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BDS3 Past Paper By Topic (05-19) > Dental materials science > Flashcards

Dental materials science Flashcards

1
Q

State reaction of heat-cured acrylic resin.

A
  1. Initiation reaction (With heat, benzoyl peroxide(initiator) is split into benzoyloxy radicals and then further into phenyl radicals) 2. Chain propagation 3. Chain termination Anson: images available on document page 102
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2
Q

Disadvantages of cold cure acrylic resin for temporary crown construction (2.5)

A

Poor colour stability Less strong Less surface hardness More initial deformation and creep Anson: Full table of comparison between heat and cold-cure acrylic on page 102

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3
Q

Why is there setting expansion of gypsum? (1)

A

Because of the formation of gypsum crystals Long version: Hemihydrate dissolves in water to form dihydrate Dihydrate becomes supersaturated and precipitate by nucleation Dihydrate crystals grow to form spherulites Contacts are not geometrically perfect Million points of contact generate force to become crystal growth pressure Expansion

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4
Q

What factors affecting proper amount of water to be mixed with powder? (1)

A

Whether a catalyst is added (KE solution) Human error, e.g. parallax error when reading cylinder

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5
Q

What properties, aspects or behaviour affect the dimensional accuracy and stability of : i. impression (1)

A

time

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6
Q

What properties, aspects or behaviour affect the dimensional accuracy and stability of : ii. model poured from the impression (1)

A

Water and powder ratio

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7
Q

What properties, aspects or behaviour affect the dimensional accuracy and stability of : iii. wax pattern (1)

A

Time

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8
Q

What properties, aspects or behaviour affect the dimensional accuracy and stability of : iv. investment mould made from that pattern (2)

A

Investment mixing ratio; time from wax pattern

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9
Q

What properties, aspects or behaviour affect the dimensional accuracy and stability of : v. casting (2)

A

temperature of casting improper reservoir location

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10
Q

• List the key stages of the setting reactions of heat-cured acrylic? [3]

A

Sandy, stringy, doughy, rubbery

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11
Q

a. What is the setting reaction of alginate impression? (1)

A

• (CaSO4).2H2O ↔ Ca2+ + SO42- + 2H2O • Na(Alg) + Ca2+ –> Ca(Alg) + Na+ (egg box structure) • Sink: 3Ca2+ + 2PO43- –> Ca3¬(PO4)2 Anson: more textbook info on page 102

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12
Q

What is “coordination complex”? (2)

A

A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bond

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13
Q

“Syneresis” molecular theory. (3)

A

Syneresis is the exudation of a solvent from a gel structure. The gel structure will undergo shrinkage too. (Properties of gels: hydration, swelling, syneresis, thixotropic)

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14
Q

According to the Arrhenius Equation, it is practical and useful to store alginate impression in refrigerator one day before pouring. However it is unacceptable. Explain both sides of the statement.(4)

A

Alginate is unstable under moisture environment (i.e. in fridge) According to equation, reaction rate slows down if temperature is lower, to increase working time • When the alginate impression is put in the refrigerator where the temperature is low, the kinetic energy of the molecules would be low, hindering them from overcoming the barrier called activation energy for reaction to occur, thus reaction and any change in the impression should be minimal, according to the Arrhenius Equation. • However, the refrigerator provides a moist environment where the temperature is not absolutely low enough to avoid any occurrence of reactions. Since alginate is hygroscopic and will undergo imbibition, it may absorb moisture in refrigerator, causing dimensional changes (expansion), making such storage unacceptable.

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15
Q

What are dental plaster and dental stone? Describe their setting reactions.

A

Dental plaster

  • Type II plaster
  • beta form of calcium sulfate hemihydrate (CaSO4. 1/2H2O)
  • Lower strength
  • Larger crystal size
  • Irregular shape
  • Loosely packed w/ ample space btwn crystals
  • More W/P ratio

Dental stone

  • Type III plaster
  • Alpha form of calcium sulfate hemihydrate (CaSO4. 1/2H2O)
  • Higher strength
  • Smaller crystal size
  • Prismatic shape
  • Closely packed w/ little space btwn crystals
  • Less W/P ratio

Setting reaction

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16
Q

Given: Non-aqueous elastomeric impression materials

  1. Classify them according to composition and curing method. Which are available in polyclinic?
A

By composition: polysulphide, polyether, polysiloxane(condensation silicone/addition silicone), ‘hybrid’

By curing method: Condensation polymerization (polysulphide and condensation silicone)/// Addition polymerization (polyether and addition silicone)

Available in clinic (?): Impregum – polyether///Condensation/addition silicone

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17
Q

Given: Non-aqueous elastomeric impression materials

b. Give reasons why they are needed for crown impressions?

A

High degree of accuracy

Good tear resistance

Dimensionally stable

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18
Q

Given: Non-aqueous elastomeric impression materials

c. What would you do after taking impression? What disinfectant the polyclinic has?

A

Disinfection of Impregum (Polyether)

  1. Rinse with water then shake off surface water
  2. Dip in 0.8% Sodium Hypochlorite solution for 1-2s
  3. Rinse with water then shake off surface water
  4. Dip again in 0.8% Sodium Hypochlorite solution for 1-2s
  5. DO NOT cover it with damp gauze, as it will absorb water
  6. Wait 10 mins in open air
  7. Rinse with water and shake off surface water
  8. Place in polyethene bag labelled “disinfected”
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19
Q

Given: Non-aqueous elastomeric impression materials

d. In a hot weather, how would you slow down the rate of setting of impression materials?

A

Use cold water Make sure impression material is is stored in dry place

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20
Q

What are the factors affecting setting of gypsum? (3)

A
  • Temperature of water (colder = longer setting time)
  • Spatulation (Rapid spatulation accelerates setting time and produces the greatest strength
  • Water-powder ratio (if high proportion of water is used, setting time is retarded + crushing strength is lowered)
21
Q

What happens if you use tap water to mix gypsum? (2)

A

Tap water contains many cations and anions, affecting the activation energies on nucleation and growth.

Alter setting rates, affect expansion and affect crystal quality

22
Q

Describe the Craze-Crack Progression in acrylic? (4) How to Prevent it? (1)

A
  1. Formation of voids due to tensile stress on polymer chains at near surface
  2. Voids join together, form fibrils. At the same time, voids form at the inner part
  3. Fibrils continue in an increasingly large volume to have a characteristics half lenslike shape à crazes
  4. Fibrils rupture successively à cracks

Addition of EGDMA or DEGDMA can reduce craze formation à act as crosslinking agent

23
Q

What is gypsum?(1) Theoretically it should undergo volumetric contraction.

However practically it expands in volume. Explain the phenomenon? (3)

A

Gypsum is soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4· 2H2O

Expands due to the outgrowth of crystals from the nuclei (crystallization of the dihydrates)

24
Q

Name the types of Gold alloys and give the description. (4)

A

Type 1 alloy contains the most Au and is used for single surface inlays (low stress)

Type 2 alloy is used for complex inlays and crowns

Type 3 alloy is used for inlays/onlays, crowns and bridges

Type 4 alloy is used for cast posts and cores, long span bridges, partial denture clasps

(Type 3 and 4 alloys could be hardened by heat treatment. Zn is added to them as a deoxidizer)

Anson: See page 104 for other tables + images

25
Q

How to control the degree of expansion of gypsum? (2)

A
  • Potassium sulphate (accelerator) 4% - reactions with water and hemihydrate to form “syngenite” à reduce expansion and accelerate setting reaction
  • Sodium chloride – provide additional site for nucleation à increase rate of reaction
    • Higher density of crystal à limit crystal growth
26
Q

After removing the cast from alginate impression, some yellow dust/fragments are left in the impression, why? Give 2 scientific reasons (2)

A
  • Water was not removed from the deepest parts of the impression before it was poured – water/powder ratio too high in these areas
    • Impression material still covered with mucin films – very hard to remove even with thorough rinsing (will inhibit setting reaction of gypsum); need to rinse impression in slurry water before it is poured
27
Q

4 differences in physical property of acrylic denture base and acrylic pontic teeth (2)

A
  • Acrylic pontics have higher compressive strength due to its glass fibre reinforcement
  • Acrylic pontics have higher wear resistance
  • Acrylic denture base is more flexible than acrylic pontic teeth
  • Acrylic denture base is pink in colour while acrylic pontic teeth are more more opaque and tooth-coloured
28
Q

What is the difference between zirconium and zirconia? Which of them is more suitable to be a dental restorative materials and why? (2)

A

Cubic zirconia is manmade crystalline zirconium dioxide, ZnO2

Zirconium is the element.

Zirconia is more suitable, as it’s white and harder; zirconium is a metal and is soft, malleable, greyish white.

29
Q

What is (2):

FSZ

A

fully stabilised zirconium oxide

30
Q

What is (2):

PSZ

A

PSZ: partially stabilized zirconium oxide

31
Q

What is (2):

TZP

A

Tetragonal Zirconia Polycrystal

32
Q

What is (2):

Y-TZP:

A

Yttria-stabilized Tetragonal Zirconia Polycrystal ceramics

33
Q

What are the indications of zirconia as a restorative material? (1)

A
  • Single posterior crowns
  • Up to 4-unit posterior bridges
  • Multiple tooth restorations
  • As a replacement for gold crowns
  • Masking of moderate to severe discoloration of underlying tooth structure
  • As an alternative to a metal occlusal surface when a patient desires a metal- free restoration
  • When the patient has parafunctional issues
  • When you need to achieve maximum strength in an all-ceramic restoration
34
Q

List two inorganic acids used in dentistry (i.e. one for intra-oral use and the other for extra-oral use) (2)

A

Acid etch (composite) – 37% phosphoric acid

Hydrofluoric acid (to etch internal surface of ceramic restorations)

35
Q

Define polymer, plastic and resin (3)

A

Polymer: a macromolecule composed of many repeating units

Plastic: “An acrylate polymer belongs to a group of polymers that in general may be referred to as plastics”(resistance

to breakage, transparency, and elasticity - Matinlinna) a synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon, etc., that can be moulded into shape while soft, and then set into a rigid or slightly elastic form.

Resin: a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers.

36
Q

Resin luting cement majorly include resin and filler, what are the composition and function of resin and filler? (4)

A

Resin – undergoes polymerization to become solid after light curing or chemical curing

Filler – to control the viscosity of the paste

37
Q

Describe a test to test one of the properties of resin luting cement on ceramic crown. Write down the strength the test aim to determine and use a diagram/photograph/drawing to explain. (3)

A

Shear bond strength. Bond ceramic crown to resin luting cement. Apply a load at the interface between resin composite cylinder and the ceramic. Use a loading blade that is as close as possible to the bonding interface, and move parallel to this interface to create a localized stress concentration at the loaded area.

38
Q

What does glass transition temperature mean? How would that affect a heat-cures acrylic denture being thrown into boiling water?

A

Glass transition temperature: The temperature where amorphous polymers transit from a hard and brittle “glassy” state into a viscous or rubbery state

-Makes it more flexible, elastic, rubbery, less dimensional stability

Other note: A polymer which is used to construct a denture base should have a value of glass transition temperature (Tg) which is high enough to prevent softening and distortion during use. Although the normal temperature in the mouth varies only from 32°C to 37°C, account must be taken of the fact that patients take hot drinks at temperatures up to 70°C and, despite advice, sometimes clean dentures in very hot or even boiling water.

39
Q

State the laboratory procedure of heat cured acrylic in making a denture base.

A
  • Wax up the saddle area and set up the teeth on the duplicated master cast
  • Sent to clinic for try-in wax-up
  • Sent back to lab to convert wax to acrylic:
  • Flasking and packing
    • Embed the cast into plaster, while leaving the wax and acrylic teeth uncovered
    • After embedding, put the flask into hot water to remove the wax
  • Acrylic processing
    • Mix the PMMA power and MMA liquid (change from sandy > stringy > dough)
    • Pack the acrylic when it becomes dough into space prepared for acrylic base
    • Put flask under hydraulic pressure and put in water bath(72 degree for 7 hrs > 100 degree for 1 hr) and then allow cooling
  • Deflasking
    • Use hammers, air chisel and plater cutter to get the denture and model (should be together)
  • Check processing errors and occlusion (using articulator)
  • Plaster removal
    • When OVD is correct, separate the denture from the model with an air chisel
  • Trim and smooth acrylic (metal burs à sand paper disc àsilicone polisher)
  • Shining/polishing of acrylic (using tripoli and calico mops)
  • Delivery – denture is ultrasonically cleaned in distilled water, and the wet denture is sealed in a plastic bag for delivery into the clinic
40
Q

How to bond ceramic to metal in a CMC?

A
  • Heating of metal construction à diffusion of certain atoms on the surface à react with atmospheric oxygen to create oxides (remain on cast surface)
    • Atoms of silica in the ceramic are bound with these oxides à metal-tracer-ceramic compound
  • Use oxidation heat treatment (OHT) to remove entrapped gas and eliminate surface contaminants
  • Roughen surface by sandblasting à increase surface area (air abrasion increases wettability too) à mechanical interlocking
  • Apply bonding agent
  • Compressive forces
    • Metal must have similar but silghtly higher coefficient of thermal expansion than ceramic veneered over it
    • Metal contract faster during cooling and hence cause ceramic draw toward the metal coping → under compression
    • Ceramic is strong when under compression but weak under tension
  • Van der Waals forces
    • Minor contribution between charged molecules
41
Q

Why porcelain will fracture? (3)

A

Patient

Technical mistakes

Dentist

Environmental factors

Inherent material properties

42
Q

How does a patient cause porcelain to fracture?

A
  • Trauma, parafunctional habits à Cracks à fracture
    • Mechanical failure – primarily due to difference in modulus between the metal and ceramic materials
43
Q

How do technical mistakes cause porcelain to fracture?

A
  • occasional presence of pores inside ceramic
  • microcracks within the porcelain
  • Inadequate thickness
    • Minimum 0.7 mm, but ideally 1 – 1.5 mm, more importantly is uniform thickness
    • But never exceed 2 mm due to weakness under tension and stress conc at bond interface
44
Q

How does a dentist cause porcelain to fracture?

A
  • Inadequate tooth preparation à too little interocclusal space
  • Incorrect occlusal registration
45
Q

How do environmental factors cause porcelain to fracture?

A

Moisture in the mouth – 20-30% of reduction in metallic-ceramic strength (silicon-oxygen bond becomes weaker in the presence of water) - Since silicate bond in glass ceramic is prone to hydrolysis by environment moisture in presence of mechanical stress àlead to propagation of fracture along microcracks

46
Q

How do Inherent material properties cause porcelain to fracture?

A

low tensile strength; brittle; low fracture toughness; not good at resisting fracture under the presence of cracks

47
Q

What will be consequence if the condition(fractured porcelain) is left untreated (4)

A
  • Recurrent caries
  • Hypersensitivity due to thermal conduction
  • Loss of vitality
  • Galvanic shock
  • Continuous breakage of crown
  • Tooth fracture
  • Loss of occlusal contact causing overeruption of opposing tooth
48
Q

3 treatment options (Fractured porcelain) (3)

A
  • Simple repair with composite restoration (sandblasting with silica-coated alumina, silane coupling agent)
  • Reapply broken piece of porcelain with resin cement
  • Replace the whole crown
  • Extraction and replace with implant
49
Q

What is modulus of elasticity and proportional limit? (2)

A

Modulus of elasticity: a quantity that measures an object or substance’s resistance to being deformed elastically when a stress is applied to it. It is the slope of a stress/strain curve of an object. (stiffer à high elastic modulus)

Proportional limit: the maximum stress that a dental material sustains without any deviation; the highest stress at which the stress-strain curve is a straight line

BDS3 Past Paper By Topic (05-19) (24 decks)

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