General

Question 1

3 types of forces applied to materials

Answer 1

• compressive
• tensile
• shear

Question 2

Stages of material selection effected by material characteristics

Answer 2

1. selection (adequate)
2. preparation required (surface on/cavity design)
3. Placement (curing/increments)
4. Performance (fit/longevity)
5. Patient expectations (aesthetics/durability)

Question 3

5 restorative materials

Answer 3

1. Amalgam
2. Composite
3. Glass ionomer cements
4. Compomers
5. Porcelain

Question 4

4 impression materials

Answer 4

1. Impression compound (non-elastic)
2. Impression paste (non elastic)
3. Hydrocolloids (alginate)
4. Elastomers (polysulphides, polyethers, silicones)

Question 5

3 properties of any material

Answer 5

• chemical
• physical
• mechanical

Question 6

3 outcomes of forces applied to materials

Answer 6

• stretch/compression
• deform (temp/permanent)
• fracture (failure)

Question 7

What is stress to a material

Answer 7

Force/area where force applied = pascal (Pa) - changed to giga pascals (GPa)

Question 8

What is strain to a material

Answer 8

Change in length / original length = strain (ratio of change in length)

Question 9

Explain the proportionate limit (stress strain graph)

Answer 9

Where stress and strain changes become non linear (leads to fracture - fracture stress)

Question 10

Opposites of each mechanical property of materials (I.e hard and soft)

Answer 10

• Strong and weak
• rigid and flexible
• ductile and brittle

Question 11

Explain the elastic modulus of a material

Answer 11

Lower gradient of stress strain graph means a lower elastic modulus

Question 12

causes of failure to a material (7)

Answer 12

1. fracturing (large force causing catastrophic destruction to structure)
2. impact (large force causing fracturing)
3. fatigue (repetitive small stresses causing fracture)
4. permanent deformation (no fracturing, when releasing stress > elastic limit)
5. abrasion (surface removed by grinding)
6. lack of hardness (allows to be indented)
7. de-bond (forces enough to break bond)

Question 13

physical properties of materials

Answer 13

• viscosity
• thermal conductivity
• thermal expansion
• density
• radiodensity

Question 14

chemical properties of materials

Answer 14

• setting mechanism
• setting time
• corrosive potential

Question 15

explain the acid etch technique

Answer 15

• 35% phosphoric acid acts on Ca2+ of hydroxyapatite
• roughens surface by decalcifying causing exposure of collagen fibres for more molecular entanglement and removes smear layer increasing surface energy

Question 16

why is it hard to bond to dentine

Answer 16

• more organic (water puddles from tubules but material hydrophobic)
• low surface energy
• inconsistent structure (soft then hard)
• smear layer

Question 17

describe technique to bond to dentine

Answer 17

1. etching = removes smear layer exposing tubules and roughens surface
2. wetting agent used to increase surface energy (bonding agent covers tubules)

Question 18

describe the 3 types of bonding (normally all used together)

Answer 18

1. mechanical (physical interlocking of structures i.e. enamel and some dentine with composite)
2. chemical adhesion (ionic bonds with mineralised part but mostly covalent bonds with organic regions
3. van der walls adhesion (dipole interaction between bonding agent and substrate)

Question 19

describe critical surface energy (why bonding agent used on dentine?)

Answer 19

liquid must have lower surface energy than surface placed on to flow and stick
- as dentine wet has lower surface energy than composite so bonding agent fixes this

Question 20

what is a surface wetting agent

Answer 20

bonding agent (as increases surface energy allowing materials to stick)

Question 21

describe the action of a wetting agent and what its 4 property requirements

Answer 21

meshes with acid cured surface via molecular entanglement
1&2. ability to flow and low viscosity
3. ability to make good contact with dentine
4. adhesion to substrate via van der walls, chemical, mechanical or all of the above

Question 22

describe what the smear layer is and how we deal with it

Answer 22

• layer of debris on dentine contaminated with bacteria
• either remove by etch [total etch] or incorporate [self etch] via bonding agent penetrating it (can allow leaking thus hydrolysis)

Question 23

two types of etching

Answer 23

1. total etch

2. self etch

Question 24

explain what total etch is

Answer 24

• 3 components
  1. dentine conditioner
  - removes smear layer (decalcifying) while hardly demineralising HA
  2. primer (bonding agent) = double headed molecule, one phobic one phylic
  3. adhesive = resin that penetrates now hydrophobic tubules and has molecular entanglement with tubules and collagen fibres = forms hybrid layer (hydrophobic with camphorquinone)

Question 25

give an example of a primer

Answer 25

HEMA (hydroxy ethyl methacrylate) - Methacrylate head = hydrophobic - hydroxyl head polar bonds to similar regions of HA

Question 26

Photoinitiator put in materials for curing

Answer 26

camphorquinone

Question 27

issues with total etch

Answer 27

- can over etch meaning collagen fibres collapse or too deep and etch as so primer cant reach mineralised dentine - too wet means is diluted and too dry means surface too smooth and only tubules as means collagen fibres collapse

Question 28

describe what self etch is

Answer 28

- solution contains etch, primer and adhesive in one (or etch separate) - incorporates smear layer into hybrid later via dissolving it - good as less technique sensitive and less chance of post op sensitivity due to no drying stage = no breakdown of dentine architecture - bad as not as strong bonding and lower pH may not etch as well - uses bifunctional molecule (acidic methacrylate monomers (etch and infiltrate dentine with hydrophilic end and while usual polymerising at hydrophobic end) -

Question 29

describe what self etch is

Answer 29

- solution contains etch, primer and adhesive in one (or etch separate) - incorporates smear layer into bonding - good as less technique sensitive and less chance of post op sensitivity due to no drying stage = no breakdown of dentine architecture - bad as not as strong bonding - uses bifunctional molecule (acidic methacrylate monomers

Question 30

what primer and solvent are used in a primer and adhesive mix for self etch

Answer 30

Bis-GMA and alcohol or acetone

Question 31

draw backs to strong self etch compared to mild

Answer 31

breaks down too much HA so not as good ionic bonding and liable to allow breakdown of collagen fibres

Question 32

blacks cavity classification

Answer 32

``` class i = pit and fissure caries ii = posterior approximal caries iii = anterior approximal caries iv = anterior approximal carries effecting incisal angle v = caries effecting cervical surfaces vi = carries effecting cusp tips ```