Casts, models and dies

Casts, models and dies Dr. Judit Borbély 2015

MODEL OR CAST and DIE CAST: the positive likeness of a patient s upper or lower jaw DIE: small model of prepared teeth

CAST TYPES I. 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.

CAST TYPES II. PRIMARY CAST Made according to a primary impression in construction of every kind of dental appliances, a special tray is constructed onto it (after outlining the borders of the special tray at the dental office).

CAST TYPES III. SECONDARY CAST Made according to a secondary impression taken by a special tray. Gives more precise details about the hard and soft tissues.

CAST TYPES IV. SECTIONAL MODEL The 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 method Kiefer method Crystal method

CAST TYPES V. 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.

CAST TYPES VI. WORKING CAST -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 prefabricated wax elements

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.

Gingival mask Images from Dr. Peter Hermann and Zoltan Hajdu

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 used 2. Acrylic 3. Polyurethan

http://www.usc.edu/hsc/dental/resources/courseware/preclinicalprosth/assetts/gypsum.pdf

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

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

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

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 H2O Dihydrate hemihydrate Mineral gypsum: CaSO4 2 H2O - When the dihydrate is heated (110 and 130 o C), water of crystallization is removed dry β-hemihydrate - irregular particles plaster 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 CaSO 4. 1/2H 2 O + water CaSO 4. 2H 2 O

β-hemihydrate α-hemihydrate

http://www.usc.edu/hsc/dental/resources/courseware/preclinicalprosth/assetts/gypsum.pdf

Types of plasters and stones Type I.: Impression plaster History is used to make impressions of edentulous patients hard and brittle does not deform elastically, could not be used on dentulous patients because the undercuts of the teeth it has relatively short setting time and low expansion Plaster of paris

Types of plasters and stones Type II.: Laboratory or model plaster requirements are minimal because it is not used intraorally or for any precise operations to make study cast or mount stone cast in the articulator or in the denture flask it is stronger than type I.

Types of plasters and stones Type III.: Laboratory stone/ Dental stone common stone used to make casts of impressions it is harder and stronger than type II. diagnostic cast, opposing arch, RPD additives used in the formulation, the expansion 0,15 and 0,20 % dry strength is 62 MPa

Types of plasters and stones Type IV.: Die stone/ low expansion high-strength 79 MPa high hardness low expansion 0,08%

Types of plasters and stones Type V.: Die stone/ high expansion High-strength, high expansion die stone it is the result of need for dies with increased expansion (0,3%) to compensate for the greater shrinkage that occurs in many of newer, high-melting alloys used for dental castings

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 impression 3. 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.

3D printing of stereolitographic models

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