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CDS Bds2 > Dental Materials Sciences > Flashcards

Dental Materials Sciences Flashcards

1
Q

Mention restorative materials

A
  • Metals/Alloys→ Amalgam, Cobalt Chromium, Titanium, Gold & Stainless Steel.
  • Composites
  • Glass Ionomer Cements
  • Compomers
  • Porcelain
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2
Q

Mention impression materials

A

Hydrocolloids (e.g., alginate)
Elastomers (e.g., polyethers, silicones)
Impression compound & paste

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3
Q

What is negative & positive replica?

A

Impression – negative replica
Dental stone (gypsum) \ study cast - positive replica

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4
Q

What is Mechanical properties?

A

A force that applied to a material may cause:
- Stretch/compress
- Deform (Change shape) - this may be temporary or permanent
- Fracture (failure)

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5
Q

What are the three primary types of force acting on dental materials?

A

Compressive (squeezing)
Tensile (stretching)
Shear (sliding)

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6
Q

What is stress in mechanical properties?

A

Stress is a physical quantity that describes forces present during deformation
Stress (Pa (Pascals)) = Force (F=mg)/ Unit Area

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7
Q

What is the unit of measurement for stress?

A

Pascals (Pa)

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8
Q

What is Strain?

A

Change in length / Original length

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9
Q

What is {Strain - Stress Curves} ?

A

Used to identify how a material performs under pressure

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10
Q

FS stands for?

A

Refers to the point at which the material fractures.

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11
Q

PL stands for?

A

Refers to the limit at which after stress is removed, the material can return to its original shape.

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12
Q

What kind of forces are the teeth typical exposed to?

A
  • Compressive forces - via biting
  • Abrasive & frictional forces - via Grinding & chewing
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13
Q

What is Elastic (Young’s) Modulus?

A
  • YM = Stress/strain
  • a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression
  • Effectively how rigid/stiff a material is
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14
Q

Fracture

A

Large force causing catastrophic destruction of material’s structure

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15
Q

Hardness

A

Ability of surface to resist indentation

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16
Q

Abrasion

A

Material surface removal due to grinding

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17
Q

Abrasion Resistance

A

Ability to withstand surface layers being removed

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18
Q

Fatigue

A

Repetitive ‘small’ stresses causing material fracture

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19
Q

Creep

A

Gradual dimensional change due to repetitive small forces

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20
Q

Deformation

A

Applied stress cause permanent change in materials dimension when greater than the elastic limit of material

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21
Q

De-bond

A

Applied forces sufficient to break material-tooth bond

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22
Q

Impact

A

Large, sudden force causing a fracture

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23
Q

Chemical properties

A
  • Setting mechanism.
  • Setting time.
  • Corrosive Potential.
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24
Q

Physical properties

A
  • Viscosity.
  • Thermal Conductivity & Expansion.
  • Density
  • Radiodensity.
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25
Q

What is Amalgam?

A

An alloy of:
- Mercury (Liquid component)
- Silver, Tin, Copper & Other trace metals (Powder component)

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26
Q

What are the compositions of Amalgam?

A
  • Powder (50% by weight): Silver/Tin- Copper- Zinc- mercury (Hg)
  • Liquid (50% by weight): mercury (Hg)
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27
Q

What are the two main classifications of dental amalgam?

A

Composition-based (Traditional & Copper-Enriched) and Particle shape-based (Lathe-cut & Spherical).

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28
Q

What is the function of silver and tin in amalgam?

A

They form an intermetallic compound.

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29
Q

Why is copper added to dental amalgam?

A

It increases strength and hardness.

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30
Q

What is the role of zinc in amalgam?

A

Acts as a scavenger of oxygen, ensuring clean castings.

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31
Q

How does mercury contribute to the amalgam reaction?

A

It reacts with the powder metals to form the amalgam matrix or final set material.

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32
Q

What are the two main types of amalgam particle shapes?

A

Lathe-cut and Spherical.

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33
Q

Which amalgam particle type provides stronger proximal contacts?

A

Lathe-cut particles.

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34
Q

How are spherical amalgam particles formed?

A

By spraying molten metal into an inert atmosphere.

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35
Q

List two advantages of spherical particles in amalgam.

A

Higher early compressive strength and easier carving.

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36
Q

Amalgam setting reaction

A

Silver/Tin + Mercury → Silver/tin + Silver/mercury + Tin/Mercury

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37
Q

Gamma phases

A

Gamma - Has good strength & corrosion resistance (Unreacted particles)

Gamma-1 - Has good corrosion resistance (Forms amalgam matrix)

Gamma-2 - Has poor corrosion resistance (Forms amalgam matrix)

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38
Q

What are the three gamma phases in amalgam?

A

Gamma (Ag-Sn), Gamma-1 (Ag-Hg), and Gamma-2 (Sn-Hg).

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39
Q

Which gamma phase has the best corrosion resistance?

A

Gamma-1.

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40
Q

Which gamma phase has the worst corrosion resistance?

A

Gamma-2

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41
Q

What happens if zinc-containing amalgam is contaminated with saliva or blood during placement?

A

It reacts with moisture to form hydrogen gas bubbles, causing expansion.

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42
Q

How long does it take for amalgam to fully set?

A

Approximately 24 hours.

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43
Q

List three factors that decrease the strength of amalgam.

A

Undermixing, slow rate of packing, and corrosion.

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44
Q

What H2 bubbles that formed in Zinc reaction can do?

A
  • Cause pressure build up → Leading to expansion
  • Downward pressure → Causing pulpal pain
  • Upward pressure → Restoration sits above occlusal surface
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45
Q

Why is high-copper amalgam preferred over traditional amalgam?

A

It eliminates the Gamma-2 phase, reducing corrosion and cree

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46
Q

What are the two types of high-copper amalgam?

A

Dispersion-modified and single-composition.

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47
Q

What is a key advantage of single-composition high-copper amalgam?

A

It has a single setting reaction, with copper integrated into the silver-tin alloy particles.

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48
Q

Pros of Copper Enriched

A
  • Higher early strength.
  • Less Creep.
  • Higher resistance to corrosion.
  • Increased Margin durability.
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49
Q

How can corrosion in amalgam be reduced?

A

Using copper-enriched amalgam and polishing the margins.

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50
Q

How does amalgam’s thermal expansion compare to tooth structure?

A

It expands 3 times more than tooth structure.

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51
Q

Why might a liner or varnish be needed under an amalgam restoration?

A

To protect the pulp from thermal conductivity.

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52
Q

What is the main bonding mechanism for amalgam in cavity preparation?

A

Mechanical retention

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53
Q

What is Permite, and why is it used?

A

A non-Gamma-2, spherical, and lathe-cut material with high compressive strength and low microleakage.

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54
Q

What are the key requirements of a good dental adhesive?

A
  • Provide high bond strength to tooth tissues
  • Immediate high strength bond
  • Durable bond
  • Impermeable bond
  • Easy to use
  • safe
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55
Q

Why is bonding to enamel easier than dentine?

A

Due to structure of enamel:
Enamel is densely packed, highly mineralized (95%), and dry

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56
Q

What is the purpose of acid etching on enamel?

A

To roughen the enamel surface for micromechanical interlocking of resin filling materials.

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57
Q

Which acid is most commonly used for etching enamel?

A

Phosphoric acid.

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58
Q

What is the usual concentration of phosphoric acid used for etching enamel?

A

30-50% phosphoric acid

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59
Q

Why must enamel be dry after etching?

A

Moisture prevents the resin from flowing into the etched surface.

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60
Q

How does etching affect the surface energy of enamel?

A
  • Increases surface energy
  • Improves wettability, allowing bonding agents to adhere better
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61
Q

What type of bonding agent is commonly applied to etched enamel?

A

A low-viscosity Bis-GMA bonding agent.

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62
Q

What makes bonding to dentine more difficult than enamel?

A
  • Dentine has permeable tubules
  • Contains fluid from pulp, making it wet
  • Lower surface energy than enamel
  • Hydrophilic nature (while most bonding agents are hydrophobic)
  • Presence of the smear layer
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63
Q

What is the smear layer, and how does it affect bonding?

A
  • A thin layer of debris left on dentine after preparation (0.5-5μm thick)
  • Contains bacteria and organic material
  • Can interfere with bonding
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64
Q

What are the main requirements of a dentine bonding agent?

A
  • Ability to flow
  • Intimate contact with dentine surface
  • Low viscosity
  • Adhesion via mechanical, chemical, and van der Waals forces
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65
Q

What are the three mechanisms of adhesion to dentine?

A

Mechanical, Chemical, and Van der Waals forces.

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66
Q

How does dentine bonding occur?

A
  • Mechanical bonding: Interlocking with dentine tubules
  • Chemical bonding: Ionic and covalent interactions with dentine components
  • Van der Waals forces: Electrostatic interactions
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67
Q

Why is wet dentine challenging for bonding?

A

It has a low surface energy, making it difficult for bonding agents to adhere.
Liquid CSE < Surface CSE = adequate adhesion

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68
Q

What is the gold standard for dentine bonding?

A

Total Etch technique

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69
Q

What is the main etchant used in Total Etch?

A

35% phosphoric acid

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70
Q

What are the three main components of a total etch bonding system?

A
  • Dentine conditioner (etchant): Removes smear layer, opens tubules
  • Primer: Hydrophilic/hydrophobic coupling agent
  • Adhesive: Hydrophobic resin, forms hybrid layer
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71
Q

What is the purpose of the dentine conditioner?

A

It removes the smear layer and opens up dentinal tubules.

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72
Q

What is the function of the primer in total etch bonding?

A

It contains hydrophilic and hydrophobic components that prepare the dentine for bonding.

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73
Q

What does the adhesive do in the total etch technique?

A

It penetrates the dentine, forms a micromechanical bond, and creates the hybrid layer.

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74
Q

What happens if dentine is over-etched?

A

Collagen fibers collapse, preventing resin penetration.

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75
Q

Why must dentine be neither too dry nor too wet for bonding?

A
  • Too dry → Collagen collapses.
  • Too wet → Primer is diluted, reducing bond strength.
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76
Q

How does self-etch bonding differ from total etch?

A
  • Self etch does not remove the smear layer but infiltrates and incorporates it
  • Less technique-sensitive than Total Etch
  • Weaker bond strength compared to Total Etch
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77
Q

Why is self-etch less technique-sensitive?

A

It eliminates the need to control moisture levels precisely.

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78
Q

What is the primary disadvantage of self-etch bonding?

A

The bond strength is weaker, especially to enamel.

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79
Q

Why is self-etch preferred for dentine over enamel?

A

It partially demineralizes dentine, preserving hydroxyapatite around collagen for stability.

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80
Q

Why does mild self-etch bonding work better on dentine than strong self-etch?

A
  • Mild self-etch partially demineralizes dentine, leaving hydroxyapatite for additional ionic bonding
  • Protects collagen from degradation
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81
Q

What is a disadvantage of self-etch in dentine bonding?

A

Etching by-products are not washed away, weakening the bond.

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82
Q

Why might a mild self-etch system fail to bond properly?

A

It may not fully penetrate the smear layer.

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83
Q

What is Critical Surface Energy (CSE)?

A

The surface tension of a liquid that determines its ability to spread on a solid surface

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84
Q

How does CSE affect adhesion?

A

Liquid CSE < Surface CSE → Good adhesion
Liquid CSE > Surface CSE → Poor adhesion

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85
Q

What is molecular entanglement in dentine bonding?

A

When adhesive penetrates dentine and forms long-chain polymers that interlock with the collagen matrix

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86
Q

What is the fundamental mechanism of bonding to dentine?

A

Molecular Entanglement – resin replaces minerals in the dental tissue, forming an interlocking bond.

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87
Q

According to Peumans et al. (2010), how can self-etch bonding be improved?

A

By etching enamel with phosphoric acid first.

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88
Q

Why do some restorative materials require lining materials?

A

To compensate for poor contact with the tooth surface, prevent chemical irritation to the pulp, and reduce microleakage.

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89
Q

What is the main function of a lining material?

A

It prevents gaps and acts as a protective barrier between the tooth and the restorative material.

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90
Q

How does a cavity base differ from a cavity liner?

A
  • Cavity base: Thick mix, replaces dentine, reduces bulk of restorative material, used more in metal restorations.
  • Cavity liner: Thin coating over exposed dentine, adheres to tooth structure, provides antibacterial action, promotes pulp health.
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91
Q

What are the three main purposes of a cavity liner?

A
  • Pulpal protection (prevents chemical, thermal, and bacterial irritation)
  • Therapeutic (reduces inflammation, promotes healing)
  • Palliative (reduces patient symptoms in reversible pulpitis)
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92
Q

How does a liner protect the pulp from chemical irritation?

A

It prevents unreacted chemicals in the filling material from reaching the pulp.

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93
Q

How does a liner protect against thermal damage?

A

It acts as an insulator, preventing heat from metal fillings or exothermic reactions from reaching the pulp.

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94
Q

How does a liner help prevent microleakage?

A

It seals the space between the restoration and the cavity walls, preventing bacterial and endotoxin infiltration.

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95
Q

What are the ideal properties of a liner?

A
  • Easy to use: Long working time, short setting time
  • Thermal properties: Low conductivity, similar expansion & diffusivity as dentine
  • Mechanical properties: High compressive strength
  • Radiopacity: Allows distinction from tooth structure
  • Low solubility: Should not dissolve over time
  • Marginal seal: Forms chemical bond to dentine
  • Cariostatic: Fluoride-releasing, antibacterial
  • Biocompatibility: Non-toxic, pH-neutral, no excessive heat during setting
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96
Q

Setting Calcium Hydroxide (Liner)

What is the composition of setting calcium hydroxide?

A

A base and a catalyst that react via a chelation reaction.

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97
Q

How does calcium hydroxide kill bacteria?

A

It creates a high initial pH (~12), which is hostile to cariogenic bacteria.

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98
Q

How does calcium hydroxide promote dentine formation?

A

It irritates the odontoblast layer, triggering necrosis and the formation of tertiary dentine.

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99
Q

What are the disadvantages of calcium hydroxide liners?

A

Low compressive strength, unstable, highly soluble (dissolves if leakage occurs or in moist conditions).

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100
Q

What are the types of zinc oxide-based cements, and which ones are no longer used?

A
  • No longer used: Zinc phosphate, Zinc polycarboxylate
  • Still used: Zinc Oxide Eugenol (ZOE), Resin Modified ZOE, Ethoxybenzoic Acid (EBA) ZOE
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101
Q

What is the setting reaction of zinc phosphate cement?

A

Acid-base reaction between zinc oxide (powder) and phosphoric acid (liquid), followed by a hydration reaction forming a crystalline phosphate matrix.

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102
Q

Why does zinc phosphate cement become stronger over time?

A

It absorbs water, making the matrix less porous and increasing strength.

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103
Q

What are the disadvantages of zinc phosphate cement?

A

Low initial pH (causing pulpal irritation), exothermic setting reaction, no adhesion to tooth/restoration, non-cariostatic, brittle.

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104
Q

What makes zinc polycarboxylate different from zinc phosphate cement?

A

It replaces phosphoric acid with polyacrylic acid, allowing bonding to the tooth.

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105
Q

What are the advantages of zinc polycarboxylate cement over zinc phosphate?

A

Less exothermic, neutral pH quicker, bonds to tooth, does not penetrate dentine deeply.

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106
Q

What are the disadvantages of zinc polycarboxylate cement?

A

Difficult to mix, low compressive strength, soluble in the oral environment.

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107
Q

Why are zinc phosphate and zinc polycarboxylate cements rarely used as linings today?

A

Poor adhesion, irritation, and brittleness; mainly used for temporary crowns.

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108
Q

What is the setting reaction of Zinc Oxide Eugenol (ZOE)?

A

Acid-base reaction between zinc oxide and eugenol, forming a zinc eugenolate matrix.

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109
Q

What are the advantages of ZOE?

A

Quick setting, low thermal conductivity, radiopaque, pulpal soothing effect.

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110
Q

What are the disadvantages of ZOE?

A

Low strength (20MPa), high solubility (breaks down over time), eugenol inhibits resin-based materials.

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111
Q

Why should ZOE not be used under composite restorations?

A

Eugenol softens and discolors resin-based materials.

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112
Q

How does adding resin improve ZOE?

A

It increases compressive strength (40MPa) and decreases solubility.

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113
Q

How does Ethoxybenzoic Acid (EBA) improve ZOE?

A

Encourages crystalline structure, increasing strength (60MPa) and reducing solubility.

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114
Q

Why is glass ionomer the most widely used lining material?

A

Bonds to dentine and composite, releases fluoride, and seals the cavity.

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115
Q

What are the advantages of glass ionomer liners?

A
  • Thermal properties close to dentine
  • High compressive strength (>170MPa)
  • Radiopaque
  • Chemical bond to enamel/dentine for strong marginal seal
  • Reduces microleakage and post-treatment sensitivity
  • Least soluble cement
  • Releases fluoride and antibacterial compounds
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116
Q

What is a disadvantage of resin-modified glass ionomer (RMGIC)?

A

Unreacted HEMA may be cytotoxic to the pulp.

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117
Q

Why is RMGIC unique among linings?

A

It can bond to restorative materials and may even bond amalgam to the tooth.

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118
Q

What is the recommended liner for amalgam or large composite cavities?

A

RMGIC (e.g., Vitrebond).

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119
Q

When should calcium hydroxide be used?

A

For deep cavities near the pulp (direct/indirect pulp capping).

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120
Q

What must be done after applying calcium hydroxide before placing a final restoration?

A

Cover it with RMGIC.

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121
Q

What are composite resins used for?

A

Filling cavities, replacing abraded tissue, repairing/replacing failed restorations.

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122
Q

Which cavity classes are composite resins commonly used for?

A

Class III, IV, V, and limited occlusal wear in Class II.

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123
Q

When is composite resin preferred over other materials?

A

When aesthetics are important.

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124
Q

What are the three main components of composite resin?

A

Filler particles, resin, camphorquinone

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125
Q

What is the function of Camphorquinone in composite resins?

A

It acts as a photo-initiator for polymerization or light curing.

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126
Q

What color light activates camphorquinone?

A

Blue light

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127
Q

What role do Low Weight Dimethacrylates play in composite resins?

A

They control mechanical properties.

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128
Q

What does the Silane Coupling Agent do?

A

It ensures bonding between filler particles and resin.

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129
Q

What types of filler particles are used in composite resins?

A

Basic silica, quartz, or various silicates like lithium aluminum silicate.

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130
Q

How does filler particle type affect composite resin properties?

A

It influences hardness, rigidity, and abrasion resistanc

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131
Q

Why are hybrid composites mechanically superior?

A

They contain both large and small filler particles, providing strength and aesthetics.

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132
Q

What is the most common resin monomer used in composites?

A

BIS-GMA (Bisphenol A Glycidyl Methacrylate).

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133
Q

Why is BIS-GMA used in composite resin?

A

It contains C=C bonds, allowing for crosslinking and polymerization

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134
Q

What happens to the C=C bonds during polymerization?

A

They break, allowing monomer molecules to join and form a polymer.

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135
Q

What are the four classifications of composite resin based on filler type?

A

Conventional, Micro-filled, Submicron, Hybrid.

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136
Q

What is the difference between flowable and condensable composites?

A

Flowable has lower filler content (more shrinkage), while condensable has an ‘amalgam-like’ consistency

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137
Q

What are the two curing methods for composite resins?

A

Light-cured and self-cured.

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138
Q

List three potential problems with light curing.

A

Light-material mismatch, premature polymerization, and polymerization shrinkage

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139
Q

Which type of composite resin is used for anterior restorations?

A

Micro-filled or Hybrid.

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140
Q

Which handling characteristic of composite resins has the highest viscosity?

A

Flowable composite.

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141
Q

What initiates polymerization in light-cured composites?

A

Camphorquinone activated by blue light (430-490 nm).

142
Q

What is the depth of cure for composite resin?

A

Typically 2mm.

143
Q

Why is composite resin placed in increments?

A

To ensure complete curing and prevent shrinkage stress.

144
Q

What type of photo-initiator is used in bulk-fill composites?

A

Lucirin, which is activated by UV light.

145
Q

What can happen if the curing light spectrum does not match the composite material?

A

Inefficient curing, leading to under-polymerization.

146
Q

How can polymerization shrinkage be minimized?

A

By using small increments and curing from different angles.

147
Q

Why should the curing light not be directed at the eyes?

A

Blue LED light can cause ocular damage.

148
Q

What is the fracture strength of composite resin?

149
Q

What is the Young’s modulus and Proportional Limit (PL) of composite resin?

A
  • YM: 15 GPa (15,000 MPa)
  • PL: 300 MPa
150
Q

What happens when stress exceeds the Elastic Limit (EL) of composite?

A

The composite deforms and does not return to its original shape.

151
Q

Why is composite preferred over amalgam in deciduous teeth restorations?

A

Because it bonds better, reduces microleakage, and wears at the same rate as natural tooth structure.

152
Q

Why is a good bond between composite and tooth tissue important?

A

It prevents microleakage and stress concentration, reducing the risk of fractures.

153
Q

Why is low thermal conductivity important for composite resins?

A

To protect the pulp from heat damage.

154
Q

What is the major drawback of composite resin’s thermal expansion coefficient?

A

It is higher than tooth tissue, increasing the risk of microleakage.

155
Q

What are the advantages and disadvantages of conventional composite?

A

Strong but difficult to finish and prone to staining.

156
Q

What are the advantages and disadvantages of microfine composite?

A

Smoother surfaces and better aesthetics but weaker mechanical properties.

157
Q

What property of composite resins allows for easy shade matching?

A

Their wide range of shades and translucency.

158
Q

Why is a hybrid composite a good balance between strength and aesthetics?

A

It contains both large and small filler particles.

159
Q

Why are hybrid composites widely used today?

A

They offer a balance between strength and aesthetics with improved fillers and coupling agents.

160
Q

How is hardness measured in composite resin?

A

Using an indentor to create a notch, then measuring the depth of indentation.

161
Q

Name three clinical factors that affect composite wear.

A

Cavity design, occlusion, and curing efficiency.

162
Q

What is the thermal diffusivity of hybrid composite compared to dentine?

A

Hybrid composite = 0.005 cm²/s, Dentine = 0.002 cm²/s.

163
Q

What aesthetic properties make composite a good restorative material?

A

A range of shades, translucency options, stain resistance, and polishability.

164
Q

Why must prosthetic materials have a high Young’s modulus?

A

To ensure rigidity and withstand biting forces.

165
Q

Why is a high softening temperature important for prosthetic materials?

A

To prevent distortion from hot fluids and cleaning.

166
Q

Why should prosthetic materials have a high proportional limit?

A

To prevent deformation under large biting forces.

167
Q

Why do we want high thermal conductivity in acrylic resin materials?

A

To prevent scalding and allow the patient to feel heat.

167
Q

Why is low density an important property for prosthetic materials?

A

To improve retention, especially for upper dentures.

168
Q

What are the aesthetic requirements of prosthetic materials?

A

They should match the colour and translucency of natural tissues.

169
Q

Why must prosthetic materials be non-toxic and non-irritant?

A

To ensure biocompatibility and avoid adverse reactions.

170
Q

What is free radical addition polymerization?

A

A reaction where monomers join without eliminating smaller molecules.

170
Q

What type of polymerization does PMMA undergo?

A

Free radical addition polymerization.

171
Q

What is the initiator in PMMA polymerization?

A

Benzoyl peroxide

172
Q

How many free radicals does each benzoyl peroxide molecule release?

A

Two free radicals

173
Q

At what temperature does PMMA polymerization activate?

A

Above 72°C

174
Q

What are the four steps of polymerization?

A
  • Activation – Benzoyl peroxide provides free radicals.
  • Initiation – Free radicals break monomer C=C bonds.
  • Propagation – Polymer chains grow.
  • Termination – Polymerization stops.
175
Q

Why is a powder-liquid mix used in heat-cured acrylic?

A

To reduce heat of reaction, minimize shrinkage, and improve handling.

176
Q

What is the powder-to-liquid ratio in heat-cured acrylic?

A

By volume: 3 - 3.5 to 1
By weight: 2.5 to 1

177
Q

Heat-Cured Acrylic Composition

What does the powder component contain?

A

PMMA particles, benzoyl peroxide (initiator), plasticizers, and co-polymers.

178
Q

Heat-Cured Acrylic Composition

What does the liquid component contain?

A

Methacrylate monomers, pigments, hydroquinone (inhibitor), and co-polymers.

179
Q

What happens if acrylic resin is cured too quickly?

A

Gaseous porosity occurs due to excessive heat.

180
Q

Why must acrylic be slowly cooled after curing?

A

To reduce internal stresses that weaken the material.

181
Q

How does internal stress affect acrylic resin?

A

Reduces strength, increases fatigue failure, and causes warping.

182
Q

What are the effects of under-curing?

A

Free monomers remain, leading to weaker mechanical properties.

183
Q

What happens if the monomer ratio is incorrect?

A

Too much monomer → Contraction porosity.
Too little monomer → Granularity porosity.

184
Q

How does porosity affect the final dental material?

A

Reduces strength, affects aesthetics, and absorbs saliva, causing hygiene issues.

185
Q

When and where does gaseous porosity occur?

A

When the monomer boils above 100°C, often in bulkier denture areas.

186
Q

What factors contribute to polymerization shrinkage?

A

Excess monomer, insufficient packing, and lack of pressure.

187
Q

What are the advantages of PMMA as a dental material?

A

Non-toxic, stable, good aesthetics, and unaffected by oral fluids.

188
Q

What are the disadvantages of PMMA?

A

High thermal expansion, low thermal conductivity, and poor mechanical properties.

189
Q

Why does the low density of PMMA not always benefit denture applications?

A

Increased bulk is needed to compensate for weak mechanical properties.

190
Q

What happens if an acrylic appliance is exposed to boiling water?

A

The material may warp and lose its shape.

191
Q

Why do porcelain teeth have a higher risk of creating gaps compared to acrylic teeth in acrylic denture?

A

Porcelain has a lower thermal expansion coefficient than PMMA.

192
Q

What is the aim of impression materials?

A

To produce an accurate negative reproduction of the surface and shape of oral soft and hard tissues.

193
Q

Why is the accuracy of an impression important?

A

Treatment outcomes depend on the quality & accuracy of the initial impression.

193
Q

What material is used to fill an impression to produce a cast?

A

Dental stone

194
Q

What are the uses of a stone cast?

A

Evaluating ortho & occlusal relationships, production of RPD & other restorations.

195
Q

What is a muco-static impression material?

A

A material that does not displace the mucosa and records its resting position.

196
Q

Give examples of muco-static materials.

A

Elastomers: Polyvinyl Siloxane (PVS), Polyether.

197
Q

What is a muco-compressive impression material?

A

A material that displaces the mucosa and records its shape under load.

198
Q

Give examples of muco-compressive materials.

A

Silicone putty (heavy-bodied PVS), Hydrocolloids (Alginate).

199
Q

What are elastic impression materials?

A

Materials that can stretch and deform without breaking and return to their original shape.

200
Q

What are non-elastic impression materials?

A

Materials that cannot stretch and typically break/tear under stress.

201
Q

Are muco-static materials elastic or non-elastic?

202
Q

Are muco-compressive materials elastic or non-elastic?

A

Non-elastic

203
Q

Why are non-elastic impression materials not recommended for master impressions?

A

Due to limited accuracy and risk of breaking/tearing upon removal.

204
Q

What is viscoelastic behaviour?

A

A material exhibiting both viscous (fluid-like) and elastic (solid-like) properties.

205
Q

What happens when an impression material undergoes elastic recovery?

A

After removal, the material recovers, but some permanent strain remains.

206
Q

How does removal speed affect permanent strain?

A

Faster removal → Less permanent strain

207
Q

For Accuracy

What properties ensure an accurate impression?

A

Viscosity, setting mechanism, low thermal expansion, viscoelastic behavior, high tear strength.

208
Q

For Patient Comfort

What properties ensure patient comfort?

A

Non-toxic, non-irritant, acceptable taste/smell, short setting time, easy removal.

209
Q

For Operator Convenience

What properties ensure operator convenience?

A

Quick/simple technique, good working/setting times, ability to be decontaminated, cost-effective

210
Q

What is a colloid?

A

A two-phase system of fine particles dispersed in another phase.

211
Q

What is a hydrocolloid?

A

A colloid with water as the dispersing medium.

212
Q

What is the most commonly used irreversible hydrocolloid?

213
Q

What are the main components of alginate?

A

Sodium Alginate
Calcium Sulphate
Trisodium Phosphate
Fillers
Modifiers

214
Q

Describe the setting reaction of alginate.

A

Sodium Alginate reacts with Calcium Sulphate, forming Calcium Alginate, which cross-links to create a solid gel.

215
Q

Why does alginate continue to develop elastic properties after setting?

A

Cross-linking continues after the apparent set, improving elasticity.

216
Q

What temperature of water should be used when mixing alginate?

217
Q

What type of tray should be used with alginate?

A

Perforated tray

218
Q

How should an alginate impression be removed?

A

With a sharp pull to ensure elastic recovery and minimize permanent deformation.

219
Q

What is the primary use of gypsum in dentistry?

A

To produce a positive replica of dentition from an impression.

220
Q

Why is gypsum used in dental prosthesis manufacturing?

A

It enables assessment of the dentition and the fabrication of prostheses.

220
Q

What does gypsum record?

A

Position, shape, and dimensions of teeth and surrounding soft tissue.

221
Q

What is the chemical composition of gypsum used in dentistry?

A

Calcium sulfate dihydrate (CaSO₄ · 2H₂O).

222
Q

What happens to gypsum when heated?

A

It converts to calcium sulfate hemihydrate (CaSO₄ · ½H₂O), creating a powder.

223
Q

What happens during the setting reaction of gypsum?

A

The reaction is reversed, and calcium sulfate dihydrate reforms.

224
Q

What are the three types of gypsum?

A
  • Plaster (β-hemihydrate)
  • Dental stone (α-hemihydrate)
  • Densite (improved stone).
225
Q

How is plaster produced?

A

By heating gypsum in an open vessel.

226
Q

What is the structure of plaster crystals?

A

Irregular and porous.

227
Q

How is dental stone produced?

A

By heating gypsum in an autoclave.

228
Q

What is the structure of dental stone crystals?

A

Regular and non-porous.

229
Q

How is densite (improved stone) produced?

A

By heating gypsum in the presence of calcium chloride.

230
Q

What is the structure of densite crystals?

A

Compact and smooth.

231
Q

What is the chemical reaction for the setting of gypsum?

A

Calcium sulfate hemihydrate + Water → Calcium sulfate dihydrate + Water

232
Q

What happens when water is added to gypsum powder?

A

Hemihydrate dissolves, forming dihydrate crystals.

233
Q

What do dihydrate crystals do during the setting process?

A

They precipitate and grow around impurities.

234
Q

What do dihydrate crystals do during the setting proces

A

They precipitate and grow around impurities.

235
Q

What marks the initial set of gypsum?

A

Dihydrate crystals grow and come into contact, starting the expansion process.

236
Q

At what stage can gypsum be carved?

A

During the initial set.

237
Q

What marks the final set of gypsum?

A

The material hardens, and water evaporates, making it porous.

238
Q

What is the water-to-powder ratio for plaster?

A

60ml water to 100g powder.

239
Q

What is the water-to-powder ratio for dental stone?

A

35ml water to 100g powder.

240
Q

How does increasing the powder-to-water ratio affect gypsum setting time?

A

It decreases setting time and increases expansion.

241
Q

What is the compressive strength of gypsum?

A

Ranges from 28-38 MPa, developing to 75 MPa over 24 hours.

242
Q

Which type of gypsum has the greatest strength?

A

Densite (Improved Stone).

243
Q

Why does gypsum have a rough surface?

A

Because it is porous, with surface roughness of 28-40 µm.

244
Q

What is the main disadvantage of gypsum’s surface hardness?

A

It has low resistance to abrasive forces.

245
Q

How does spatulation affect the setting time of gypsum?

A

Increased spatulation breaks down growing crystals, reducing setting time.

246
Q

How does gypsum expand during setting?

A

Expansion is low, minimizing dimensional inaccuracies.

247
Q

What is the flexural strength of gypsum?

A

15-20 MPa.

248
Q

How does potassium sulfate affect gypsum setting time? Why?

A

It decreases setting time by producing syngenite, which rapidly crystallizes and promotes further crystal growth.

249
Q

How does borax affect gypsum setting time? Why?

A

It increases setting time by forming calcium borate, which deposits on dihydrate crystals and delays setting.

250
Q

What are the advantages of gypsum in dentistry?

A

Dimensionally accurate and stable.
Low expansion rate for stone and densite.

251
Q

What are the disadvantages of gypsum?

A
  • Low tensile strength and poor abrasion resistance.
  • Very brittle.
  • Less surface detail compared to elastomer impression materials.
  • Poor wetting on some impression materials.
252
Q

What does the gradient of the stress-strain curve represent?

A

The Rigidity of the material.

253
Q

What does a higher rigidity indicate?

A

A higher rigidity means that more stress produces less strain on the material.

254
Q

What is the Elastic Limit (EL)?

A

The maximum stress a material can withstand without plastic deformation.

255
Q

What is the Ultimate Tensile Strength (UTS)?

A

The highest stress a material can withstand before breaking.

256
Q

What is the Fracture Strength (FS)?

A

The point at which the material fractures.

257
Q

What does Ductility represent?

A

How much plastic deformation a material can undergo under tensile stress before breaking.

258
Q

What does Malleability represent?

A

How much plastic deformation a material can undergo under compressive stress before breaking.

259
Q

What is an Alloy?

A

combination of metal atoms in a crystalline structure.

260
Q

What are the 3 types of Crystal Structures?

A
  • Cubic
  • Face-Centred Cubic (FCC)
  • Body-Centred Cubic (BCC)
260
Q

What are the 3 main factors affecting mechanical properties of a metal/alloy?

A
  • Crystalline Structure
  • Grain Size
  • Grain Imperfections
261
Q

What happens when a molten metal cools?

A

Crystals form from Nuclei of Crystallisation.

262
Q

What do crystals grow into?

A

Dendrites → Grains

263
Q

What are Grain Boundaries?

A

Regions where grains make contact.

265
Q

How does Slow Cooling affect grain size?

A

Produces fewer nuclei → Large coarse grains.

266
Q

What are Nucleating Agents?

A

Impurities or additives that act as extra nuclei → Producing smaller grains.

267
Q

How do Small Grains affect properties?

A
  • Increase: Elastic Limit, UTS, Hardness
  • Decrease: Ductility
268
Q

What is a Dislocation?

A

A defect in the crystal lattice that weakens the structure.

269
Q

What happens when a force is applied to a metal with dislocations?

A

Dislocations propagate along the lattice plane until they reach the grain boundary → This is called Slip.

270
Q

How do Small Grains impede dislocation movement?

A

Small grains = More Grain Boundaries → Dislocations are blocked.

271
Q

What is Cold Working?

A

Shaping metal by applying force at low temperatures.

271
Q

How does Cold Working affect metal properties?

A
  • Increases: Elastic Limit, UTS, Hardness
  • Decreases: Ductility, Corrosion Resistance
272
Q

What is Residual Stress?

A

Internal stresses in the metal caused by dislocation buildup during Cold Working.

273
Q

What is Annealing?

A

A heating process that allows atoms to rearrange and eliminate residual stress.

274
Q

What is Recrystallisation?

A

A heating process that resets grain structure and reverses the effects of Cold Working.

275
Q

What type of grains does Quenching produce?

A

Small fine grains

276
Q

What is the key influence on grain size?

A

Rate of Cooling

277
Q

What happens to Ductility if grain size decreases?

A

It decreases

278
Q

What process pushes dislocations to grain boundaries?

A

Cold Working

279
Q

What is the main disadvantage of Cold Working?

A

Residual Stress

280
Q

What is an Alloy?

A

A combination of 2 or more metals, or a combination of metal(s) with a metalloid (e.g., Silicon or Carbon).

281
Q

What are the advantages of alloys?

A

Superior mechanical properties such as high Elastic Limit, Fracture Strength, Strength & Rigidity, Corrosion Resistance, and lower melting points.

282
Q

Name some examples of alloys used in dentistry.

A

Gold alloys, Amalgam, Stainless Steel, Cobalt-Chromium.

283
Q

What is a phase in alloys?

A

A physically distinct homogenous structure.

284
Q

What is a solid solution?

A

A homogenous mixture of two metals at an atomic scale forming one phase.

285
Q

What are the possible interactions between two molten metals during crystallisation?

A
  • Insoluble (2 Phases)
  • Soluble (1 Phase forming a solid solution)
  • Formation of Intermetallic Compound (e.g., Amalgam)
286
Q

What are the two types of solid solutions?

A

Substitutional Solid Solution and Interstitial Solid Solution.

287
Q

What is a substitutional solid solution?

A

Atoms of metal 1 replace atoms of metal 2 in a crystal lattice, requiring similar size, valency, and crystal structure.

288
Q

What is an interstitial solid solution?

A

Smaller atoms fit into the spaces within the lattice of larger atoms.

289
Q

How does the crystallisation process differ between pure metals and alloys?

A

Pure metals crystallise at one temperature, while alloys crystallise over a temperature range.

290
Q

What are the effects of slow cooling on alloys?

A

Produces large grains and homogenous composition.

291
Q

What are the effects of fast cooling on alloys?

A

Produces small grains, heterogenous composition (coring), and improves mechanical properties but reduces corrosion resistance.

292
Q

How can coring be removed from alloys?

A

Through homogenising annealing, where the alloy is reheated below recrystallisation temperature to allow atoms to diffuse and create a homogenous structure.

293
Q

Why do alloys have increased strength compared to pure metals?

A

Alloys distort the grain lattice due to different-sized atoms, impeding dislocation movement and increasing fracture resistance.

294
Q

What is a eutectic alloy?

A

An alloy where metals are soluble in liquid state but insoluble in solid state, forming two physically distinct grains.

295
Q

What is a partially soluble alloy?

A

An alloy producing two distinct grains, each containing both metals but in different concentrations.

296
Q

What is precipitation hardening?

A

The process of annealing a partially soluble alloy to increase strength and surface hardness

297
Q

What are three examples of partial denture alloys?

A
  • Type IV Gold
  • Co-Cr
  • Titanium
297
Q

What are the desired properties of a denture base?

A
  • High YM (To maintain shape)
  • High EL (To avoid deformation)
298
Q

What are the desired properties of a denture clasp?

A
  • Lower YM (To allow flexure over tooth)
  • High EL (Maintain elasticity)
299
Q

What type of gold alloy is used for partial dentures?

300
Q

What is the gold percentage range in Type IV gold alloy?

301
Q

What metals make up Type IV gold alloy?

A

Gold, Silver, Copper, Zinc, Palladium, Platinum

302
Q

What are the effects of adding Copper to gold alloy?

A
  • Formation of solid solution
  • Solution & order hardening
  • Reduces melting point
  • Little to no coring
  • Imparts red colour
  • Reduces density
303
Q

What are the effects of adding Silver to gold alloy?

A
  • Formation of solid solution
  • Solution hardening
  • Precipitation hardening with copper
  • Allows tarnishing
  • Absorbs gas (CO2)
  • Whitens the alloy
304
Q

Why does Au-Ag show little/no coring?

A

Solidus & liquidus lines are close together

305
Q

What are the effects of adding Platinum to gold alloy?

A
  • Formation of solid solution
  • Solution hardening
  • Produces fine grain structure
  • Coring can occur
    *
306
Q

What are the effects of adding Palladium to gold alloy?

A
  • Similar to platinum
  • Less coring
  • Produces more coarse grains
307
Q

What is the function of Zinc in gold alloys?

308
Q

What is the function of Nickel in gold alloys?

A

Increase hardness & strength

309
Q

What is the function of Indium in gold alloys?

A

Produce fine grain structure

310
Q

What are gold alloys generally suited for in a denture: base or clasp?

311
Q

Why are gold alloys not typically used for denture bases?

A

Thickness required → Expensive

312
Q

What is Co-Cr frequently used for in RPD?

A

Connectors

313
Q

What is the composition of Co-Cr alloy?

A

Cobalt, Chromium, Nickel

314
Q

What is the effect of Chromium in Co-Cr alloy?

A

Corrosion resistance (Passive layer)

315
Q

What is the effect of Nickel in Co-Cr alloy?

A
  • Improves ductility
  • Slightly reduces strength
316
Q

What are the general properties of Co-Cr alloy?

A

Harder than gold
Low ductility
Difficult to adjust

316
Q

What are the uses of Titanium in dentistry?

A
  • Implants
  • Crown & Bridge
  • Partial Denture
317
Q

What are the benefits of Titanium?

A
  • Good biocompatibility
  • Good corrosion resistance
  • Can be joined by laser welding
318
Q

What does viscosity measure in impression materials?

A

The material’s ability to flow and make close contact with tissues.

319
Q

What does viscosity measure in impression materials?

A

The material’s ability to flow and make close contact with tissues.

334
Q

Why is viscosity important for impression materials?

A

It determines how well the material can record surface detail.

335
Q

What is wettability in impression materials?

A

The ability of a material to make intimate contact with hard and soft tissues.

336
Q

How is wettability measured?

A

By the contact angle (Lower angle = better wetting).

337
Q

What is the ISO standard for surface reproduction?

A

Grooves of 50 microns must be replicated.

338
Q

What is elastic recovery in impression materials?

A

The ability of the material to return to its original shape after removal.

339
Q

How does load time affect permanent deformation?

A

Reduced load time → Less strain → Less deformation

340
Q

Do we want high or low viscoelasticity?

A

Low viscoelasticity (Small permanent deformation).

341
Q

What is tear strength?

A

The amount of stress a material can withstand before fracturing.

342
Q

Why is tear strength important in impression materials?

A

It helps prevent the impression tearing when removed from undercuts.

343
Q

What kind of rigidity is ideal for impression materials?

A

Low rigidity → More flexible for easier removal.

344
Q

Which material type has longer working time?

345
Q

Which material has better elastic recovery?

A

Addition Silicone (PVS)

346
Q

Which material has higher tear strength?

A

Addition Silicone (PVS)

347
Q

What are ISO standards?

A

International guidelines that assess the properties of materials to ensure they meet safety & effectiveness requirements.

348
Q

What size grooves must impression materials replicate according to ISO?

A

50 microns or 20 microns depending on material type.

349
Q

What is the best addition silicone brand on the market?

CDS Bds2 (4 decks)

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