2 Alginate & Gypsum

why are mounted diagnostic casts on an articulator important?

• complement tool in addition to clinical exam

• legal proof of pt’s current occlusion

• allow clinician to

  • modify occlusal pre-treatment plan prior to establish definitive Tx plan

  • evaluate occlusion in centric and eccentric positions

  • determine inter-arch distance, teeth vs alveolar ridge

  • make alteration without pt being there physically

what materials are most commonly used to make impressions?

Hydrocolloids and synthetic elastomeric polymers

what is alginate made from?

cell wall constituents of brown algae (chemistry involves salt of alginic acid Na or Ca alginate)

what can be evaluated with casts of the mouth?

• relative alignement between maxillary/mandibular arches

• occlusal relationship

• fabrication of restoration and prostheses

The impression material is usually carried to the mouth by means of …?

a tray (plastic or metal; pre-fabricated or individual)

what are desireable qualities to consider when picking impression materials?

• Pleasant odor, taste, and color

• Absence of toxic or irritant constituents Adequate shelf life

• Economic

• Easy to use

• Good setting characteristicsSatisfactory consistency and texture

• Readily wets oral tissues

• Resistance to permanent distortion

• Adequate strength

• Dimensional stability over temperature and humidity normally found in clinical and laboratory setting

• Compatibility with cast and die materials

• Accuracy in clinical use

• Readily disinfected without loss of accuracy

• No release of gas during the setting of the impression or cast and die materials

what are different types of impression materials?

• Alginate hydrocolloid (A.K.A. irreversible hydrocolloid)

• Agar hydrocolloid (A.K.A. reversible hydrocolloid)

• Elastomeric materials

• Zinc oxide-eugenol materials

• Gypsum

• Compound impression materials

what are the most widely used impression material in dentistry?

Alginates

what are some advantages of alginate hydrocolloids?

• hydrophilic

• accurate if handled properly

• inexpensive

• easy manipulation

• pleasant taste

• able to displace blood and body fluids

• easily pourable

• can use with stock trays

what are some disadvantages of alginate hydrocolloids?

• dimensionally unstable (syneresis vs imbibition)

• low tear resistance

• must be poured immediately

• limited detail reproduction

• can only be used for a single cast

• incompatible w epoxy resin die materials

what 2 main chemical reactions occur when alginate hydrocolloid powder is mixed with water to obtain a paste?

Na3PO4 = retarder (delays reaction so we have more time to work w it)

CaSO4 = active ingredient

describe the process of mixing alginate and water.

1. powder lightly shaken for aeration

2. add water to mixing bowl first

   1. 1 scoop of powder used for 1 measure of water

3. 1 minute of thorough mixing for regular setting

   1. 45 seconds for fast setting

4. set time = 3.5 minutes

1 scoop of alginate powder is __ g

16

1 measure of water is __ ml

38 ml

what happens if you use a lower water to powder ratio?

• increased strength

• increased tear strength

• “better” consistency

• decreased working time

• decreased setting time

• decreased flexibility

what happens if you use a higher water to powder ratio?

• decreased strength

• decreased tear strength

• runny consistency

• increased working time

• increased setting time

• increased flexibility

how does cold vs hot water affect alginate?

colder water increases working/setting times

t/f: Insufficient mixing results in a grainy mix and poor recording of details

true

Disinfection can be accomplished by immersion in …?

sodium hypochlorite or iodophors

If needed, impression can be stored in 100% humidity for __ minutes prior to pouring

30

Storage in either air (syneresis) or water (imbibition) results in significant dimensional change. what is syneresis? water is imbibition?

Syneresis: Loss of water due to dehydration that causes shrinkage

Imbibition: Tendency to absorb water that results in swelling

what are some common problems that are related to inadeqaute working or setting time?

• temperature of mixing water (ideally 65-75 F)

• incomplete spatulation

• incorrect water-powder ratio

• improper storage of alginate powder

what are some common problems that occur with impressions?

• distortion

• inadequate working/setting time

• tearing

distortion can occur due to…?

• movement of the tray during setting of the material

• premature removal of impression

• improper storage of impression after removal

• length of time between impression making and pouring

• Delayed cast separation

tearing can occur to impression due to…?

• premature removal from the mouth

• speed of impression removal

• incorrect water-powder ratio

• undercuts

• not enough material

Consistency is related to what 3 factors?

(1) water-powder ratio,

(2) water temperature,

(3) spatulation

Porosity can be related to incorrect ________

spatulation

Poor stone surface is related to an extended period of contact between the set gypsum and the alginate (separation should occur between ___ minutes after pouring)

45 to 60

describe infection control guidlines for any impression

in operatory:

1. Rinse impression using cold running water to remove blood and debris

2. Rub impression using Cavi Wipe

3. Wrap it with Cavi-Wipe and place in plastic bag

in laboratory:

1. Place impression on a paper towel

2. Spray with Cavicide in 4th Fl Lab and leave it for 3 mins

3. Rinse impression using cold running water

4. Pour impression with Type III or IV dental stone

should alginate impressions be boxed?

no!

what happend?

left out too long

is this a good impression?

yes (9 out of 10)

Gypsum products generally refer to the various forms of …?

calcium sulfate (hydrous and anhydrous)

gypsum products are manufactured by the calcination of …?

calcium sulfate dihydrate (CaSO 4 . 2H 2 0)

OR

calcining “synthetic” or “chemical” gypsum, a by-product of the manufacture of phosphoric acid

how are gypsum products classified?

by International Organization for Standardization (ISO)

Both plaster and stone are products of…?

partial dehydration of gypsum (Differences in properties result from differences in the physical nature of the powders)

Traditional hemihydrate plaster is produced by what process?

dry calcination (sometimes refer to Beta Hemihydrate)

Medium- and high- strength plasters (stones) manufactured by wet calcination have a stronger/weaker set mass (sometimes refer to Alpha Hemihydrate)

stronger

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