dental materials theory - unit 6

Alginate impressions

Alginate impressions

Are preliminary impressions that are irreversible hydrocolloid - used more often that any other single type of impression material

Primary use of alginate impressions

To create a positive reproduction (diagnostic casts/ study models) of an entire dental arch or a segment of it in either stone or plaster

Irreversible hydrocolloid

Hydro - water

Colloid - gelatinous substance: a material used to obtain preliminary impressions and final impressions

Different types of hydrocolloids

Can change the physical change from sol (solution) to gel (solid), which can be irreversible or reversible (changed by thermal factors)

Types of impressions taken in the dental office

1. Preliminary impression (alginate)

2. Final impressions

3. Bite registration

Preliminary impression

AN alginate impression - can be taken by DA II, DH, and DDS

• used to create a reproduction of the teeth and surrounding tissues

Final impressions

Different material, not water - can only be taken by DDS

• used to produce the most accurate reproduction of the teeth and surrounding structures

Bite registration

Can be taken by DDS or DH (wax bites can be taken by DA)

• taken to produce a reproduction of the occlusal relationship of the maxillary and mandibular teeth

Uses of alginate impression material/ items they assist in the fabrication of

1. Mouthguards

2. Study models

3. Temporary restorations

4. Orthodontic appliances (retainers)

5. Custom impression trays

6. Opposing models in fixed and removeable prosthetics

Advantages for taking preliminary impressions

• Low coast

• Easy to manipulate

• Flexibility in the set material

• Produces the necessary accuracy for the purpose

Disadvantages for taking preliminary impressions

• Susceptibility to atmospheric conditions

• Only used with gypsum products

• Provides limited definition of anatomy

Packaging and self life of alginate

Powder is packaged in bulk or pre-weighed individual containers

Deterioration of alginate impression materials

• Can occur rapidly at elevated temperatures and in the presence of moisture

• Material can become thin during mixing - results in reduced strength and higher permanent deformation when removed from mouth

• Do not store in warm cupboards and replace after one year (/ check expiry date)

• Close lid after dispensing

Ingredients in alginate powder composition

POWDER

1. Potassium Alginate

2. Calcium Sulfate

3. Trisodium phosphate

4. Diatomaceous Earth

5. Potassium titanium fluoride

6. Zinc oxide

7. Organic glycol

Use of potassium alginate powder

Powder in alginate composition - 20%

A thickening agent, aids in setting and allows material to dissolve in water

• made from seaweed

• works with calcium sulfate powder

Use of calcium sulfate

Powder in alginate composition

Reacts with potassium salt to form gel

Trisodium phosphate

Powder in alginate composition

• Retard - slow down gelation, controls setting time

Diatomaceous Earth

Powder in alginate composition - 50-60%

• filler which adds bulk and controls consistency

Potassium titanium fluoride

Powder in alginate composition

• controls interfering effect of alginate on setting and surface strength and controls pH

Zinc oxide

Powder in alginate composition

• adds bulk

Organic glycol

Powder in alginate composition

• disinfectant

Key component of alginate composition

Sodium phosphate reacts with → calcium ions forming → calcium phosphate (alginate)

• Sodium phosphate becomes a retardant

• Calcium phosphate becomes an irreversible hydrocolloid

• Hydrogel - Created when the calcium alginate that is formed is combined with water

  • An irreversible Hydrocolloid

Properties of dental materials that pertain to alginate impression material

1. Working and setting times

2. Water/powder proportions

3. Types of alginate

4. Strength

5. Syneresis and Imbibition

Working time

Time allowed for mixing the alginate, loading the tray, and positioning the tray in the client’s mouth

basically: Mixing time + loading time + seating time

Mixing time - time required to obtain smooth creamy consistency

Loading time - time required to load tray

Seating time - time required to seat tray in clients mouth (must be seated in before initial set occurs)

Setting time

time required for chemical action to be completed - after which impression is ready to be removed from clients mouth

• measured from beginning of mixing (when powder touches water) until gelation/ setting occurs

  • gelation must not be disturbed once it starts - impression will be unacceptable

• Occurs as a result of a chemical reaction that is controlled/altered by water temperature and water/powder ratio

Water/powder proportions

MUST BE EXACT - EFFECTS SETTING TIME

hot water - speeds up setting, inadequate working time

cold water - slows down setting, uncomfortable

Tooth much water - runny, can lead to gagging

too little water - too thick, no flowability

Types of alginate

Can be

Fast set - 45s mixing, 15s loading, 15s seating, total working: 1 ¼m
Regular set - 1m mixing, 15s loading, 45 seating, total 2m

Strength of alginate

A function of the rate which impression is deformed without tearing

• incorrect water/powder ratio can affect flexibility of set alginate

• Tear and compressive strength are increased if impression is left in the mouth 2-3 minutes after gelation

Pouring alginate

Should happen as soon as possible after mixing - material undergoes dimensional changes as time elapses

Synerisis

Refers to loss of water by evapouration due to expssure to the air - impression shrinks

Imbibition

Impression absorbs water and expands when place in water

Criteria for alginate impressions

1. Impression should be free of voids, air bubbles, and tears

2. All peripheral borders should be clearly defined

3. Anatomic areas should be clearly defined (teeth, tongue
   Max - frenum, rugae, palate, tuberosity
   mand - frenum, peripheral borders, retromolar pads
   
   

4. Uniform thickness without exposed tray surfaces.

   
   

What is required in obtaining an alginate impression

1. dispensing and manipulation of alginate

2. Selecting and preparing stock alginate trays

3. preparing client

4. manipulation

dispensing and manipulation of alginate

Correct water/ powder ratio for a good mix (keeping in mind the temperature of water)

2:2 - mand

3:3 - max

• line up in the middle

• seat posterior then anterior

• smooth creamy material

What is required in obtaining an alginate impression

Selecting and preparing stock alginate trays

Max -

• completely covers tuberosity (behind back teeth)

• covers anterior teeth

• enough width in molars

• alveolar process in molar region

Mand-

• covers all teeth and retromolar pad area

• Allow for centering teeth labially ad lingually in the tray

What is required in obtaining an alginate impression

Infection control

• gloves, mask, eyewear

• Rinse (cool water) saliva and debris off impression, disinfect and wrap in baggy

• Clean debris from bowl and spatula - disinfect and wrap in plastic, spatual in sterilizer

• Clean alginate from trays - dispose in biological waste

• Trays go in ultrasonic then sterilized