Dental Materials Sciences Flashcards
Mention restorative materials
- Metals/Alloys→ Amalgam, Cobalt Chromium, Titanium, Gold & Stainless Steel.
- Composites
- Glass Ionomer Cements
- Compomers
- Porcelain
Mention impression materials
Hydrocolloids (e.g., alginate)
Elastomers (e.g., polyethers, silicones)
Impression compound & paste
What is negative & positive replica?
Impression – negative replica
Dental stone (gypsum) \ study cast - positive replica
What is Mechanical properties?
A force that applied to a material may cause:
- Stretch/compress
- Deform (Change shape) - this may be temporary or permanent
- Fracture (failure)
What are the three primary types of force acting on dental materials?
Compressive (squeezing)
Tensile (stretching)
Shear (sliding)
What is stress in mechanical properties?
Stress is a physical quantity that describes forces present during deformation
Stress (Pa (Pascals)) = Force (F=mg)/ Unit Area
What is the unit of measurement for stress?
Pascals (Pa)
What is Strain?
Change in length / Original length
What is {Strain - Stress Curves} ?
Used to identify how a material performs under pressure
FS stands for?
Refers to the point at which the material fractures.
PL stands for?
Refers to the limit at which after stress is removed, the material can return to its original shape.
What kind of forces are the teeth typical exposed to?
- Compressive forces - via biting
- Abrasive & frictional forces - via Grinding & chewing
What is Elastic (Young’s) Modulus?
- 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
Fracture
Large force causing catastrophic destruction of material’s structure
Hardness
Ability of surface to resist indentation
Abrasion
Material surface removal due to grinding
Abrasion Resistance
Ability to withstand surface layers being removed
Fatigue
Repetitive ‘small’ stresses causing material fracture
Creep
Gradual dimensional change due to repetitive small forces
Deformation
Applied stress cause permanent change in materials dimension when greater than the elastic limit of material
De-bond
Applied forces sufficient to break material-tooth bond
Impact
Large, sudden force causing a fracture
Chemical properties
- Setting mechanism.
- Setting time.
- Corrosive Potential.
Physical properties
- Viscosity.
- Thermal Conductivity & Expansion.
- Density
- Radiodensity.
What is Amalgam?
An alloy of:
- Mercury (Liquid component)
- Silver, Tin, Copper & Other trace metals (Powder component)
What are the compositions of Amalgam?
- Powder (50% by weight): Silver/Tin- Copper- Zinc- mercury (Hg)
- Liquid (50% by weight): mercury (Hg)
What are the two main classifications of dental amalgam?
Composition-based (Traditional & Copper-Enriched) and Particle shape-based (Lathe-cut & Spherical).
What is the function of silver and tin in amalgam?
They form an intermetallic compound.
Why is copper added to dental amalgam?
It increases strength and hardness.
What is the role of zinc in amalgam?
Acts as a scavenger of oxygen, ensuring clean castings.
How does mercury contribute to the amalgam reaction?
It reacts with the powder metals to form the amalgam matrix or final set material.
What are the two main types of amalgam particle shapes?
Lathe-cut and Spherical.
Which amalgam particle type provides stronger proximal contacts?
Lathe-cut particles.
How are spherical amalgam particles formed?
By spraying molten metal into an inert atmosphere.
List two advantages of spherical particles in amalgam.
Higher early compressive strength and easier carving.
Amalgam setting reaction
Silver/Tin + Mercury → Silver/tin + Silver/mercury + Tin/Mercury
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)
What are the three gamma phases in amalgam?
Gamma (Ag-Sn), Gamma-1 (Ag-Hg), and Gamma-2 (Sn-Hg).
Which gamma phase has the best corrosion resistance?
Gamma-1.
Which gamma phase has the worst corrosion resistance?
Gamma-2
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.
How long does it take for amalgam to fully set?
Approximately 24 hours.
List three factors that decrease the strength of amalgam.
Undermixing, slow rate of packing, and corrosion.
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
Why is high-copper amalgam preferred over traditional amalgam?
It eliminates the Gamma-2 phase, reducing corrosion and cree
What are the two types of high-copper amalgam?
Dispersion-modified and single-composition.
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.
Pros of Copper Enriched
- Higher early strength.
- Less Creep.
- Higher resistance to corrosion.
- Increased Margin durability.
How can corrosion in amalgam be reduced?
Using copper-enriched amalgam and polishing the margins.
How does amalgam’s thermal expansion compare to tooth structure?
It expands 3 times more than tooth structure.
Why might a liner or varnish be needed under an amalgam restoration?
To protect the pulp from thermal conductivity.
What is the main bonding mechanism for amalgam in cavity preparation?
Mechanical retention
What is Permite, and why is it used?
A non-Gamma-2, spherical, and lathe-cut material with high compressive strength and low microleakage.
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
Why is bonding to enamel easier than dentine?
Due to structure of enamel:
Enamel is densely packed, highly mineralized (95%), and dry
What is the purpose of acid etching on enamel?
To roughen the enamel surface for micromechanical interlocking of resin filling materials.
Which acid is most commonly used for etching enamel?
Phosphoric acid.
What is the usual concentration of phosphoric acid used for etching enamel?
30-50% phosphoric acid
Why must enamel be dry after etching?
Moisture prevents the resin from flowing into the etched surface.
How does etching affect the surface energy of enamel?
- Increases surface energy
- Improves wettability, allowing bonding agents to adhere better
What type of bonding agent is commonly applied to etched enamel?
A low-viscosity Bis-GMA bonding agent.
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
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
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
What are the three mechanisms of adhesion to dentine?
Mechanical, Chemical, and Van der Waals forces.
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
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
What is the gold standard for dentine bonding?
Total Etch technique
What is the main etchant used in Total Etch?
35% phosphoric acid
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
What is the purpose of the dentine conditioner?
It removes the smear layer and opens up dentinal tubules.
What is the function of the primer in total etch bonding?
It contains hydrophilic and hydrophobic components that prepare the dentine for bonding.
What does the adhesive do in the total etch technique?
It penetrates the dentine, forms a micromechanical bond, and creates the hybrid layer.
What happens if dentine is over-etched?
Collagen fibers collapse, preventing resin penetration.
Why must dentine be neither too dry nor too wet for bonding?
- Too dry → Collagen collapses.
- Too wet → Primer is diluted, reducing bond strength.
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
Why is self-etch less technique-sensitive?
It eliminates the need to control moisture levels precisely.
What is the primary disadvantage of self-etch bonding?
The bond strength is weaker, especially to enamel.
Why is self-etch preferred for dentine over enamel?
It partially demineralizes dentine, preserving hydroxyapatite around collagen for stability.
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
What is a disadvantage of self-etch in dentine bonding?
Etching by-products are not washed away, weakening the bond.
Why might a mild self-etch system fail to bond properly?
It may not fully penetrate the smear layer.
What is Critical Surface Energy (CSE)?
The surface tension of a liquid that determines its ability to spread on a solid surface
How does CSE affect adhesion?
Liquid CSE < Surface CSE → Good adhesion
Liquid CSE > Surface CSE → Poor adhesion
What is molecular entanglement in dentine bonding?
When adhesive penetrates dentine and forms long-chain polymers that interlock with the collagen matrix
What is the fundamental mechanism of bonding to dentine?
Molecular Entanglement – resin replaces minerals in the dental tissue, forming an interlocking bond.
According to Peumans et al. (2010), how can self-etch bonding be improved?
By etching enamel with phosphoric acid first.
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.
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.
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.
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)
How does a liner protect the pulp from chemical irritation?
It prevents unreacted chemicals in the filling material from reaching the pulp.
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.
How does a liner help prevent microleakage?
It seals the space between the restoration and the cavity walls, preventing bacterial and endotoxin infiltration.
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
Setting Calcium Hydroxide (Liner)
What is the composition of setting calcium hydroxide?
A base and a catalyst that react via a chelation reaction.
How does calcium hydroxide kill bacteria?
It creates a high initial pH (~12), which is hostile to cariogenic bacteria.
How does calcium hydroxide promote dentine formation?
It irritates the odontoblast layer, triggering necrosis and the formation of tertiary dentine.
What are the disadvantages of calcium hydroxide liners?
Low compressive strength, unstable, highly soluble (dissolves if leakage occurs or in moist conditions).
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
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.
Why does zinc phosphate cement become stronger over time?
It absorbs water, making the matrix less porous and increasing strength.
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.
What makes zinc polycarboxylate different from zinc phosphate cement?
It replaces phosphoric acid with polyacrylic acid, allowing bonding to the tooth.
What are the advantages of zinc polycarboxylate cement over zinc phosphate?
Less exothermic, neutral pH quicker, bonds to tooth, does not penetrate dentine deeply.
What are the disadvantages of zinc polycarboxylate cement?
Difficult to mix, low compressive strength, soluble in the oral environment.
Why are zinc phosphate and zinc polycarboxylate cements rarely used as linings today?
Poor adhesion, irritation, and brittleness; mainly used for temporary crowns.
What is the setting reaction of Zinc Oxide Eugenol (ZOE)?
Acid-base reaction between zinc oxide and eugenol, forming a zinc eugenolate matrix.
What are the advantages of ZOE?
Quick setting, low thermal conductivity, radiopaque, pulpal soothing effect.
What are the disadvantages of ZOE?
Low strength (20MPa), high solubility (breaks down over time), eugenol inhibits resin-based materials.
Why should ZOE not be used under composite restorations?
Eugenol softens and discolors resin-based materials.
How does adding resin improve ZOE?
It increases compressive strength (40MPa) and decreases solubility.
How does Ethoxybenzoic Acid (EBA) improve ZOE?
Encourages crystalline structure, increasing strength (60MPa) and reducing solubility.
Why is glass ionomer the most widely used lining material?
Bonds to dentine and composite, releases fluoride, and seals the cavity.
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
What is a disadvantage of resin-modified glass ionomer (RMGIC)?
Unreacted HEMA may be cytotoxic to the pulp.
Why is RMGIC unique among linings?
It can bond to restorative materials and may even bond amalgam to the tooth.
What is the recommended liner for amalgam or large composite cavities?
RMGIC (e.g., Vitrebond).
When should calcium hydroxide be used?
For deep cavities near the pulp (direct/indirect pulp capping).
What must be done after applying calcium hydroxide before placing a final restoration?
Cover it with RMGIC.
What are composite resins used for?
Filling cavities, replacing abraded tissue, repairing/replacing failed restorations.
Which cavity classes are composite resins commonly used for?
Class III, IV, V, and limited occlusal wear in Class II.
When is composite resin preferred over other materials?
When aesthetics are important.
What are the three main components of composite resin?
Filler particles, resin, camphorquinone
What is the function of Camphorquinone in composite resins?
It acts as a photo-initiator for polymerization or light curing.
What color light activates camphorquinone?
Blue light
What role do Low Weight Dimethacrylates play in composite resins?
They control mechanical properties.
What does the Silane Coupling Agent do?
It ensures bonding between filler particles and resin.
What types of filler particles are used in composite resins?
Basic silica, quartz, or various silicates like lithium aluminum silicate.
How does filler particle type affect composite resin properties?
It influences hardness, rigidity, and abrasion resistanc
Why are hybrid composites mechanically superior?
They contain both large and small filler particles, providing strength and aesthetics.
What is the most common resin monomer used in composites?
BIS-GMA (Bisphenol A Glycidyl Methacrylate).
Why is BIS-GMA used in composite resin?
It contains C=C bonds, allowing for crosslinking and polymerization
What happens to the C=C bonds during polymerization?
They break, allowing monomer molecules to join and form a polymer.
What are the four classifications of composite resin based on filler type?
Conventional, Micro-filled, Submicron, Hybrid.
What is the difference between flowable and condensable composites?
Flowable has lower filler content (more shrinkage), while condensable has an ‘amalgam-like’ consistency
What are the two curing methods for composite resins?
Light-cured and self-cured.
List three potential problems with light curing.
Light-material mismatch, premature polymerization, and polymerization shrinkage
Which type of composite resin is used for anterior restorations?
Micro-filled or Hybrid.
Which handling characteristic of composite resins has the highest viscosity?
Flowable composite.
What initiates polymerization in light-cured composites?
Camphorquinone activated by blue light (430-490 nm).
What is the depth of cure for composite resin?
Typically 2mm.
Why is composite resin placed in increments?
To ensure complete curing and prevent shrinkage stress.
What type of photo-initiator is used in bulk-fill composites?
Lucirin, which is activated by UV light.
What can happen if the curing light spectrum does not match the composite material?
Inefficient curing, leading to under-polymerization.
How can polymerization shrinkage be minimized?
By using small increments and curing from different angles.
Why should the curing light not be directed at the eyes?
Blue LED light can cause ocular damage.
What is the fracture strength of composite resin?
350 MPa.
What is the Young’s modulus and Proportional Limit (PL) of composite resin?
- YM: 15 GPa (15,000 MPa)
- PL: 300 MPa
What happens when stress exceeds the Elastic Limit (EL) of composite?
The composite deforms and does not return to its original shape.
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.
Why is a good bond between composite and tooth tissue important?
It prevents microleakage and stress concentration, reducing the risk of fractures.
Why is low thermal conductivity important for composite resins?
To protect the pulp from heat damage.
What is the major drawback of composite resin’s thermal expansion coefficient?
It is higher than tooth tissue, increasing the risk of microleakage.
What are the advantages and disadvantages of conventional composite?
Strong but difficult to finish and prone to staining.
What are the advantages and disadvantages of microfine composite?
Smoother surfaces and better aesthetics but weaker mechanical properties.
What property of composite resins allows for easy shade matching?
Their wide range of shades and translucency.
Why is a hybrid composite a good balance between strength and aesthetics?
It contains both large and small filler particles.
Why are hybrid composites widely used today?
They offer a balance between strength and aesthetics with improved fillers and coupling agents.
How is hardness measured in composite resin?
Using an indentor to create a notch, then measuring the depth of indentation.
Name three clinical factors that affect composite wear.
Cavity design, occlusion, and curing efficiency.
What is the thermal diffusivity of hybrid composite compared to dentine?
Hybrid composite = 0.005 cm²/s, Dentine = 0.002 cm²/s.
What aesthetic properties make composite a good restorative material?
A range of shades, translucency options, stain resistance, and polishability.
Why must prosthetic materials have a high Young’s modulus?
To ensure rigidity and withstand biting forces.
Why is a high softening temperature important for prosthetic materials?
To prevent distortion from hot fluids and cleaning.
Why should prosthetic materials have a high proportional limit?
To prevent deformation under large biting forces.
Why do we want high thermal conductivity in acrylic resin materials?
To prevent scalding and allow the patient to feel heat.
Why is low density an important property for prosthetic materials?
To improve retention, especially for upper dentures.
What are the aesthetic requirements of prosthetic materials?
They should match the colour and translucency of natural tissues.
Why must prosthetic materials be non-toxic and non-irritant?
To ensure biocompatibility and avoid adverse reactions.
What is free radical addition polymerization?
A reaction where monomers join without eliminating smaller molecules.
What type of polymerization does PMMA undergo?
Free radical addition polymerization.
What is the initiator in PMMA polymerization?
Benzoyl peroxide
How many free radicals does each benzoyl peroxide molecule release?
Two free radicals
At what temperature does PMMA polymerization activate?
Above 72°C
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.
Why is a powder-liquid mix used in heat-cured acrylic?
To reduce heat of reaction, minimize shrinkage, and improve handling.
What is the powder-to-liquid ratio in heat-cured acrylic?
By volume: 3 - 3.5 to 1
By weight: 2.5 to 1
Heat-Cured Acrylic Composition
What does the powder component contain?
PMMA particles, benzoyl peroxide (initiator), plasticizers, and co-polymers.
Heat-Cured Acrylic Composition
What does the liquid component contain?
Methacrylate monomers, pigments, hydroquinone (inhibitor), and co-polymers.
What happens if acrylic resin is cured too quickly?
Gaseous porosity occurs due to excessive heat.
Why must acrylic be slowly cooled after curing?
To reduce internal stresses that weaken the material.
How does internal stress affect acrylic resin?
Reduces strength, increases fatigue failure, and causes warping.
What are the effects of under-curing?
Free monomers remain, leading to weaker mechanical properties.
What happens if the monomer ratio is incorrect?
Too much monomer → Contraction porosity.
Too little monomer → Granularity porosity.
How does porosity affect the final dental material?
Reduces strength, affects aesthetics, and absorbs saliva, causing hygiene issues.
When and where does gaseous porosity occur?
When the monomer boils above 100°C, often in bulkier denture areas.
What factors contribute to polymerization shrinkage?
Excess monomer, insufficient packing, and lack of pressure.
What are the advantages of PMMA as a dental material?
Non-toxic, stable, good aesthetics, and unaffected by oral fluids.
What are the disadvantages of PMMA?
High thermal expansion, low thermal conductivity, and poor mechanical properties.
Why does the low density of PMMA not always benefit denture applications?
Increased bulk is needed to compensate for weak mechanical properties.
What happens if an acrylic appliance is exposed to boiling water?
The material may warp and lose its shape.
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.
What is the aim of impression materials?
To produce an accurate negative reproduction of the surface and shape of oral soft and hard tissues.
Why is the accuracy of an impression important?
Treatment outcomes depend on the quality & accuracy of the initial impression.
What material is used to fill an impression to produce a cast?
Dental stone
What are the uses of a stone cast?
Evaluating ortho & occlusal relationships, production of RPD & other restorations.
What is a muco-static impression material?
A material that does not displace the mucosa and records its resting position.
Give examples of muco-static materials.
Elastomers: Polyvinyl Siloxane (PVS), Polyether.
What is a muco-compressive impression material?
A material that displaces the mucosa and records its shape under load.
Give examples of muco-compressive materials.
Silicone putty (heavy-bodied PVS), Hydrocolloids (Alginate).
What are elastic impression materials?
Materials that can stretch and deform without breaking and return to their original shape.
What are non-elastic impression materials?
Materials that cannot stretch and typically break/tear under stress.
Are muco-static materials elastic or non-elastic?
Elastic
Are muco-compressive materials elastic or non-elastic?
Non-elastic
Why are non-elastic impression materials not recommended for master impressions?
Due to limited accuracy and risk of breaking/tearing upon removal.
What is viscoelastic behaviour?
A material exhibiting both viscous (fluid-like) and elastic (solid-like) properties.
What happens when an impression material undergoes elastic recovery?
After removal, the material recovers, but some permanent strain remains.
How does removal speed affect permanent strain?
Faster removal → Less permanent strain
For Accuracy
What properties ensure an accurate impression?
Viscosity, setting mechanism, low thermal expansion, viscoelastic behavior, high tear strength.
For Patient Comfort
What properties ensure patient comfort?
Non-toxic, non-irritant, acceptable taste/smell, short setting time, easy removal.
For Operator Convenience
What properties ensure operator convenience?
Quick/simple technique, good working/setting times, ability to be decontaminated, cost-effective
What is a colloid?
A two-phase system of fine particles dispersed in another phase.
What is a hydrocolloid?
A colloid with water as the dispersing medium.
What is the most commonly used irreversible hydrocolloid?
Alginate
What are the main components of alginate?
Sodium Alginate
Calcium Sulphate
Trisodium Phosphate
Fillers
Modifiers
Describe the setting reaction of alginate.
Sodium Alginate reacts with Calcium Sulphate, forming Calcium Alginate, which cross-links to create a solid gel.
Why does alginate continue to develop elastic properties after setting?
Cross-linking continues after the apparent set, improving elasticity.
What temperature of water should be used when mixing alginate?
18-24°C
What type of tray should be used with alginate?
Perforated tray
How should an alginate impression be removed?
With a sharp pull to ensure elastic recovery and minimize permanent deformation.
What is the primary use of gypsum in dentistry?
To produce a positive replica of dentition from an impression.
Why is gypsum used in dental prosthesis manufacturing?
It enables assessment of the dentition and the fabrication of prostheses.
What does gypsum record?
Position, shape, and dimensions of teeth and surrounding soft tissue.
What is the chemical composition of gypsum used in dentistry?
Calcium sulfate dihydrate (CaSO₄ · 2H₂O).
What happens to gypsum when heated?
It converts to calcium sulfate hemihydrate (CaSO₄ · ½H₂O), creating a powder.
What happens during the setting reaction of gypsum?
The reaction is reversed, and calcium sulfate dihydrate reforms.
What are the three types of gypsum?
- Plaster (β-hemihydrate)
- Dental stone (α-hemihydrate)
- Densite (improved stone).
How is plaster produced?
By heating gypsum in an open vessel.
What is the structure of plaster crystals?
Irregular and porous.
How is dental stone produced?
By heating gypsum in an autoclave.
What is the structure of dental stone crystals?
Regular and non-porous.
How is densite (improved stone) produced?
By heating gypsum in the presence of calcium chloride.
What is the structure of densite crystals?
Compact and smooth.
What is the chemical reaction for the setting of gypsum?
Calcium sulfate hemihydrate + Water → Calcium sulfate dihydrate + Water
What happens when water is added to gypsum powder?
Hemihydrate dissolves, forming dihydrate crystals.
What do dihydrate crystals do during the setting process?
They precipitate and grow around impurities.
What do dihydrate crystals do during the setting proces
They precipitate and grow around impurities.
What marks the initial set of gypsum?
Dihydrate crystals grow and come into contact, starting the expansion process.
At what stage can gypsum be carved?
During the initial set.
What marks the final set of gypsum?
The material hardens, and water evaporates, making it porous.
What is the water-to-powder ratio for plaster?
60ml water to 100g powder.
What is the water-to-powder ratio for dental stone?
35ml water to 100g powder.
How does increasing the powder-to-water ratio affect gypsum setting time?
It decreases setting time and increases expansion.
What is the compressive strength of gypsum?
Ranges from 28-38 MPa, developing to 75 MPa over 24 hours.
Which type of gypsum has the greatest strength?
Densite (Improved Stone).
Why does gypsum have a rough surface?
Because it is porous, with surface roughness of 28-40 µm.
What is the main disadvantage of gypsum’s surface hardness?
It has low resistance to abrasive forces.
How does spatulation affect the setting time of gypsum?
Increased spatulation breaks down growing crystals, reducing setting time.
How does gypsum expand during setting?
Expansion is low, minimizing dimensional inaccuracies.
What is the flexural strength of gypsum?
15-20 MPa.
How does potassium sulfate affect gypsum setting time? Why?
It decreases setting time by producing syngenite, which rapidly crystallizes and promotes further crystal growth.
How does borax affect gypsum setting time? Why?
It increases setting time by forming calcium borate, which deposits on dihydrate crystals and delays setting.
What are the advantages of gypsum in dentistry?
Dimensionally accurate and stable.
Low expansion rate for stone and densite.
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.
What does the gradient of the stress-strain curve represent?
The Rigidity of the material.
What does a higher rigidity indicate?
A higher rigidity means that more stress produces less strain on the material.
What is the Elastic Limit (EL)?
The maximum stress a material can withstand without plastic deformation.
What is the Ultimate Tensile Strength (UTS)?
The highest stress a material can withstand before breaking.
What is the Fracture Strength (FS)?
The point at which the material fractures.
What does Ductility represent?
How much plastic deformation a material can undergo under tensile stress before breaking.
What does Malleability represent?
How much plastic deformation a material can undergo under compressive stress before breaking.
What is an Alloy?
combination of metal atoms in a crystalline structure.
What are the 3 types of Crystal Structures?
- Cubic
- Face-Centred Cubic (FCC)
- Body-Centred Cubic (BCC)
What are the 3 main factors affecting mechanical properties of a metal/alloy?
- Crystalline Structure
- Grain Size
- Grain Imperfections
What happens when a molten metal cools?
Crystals form from Nuclei of Crystallisation.
What do crystals grow into?
Dendrites → Grains
What are Grain Boundaries?
Regions where grains make contact.
How does Slow Cooling affect grain size?
Produces fewer nuclei → Large coarse grains.
What are Nucleating Agents?
Impurities or additives that act as extra nuclei → Producing smaller grains.
How do Small Grains affect properties?
- Increase: Elastic Limit, UTS, Hardness
- Decrease: Ductility
What is a Dislocation?
A defect in the crystal lattice that weakens the structure.
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.
How do Small Grains impede dislocation movement?
Small grains = More Grain Boundaries → Dislocations are blocked.
What is Cold Working?
Shaping metal by applying force at low temperatures.
How does Cold Working affect metal properties?
- Increases: Elastic Limit, UTS, Hardness
- Decreases: Ductility, Corrosion Resistance
What is Residual Stress?
Internal stresses in the metal caused by dislocation buildup during Cold Working.
What is Annealing?
A heating process that allows atoms to rearrange and eliminate residual stress.
What is Recrystallisation?
A heating process that resets grain structure and reverses the effects of Cold Working.
What type of grains does Quenching produce?
Small fine grains
What is the key influence on grain size?
Rate of Cooling
What happens to Ductility if grain size decreases?
It decreases
What process pushes dislocations to grain boundaries?
Cold Working
What is the main disadvantage of Cold Working?
Residual Stress
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).
What are the advantages of alloys?
Superior mechanical properties such as high Elastic Limit, Fracture Strength, Strength & Rigidity, Corrosion Resistance, and lower melting points.
Name some examples of alloys used in dentistry.
Gold alloys, Amalgam, Stainless Steel, Cobalt-Chromium.
What is a phase in alloys?
A physically distinct homogenous structure.
What is a solid solution?
A homogenous mixture of two metals at an atomic scale forming one phase.
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)
What are the two types of solid solutions?
Substitutional Solid Solution and Interstitial Solid Solution.
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.
What is an interstitial solid solution?
Smaller atoms fit into the spaces within the lattice of larger atoms.
How does the crystallisation process differ between pure metals and alloys?
Pure metals crystallise at one temperature, while alloys crystallise over a temperature range.
What are the effects of slow cooling on alloys?
Produces large grains and homogenous composition.
What are the effects of fast cooling on alloys?
Produces small grains, heterogenous composition (coring), and improves mechanical properties but reduces corrosion resistance.
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.
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.
What is a eutectic alloy?
An alloy where metals are soluble in liquid state but insoluble in solid state, forming two physically distinct grains.
What is a partially soluble alloy?
An alloy producing two distinct grains, each containing both metals but in different concentrations.
What is precipitation hardening?
The process of annealing a partially soluble alloy to increase strength and surface hardness
What are three examples of partial denture alloys?
- Type IV Gold
- Co-Cr
- Titanium
What are the desired properties of a denture base?
- High YM (To maintain shape)
- High EL (To avoid deformation)
What are the desired properties of a denture clasp?
- Lower YM (To allow flexure over tooth)
- High EL (Maintain elasticity)
What type of gold alloy is used for partial dentures?
Type IV
What is the gold percentage range in Type IV gold alloy?
60-70%
What metals make up Type IV gold alloy?
Gold, Silver, Copper, Zinc, Palladium, Platinum
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
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
Why does Au-Ag show little/no coring?
Solidus & liquidus lines are close together
What are the effects of adding Platinum to gold alloy?
- Formation of solid solution
- Solution hardening
- Produces fine grain structure
- Coring can occur
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What are the effects of adding Palladium to gold alloy?
- Similar to platinum
- Less coring
- Produces more coarse grains
What is the function of Zinc in gold alloys?
Scavenger
What is the function of Nickel in gold alloys?
Increase hardness & strength
What is the function of Indium in gold alloys?
Produce fine grain structure
What are gold alloys generally suited for in a denture: base or clasp?
Clasp
Why are gold alloys not typically used for denture bases?
Thickness required → Expensive
What is Co-Cr frequently used for in RPD?
Connectors
What is the composition of Co-Cr alloy?
Cobalt, Chromium, Nickel
What is the effect of Chromium in Co-Cr alloy?
Corrosion resistance (Passive layer)
What is the effect of Nickel in Co-Cr alloy?
- Improves ductility
- Slightly reduces strength
What are the general properties of Co-Cr alloy?
Harder than gold
Low ductility
Difficult to adjust
What are the uses of Titanium in dentistry?
- Implants
- Crown & Bridge
- Partial Denture
What are the benefits of Titanium?
- Good biocompatibility
- Good corrosion resistance
- Can be joined by laser welding
What does viscosity measure in impression materials?
The material’s ability to flow and make close contact with tissues.
What does viscosity measure in impression materials?
The material’s ability to flow and make close contact with tissues.
Why is viscosity important for impression materials?
It determines how well the material can record surface detail.
What is wettability in impression materials?
The ability of a material to make intimate contact with hard and soft tissues.
How is wettability measured?
By the contact angle (Lower angle = better wetting).
What is the ISO standard for surface reproduction?
Grooves of 50 microns must be replicated.
What is elastic recovery in impression materials?
The ability of the material to return to its original shape after removal.
How does load time affect permanent deformation?
Reduced load time → Less strain → Less deformation
Do we want high or low viscoelasticity?
Low viscoelasticity (Small permanent deformation).
What is tear strength?
The amount of stress a material can withstand before fracturing.
Why is tear strength important in impression materials?
It helps prevent the impression tearing when removed from undercuts.
What kind of rigidity is ideal for impression materials?
Low rigidity → More flexible for easier removal.
Which material type has longer working time?
Polyether
Which material has better elastic recovery?
Addition Silicone (PVS)
Which material has higher tear strength?
Addition Silicone (PVS)
What are ISO standards?
International guidelines that assess the properties of materials to ensure they meet safety & effectiveness requirements.
What size grooves must impression materials replicate according to ISO?
50 microns or 20 microns depending on material type.
What is the best addition silicone brand on the market?
Virtual