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Basic Science > Biomaterials > Flashcards

Biomaterials Flashcards

1
Q

Define a biomaterial?

A
  • Any substance or combination of substance ( other than a drug) synthetic or natural in origin, that can be used for any period of time as a whole or part of any system that treats, augments or replaces any tissue , organ or function of the body
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2
Q

What are the most common used metal alloys in orthopaedics?

A
  • Stainless steel
  • titanium alloy
  • Colbalt chrome
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3
Q

What is the microstructure of metal elements?

A
  • Crystalline lattice microstructure
  • either
    • Body centred cubic ( each atom in contact with 8 atoms)
    • **Face centred cubic **(each atom incontact 12)
    • Hexagonal closed packed (each atom incontact with 12)
    • density > with FCC/HCP due to > no atoms
    • Plastic deformation - easiest in FCC- as 3d packing of this unit allow easy movement of crystalline units
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4
Q

What is the name of the clusters of cyrstalline units?

A
  • Grains
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5
Q

What are the areas between ajoining grains called?

A
  • Grain boundaries
  • these are imperfections in the solid structure
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6
Q

What are the defects in the grain microstructure called?

A
  • Vacancies and dislocations
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7
Q

Name the defects in the macrostructure of metal?

A
  • Scratches and voids
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8
Q

How does grain size effect a materials property?

A
  • the smaller the grain size the more uniform the material
  • during manufacturing the grain size is examined as high grain size are assoc with earlier fatigue and failure of the material
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9
Q

How is an alloy formed?

A
  • Usually formed by adding molten alloying metal elements to the principle metal element in its molten state. as this mixture solidifies such as chromium and nickel iin the case of stainless steel , subsitute for iron in the FCC arrangment.
  • Can occur as sizes of metal atoms involved similar
  • if non metallic elements are added, these often try and fit in free spaces in the crystalline structure
  • if high conc of added elements as the mixture solidifies an inital precipitate is formed with the usual crystalline structure = Alpha phase
  • then a second perciptate forms = Beta phase which has a high concentration of added elements which causes it to form a different crystalliine structure
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10
Q

What are the mechanical propertied of most metals?

A
  • Ductile
  • Stiff
  • Hard
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11
Q

What is work hardening?

A
  • A process that involves repeatedly tensile loading a material until it plastically deforms and reduces its cross sectional area and then removing the load before the material fails
  • in reality this is done between rollers
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12
Q

What is work hardening called when preformed at room temperature?

A
  • Cold working
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13
Q

What does cold working do the material?

What is this called

A
  • Effect of making strains ( dislocations) in the microstructure of the material that build during cold working permanently
  • -> Increase Yield stress
  • Decrease Ductility
    • as local strains become permanent

​

  • Strain hardening
    • ​often used in the manufacturing of stainless steel as the SS can be made stronger with only a small amount of ductility being sacrified
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14
Q

What other process can reduce grain size like cold working?

A
  • Annealing
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15
Q

What is annealing?

A
  • A process whereby the metal is heated to a certain level at which the mechanical properties revert to the original characteristics before cold working was preformed
  • annealing releaves internal stress and increases ductility
  • combining annealing with cold working reduces grain size
    • so increases the strength
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16
Q

What is fatigue failure?

A
  • the growth of cracks , in a structure subjected to repetitive load below the failure load of that structure
  • crack initation and propagation are important processes leading to ultimate failure of the metal
  • the higher the intensity of loading , the lower the number of loading cycles required for failure
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17
Q

what is the endurance limit?

A
  • As the stress at which the metal ( or material) can withstand 10 million cycles without experiencing failure
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18
Q

How do you explain the electrical and thermal conductivity of metals?

A
  • The metal atoms in the lattice lose their electrons and become metal ions in the crystal structure
  • the electrons are free to circulate about the ions in the crystal structure ( free electron model)
  • it also explaind their high chemical reactivity
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19
Q

What is stainless steel comprised of?

A
  • Carbon
  • Iron
  • Chromium added to above to ie steel
    • forms an oxide layer on surface to protect from corrosion
  • “CIC”
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20
Q

What is the commonest form of Stainless steel we use in orhtopaedics?

A
  • 316L
  • 3 % Molybdenum
  • 16% Nickel
  • added to Carbon Iron, Chromium
  • Low carbon - <0.03
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21
Q

Why is low carbon important?

A
  • Improves corrosive resistance further
  • Increasing carbon
    • weakens material
    • carbon reacts with chromium -> brittle carbides-> exposure of metal to corrosion and failure
    • this process = Sensitisation
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22
Q

what is the structure of SS 316L?

A
  • Variety of crystalline structures
    • Face centered cubic
    • Body centered cubic
  • aka Austenite
    • ​FCC structure tends to form at higher temps but the addition of Nickel to SS stabilises the FCC at room temp - that’s why often called austentic steel
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23
Q

What are the advantages of SS 316L?

A
  • Strength ( cold worked 30% to improve)
  • Ductility ( only marginally sacrified with cold working, structure of FCC is stable)
  • Reasonable resistance to corrosion ( addition of Chromium & carbon)
  • Resonable biocompatibility
  • Relatively cheap
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24
Q

What are the disadvantages of SS 316L?

A
  • Susceptibility to crevice corrosion
    • occurs in o2 depleted regions
    • e.g under head of screw
  • Susceptibilty to stress corrosion
    • application of a low level constant stress in a corrosive environment not normally assoc with metal failure
    • -> higher susceptibility of crack initation and propagation
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25
Q

What is common titanium alloy used?

A
  • titanium 64
26
Q

What does Ti 64 contain?

A
  • Titanium ( 89%) with
  • Aluminium 6%
  • Vanadium 4%

“TAV 64”

27
Q

What crystalline structure is Ti?

What advantage is this?

A
  • During alpha phase= Hexagonal close packed
  • During Beta phase = Body Centred Cubic
  • This biphasic structure confers the advantage of improved fatigue resistance on Ti
28
Q

What is the advantage of Ti 64?

A
  • Excellent resistance to corrosion
    • better than stainless steel
    • forms an oxide layer by Passivation
  • Excellent biocompatibility
    • oxide layer TiO2 integrates with bone well
  • Lower Young’s modulus
    • 100 cf stainless steel 200
    • tibial plate constructed from Ti will be half as rigid cf SS
    • so prevents stress sheilding
  • ​Excellent fatigue resistance
    • ​biphasic structure (HCP alpha, BCC beta)
29
Q

What is the disadvantage of Ti 64?

A
  • Notch sensitivity
    • scratching reduces fatigue resistance -> fatigue failure
  • Susceptible to wear
    • ​softer material cf SS /Colbalt chrome so not used as bearing surface
  • Systemic cytotoxic vanadium ion release
    • ​prevelant in soft material
  • accelerated particle- induced wear from titanium prosthesis
  • Relatively expensive
30
Q

What does colbalt chrome alloys contain?

A
  • Colbalt
  • Chrome
  • small amounts of
    • Carbon
    • molybdenum
    • nickel
    • tungsten
  • ‘CCC MNT’
31
Q

How was colbalt chrome orginally made?

A
  • cast wraught
    • pouring molten colbalt chrome into prefabricated ceramic moulds.
    • alloy cools and mould is broken away, leaving th desired shape
    • difficulty was molten CoCr cools slowly, large grain sizes tend to form.
    • If too much carbon was added -> carbides
    • -> limiting strength and reducing resistance to corrosion
32
Q

What is forging of CoCr?

A
  • Cold working and annealing
  • alloy bar is cast
  • bar is then forged to improve strength and reduce grain size
  • -> superior alloy
  • disadvantage is that the bar require further machining in order to achieve the desired shape = expensive
33
Q

What is the most recent method of making Cocr?

A
  • Powder metallurgy
    • sieving fine powder of the alloy, with the smallest particles being compressed together into the die cast of the desired final shape
    • => smaller grain size
    • => less carbide formation
34
Q

What is the structure of Cocr?

A
  • Adopt a FCC structure at room temp
  • although actually biphasic material with FCC ( alpha phase) and HCP ( beta phase)
35
Q

What are the advantages/disadv of CoCr?

A

adv

  • Excellent resistance to corrosion
    • esp crevice corrosion
  • Excellent biocompatibility in long term studies
  • Strength
    • can be strongest of all alloys
  • Mechanical properties can be altered by manufacturing methods

disadv

  • expensive to manufacture
36
Q

What are ceramics?

A
  • Compounds of metallic elements
    • aluminium
    • zirconium
    • silicon
  • ​Bound ionically & or covalently with non metallic elements
37
Q

Name some common ceramics?

A
  • Aluminium oxide ( alumina)
  • Silicon oxide ( silica)
  • Zirconium oxide ( zirconia)
  • Hydroxyapatite (HA)
38
Q

What is the structure of ceramics?

A
  • Ionically ( electron fee) or covalently bounded
  • granular structure
  • properties dependent on grain size and phases
  • these depend on manufacturing
39
Q

How are ceramics made?

A
  • mixing powdered ceramic and water
  • pressing them into Prefabricated casts
  • casts removed and the ceramic is then heated to a high temperature ( sintered) in order to achieve a higher denisty in the granular structure
40
Q

What is affected by sintering times?

A
  • Porosity
  • increase sintering times -> increased porosity
  • decreases strength
41
Q

Describe the chemical properties of ceramic?

A
  • chemically Inert
    • as metal has reacted with oxygen to return to a lower energy state
  • Insoluble
  • = best biocompatability of all materials
42
Q

Describe the mechanical properties of ceramic?

A
  • Hard materials- > Very Resistant to wear
  • HIgh elastic modulus ( high stiffness)
  • Osteoconductive

but

  • Brittle- no plastic deformation
    • catastrophic failure
43
Q

What is the use of ceramics in orthopaedics?

A
  1. Bearing surface for femoral head component
    • due to hardwearing properties e.g zirconium/alumina
  2. Coating metal implants to increase biocompatability
    • as osteoconductive properties e.g hydroxyaapetite
44
Q

What is the formular of hydroxyapatite?

A

Ca 10 ( PO4)6(OH)2

the mineral basis of bone

45
Q

What is the problem of coating surfaces with hydroxyapatite?

A
  • Brittle
  • coating tends to strip off implants
46
Q

What is polymer?

A
  • Consists of many repeating units of a basic sequence ( monomer)
47
Q

What is a monomer?

A
  • consists of a long-backbone carbon chain linked by covalent bonds
  • each carbon atom has 4 bonding sites available
48
Q

What is the structure of ethylene?

A

H H

I I

                                         C ==  C

                                         I          I

                                         H        H
49
Q

How are polmers formed?

A
  • By addition or condensation polymerisation
  • addition polymerisation
    • when a free radical is added to a monomer that contains a double carbon bond
    • the free radical break the carbon bond & occupies the binding site
    • => one carbon having a free binding site which then proceeds to react with another free radical, and the reaction progresses
  • Condensation polymerisation
    • when momers with symmetrical end groups react , losing water ( condensation) or carbon dioxide molecule then bonding together
50
Q

what is the structure of methylmethacrylate?

A

H CH2

     I              I

     C======C

     I              I

     H           C= O

                   I

                   CH<sub>2</sub>
51
Q

name a commonly used polymer in orthopaedics?

A
  • Polymethylmethacrylate- bone cement
  • Ultra- high- molecular- weight - polyethylene (UHMWPE) for acetabular cups
52
Q

What factors increase the strenght of polymer?

A
  1. Cross linking or entanglement
    • only weak van der waals forces stop movement of polymer chains reltive to each other.
  2. Length and consequence molecular weight
    • as weight increases, shear strength, tensile stength and wer resistance increases
  3. Crystallinity
    • when organised arrangement of the chains in 3d pattern ->high density structure with close proximity of elements and higher van der waals forces
  4. Amorphous
    • random entanglement of chains and induction of further cross linking of polymer
    • the longer the chains the more this happens
53
Q

What is in the liquid to make PMMA?

A
  • Activator= N,N-dimethyl-p-toluidine
  • **Inhibitor= Hydroquinone ( **prevents polymerisation within bottle)
  • Monomer = methylmethacrylate
  • Chorophyll

“AIMC”

54
Q

what is the powder to make PMMA?

A
  • **Radioopacifer= Barium or zirconium dioxide **
  • Antibtiotics= Gentamycin
  • Initator= Benzoyl peroxide ( the initator of polymerisation)
  • polymethyl methacrylate (PMMA) copolymer
55
Q

What happens to cement if the temperature or humidity increase?

A
  • The working time decreases
56
Q

what is the normal powder to liquid ratio when preparing cement?

A
  • 2ml of powder to 1mg liquid
  • if more liquid is added it reduces the P/L ratio and so increases the setting time
57
Q

When is the cement inserted?

A
  • As soon as the cement starts setting, it becomes pseudoplastic ie the viscosity lowers as the flow rate increases
58
Q

How is heat induced tissue necrosis prevented?

A
  • polymerisation reactio is exothermic
  • the amount of heat generated depends on surface area and thickness of mantle
  • increased thickness ir decerased surface area -> increase in temperature generated
  • rationale for uniform cement mantle
59
Q

Why is vaccum mixing used?

A
  • Reduces voids
  • so decreases porosity of the cement
  • so increases strength of cement
60
Q

Do antibiotics affect cement properties?

A
  • No but they must be stable to heat ( exothermic reaction)
61
Q

Describe the stages of mixing?

A
  • Mixing phase
  • waiting phase
  • working phase
  • setting phase

Basic Science (31 decks)

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