Prosthodontic Materials Flashcards
HYDROCOLLOIDS
Note Agar Reversible Hydrocolloids is no longer being used and thus, we are not learning it.
But, why did alginate replace agar?
Alginate was easier to manipulate, more comfortable for the patient and relatively inexpensive - Anusavice and Phillips (2003)
It is also less technique sensitive and requires much less specialised equipment. Furthermore, cross-contamination can be an issue when using agar.
HYDROCOLLOIDS
Classification
Classified by Elasticity
Elastic Materials
- Synthetic elastomers = PS, PE, PVS
- Hydrocolloids = Reversible (agar), Irreversible (alginate)
Non-Elastic Material
- Impression plaster
- Impression compound
- ZOE pastes
- Impression waxes
HYDROCOLLOIDS
Irreversible Alginate
1) What?
2) Setting Reaction?
1) What?
Brown marine alginic extract (structural formula is based on alginic acid)
2) Setting reaction of alginate irreversibly drives alginate sol to a gel by forming cross-linked 3D networks of alginate chains using calcium ions. This gives it elastic properties.
Na(x)Alg + x/2CaSO4 –> n/2Na2SO4 + Ca(n/2)Alg
COONa COONa COONa COONa
—l———l———l—- —l——–l——–l—-
COONa COO-
–> Ca2+
COONa COO-
—l———l———l—- —l——–l——–l—-
COONa COONa COONa COONa
HYDROCOLLOIDS
Irreversible Alginate
3) Components
3) Components Sodium Alginate - 18% - Base structure of alginate Calcium Sulphate Dihydrate - 14% - Provides calcium Sodium Phosphate - 2% - Controls working time by producing competing reaction Potassium Sulphate - 10% - Reduces setting expansion and time by promoting crystal growth Filler - Can vary but can be as high as 75%. - Control consistency (Can increase viscosity of mix and be an elastic modulus of set alginate) - Note high silica content of diatomaceous earth which may cause respiratory irritation (silicosis) Sodium Silicofluoride - 4% - Controls pH
HYDROCOLLOIDS
Irreversible Alginate
4) Additives
To delay gel formation
Retarders
- Trisodium phosphate will reduce elastic deformation and slow down
fast formation of calcium alginate chains by forming competing
reaction for Ca ions to produce Ca3(PO4)2 precipitate
Borax
- Simply slows down rate of reaction
To speed up gel formation
Accelerators
- Potassium titanium fluoride can be added to increase setting and hardness of dental stone pouring into the impression
HYDROCOLLOIDS
Alginate Precautions
Temperature of water.
Storage
1) Temperature of Water.
Temperature will change setting time of impression.
If water if too cold, it will be expected that we have to leave the impression longer in the mouth, increasing discomfort and possibility of tray movement.
If water is too warm, premature gelation can occur leading to poor seating.
2) Storage
Best stored at 100% humidity but note that covering with soaked tissues may alter water content of impression and ruin dimensional stability.
HYDROCOLLOIDS
Pouring Up
When should we pour up?
Powers JM et al (2006)
- For maximum accuracy, the model material should be poured into
alginate impression as soon as possible.
This is because, during storage, the impression is prone to effects of syneresis and imbibition.
Chiayi Shen (2003) - One removed from the mouth and exposed to air at room temperature, some shrinkage associated with syneresis and evaporation is bound to occur. Conversely, if the impression is immersed in water, swelling as a result of imbibition occurs.
Syneresis = Water displacement from gel, appearing as an exudate on surface of impression causing tightening of fibrils during transformation leading to contraction.
Imbibition = Water uptake by gel (particularly if gel is dried due to extended storage period or if impression is stored in water) resulting in expansion and distortion of impression material.
WAXES IN DENTISTRY
What?
Waxes are thermoplastic materials used primarily to form ‘patterns’ for casting in dentistry. They have a high co-efficient of thermal expansions.
WAXES IN DENTISTRY
Clinical Uses? (4)
- Altering and adapting impression trays (peripheral wax)
- Bite registration (moyco wax)
- Blocking out undercuts
- Direct waxing for cast/metal restoration
WAXES IN DENTISTRY
Laboratory Uses (5)
- Indirect pattern for casting
- Baseplate and biterims for RDPs
- Diagnostic wax up
- To support and secure cast during articulation (sticky wax)
- Beading and boxing technique
WAXES IN DENTISTRY
Wax Patterns
1) What?
2) Uses?
1) What?
A precursor of the finished cast restoration that will be placed on the prepared tooth
2) It can be used for:
- Final cast restorations (chrome cobalt frameworks and indirect
restorations such as inlays)
- Polymer restorations (prosthesis and appliances)
WAXES IN DENTISTRY
Types of Wax Based on Components
- Natural
- Mineral = Parafinn
- Animal = Beewax
- Plant = Carnauba and Candelilla Wax
- Synthetic
- Additional components
- E.g. gums, resin, oils and fats
WAXES IN DENTISTRY
Types of Waxes Based on Use (7)
- Utility Wax (Periphery Wax)
- Room temp = easy to manipulate
- Used to adjust impression trays and cover sharp brackets/wires
during orthodontics
- Sticky Wax
- Room temp = Hard but brittle. Soft and sticky when heated
- Used to temporarily join metal, gypsum and resin during
fabrication and repair
- Modelling Wax (Baseplate Wax)
- Layered to produce the form on which denture teeth are set
- Impression Wax
- Normally used to correct small imperfections in impressions (esp
in ZOE impressions. - Relatively soft and flows at mouth temperature
- Contains low melting paraffin wax and beeswax (3:1)
- Normally used to correct small imperfections in impressions (esp
- Processing Wax
- Room temp = easy to manipulate
- Used to form bases of gypsum of models
- Bite Registration Wax
- Used to take interocclusal records for articulation of models
- Very susceptible to distortion
- Inlay Casting Wax
- Waxes used to make patterns for cast restorations
- Dipping wax = Used to make top of crowns, inlay and onlays.
- Spool ‘wire’ wax = Used to produce lingual bars and sprues.
WAXES IN DENTISTRY
Major Requirements of Wax Patterns (3)
- Dimensional stability
- Must conform to the shape, size and contour of the appliance/
restoration to be constructed - After formation of the casting mould, it should be possible to remove
wax without residue.
Note that the final restoration can be no better than its wax pattern!!!
SECONDARY IMPRESSION MATERIAL
Desirable Qualities
- Pleasant odour, taste and aesthetic colour
- Adequate shelf life for storage and distribution
- Easy to use with minimum equipment
- Adequate strength so it will not break/tear on removal
- Wets oral tissues
- Setting characteristics that meet clinical requirements
- Elastic properties to resist permanent deformation
- Dimensional stability of temperature and humidity ranges
- Accurate
- Readily disinfected
- Economically viable
SECONDARY IMPRESSION MATERIAL
Types
Elastic
- Elastomers (e.g. PS, VPS, PE)
- Hydrocolloids (e.g. Irreversible, Reversible)
Non-Elastic
- Impression compound, plaster, waxes
- ZOE
SECONDARY IMPRESSION MATERIAL
Elastomers (PS)
Advantages & Disadvantages
Polysulphides Ad - Inexpensive - High tear resistances - Enhanced elastic properties - Dimensionally stable but less so than VPS and PE
Dis
- Unpleasant sulphide odour and taste
- Long setting time (10mins)
- Reduced working time
- Hard to handle (sticky)
SECONDARY IMPRESSION MATERIAL
Elastomers (PE)
Advantages & Disadvantages
Polyether Ad - More dimensionally stable than PS - Adequate setting time (5mins) - Can be poured up to 24hrs later - Stable when dry
Dis
- Very stiff and rigid (very hard to remove from mouth)
- Hydrophillic (more prone to dimensional changes due to increased
risk of imbibition)
SECONDARY IMPRESSION MATERIAL
Elastomers (VPS)
Advantages & Disadvantages
Vinyl Polysiloxanes Ad - High dimensional stability - Very accurate - Can be poured up to a week later - Hydrophobic (less imbibition) - Adequate working time (3-5mins) - Available in many different consistencies
Dis
- High cost
ACRYLIC MATERIALS
Types:
Based on Area of Application
- Denture bases
- Artificial denture teeth
- Relining materials (splints)
- Mouth guards
- Provisional restorations (temp crowns/FDPs)
Based on Curing Types
- Heat cure
- Light cure
- Chemical cure (self cure)
ACRYLIC MATERIALS
Acrylics in Denture Bases Construction
Use?
Types?
Use:
- Helps retain artificial teeth
- Replaces missing soft and hard tissues
- Distribute load over wide area
- Creates effective seal for retention
Types: - Heat cured - Self cured (autopolymerising) - Thermoplastic - Light activated - Microwave cured Note: Type 1&2 we mix a powder and liquid to cause monomer diffusion and softening of the surface of the powder in the following stages: Sandy, Stringy/Sticky, Dough, Rubbery.
ACRYLIC MATERIALS
Acrylics in Denture Bases Construction
Desired Quality?
Sai Sathey Pulls Bitches Easily And Must Catch FISH
- Strength, durability and colour stability
- Satisfying thermal properties
- Processing accuracy and dimensional stability
- Biocompatibility: absense of taste and odour
- Ease of fabrication
- Adequate fatigue strength and high fatigue limit
- Moderate cost and good shelf life
- Chemical stability, insolubility
- Flexural Strength
- Impact Strength
- Sufficient abrasion resistance
- High modulus of elasticity
ACRYLIC MATERIALS
Applications in Clinical and Laboratory Scenarios
Clinical Applications:
- Direct resin pattern
- Temporization (key use)
- Denture relining
- Picking attachments with implant overdentures
Laboratory Application
- Fabrication of RDP/FDP
- Acrylic splints or orthodontic appliances
- Temporary FDPs
- Rebasing dentures
- Secondary impression tray
- Denture repair
- Maxillofacial prosthesis
GYPSUM
1) What?
2) Calcination (less important, more of a ‘now-you-know’ kinda thing)
3) Rehydration*
1) What?
Dental plaster and stone are gypsum products used to produce study models.
2) Calcination
2CaSO4.2H2O –> (CaSO4)2.H2O + 3H2O
Method of which calcium sulphate hemihydrate is produced from gypsum or calcium sulphate dihydrate. Note that gypsum products are commonly called ‘hemihydrate powders’. This reaction is driven by heat. When excessive heat is used, anhydrite powders are produced and this reacts differently when mixed with water and produces ‘dead-burnt plaster.
3) Rehydration
When mixing gypsum products with water, the opposite reaction takes place
(CaSO4)2.H2O + 3H2O –> 2CaSO4.2H20
GYPSUM
Die Stone Production
less important, more of a ‘now-you-know’ kinda thing
Calcination occurs under certain conditions (in calcium chloride solution, under pressure). This produces shorter particles with more cube-like shapes. Residual calcium chloride is then removed by washing in a heated bath, leaving gypsum product that is still a calcium sulphate alpha-hemihydrate but with more refined particles. These modified particles require less water in mixing and produce harder stone (die stone).
GYPSUM
Classification of Gypsum Products Type 1 & 2 Use - Compressive Strength - Water/Powder Ratio - Expansion
Type 1: Impression Plaster - β gypsum U - Impression material CS - 10-20MPa W/P - 1:2 E - 0.15%
Type 2: Casting/Model Plaster - β gypsum U - Basic study models CS - 10-25MPa W/P -1:2-3 E - variable but up to 0.3%
Classification of Gypsum Products Type 3 & 4 & 5 Use - Compressive Strength - Water/Powder Ratio - Expansion
Type 3: Stone - α gypsum U - Articulated/working models and processing CS - 25-70MPa W/P - 1:3 E - 0.2%
Type 3: High Strength Die Stone w/ Low Expansion - α gypsum U - Orthodontic and CAD models CS - 70-150 MPa W/P - 1:4 E - 0.1%
Type 3: High Strength Die Stone w/ High Expansion - α gypsum U - Casting metal CS - 70-150MPa W/P - 1:5 E - 0.3%
GYPSUM POWDER + WATER –> SLURRY
1) Reaction Type?
2) What occurs?
1) Reaction type:
Exothermic reaction
2) What occurs?
Precipitation of needle-like crystals of calcium sulphate dihydrate form around a foci of crystallisation (in 3D, it would look spherulitic like a roll-up hedgehog due to interlocking mesh of crystals).
SLURRY –> PLASTER
1) Setting Process
2) Tensile Strength
The measure of the fracture resistance of a material placed under loading; Stress = Force (applied load) / Area
1) Setting Process
a. Fluid (pouring into mould)
b. Plastic (basic shaping, building up without folds or crumbling)
c. Friable (plasticity is lost but material can still be manipulated)
d. Initial Set (carvable with sharp blade and will crumble)
e. Final Set (increases in strength)
2) Tensile Strength
Though plaster has a good compressive strength, it has low tensile strength. Therefore in thin areas of models (e.g. incisors) they are brittle and much more susceptible to damage. Dental stones have much higher tensile strength and thus is more preferred as working models.
SLURRY –> PLASTER
3) Setting Variables
3) Setting Variables
Spatulation (Mixing) Time
- ↑mixing time = ↓setting time & ↑setting expansion
- Over/Under Spatulation = Weakening of plaster
- This is due to breaking up of crystal formation and increases number
of crystallisation sites.
Age of the Product
- ↑Age = ↑Setting time
- If not stored in a sealed, dry container in a cool environment, water
vapour is adsorbed and surface hydration of powder occurs.
- Note that any residual anhydrous calcium sulphate in the powder
will preferentially hydrate, becoming a source for crystal growth and
will alter setting properties of mixed gypsum
SLURRY –> PLASTER
3) Setting Variables: Influences of Water
3) Influences of Water
Increasing W/P Ratio (Adding Water)
- ↑Water = ↑Setting time & smoothness/pourability of the mix
↓Set compressive strength
Immersion of Setting Material in Water
- Leads to hygroscopic expansion where crystals continue to grow due
to reduces surface tension –> greater expansion of setting product
Wet Compressive Strength
- Strength of a setting stones are lower than that of their set strength.
Therefore, always wait until the model is dry before using it.
Soaking a Dry Model
- Water reduces strength of the dry model
Solubility of Set Material in Water
- Increases with temperature.
- Note that when removing wax from a model by pouring hot water
onto it, it can result in some loss of the models surface accuracy
SLURRY –> PLASTER
3) Setting Variables: Additives
3) Additives
Sodium Chloride
- ↓ Setting expansion as it increases sites for crystal formation
- Low Conc ( Syngenite
- Causes ↓Setting expansion and time as it acts as a stimulus for
crystal growth
Calcium Sulphate Dihydrate
- ↓working time and setting time (increases site of crystal formation)
- Used as an accelerator
Borax
- Retards setting as it causes formation of calcium tetraborate on
the crystals inhibits their further growth.
Potassium Citrate & Sodium Citrate
- Retards setting
