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Year 2 Flashcards

0
Q

What are the disadvantages of amalgam?

A
  1. Poor aesthetic as it is metallic and not tooth coloured
  2. It lacks adhesion which requires retentive features in its cavity preperation. This destroys healthy tooth structure
  3. Amalgam corrodes in the oral environment as it is a metal and it reacts with oxygen that leads to weakening of the material leading to marginal leakage
  4. Restorations in the oral environment experiences repeated occlusal loading and this subjects the amalgam to creep (up to 3%). This also leads to marginal ditching
  5. Mercury hazard; there are indications of some mercury vapour released upon placement of freshly mixed amalgam and also from wear and corrosion of the restoration
  6. Galvanic effects; reaction between metals in restoration with different degree of electro negativity resulting in a strong metallic taste in the mouth
  7. Because dental amalgam is brittle is has to placed in bulk which means the cavity has to be cut deep and cavo surface angle of 90 degrees. This leads to more tooth destruction
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1
Q

Why do we use amalgam?

A
  1. Inexpensive (compared to resin composite)
  2. Ease of use and manipulation (has less steps in procedure compared to composite)
  3. Proven track records and familiarity of more than 100 years
  4. Resin free; less likelihood of allergies than composite
  5. Durable as it has high strength, rigidity and wear retention (if placed with the right techniques)
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2
Q

How are alloy particles on dental amalgam produced?

A
  1. Lathe cut amalgam; machining of solid ingot of the alloy on a lathe. The alloy powder is then homogenised by heat by placing the powder in boiling water
  2. Spherical; various ingredients of alloy are melted together and sprayed into an inert atmosphere where droplets solidify as small spherical pellets. This process is know as atomisation. Particles could me fine cut (35 micrometer) or micro cut (25 micrometer)
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3
Q

How does properties of amalgam vary?

A
  1. It’s properties depends on the composition of alloy
  2. It’s handling properties depend on the shape and size of alloy particles;
    a) NARROW head condenser is used for IRREGULAR shaped alloy particles
    b) WIDE head condenser is used for SPHERICAL shaped alloy particles
    * if your body is like a SPHERE may be you should wear WIDE fit clothes
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4
Q

What is the alloy composition for both traditional/conventional and blended amalgam?

A 
Conventional
1. Silver (Ag); 67-74%
2. Tin (Sn); 25-27%
3. Copper (Cu); 0-6%
4. Zinc (Zn); 0-2%
*silver and tin are main combination producing an intermetallic compound Ag3Sn known commonly as GAMMA
Blended/dispersion modified
1. Silver (Sn); 70%
2. Tin (Ag); 16%
3. Copper (Cu); 13%
4. Zinc (Zn); 1%
*dispersion modified is when 2 parts conventional lathe-cut alloy is mixed with 1 part of Ag-Cu eutectic spheres
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5
Q

Describe the setting reaction of conventional dental amalgam

A

Ag3Sn (GAMMA) + Hg > Ag2Hg3 (GAMMA 1) + Sn7Hg (GAMMA 2) + Ag3Sn (unreacted alloy)
*gamma 1 + gamma 2 = amalgam matrix component
This process is done by vigorously mixing the alloy powder with liquid mercury

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6
Q

Describe properties of the solid phases of conventional dental amalgam

A 
Gamma
1. Ag3Sn
2. Original alloy
3. Some remains unreacted
4. Hard an strong (contribute to the overall strength of amalgam)
5. Highest tensile strength
Gamma 1
1. Ag2Hg3
2. Small crystals
3. Brittle phase
4. Intermediate strength
5. Low melting point (127C)
Gamma 2
1. Sn7Hg
2. Long blade like crystals
3. Soft phase
4. Low tensile strength
5. Prone to corrosion
6. High flow
*This phase is bad and should be eliminated. That's why we use high Cu (dispersion modified) amalgam
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7
Q

Describe the setting reaction of dispersion modified amalgam

A
  1. Stage 1 (similar to conventional amalgam)
    Ag3Sn + Hg > Ag2Hg3 + Sn7Hg + Ag3Sn
  2. Stage 2 (reaction of gamma 2)
    Sn7Hg + Ag-Cu (eutectic spheres) > Cu6Sn5 + Ag2Hg3 (production of gamma 1)
    This helps to remove if not all, most of gamma 2
    *however in excess Hg gamma 2 will not be completely removed
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8
Q

State the physical properties of amalgam

A
  1. High thermal conductivity
  2. High electrical conductivity
  3. > 2mm aluminium radiopacity (enamel and dentine have 2mm Al radiopacity)
  4. Thermal expansion; 25ppm/C (x3 of enamel thus reacts faster than dental tissues which could lead to marginal leakage)
  5. Lustrous and shiny colour
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9
Q

State the erosion reaction of dental amalgam

A

Primarily involved the gamma 2;

Sn7Hg + saliva (H20, Cl) > SnO2 + Sn(OH)6 (tin salts) + Hg (free mercury)

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10
Q

Describe the properties of amalgam in relation to time and strength

A
  1. It is weak 1 hour after placing the material as it hasn’t reach it’s optimum strength and is considered soft
  2. After 24 hours it reaches 90% of its strength
  3. After 7 days it reaches its maximum strength
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11
Q

When is a mercury toxicity most likely to occur?

A
  1. Direct contact with mercury
  2. Inhalation of mercury vapour due to mercury spillage in the surgery
  3. Placement of new and removal of old restoration (not likely to reach toxic levels)
  4. Hypersensitivity to mercury
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12
Q

What are the environmental problems related to amalgam?

A
  1. Left over amalgam from capsules that are not disposed properly
  2. Mercury spillage
  3. Residual mercury in capsule
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13
Q

Compare the strength of amalgam and composite

A
  1. Initially composite performs better but deteriorates over time
  2. Amalgam initially performs worse but gradually stabilises and performs better
    However the strength of amalgam is subject to the operators skill. Poor condensation leads to porosity and presence of excess mercury which reduces th strength of amalgam. Poor condensation also causes marginal adaptation to be poor and this increases the potential of marginal leakage. Undertrituration results in dry amalgam that won’t pack properly
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14
Q

List the ideal property requirements for denture based material

A
  1. Ease of fabrication and repair
  2. Biocompatible
  3. Ease of cleaning
  4. Resistance to bacterial growth
  5. No taste or odour; does not comprise quality of life
  6. Clinically, dimensionally, mechanically, thermally stable in oral fluids, drinks and moisture (as oral environment is aggressive and constantly under attack)
  7. Low cost and good shelf life
  8. Radiopacity; denture can fracture which could the be inhaled or swallowed. We need to be able to detect where it is
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15
Q

List the thermal properties of denture based material

A
  1. High softening temperature; hot drinks can have temperature up to 60C
  2. High thermal diffusity; dentures isolates area of soft tissues from any sensation. If the thermal diffusity is low we won’t be able to convey the message fast enough and the food/drink would have scalded the mouth
  3. Expansion coefficient similar to that of artificial teeth; so that changes in base causes similar change to teeth
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16
Q

Describe the strain and stress curve

A
  1. When we increase stress the strain on the material increases
  2. The first part of the curve is a straight line known as “elastic modulus” of the material where you have direct representation of the stress-strain relationship
  3. It then reaches a point “proportional limit” where it slowly loses elasticity
  4. At “yield stress” it reaches its “elastic limit” and loses elasticity
  5. Region from starting point to “yield stress” is known as “elastic region”. When we apply steed and remove it, the material returns to its original dimension
  6. Region from “yield stress” onwards is knows as “plastic region”. Any stress applied is permanent (the material loses stability)
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17
Q

What are the mechanical requirements of denture based material

A
  1. High elastic modulus to prevent permanent deformation (when chewing forces are removed it will retain its original structure). Enables us to use it in thin section
  2. High proportional limit to prevent permanent deformation
  3. High flexural/tensile strength to resist intraoral forces
  4. High fatigue strength (fatigue: constant applying of small forces that eventually break the material). In the oral environment we experience forces that are continuous. We need the material to withstand the multiple loading
  5. High impact strength to withstand fracture if dropped
  6. Hard, abrasion resistant surface so it is polishable
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18
Q

State 3 basic characteristic of methyl metracrylate (MMA)

A
  1. Clear colourless liquid with intense odour
  2. Inhibited by hydroquinone/or it’s derivates
  3. Can be polymerised (forms C=C)
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19
Q

What happens when we combine materials of different moduli?

A

Different moduli = different differential stress = materials will fail and detach from one another as they don’t respond the same way to stress
*to combine material with different moduli you need good adhesion

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20
Q

How is PMMA for surgical use designed?

A

Two part formulation of powder and 1 part liquid ready for “dough processing”

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21
Q

State the volume shrinkage of 100% MMA?

A

1 MMA monomer > 1 PMMA polymer ~21% shrinkage (huge gaps leads to dimensional change that would be too much to compensate)

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22
Q

State the volume shrinkage of MMA in two part powder/liquid mixture

A

2 part powder (already polymerised PMMA) + 1 part MMA liquid = 6-7% shrinkage
*techniques could be used to further reduce this shrinkage

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23
Q

State the principal constituent of acrylics

A

Powder
1. Acrylic polymer beads (PMMA); long chains of molecules that react with one another
2. Initiator; benzoyl peroxide
3. Pigments
4. Dyes
5. Opacifiers
Liquid
1. MMA monomer
2. Cross linking agent; EGDMA which connects one polymer chain to another making the material stronger
3. Inhibitor; hydroquinone which prevents the monomers from polymerising on its own (extends shelf life)

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24
Q

What happens go PMMA spheres upon contact with liquid MMA?

A
  1. The PMMA spheres swell up
  2. Their consistency change and they become sticky
  3. The mutually adhere to one another
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25
Q

State the 4 physical stages of mixing acrylic

A
  1. Sandy mixture; momomer and PMMA spheres are like sand and water
  2. Tacky mixture; diffusion of MMA into PMMA spheres has begun
  3. Dough stage; should be packed into flask to minimise shrinkage and porosity
  4. Rubbery stage; polymerisation reaction has gone too far
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26
Q

Describe the radical polymerisation activation phase (the follows the physical mixing of powder and liquid)

A
  1. Benzoyl peroxide is the initiator

2. At 60C it undergoes thermal decomposition (gets cleaved in the middle) to produce 2x free radicals

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27
Q

Describe the 4 stages of free radical polymerisation

A
  1. Activation; either by mixing or heat producing free radicals
  2. Initiation; attack of first free radical on monomer molecules (when the radicals attack monomer they cause the monomers to come together and form polymers)
  3. Propagation; the auto accelerating growth of one linear polymer chain (more monomers come together)
  4. Termination; the reaction comes to a stop by various radical annihilation (you have no free radicals) or inhibition of segmental movement (you have no free monomers). The chains of polymers with a free radical at the end come together to form a long chain and the free radicals become inactive
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28
Q

What causes shrinkage during polymerisation?

A

When MMA form polymers they come together and there is a gap where the monomers used to be. This causes shrinkage

29
Q

State the problems associated with exothermic nature of the reaction (Heat of polymerisation: 57 kJ/per mole MMA)

A
  1. We need to heat the mixture to 60C to initiate the benzoyl peroxide
  2. As soon as the reaction starts the temperature increases rapidly. We want it to be less than 100C but it will go higher than that (bp of MMA is 100C)
  3. Once temperature exceeds its bp the MMA will boil and this causes porosity
    * the way to overcome this is to increase the pressure. This causes the bp of MMA to be higher and thus it wouldn’t matter if the reaction exceeds 100C
30
Q

State the arrangement of polymer chains in denture based acrylic

A

Amorphous PMMA + 5% EGDMA cross linking agent

*the new polymer is pseudo chemically bonded to old polymer

31
Q

What are types of porosity in acrylic?

A
  1. Gaseous; monomer has boiled inside mix and this causes fine bubbles esp in thick section
  2. Granular; insufficient monomer was added to polymer (insufficient to wet the spheres)
  3. Shrinkage porosity; irregular voids on surface and through out material because;
    a) Pressure in flask is too low
    b) Insufficient material in flask (more material than actually needed in flask reduced overall shrinkage to only 1-2%)
32
Q

What are problems associated with acrylic (patient related problems)?

A
  1. Insufficient curing leads to excess monomer remaining. This increases the risk of the monomers leaking out and causes irritation (burning mouth syndrome)
  2. The material absorbs water (~2%) and this could be good as it compensates for the 2% shrinkage. However the water can also leak out causing dehydration of material which leads to surface crazing (cracks on surface of material). This could leads to failure of material or cause organism (candida albicans) to colonise the surface
  3. Cleaning product can cause harm to the material;
    a) Hot water; compromises the stability of material
    b) Acetone; has plactisicing effect (makes material softer) thus it would lose strength and won’t be able to withstand impact forces
    c) Sodium hypochlorite; corrodes metal
33
Q

Define cohesion and adhesion

A

Adhesion; force that binds two DISSIMILAR materials together

Cohesion; attraction between SIMILAR atoms or molecules within a material

34
Q

What are advantages of adhesion?

A
  1. Offsets polymerisation shrinkage
  2. Conserves tooth structure
  3. Enables aesthetic restoration to be carried out
35
Q

When should adhesive be used?

A
  1. Replaces carious and fractured tooth
  2. Fills erosion and abrasion defects in cervical area
  3. Cements crowns
  4. Bonds orthodontic brackets
  5. Treat dentinal hypersensitivity
  6. Repair fractures porcelain, amalgam and composite restorations
  7. Core build up foundation
  8. May decrease incident of post op sensitivity
36
Q

What are two main factors of bonding

A
  1. Adhesion (chemical)

2. Attachment (mechanical)

37
Q

Proper adhesion requires intimate contact between adhesive and substrate. This contact is affected by?

A
  1. Wettability of the substrate surface
  2. Viscosity of the adhesive
  3. Surface roughness (morphology) of the substrate
38
Q

Explain the principles of surface roughness

A
  1. Increase surface roughness can increase potential surface area for bonding and thus improve it (greater surface area = more contact points = more chemical bonds)
  2. However it also creates air entrapment (which reduces effectiveness of bonding)
  3. High viscosity adhesive is prone to cause air entrapment as their stiffness bridges the small cracks as opposed to flowing into them
39
Q

Explain the principles of wettability

A
  1. In order for an adhesive to bond between two materials it must make intimate contact with the two solid surfaces and “wet” them. It must also penetrate the surface porosities
  2. To achieve good wettability;
    a) The contact angle must be low (contact angle is the angle between the liquid surface and the solid surface). It relates to the ability of the adhesive to spread and cover surface of the substrate. In other words contact angle is the measure of surface wettability
    b) The liquid must have a critical surface energy lower than that of the solid (this will enable it to readily spread over the substrate)
    * tooth surface contamination (saliva and smear layer) must be removed as a clean surface increases surface energy of substrate and reduces the contact angle
    c) The liquid should have a low viscosity so that it can spread easily but not too runny that it is impossible to control
40
Q

State the microscopic process for formation of adhesive joint

A
  1. Clean, stable and rough surface
  2. Good wettability by adhesive
  3. High contact points (able to penetrate and create unified contact)
  4. Strong cured adhesive (gives mechanical strength against shear force but creates setting shrinkage)
41
Q

Based on principles of wettability state ideal characteristic of adhesive

A
  1. Low viscosity
  2. High surface wetting; adhesive surface tension < substrate surface energy
  3. High surface tension to promote capillary action (to fill pores) and dentine tubules
42
Q

Define penetration coefficient

A

Penetration coefficient is the ability of a liquid to fills cracks and crevices
PC = [(surface tension) (cos of contact angle)] / (2 x viscosity)

43
Q

Describe structure of dental adhesion joint

A

(Adherent 1) | Adhesive || (Adherent 2)
|; interface 1 (enamel and dentin)
||; interface 2 (composite, amalgam etc)

44
Q

State the steps in forming good adhesion

A
  1. Clean surface of adherent
  2. Good wetting of adherent surface by adhesive
  3. Initiate adaptation between adhesive and adherent
  4. Bonding (physical, chemical and mechanical)
  5. Good curing
    * mechanical bond is the strongest
45
Q

State the consequence if material is unable to penetrate cracks

A
  1. Initially not much problem and the restoration will be in place
  2. Materials in oral environment are always subjected to forces and change in temperature. This will put pressure on the material
  3. Crack formation > crack propagation > detachment (as there won’t be contact between material and original surface)
46
Q

Describe the acid etch technique

A
  1. Uses acid in liquid/gel form; usually phosphoric acid
  2. Creates micro pores of 5-50 microns deep which increases surface roughness of enamel at microscopic level. This increases it’s surface energy which improves wettability
  3. Etch time of 10-60s (varies from one enamel to another)
  4. Surface changes from smooth and glossy to frosty
  5. Removed with water and air dried for 20s
    * use copious amount of water because if the acid is still there it will continue to work and damage the surface
47
Q

What are the conditions for efficient dentine bonding?

A
  1. Clean surface
  2. Remove saliva and moisture
  3. Ensures good wettability
  4. Have hydrophilic properties
  5. Maximise intimate adaptation
  6. Appropriate viscosity
  7. Start reaction by curing (good irradiance will provide good curing and good micro mechanical retention)
48
Q

Why is adhesion to enamel easier than adhesion to dentine?

A
  1. Enamel is primarily hydroxyapatite (high surface energy)
  2. Dentine is made up of two distinct substrates; hydroxyapatite and collagen (low surface energy)
  3. Dentine has a complex structure as it is permeated by tubules through out
  4. Has water which is a competitor with adhesive material bonding to it (water + adhesive not dentine + adhesive)
  5. A smear layer is formed when dentine is cut which cannot be physically removed
  6. Dentine is hydrophilic (fluid is pumped from pulp and it is always wet) and requires hydrophilic monomers when most adhesive are hydrophobic
    * there is also a variation in size and shape of dentine tubules (largest and most concentrated near the pulp)
49
Q

Describe characteristics of smear layer

A
  1. Produced by instrumentation
  2. Consists of dentine debris and bacteria
  3. Reduces dentine permeability by 86%
  4. Thickness of 0.5-5.0 microns
  5. Very soluble in weak acids
  6. Blocks tubule openings
  7. To achieve strong bond you either strengthen it in situ or remove entirely
50
Q

What are MPBS components?

A 
Conditioners;
1. Remove/modify smear layer
2. Acid solution of H3PO4 or diluted HNO3
3. Rinsed off after application
Primers
1. Solution of adhesion promoter
2. Di functional molecule such as HEMA
3. Dissolved in water, ethanol, acetone
Bonding agent/sealers
Conventional enamel bonding agent (mixture of Bis GMA and HEMA)
51
Q

How are MPBS components applied?

A

Conditioners
1. Applied for few seconds
2. Removed with copious water (to ensure it does not continue or attack dentine)
3. Strips away smear layer
4. Demineralise outer dentine (hydroxyapatite) of about 4 micrometers deep
5. Leaves behind a collapsed collagen (once air dried to remove water)
Primers
1. Leave adhesion promoter (coupling agent) physically adsorbed on dentine with hydrophobic (metracrylate) group exposed to create a favourable wetting surface for bonding agent
2. It consists of a bifunctional monomer (coupling agent) that is able to combine two distinctively different material
3. It contains solvent (acetone) that readily evaporates with water from dentine surface and thus is known as “water chasers. As they displace the water molecules they carry the coupling agent along as it penetrates the dimineralised dentine
4. Eg; HEMA, BDPM
5. Applied by brush sometimes with several coats
6. Coupling agent consists of three component; methcrylate group - spacer - reactive group. It links the collagen (reactive group) with resin (metracrylate group)
7. Permeates the collapsed college network and re expands it via binding of the reactive group (of the coupling agent) to the collagen
8. Primer must be able to penetrates fully into and saturate the dimineralised collagen layer
Sealer/Bonding resin
1. Equivalent to enamel bonding agent
2. May consist of hydrophobic monomers alone (Bis GMA/TEGDMA) or may include adhesion promoters (HEMA)
3. Applied by brush to cover treated surface
4. Monomers penetrate into tubules and reacts with metracrylate group of the coupling agent in the collagen
5. Light curing; polymers cross link with collagen forming a hybrid layer. Provides micro mechanical adhesion and tubule sealing properties
*Resin composite may now be placed and cured

52
Q

Describe the dentine bonding process

A
  1. Acid etch (by conditioner); removed smear layers and dissolves hydroxyapatite
  2. Slightly dry the tooth which collapses remaining collagen framework (but do not dry excessively)
  3. Permeation of the tubules (by primer) swells up collagen and primer binds reactive group of coupling agent to dentine
  4. Bonding agent applied causes monomer penetration inside collagen via binding to metracrylate group of coupling agent
  5. Upon curing, monomer polymerises with collagen forming a hybrid layer
53
Q

How is the hybrid layer formed and how does it promote bonding?

A
  1. A sequence of treatments that allow monomers to permeate dentinal collagen
  2. On polymerisation an inter penetrating pair of network is formed from polymer chains;
    a) of collagen and dimethacrylate resin
    b) results in micro mechanical bond >20 Mpa
    c) without specific tag formation
  3. The affected dentine layer is known as hybrid layer
    * bond strength in dentine is determined by quality of hybrid layer and it’s quality is dependent on ability of primer and bonding resin to (a) infiltrate (b) cross link inside collage mesh
54
Q

What are MMPs and what do they do?

A
  1. Metallic metalloproteinase
  2. Converts insoluble collagen fibrils > soluble gelatin peptides
  3. Causes a loss of continuance between hybrid layer and anchoring mineralised collage fibrils in underlying dentine
  4. Leads to loss of retention
  5. Dormant but activated by acid etching
55
Q

How do we stop MMP?

A
  1. MMP require calcium to maintain their tertiary structure and zinc ions for their catalytic hydrolyse activity
  2. By treating acid etched dentine with divalent cation chelators like EDTA we can inactivate the MMP
  3. Chlorhexidane can also be added in primer to prevent their action
56
Q

State the different types of dentine bonding agent

A
  1. Etch and rinse separately
    a) Acid + primer + bond
    b) Acid + primer/bond
  2. Self etch adhesive
    a) Acid/primer + bond
    b) All solution in one
57
Q

Describe properties of glass ioniser cement

A
  1. Two types of component;
    a) Liquid (acidic solution); poly acid (acrylic acid/itaconic acid copolymer or maleic acid/tartaric acid)
    b) Powder (base); silica (SiO2), alumina (Al2O3), calcium fluoride (CaF2)
  2. Acid penetrates glass particles and release metal ions (Al, Ca)
  3. Metal ions reacts with polyacylate anions to produce cross linked polyacrylate
    * Ca (divalent) is released from glass faster than Al (trivalent) and thus it’s a two stage reaction
  4. Fluoride release occurs in two stage; initial burst and slow term release over months (release is more rapid at low pH). GIC restorations can be recharged by topical application of fluoride
  5. Used in anterior filling, fissure sealing and cementing
58
Q

State benefits of GIC

A
  1. Direct bond with dentine and enamel
  2. Bonds to moist tooth
  3. Fluoride release
  4. Excellent marginal seal
  5. Thermal expansion similar to that of tooth
  6. Biocompatible; ideal match for dentine
59
Q

State limitations of GIC

A
  1. No command set (lack of control)
  2. Slow development of modulus and strength
  3. Low fracture toughness
  4. Low abrasion (wear) resistance
60
Q

State properties of resin modified GIC

A
  1. Powder; fluroaluminosilicate glass
  2. Liquid; poly acid (poly acrylic acid or HEMA initiators)
  3. Setting reaction is slower due to presence of resin
  4. Used as filling, bases or liners
61
Q

State advantages and disadvantages of resin modified GIC

A

Advantages
1. Fluoride effect
2. Chemical/molecular bond to dentine and enamel
3. Low solubility
4. Prolonged working time
Disadvantage
1. Swell up thus can split some ceramic crowns

62
Q

Compare GIC and composite bonding

A 
GIC 
1. Ion exchange
2. Chemical bond
3. Hyper mineralised zone
4. No primer required 
Composite
1. Etching demineralise tooth 
2. Multiple primer/bonding steps
3. Mechanical bond
4. No acid base reaction
5. No ion exchange
6. Primer acts as fluoride barrier
63
Q

Describe composition of composite

A

Two major component

  1. Matrix; monomer (Bis-GMA), initiator, inhibitor (prevents monomer from polymerising), pigments
  2. Reinforcements;
    a) Inorganic filler; glass, quartz, colloidal silica
    b) Coupling agent
64
Q

Describe classification of resin composite

A
  1. Filler size; homogenous (ceramic particles), hybrid (more than 1 range of size), heterogenous (micro silica in prepolymerised resin)
  2. Matrix monomer type
  3. Viscosity
65
Q

Why do we include fillers in composite?

A
  1. Reduce polymerisation shrinkage
  2. Reduce dimensional change with temperature
  3. Improve hardness and compressive strength
  4. Increase stiffness
  5. Increase strength and fracture resistance
  6. Increase radiopacity
  7. Enhance aesthetics
66
Q

Describe stages of light curing

A
  1. Initiation; activation of camphorquinone by using light with wavelength of ~470nm. Absorption of light excites it and produces free radicals
  2. Propagation; free radicals start attacking monomers and the monomers links together to form polymers (C=C becomes C-C)
  3. Termination; when there are no more free radicals the polymerisation reaction will stop
67
Q

What causes shrinkage?

A
  1. When monomers becomes polymers there is a gap in the space where the monomers used to occupy
  2. Depends on shrinkage stress. Low shrinkage stress means that the restoration will stay in place. High shrinkage stress means that the material will start pulling away > marginal leakage
68
Q

How can stress be offset in a clinical placement?

A
  1. Increment technique
  2. Light irritation protocols (soft start light curing unit first then full blast thus slowing down the rate of reaction)
  3. The use of low modulus lining material to act as stress absorber
69
Q

Discuss macro filler, homogenous/heterogenous micro filler and their properties

A

If a material has >60% filler it is considered packable and if it has t exhibit a smooth finish upon polishing

  1. Homogenous micro filler
    a) 0.01-0.05 micrometer of colloidal silica
    b) It exhibits a smooth finish upon polishing
    c) The microfiller particles has a large surface area in contact with the resin which meant a large amount of resin is need to wet the fillers and this causes a change in handling propertiess
    d) If the same fluid consistency was to be maintained with micro fillers it is hard to obtain a high filler loading (only 20%)
  2. Heterogenous micro filler
    a) Prepolymerised micro filler + micro filler
    b) This increases the surface loading
70
Q

Discuss nano fillers and their properties

A
  1. Must include components with at least one dimensional of the order that is less than 100nm
  2. Displays superior gloss retention without comprising mechanical properties
  3. More studies needed
  4. Some issues with their ability to cross glove barrier and even enter cells
71
Q

Discuss hybrid fillers and their properties

A

Hybrid filler

  1. Combination of macro filler and micro filler
  2. Large glass particles with small colloidal silica particles
  3. Combines their characteristics (good filler loading and good aesthetics)

Symposia (5 decks)

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