Fundamentals of tooth preparations - Semmelweis Egyetem · CASTING OF IMPRESSIONS II. the main...

Casts, models and diesCasts, models and dies

Dr. Judit Borbély2017

Information

Study cast -made according to a study

impression made previously

-should be mounted on an articuator

Besides revealing the shape of the hard and soft tissues, the mounted casts permit the cast’s relation to each other and occlusion to be studied in detail, even from lingual aspect. It helps to decide the patient’s treatment plan.

MASTER CAST

It is the cast what is first poured out with stone

after taking impression at the

dental office. Ready appliances are also

transported on it.

-a copy what is made from the master cast with the

help of duplicating material

-used for making the wax pattern for the metal framework of a removable partial denture on its surface with pre-fabricated wax elements

WORKING CAST

SECTIONAL MODELThe dies of the prepared

teeth or roots can be removed separately from the cast to make a very accurate or precise wax

pattern for making a fixed dental appliance.

Several methods are used for making sectional casts:

Pindex methodKiefer methodCrystal method

MODEL OR CAST and DIE

CAST: the positive likeness of a patient’s upper or lower jaw

DIE: small model of prepared teeth

DIES I. DIE: small model or cast with a base Produced from an impression (mainly

elastomeric types) of the crown or root surface of the tooth

Used in construction of fixed appliances (inlays, crowns, bridges)

Essential function of the die is to allow a restoration to be constructed in the laboratory which has an accurate internal or fitting surface.

DIES II. the base of the die provides a means of locating

the die within a larger localizing model which includes the other teeth on the arch adjacent to the prepared tooth.

the die can be removed and replaced in its original position

Materials for preparing dies:1.die stone: most widely used2. Acrylic3. Polyurethan

The desirable properties of model and die materials I.

accurate reproduction of all the details of the impression

sufficient strength and hardness to withstand work done upon them

ease of manipulation and production of model or die in a reasonable time

The desirable properties of model and die materials II. good colour contrast

with other materials suitable for use with all

types of impression materials

The most widely used model material is STONE ( derived from gypsum ).

Manufacturing

CaSO4 · 2 H2O CaSO4 · ½ H2O + 1 ½ H2ODihydrate hemihydrateMineral gypsum: CaSO4 · 2 H2O - When the dihydrate is heated (110 and 130

oC), water of crystallization is removed dry → β-hemihydrate - irregular particlesplaster of Paris, model stone autoclave → α-hemihydrate - more perfect crystals model stone, die stone When calcium sulphate hemihydrate particles are mixed with water, the

reverse of the above reaction occurs

CaSO4 . 1/2H2O + water CaSO4 . 2H2O

β-hemihydrate

α-hemihydrate

http://www.usc.edu/hsc/dental/Resources/Courseware/preclinicalprosth/assetts/Gypsum.pdf

CASTING OF IMPRESSIONS I. during casting all impressions must be handled with great care to

avoid damage and weakening of the cast impressions taken in compound, zinc oxide paste, alginate or

elastomers do not require treating with separating medium, stone does not stick to them

plaster impressions must be painted with with a soap or detergent solution (waterglass)

all equipment used for mixing the stone must be clean (any material remaining from the previous mix in the mixing bowl must be removed, as it may impair the surface of the cast as well as accelerating the setting time)

the water-stone ratio should be that advised by the makers of the particular stone being used

CASTING OF IMPRESSIONS II.

the main problem in casting impressions is the exlusion of air from the mix since its presence weakens the cast

three methods may be used to eliminate as much air as possible:1. mechanical spatulation (for at least 30 seconds till consistency is smooth and creamy in a flexible bowl with a stiff bladed spatula)2. vibration table: during filling up of the impression vibrating is recquired until the stone fills all the deepest parts of the impression3. vacuum mixer

Disinfection of stone casts or dies Although the impression itself should be

disinfected before being handled further by the dental office staff or the laboratory technicians, whenever there is a question about whether or not this has been done, the stone cast or die also can be treated.

Soaking the gypsum, iodophor, glutaraldehyde, phenol.

In addition, stone materials are available that contain disinfecting agents.

Pindex-method

Pindex-method

http://www.usc.edu/hsc/dental/Resources/Courseware/preclinicalprosth/assetts/Gypsum.pdf

Kiefer method

Images from: Dr. Peter Hermann, Zoltan Hajdu: Sectional casts

Crystal method

Images from: Dr. Peter Hermann, Zoltan Hajdu: Sectional casts

Gingiva mask

Images from Dr. Peter Hermann and Zoltan Hajdu

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