DMS

Question 1

MCC

1. 3 porcelain features
2. 3 types of bond
3. Most common metal alloy used
4. Purpose of metal oxide bond

Answer 1

1. Best aesthetics, hard, rigid, forms microcracks at fitting surface
2. Chemical, mechanical, stressed skin
3. CoCr
4. Helps eliminate cracks on porcelain surface

Question 2

Steel

1. Maximum carbon %
2. What is iron and describe phase changes
3. 4 types on FeC phase diagram
4. How is pearlite formed
5. When is martensite formed

Answer 2

1. <2%
2. Allotropic - undergoes two solid state phase changes with temperature
   <900C - BCC lattice structure, low carbon solubility
   900-1400C - FCC lattice structure, high carbon solubility
   >1400C - BCC lattice structure, low carbon solubility
3. Austenite (above 720C), cementite (Fe3C), ferrite (low temperature), pearlite (eutectoid mixture of cementite and ferrite)
4. Slow cool austenite/temper martensite
5. Quench austenite/when no time for carbon to diffuse

Question 3

Stainless steel

1. Why stainless
2. Composition and function of components of 18/8
3. Describe weld decay and how it is stabilised
4. Define cold work

Answer 3

1. Chromium >13%
2. 72% iron - forms steel with carbon
   18% chromium - increases corrosion resistance
   8% nickel - increases UTS and corrosion resistance
   1.7% titanium - stabilises weld decay
   0.3% carbon - forms steel with iron
3. Where chromium carbides precipitate at grain boundaries, making the material more brittle and susceptible to corrosion. Occurs around 500-900C
   Stabilised/prevented by incorporation of titanium and low carbon content
4. Strengthening of the metal by plastic deformation. Work being done on the metal at low temperatures (bending, swaging), that causes dislocations to correct at grain boundaries (slip). The metal is strengthened and develops resistance to dislocation formation. Used for shaping partial denture clasps and ortho wires

Question 4

PMMA

1. 3 good features
2. 2 bad
3. Reaction name and stages
4. Heat-cured vs self-cured – 2 benefits/bad bits about each

Answer 4

1. Non-toxic, non-irritant, high softening temperature, good aesthetics, high abrasion resistance
2. Poor mechanical propertjes, low thermal conductivity
3. Free radical addition polymerisation
   Activation, initiation, propagation, termination
4. Heat-cured - better mechanical properties (stronger), less unreacted monomer; curing can cause porosity, longer time
   Self-cured - quicker, cheaper; poorer mechanical properties, more unreacted monomer

Question 5

Investment materials

1. 3 ideal features
2. 2 components and functions
3. Define hygroscopic expansion and 3 ways it is increased
4. 4 types and 3 features of 3

Answer 5

1. Porous, expands, easily removed from cast, smooth surface, strong
2. Binder - forms coherent mass
   Refractory - expansion, withstands high temperatures
3. Water molecules attracted between crystals by capillary forces, forcing crystals apart
   Increased by lower powder/water ratio (more water to powder), higher water temperature, longer immersion time, higher silica content
4. Dental stone/plaster
   Gypsum-bonded - smooth surface, porous, adequate strength. Calcium sulphate hemihydrate combines with water to form calcium sulphate dihydrate
   Phosphate-bonded - porous, easy to use, stronger
   Silica-bonded - sufficient strength, not porous, complicated manipulation

Question 6

Elastomers

1. 2 types
2. What makes hydrophilic better
3. Ideal elastic behaviour
4. Actual elastic behaviour
5. 4 key impression material features

Answer 6

1. Addition silicones, polyethers
2. Incorporation of non-ionic surfactant (wets tooth surface)
3. Material applied and set. Upon removal, material reaches max. strain almost instantly, Strain held during removal When fully removed, material instantly returns to original strain and pre-removal shape. No permanent deformation
4. Material applied and set. Upon removal, material gradually increases to just below max. strain. When fully removed, material quickly (instantly) returns to almost original strain. Permanent deformation/strain and permanent change in dimension
5. Low viscosity, low viscoelasticity, high tear resistance, high elastic recovery, good wettability, good surface detail, able to flow under pressure

Question 7

Luting agents

1. 3 key features
2. 3 types

Answer 7

1. Strong, good aesthetics, biocompatible, good marginal seal, low viscosity, low thermal conductivity, easy to use
2. Conventional dental cements, GIC, composite resins, self-adhesive composites, surface-modifying chemicals

Question 8

Cements

1. 2 types
2. 2 good features
3. 2 bad features
4. Setting description of one material

Answer 8

1. Zinc phosphate, zinc polycarboxylate
2. Cheap, easy to use
3. Low initial pH, exothermic setting reaction, brittle, opaque, don’t bond to tooth/not adhesive
4. Acid base reaction - AlO prevents recrystallisation leaving an amorphous matrix of acid salt surrounding unreacted ZnO powder. Matrix almost insoluble, but porous and contains free water (increasing strength) from setting reaction

Question 9

GIC

1. 2 components
2. Setting reaction and stages (describe)
3. How bonds to tooth
4. 3 good features
5. RMGIC – 2 components, 2 pros, 2 cons
6. 2 cure types
7. When is GIC used

Answer 9

1. Polyacrylic acid, tartaric acid, glass powder, silica
2. MO.SiO2 + H2A –> MA + SiO2 + H2O
   Dissolution - acid added to solution. H ions interact and attack glass surface. Glass ions are released and leach out, leaving a layer of silica gel around unreacted glass
   Gelation - bivalent Ca ions crosslink with polyacid by chelation with carboxyl groups
   Maturation/hardening - trivalent Al ions ensure good cross linking, increasing strength
3. Ion exhange with calcium in enamel and dentine and hydrogen bonding with collagen in dentine. Strong, durable bond
4. No/limited setting contraction, F release, strong bond to tooth, easy to use, durable
5. Fluoro-alumino-silicate glass, barium glass, HEMA, polyacrylic acid, tartaric acid
   Stronger, good bond to tooth, longer working time, set on demand
   Setting contraction, moisture-sensitive, unreacted cytotoxic HEMA
6. Dual-cure (acid-base reaction, light cure - camphorquinine)
   Tri-cure (acid base reaction, light cure, redox reaction)
7. MCC, metal post, zirconia crown, gold restoration

Question 10

Composite luting agent

1. What used to bond (describe this chemically)
2. 2 benefits
3. How bonds to porcelain
4. How bonds to metal
5. How bonds to non-precious metal
6. How bonds to precious metal
7. Where/why use light cure
8. Why/where dual cure
9. What is useful about self-adhesive composite

Answer 10

1. Surface wetting agent/silane coupling agent - hydrophilic end forms bond between oxide groups in silane and porcelain surface. Hydrophobic end reacts with composite resin, forming a bond. Similar to DBA
2. Better physical properties and aesthetics, less soluble
3. Etch with HF and use silane coupling agent
   Tooth-DBA-composite luting resin-silane coupling agent-porcelain
4. Sandblast/etch to roughen surface to form micro mechanical bond
5. Use metal bonding agent (MDP/4-META)
   Tooth-DBA-composite luting agent-metal bonding agent-non-precious metal
6. Allow oxide formation (complicated, technique sensitive)
7. If restoration is thin (veneer)
8. Thick restoration, metal restorations, most crowns, bridges and posts. fibre post, composite inlay, porcelain inlay
9. Incorporates metal coupling agent

Question 11

Ceramics

1. Define
2. 2 components of feldspathic (reason for one of them)
3. What happens when 1150-1500C
4. Crown fabrication process
5. Describe sintering
6. 3 good features
7. 2 types of materials for cores
8. Describe static fatigue

Answer 11

1. Solid material comprising of an inorganic compound of metal, non-metal or metalloid atoms held in ionic and covalent bonds
2. Feldspar - fluxing agent (lowers fusion and softening temperature of glass) and silica replace kaolin
3. Forms leucite, a powder of known chemical and physical properties
4. Powder fritted (rapidly cooled) and milled into fine powder. Binder and distilled water added and mixed together. Applied to die, built-up into restoration/crown, fusion in furnace (sintering), staining/glazing, finishing
5. When ceramic particles begin to fuse into a solid mass. Occurs above glass transition temperature. Glass phase softens and fuses (controlled diffusion), forming a solid mass. 20% material contraction
6. Best aesthetics, less staining, biocompatible, similar thermal properties to tooth, low thermal diffusivity, hard, high compressive strength
7. Alumina, zirconia
8. Time dependent reduction in strength, even in absence of applied load. Likely due to hydrolysis of Si-O groups within material, over time in aqueous environment

Question 12

Alumina

1. 2 good features
2. Luting (silica-containing ceramics)
3. 2 types

Answer 12

1. High flexural strength, good aesthetics, cheap, alumina particles act as crack stoppers
2. HF and silane coupling agent
3. IN-CERAM, PROCERA

Question 13

Zirconia

1. 2 good features
2. Luting
3. What type used (describe this)

Answer 13

1. Hard, strong, excellent fit
2. Inert fitting surface and strong enough to be self-supporting, so conventional dental cement
3. Yttria-stabilised zirconia
   Normal zirconia is monoclinic crystal at room temperature. When a crack forms (and stress at crack tip reaches critical level), crystal structure transforms into a monoclinic structure, causing the material to expand slightly and close up the crack tip

Question 14

Cast and pressed ceramics

1. Describe process
2. Describe 2 stages of ceraming
3. What ceramics are used

Answer 14

1. Wax-up, investment, cast from heated ingot of ceramic, no sintering occurs, ceraming, staining
2. Crystal formation - maximum number of crystal nuclei formed
   Crystal growth - to maximise physical properties
3. Glass ceramics