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