Biomaterials Flashcards
What is material:
strength?
resilience?
toughness?
tensile strength?
compressive strength?
Strength = ability to absorb a lot of stress before breaking (total stress absorbed before #)
Resilience = stress absorbed that can be released again
Toughness = energy absorbed in total until the material fractures
Tensile strength = ability to be pulled or stressed
Compressive strength = ability to withstand stress when squashed.
Why does thermal conductivity matter in restorations?
Restorative materials should expand when heated as much as enamel and dentine.
If not, gaps will form at the edge of restorations, allowing bacterial invasion.
What is the elastic modulus?
What is the elastic region and plastic region?
Young’s modulus.
Measure of how flexible or rigid a material is.
Elastic region: if material is released, it return to its original shape with no deformities.
Plastic region: material is permanently deformed and does not go back to original shape/size
What is corrosion?
The deterioration of intrinsic properties in a material due to environment.
- Due to oxidation in metals.
Can cause failure of metals and alloys due to:
> Loss of material bulk
> Deterioration of mechanical properties
> Leaching of corrosive products.
Conducting material + reactive material + electrolyte = corrosion
What is passivation?
Protecting coating of the material, rendering the material inert.
Why may a filling break which has been there for years?
Because small forces applied over the years, the material became fatigued and fractures due to molecular change in the material (as supposed to a single event with great force).
If apply a stress to material below the yield stress, eventually will become fatigued and fracture.
What is galvanic corrosion?
Movement of electrons from anode to cathode.
- the more electronegative material is the anode, this material corrodes.
- requires two or more inhomogenous metals/alloys to be in contact.
How can you reduce pit corrosion?
Polish amalgam surface to remove oxidation layer.
What is:
- tarnish?
- discolouration?
- dissolution?
- erosion?
- sorption?
- Tarnish = surface discolouration on the metals/alloys due to sulphides and chlorides
- Discolouration: due to extrinsic and intrinsic staining
- Dissolution: solubility, measure of the extent to which material will dissolve
- Erosion: dissolution and surface wear
- Sorption: usually with polymers, associated with dimensional change and flexibility loss.
What metals does amalgam contain? %? purpose?
- Mercury (Hg) = 44-48%
- ~ 400mg in one capsule - Silver (Ag) = 30-35%
- Tin (Sn) = 9-15%
- decreases reaction rate = increased clinical working time - Copper (Cu) = 0-15%
- eliminates gamma-2 phase by converting it to gamma-1
What is amalgam? What are the good and bad properties of amalgam?
An alloy, a mixture of metals (mercury, silver, tin, copper).
Good properties:
- High tensile and compressive strength
- Good working time
- Good longevity
Bad properties:
- Poor aesthetics
- Not adhesive - relies on mechanical retention
- Destructive cavity prep
- Mercury use
How does amalgam set?
What does adding copper do?
Trituration initiated when the alloy powder encounters the elemental mercury (Hg) in the amalgamator:
Ag3Sn + Hg –> Ag3Sn (y) + Ag2Hg3 (y1) + Sn7-8Hg (y2)
y = gamma phase = Ag3Sn
y1 = gamma -1 phase = Ag2Hg3
y1 = gamma -2 phase = Sn7-8Hg
Adding copper impacts the setting reaction:
Converts gamma 2 to gamma 1
Why is copper in amalgam fillings useful?
Adding copper converts gamma 2 to gamma 1.
Sn7-8Hg + AgCu –> Ag2Hg3 + Cu6Sn5
Useful as gamma 2 is most prone to corrosion since highest in electronegativity.
- Eliminating the gamma 2 reduces the chances of galvanic corrosion or creep (protrusion of the amalgam from the cavity).
High copper amalgam has low creep value due to little/no gamma-2 phase.
What are the properties of amalgam?
High compressive and tensile strength
Strong
Prone to corrosion (galvanic corrosion)
Good conductor
Amalgam is prone to creep
Under trituration can result in amalgam that sets too quickly.
y-gamma and y-gamma 1 have high corrosion resistance. y-gamma 2 has low corrosion resistance.
y-gamma has high strength. y-gamma 1 and y-gamma 2 have low strength
What is amalgam creep?
Protrusion of the amalgam from the cavity. It is the physical property of a material to deform over time due to constant application of force or stress.
This can lead to flow of material –> ditching –> secondary caries.
What are the advantages and disadvantages of amalgam bonding agents?
They reduce microleakage and allow adhesion to dental hard tissues as well as increasing strength.
But time consuming and costly.
What are the different types of amalgam cut?
- Lathe cut = made into an ingot and cut into a lathe –> irregular chippings
- Spherical = molten and sprayed where it solidifies to form round droplets
- Admix amalgam contains both lathe-cut and spherical cut particles.
What is light-cured resin-based composites?
A filler polymer material that is strong and durable with outstanding aesthetics. Reason to fail - 2* caries.
What is the composition of light-cured resin-based composites?
- Filler particles
> Silica or quartz
> they are the largest component (silicon dioxide and glass filler particle)- (silica and quartz are chemically identical (both silicone dioxide) but with different crystalline structures (quartz is a specific crystal structure of silica))
- Large resin monomers (high viscosity, large molecules)
> Urethane dimethacrylate (UDMA)
> Bisphonol-A glycidyl methacrylate (Bis-GMA) - Smaller resin monomers that act as ‘dilutents’ to reduce viscosity
> TEGDMA (triethylene glycol dimethacrylate)
> MMA (methyl methacrylate) - Coupling agent
> Silane molecules
- bonds the resin to filler (by establishing covalent bond) - Heavy metal-containing fillers (for radiopacity)
- Photoinitiator/activator
> Photoinitiator - Camphorquinone- produces free radicals to allow the polymerisation to start
> Chemical activator where light can’t access - benzyl peroxide
- produces free radicals to allow the polymerisation to start
- Inhibitor/retarding agent
> hydroquinone
- prevents premature polymerisation
What are compomers?
They are a material combination of composite and glass ionomer. They require bonding agents for good dental surface retention.
Also called Polyacid-modified composite resin.
What are hybrid ionomers?
Contain 80% glass ionomer and 20% composite.
Don’t require a bonding agent for retention
They release fluoride, bonding agent may reduce the fluoride received.
Good for high caries patients.
Useful when light curing access is difficult.
How do resin based composites set?
Resin based composites exhibit a command set (usually 20-40 sec).
Polymerisation is initiated when the composite is exposed to blue light (470nm)
The photons stimulate camphorquinone to release free radicals.
This initiates the polymerisation of the resin monomers.
Polymerisation inhibited by oxygen (apply translucent strip on top of restoration surface to exclude oxygen).
Exhibit a limited depth of cure, some light is absorbed, transmitted or scattered.
What can incomplete polymerisation of composite result in?
Reduced strength and bond strength
Decreased marginal integrity and increased wear.
Water sorption and leaching of uncured monomers.
What factors influence the depth of cure?
- Shade
- Filler loading
- Filler size
- Powder/intensity of curing light
- Distance between curing light and composite
- Time of cure
- Oxygen inhibition (inhibits depth of cure
How do you compensate for limited depth of cure and what is another advantage of this?
To compensate for limited depth of cure, cure using increments.
This also reduces setting shrinkage (typically 2-4% shrinkage).
(when monomers join together to form the polymer, they get closer together. Thus, less gaps = shrinkage).
What are the properties of composites?
- Poor compressive strength (weakest after amalgam, dentine and enamel).
- Relatively high tensile strength (stronger than enamel)
- Can have a good adhesive. bond to enamel and dentine
- Good aesthetics
- Moderate wear resistance (but can be worn by opposing teeth)
- Staining (at margins or at rough surface)
- Shrinkage of 2-4% during polymerisation, with shrinkage reduced by increased filler (causes lower coefficient of thermal expansion though)
- Water sorption (leads to deterioration)
What is the composite bond strengths to enamel and dentine?
Enamel - 20-30 MPa
Dentine - 3-11 MPa
What layer can impact the bonding of a restoration material?
Smear layer
- Tissue debris over the tooth surface when tooth tissue is cut. Contains debris produced by reduction or instrumentation of enamel, dentine or cementum.
It is calcific in nature and obstructs the bonding of restoration material with underlying tooth tissue.
What is the hybrid layer in composite restorations?
Area which the resin of restorative material has interlocked with collagen of dentine by providing micromechanical retention.
What acid does etch contain?
Function of etch?
Phosphoric acid 36%
Increases surface area for bonding
What does enamel bonding agents contain?
Enamel bonding agents (e.g. Bis-GMA) consists of unfilled resin and is extremely moisture sensitive.
What do dentine conditioners do?
Dentine conditioner (acid/conditioner) remove/dissolve smear layer by removing most of the hydroxyapatite and expose a microporous network of collagen resulting in diffusion-based bonding.
Conditioned dentine surface is difficult to wet with bonding agents
> Primers increase the surface free energy (wet) dentine
> Primers increase the wettability of dentine surface which allows the resin to spread and penetrate the tubular dentine
> Improving the bonding of the subsequently applied adhesive resin
> The resin used in them are hydrophobic
What do dentine bonding agents do?
Form a link between the resin primer and the restorative material .
Form resin tags in dentine tubules.
Stabilise the hybrid layer.
What is Glass Ionomer cement?
Acid-base cement material composed mostly of glass filler particles in a polymer matrix.
What are GIC’s used as?
- A restorative material, mainly for where moisture control is difficult.
- For wear cavities close to gingival margin
- Lining material under amalgam and composite (lining/base)
- Luting material (attaches prosthesis to tooth structure).
- Temporary restoration
- As pit and fissure material (e.g. Fuji Triage)
- As permanent direct restorative material in primary and permanent teeth.
What is the composition of GIC? (4) (2 additionals)
- Polyacid (typically polyacrylic acid)
- Glass filler particles (silica-alumina-calcium fluoride flux)
- Tartaric acid - initially retards the setting, so viscosity is lower for longer, followed by a much more rapid set (snap set).
- Water - needed to initiate the process.
Additional/minor components in GIC glass fillers are:
> sodium and aluminium fluoride (act as a fluoride source)
> calcium and aluminium phosphates (act as a fluxes)
What is snap-set of GIC due to?
Tartaric acid
Extends working time
Shorter setting time
How does GIC set?
- Dissolution
- Polyacrylic + water –> poly acid
> Dissociates so hydrogen ion are released.
> H+ ions attack the glass filler particles and cause them to start to dissolve.
> Ions of Na, Ca, Al, F ions dissolve leaving a layer of silica outside the filler particles, but Zn does not get released when GIC sets.
> When F- ions leach out the GIC, hydroxyl ions replace them. - Gelation
- Ca2+ released from dissolution cross-link the negatively charged polyacid chains
- Ca2+ is divalent, and so 2 polyacid chains to 1 Ca2+ ion. - Hardening
- Al3 are also released from dissolution stage
- Al ions are released more slowly than Ca ions and so Ca ions initially link polyacid chains.
- Al is trivalent and so can displace the Ca ions - linking 3 polyacid chains per 1 Al ion.
What is the role of tartaric acid in GIC?
It is a chelator, so can sequester other ions, removing them from other chemical reactions.
Chelates Ca2+ during dissolution phase.
Make it less likely that polyacid chain will be linked by Ca ions compared to Al ions.
Makes gelation phase shorter and hardening phase occur more quickly.