working casts and dies BDS 8187.pdf

Is the positive reproduction of the prepared teeth, ridge areas and other parts of the dental arch , obtained from pouring the final impression Is the positive reproduction of the prepared tooth and consists of a hard substance with sufficient accuracy (usually stone, resin or metal). 1- It mus...

Is the positive reproduction of the prepared teeth, ridge areas and other parts of the dental arch , obtained from pouring the final impression Is the positive reproduction of the prepared tooth and consists of a hard substance with sufficient accuracy (usually stone, resin or metal). 1- It must reproduce both prepared and unprepared tooth surfaces and should be free of any voids or defects 2- All soft tissues should be reproduced in the working cast, including all edentulous spaces and residual ridge contours that will be involved in the fixed prosthesis 1- Finish line complete 2- The remaining unprepared tooth structure immediately cervical to the finish line should be easily detected on the die, ideally with 0.5 to 1 mm visible to adjust the contour of the tooth & allow margin adaptation I- Ideal time for impression pouring based on the impression material used Agar agar: should be poured as soon as possible Dimensionally unstable; undergoes synersis & imbibition Polysulfide and condensation silicone: should be poured within one hour Poor dimensional stability; due to loss of water or alcohol as a byproduct Addition silicone and polyether: Pouring can be delayed up to 7 days and multiple pours can be done Excellent dimensional stability; no byproduct from the polymerization reaction  High-strength stone (type IV) or high-strength, high-expansion stone (type V) (die stone) should be used for pouring the impression as the die and cast should have a hard surface to prevent surface abrasion when the wax pattern is fabricated GYPSUM PRODUCTS: Type I: Impression Plaster Type II: Model plaster Type III: Dental stone Type IV: High strength dental Stone Type V: Improved Stone, Die Stone 1- Follow the manufacturer’s instructions for the correct water/powder ratio 2- Mix water and stone by hand with spatula until the powder is completely wet then use vacuum. 3- Impressions should be completely dry and the surface should be free of visible water 4- Apply surfactant on the impression by spraying to reduce the number of voids trapped in the cast 5- Place a small amount of stone above the preparation and the impression is vibrated until stone reaches the “bottom” (incisal or occlusal surface) of the preparation 6- Add stone in small increments. If a large amount of stone is dropped, air will be trapped and voids will result in the cast. 7- Build the stone up to a height approximately one inch over the preparation 8- Add small increments of stone to the distal end of one side of the impression, then slowly raise the distal end of the impression, so that stone will move mesially flowing from tooth to tooth and filling each of them from the bottom. 9- Add more stone and vibrate until all the teeth in the arch are filled The working cast and the sectional cast (separate die) can be obtained from separate impressions or by pouring the impression twice  If a double pour is used, the first pour is used for fabrication of the die.  The double pour can be used only with elastomeric impressions because hydrocolloid is torn and distorted too much to be used for an accurate second pour  Full arch cast for adjusting proximal contact and occlusion Sectional cast (Separate die) for wax coping and margins  Advantages: Ease of fabrication Keep the relationship between abutments fixed and immovable Disadvantages: Fragile wax patterns are difficult to transfer between the two parts (from cast to die), so distortion of some of internal adaptation will occur The second pour of the impression may be different (slightly larger) than the first, therefore, it may be necessary to relieve the stone slightly to seat the pattern One die is used for proximal contacts, occlusion and margins o Dies must return to their exact positions on the cast. o Dies must remain stable even when inverted o Dies should be accurately related to adjacent and opposing teeth  Advantages No need to transfer wax pattern from dies to working cast. Eliminate errors between separate die & working cast caused by impression distortion between pours. Disadvantages The risk of introducing an error if the die doesn’t reseat accurately in the working cast. Debris may prevent complete reseating of the dies. A. Pindex System The Pindex machine drill parallel holes from the underside of the trimmed cast by using a reverse drill press, creating a master cast with dies that can be removed & replaced repeatedly with great precision. 1) The cast is trimmed to remove all rough & irregular areas 2) Mark the location of each pin on the occlusal surface. Two pins are needed (Long pin facially / short pin lingually) 3) The cast is put on the table and a light indicates the location of the drill ,then the cast is held firmly 4) Press the table downwards which activates the drill that penetrates into the cast. 5) Try in the pins & cement them to place using cyano-acrylate 6) Position the plastic sleeves (white sleeves for long pin / grey sleeves on the short one) and wax is applied to the sleeve end to facilitate removal of the dies 7) Coat the bottom of the cast with separating medium and place the cast in a special former to form the base. 8) Section the dies by using cutting saws. It is a snap apart three pieces plastic tray with internal orienting grooves & notches used to reassemble the working cast & dies 1) Pour the full arch impression with die stone, restricted to u-shaped arch i.e. with no stone on the center. 2) After setting , trim the buccal & lingual side to fit in the di-lok tray , then one or two horizontal grooves are placed on the inner & outer aspect of the cast to hold the cast in stone in the tray. 3) Mix stone and put it into the tray then seat the cast into the tray so that the cervical lines of the teeth are 4mm above the edge of the tray. 4) After stone hardening, disassemble the tray. 5) By using a saw blade , cut between the prepared tooth & the adjacent one till approximately 3/4 the way of the stone base then by finger pressure break the die from the cast. 6) Reassemble the dies & other parts of the cast in the tray Uses special plates which are aligned and drilled to provide removable dies. 1) Trimmed impression is placed on the alignment fixture and a clear plate is positioned over it 2) Dowel pin locations are marked on the clear plate. 3) Holes for dowel pins are drilled as marked on the plate. 4) Dowels are inserted in the baseplate. 5) Impression is poured and stone is placed around the dowel pins and the plate is replaced over the poured impression. 6) After stone setting , cast is removed from baseplate and trimmed. 7) Sectioning of the dies is done by using cutting saw. only one pour fast simple and easy method for producing dies and model I-Ditching It is a circumferential groove 0.3mm in depth below the finish line Function: Accentuate finish line , therefore, allow better wax pattern construction The finish line should be colored with red pencil without any pressure to facilitate carving the margin accurately. Painting the die with a die spacer (20-40µm in thickness) to: o Provide space for cement o Allow complete seating of restoration during cementation. A band of 1mm must be left unpainted at die margin to increase margin adaptation. Aims: The educational aims of this lecture are: To explain when and how to pour impressions to produce good quality working casts To detail different techniques of producing dies Objectives: On completion of this lecture, the student should have: An understanding of how to pour impressions (within recommended times) An understanding of how to produce a die using different techniques  Fundamentals of fixed prosthodontics, Herbert T. Shillingburg, 2012 (chapter 18) Fixed prosthodontics, principles and clinics, H.W. Anselm Wiskott, 2011 (chapter 16, 18) Contemporary fixed prosthodontics, Rosenstiel, Land, Fujimoto, 4th edition, 2006 (chapters 2, 17)