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biomaterials > revision > Flashcards

revision Flashcards

1
Q

What is a biomaterial?

A

Any substance (other than drugs) or combination of substances, synthetic or natural in origin, which can be used for any period of time, as a whole or as part of a system which treats, augments, or replaces any tissue, organ or function of the body

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2
Q

State common examples of metals that are used as biomaterials

A
  • Stainless steels
  • Cobalt chromium alloys
  • Titanium and its alloys
  • Tantalum
  • Precious metals (gold and silver)
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3
Q

What are the advantages and disadvantages associated with metal biomaterials

A

Advantages: good ductility, good mechanical properties (high strength), biocompatible
Disadvantages: high elastic modulus (stress shielding effect and bone loss), inadequate corrosion resistance (release of fine metallic ions results in adverse tissue effect and decreases implant life), inadequate wear resistance (release of wear debris leads to both toxic and inflammatory response and loosening of component), no bioactivity (soft tissue interlayer)

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4
Q

Give examples of where metals are used as biomaterials in orthopaedics

A

Examples: joint replacements, bone plates and screws, dental root implants, suture wires, cochlear implants, piercings

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5
Q

What type of stainless steel is most commonly used – state its main compositional features

A 
Nickel/chrome/molybdenum alloys are used for orthopaedic implants. 
Orthopaedic implants are austenite:
o	Strong but not too brittle
o	Carbon content < 0.08%
Most surgical equipment is martensite:
o	Much harder than austenitic steel
o	Easier to keep sharp
o	Carbon content 0.15-1.2%
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6
Q

Why is the carbon content reduced in stainless steels?

A

Reduce carbide (Cr23C6) formation at grain boundary
< 9% Cr content enhances corrosion
Carbide impairs formation of surface oxide
Better resistance to in vivo corrosion

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7
Q

How are the mechanical properties of stainless steel improved?

A

Improved through cold working (a.k.a strain hardening) –

Excessive number of dislocations are induced prior to in-vivo use, newer dislocations will be harder to induce.

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8
Q

What are the pros and cons associated with cold working?

A

Cold working:

  • Pros: increased yield strength, ultimate tensile strength, fatigue strength
  • Cons: reduced ductility
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9
Q

What are the two main forms of cobalt chrome alloys used in orthopaedics?

A
  • Cast (CoCrMo)

- Wrought (CoNiCrMo)

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10
Q

What additional elements are added to cobalt chrome, and what is their function?

A
  • Molybdenum: Produces finer grains
  • Chromium: provides corrosion resistance
  • Nickel: Stabilises the F.C.C structure at R.T
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11
Q

What are the manufacturing issues associated with cobalt chrome alloys?

A 
Casting defects can occur, these can be: 
-	Stress concentrations	
o	Propensity to fatigue failure
-	Defect location critical
-	Change in cast standard
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12
Q

What are the advantages of hot isostatic pressing?

A
  • Highest achievable density of material
  • Higher static strength
  • No segregation or grain growth during manufacturing
  • Higher dynamic/ yield and tensile strength
  • Homogeneous annealed microstructure
  • Maximum abrasion resistance
  • Higher corrosion resistance
  • Reduced porosity
  • Improved fatigue resistance
  • Reduction of micro shrinkage of casting
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13
Q

What form of Ti-alloy is used in orthopaedics?

A
  • Commercially, pure (CPTi)

- Ti-6Al-4V alloy

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14
Q

What are the roles of the additional elements in Ti-alloys?

A
  • Aluminium: stabilises the α phase
  • Vanadium: stabilises the β phase
  • Combined effect = balanced strength/ductility
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15
Q

What are the advantages and disadvantages of Ti-alloys?

A 
Advantages:
-	High biocompatibility
-	Low young’s modulus
-	Excellent corrosion resistance
-	Low density 
Disadvantages:
-	Poor fretting fatigue resistance and poor tribological properties due to its low hardness
-	High coefficient of friction, severe adhesive wear with a strong tendency to seizing and low abrasion resistance
-	Toxic effect of Al and V on long term
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16
Q

How does corrosion occur through chromium oxide and titanium oxide?

A

Chromium:
- Increase Cr raises the transition temperature for FCC – HCP
o εCo stabilizers
- stabilize the hcp phase by reducing the stacking fault energy of the crystal structure
- improves oxidation and corrosion resitance by forming a passive oxide film
- strengthens by forming of Cr7C3 within the hcp zones and Cr23C6

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17
Q

What kind of oxide structure minimises the corrosion process?

A

Lots of possible answers – compact, tightly adherent, no porosity, rapid formation

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18
Q

Why is corrosion expected to occur faster in vivo compared with in vitro?

A

Presence of proteins accelerates corrosion

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19
Q

What are the concerns associated with metals in orthopaedic implants?

A
  • Release of metal particles in the body
  • Wear and corrosion at the connection between the metal ball and taper of the stem may also occur and may enter the bloodstream
  • Different people will react to these metal ions and particle in different ways, not possible to predict who will experience a reaction, what type of reaction they might have, when the reaction will occur, or how severe the reaction will be
  • The metal particles around some implants can cause damage to bone and / or tissue surrounding the implant and joint.
  • Bearing surfaces -> wear particle generation, taper interface -> fretting/crevice corrosion, Cement -> fatigue/ fretting/ corrosion
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20
Q

What type of implant was popular during the 2000’s, and why has its use declined?

A

Metal on metal implants were popular

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21
Q

Describe two methods by which immediate secure implant / bone fixation be achieved without the use of bone cement?.

A
  • Bone screws
  • Press-fit between implant and bone
  • Specific design features that encourage bone growth into or onto the implant
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22
Q

What are the advantages and disadvantages of adding radiopacifier to bone cement?

A

Advantages:
- The implant will be visible on xrays and other medical imaging devices

Disadvantages:
- Can be detrimental to some of the physical, mechanical and biological properties

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23
Q

What microstructural features affect the mechanical performance of bone cement?

A

Radiopacifiers and pores can initiate cracks leading to early failure

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24
Q

How do UHMWPE particles lead to bone resorption?

A

Body tries to get rid of the particles by oxidising them using white blood cells but as the cells release oxidising agents they kill bone

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25
Q

What happens to UHMWPE that had been gamma irradiated in air?

A

Gamma irradiation in UHMWPE causes a cleavage of chemical bonds of the PE chains, which create free radicals. The free radicals are highly reactive species that tend to produce a complex series of reactions. These reactions release hydrogen atoms.

Crosslinking occurs with the release of hydrogen atoms

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26
Q

Name three advantages of E-poly compared to highly cross linked polyethylene.

A

Vitamin e mops up free radicals, contains a naturally occurring antioxidant and is very wear resistant

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27
Q

What effect does particle size have on biological activity?

A

Lager molecules have multiple contact points with a surface increasing activity

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28
Q

What are the two main forms of ceramic based implants?

A

Oxides, non-oxides and composite materials

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29
Q

What are the drawbacks of ceramics?

A
  • Ceramic implants may “shatter” if impacted by larger force causing complex surgery to remove the parts. (high brittleness)
    o This could lead to further bone resection and a thicker component
  • High elastic modulus  leads to stress shielding effect and bone loss
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30
Q

Name two of the most common resorbable polymers

A
  • Poly lactic acid (PLA)

- Polycaprolactone (PCL)

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31
Q

How do resorbable polymers degrade?

A

Biodegradation occurs when hydrolysis yields short chains (oligomers) that are metabolised by cells
General Degradation sequence:
- Water absorbed (mass increase)
- Hydrolysis begins (fragmentation)
- Hydrolysis continues (mass loss)
- Resorption & metabolism of oligomers

32
Q

What factors increase the rate of biodegradation?

A

Depends on several factors:

  • polymer degradation rate (higher => surface erosion)
  • Water diffusion rate (lower =surface erosion)
  • Polymer dimensions (thicker = ? surface erosion)
33
Q

What negative effects can arise during degradation of resorbable polymers?

A

Inflammatory acidic hydrolysis products released
- Normal bone maintenance balances bone formation and Resorption
o Relies on osteoblasts and osteoclasts
- Upset balance  bone loss (osteolysis) seen as cortical thinning/abnormal bone growth (lesions)

34
Q

Why do resorbable polymers have poor bioactivity?

A

Because they release wear particles causing inflammatory responses in vivo. They are not mechanical strong so the likelihood of this increases. Moisture makes the polymer swell as it absorbs the water, causing the mechanical properties to deteriorate and eventually fail.

35
Q

How do UHMWPE particles lead to bone resorption?

A

Body tries to get rid of the particles by oxidising them using white blood cells but as the cells release oxidising agents they kill bone

36
Q

What happens to UHMWPE that had been gamma irradiated in air?

A

Gamma irradiation in UHMWPE causes a cleavage of chemical bonds of the PE chains, which create free radicals. The free radicals are highly reactive species that tend to produce a complex series of reactions. These reactions release hydrogen atoms.

37
Q

What is the effect of ageing on wear?

A

Makes polymer less wear resistant by plasticising matrix

38
Q

Name three types of wear that can occur between articulating surfaces

A
  1. Adhesive wear: wear due to localized bonding between contacting solid surfaces leading to material transfer between two surfaces or loss from either surface
  2. Fatigue wear: wear where a number of cycles is needed to generate debris. The fatigue process in metals may induce the generation of surface and subsurface cracks, which after a critical number of cycles results in severe damage, such as large fragments leaving the surface.
  3. Three body (abrasive) wear: the removal of material from a surface by a harder material impinging on or moving along the surface under load.
39
Q

How does the wear of different classes of materials compare with that of the traditional metal/polymer combination?

A 
Cup | Head | Wear Volume, mm/year
UHMWPE | Metal | 56
Cross Linked UHMWPE | Metal | 2.8
Metal | Meta| | 0.9
Ceramic | Ceramic | 0.004
40
Q

What are the advantages and disadvantages of sterilisation?

Gamma Irradiation

A 
Gamma Irradiation-
Pros:
•	Increase cross linking and strength, thus increased resistance to wear
•	Gamma rays release free radicals, which release hydrogen atoms
•	High penetration ability
•	Low temperature 
•	Effective
•	Easy to control
•	No residue
Cons:
•	Induce structural properties changes
•	Dose rate is lower than electron beams
•	Take a long time
41
Q

What are the advantages and disadvantages of sterilisation?

Ethylene Oxide

A

Pros:
• A colourless and odourless gas
• Good diffusion and absorption in most heat sensitive plastic materials
• Works at 25 degrees and 55 degrees (low temp = low cost)
• Can pass through the membrane of the packing containing the elements
Cons:
• Slow process (~60 hours)
• Pure EtO gas boiling point is 10.73 degrees at stmospheric pressure
• Bad handling may cause adverse health effects:
- Flammable
- Toxic
- Carcinogenic
• Induce structural property change

42
Q

What are the advantages and disadvantages of sterilisation?

Gas Plasma

A 
Pros:
•	Low temperature
•	Improved cell interaction
•	Increasing wettability on surface of biodegradable polymers
•	Fast
•	No toxic chemicals 
•	No environmental impact 
Cons:
•	May cause changes in chemical and mechanical properties of polymers
•	Leave reactive species
43
Q

What are the advantages of using PEEK as a load bearing material?

A

PEEK plastic toughness and rigidity, and to strike a balance. Especially its alternating stress of excellent fatigue resistance exhibits good creep resistance. With a high Tg (glass transition temp.), along with good balance of properties, allows it to withstand high loadings for long periods at high temperature without permanent deformations

44
Q

Why is PEEK so inert?

A

It is non –cytotoxic, non-mutagenic, non-immunogenic, - non carcinogenic. Low tissue adherence, simplified retrieval

45
Q

What are the limitations of using PEEK in joint replacement?

A
  1. Expensive, applicable for highly demanding application
  2. Processing at high temperatures
  3. Low resistance to UV light
46
Q

How can fixation of PEEK implants be achieved?

A

Limited fixation of PEEK so bioactive coatings may be applied:
o Ti + HA coatings
 Interference fit
 Bone ingrowth
Limited fixation once HA is resorbed. This relies on mechanical interlock
Bone cement acts as a grout, transferring load to supporting bone. Compliance at PEEK interface could compromise fixation.
Mechanical bone screws could be used.

47
Q

What are the benefits of fibre reinforcing PEEK?

A
  • Biocompatible
  • Low wear rate
  • Chemical stability
  • Imaging capability
  • Tailored stiffness
48
Q

What are the difficulties associated with the development of carbon composite implants?

A

Examples - poor surgeon acceptance as it is a completely new concept, previous designs have failed catastrophically, difficult to manufacture

49
Q

What potential benefits do composite materials have to offer?

A
  • Excellent biocompatibility
  • Supporting cell activity
  • Excellent mechanical properties
50
Q

Why can’t conventional plasma spray coating techniques be used on polymer matrix composites?

A

Too hot, will melt polymer

51
Q

What can be used as an alternative to plasma spraying for coating polymer matrix composite implants?

A

Adhesively bonded hydroxyapatite (HA), solution deposited HA

52
Q

What are the failure mechanisms of carbon fibre composite materials?

A
  • The longer the fibre, the lower the strength because the more likely it is to contain a defect of a particular size. Defects in the fibre
  • Fibre fracture
  • Damage during tensile load and shear load
  • In compression, fibre buckling and the formation of kink bands. Brittle fibres formed.
  • In carbon fibre, shear failure can also occur in compression
53
Q

What effect does moisture have on a polymeric matrix material?

A

Moisture absorption from environment causes swelling in the polymer as well as a decrease of Tg

54
Q

What are Macrophages?

A

A large phagocytic cell found as a mobile white blood cell, especially at sites of an infection

55
Q

Rank metals, ceramics, and polymers in terms of resistance to corrosion

A

Corrosion resistance:
Ceramics – High
Polymers – Medium
Metals – Low

56
Q

What degradation products are possible from metals, ceramics and polymers?

A

Metals: metal corrosion products usually ceramic (oxide/hydroxide/chloride)
Ceramics: Little evidence of alumina, zirconia
Polymers: contain harmful components.

57
Q

Describe the difference in ion release behaviour between cobalt and chromium.

A

both escape the joint capsule but chromium has very high release evidenced by the amount found in blood. Cobalt is found everywhere but not at such high concentrations – this is because it is excreted easily.

58
Q

Describe the difference in ion release behaviour between titanium and aluminium.

A

titanium stays local to the implant, aluminum escapes and gets into blood in relatively high quantities

59
Q

Describe the ion release behaviour of nickel from a cobalt chromium alloy. What are the implications of nickel release?

A

Nickel can be found everywhere but stays local to the implant. Cobalt chromium ? Nickel can cause sensitivity reactions in patients

60
Q

Describe a simple method for determining the corrosion potential of metals.

A

Use of the pourbaix diagram/pH diagram:
- Maps out possible stable phases of an aqueous system
- Limitations:
o Apply to pure metals in pure water
o Local micro conditions vary
o Equilibrium takes time to reach
o Alloy materials may behave differently
o Active cells affect processes
o Physiological conditions vary between patients
o Determine whether corrosion occurs but not rate

61
Q

Summarise the adsorption processes that takes place when an implant is introduced into the body.

A

Within 1 sec, in vivo initial proteins absorbed
Within minutes – layers of proteins formed
Absorption of proteins and cells on surface occurs in 3 steps:
 Transport to interface
 Absorption reaction
 Conformation and adherence of cells
Within 5 minutes – 75% complete
Equilibrium after 1 hour

62
Q

Describe conformation and denaturation

A

Conformation: when a protein conforms/retains its natural state
Denaturation: when a protein’s molecular structure is modified not permitting it to retain its original state

63
Q

What factors affect the degree of conformation and denaturation?

A

Absorbed proteins undergo conformational changes:
Low structural stability  unfolding  further bond  formation  denaturation

Denaturation dependent on:
Protein – Heat - Time – Solvent

64
Q

What is the isoelectric point?

A

The pH at which a particular molecule carries no net electric charge
Proteins contain both acidic and basic functional groups

65
Q

How might a charge on an implant affect a protein?

A
  • Amino acids that make up proteins may be positive, negative, neutral
  • At a pH below their pl, proteins carry a net positive charge; above their pl they carry a net negative charge
66
Q

What does oxide stability depend on?

A

Crystallinity, porosity, adherence, heat of formation, ability to reform

67
Q

What are the unique properties of titanium oxide that make it more biocompatible than other oxides?

A

The unique property is that titanium oxide forms a thin passive oxide layer. This oxide layer in turn leads to a high resistance to corrosion. The protective passive layer is retained at pH values of the human body due to titanium having an oxide isoelectric point.
The structure must inhibit ion/electron migration across oxide interface

68
Q

Why does increasing the oxide layer on a titanium implant improve biocompatibility?

A

The unique property is that titanium oxide forms a thin passive oxide layer. This oxide layer in turn leads to a high resistance to corrosion. The protective passive layer is retained at pH values of the human body due to titanium having an oxide isoelectric point.
The structure must inhibit ion/electron migration across oxide interface

69
Q

What factors can affect the effectiveness of a passivating film?

A

Difficult under low dissolved oxygen concentrations

70
Q

How does the modulus of resorbable polymers compare with that of cancellous bone?

A

Dense biodegradable polymers are closest to cancellous bone modulus strengths.

71
Q

What is denaturing, and what is it dependent on?

A

Conformation: when a protein conforms/retains its natural state
Denaturation: when a protein’s molecular structure is modified not permitting it to retain its original state
Absorbed proteins undergo conformational changes:
Low structural stability  unfolding  further bond  formation  denaturation
Denaturation dependent on:
Protein – Heat - Time – Solvent

72
Q

Why is in-vitro testing important?

A
  • Quick turnover (days)
  • High throughput screening
  • Standardized with appropriate protocols
  • Reduce the use of mice tests
73
Q

What is a systemic effect – give an example

A

Are defined as those effects occurring in tissues distant from the site of contact between the body and the medical device or biomaterial.
Systemic effects can be associated with leachable chemicals or degradation products released from a medical device following exposure to biological fluids and/or inflammatory cells.
Eg) anaemia (shortage of red blood cells)

74
Q

Why are in-vivo tests essential?

A 
Pros:
-	Simulate real body condition 
-	Clinically relevant 
Cons:
-	Expensive 
-	Time consuming 
-	Ethical and regulatory issues
75
Q

What does animal testing aim to do and what are the concerns?

A

Animals display similar human physiology, which provides information about how the human body will react to certain substances.

Concerns are ethical and that although similar not exactly the same and cannot be guaranteed for human results be the same

76
Q

What are morphological assay methods?

A

Morphological requirements:

  • The variability between patient morphology must be met with flexibility within an implant system
  • The ‘shape’ of an implant effects the kinematics, its structural characteristics and how well it integrates with the host bone
  • Hip stems are structurally over-engineered, but are shaped to get optimum fit with the best quality
77
Q

Why are tantalum and magnesium attractive biomaterials for bone regeneration?

A

Tantalum:

  • Corrosion resistant
  • Bioactive
  • These porous Ta components offer a low modulus of elasticity, high surface frictional characteristics, and excellent osseointegration properties (i.e., bioactivity, biocompatibility, and in-growth properties)

Magnesium:

  • Good biocompatibility, biodegradability, and acceptable mechanical properties
  • Mg is taken into the body daily in substantial amounts, stimulates the growth of bone cells, and accelerates the healing of bone tissue.
  • Elastic modulus of Mg alloys are similar to natural bone, which ensures stress shielding does not occur too much

biomaterials (1 decks)

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