Dental Materials Flashcards
What are the requirements of dental materials?
- Fit for purpose - Each patient is different
- Easy to handle
- Appropriate properties
- Aesthetics
- Clinically efficacious
- Safe
How is testing for safety of a dental material done?
BASIC RESEARCH
- Pre-market testing
CLINICAL RESEARCH
- Post-market surveillance
What guidelines is pre-market testing based on?
ISO 10993 guidelines
What do you test for in vitro?
- Cytotoxicity, Genotoxicity, Estrogenicity - organ culture, tissue culture, cell culture (2D or 3D)
- Microbial test
What do you test for in vivo?
Implantation tests in animals
Systemic toxicity
Evaluation of sensitization and irritation
Pulp studies
What is the CE mark?
a certification mark that indicates conformity with health, safety, and environmental protection standards for products sold within the European Economic Area (EEA).[
What are the most common allergens in dental materials?
nickel: 25.0%; palladium: 24.4%; chromium: 16.7%; cobalt - 15.9%; stannum (Tin) - 12.5%
What types of adverse reactions to dental materials could there be?
Toxic reactions - e.g. cytotoxicity, carcinogenicity Irritant contact dermatitis - acute toxic reaction, paresthesia,cumulative insult dermatitis -Allergic contact dermatitis -Oral lichenoid reactions -Anaphylactoid reactions -Contact urticaria -Intolerance reactions
What does post-market surveillance do?
Monitors reactions after the product has gone out on the market e.g. long term drug side effects
Why are metals good conductors?
due to movement of free electrons
What is a transition?
a reorganisation of the distribution of the atoms in a material.
What is the transition temperature
The temperature at which a reorganisation of the distribution of the atoms in a material occurs
What are the 3 common transition states?
Heat a solid – melts. Cool it and it re-solidifies
Add a liquid to a solid – It solidifies (amalgams)
Heat some metals and they change their atomic structure eg gamma to alpha iron
What are the features of ceramics?
Inorganic, non-metallic compounds, usually crystalline in nature.
Occasionally term ceramic is used to encompass inorganic amorphous materials (glasses)
- Brittle, hard materials with high melting points
- Weak in tension, but have very high compressive strengths
- Ordered 3D structure of covalently and ionically bonded compounds of a metal with a non-metal.
How are ceramics made?
1: Mix
2: Sieve
3: Squash
4: Dry
5: Heat
Fine, squashed, dry powders heated up to just below melting point
All the little particles stick together and usually shrink
What are the 3 common transitions for ceramics?
- Sintering process (shrinkage)
- Hot pressing induces phase change (change in atomic arrangement) and/or a densification
- Solid liquid setting reaction
What other material is similar to ceramics?
Glass
How is glass formed?
Supercooling
No time to organise molecules
Amorphous
Forms a glass
How are glass-ceramics formed?
2 stage heat treatment - Formed by the controlled crystallisation of glass
Nucleation - small areas of order (a little below the glass transition temp)
Growth – the nuclei grow larger (below melting temp)
Can alter the time and temperature of each step to produce larger/smaller crystals
Give examples of some polymers in dentistry
acrylic resin, alginate, polyether
What is the glass transition temperature?
The temperature at which the polymer chains begin to flow past each other
What are the 3 polymer transitions?
Solid > Liquid > Solid
Monomer > Solid
Polymer liquid > Polymer solid
At what stages do you need to consider the properties of materials?
The unmixed compounds
during mixing
the set material
What are mechanical properties of a material?
Used to indicate how a material or component will respond to use i.e. in response to applied forces
What are the ISO standards?
ISO creates documents that provide requirements, specifications, guidelines or characteristics that can be used consistently to ensure that materials, products, processes and services are fit for their purpose
What are some mechanical properties?
Stress and Strain
Fatigue
Hardness
Abrasion resistance (wear)
What is fatigue?
Repetitive stress
What is hardness?
resistance to scratching
What is shear?
Two planes moving across each other in opposite directions
What is stress?
the force per unit cross-sectional area, that is acting on a material.
Stress = F/A
where F = force, A = original cross area
Units = (mega) pascals (MPa) = Newtons per square meter (N.m-2
What is strain?
the fractional change in the dimensions caused by the force
Strain does not have units as it is a ration of lengths
Strain = (L1-L0)/L0
where L1-L0 = change in length,
L0 = original length
What is resilience?
The amount of energy a material can absorb without undergoing any plastic deformation
What is toughness?
The amount of energy a material can absorb to the point of fracture
What is ductility?
Ductility is the amount of plastic strain at fracture
Describe the stress-strain curve
Stress - y axis
Strain - x axis
straight bit - elastic bit
end of straight bit - eleastic limit at yield stress
after this point it’s the plastic flow region - permanent deformation
What is the elastic (young’s) modulus?
a measurement of the stiffness, units = pascals (often mega or giga: MPa or GPa)
change in y (stress) over change in x (strain)
What is the ultimate tensile strength?
The maximum force absorbed
What are dental products often subjected to?
Fluctuating, cyclic loads over a period of time. The accumulation of stress (esp around defects or pores) can cause crack propagation leading to failure.
What are fatigue properties given as?
FATIGUE LIFE (No. cycles to failure) or FATIGUE LIMIT or FATIGUE STRENGTH (cyclic stress required to cause failure for a set number of cycles).
What are the 4 indentation techniques used to measure hardness?
Knoop, Vickers, Brinell or Rockwell
What is hardness directly proportional to?
the size of the indentation and is given as a number (the hardness number), large for soft materials and smaller for hard
What can hardnesss also be measured by (as well as indentation techniques)?
The scratch test
What are some chemical and physical properties?
Elasticity, Viscosity and Viscoelasticity - working times and setting times Durability & Degradation - solubility, corrosion, erosion Thermal Properties - thermal conductivity, thermal diffusivity, exothermic reactions Adhesion Colour and Aesthetics Biological Properties (biocompatibilty)
How do viscous materials respond to stress and strain?
resist flow and strain linearly with time when a stress is applied.
And when the load is released, they do not immediately return to the original state.
How do elastic materials respond to stress and strain?
strain when stretched & instantaneously return to their original state once the stress is removed.
What do viscoelastic materials exhibit?
time-dependent strain
What is creep?
a constant state of stress with an increasing amount of strain
What is stress relaxation?
the observed decrease in stress in response to the same amount of strain generated in the structure
What is corrosion?
the deterioration of a material, usually a metal with its environment. It is an electrochemical process and basically can be considered as destructive oxidation.
What is the chemical solubility test?
Put it in vinegar at 80C for 16h, weigh before and after
What is thermal conductivity defined as?
the rate of heat flow (Watts per meter) per unit of temperature gradient under steady state conditions, hence the units of thermal conductivity are W m-1 °C-1
What equation links thermal conductivity and diffusivity?
D = λ/( ρ Cp)
Where D is thermal diffusivity, λ is thermal conductivity, ρ is density and Cp is heat capacity
What is thermal diffusivity?
It’s a measure of readily a material conducts thermal energy to how readily it stores it
When transient heat is applied (to teeth/dental material) a proportion will be expended raising the temperature of the tooth/material, whereas the remainder will be conducted to the pulp.
Therefore materials with low thermal diffusivities are generally preferred.
Why does thermal expansion occur?
due to the increase amplitude of atomic/molecular vibrations due to the absorption of heat energy.
In dentistry it is crucial to minimise thermal expansion mismatches between restorative materials and tooth tissue and between neighbouring materials e.g. cores/substructures and crowns
What is a metal?
A crystalline structure held together by metallic bonds - sea of negative delocalised electrons surrounded positive cations
What are the two types of crystal structures in metals?
FCC - face centred cubic
BCC - body centred cubic
What are the two packing factors of the two crystal structures?
FCC - packing factor 0.74
BCC - packing factor 0.60
What is the metal microstructure?
(Usually) polycrystalline structure consisting
of grains and grain boundaries
What does etching do to a metal structure?
creates grain contrast depending on
crystallographic orientation
What is an alloy?
A combination of two or more metals
What are the 3 different types of alloys?
- substitutional solid solutions
- interstitial solid solutions
- intermetallic compounds
What are the characteristics of a substitutional solid solution?
● The different atoms have the same valency ● With the same crystal structure ● Their atomic size is within 15 %
What are the characteristics of a interstitial solid solution?
● Solute atom solvent atom ● The solute (smaller) atoms occupy the space between the solvent (larger) atoms ● Distortion of the lattice occurs to accommodate the extra atoms
What are the characteristics of a intermetallic compound?
An intermetallic compound is formed when
two or more metals combine with a discrete
composition or stoichiometric ratio
e.g. dental amalgam
● The alloy contains a silver/tin compound,
Ag3Sn, and a copper/tin compound, Cu6Sn5
What is a phase?
A phase is defined as a homogeneous
physically distinct part of a system that is
separated from other parts by a definite
physical boundary
Alloys can be single or multiple phase
What do the phases in an alloy depend on?
Composition and temperature
What are the different crystal structures in alloys called?
Phases
What is a dental amalgam mixed in?
An amalgamator
What is mixed in the amalgamator?
Powder (gamma phase - tin and silver)and copper/zinc to the mercury
What are the two types of dental amalgam?
Conventional dental amalgam and High copper content amalgams (nongamma
2)
What is in a conventional dental amalgam? (composition)
Silver Ag 67-74 %
Tin Sn 25-27 %
Copper Cu ~6 %
Zinc Zn ~2%
triple distilled mercury (Hg)
What type of alloy is the gamma phase in amalgam?
Intermetallic compound
What phase is the gamma phase?
3rd pure phase of silver and tin
What does the size and shape of the powder particles influence when mixing amalgam?
handling,
• setting reactions and
• final properties of the restoration.
What is the conventional amalgam reaction?
Ag3Sn + Hg → Ag3Sn + Ag2Hg3 + Sn7-8Hg
g + Hg → g + g 1 + g 2
Alloy + Triple distilled mercury -> (unreacted) +Alloy Silver Mercury phase + Tin mercury phase
solid + liquid solid + solid + solid
How does the powder and the mercury mix?
Outer layer of alloy particles dissolve in the mercury
What are the reasons for replacement of amalgam restorations?
- Tooth fracture
- Recurrent caries
- Gross amalgam fracture
- Marginal breakdown
What can undermine enamel in cavity prep?
Providing flat walls and floors to a cavity can undermine
enamel
What are the causes of recurrent caries?
- Contamination
● Poor Matrix techniques = proximal overhangs or poor contact points = plaque accumulation = recurrent caries
● Poor Condensation = porosity and excess Hg =reduced amalgam strength
● Marginal adaptation =poor, can = marginal leakage, recurrent caries & corrosion
What are the causes of gross amalgam fractures?
● Shallow preparations
● Non-retentive proximal boxes
● Sharp internal angles = stress concentrations
What are the causes of marginal breakdown?
● Incorrect cavo-surface angles(= (the angle formed by the junction of a wall of a tooth cavity preparation and a surface of the crown of the tooth.)
● Delayed expansion - In the case of Zn containing alloys, if the preparation site is not kept dry, Zn can
react with the saliva
Zn +H2O ® ZnO + H2
Bubbles of hydrogen form within the
amalgam. The bubbles expand, so amalgam
expands - Pressure on the pulp
● Overfilling, underfilling, overcarving
● Overfilling (without carving back) = ledge = fracture
● Underfilling or overcarving = acute amalgam angles = marginal breakdown
● Creep and corrosion of the amalgam - unavoidable intrinsic property of amalgam
What are the problems with amalgam restorations?
-Lack of aesthetics • Non-adhesive • Lack of strength and toughness • Susceptible to corrosion • Biocompatibility
What is the weakest phase in amalgam?
y2/ gamma 2
What are dispersed phase amalgams called (addition of Cu)?
Admix
What is the admix made of?
AgSn alloy (lathe cut) and AgCu (spherical)
How does the dispersed phase amalgam setting reaction differ from the traditional amalgam setting reaction?
first step same - y + Hg -> y + y1 + y2
Then: y2 + Ag/Cu -> Cu5Sn5 + y1
finally: y + (Ag-Cu-Sn) + y1 (gets rid of gamma 2)
How does the addition of copper change the properties of the amalgam?
gets rid of susceptibility to corrosion
What is a galvanic cell?
an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell
What are the benefits of dispersed phase amalgam (high Cu)?
- Reach full strength more quickly
- Easier to condense
- Easier to carve and polish
- Lower mercury content
- Higher strength
- More resistant to corrosion
What is the setting reaction for high Cu single phase amalgams?
AgSnCu + Hg -> AgSnCu + y1 + Cu6Sn5
What are the variations in dental amalgams referring to different types of cuts?
Dental amalgams: variations • Conventional amalgams – lathe cut Ag3Sn – spherical Ag3Sn • High Cu dispersed phase amalgams – Lathe cut Ag3Sn+ spherical AgCu – Lathe cut AgSnCu+ spherical AgSnCu • High Cu single phase amalgams – All spherical AgSnCu – Lathe cut AgSnCu
What are the potential symptoms and hazards of Hg Exposure
– Respiratory failure – Kidney impairment – Cognitive disturbances – Reduced visuoperceptual and constructional skills – Memory loss – Hypertension – Headaches
What are the potential potential sources of
mercury contamination?
– spills
– leaky dispensers or capsules
– removing or polishing amalgams
– sterilising Hg-contaminated instruments
How can you minimise mercury contamination?
Therefore:
– Use pre-capsulated alloys
– Avoid direct skin contact with dental amalgam
– Use high volume evacuation when finishing or removing amalgams
– Wear a mask when removing amalgams
– Store bulk mercury in unbreakable containers
– Store amalgam scrap under radiographic fixer solution
– Dispose of contaminated items in a sealed
bag
– Report and clean up spilled mercury
immediately using a clean-up kit
– Wear professional clothing only in the
dental surgery
How to store and dispose of mercury/dental amalgam?
– Store bulk mercury in unbreakable containers – Store amalgam scrap under radiographic fixer solution – Dispose of contaminated items in a sealed bag – Report and clean up spilled mercury immediately using a clean-up kit – Wear professional clothing only in the dental surgery
What is the performance criteria for dental resin?
Mechanically Strong Physically Stable Easily Manipulated Excellent Aesthetic Qualities Chemically Stable (in the mouth and under storage conditions) Biocompatible Reasonable Cos
When was Polymethylmethacrylate (PMMA) introduced and name some uses?
1936
(a) Bone cements
(b) Contact and intraocular lens
(c) Screw fixation in bone
(d) Filler for bone cavities and skull defects
(e) Vertebrae stabilization in osteoporotic
patients
What are the pros and cons of PMMA?
- bio compatible
- acceptable costs of material and processing method
- aesthetic
- good mechnical properties - strength, rigidity, wear resistance,
- suitable manipulation, processing is easy, easy mixing
colour stability
shrinking problems
adhesion problems
What is a polymer?
A polymer is a large, chain-like
molecule made up of monomers,
which are small molecules
When were Self-curing dimethacrylates invented?
1950s
What decides the property of a polymer?
- how their molecules are arranged (polymer structure)
* the strength of the forces between these molecules (intermolecular forces)
What does strength and flexibility of a polymer depend on?
•Chain length: in general, the longer the chains the stronger the
polymer
•Side groups – intramolecular forces (e.g. hydrogen bonding) give
stronger attraction between polymer chains, making the polymer
stronger
•Branching - straight, unbranched chains can pack together more
closely than highly branched chains, giving polymers that have
higher density, are more crystalline and therefore stronger
•Cross-linking - if polymer chains are linked together extensively by
covalent bonds, the polymer is harder and more difficult to melt
What are the useful properties of amalgam?
Good compressive strength
• Good balance of working & setting times
• Fair resistance to marginal leakage
• Minimal dimensional change on setting
• Relatively resistant to contamination
• Nearly 200 years of scientific development
• Low cost
What are the limitations of amalgam?
Not adhesive to tooth substance • Brittle in thin sections • Alloy / mercury proportions critical • Detectable levels of creep • Corrosion can weaken • Affected by water during setting reaction • Toxic components • Unaesthetic • Not conducive to minimal intervention approach
What are the indications for amalgam?
Posterior teeth • When too late for prevention • When an adhesive restoration not indicated • Occlusal load • Size of restoration • Isolation from moisture
How can amalgam be retained?
- Mechanical ‘lock’ of the restoration
- Undercuts – can be very subtle
- Occlusal lock
How can you get resistance to occlusal forces?
- At least 2mm thick
- No feather edges
- Cavosurface angle
- Consider effects of creep
- Floor perpendicular to occlusal forces
- Smooth internal angles
What are the hazards of amalgam?
• Mercury • Neurotoxic • Adversely affects renal function • Culmulative in the body & environment • Alloy components • Tin and silver compounds considered hazardous to the environment
How to manage risk of amalgam?
- PPE
- Avoid in pregnancy
- Use capsules
- Reduces exposure
- Optimises trituration
- Aspirator & isolation
- Proper disposal
How to prepare and place the restoration?
Isolate the cavity - free of debris and dry
Triturate the amalgam - amalgamator
Place and condense the amalgam - amalgam carrier, condensers/pluggers,
Condensing - add layers and press together
Burnishing - pear ended/ ladmore burnisher, iinitiate fissures and condense margins
carving - removes mercury rich layer, develops anatomy
Finishing: Adjust occlusion & morphology • Reduces corrosion • Aesthetics • …Wait for 24 hours
What advantages are there to smaller filler sizes?
- Increased resistance to wear and potentially greater strength
- Increased ability to polish
- Improved translucency
- Combined with changes in filler volume, potential to decrease polymerisation shrinkage.
What are the 3 phases in a composite resin system?
Organic phase
Dispersed phase
Interfacial phase
What does the organic phase (resin matrix) form?
Forms the polymer backbone to provides tensile strength
What does the dispersed phase form? (inorganic filer)
Improves mechanical properties (wear and compressive strength) and decreases shrinkage.
What does the interfacial phase form? (coupling agent)
Provides adhesive bond between organic and dispersed inorganic phases
What are the advantages of nanofilled over conventional composites?
Potentially superior fracture toughness, high strength, and excellent wear resistance.
High polishability and very low polymerization shrinkage.
Improve the continuity between nanoscopic tooth structure and nanofillers to provide a more stable and “natural” interface
High surface to volume ratio allow high filler loading to give workable consistencies
Nanosized fillers are able to scatter or absorb visible light which increase translucency
