Materials Flashcards
1
Q
What do dentists want in restorative materials
A
. Safe . Cost . Command set . Easy to use/simple procedure . Good marginal seal . Can detect recurrent caries . Can view on an x-ray
2
Q
What do patients want in their restorative materials
A
. Safe
. Cheap
. Last forever
. Aesthetics
3
Q
Steps to ensuring new dental materials are safe for use
A
. Test in vitro to check for cytotoxicity, genotoxicity and estrogenicity.
. Test in vivo for systemic toxicity, irritation and repeated exposure effects.
. Continued post-market monitoring.
. Has a CE mark after all this.
4
Q
Common adverse reactions from dental materials
A
. Contact dermatitis (allergic and irritant) . Oral lichenoid . Anaphylaxis reaction. . Intolerance reactions. Toxic reactions e.g. cytotoxicity.
5
Q
What are the functions of restorations
A
. To restore function . Stop pain . Stop further decay . Protect the rest of the tooth. . Aesthetics
6
Q
Methods of detected caries
A
. Clinically - need good lighting, dry clean teeth and magnification.
. X-rays
. Transmitted light for inter-proximal
. Separating teeth and taking an impression.
. Laser fluorescence for occlusal caries.
7
Q
Types of instruments used to remove caries
A
Rotatory instruments Hand instruments Sonic/ultrasonic tips Pulsed Laser Air abrasion Chemo-mechanical
8
Q
Air abrasion
A
Particles sprayed at high speed and used to cut cavities and remove occlusal caries.
Can’t be used for people with airway problems e.g. asthma.
No tactile feedback so can over-cut.
9
Q
Pulsed laser
A
For cutting cavities.
No tactile feedback (dentist relies on sound)
Minimally invasive
Slow removal of tooth structure
10
Q
Chemo-mechanical caries removal
A
Disrupts the collagen fibers and make sit easier to remove caries.
Selective caries removal.
Minimally invasive.
Needs an open cavity.
11
Q
Different groups of materials
A
Metals Polymers Composites Glasses Ceramics
12
Q
Tm
A
Transition temperature. When the atoms in the material reorganize.
13
Q
Metals properties
A
Conductors, malleable (can be shaped cold), can be cast, strong, solid at room temp normally bc of crystal lattice.
14
Q
Transitions in metals
A
solid -> solid (can change structure and size but still a solid)
solid + liquid -> solid
Solid -> liquid -> solid
15
Q
Silica tetrahedron
A
Building block for silica, and glasses and ceramics.
Si bonded to 3 oxygen’s.
All the bonds have identical lengths and angles.
16
Q
Ceramics
A
Homogeneous powder heated, dried and squashed (and shrunk). Can be squashed more by dry pressing, injection moulding.
17
Q
Transition in ceramics
A
Solid -> solid (e.g. after hot pressing)
Powder -> solid
solid + liquid -> solid
18
Q
Glasses
A
The random atomic arrangement makes it transparent.
Supercooled liquid - cooled in a special way that avoids nucleation/crystallization and no organisation of atoms (amorphous)
Tg is when it becomes a solid instead of Tm (Tg is cooler).
19
Q
Glass-ceramics properties
A
Made by controlled crystallization of glass.
Not much decrease in volume during ceramming like with normal ceramics.
Less brittle bc cracks hit the crystals and lose energy.
More translucent.
Can be cast.
20
Q
Glass-ceramics process of making
A
Heated v slowly until nucleation and then heated more to allow crystals to grow. Temperature stays below melting point.
Changing the time and temperature of the steps changes the size of the crystals.
21
Q
Composite transitions
A
Depends on what they are made of so can’t make generalisations.
22
Q
How is adhesion useful in dentistry
A
. Reinforces tooth structure . Conserves tooth structure . New treatments e.g. veneers and crowns . Reduced post operate sensitivity . Reduces marginal leakage/infiltration.
23
Q
Adhesion vs cohesion
A
Force that binds 2 different materials at a molecular level (<0.7nm) vs the same material.
24
Q
solid-solid vs solid-liquid adhesion
A
solid-solid = both have rough surfaces so not complete intimate contact at a molecular level just certain areas of contact and lots of force on these areas. Liquid can flow over the rough areas (if the surface is clean) and means there is complete intimate contact across the whole surface. Low force but large contact areas and secondary bonds.
25
Criteria for adhesion
- Complete intimate contact at a molecular level.
- Complete wettability.
- Forces of attraction.
26
What is surface tension
on the surface of a liquid there's an imbalance of forces bc no forces outwards so overall force is into the molecule.
27
How is complete wettability achieved
When critical surface tension of liquid < critical surface energy of a solid, solid pulls the molecule towards it and reduces the contact angle so it is more flat onto the solid's surface.
28
Different mechanisms of adhesion
- Molecular entanglement
- Micro-mechanical adhesion.
- Physical adhesion
- Chemical adhesion
29
Molecular entanglement
A porous solid surface is created by removing a component of that material e.g. removing the HA from dentine. The liquid (+complete wettability) flows into these holes and then is solidified e.g. polymerises, creating a hybrid layer.
30
Micro-mechanical adhesion
Rough solid surface with undercuts which a liquid with complete wettability on that surface can flow into and then harden. Need a clean surface.
31
Physical adhesion
Secondary bonds e.g. VdW and H-bonds and polar/permanent and non-polar/temporary dipole interactions. Reversible but be precursor to chemical adhesion by bringing the 2 materials together.
32
Chemical adhesion
Strong permanent bonds - covalent, ionic and metallic bonds.
33
What are physical properties of materials
Stress, Strain, Hardness, Fatigue, Abrasion resistance/wear
34
Young modulus
Young modulus = gradient of line and stiffness of materials (high = stiffer).
35
Elastic limit and resilience
Elastic limit = when material won't go back to original shape after stress removed. Resilience = area under the straight line and is the amount of energy material can absorb before deforming.
36
Fracture strength
Maximum strain before the material breaks.
37
Ultimate tensile strength
max stress applied e.g. highest point on the graph
38
Fatigue
Due to the accumulation of stress e.g. cycles of stress. Given as fatigue strength (cycle stress for failure for a set number of cycles) or fatigue life (no. of cycles to failure)
39
Hardness/wear
Withstanding surface compression F, proportional to the size of indent and given as a number (larger for soft material). Can be measured by scratch test for abrasion resistance.
40
Chemical and physical properties
elasticity, viscosity, elastoviscosity, durability/degradation, thermal properties.
41
Elasticity
Ability to return back to original shape instantly when stress removed (Newtonian liquid)
42
Viscosity
Resists flow. Doesn't return back to original shape instantly when stress removed.
43
Viscoelasticity
Time-dependent
44
Durability/degradation and dental examples
The ability of a material to withstand its environment, e.g. acid attack/erosion, dissolving, corrosion (electrochemical)
45
Thermal properties
Thermal expansion, exothermic reactions, thermal conductivity and thermal diffusivity
46
Thermal expansion in dentistry
Materials need to have similar thermal expansion as tooth material so that they don't cause cracks when they expand
47
Thermal conductivity
conducting heat via a temp gradient
48
Thermal diffusivity
material's ability to conduct heat compared to its ability to store thermal energy e.g. when heat applied to a material, some heats up material and some conducted. In dentistry, want less to be conducted e.g. to the pulp.
49
Types of metal crystalline structures
Face centred cubic and body centred cubic - FCC is denser.
50
Properties of metals
Conduct heat and electricity bc of free electrons. Strong and normally solid bc of metallic bonds and malleable bc electrons form a shield around the anodes and stop them repelling and shattering.
51
Types of metals/alloys in dentistry
Titanium, amalgam, cobalt chronium, stainless steel.
52
How can the grains in metals' microstructure be viewed
Acid etch to remove the grain boundaries and then can view the grains which appear as different colours bc of their diff orientations.
53
Types of alloy structures
Interstitial solid solution
Intermetallic solid solution
Substitutional solid solution
54
Interstitial solid solution
When solute atom < solvent atom, so fill in the spaces between solvent atoms
55
Substitutional solid solution
2+ atoms with same valencies, crystalline structure and similar sizes.
56
Different phases in amalgam
Gamma Phase = Ag3Sn
phase 1 = Ag-Hg
Phase 2 = Sn-Hg
57
Conventional amalgam setting reaction
Ag3Sn (powder) + Hg (liquid) --> Ag3Sn + Ag-Hg + Sn-Hg
58
Why do amalgam restorations fail
Marginal breakdown
Fracture of amalgam/tooth
Recurrent caries
59
Reasons for tooth fracture after an amalgam restoration
Remaining tooth material is weakened (amalgam has no adhesive properties so doesn't strengthen underlying tooth).
Undermined enamel
Residual caries
60
Reasons for amalgam fracture after a restoration
Shallow prep or unretentive or sharp internal angles
61
Reasons for recurrent caries beneath amalgam restorations
poor condensation = porous and excess Hg = weaker enamel and bad margins.
Overhangs/plaque retentive features.
Contamination.
62
Reasons for marginal breakdown around an amalgam restoration
Wrong cavo-surface angles.
Over/underfilling, or overcarving = ledges and fractures.
Zn reacts w water if cavity not dried and makes a gas that expands and makes the restoration expand, and sit proud.
Creep and corrosion.
63
Problems w amalgam material
. Poor aesthetics
. Corrosion (Phase 2, Sn-Hg is v negative so gets eroded away and leaves holes)
. Lack of strength and toughness - brittle in thin sections (phase 2 is weakest phase/weakest tensile strength).
. Non-adhesive - can use adhesive Panavia or retentive grooves.
. Biocompatibility
64
How can corrosion in amalgams be reduced
Remove the gamma 2 phase (Sn-Hg) by reacting with copper. This removes the very negative anode so less electron movement and less corrosion (corrosion needs a big difference in electronegativity)
65
Lathe cut vs spherical alloy shapes
Spherical Cu-Ag is better than lathe cut Sn-Ag because is easier to mix, condense and polish. It reaches its full strength quicker and has less mercury.
66
Adding copper to amalgam (reactions)
Sn-Hg +AgCu --> Cu-Sn + gamma phase 1 (Ag-Hg)
| = Gamma + Cu-Sn-Ag + Ag-Hg
67
Benefits of adding copper to amalgam
Lower mercury content
No Gamma 2 phase (less corrosion and stronger)
Spherical is easier to condense and polish and mix and reaches full strength quicker.
68
Variations of dental amalgams
Conventional (lathe cut or spherical Ag3Sn)
High Cu dispersed phase (lathe cut Ag3Sn, spherical Ag-Cu or lathe and spherical Ag-Cu-Sn)
High Cu single phase (lathe or spherical Ag-Sn-Cu)
69
Precautions when handling amalgams
```
Wear PPE
Professional clothing only worn in dental surgery
Ventilation
Measure Hg levels in people and surgery
Not in children or pregnant women
Use pre-capsulated alloys
```
70
Potential sources of mercury toxicity from amalgam
Spills/leakages
When placing/polishing/removing amalgams
When sterilising contaminated equipment
71
The clinical need for cavity bases/liners
. Protect from chemical, physical and bacterial stimuli.
. Replicate dentine function.
. Thermal insulation
. Seal dentine tubules and reduce microleakage and sensitivity.
. Stimulate tertiary dentine.
72
Ideal properties of cavity bases/liners
```
. Biocompatible
. Compatible with other materials
. Aesthetic
. Radioopaque
. Encourage reparative dentine
. Safe for dentist and patient
. Easy to apply/use
. Effective dentine seal against stimuli
```
73
Intermediate restorative materials examples
```
RM GIC
Unfilled resin
Varnish
GIC
ZOE
CaOH cement
```
74
IRM - varnish
Polymer dissolved in a solvent that evaporates to leave a thin polymer layer that protects against chemical stimuli and micro-leakages.
75
IRM - Unfilled resin
Seals dentine surface and command set using photoinitiator
76
ZOE
Zinc oxide eugenols, set by acid-base reaction b/w metal oxide and eugenol. Is unpopular bc risk of pulpal necrosis and not compatible with composites (stops their polymerisation) and doesn't stimulate tertiary dentine.
77
Calcium Hydroxide cement
Rigid and self-setting via a chelate reaction when you mix the 2 pastes. Is v alkaline so has an antibacterial function and stimulates tertiary dentine. But has low compressive strength and slowly soluble in water.
78
Types of polymer
```
Linear/thermoplastics = can be melted and reshaped.
Branched/elastomers = rubbery and stretchy and return to the original shape.
Cross-linked/thermosets = can't be reshaped.
```
79
What is crystallinity
Degree of structural order - amorphous is no order.
80
Transitions of polymers
Solid -> Liquid -> Solid
Monomer -> Solid
Liquid polymer -> Solid polymer
81
Tg
Glass transition temp = when the chains start to flow over each other and the material becomes soft and rubbery. Is below melting temp.
82
Physical properties of polymers
Depends on chain length, branching, side groups, cross-linking
83
Polymerisation mechanisms
. Condensation
. Addition (activation, initiation, propagation, termination)
. Plasticizer (added to the polymer to reduce Tg and elastic modulus and reduce attraction bw the chains).
. Blend
. Copolymer (polymer made of 2+ types of monomers)
. Composite
84
Advantages of amalgam
```
. High compressive strength
. Cost
. Resistant to corrosion and marginal breakdown
. Good working and setting times
. Minimal change in volume on setting
. Doesn't need as much moisture control
```
85
Why is it important to correctly condense amalgam
To adapt the amalgam to the cavity and remove voids and remove the Hg-rich layer.
86
How do you make an amalgam restoration retentive
```
Occlusal locks
Deep
Retentive grooves
Undercut
Cavosurface angle
Smooth internal angles
```
87
Ideal properties of models and dies
```
Cheap
Strong
Durable
Dimensionally stable
Good surface reproducibility
rigid
Doesn't react w other materials
Resistant to wear and heat
```
88
Plaster of Paris chemistry
Dehydrated gypsum + water --> gympsum
Hemihydrate + water --> dihydrate
Powder + liquid.
89
Plaster of Paris types and properties
1. Plaster = mined rock, heated and ground down.
2. Rock = heated more and under pressure to make finer, denser particles.
3. Improved rock = add smaller denser particles.
Less expansion on setting from 1-3, slower setting, more expensive, harder better properties.
90
Plaster of Paris setting reaction
Hemihydrate + water until make ppt
Add more water to the ppt (dihydrate)
Crystals form
When crystals lock the material has hardened
91
Properties of Plaster of Paris
```
Holds its shape when wet but flows when shear force/vibrated.
Diff colours
Cheap, easy to use
Good impressions
Viscous so good for mucostatic imps
Safe
maintains shape well
Can stick to itself
Easy to shape
```
Rough surface
Brittle
Makes mouth dry
Expands on setting
92
What gives the Plaster of Paris its properties
Crystal sizes, affected by the particle sizes. Larger crystals = weaker, brittle, faster setting, more expansion on setting. Small and dense is best. Larger means fewer crystals so less hard.
93
What can be used to reduce the setting expansion of PoP
Add accelerators and retarders. Snap set when overmixed so crystals are broken up and more crystals formed from them.
94
Denture base material ideal properties
```
Biocompatible
Good surface texture/reproducibility
Safe, inert, etc.
Allows the patient to feel hot and cold food.
Strong and hard and stiff
Polishable
Hygienic/resists bacterial growth.
Durable
Low density
```
95
Disadvantages of PMMA as a denture base material
Not hygienic and doesn't allow the patient to feel hot and cold food and not strong/stiff/hard/tough.
96
Chemistry/structure of PMMA
```
solid = methacrylate polymer grains, initiator
liquid = methacrylate monomers and cross-linking agent
```
97
Setting reaction of PMMA
Polymerisation/addition reaction by activation of the initiator (benzoyl peroxide if heat-cured). Cross-linking agent forms additional bonds b/w the chains = improved mechanical and physical properties.
98
Processing problems of PMMA
. Shrinkage on setting - use polymer beads bc then less polymerisation and have fewer pores and cool slower.
. Porosity bc monomer is volatile and the reaction is exothermic. Makes it opaque, weak bc stress points and rough surface. Heat slowly and under pressure.
. Processing strains if diff materials cool down at diff rates.
99
Varients of PMMA
Cold/auto curing - to repair or add extra teeth to Co-Cr denture.
High impact - has rubber so doesn't break as easily if dropped but inclusions aren't bonded so more susceptible to fatigue.
100
Adverse reactions from PMMA as a denture base material
Irritant contact dermatitis bc monomers left in the base diffuse out to tissues and irritate them. Not antibacterial so biofilm can form in cracks and on surfaces.
101
Flexible material for denture base indications
Lots of deep undercuts in the patient's mouth, trismus, only replacing a few teeth, pt keeps dropping them/they keep breaking.
102
Denture re-lining materials - when and how to use
If further bone resorption of mandible/maxilla. Turn current denture into a specialised tray and take an impression in it. Can use cold-curing materials.
103
Soft liner requirements
Low elastic modulus, high resilience, high tear strength, antibacterial, good adhesion to the denture, complete wettability, biocompatible.
104
Silicone vs acrylate based materials
Acrylate is hygienic, forms a strong bond with acrylic and has better tear strength and wettability but silicone is more resilient and harder.
105
Important properties for impression materials for dentist
```
Safe
Good working and setting times
Easy to use
Can be disinfected
Good surface reproducibility
Dimensionally stable/no thermal expansion
Can be cast
Easy to remove
Cheap
Good shelf life
```
106
Important properties for impression materials for the patient
Nice smell and odour
Doesn't have to be repeated
Sets quickly
Safe, non-toxic
107
Different impression materials used for full, partial denture or fixed prosthodontics
```
Full = ZnO eugenol, PoP.
Partial = alginate, elastomers
Fixed = elastomers, composition
```
108
Mucostatic vs mucocompressive materials
Mucostatic for fibrous ridges and mobile stuff e.g. ZnO, agar, Plaster of Paris.
Mucocompressive = viscous material to compress the oral tissues e.g. alginate.
109
The viscosity of impression materials
Shear stress/shear rate. High means it becomes more viscous when higher shear rates applied. Low is becoming less viscous and Newtonian liquid e.g. water is when it doesn't change.
110
Dimensional accuracy of impression materials
Expand or shrink on setting and becomes less accurate.
| TEC = thermal expansion coefficient, change in length per change in temp. High means it expands more.
111
Thixotropy
Viscosity with time
112
Wettability of impression materials
Increased wettability for better surface reproducibility.
| Means fewer voids and bubbles and better impression.
113
Rigid impression materials
Dental waxes, ZnO eugenol, Plaster of Paris, Composition
114
Dental waxes
Use in labs mostly. Low softening temp (just above mouth temp) and high TEC so not dimensionally stable. Not good surface reproducibility.
115
Impression composition
Useful for extending the impression tray. High TEC, viscous (muco-compressive).
116
ZnO/Eugenol
For special trays - has good dimensional stability and surface reproducibility bc v hydrophilic (can stick to patient's mouth)
117
Plaster of Paris as an impression material
Easy to use, cheap, low viscosity (mucostatic) and good dimensional stability and accuracy by low strength.
118
Elastic materials
Can engage undercuts e.g. elastomers and hydrocollides like agar and alginate
119
Elastomers
E.g. silicone, polysulphide.
Base + activator.
Mainly for crowns and bridges - good surface detail.
120
Agar
Reversible reaction, can be sterilized and good surface detail but has dimensional instability and slow setting time and poor tear resistance.
121
Alginate
Needs perforations in tray and tray adhesive bc poor storage stability (gains/loses water). Poor surface reproduction and low tear strength and lots of permanent deformation. Cheap, easy, good working times, mucostatic.
122
Problems with impressions
Rough surface bc of pores and air bubbles and affect the dimensional stability - if cooled down too fast, not mixed well or sets too quickly. Expansion or shrinkage after taking the impression affects the model accuracy.
Poor fit bc of distortion, model cast too soon or too late, patient movement.
123
What makes a good impression
Dimensionally stable and accurate -affected by storage, tray type, material.
Accurate surface reproducibility
