Lecture 4 Alginate and Stone Flashcards
what is meant by accuracy in impression materials
ability to replicate the intraoral surface detaisl
what is meant by dimensional stability in impression materials
ability to retain its absolute dimensional size over time
what is meant by tear resistance in impression materials
ability to resist tearing in thin sections
what are the 2 classes of impression material
elastic and non elastic
what are the classes of elastic material
hydrocolloids and non aqueous elastomers
what are the classes of hydrocolloids
agar reversible and alginate irreversible
what are the non aqueous elastomers
-polysulfides
-polyethers
-condensetation silicone
-addition silicone
what are the non elastic classes
-plaster (gysum)
-impression compound
-zinc oxide eugenol
-impression waxes
when was alginate made
as a substitute for the agar impression material when its supply became scarce during world war 2
what is alginate based on
a natural substance extracted from brown seaweed called anhydro-B-D-mannuronic acid or alginic acid
what are the principle factors responsible for success of alginate
easy to manipulate
-comfortable for the patient
-relatively inexpensive
what are the chief active ingredients of the irreversible hydrocolloid impression materials
sodium, potassium, or triethanolamine alginate
what is the composition of alginate with respective function
-potassium alginate- 15% - soluble alginate
-calcium sulfate - 16% - reactor
-zinc oxide - 4% - filler particles
- potassium titanium fluoride - 3% - accelerator
- diatomaceous earth - 60% - filler particles
- sodium phosphate - 2% - retarder
describe the typical sol-gel reaction
a reaction of soluble alginate with calcium sulfate and the formation of an insoluble calcium alginate gel
what can modifying the W/P ratio or mixing time have on alginate
effects properties of the gel, the tear strength, and the elasticity
how can you safely influence setting time
altering the temperature of the water
what is the polarity of alginate impression material
hydrophilic
what are the steps in making an alginate impression
- measured powder is sifted into a clean rubber bowl
- powder incorporated into water by mixing with metal or plastic spatula- mixing time about 45 seconds to 1 minute
-result from mixing should be a smooth creamy mixture that does not readily drip off the spatula when it is raised from the bowl
-before seating the material should have enough body so it will not flow out of the tray and gag the patient - mixture is placed in a suitable tray which is then placed in the mouth, ensuring the material adheres to the tray
- tray must fit patients arch so there is enough thickness of material - thickness between tray and the tissue should be at least 3 mm
when is the tear strength increased in alginate hydrocolloid
when the impression is removed with a snap
how do you disinfect impressions
soaked paper towel/plastic bag for 10 minutes
is VPS affected by disinfection
no
when do hydrocolloids expand
after 10 minutes
when do polysulfides and polyethers expand
with increased disinfectant exposure time
what are some current digital impression systems
-CEREC, omnicam (sirona)
- itero element (align technology)
- LAVA true definition scanner (midmark
what is gypum made of chemically
pure calcium sulfate dihydrate
what is gypsum dental investment
when plaster is mixed with fillers such as different forms of silica
what is the principle constituent of gypsum based products such as plasters and stones
calcium sulfate hemihydrate
what is the colloidal theory
when plaster is mixed with water, plaster enters into the colloidal state through a sol-gel mechanism
what is the hydration theory
rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material
what is the most widely accepted mechanism of setting gysum and what is it
dissolution precipitation theory- based on dissolution of plaster and instant recrystallization of gypsum followed by interlocking of the crystals to form the set solid
how does the setting reaction of gypsum occur
-dissolution of calcium sulfate hemihydrate
- formation of a saturated solution of calcium sulfate
- aggregation of less soluble calcium sulfate dihydrate
-precipitation of the dihydrate crystals
what is mixing time defined by
the time from the addition of the powder to the water until the mixing is completed
how long does it take to mix stones and plasters
20-30 seconds
what is working time defined as
the time available to use a workable mix, one that maintains a uniform consistency to perform one or more tasks
what is setting time defined as
the time that elapses from the beginning of mixing until the material hardens
what is the most effective and practical method for controlling the setting time
the addition of certain chemical modifiers to the mixture of plaster or dental stone
what is the chemical called if it decreases the setting time
an accelerator
what is the chemical called if it increases the setting time
a retarder
what is the crystallization process
an outgrowth of crystals from nuclei of crystallization. on the basis of entanglement of the dihydrate crystals, crystals growing from the nuclei can intermesh with and obstruct the growth of adjacent crystals
when sufficient crystals form to produce the outward thrust by impingement ____
setting expansion follows
what is the most effective method for controlling the setting expansion
through the addition of chemicals by the manufacturer
how can the setting expansion be reduced
by adding either potassium sulfate, sodium chloride or borax
what are the types of gypsum
-impression plaster (type 1)
- model plaster (type 2)
- dental stone (type 3)
- dental stone, high strength (type 4)
-dental stone, high strength, high expansion (type 5)
what is impression plaster type 1 made of
composed of plaster of paris, to which modifiers have been added to regulate the setting time and the setting expansion
describe model plaster type 2
-usually marketed in the natural white color
- relatively weak with compressive strength as low as 9 MPa and a tensile strength of 0.6 MPa
when are type 3 stones preferred and why
for casts used to process dentures because the stone has adequate strength for that purpose and the denture is easier to remove after processing
when do you use type 4 gypsum
a hard surface is necessary for a die stone because the cavity preparation is filled with wax that is carved flush with the margins of the die
describe the compressive strength of type 5
higher than type 4
describe the setting expansion of type 5
higher than type 4
what is maximum intercuspation
all the teeth contact in a stable way
what is centric occlusion contact
first contact with leaf gauge is in place on the anterior teeth is in posterior teeth
what is centric relation
condyle is in the most anterior superior position
