm/year
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Threshold is modified by
-Intracapsular pressure-Bone interface access-Patient reaction to debris
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Types of bearings Hard
Soft
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Femoral Head material Metallic alloys that can be used with
UHMWPE liner:-Stainless steel 316L-Cobalt chromium alloy-Titanium alloys
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Ceramics with UHMWPE Alumina
Zirconia
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Conventional UHMWPE Ram extrusion
Compression molding
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Sterilization Ethylene oxide Gas plasma Gamma radiation in air (2.5 to 4 mrad)Gamma radiation in inert atmosphere
(nitrogen, argon or vacuum)
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Gamma radiationCross-linking of polyethylene molecules
Interaction of free radicals formed during irradiation
Improved wear resistance
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Highly cross-linked UHMWPE Higher doses of radiation
Heat
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Remelting: Heating above the melting range of polyethylene
Annealing: Heating below the melting range
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UHMWPE- Semi crystalline polymer Mechanical behavior - crystalline morphology
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Benefits of cross linked polyethylene -High wear resistance-No toxicity-Relatively low cost-Multiple liner options (elevated rim etc)
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Risks of cross-linked polyethylene -Reduction in other material properties -gross
material failure
-Increased bioactivity of wear particles
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Metal on metal bearings Muller and Weber Wear depends on:1.Type of cobalt chromium alloy2.Surface finish3.Bearing clearance4.Sphericity
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Fluid film lubrication 1. Bearing size2. Clearance3. Sphericity4. Surface finish
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Benefits of Metal on metal Very high wear resistance
Favors larger diameters (lowers wear)
Long in vivo experience
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Risks of Metal on metal Increased ion levels
Delayed type hypersensitivity
Carcinogenesis
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Ceramic on ceramic bearings Alumina:1.Hardness2.High wear resistance3.Chemical inertness
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Benefits of ceramic on ceramicHighest wear resistance
No toxicity
Long in vivo experience
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Risks of ceramic on ceramic
1.Position sensitivity2.Liner chipping3.Fracture risk
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Material properties
CoC MoM MoHCLUHMWPE
HardnessMPa
2300 350 Low
# reported no +remelted
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TribologyCoC MoM HCLUHM
WPE
Wear 1 25 100
Particle size
<0.02and >0.2
0.05 0.4
Metal ion Not increased
increased Not increased
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Biologic effectsCoC MoM MoHCLUHWPE
Cell toxicity
No Yes No
Local tissue reaction
Low Low Low
Chromo. changes
NR R NR
Hypersensitivity
NR R NR
Carcin. NR * NR
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Coc MoM MoHCLUHMWPE
Squeaking + + -
Clicking + + -
Seizing - + -
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Future
*IDEAL*
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Next generation polyethylenes 1. Sequential irradiation and annealing 2. Irradiation and solid state deformation by
extrusion below melting temperature3. Vit E antioxidant containing polyethylene
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Metal on metal Increase in femoral head size –greater sliding
velocity and increased probability of fluid film lubrication – decrease in wear
Reduced risk of impingement and dislocation 32 mm and bigger
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Ceramic on metal Alumina head Cobalt chromium alloy cup Reduced wear rate 100 foldClinical studies underway
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Ceramics Alumina – Zirconia combination: for
increasing toughness of alumina75 % alumina 24 % Zirconia 1 % chromium
oxide- Greater bending strength- Lower wear rate- Added manufacturing complexity- Cost
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Take home message There is no 100 % ideal bearing surface Hard/hard and hard/soft Tribology –friction, lubrication and wear Wear causes osteolysis
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Modern bearings: - all have low wear
Young and active patients Hard/hard bearings will produce less wear debris but noise remains a problem
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For hard on hard bearings cup placement is important to reduce risk of impingement, excessive wear and fracture.
Metal on metal bearings release metal ions and corrosion products and probably should not be used for patients with impaired kidney function or women of child bearing age.