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
2
Q
Mention impression materials
A
Hydrocolloids (e.g., alginate)
Elastomers (e.g., polyethers, silicones)
Impression compound & paste
3
Q
What is negative & positive replica?
A
Impression – negative replica
Dental stone (gypsum) \ study cast - positive replica
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)
5
Q
What are the three primary types of force acting on dental materials?
A
Compressive (squeezing)
Tensile (stretching)
Shear (sliding)
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
7
Q
What is the unit of measurement for stress?
A
Pascals (Pa)
8
Q
What is Strain?
A
Change in length / Original length
9
Q
What is {Strain - Stress Curves} ?
A
Used to identify how a material performs under pressure
10
Q
FS stands for?
A
Refers to the point at which the material fractures.
11
Q
PL stands for?
A
Refers to the limit at which after stress is removed, the material can return to its original shape.
12
Q
What kind of forces are the teeth typical exposed to?
A
- Compressive forces - via biting
- Abrasive & frictional forces - via Grinding & chewing
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
14
Q
Fracture
A
Large force causing catastrophic destruction of material’s structure
15
Q
Hardness
A
Ability of surface to resist indentation
16
Q
Abrasion
A
Material surface removal due to grinding
17
Q
Abrasion Resistance
A
Ability to withstand surface layers being removed
18
Q
Fatigue
A
Repetitive ‘small’ stresses causing material fracture
19
Q
Creep
A
Gradual dimensional change due to repetitive small forces
20
Q
Deformation
A
Applied stress cause permanent change in materials dimension when greater than the elastic limit of material
21
Q
De-bond
A
Applied forces sufficient to break material-tooth bond
22
Q
Impact
A
Large, sudden force causing a fracture
23
Q
Chemical properties
A
- Setting mechanism.
- Setting time.
- Corrosive Potential.
24
Q
Physical properties
A
- Viscosity.
- Thermal Conductivity & Expansion.
- Density
- Radiodensity.
25
What is Amalgam?
An alloy of:
- Mercury (Liquid component)
- Silver, Tin, Copper & Other trace metals (Powder component)
26
What are the compositions of Amalgam?
* Powder (50% by weight): Silver/Tin- Copper- Zinc- mercury (Hg)
* Liquid (50% by weight): mercury (Hg)
27
What are the two main classifications of dental amalgam?
Composition-based (Traditional & Copper-Enriched) and Particle shape-based (Lathe-cut & Spherical).
28
What is the function of silver and tin in amalgam?
They form an intermetallic compound.
29
Why is copper added to dental amalgam?
It increases strength and hardness.
30
What is the role of zinc in amalgam?
Acts as a scavenger of oxygen, ensuring clean castings.
31
How does mercury contribute to the amalgam reaction?
It reacts with the powder metals to form the amalgam matrix or final set material.
32
What are the two main types of amalgam particle shapes?
Lathe-cut and Spherical.
33
Which amalgam particle type provides stronger proximal contacts?
Lathe-cut particles.
34
How are spherical amalgam particles formed?
By spraying molten metal into an inert atmosphere.
35
List two advantages of spherical particles in amalgam.
Higher early compressive strength and easier carving.
36
Amalgam setting reaction
Silver/Tin + Mercury → Silver/tin + Silver/mercury + Tin/Mercury
37
Gamma phases
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)
38
What are the three gamma phases in amalgam?
Gamma (Ag-Sn), Gamma-1 (Ag-Hg), and Gamma-2 (Sn-Hg).
39
Which gamma phase has the best corrosion resistance?
Gamma-1.
40
Which gamma phase has the worst corrosion resistance?
Gamma-2
41
What happens if zinc-containing amalgam is contaminated with saliva or blood during placement?
It reacts with moisture to form hydrogen gas bubbles, causing expansion.
42
How long does it take for amalgam to fully set?
Approximately 24 hours.
43
List three factors that decrease the strength of amalgam.
Undermixing, slow rate of packing, and corrosion.
44
What H2 bubbles that formed in Zinc reaction can do?
- Cause pressure build up → Leading to expansion
- Downward pressure → Causing pulpal pain
- Upward pressure → Restoration sits above occlusal surface
45
Why is high-copper amalgam preferred over traditional amalgam?
It eliminates the Gamma-2 phase, reducing corrosion and creep
46
What are the two types of high-copper amalgam?
Dispersion-modified and single-composition.
47
What is a key advantage of single-composition high-copper amalgam?
It has a single setting reaction, with copper integrated into the silver-tin alloy particles.
48
Pros of Copper Enriched
* Higher early strength.
* Less Creep.
* Higher resistance to corrosion.
* Increased Margin durability.
49
How can corrosion in amalgam be reduced?
Using copper-enriched amalgam and polishing the margins.
50
How does amalgam’s thermal expansion compare to tooth structure?
It expands 3 times more than tooth structure.
51
Why might a liner or varnish be needed under an amalgam restoration?
To protect the pulp from thermal conductivity.
52
What is the main bonding mechanism for amalgam in cavity preparation?
Mechanical retention
53
What is Permite, and why is it used?
A non-Gamma-2, spherical, and lathe-cut material with high compressive strength and low microleakage.
54
What are the key requirements of a good dental adhesive?
- Provide high bond strength to tooth tissues
- Immediate high strength bond
- Durable bond
- Impermeable bond
- Easy to use
- safe
55
Why is bonding to enamel easier than dentine?
Due to structure of enamel:
Enamel is densely packed, highly mineralized (95%), and dry
56
What is the purpose of acid etching on enamel?
To roughen the enamel surface for micromechanical interlocking of resin filling materials.
57
Which acid is most commonly used for etching enamel?
Phosphoric acid.
58
What is the usual concentration of phosphoric acid used for etching enamel?
30-50% phosphoric acid
59
Why must enamel be dry after etching?
Moisture prevents the resin from flowing into the etched surface.
60
How does etching affect the surface energy of enamel?
* Increases surface energy
* Improves wettability, allowing bonding agents to adhere better
61
What type of bonding agent is commonly applied to etched enamel?
A low-viscosity Bis-GMA bonding agent.
62
What makes bonding to dentine more difficult than enamel?
* 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**
63
What is the smear layer, and how does it affect bonding?
* A thin layer of debris left on dentine after preparation (0.5-5μm thick)
* Contains bacteria and organic material
* Can interfere with bonding
64
What are the main requirements of a dentine bonding agent?
* Ability to flow
* Intimate contact with dentine surface
* Low viscosity
* Adhesion via mechanical, chemical, and van der Waals forces
65
What are the three mechanisms of adhesion to dentine?
Mechanical, Chemical, and Van der Waals forces.
66
How does dentine bonding occur?
* Mechanical bonding: Interlocking with dentine tubules
* Chemical bonding: Ionic and covalent interactions with dentine components
* Van der Waals forces: Electrostatic interactions
67
Why is wet dentine challenging for bonding?
It has a low surface energy, making it difficult for bonding agents to adhere.
Liquid CSE < Surface CSE = adequate adhesion
68
What is the gold standard for dentine bonding?
Total Etch technique
69
What is the main etchant used in Total Etch?
35% phosphoric acid
70
What are the three main components of a total etch bonding system?
* **Dentine conditioner (etchant)**: Removes smear layer, opens tubules
* **Primer**: Hydrophilic/hydrophobic coupling agent
* **Adhesive**: Hydrophobic resin, forms hybrid layer
71
What is the purpose of the dentine conditioner?
It removes the smear layer and opens up dentinal tubules.
72
What is the function of the primer in total etch bonding?
It contains hydrophilic and hydrophobic components that prepare the dentine for bonding.
73
What does the adhesive do in the total etch technique?
It penetrates the dentine, forms a micromechanical bond, and creates the hybrid layer.
74
What happens if dentine is over-etched?
Collagen fibers collapse, preventing resin penetration.
75
Why must dentine be neither too dry nor too wet for bonding?
* Too dry → Collagen collapses.
* Too wet → Primer is diluted, reducing bond strength.
76
How does self-etch bonding differ from total etch?
* 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
77
Why is self-etch less technique-sensitive?
It eliminates the need to control moisture levels precisely.
78
What is the primary disadvantage of self-etch bonding?
The bond strength is weaker, especially to enamel.
79
Why is self-etch preferred for dentine over enamel?
It partially demineralizes dentine, preserving hydroxyapatite around collagen for stability.
80
Why does mild self-etch bonding work better on dentine than strong self-etch?
* Mild self-etch partially demineralizes dentine, leaving hydroxyapatite for additional ionic bonding
* Protects collagen from degradation
81
What is a disadvantage of self-etch in dentine bonding?
Etching by-products are not washed away, weakening the bond.
82
Why might a mild self-etch system fail to bond properly?
It may not fully penetrate the smear layer.
83
What is Critical Surface Energy (CSE)?
The surface tension of a liquid that determines its ability to spread on a solid surface
84
How does CSE affect adhesion?
Liquid CSE < Surface CSE → Good adhesion
Liquid CSE > Surface CSE → Poor adhesion
85
What is molecular entanglement in dentine bonding?
When adhesive penetrates dentine and forms long-chain polymers that interlock with the collagen matrix
86
What is the fundamental mechanism of bonding to dentine?
Molecular Entanglement – resin replaces minerals in the dental tissue, forming an interlocking bond.
87
According to Peumans et al. (2010), how can self-etch bonding be improved?
By etching enamel with phosphoric acid first.
88
Why do some restorative materials require lining materials?
To compensate for poor contact with the tooth surface, prevent chemical irritation to the pulp, and reduce microleakage.
89
What is the main function of a lining material?
It prevents gaps and acts as a protective barrier between the tooth and the restorative material.
90
How does a cavity base differ from a cavity liner?
* 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.
91
What are the three main purposes of a cavity liner?
* Pulpal protection (prevents chemical, thermal, and bacterial irritation)
* Therapeutic (reduces inflammation, promotes healing)
* Palliative (reduces patient symptoms in reversible pulpitis)
92
How does a liner protect the pulp from chemical irritation?
It prevents unreacted chemicals in the filling material from reaching the pulp.
93
How does a liner protect against thermal damage?
It acts as an insulator, preventing heat from metal fillings or exothermic reactions from reaching the pulp.
94
How does a liner help prevent microleakage?
It seals the space between the restoration and the cavity walls, preventing bacterial and endotoxin infiltration.
95
What are the ideal properties of a liner?
* 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
96
# Setting Calcium Hydroxide (Liner)
What is the composition of setting calcium hydroxide?
A base and a catalyst that react via a chelation reaction.
97
How does calcium hydroxide kill bacteria?
It creates a high initial pH (~12), which is hostile to cariogenic bacteria.
98
How does calcium hydroxide promote dentine formation?
It irritates the odontoblast layer, triggering necrosis and the formation of tertiary dentine.
99
What are the disadvantages of calcium hydroxide liners?
Low compressive strength, unstable, highly soluble (dissolves if leakage occurs or in moist conditions).
100
What are the types of zinc oxide-based cements, and which ones are no longer used?
* No longer used: Zinc phosphate, Zinc polycarboxylate
* Still used: Zinc Oxide Eugenol (ZOE), Resin Modified ZOE, Ethoxybenzoic Acid (EBA) ZOE
101
What is the setting reaction of zinc phosphate cement?
Acid-base reaction between zinc oxide (powder) and phosphoric acid (liquid), followed by a hydration reaction forming a crystalline phosphate matrix.
102
Why does zinc phosphate cement become stronger over time?
It absorbs water, making the matrix less porous and increasing strength.
103
What are the disadvantages of **zinc phosphate cement**?
Low initial pH (causing pulpal irritation), exothermic setting reaction, no adhesion to tooth/restoration, non-cariostatic, brittle.
104
What makes z**inc polycarboxylate** different from **zinc phosphate cement**?
It replaces phosphoric acid with polyacrylic acid, allowing bonding to the tooth.
105
What are the advantages of zinc polycarboxylate cement over zinc phosphate?
Less exothermic, neutral pH quicker, bonds to tooth, does not penetrate dentine deeply.
106
What are the disadvantages of **zinc polycarboxylate cement**?
Difficult to mix, low compressive strength, soluble in the oral environment.
107
Why are **zinc phosphate** and z**inc polycarboxylate** cements rarely used as linings today?
Poor adhesion, irritation, and brittleness; mainly used for temporary crowns.
108
What is the setting reaction of Zinc Oxide Eugenol (ZOE)?
Acid-base reaction between zinc oxide and eugenol, forming a zinc eugenolate matrix.
109
What are the advantages of ZOE?
Quick setting, low thermal conductivity, radiopaque, pulpal soothing effect.
110
What are the disadvantages of ZOE?
Low strength (20MPa), high solubility (breaks down over time), eugenol inhibits resin-based materials.
111
Why should ZOE not be used under composite restorations?
Eugenol softens and discolors resin-based materials.
112
How does adding resin improve ZOE?
It increases compressive strength (40MPa) and decreases solubility.
113
How does Ethoxybenzoic Acid (EBA) improve ZOE?
Encourages crystalline structure, increasing strength (60MPa) and reducing solubility.
114
Why is glass ionomer the most widely used lining material?
Bonds to dentine and composite, releases fluoride, and seals the cavity.
115
What are the advantages of glass ionomer liners?
* 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
116
What is a disadvantage of resin-modified glass ionomer (RMGIC)?
Unreacted HEMA may be cytotoxic to the pulp.
117
Why is RMGIC unique among linings?
It can bond to restorative materials and may even bond amalgam to the tooth.
118
What is the recommended liner for amalgam or large composite cavities?
RMGIC (e.g., Vitrebond).
119
When should calcium hydroxide be used?
For deep cavities near the pulp (direct/indirect pulp capping).
120
What must be done after applying calcium hydroxide before placing a final restoration?
Cover it with RMGIC.
121
What are composite resins used for?
Filling cavities, replacing abraded tissue, repairing/replacing failed restorations.
122
Which cavity classes are composite resins commonly used for?
Class III, IV, V, and limited occlusal wear in Class II.
123
When is composite resin preferred over other materials?
When aesthetics are important.
124
What are the three main components of composite resin?
Filler particles, resin, camphorquinone
125
What is the function of Camphorquinone in composite resins?
It acts as a photo-initiator for polymerization or light curing.
126
What color light activates camphorquinone?
Blue light
127
What role do Low Weight Dimethacrylates play in composite resins?
They control mechanical properties.
128
What does the Silane Coupling Agent do?
It ensures bonding between filler particles and resin.
129
What types of filler particles are used in composite resins?
Basic silica, quartz, or various silicates like lithium aluminum silicate.
130
How does filler particle type affect composite resin properties?
It influences hardness, rigidity, and abrasion resistanc
131
Why are hybrid composites mechanically superior?
They contain both large and small filler particles, providing strength and aesthetics.
132
What is the most common resin monomer used in composites?
BIS-GMA (Bisphenol A Glycidyl Methacrylate).
133
Why is BIS-GMA used in composite resin?
It contains C=C bonds, allowing for crosslinking and polymerization
134
What happens to the C=C bonds during polymerization?
They break, allowing monomer molecules to join and form a polymer.
135
What are the four classifications of composite resin based on filler type?
Conventional, Micro-filled, Submicron, Hybrid.
136
What is the difference between flowable and condensable composites?
Flowable has lower filler content (more shrinkage), while condensable has an ‘amalgam-like’ consistency
137
What are the two curing methods for composite resins?
Light-cured and self-cured.
138
List three potential problems with light curing.
Light-material mismatch, premature polymerization, and polymerization shrinkage
139
Which type of composite resin is used for anterior restorations?
Micro-filled or Hybrid.
140
Which handling characteristic of composite resins has the highest viscosity?
Flowable composite.
141
What initiates polymerization in light-cured composites?
Camphorquinone activated by blue light (430-490 nm).
142
What is the depth of cure for composite resin?
Typically 2mm.
143
Why is composite resin placed in increments?
To ensure complete curing and prevent shrinkage stress.
144
What type of photo-initiator is used in bulk-fill composites?
Lucirin, which is activated by UV light.
145
What can happen if the curing light spectrum does not match the composite material?
Inefficient curing, leading to under-polymerization.
146
How can polymerization shrinkage be minimized?
By using small increments and curing from different angles.
147
Why should the curing light not be directed at the eyes?
Blue LED light can cause ocular damage.
148
What is the fracture strength of composite resin?
350 MPa.
149
What is the Young’s modulus and Proportional Limit (PL) of composite resin?
* YM: 15 GPa (15,000 MPa)
* PL: 300 MPa
150
What happens when stress exceeds the Elastic Limit (EL) of composite?
The composite deforms and does not return to its original shape.
151
Why is composite preferred over amalgam in deciduous teeth restorations?
Because it bonds better, reduces microleakage, and wears at the same rate as natural tooth structure.
152
Why is a good bond between composite and tooth tissue important?
It prevents microleakage and stress concentration, reducing the risk of fractures.
153
Why is low thermal conductivity important for composite resins?
To protect the pulp from heat damage.
154
What is the major drawback of composite resin’s thermal expansion coefficient?
It is higher than tooth tissue, increasing the risk of microleakage.
155
What are the advantages and disadvantages of conventional composite?
Strong but difficult to finish and prone to staining.
156
What are the advantages and disadvantages of microfine composite?
Smoother surfaces and better aesthetics but weaker mechanical properties.
157
What property of composite resins allows for easy shade matching?
Their wide range of shades and translucency.
158
Why is a hybrid composite a good balance between strength and aesthetics?
It contains both large and small filler particles.
159
Why are hybrid composites widely used today?
They offer a balance between strength and aesthetics with improved fillers and coupling agents.
160
How is hardness measured in composite resin?
Using an indentor to create a notch, then measuring the depth of indentation.
161
Name three clinical factors that affect composite wear.
Cavity design, occlusion, and curing efficiency.
162
What is the thermal diffusivity of hybrid composite compared to dentine?
Hybrid composite = 0.005 cm²/s, Dentine = 0.002 cm²/s.
163
What aesthetic properties make composite a good restorative material?
A range of shades, translucency options, stain resistance, and polishability.
164
Why must prosthetic materials have a high Young’s modulus?
To ensure rigidity and withstand biting forces.
165
Why is a high softening temperature important for prosthetic materials?
To prevent distortion from hot fluids and cleaning.
166
Why should prosthetic materials have a high proportional limit?
To prevent deformation under large biting forces.
167
Why do we want high thermal conductivity in acrylic resin materials?
To prevent scalding and allow the patient to feel heat.
167
Why is low density an important property for prosthetic materials?
To improve retention, especially for upper dentures.
168
What are the aesthetic requirements of prosthetic materials?
They should match the colour and translucency of natural tissues.
169
Why must prosthetic materials be non-toxic and non-irritant?
To ensure biocompatibility and avoid adverse reactions.
170
What is free radical addition polymerization?
A reaction where monomers join without eliminating smaller molecules.
170
What type of polymerization does PMMA undergo?
Free radical addition polymerization.
171
What is the initiator in PMMA polymerization?
Benzoyl peroxide
172
How many free radicals does each benzoyl peroxide molecule release?
Two free radicals
173
At what temperature does PMMA polymerization activate?
Above 72°C
174
What are the four steps of polymerization?
* Activation – Benzoyl peroxide provides free radicals.
* Initiation – Free radicals break monomer C=C bonds.
* Propagation – Polymer chains grow.
* Termination – Polymerization stops.
175
Why is a powder-liquid mix used in heat-cured acrylic?
To reduce heat of reaction, minimize shrinkage, and improve handling.
176
What is the powder-to-liquid ratio in heat-cured acrylic?
By volume: 3 - 3.5 to 1
By weight: 2.5 to 1
177
# Heat-Cured Acrylic Composition
What does the powder component contain?
PMMA particles, benzoyl peroxide (initiator), plasticizers, and co-polymers.
178
# Heat-Cured Acrylic Composition
What does the liquid component contain?
Methacrylate monomers, pigments, hydroquinone (inhibitor), and co-polymers.
179
What happens if acrylic resin is cured too quickly?
Gaseous porosity occurs due to excessive heat.
180
Why must acrylic be slowly cooled after curing?
To reduce internal stresses that weaken the material.
181
How does internal stress affect acrylic resin?
Reduces strength, increases fatigue failure, and causes warping.
182
What are the effects of under-curing?
Free monomers remain, leading to weaker mechanical properties.
183
What happens if the monomer ratio is incorrect?
Too much monomer → Contraction porosity.
Too little monomer → Granularity porosity.
184
How does porosity affect the final dental material?
Reduces strength, affects aesthetics, and absorbs saliva, causing hygiene issues.
185
When and where does gaseous porosity occur?
When the monomer boils above 100°C, often in bulkier denture areas.
186
What factors contribute to polymerization shrinkage?
Excess monomer, insufficient packing, and lack of pressure.
187
What are the advantages of PMMA as a dental material?
Non-toxic, stable, good aesthetics, and unaffected by oral fluids.
188
What are the disadvantages of PMMA?
High thermal expansion, low thermal conductivity, and poor mechanical properties.
189
Why does the low density of PMMA not always benefit denture applications?
Increased bulk is needed to compensate for weak mechanical properties.
190
What happens if an acrylic appliance is exposed to boiling water?
The material may warp and lose its shape.
191
Why do porcelain teeth have a higher risk of creating gaps compared to acrylic teeth in acrylic denture?
Porcelain has a lower thermal expansion coefficient than PMMA.
192
What is the aim of impression materials?
To produce an accurate negative reproduction of the surface and shape of oral soft and hard tissues.
193
Why is the accuracy of an impression important?
Treatment outcomes depend on the quality & accuracy of the initial impression.
193
What material is used to fill an impression to produce a cast?
Dental stone
194
What are the uses of a stone cast?
Evaluating ortho & occlusal relationships, production of RPD & other restorations.
195
What is a muco-static impression material?
A material that does not displace the mucosa and records its resting position.
196
Give examples of muco-static materials.
Elastomers: Polyvinyl Siloxane (PVS), Polyether.
197
What is a muco-compressive impression material?
A material that displaces the mucosa and records its shape under load.
198
Give examples of muco-compressive materials.
Silicone putty (heavy-bodied PVS), Hydrocolloids (Alginate).
199
What are elastic impression materials?
Materials that can stretch and deform without breaking and return to their original shape.
200
What are non-elastic impression materials?
Materials that cannot stretch and typically break/tear under stress.
201
Are muco-static materials elastic or non-elastic?
Elastic
202
Are muco-compressive materials elastic or non-elastic?
Non-elastic
203
Why are non-elastic impression materials not recommended for master impressions?
Due to limited accuracy and risk of breaking/tearing upon removal.
204
What is viscoelastic behaviour?
A material exhibiting both viscous (fluid-like) and elastic (solid-like) properties.
205
What happens when an impression material undergoes elastic recovery?
After removal, the material recovers, but some permanent strain remains.
206
How does removal speed affect permanent strain?
Faster removal → Less permanent strain
207
# For Accuracy
What properties ensure an accurate impression?
Viscosity, setting mechanism, low thermal expansion, viscoelastic behavior, high tear strength.
208
# For Patient Comfort
What properties ensure patient comfort?
Non-toxic, non-irritant, acceptable taste/smell, short setting time, easy removal.
209
# For Operator Convenience
What properties ensure operator convenience?
Quick/simple technique, good working/setting times, ability to be decontaminated, cost-effective
210
What is a colloid?
A two-phase system of fine particles dispersed in another phase.
211
What is a hydrocolloid?
A colloid with water as the dispersing medium.
212
What is the most commonly used irreversible hydrocolloid?
Alginate
213
What are the main components of alginate?
Sodium Alginate
Calcium Sulphate
Trisodium Phosphate
Fillers
Modifiers
214
Describe the setting reaction of alginate.
Sodium Alginate reacts with Calcium Sulphate, forming Calcium Alginate, which cross-links to create a solid gel.
215
Why does alginate continue to develop elastic properties after setting?
Cross-linking continues after the apparent set, improving elasticity.
216
What temperature of water should be used when mixing alginate?
18-24°C
217
What type of tray should be used with alginate?
Perforated tray
218
How should an alginate impression be removed?
With a sharp pull to ensure elastic recovery and minimize permanent deformation.
219
What is the primary use of gypsum in dentistry?
To produce a positive replica of dentition from an impression.
220
Why is gypsum used in dental prosthesis manufacturing?
It enables assessment of the dentition and the fabrication of prostheses.
220
What does gypsum record?
Position, shape, and dimensions of teeth and surrounding soft tissue.
221
What is the chemical composition of gypsum used in dentistry?
Calcium sulfate dihydrate (CaSO₄ · 2H₂O).
222
What happens to gypsum when heated?
It converts to calcium sulfate hemihydrate (CaSO₄ · ½H₂O), creating a powder.
223
What happens during the setting reaction of gypsum?
The reaction is reversed, and calcium sulfate dihydrate reforms.
224
What are the three types of gypsum?
* Plaster (β-hemihydrate)
* Dental stone (α-hemihydrate)
* Densite (improved stone).
225
How is plaster produced?
By heating gypsum in an open vessel.
226
What is the structure of plaster crystals?
Irregular and porous.
227
How is dental stone produced?
By heating gypsum in an autoclave.
228
What is the structure of dental stone crystals?
Regular and non-porous.
229
How is densite (improved stone) produced?
By heating gypsum in the presence of calcium chloride.
230
What is the structure of densite crystals?
Compact and smooth.
231
What is the chemical reaction for the setting of gypsum?
Calcium sulfate hemihydrate + Water → Calcium sulfate dihydrate + Water
232
What happens when water is added to gypsum powder?
Hemihydrate dissolves, forming dihydrate crystals.
233
What do dihydrate crystals do during the setting process?
They precipitate and grow around impurities.
234
What do dihydrate crystals do during the setting proces
They precipitate and grow around impurities.
235
What marks the initial set of gypsum?
Dihydrate crystals grow and come into contact, starting the expansion process.
236
At what stage can gypsum be carved?
During the initial set.
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What marks the final set of gypsum?
The material hardens, and water evaporates, making it porous.
238
What is the water-to-powder ratio for plaster?
60ml water to 100g powder.
239
What is the water-to-powder ratio for dental stone?
35ml water to 100g powder.
240
How does increasing the powder-to-water ratio affect gypsum setting time?
It decreases setting time and increases expansion.
241
What is the compressive strength of gypsum?
Ranges from 28-38 MPa, developing to 75 MPa over 24 hours.
242
Which type of gypsum has the greatest strength?
Densite (Improved Stone).
243
Why does gypsum have a rough surface?
Because it is porous, with surface roughness of 28-40 µm.
244
What is the main disadvantage of gypsum’s surface hardness?
It has low resistance to abrasive forces.
245
How does spatulation affect the setting time of gypsum?
Increased spatulation breaks down growing crystals, reducing setting time.
246
How does gypsum expand during setting?
Expansion is low, minimizing dimensional inaccuracies.
247
What is the flexural strength of gypsum?
15-20 MPa.
248
How does potassium sulfate affect gypsum setting time? Why?
It decreases setting time by producing syngenite, which rapidly crystallizes and promotes further crystal growth.
249
How does borax affect gypsum setting time? Why?
It increases setting time by forming calcium borate, which deposits on dihydrate crystals and delays setting.
250
What are the advantages of gypsum in dentistry?
Dimensionally accurate and stable.
Low expansion rate for stone and densite.
251
What are the disadvantages of gypsum?
* Low tensile strength and poor abrasion resistance.
* Very brittle.
* Less surface detail compared to elastomer impression materials.
* Poor wetting on some impression materials.
252
What does the gradient of the stress-strain curve represent?
The Rigidity of the material.
253
What does a higher rigidity indicate?
A higher rigidity means that more stress produces less strain on the material.
254
What is the Elastic Limit (EL)?
The maximum stress a material can withstand without plastic deformation.
255
What is the Ultimate Tensile Strength (UTS)?
The highest stress a material can withstand before breaking.
256
What is the Fracture Strength (FS)?
The point at which the material fractures.
257
What does Ductility represent?
How much plastic deformation a material can undergo under tensile stress before breaking.
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What does Malleability represent?
How much plastic deformation a material can undergo under compressive stress before breaking.
259
What is an Alloy?
combination of metal atoms in a crystalline structure.
260
What are the 3 types of Crystal Structures?
* Cubic
* Face-Centred Cubic (FCC)
* Body-Centred Cubic (BCC)
260
What are the 3 main factors affecting mechanical properties of a metal/alloy?
* Crystalline Structure
* Grain Size
* Grain Imperfections
261
What happens when a molten metal cools?
Crystals form from Nuclei of Crystallisation.
262
What do crystals grow into?
Dendrites → Grains
263
What are Grain Boundaries?
Regions where grains make contact.
264
How does Slow Cooling affect grain size?
Produces fewer nuclei → Large coarse grains.
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What are Nucleating Agents?
Impurities or additives that act as extra nuclei → Producing smaller grains.
266
How do Small Grains affect properties?
* Increase: Elastic Limit, UTS, Hardness
* Decrease: Ductility
267
What is a Dislocation?
A defect in the crystal lattice that weakens the structure.
268
What happens when a force is applied to a metal with dislocations?
Dislocations propagate along the lattice plane until they reach the grain boundary → This is called Slip.
269
How do Small Grains impede dislocation movement?
Small grains = More Grain Boundaries → Dislocations are blocked.
270
What is Cold Working?
Shaping metal by applying force at low temperatures.
270
How does Cold Working affect metal properties?
* Increases: Elastic Limit, UTS, Hardness
* Decreases: Ductility, Corrosion Resistance
270
What is Residual Stress?
Internal stresses in the metal caused by dislocation buildup during Cold Working.
270
What is Annealing?
A heating process that allows atoms to rearrange and eliminate residual stress.
270
What is Recrystallisation?
A heating process that resets grain structure and reverses the effects of Cold Working.
270
What type of grains does **Quenching** produce?
Small fine grains
270
What is the key influence on grain size?
Rate of Cooling
270
What happens to Ductility if grain size decreases?
It decreases
271
What process pushes dislocations to grain boundaries?
Cold Working
272
What is the main disadvantage of Cold Working?
Residual Stress
273
What is an Alloy?
A combination of 2 or more metals, or a combination of metal(s) with a metalloid (e.g., Silicon or Carbon).
274
What are the advantages of alloys?
Superior mechanical properties such as high Elastic Limit, Fracture Strength, Strength & Rigidity, Corrosion Resistance, and lower melting points.
275
Name some examples of alloys used in dentistry.
Gold alloys, Amalgam, Stainless Steel, Cobalt-Chromium.
276
What is a phase in alloys?
A physically distinct homogenous structure.
277
What is a solid solution?
A homogenous mixture of two metals at an atomic scale forming one phase.
278
What are the possible interactions between two molten metals during crystallisation?
* Insoluble (2 Phases)
* Soluble (1 Phase forming a solid solution)
* Formation of Intermetallic Compound (e.g., Amalgam)
279
What are the two types of solid solutions?
Substitutional Solid Solution and Interstitial Solid Solution.
280
What is a substitutional solid solution?
Atoms of metal 1 replace atoms of metal 2 in a crystal lattice, requiring similar size, valency, and crystal structure.
281
What is an interstitial solid solution?
Smaller atoms fit into the spaces within the lattice of larger atoms.
282
How does the crystallisation process differ between pure metals and alloys?
Pure metals crystallise at one temperature, while alloys crystallise over a temperature range.
283
What are the effects of slow cooling on alloys?
Produces large grains and homogenous composition.
284
What are the effects of fast cooling on alloys?
Produces small grains, heterogenous composition (coring), and improves mechanical properties but reduces corrosion resistance.
285
How can coring be removed from alloys?
Through homogenising annealing, where the alloy is reheated below recrystallisation temperature to allow atoms to diffuse and create a homogenous structure.
286
Why do alloys have increased strength compared to pure metals?
Alloys distort the grain lattice due to different-sized atoms, impeding dislocation movement and increasing fracture resistance.
287
What is a eutectic alloy?
An alloy where metals are soluble in liquid state but insoluble in solid state, forming two physically distinct grains.
288
What is a partially soluble alloy?
An alloy producing two distinct grains, each containing both metals but in different concentrations.
289
What is precipitation hardening?
The process of annealing a partially soluble alloy to increase strength and surface hardness
290
What are three examples of partial denture alloys?
* Type IV Gold
* Co-Cr
* Titanium
290
What are the desired properties of a denture base?
* High YM (To maintain shape)
* High EL (To avoid deformation)
291
What are the desired properties of a denture clasp?
* Lower YM (To allow flexure over tooth)
* High EL (Maintain elasticity)
292
What type of gold alloy is used for partial dentures?
Type IV
293
What is the gold percentage range in Type IV gold alloy?
60-70%
294
What metals make up Type IV gold alloy?
Gold, Silver, Copper, Zinc, Palladium, Platinum
295
What are the effects of adding Copper to gold alloy?
* Formation of solid solution
* Solution & order hardening
* Reduces melting point
* Little to no coring
* Imparts red colour
* Reduces density
296
What are the effects of adding Silver to gold alloy?
* Formation of solid solution
* Solution hardening
* Precipitation hardening with copper
* Allows tarnishing
* Absorbs gas (CO2)
* Whitens the alloy
297
Why does Au-Ag show little/no coring?
Solidus & liquidus lines are close together
298
What are the effects of adding Platinum to gold alloy?
* Formation of solid solution
* Solution hardening
* Produces fine grain structure
* Coring can occur
*
299
What are the effects of adding Palladium to gold alloy?
* Similar to platinum
* Less coring
* Produces more coarse grains
300
What is the function of Zinc in gold alloys?
Scavenger
301
What is the function of Nickel in gold alloys?
Increase hardness & strength
302
What is the function of Indium in gold alloys?
Produce fine grain structure
303
What are gold alloys generally suited for in a denture: base or clasp?
Clasp
304
Why are gold alloys not typically used for denture bases?
Thickness required → Expensive
305
What is Co-Cr frequently used for in RPD?
Connectors
306
What is the composition of Co-Cr alloy?
Cobalt, Chromium, Nickel
307
What is the effect of Chromium in Co-Cr alloy?
Corrosion resistance (Passive layer)
308
What is the effect of Nickel in Co-Cr alloy?
* Improves ductility
* Slightly reduces strength
309
What are the general properties of Co-Cr alloy?
Harder than gold
Low ductility
Difficult to adjust
309
What are the uses of Titanium in dentistry?
* Implants
* Crown & Bridge
* Partial Denture
310
What are the benefits of Titanium?
* Good biocompatibility
* Good corrosion resistance
* Can be joined by laser welding
311
What does viscosity measure in impression materials?
The material's ability to flow and make close contact with tissues.
312
What does viscosity measure in impression materials?
The material's ability to flow and make close contact with tissues.
327
Why is viscosity important for impression materials?
It determines how well the material can record surface detail.
328
What is wettability in impression materials?
The ability of a material to make intimate contact with hard and soft tissues.
329
How is wettability measured?
By the contact angle (Lower angle = better wetting).
330
What is the ISO standard for surface reproduction?
Grooves of 50 microns must be replicated.
331
What is elastic recovery in impression materials?
The ability of the material to return to its original shape after removal.
332
How does load time affect permanent deformation?
Reduced load time → Less strain → Less deformation
333
Do we want high or low viscoelasticity?
Low viscoelasticity (Small permanent deformation).
334
What is tear strength?
The amount of stress a material can withstand before fracturing.
335
Why is tear strength important in impression materials?
It helps prevent the impression tearing when removed from undercuts.
336
What kind of rigidity is ideal for impression materials?
Low rigidity → More flexible for easier removal.
337
Which material type has longer working time?
Polyether
338
Which material has better elastic recovery?
Addition Silicone (PVS)
339
Which material has higher tear strength?
Addition Silicone (PVS)
340
What are ISO standards?
International guidelines that assess the properties of materials to ensure they meet safety & effectiveness requirements.
341
What size grooves must impression materials replicate according to ISO?
50 microns or 20 microns depending on material type.
342
What is the best addition silicone brand on the market?
Virtual
343
What are the two main types of Glass Ionomer Cements (GIC)?
Conventional GIC and Resin Modified GIC (RMGIC)
344
What is the difference between anhydrous and original Conventional GIC?
Anhydrous GIC has freeze-dried acid in the powder and is mixed with distilled water, making it easier to handle and mix.
345
What are the two curing mechanisms for Resin Modified GIC?
Self-cure and Light-cure.
346
List four main uses of GIC.
Restorative (filling material), Core build-up, Lining (under fillings), Luting (cementing indirect restorations).
347
Why is GIC commonly used in deciduous teeth but less in permanent teeth?
GIC cannot withstand occlusal forces in adult teeth but is suitable for primary teeth.
348
What are the two key components of Conventional GIC?
Acid (liquid) and Base (glass powder).
349
Which acids are used in Conventional GIC?
Polyacrylic acid (Acrylic + Itaconic acid or Acrylic + Maleic acid) and Tartaric acid.
350
What is the role of Tartaric acid in GIC?
It controls the setting characteristics of the material.
351
What effect does increasing silica content have on GIC?
It increases translucency.
352
What element is added to GIC to improve radiopacity?
Strontium and lithium salts.
353
What are the three phases of the GIC setting reaction?
Dissolution, Gelation, and Hardening.
354
What happens during the dissolution phase?
Acid releases H+ ions, which attack the glass surface, releasing Ca, Al, Na, and F ions.
355
What crosslinks with polyacid molecules during the gelation phase?
Calcium ions (Ca²⁺) form calcium polyacrylate.
356
What ensures stronger crosslinking in the hardening phase?
Trivalent aluminium ions (Al³⁺), forming aluminium polyacrylate.
357
Why must GIC be protected from moisture and desiccation after placement?
Moisture can cause water absorption, and excessive drying leads to strength reduction.
358
How can GIC be protected after placement?
Using varnishes (Copal ether, Acetate), resins (dentine/enamel bonding agents), or gels (Vaseline).
359
How does GIC adhere to enamel and dentine?
Chelation between carboxyl groups and calcium ions on the tooth surface.
360
Why is GIC less aesthetic than composite?
It lacks translucency.
361
What is the primary advantage of GIC over composite in terms of bonding?
It bonds chemically to enamel and dentine without an intermediate material.
362
What mechanical properties of GIC make it inferior to composite?
Poor tensile strength, lower compressive strength, poor wear resistance, and higher solubility.
363
How does GIC compare to dentine in terms of thermal expansion?
It has similar thermal expansion to dentine.
364
What is the significance of fluoride release in GIC?
It helps prevent secondary caries.
365
How does GIC act as a fluoride reservoir?
It can take up fluoride from the environment and re-release it when surrounding fluoride concentration drops.
366
List three advantages of GIC.
Stable chemical bond to enamel/dentine, fluoride release, no contraction on setting.
367
List three disadvantages of GIC
Brittle, poor wear resistance, susceptible to moisture during setting.
368
What are cermets, and why are they not widely used?
Cermets are GICs with added silver to improve toughness, but they provided no clinical advantage.
369
What was the purpose of developing RMGIC?
To improve the physical properties and aesthetics of Conventional GIC.
370
What is the major difference in the composition of RMGIC compared to Conventional GIC?
RMGIC contains resin components such as HEMA and photo-initiators.
371
What are the two setting mechanisms of RMGIC?
Dual curing and Tri curing.
372
Why does RMGIC need to be placed in layers?
It is opaque, limiting light penetration, which affects curing depth.
373
What are two disadvantages of RMGIC?
Polymerization contraction and monomer leaching (HEMA is toxic to pulp if not fully polymerized).
