1/31: Alginate and Stone Flashcards
What are features of ideal impression material?
- easy to mix and handle
- suitable working time
- suitable setting time
- compatible with stone
- not toxic or allergenic to the patient
- dimensionally stable on setting
- accurate to record the fine details
- has acceptable odor and taste
- adequate strength
- adequate shelf life
- economical
- ready to be disinfected without loss of accuracy
- fluid or plastic when inserted into the mouth
- it must be an exact record of all the aspects
- other teeth and tissue surrounding
- it must be free from air bubbles especially in the finish line area
What are the three key properties of impression materials?
Accuracy
Dimensional stability
tear resistance
What does accuracy mean referring to impression materials?
Ability to replicate the intraoral surface details
What does dimensional stability mean referring to impression materials?
Ability to retain its absolute dimensional size over time
What are the two types of impression materials?
Nonelastic and elastic
What are examples of nonelastic impression material?
Plaster
Impression compound
Zinc oxide eugenol
Impression waxes
What are two types of elastic impression material?
Hydrocolloids
Non-aqueous elastomers
What are the two types of hydrocolloid elastic impression material?
Agar reversible
Alginate irreversible
What are the types of non-aqueous elastomers?
Polysulfides
Polyethers
Condensation silicone
Addition silicone
What was alginate (irreversible hydrocolloid) developed as a substitute for?
Agar impression material when its supply became scarce during WWII
What is alginate based on?
Natural substance extracted fro certain brown seweed
Substance called anhydro-b-d-mannuronic acid or alginic acid
What are the principal factors responsible for the success of alginate impression material?
Easy to manipulate
Comfortable for the patient
Relatively inexpensive since it does not require elaborate equipment
What are the chief active ingredients of the irreversible hydrocolloid impression materials?
Soluble alginates, such as potassium or triethanolamine alginate
What are the components of alginate?
From most to least:
Diatomaceous earth
Calcium sulfate
Potassium alginate
Zinc oxide
Potassium titanium fluoride
Sodium phosphate
The typical sol-gel reaction can be described as a reaction of soluble alginate with calcium sulfate and the formation of an insoluble _________
Calcium alginate gel
Altering the W/P ratio or the mixing time can have marked effects on the property of?
Gel, tear strength and elasticity
A clinician can influence the setting time by altering the ____________
Temp of the water
Alginate impression materials are ________, so most tissue surfaces are not a problem
Hydrophilic
What is an ideal mixing time of alginate?
45 sec- 1 min
The result of a mix of alginate should be what consistency?
Smooth, creamy, doesn’t readily drip off the spatula when raised from the bowl
What are the two types of impression trays?
Rim lock tray
Perforated tray
The thickness of the alginate impression between the tray and the tissues should be at least _______mm
3mm
Alginate hydrocolloid materials are ________ dependent
Strain-rate dependent
Thus the tear strength is increased when the impression is removed with a snap
What are disinfection methods for impressions?
Disinfectant-soaked paper towel/plastic bag, 10 min
VPS not adversely affected
How can hydrocolloids expand?
Imbibition: expansion after 10 min
How can polysulfide, polyether expand?
Potential expansion, increase exposure time
What are current digital impression systems?
CEREC primescane, omnicam or bluecam
Itero element
LAVA true definition scanner
What is the gypsum produced for dental applications?
Pure calcium sulfate dihydrate (CaSO4)
What are gypsum products used in dentistry for?
Preparation of study models for oral and maxillofacial structures and is important auxiliary materials for dental lab operations that are involved in the production of dental prostheses
When plaster is mixed with fillers, such as different forms of silica, it is known as __________
Gypsum dental investment
What is the principal constituent of gypsum based products such as dental platers and stones?
Calcium sulfate hemihydrate
Various gypsum products require different amount of water and these differences are accounted for pincipally by _______ and _______
Shape and compactness of the crystals (regualted by manufacturer)
What does the colloidal theory propose?
When mixed with water, plaster enters into the colloidal state through a sol-gel mechanism
What does the hydration theory suggest?
Rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material
What is the most widely accepted mechanism for setting gypsum?
Dissolution-precipitation theory
What is the dissolution precipitation theory based on?
Dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid
The setting reaction of gypsum occurs by:
Dissolution of calcium sulfate hemihydrate -> formation of a saturated solution of calcium sulfate -> aggregation of less soluble calcium sulfate dihydrate and -> precipitation of the dihydrate crystals
What is mixing time defined as?
Time from the addition of the powder to the water until the mixing is completed
Mechanical mixing of stones and palsters is usually completed in _______ sec
20-30
What is working time?
The time available to use a workable mix, one that maintains a uniform consistency to perform one or two tasks
What is setting time?
The time that elapses from the beginning of mixing until the material hardens
What is the most effective and practical method for contorlling the setting time?
Addition of certain chemical modifiers to the mixture of plaster or dental stone
If the chemical added decreases the setting time it is known as an ______
Accelerator
If the chemical added increases the setting time it is known as an ______
Retarder
What is the crystallization process?
Outgrowth of crystals from nuclei of crystallization
When sufficient crystals form to produce the outward thrust by impingement, _____________ follows
Setting expansion
What is the most effective method for controlling the setting expansion?
Through the addition of chemicals by the manufacturer
How can setting expansion be reduced?
By adding either potassium sulfate, sodium chloride, or borax
What are the two types off gypsum?
Impression plaster (type I)
Model plaster (type II)
Dental stone (type III)
Dental stone, high strength (Type IV)
Dental stone, high strength, high expansion (Type V)
What is impression plaster composed of?
Plaster of paris to which modifiers have been added to regulate the setting time and setting expansion
**rarely used anymore for dental impressions
What is model plaster marketed as?
Natural white color, thus contrasting with stones that are generally colored
What is the strength of model plaster?
Relatively weak as evidenced by a compressive strength as low as 9MPA and a tensile strength of 0.6MPA
What stones are preferred for casts used to process dentures? why?
Type III; the stone has adequate strength for that purpose and the denture is easier to remove after processing
What surface is necessary for a die stone (dental stone, high strength type IV)? Why?
Hard; The cavity preparation is filled with wax that is carved flush with the margins of the die
What dental stone product exhibits an even higher compressive strength than type IV stone?
Type V stone
What is the setting expansion of type V?
Increased from a maximum of 0.10% to 0.30%
What is the rationale for the increase in setting expansino limits?
The newer alloys such as base metal have a greater casting shrinkage than do the traditional noble metal alloys
Thus higher expansion is required in the stone used for the die to aid in compensation for the alloy soldification shrinkage
What is maximum intercuspation?
All the teeth contact in a stable way
What is centric relation?
First contact when leaf guage is in place on the anterior teeth
Where is the condyle in centric relation?
Most anterior-superior position
