alginate lecture

8/2/2019 Alginate Lecture

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Using Irreversible Hydrocolloid and Gypsum

Materials to Fabricate Diagnostic Casts

Irreversible Hydrocolloid (alginate)

Making an impression

Gypsum Products (dental plaster & stone)

Making Diagnostic casts

Patient

Impression

Patient

replica

Articulator a mechanical instrument that represents the

temporalmandibular joint and jaws to which maxillary andmandibular casts are attached to simulate some or all of themandibular movements

Articulator

Articulate- 1. to join together as a joint 2.

the relating of contacting surfaces of the

teeth or their artificial replicas in the maxilla

to those in the mandible

What are Diagnostic Casts and Why do We

Make Them?

Diagnostic Casts: Artificial stone replicas of

the patients oral and dental anatomy used by

the dentist to:

Plan treatment for patients

Help fabricate devices needed in

rendering treatment

Keep a record of preoperative

conditions

Irreversible Hydrocolloid

Impression Material

Also called alginate impression material

Used to make dental impressions for:

preliminary impressions for complete dentures

partial dentures with clasps

orthodontic models and study modelsNot used for:

crown and bridge

impressions

(lack of accuracy)


Impression Material

Alginic acid / salt of alginic acid

Soluble in water


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Impression Material

Alginic acid is soluble in water andforms a sol

sol: resembles a solution, but ismade up of colloidal particlesdispersed in a liquid;


Impression Material

Setting Reaction

sol to gel transformation

Temperature controlled (reversible)

Chemically controlled (irreversible)

gel entangled framework of solid colloidalparticles in which liquid is trapped in theinterstices and held by capillary forces (Jello)


Impression Material

Reactants: sodium or potassium alginate (12-15%)

calcium sulfate dihydrate (8-12%)

Retarder: sodium phosphate (2%)

(sodium phosphate content adjusted by manufacturerto make either regular or fast-set alginate)

(we will use Jeltrate brand regular-set alginate in lab)

Reinforcing filler: diatomaceous earth (70%)(for strength and body)

(controls the stiffness of the set gel)

Potassium sulfate or alkali zinc fluorides (~10%)

(provides good surfaces on gypsum dies)

For esthetics: coloring and flavoring agents (traces)

Chemical Reactions

Powder is mixed with water to obtain a paste. 2main reactions occur when powder reacts withwater during setting:

First rxn. provides adequate working time: (Retarder)

2Na3PO4 + 3CaSO4 ----> Ca3(PO4)2 + 3Na2SO4

After the sodium phosphate has reacted, remaining calciumsulfate reacts with sodium alginate to form an insolublecalcium alginate, which forms a gel with the water:

H2O

Na alginate + CaSO4 --------> Ca alginate + Na2SO4(powder) (gel)

Manipulation

Generally easy to use

Powder (supplied in can) shaken up before use for aeration

Water to Powder (W/P) ratio to be carefully followed asspecified by the manufacturer

Lower W/P ratio increases strength, tear resistance, andconsistency; decreases working and setting times andflexibility

Lower water temperature increases working and settingtimes

Insufficient mixing produces a grainy mixture and poorrecording of detail

Adequate spatulation produces smooth, creamy mix withminimum of voids


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Definitions

Mixing time

Time to complete uniform blending of components

Working timeTime from beginning of the mix until the setting reaction

dominates

Setting time

Time from beginning of the mix until the setting reactionis over

Regular set alginate: 1 min, 2.5 min, 3.5 min

Fast set alginate: 45 sec, 1.5 min, 2.5 min

Problems with Alginate Impressions

Dimensional stability:

Because it is a gel, it undergoes shrinkage or expansionupon loss or gain of water.

Syneresis

Loss of water to the surroundings

Imbibition

Pick up of water from the surroundings

For least dimensional change/avoid distortion:

Store impressions in 100% relative humidity

Pour quickly after removal from mouth

Advantages of Alginate

Economical

Easy to use

Quick setting

Fair taste

Hydrophilic

Can displace blood and saliva

Stock trays

Disadvantages of Alginate

Limited detail reproduction

Low tear resistance

Single pour only

Quick pouring required

Low dimensional stability

Use of Alginate Impression Material Armamentarium Use of Alginate Impression

MaterialImpression Trays

Types:

Metal stock trays

Disposable plastic trays

Must have retentive features:

Mechanical - rim lock, perforated

Chemical - adhesive spray


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Use of Alginate Impression

Material Tray Selection:

Select the largest tray that will fit comfortably into thepatient's mouth

* Alginate requires greater bulk of material to produce themost accurate impression.

ADA specifications:


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Making the impression

Loading the traySwipe alginate intotray so as to notincorporate air

Wipe impressionmaterial ontoocclusal surfaces ofteeth (and into highpalate).


Seat the tray

Align the labialfrenum withfrenum space onfront of tray.Seat trayposterior toanterior. Havepatient relaxfacial musclesand extend lipsover edges oftray. Do NotOverseat.


Approx. 1 min. after stickiness vanishes, (about

3.5 min. from beginning of mix), remove tray

with one quick motion. (Teasing or wiggling set

impression from the mouth will cause excessive

distortion.)

Rinse, disinfect and store impression in humid

environment (can wrap in moist paper towel)

Pour with stone within 15 minutes

Alginate Impressions

maxillary mandibular

Troubleshooting

Inadequate working or setting time:temperature of the water, incomplete spatulation

W/P too low

improper storage of alginate powder

Distortion:tray movement during gelation or removed from mouth prematurely

weight of tray compressing or distorting alginate

impression not poured up immediately

Tearing:removing impression from mouth before adequately set

thin mixes (high W/P ratio)

presence of undercuts (blocking out these areas before an impressionmay help)

inadequate amount of impression material in tray (avoided by minimum3 mm of impression material between tray and oral tissues)

Loss of detail:removed from mouth prematurely

Troubleshooting

Consistency:preset mix is too thin or thick

the W/P ratio is incorrect (avoid by fluffing powder before measuring;

do not overfill powder dispenser)

inadequate mixing (avoided by vigorous spatulation and mixing for

recommended time)

using hot water: grainy and prematurely thick mix

Dimensional change:

delay in pouringalginate impression stored in air: results in distorted, undersized cast

due to alginate impressions losing water when stored in air

Porosity:whipping air into the mix during spatulation (proper mixing: after initial

wetting of powder by the water, mix alginate so as to squeeze the

material between the spatula blade and the side of the rubber bowl)

Poor stone surface (of cast)set gypsum remaining in contact with the alginate for too long a period of

time


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Gypsum Materials

Lab casting procedures

Diagnostic casts

Gypsum Materials

Dental gypsum products are available in 5

forms (ADA types I to V):

Impression plaster (Type I)

Model plaster (Type II)

Dental Stone (Type III)

High-strength dental stone(Type IV)

High-strength, high-expansion stone (V)

What is gypsum?

Gypsum is the dihydrate form of calcium sulfate,CaSO4 . 2H2O, found in a compact mass innature. Dental gypsum products are manufacturedby driving off part of the water of the calciumsulfate dihydrate to form calcium sulfatehemihydrate. This process is referred to ascalcination.

The gypsum components of these materials areidentical chemically; differences in these materialsis attributed to calcination.

Calcination

heat or

other means

Mineral gypsum ---------------- > Model plaster + Water

(CaSO4. 2H2O) Dental stone

High-strength dental stone

(CaSO4.1/2H2O)

Reverse reaction

When calcium sulfate hemihydrate (dental plaster,stone, etc.) is mixed with water, the reverse reactiontakes place, and the hemihydrate is converted backto the dihydrate:

CaSO4.1/2H2O + 11/2H2O ---> CaSO4. 2H2O + 3900cal/g mol

Heat is released in this reaction (exothermicreaction).

Gypsum Materials

While setting, growth and subsequent interlocking

of gypsum crystals occur. Interlocking contributes

to strength and dimensional change of the

gypsum.

Physical and mechanical properties of the gypsum

mass can be influenced by manipulative

procedures that influence the difference in

solubility and growth of the dihydrate crystals.


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Types of Gypsum Products

Dental Plaster

Type I - impressionplaster

Type II - model(laboratory) plaster (usedfor mounting casts)

plaster is composed ofthe form of calciumsulfate hemihydratecrystals

Crystals of model plaster.

p.337 Restorative Dental Materials.

9th ed. Ed. RG Craig.


Dental Plaster

plaster is weaker than dentalstone due to:

1.) porosity of the particles,requiring more water fora plaster mix

2.) irregular shapes ofparticles prevent themfrom fitting togethertightly

Crystals of model plaster.

p.337 Restorative Dental Materials.

9th ed. Ed. RG Craig.


Dental Stone

Type III - dental stone(diagnostic casts)

Type IV - high strengthdental stone (workingmodels)

Type V - high-strength,high-expansion dentalstone

Stone is thehemihydrate form

Crystal structure typical of dental

stone. p.337 Restorative Dental

Materials. 9th ed. Ed. RG Craig.


Dental Stone

- dense, regularly shaped,relatively nonporouscuboidal crystal material

- require less water, andare approx. 2.5 timesstronger than plaster

- stone is widely used inmaking casts and moldsrequiring high crushingstrength and abrasion

resistanceCrystal structure typical of dental

stone. p.337 Restorative Dental

Materials. 9th ed. Ed. RG Craig.


high-strength stone: (Type IV)

-harder cast material than dental stone

-composed of modified form of calcium hemihydratecrystals

-crystals are slightly larger and more dense than those ofdental stone

-because of increased strength and resistance to abrasion,are used for inlay, and crown and bridge casts

-may be referred to as die stones

high-strength, high-expansion dental stone: (Type V)

- with high expansion, it is especially suited for polyetheror polyvinyl impression materials

Working Model

Mark Margins

Separator

Composite Technique


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Gypsum materials

Principal difference between plaster, stone, and high-

strength stone is in the shape and form of thehemihydrate crystals. Crystals ofdental stone andhigh-strength dental stone are more dense and regularin shape. This makes it possible to obtain the sameconsistency with less excess water with the stones thanwith plaster.

If gypsum needs to be soaked in water (cast duplication),soaking should be done in water saturated with plasterslurry, only long enough to achieve desired degree ofwetting (otherwise gypsum can dissolve).

The greatest disadvantage of gypsum products isrelatively poorresistance to abrasion. One way toimprove abrasion resistance is by adding gypsumhardeners like colloidal silica or synthetic resin.

Differences in gypsum products

Many properties are either inversely or directly

related to the W/P ratio:

Directly proportional: manipulation and setting

times

Inversely related: strength and setting expansion

Differences in gypsum products

Because of their lower water requirement,

raw hemihydrate used to produce stones and

die stones have a higher inherent setting

expansion in normal mixes than plaster.

This effect is masked by the additives

used in their formulation.

Mixing gypsum materials

Hand mixing

Vacuum mixing (ideal;

results are better)

-Reduces porosity

-Increases strength

Largesmall


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Incremental pouring

Reduces bubble incorporation

Use dental vibrator to decrease bubble incorporation

As material is poured into impression, it is best to start at one distal end

(molar area) and allow material to flow into each tooth socket individually

around the arch to the other distal end

A small instrument may be used to tease bubbles out of material before it

sets

Scrape excess into trash then rinse

Do Not Clog Sink

2-Pour Technique

The 2-Pour technique involves pouring

gypsum material into the impression to

cover all surfaces of the impression and

allowing the initial pour to set prior to

adding more gypsum material to serve as a

base for the cast.

2-Pour Technique

1st Pour:

Place gypsum material in tooth areas first, thenproceed to cover the palate/tongue space areas

Place impression tray-side down onto a flatsurface, or hang to avoid distortion, and allow to

set for 20-40 minutes. (Remember that distortionof the cast can occur if the impression material isunsupported by the tray.)

You may want to leave small mounds of materialon top of the 1st pour to serve as retentive featuresfor 2nd pour.


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2-Pour Technique

2nd Pour:

Make sure initial pour is slightly moist

Mix material and place on a flat surface.

Invert the 1st pour on top of the 2nd mix

and incorporate around the 1st pour

material.

Allow to set 30 minutes and separate the

impression from the cast.


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Diagnostic Casts

(Evaluation Criteria)

All surfaces of the teeth,

gingival tissues &

artificial soft tissues

should be accurately

reproduced and free of

voids and nodules

The surfaces of the cast

(including the base)

should be hard, dense,

without voids and free of

any grinding sludge left

by the trimmer.

Diagnostic Casts


The side walls and land

areas of the cast should be

trimmed appropriately to

duplicate the shape of your

existing typodont model.

The base (bottom) of the

cast should be flat and not

rock when placed on a flat

surface. Teeth, soft tissues,

and land areas are to be

poured in Type III dental

stone; the base is to be

poured in Type II laboratoryplaster.

Diagnostic Casts


The base of the cast is

between 14 and 16 mm thick

from the mid-palatal or mid-

tongue space area. The

tongue space should be

trimmed flat and smooth

duplicating the typodont.

The casts should be trimmed

so that the occlusal planes are

parallel with the tabletop.

When placed on their backs,

the casts should articulateproperly.


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In 2 weeks

alginate lecture

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