جامعة تكريت كلية طب االسنان مادة معاجلة االسنان املرحلة اخلامسة أ.م. هدى عباس عبد اهلل

Transcription

1 جامعة تكريت كلية طب االسنان مادة معاجلة االسنان املرحلة اخلامسة أ.م. هدى عباس عبد اهلل

2 Lec.8 Crown and bridge Working Cast and waxing د.هدى عباس Working cast It is a replica of the prepared tooth (teeth), ridge area & the other part of the dental arch. It is produced from the final impression, on which we will make the wax pattern (Lab. work) of the final restoration. Requirement of the good working cast:- 1. Must be free from the air bubbles especially at the area of the gingival finish line & the occlusal surface. 2. Must reproduce all details that have been captured by the impression. 3. Must be free from the any distortion. 4. It should be trimmed to ensure access for the carving wax pattern (at the margin). Die:- It is the (+ve) reproduction of the model of the individual (single) prepared tooth on which we will do wax pattern (its produce from the final impression). Requirement of the dies:- 1. Must be reproduced the preparation exactly. 2. Must be free from the air bubbles. 3. Must return to its exact position on the cast when it removed. 4. Must be stable, even when the cast inverted. Type of the dies:- 1. Silver plated die 2. Copper plated die. 3. Amalgam dies. 4. Stone die. The two critical properties of the die material are the dimensional stability or accuracy & resistance to abrasion, during construction of the wax pattern, are adequately achieved by the gypsum material (die stone). Advantages of the stone dies are 1. Easy to be prepared. 2. Can be used with all type of impression material. 3. Cheep. 4. Need less requirements and easy to manipulate.

3 Types of dental gypsum products (ADA specification) 1. Type І: impression plaster 2. Type ІІ: model plaster 3. Type ІІІ: dental stone 4. Type ІV: high strength dental stone (die stone) Types of die: 1. Working cast with separated dies: the final impression is poured twice to obtain two working cast, one of these cast is sectioned to obtain the individual dies that are separated from the cast, while the other working cast is articulated on the articulator. 2. Working cast with removable dies: the final impression is poured for one time only to construct a working cast that involves removable dies. a. Straight dowel pin b. Curved dowel pin c. Pindex system d. Di-lok tray Construction of a stone die ( Working cast with removable dies) Dowel pins: these are ready-made metal pins which are used as a means of orienting the die to the original working cast, which allow the dies to be easily removed and accurately replaced into the working cast. The dowel pin is tapered and cylindrical with one flat side for positive seating. Clinical procedure : 1. dry the impression 2. A dowel pin (D.P.) is used for each prepared tooth. it is placed over the center of the prepared tooth parallel to its long axis. A bobby pin is used to hold the D.P. in this position by placing it between its arms. The bobby pin is positioned buccolingually across the impression so that the dowel pin is centered directly over the prepared tooth. Then a straight paper pin is inserted between the arms of the bobby pin and into the impression buccaly and lingually. The D.P.is then stabilize within the bobby pin and the bobby pin itself against the straight pin with sticky wax

4 3. the impression is placed over the vibrator and dental stone is added in small increments to about 2 mm above the cervical margin. The dental stone should cover the serrated end of the dowel pin. 4. A retentive means is placed in the poured stone before its setting such as paper clips to provide retention to the second layer of the stone that is going to be poured later 5. When the first layer of the stone has set, the bobby pin(pins) and the paper pin(pins) are removed from the impression. A ball of soft wax is placed on the tip of each dowel pin 6. The surface of the first layer of the stone is lubricant with a separating medium and a second layer of stone is poured (base) that should cover the dowel pin completely. 7. After complete setting of the second layer of the stone, the cast is removed from the impression. Then using a sharp knife the wax ball which is placed on the tip of each D.P. is removed. 8. A saw is used to sectioned the proximal sides (mesial and distal) of each prepared tooth bucco-lingually to obtain the die. The cutting should be through the first layer only, and the cutting should be diverge toward the occlusal surface to facilitate the removal of the die. 9. The end of the D.P. is tapped gently with hand instrument to loosen the die. Waxing and Spruing Ditching: - is the trimming of excess stone gingivelly to the margin of the preparation. So that making the margin clear, it is done by using sharp hand instrument. Lost wax technique The process consist of surrounding the wax pattern with mold of heat resistance investment material, eliminating the wax by the heating, and introducing molten metal in to the mold cavity through channel called ( sprue). Wax pattern: it is the precursor of the final cast restoration were the wax placed on the prepared tooth, it should be clear, smooth, should be duplicate accurately the final anatomical feature of the original tooth & should be free from any debris. The information needed to duplicate the anatomical feature is taken from the adjacent teeth, the opposing teeth &from the general knowledge of the dental anatomy.

5 Wax is used to construct the wax pattern because: I) It is easy to manipulated. II) Inexpensive (cheep). III) Easily to eliminate it from the mold cavity during the burn out procedure. The type of the wax used in the construction of the wax pattern is inlay casting wax. According to the ADA we have two types of the inlay casting wax:- 1) Type one I inlay casting wax:- It is used for the intraoral waxing technique, it has higher melting temperature 2) Type II inlays casting wax:- It is used for the extra oral waxing technique and has lower melting temperature; it is used to construct the wax pattern on the cast. Requirements of good inlay casting wax:- 1. It must flow readily when heated without chipping. 2. It must rigid when cooled. 3. It must be capable of carving without chipping or smearing over when it is done over the fine margin. Techniques of construction of wax pattern (w. p):- 1. Intra oral technique (direct technique). In which the w.p. is constructed inside the patient mouth using type I inlay wax, mostly we use this technique in construction of the posterior inlay restoration & anterior post-crown. 2. Extra oral technique (indirect technique). Here we use type II inlay wax to construct the w. p. on the die of the die of the working cast. The advantage of the indirect over the direct tech.:- 1. It reduces the chair time so it saves the dentist time & more comfortable to the patient. 2. It offer opportunity for the visualization of the restoration & readily (direct) access to waxing the margin. 3. It is probably the easiest mean of fabricating cast restoration.

6 Steps in waxing procedure: 1. Cover all the preparation in one time with molten wax to make a coping of wax this can be obtained by immersing the preparation in the dish contain molten wax, so now obtain the 1 st layer of the wax pattern. 2. Add wax to the proximal surface (wall) of the restoration in relation to the adjacent teeth (to build the contact area in relation to the adjacent teeth). 3. Build the axial wall (buccal & lingual) to the normal counter. 4. Build the occlussal surface of the restoration following ( in relation ) curve of spee & curve of Wilson, then check & adjust the occlussal relation with opposing teeth in centric & eccentric relation 5. Check again all the margins of the restoration (w.p) to ensure that the margin of the (w. p) have no over or under extension, so the wax pattern is now ready for the next step. Sprue It is small diameter pin made of wax, plastic or metal, one end of it attached to the wax pattern while the other end attached to the crucible former. It provide a channel after burn out procedure to act as inlet for the gold which is forced in the mold cavity. Requirement of the sprue 1. It must allow the molten wax to escape from the mold cavity. 2. It must allow the molten metal to flow into the mold cavity with as little turbulence as possible. 3. The metal within the sprue must remain molten slightly longer than the alloy that has filled the mold. This will provide a reservoir to compensate for the shrinkage that occurs during solidification of the metal casting.

7 Dimensions and location of the sprue Diameter : the diameter of the sprue should be as large as possible to improve the flow the molten metal into the mold cavity and to provide a reservoir to compensate for the shrinkage of the metal during solidification. Length: the length of the sprue should be adjusted so that when we attach it to the crucible former the margin of the wax pattern should be about 6 mm away from the end of the casting ring. it is made so that the wax pattern will be in the center of the casting ring and surrounded by a uniform thickness of the investment material. Location: the attachment of the sprue with the wax pattern should be to the bulkiest area of the wax pattern and should be at an angle to allow the incoming metal to pass freely to all portions of the mold cavity without any turbulence. The attachment should be at the non-centric area.