Download - Dental Materials Theory Gypsum Products
DENTAL MATERIALS THEORY
GYPSUM PRODUCTSZC. BSC. RDH
Objective # 4 a, b, c, d, e Ch. 16
Readings
Read Chapter 15 and 16 thoroughly
Watch videos!
How to take Alginate Impressions?
http://www.youtube.com/watch?v=ZfMt4Bq-Yjc
How to pour dental impressions?
http://www.youtube.com/watch?v=GIFzNsvGrpc
How to trim dental models?
https://www.youtube.com/watch?v=A9os3yy51U4
Gypsum
Gypsum is a material widely found in nature and
has been widely used in dentistry since 1756.
Dental casts and dies are used as replicas of the
hard and soft tissues of the clients oral cavity
Impressions are filled with gypsum powder that is
mixed with water to form a flowable mass
Once hardened, this will be the positive
reproduction of hard and soft tissues
Gypsum
These replicas are used to plan and track treatment
and laboratory procedures
Key Terms
Diagnostic casts are generally made from
dental plaster or stone and are used for patient
education, treatment, planning and tracking the
progress of treatment and example of this
would be orthodontic treatment
Working casts are made from dental stone that
is strong enough to withstand the stresses of
fabricating an indirect restoration or prosthesis.
Die Stone is the densest form of gypsum product
used in dentistry
Terms cont’d
Casts are hard, stable, positive reproduction of
hard and soft tissue of the client’s oral cavity made
from gypsum products in dental offices referred to
as dental models
Model plaster the weakest most porous form of
gypsum product used in dentistry
Dental stone the stronger less porous form of
gypsum product used in dentistry
Uses or gypsum products
Diagnostic casts aka study models – used to plan treatment and observe oral structures. Eg. Orthodontists use to plan and treat alignment of teeth.
Working casts aka working models – used to fabricate appliances such as ortho retainer, breaching tray, or removable prosthesis such as partial or full denture.
Dies are replicas of individual teeth or groups of teeth used to fabricate crowns and bridges.
Diagnostic Casts
Terms cont’d
Pouring this refers to the process of vibrating the
flowable gypsum product into the impression taken,
this produces a cast that is the exact replica of the
impression.
Trimming is the process of removing the excess
hardened gypsum from the cast
Impression
Impression
Properties and Behaviors of Gypsum
Products: Chemical Properties
Chemically the mineral gypsum is a dihydrate of calcium sulphate – mined as a solid mass
To produce a powder of calcium sulphate the dihydrate is heated to cause it to lose water and then ground into a powder – hemihydrate
When water is reintroduced with the powder, it is capable of flowing and pouring into the impression
When the chemical reaction is complete the hemihydrate is converted into the dihydrate and is a solid mass
The byproduct of the chemical reaction is heat also known as exothermic reaction
http://www.youtube.com/watch?v=tmbk8Pfau0I
CaSO4. 2H2O 1100-1300 C CaSO4. 1/2H2O
gypsum plaster or stone
(calcium sulfate dihydrate) (calcium sulfate hemihydrate)
Impressions
Physical Properties
Physically gypsum products are manufactured as
plaster, stone, high-strength stone and gypsum-
bonded investment.
The properties and behaviour determine the
specific use of the gypsum product.
Hardness and Strength
The main difference in the physical forms is
dependent on the variations of size, shape
and porosity of the powder
The larger more porous the particles of
powder are; the weaker and less resistant to
abrasion the final product becomes
Two factors that contribute to the strength
and hardness of the final product is the
shape and porosity of the particles and or
the amount of water used, more water the
weaker the cast, less water the stronger
Reproduction of Detail
The greater the porosity of the final gypsum
product the less surface detail is produced.
The compatibility of the impression material and the
die material can influence the surface detail that is
being reduced.
Follow the manufactures directions to ensure the
compatibility between products
Solubility
Gypsum products are not highly soluble in water
but -
Exposure to water for long periods of time should
still be avoided with gypsum products
Solubility is related to the porosity of the material
for example plaster is much more soluble than
stone.
Classification of Gypsum Products
The use of the final product will aid in determining
which gypsum product will be most suitable for
strength
Type I - Impression Plaster: rarely used and
replaced with less rigid elastic impression materials
Type II - Model Plaster: frequently used for
diagnostic casts and articulation of stone casts
relatively durable but weak in comparison to dental
stone, least costly, white in color.
Classificaton Cont’d
Type III - Dental Stone: ideal or denture making, ortho
models and casts requiring high strength, about 2.5
times stronger than plaster, usually yellow in color.
Type IV - Dental Stone, High Strength: referred to as
die stones because they are suited for fabricating wax
patterns of cast restorations, 2 times stronger than type
III and often pink or green in color
Classification cont’d
Type V - High Strength, High-Expansion Dental
Stone, this was created to respond to a need for an
even higher strength and high expansion stone, most
costly and blue or green in color.
Manipulation
The selection of the gypsum product is made based on the desired properties of the material and what the product is being used for.
The following steps are involved in the manipulation of the gypsum products:
1. proportioning – water /powder ratio
2. mixing
3. initial set time/working time
4. final set time
How to pour an impression:
http://www.youtube.com/watch?v=G_1NWimPF2w
Manipulation Factors
Factor Working Time Viscosity Strength
Increase W/P ratio Increase Decrease Decrease
Decrease W/P ratio Decrease Increase Decrease
Increase rate of
spatulation
Decrease Increase No effect
Increase temperature
of water
Decrease Increase No effect
Decrease
temperature of water
Increase Decrease No effect
Controlling of Setting Time
It is impossible to accelerate the final set of a mixture
without also accelerating the initial set, thereby
decreasing the working time.
1. altering the water/powder ratio
2. spatulation
3. accelerators and retarders
Specific instructions for pouring up impressions will be
reviewed before the skill is introduced in Materials Lab.
Pour Methods
Diagnostic and Working casts have two parts the anatomic portion and the base
Double Pour Method – two separate mixes, the anatomic portion is poured and left in the upright position, 10 minutes later the base is poured
Single Step Method – one mix, following the impression being poured the remaining material is used for the base
Boxing Method – a strip of wax is used to surround the impression forming a wall to pour the gypsum into.
http://www.youtube.com/watch?v=VxwxN3YLMr0
Storage
Gypsum products should be stored in air tight
moisture proof containers to avoid absorption of
water from the environment.
Humidity will adversely affect the powder it will
initially accelerate the setting reaction and after
prolonged exposure the setting reaction will be
slowed
Clean up
All mixing and handling equipment must be
meticulously kept clean
All bowls, spatulas, and glass slabs should be
cleaned immediately following use
All excess material should not be RINSED DOWN
THE DRAINS AS THE MATERIALS SETS AND CLOGS
THE DRAINS. Place all excess in the trash.
Infection Control
Disinfection of impressions prior to pouring is crucial, the left behind saliva, blood and bacteria may hinder the setting time of the gypsum.
If the impression has not been disinfected the equipment surrounding the pouring is contaminated and the operator is required to wear protective attire, gloves, glasses, mask
The casts once poured from a non disinfected impression should be set aside for 24 hours. Then they should be sprayed with a disinfecting solution
Separating the impression from cast
Upon setting the cast and impression must be
separated
Trim excess away from the periphery of the tray
and gently ease the lab knife under the impression
and lift the tray in several areas before lifting off
of the cast.
DO NOT ROCK THE IMPRESSION BACK AND
FORTH THIS RESULTS IN BREAKAGE OF THE TEETH
Trimming
To produce an attractive and symmetrical model with easy access to all sides of the model, the cast needs to be trimmed.
The bases on dental stone need to soak for about 5 to 10 minutes before trimming the anatomical structures should not be soaked
The cast should be trimmed so that the base is one third and the anatomical portion is two thirds of the total depth.
The occlusal plane should be parallel with the base
This will be demonstrated in the Materials Lab
Trimming
Trimming
Classification of Waxes (Cont.)
Processing waxes Boxing wax
Used to form a box around impressions of the mouth when a cast is made
Utility wax Comes in long ropes that are easily manipulated at room
temperature May be used with boxing wax to aid in the pouring of an
impression
Sticky wax Comes in orange sticks that at room temperature are hard
and brittle, but when heated under flame become soft and sticky
36
Manipulation of Waxes
Softening of wax should be done:
Evenly in dry heat
With warm hands
With a warm water bath
By flame
Store waxes at or slightly below room temperature
to avoid distortion
37
Review and KNOW!
Homework will be posted within the next 2 days
Must Read Procedures from page 292 to 299
Must know criteria for evaluation of poured diagnostic casts and evaluation of trimmed diagnostic casts
Questions?39