1. Group A - Structure and physical properties
   • Stress-strain curve, modulus of elasticity, proportional limit, yield strength
   • Atomic bonds and crystal lattices
   • Thermal expansion and insulation
   • Adhesion, cohesion, wetting/contact angle, and separating media
2. Group B - Mechanical and thermal behavior
   • Hardness, brittleness, resilience, toughness
   • Masticatory stresses, creep and flow
   • Thermal expansion calculations and conductivity
   • Malleability, ductility, and latent heat of fusion
3. Group C - Corrosion, classification, and biocompatibility
   • Dental material science and ideal requirements
   • Tarnish, corrosion, galvanism, and prevention
   • Classification by property, behavior, application, and biological perspective
   • Biomaterials, biointegration, and osseointegration
4. Group D - Biological safety and polymers/resins
   • Local/systemic effects and occupational hazards
   • Biocompatibility and hazardous materials
   • Addition polymerization, dental resins, cross-linking
   • Denture indication and corrosion prevention

1. Group A - Structure and physical properties
   • Stress-strain curve, modulus of elasticity, proportional limit, yield strength
   • Atomic bonds and crystal lattices
   • Thermal expansion and insulation
   • Adhesion, cohesion, wetting/contact angle, and separating media
2. Group B - Mechanical and thermal behavior
   • Hardness, brittleness, resilience, toughness
   • Masticatory stresses, creep and flow
   • Thermal expansion calculations and conductivity
   • Malleability, ductility, and latent heat of fusion
3. Group C - Corrosion, classification, and biocompatibility
   • Dental material science and ideal requirements
   • Tarnish, corrosion, galvanism, and prevention
   • Classification by property, behavior, application, and biological perspective
   • Biomaterials, biointegration, and osseointegration
4. Group D - Biological safety and polymers/resins
   • Local/systemic effects and occupational hazards
   • Biocompatibility and hazardous materials
   • Addition polymerization, dental resins, cross-linking
   • Denture indication and corrosion prevention

Group A - Exam-ready answers

1(a). Define stress and strain. Classify stress with figure.

Classification of stress

1. Tensile stress - force tends to pull a material apart or elongate it.
   Example: Orthodontic wire under stretching force.
2. Compressive stress - force tends to shorten or crush a material.
   Example: Mastication force acting on an amalgam restoration.
3. Shear stress - force causes one part of the material to slide over another.
   Example: Shearing of a restoration during chewing.
4. Torsional stress - stress due to twisting force.
   Example: Endodontic file during rotation.
5. Flexural stress - combination of tensile and compressive stresses produced by bending.
   Example: Denture base during function.

Tensile:       ← [ specimen ] →
Compression:   → [ specimen ] ←
Shear:          → [ specimen ]
               ← [ specimen ]
Torsion:        ↻ [ specimen ] ↺

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1(b). What is modulus of elasticity? Importance in dentistry.

Importance in dentistry

• Restorative materials should have a modulus close to the tooth structure they replace.
• A high-modulus material is useful where rigidity is required, such as cast restorations, bridges, orthodontic wires, and denture clasps.
• Low-modulus materials are more flexible and may be useful where shock absorption is desirable.
• In a crown or restorative material, an unsuitable modulus may concentrate stress in enamel, dentin, cement, or the restoration.
• Materials with a high elastic modulus generally transmit more stress to tooth structure, as described in a review of tooth mechanical properties.

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2(a). Draw and interpret proportional limit, yield point, and ultimate strength from stress-strain curve.

Stress
  ↑
  │                           D = Ultimate strength
  │                          /\
  │                         /  \
  │                C = Yield    \ E = Fracture
  │               /
  │       B = Proportional limit
  │      /
  │     /
  │____/________________________________→ Strain
      A

• A to B: Elastic region
  Stress is directly proportional to strain. If the load is removed, the material returns to its original dimensions.
• B: Proportional limit
  Greatest stress at which stress and strain remain directly proportional. It marks the end of the linear part of the curve.
• C: Yield point / elastic limit
  Stress beyond which permanent deformation begins. On removal of force, the material will not completely return to its original shape.
• D: Ultimate strength
  Maximum stress that a material can withstand before failure begins.
• E: Fracture point
  Point at which the material breaks.

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2(b). Name interatomic primary bonds. Describe ionic bond with figure.

Primary interatomic bonds

1. Ionic bond
2. Covalent bond
3. Metallic bond

Ionic bond

Na  → loses 1 electron →  Na⁺
Cl  ← gains 1 electron ←  Cl⁻

Na⁺  ← electrostatic attraction →  Cl⁻

Characteristics

• Strong bond.
• Usually found in crystalline materials.
• Ionic solids are hard and brittle.
• Poor electrical conductors in solid state, but conduct electricity in molten or dissolved form.

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3(a). Define space lattice. How many space lattices are of dental interest? Mention names with examples.

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3(b). Explain each cubic structure of space lattice with diagram.

1. Simple cubic lattice

• Effective atoms per unit cell: 1
• Coordination number: 6
• Packing efficiency: low
• Rare among metals.

      ●──────●
     /|     /|
    ●──────● |
    | |    | |
    | ●────|-●
    |/     |/
    ●──────●

2. Body-centered cubic lattice, BCC

• Effective atoms per unit cell: 2
• Coordination number: 8
• Example: Iron, chromium.

      ●──────●
     /|     /|
    ●──────● |
    |   ●   | |
    | ●────|-●
    |/     |/
    ●──────●

3. Face-centered cubic lattice, FCC

• Effective atoms per unit cell: 4
• Coordination number: 12
• Closely packed structure.
• Examples: Gold, silver, copper, nickel, platinum.

      ●──────●
     /|  ●  /|
    ●──────● |
    |  ●   |●
    | ●────|-●
    |/  ●  |/
    ●──────●

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4(a). Define coefficient of thermal expansion and state its significance.

Significance in dentistry

• A restorative material should have a coefficient of thermal expansion close to that of enamel and dentin.
• If the restoration expands or contracts much more than tooth structure during hot or cold intake, marginal gaps can form.
• This can result in microleakage, recurrent caries, marginal staining, pulpal irritation, and fracture of restoration or tooth.
• It is important for amalgam, composite resin, ceramics, luting cements, denture-base materials, and porcelain-fused-to-metal restorations.
• For metal-ceramic restorations, the metal should have a slightly higher CTE than porcelain, so porcelain remains under compression on cooling.

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4(b). Why is a thick insulating base given under a metallic restoration?

• Reduce transmission of thermal changes from the restoration to the pulp.
• Prevent pulpal pain and thermal shock.
• Protect odontoblasts and pulp from injury.
• Replace lost dentin, which normally provides thermal insulation.
• Reduce sensitivity under large amalgam or cast-metal restorations.

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5(a). What physical factors may affect contact angle? Explain any two.

Factors affecting contact angle

1. Surface energy of the solid
2. Surface cleanliness
3. Surface roughness
4. Surface tension of the liquid
5. Viscosity of the liquid
6. Temperature
7. Contamination by saliva, oil, moisture, or debris
8. Chemical nature of solid and liquid

1. Surface cleanliness

2. Surface tension of liquid

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5(b). Explain adhesion and cohesion with figure.

Adhesion:
Liquid molecules attracted to solid surface

     Liquid
   ● ● ● ● ●
 ─────────────  Solid surface
   ↑ attraction between different materials

Cohesion:
● ↔ ● ↔ ● ↔ ●
Attraction between similar liquid molecules

Relation to wetting

• Adhesive force greater than cohesive force: liquid spreads, contact angle is low, wetting is good.
• Cohesive force greater than adhesive force: liquid forms a bead, contact angle is high, wetting is poor.

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6(a). Define and classify separating media used in dentistry.

Classification with examples

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6(b). Factors that play an important role in adhesion

1. Good wetting - Low contact angle permits intimate contact between adhesive and adherend.
2. Clean surface - Surface must be free from saliva, oil, moisture, smear layer, debris, and contaminants.
3. High surface energy of adherend - A high-energy surface promotes spreading and bonding.
4. Low surface tension of adhesive - Enhances spreading over the surface.
5. Surface roughness and micromechanical retention - Etching, sandblasting, and roughening increase surface area and provide retention.
6. Appropriate viscosity - Adhesive must flow into microscopic irregularities without being too thick.
7. Chemical compatibility - Chemical bonds improve adhesion, for example glass-ionomer cement bonding to tooth calcium.
8. Adequate film thickness - A thin, uniform adhesive layer is preferable.
9. Proper pressure during application - Helps intimate adaptation and removes trapped air.
10. Correct setting or polymerization - Proper curing time, light intensity, and isolation are necessary.
11. Minimal thermal and mechanical stress - Large mismatch in thermal expansion or repeated loading can weaken the bond.

Science of Dental Materials: Complete SAQ Answers

Group A

1(a). Stress and strain. Classification of stress

1. Tensile stress: Pulling force that elongates a material.
   Example: Orthodontic wire under tension.
2. Compressive stress: Pushing force that shortens a material.
   Example: Amalgam during mastication.
3. Shear stress: Force acting parallel to a surface, causing sliding.
   Example: Shearing force on a restoration.
4. Torsional stress: Twisting stress.
   Example: Rotary endodontic file.
5. Flexural stress: Stress caused by bending, with compression on one side and tension on the other.
   Example: Denture base during function.

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1(b). Modulus of elasticity and importance

• Indicates resistance to elastic deformation.
• A high modulus means a rigid material, such as cast metal or ceramic.
• A low modulus means a flexible material, such as an elastomeric impression material.
• Restorative material should preferably have modulus close to tooth structure.
• Important in bridges, clasps, orthodontic wires, posts, restorations, and denture bases.
• Excessively high modulus can transmit stress to enamel and dentin; lower modulus permits more deformation.

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2(a). Stress-strain curve: proportional limit, yield point, ultimate strength

Stress
  ↑
  │                              D = Ultimate strength
  │                             /\
  │                            /  \
  │                    C = Yield    \ E = Fracture
  │                   /
  │         B = Proportional limit
  │        /
  │       /
  │______/________________________________→ Strain
       A

• A-B: Elastic region: Material returns to original form when load is removed.
• B, proportional limit: Greatest stress at which stress is directly proportional to strain.
• C, yield point / elastic limit: Beyond this point, permanent deformation starts.
• D, ultimate strength: Maximum stress sustained by the material before failure.
• E, fracture point: Point at which the material breaks.

---

2(b). Primary interatomic bonds. Ionic bond

1. Ionic bond
2. Covalent bond
3. Metallic bond

Na  → loses electron → Na⁺
Cl  ← gains electron ← Cl⁻

Na⁺  ← electrostatic attraction →  Cl⁻

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3(a). Space lattice and cubic lattices of dental interest

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3(b). Cubic structures

1. Simple cubic

• Effective atoms per unit cell: 1
• Coordination number: 6
• Rare in metals

 ●─────●
/|    /|
●────● |
| |   | |
| ●───|-●
|/    |/
●─────●

2. Body-centered cubic, BCC

• Effective atoms per unit cell: 2
• Coordination number: 8
• Examples: Iron and chromium

 ●─────●
/|    /|
●────● |
|  ●  | |
| ●───|-●
|/    |/
●─────●

3. Face-centered cubic, FCC

• Effective atoms per unit cell: 4
• Coordination number: 12
• Examples: Gold, silver, copper, nickel, platinum

 ●─────●
/|  ● /|
●────● |
| ●   |●
| ●───|-●
|/ ●  |/
●─────●

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4(a). Coefficient of thermal expansion and significance

• A restorative material should have CTE close to enamel and dentin.
• A major CTE mismatch causes repeated expansion and contraction at restoration margins.
• It can cause marginal gap, microleakage, postoperative sensitivity, staining, recurrent caries, and fracture.
• Important in amalgam, composite, ceramic, cement, denture base, and metal-ceramic crowns.
• In metal-ceramic restorations, metal should have slightly higher CTE than porcelain to place porcelain under compression during cooling.

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4(b). Why a thick insulating base is used under metallic restoration

• Reduce thermal transmission to the pulp.
• Prevent pulpal irritation and thermal shock.
• Prevent postoperative sensitivity.
• Replace lost dentin, which normally insulates the pulp.
• Protect the dentin-pulp complex.

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5(a). Factors affecting contact angle. Explain two

• Low contact angle: Good wetting.
• High contact angle: Poor wetting.

1. Surface energy of solid
2. Surface cleanliness
3. Surface roughness
4. Surface tension of liquid
5. Viscosity of liquid
6. Temperature
7. Surface contamination
8. Chemical nature of surfaces

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5(b). Adhesion and cohesion

Adhesion:
Liquid
● ● ● ● ●
────────── Solid surface
 ↑ attraction between unlike materials

Cohesion:
● ↔ ● ↔ ● ↔ ●
Attraction between like molecules

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6(a). Separating media: definition and classification

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6(b). Factors affecting adhesion

• Clean, dry, uncontaminated surface.
• Good wetting and low contact angle.
• High surface energy of adherend.
• Low surface tension of adhesive.
• Appropriate viscosity of adhesive.
• Surface roughness and micromechanical retention.
• Acid etching or sandblasting where indicated.
• Chemical compatibility between adhesive and surface.
• Thin and uniform adhesive film.
• Adequate pressure during placement.
• Correct curing/polymerization.
• Prevention of moisture contamination.
• Similar thermal expansion characteristics to avoid bond failure.

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Group B

7(a). Hardness and clinical significance

• Restoration should resist wear during mastication.
• It should not excessively wear opposing natural teeth.
• Hardness helps in polishing and finishing procedures.
• Hardness is related to resistance to scratching by instruments.
• High hardness is desirable in dental burs, cutting instruments, abrasives, and ceramics.
• It helps compare restorative materials with enamel and dentin.

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7(b). Brittleness, examples, and reason

• Porcelain
• Dental stone
• Plaster of Paris
• Zinc phosphate cement
• Glass-ionomer cement
• Amalgam
• Acrylic resin under sudden impact
• Some ceramics

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8(a). Why proportional limit of restoration should be higher than masticatory stress

• It prevents permanent deformation under normal chewing force.
• It preserves anatomical contour and occlusion.
• It prevents marginal breakdown.
• It prevents fracture and loss of contact points.
• It maintains adaptation and reduces microleakage.
• It increases service life of restoration.

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8(b). Resilience and toughness from stress-strain curve

• Represented by the area under the stress-strain curve up to the proportional limit.
• Important for orthodontic wires and denture clasps.

• Represented by the total area under the stress-strain curve up to fracture.
• A tough material resists crack propagation and fracture.

Stress
  ↑
  │                        /\
  │                       /  \
  │                      /    \ fracture
  │                     /
  │                    /
  │___________________/____________→ Strain
       Resilience       Total area = Toughness

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9(a). Stainless-steel wire stretched to 1.001 m: strain and CTE

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9(b). Why metal is a good conductor of heat and electricity

Positive metal ions:  +  +  +  +
Free electrons:      e⁻ e⁻ e⁻ e⁻

• Metallic restorations transmit heat/cold to pulp.
• Insulating bases may be needed in deep cavities.
• Metals can cause galvanic current when dissimilar metals contact in saliva.

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10(a). Creep and flow. Importance

• Excessive creep in amalgam causes marginal breakdown and ditching.
• Causes loss of contour and occlusal anatomy.
• Affects long-term performance of restorations.
• Low creep is desirable in amalgam.

• High flow in wax may distort a wax pattern.
• Impression materials require controlled flow.
• Flow in wax increases with temperature.
• Low flow is desirable in inlay wax at oral temperature.

---

10(b). Clinical significance of hardness

• Indicates resistance to wear during mastication.
• Determines scratch resistance during finishing and polishing.
• Helps select cutting instruments and abrasives.
• Affects abrasion resistance of a restoration.
• A very hard restoration may abrade opposing enamel.
• Important for enamel, ceramics, cast alloys, composite resin, burs, and stones.

---

11(a). Covalent bond with example and figure

H·  +  ·H  →  H:H
             H-H

• Strong and directional bond.
• Electrons are shared rather than transferred.
• Common in organic compounds and polymers.
• Carbon chains in acrylic resin are linked by covalent bonds.

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11(b). Ideal separating medium and why it is ideal

• Produces a thin, continuous, smooth film.
• Separates acrylic resin from gypsum mould.
• Prevents acrylic resin from adhering to or penetrating gypsum.
• Does not discolor acrylic resin.
• Is water soluble and easy to apply.
• Allows easy deflasking and produces a smooth denture surface.
• Does not interfere with polymerization.

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12(a). Malleability and ductility. Measurement

• Malleability: Assessed by the ability to form thin sheet or by compression test.
• Ductility: Measured by percentage elongation or percentage reduction in area in a tensile test.

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12(b). Latent heat of fusion

• Important during melting and casting of dental alloys.
• Determines heat required to melt metal for casting.
• During solidification, this heat is released.

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Group C

1(a). Dental material science: definition

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1(b). Tarnish and causes in oral cavity

• Saliva and moisture
• Food debris and plaque
• Sulfur compounds from food or bacteria
• Oxidation
• Chloride ions in saliva
• Low pH due to acidic food or bacterial activity
• Poor oral hygiene
• Different metals in the mouth
• Surface roughness and scratches
• Heat treatment or contamination during processing

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2(a). Corrosion: definition, classification, examples

Classification

1. Dry or chemical corrosion
   Direct reaction of metal with gases, usually oxygen, without an electrolyte.
   Example: Oxidation of metal at high temperature during casting.
2. Wet or electrochemical corrosion
   Occurs in presence of electrolyte, such as saliva.
   Example: Corrosion of amalgam or base-metal alloys in the oral cavity.
3. Galvanic corrosion
   Occurs when two dissimilar metals contact in saliva, forming an electrochemical cell.
   Example: Gold restoration near amalgam restoration.
4. Concentration-cell corrosion
   Occurs due to differences in concentration of oxygen, ions, or saliva around the same metal.
   Example: Corrosion in crevices under a restoration.
5. Crevice corrosion
   Localized corrosion in narrow spaces with stagnant fluid.
   Example: Under plaque or at restoration margins.
6. Stress corrosion
   Corrosion accelerated by tensile stress.
   Example: Corrosion fatigue or cracking of metal appliances.
7. Pitting corrosion
   Localized deep corrosion producing pits.
   Example: Chloride-associated pitting in some alloys.

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2(b). Factors enhancing corrosion in oral cavity

• Presence of saliva as electrolyte.
• Low oral pH and acidic diet.
• Temperature changes.
• Oral bacteria and plaque.
• Food debris.
• Chloride and sulfide ions.
• Dissimilar metallic restorations.
• Large difference in electrode potentials.
• Rough or scratched surface.
• Crevices and poor polishing.
• Improper alloy composition.
• Porosity in casting.
• Mechanical stress and fatigue.
• Poor oral hygiene.
• Frequent intake of hot, cold, acidic, or carbonated beverages.

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3(a). Two restorations: one directly made and another made outside mouth

1. Direct restoration: Restoration fabricated and placed directly in the prepared cavity in the mouth.
   Example: Amalgam restoration, direct composite resin restoration.
2. Indirect restoration: Restoration fabricated outside the mouth on a cast or die, then cemented or bonded to tooth.
   Example: Cast gold inlay, ceramic crown, metal-ceramic crown.

Direct:   Tooth cavity → material inserted directly → restoration

Indirect: Tooth preparation → impression/scan → lab fabrication
          → cementation in mouth

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3(b). Effects of galvanism and prevention

• Sharp pain or electric shock sensation.
• Metallic taste.
• Pulpal irritation.
• Increased corrosion of the less noble metal.
• Tarnish and discoloration.
• Tissue irritation in susceptible persons.
• Possible acceleration of restoration failure.

• Avoid contact between dissimilar metals.
• Use similar or compatible alloys.
• Place an insulating base under metallic restoration where appropriate.
• Avoid placing new amalgam adjacent to a different metallic restoration.
• Polish restorations smoothly.
• Maintain good marginal adaptation.
• Use non-metallic restorative materials where suitable.

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4(a). Dental materials and ideal requirements

Biological

• Non-toxic
• Non-irritant to pulp and oral tissues
• Non-allergenic
• Non-carcinogenic
• Biocompatible
• No adverse systemic effect

Mechanical

• Adequate strength
• Good fatigue resistance
• Appropriate hardness
• Good wear resistance
• Low creep
• Adequate toughness

Physical

• Dimensional stability
• Low thermal conductivity when needed
• CTE similar to tooth
• Adequate radiopacity
• Low solubility and water sorption

Chemical

• Corrosion resistant
• Tarnish resistant
• Stable in saliva
• Insoluble in oral fluids

Handling and esthetics

• Easy manipulation
• Adequate working and setting time
• Good adhesion or retention
• Color similar to tooth when required
• Easily finished and polished
• Economical and available

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4(b). Classification based on property and behavior

Based on physical behavior

1. Rigid materials
   • Metals
   • Ceramics
   • Gypsum products
   • Resin after polymerization
2. Elastic materials
   • Alginate
   • Agar
   • Elastomeric impression materials
3. Plastic materials
   • Impression compound
   • Modelling wax
   • Acrylic resin dough
4. Viscoelastic materials
   • Waxes
   • Some impression materials
   • Resins during manipulation

Based on physical property

• Metallic materials: Gold alloy, amalgam, stainless steel.
• Ceramic materials: Porcelain, zirconia.
• Polymeric materials: Acrylic resin, composite resin.
• Composite materials: Resin composite, glass-ionomer cement.
• Dental cements: Zinc phosphate, glass ionomer, polycarboxylate cement.

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5(a). Physical factors affecting dental pulp. Explain two

• Thermal conductivity
• Thermal diffusivity
• Electrical conductivity
• Coefficient of thermal expansion
• Polymerization shrinkage
• Exothermic heat during setting
• Permeability and microleakage
• Pressure from condensation or manipulation

1. Thermal conductivity

2. Thermal expansion

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5(b). Classification based on application with examples

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6(a). Biointegration and osseointegration

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6(b). Biomaterial and classification from biological perspective

Classification by biological response

1. Bioinert materials
   Cause minimal tissue reaction and are separated from tissue by a fibrous capsule.
   Examples: Alumina, zirconia, some stainless-steel alloys.
2. Bioactive materials
   Produce a specific biological response and may bond with tissue.
   Examples: Bioactive glass, hydroxyapatite, calcium phosphate, some glass-ionomer materials.
3. Bioresorbable materials
   Gradually dissolve or are absorbed and replaced by body tissue.
   Examples: Tricalcium phosphate, some bone graft materials, absorbable sutures.
4. Biotolerant materials
   Tolerated by tissue but may be isolated by fibrous tissue.
   Examples: Some polymers and metallic alloys.

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Group D

1(a). Critical tissues and organs susceptible to adverse effects of dental materials

Local tissues

1. Oral mucosa
   • Irritation, ulceration, contact stomatitis, lichenoid reaction.
2. Gingiva and periodontal tissues
   • Inflammation, discoloration, allergic reaction.
3. Dental pulp
   • Thermal injury, chemical irritation, postoperative sensitivity, pulpal inflammation.
4. Dentin and enamel
   • Demineralization, marginal leakage, crack formation.
5. Bone
   • Implant-related reaction, inflammation, failure of osseointegration.
6. Skin
   • Contact dermatitis in dental personnel from resin, latex, eugenol, acrylates, or metals.

Systemic organs

1. Kidneys
   • Can be affected by toxic substances such as mercury in excessive exposure.
2. Liver
   • Detoxifies absorbed chemicals and metal ions.
3. Central nervous system
   • Potentially affected by high exposure to mercury and organic solvents.
4. Respiratory system
   • Dust from silica, acrylic, gypsum, polishing materials, and metal grinding.
5. Immune system
   • Allergy or hypersensitivity to nickel, cobalt, latex, resin monomers, or eugenol.
6. Reproductive system
   • Possible risk from chronic exposure to certain chemicals, solvents, and radiation.

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1(b). Occupational hazards for dental personnel

Physical hazards

• Radiation exposure from X-rays.
• Noise from handpieces and laboratory equipment.
• Eye injury from curing light, lasers, flying particles, and chemicals.
• Musculoskeletal problems from poor posture.
• Needle-stick and sharp injuries.

Chemical hazards

• Mercury vapor from amalgam.
• Methyl methacrylate monomer.
• Resin monomers and bonding agents.
• Disinfectants and sterilizing chemicals.
• Latex allergy.
• Silica, gypsum, acrylic, and metal dust.

Biological hazards

• Hepatitis B and C.
• HIV.
• Tuberculosis.
• Influenza and other respiratory infections.
• Aerosol-borne infections.

Psychological hazards

• Stress, fatigue, anxiety, burnout.

Prevention

• PPE: gloves, mask, protective eyewear, apron.
• Good ventilation and suction.
• Proper mercury hygiene.
• Correct posture and ergonomic practice.
• Vaccination, especially hepatitis B.
• Sterilization and infection-control procedures.
• Safe sharps disposal.
• Use of dust extraction in laboratory.

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2(a). Galvanic corrosion with prevention

Amalgam / base metal = Anode      Gold = Cathode
          ↓ corrosion

Anode ───── saliva electrolyte ───── Cathode
        Electron flow through metallic contact

• Avoid direct contact of dissimilar metals.
• Use similar metals or compatible alloys.
• Use non-metallic restorations where indicated.
• Maintain good polishing and finishing.
• Avoid crevices and defective margins.
• Use insulating base under deep metallic restoration where indicated.
• Replace incompatible restoration if persistent galvanic pain occurs.

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2(b). Addition polymerization and steps

Steps

1. Activation
   • Initiator is activated by heat, light, or chemical activator.
   • Benzoyl peroxide forms free radicals.
2. Initiation
   • Free radical attacks double bond of monomer and forms active monomer radical.
3. Propagation
   • Active chain reacts with more monomer molecules.
   • Polymer chain length increases.
4. Termination
   • Growth stops when two active radicals combine or are deactivated.

Initiator → Free radical

Free radical + Monomer → Active monomer radical

Active chain + Monomer → Growing polymer chain

Two active chains combine → Termination

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3(a). Biocompatibility and hazardous dental materials

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3(b). Denture indicated for two patients

Patient 1: 65 years old, no natural teeth in upper and lower arches

Patient 2: 50 years old, some natural teeth remain but several are missing

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4(a). Define polymer, monomer, polymerization, and free radical

R· = free radical

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4(b). Features of cross-linking of polymers

• Increases strength and rigidity.
• Increases hardness.
• Improves heat resistance.
• Reduces solubility.
• Reduces flow and creep.
• Reduces water sorption.
• Improves resistance to crazing.
• Reduces deformation.
• Makes material less soluble in organic solvents.
• Excessive cross-linking may make material brittle and difficult to repair.

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5(a). Requisites of dental resin

Biological requirements

• Non-toxic
• Non-irritant
• Non-allergenic
• Tasteless and odorless

Physical requirements

• Good esthetics and color stability
• Translucency similar to natural tissue when needed
• Low water sorption and solubility
• Dimensional stability
• Low thermal conductivity
• Appropriate thermal expansion
• Radiopacity where needed

Mechanical requirements

• Adequate strength
• High impact resistance
• Adequate hardness and wear resistance
• Good fatigue resistance
• Low creep
• Easy to polish

Handling requirements

• Easy to manipulate
• Adequate working and setting time
• Minimal polymerization shrinkage
• Low residual monomer
• Easily repaired
• Economical

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5(b). Applications of resin in dentistry

• Denture bases.
• Artificial denture teeth.
• Temporary crowns and bridges.
• Composite resin restorations.
• Pit and fissure sealants.
• Adhesive bonding agents.
• Luting resin cements.
• Orthodontic appliances and retainers.
• Orthodontic brackets.
• Custom impression trays.
• Provisional restorations.
• Occlusal splints and mouthguards.
• Maxillofacial prostheses.
• Soft liners and tissue conditioners.
• 3D-printed dental models, guides, and provisional restorations.

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6(a). Induction and propagation steps of addition polymerization

1. Induction period

2. Initiation

3. Propagation

R· + M → RM·

RM· + M → RMM·

RMM· + M → RMMM·

4. Termination

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6(b). Prevention of corrosion in oral cavity

• Select corrosion-resistant materials and alloys.
• Use noble alloys, titanium, or passivating alloys where suitable.
• Avoid coupling dissimilar metals.
• Avoid direct contact between amalgam and gold/alloys of very different electrode potential.
• Provide smooth, polished restoration surfaces.
• Avoid roughness, porosity, cracks, and crevices.
• Ensure proper casting, finishing, and polishing.
• Ensure good marginal adaptation.
• Maintain oral hygiene and remove plaque.
• Reduce acidic oral environment by controlling diet and caries activity.
• Use fluoride and preventive care as indicated.
• Avoid contaminated alloys and improper heat treatment.
• Use insulating material or non-metal restoration in selected situations.