Working Casts & Dies PDF

Working Casts & Dies Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry o Inconvenient, difficult, time consuming, and impossible to make all wax patterns directly in patient’s mouth o Master cast ( Working cast )  The requirements of a cast and-die system  The available mate...

Working Casts & Dies Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry o Inconvenient, difficult, time consuming, and impossible to make all wax patterns directly in patient’s mouth o Master cast ( Working cast )  The requirements of a cast and-die system  The available materials and selection criteria  The available methods and die systems  Mounting casts on articulators  Definitive Casts versus Diagnostic Casts  Technique for closed mouth impression The requirements of a cast for the fixed restoration : 1. Prepared and unprepared tooth surfaces 2. Adjacent to the preparation (free of voids) 3. Teeth in anterior guidance and articulation of the opposing casts 4. All edentulous spaces involved in the fixed prosthesis  The requirements of Die for the fixed restoration 1.Reproduce the prepared tooth exactly 2. No bubbles or voids 3. 0.5 to 1 mm visible unprepared tooth structure immediately cervical to the finish line 4.Adequate access to the margin  1) Gypsum Dimensional accuracy Resistance to abrasion Inexpensive Easy to use Consistent results  Gypsum Gypsum's greatest disadvantage is its relatively poor resistance to abrasion to overcome this: 1. "gypsum hardeners" E.g. Colloidal Silica little effect on the hardness of the stone improve abrasion resistance ( by 100%) slight increase in setting expansion(not clinically Significant) 2. Impregnate the die in a low-viscosity resin such as cyanoacrylate Improved abrasion resistance Care must be taken to the resin film thickness  3. Apply additives used in industrial applications 4. Use of a Arabic gum, calcium hydroxide mixture 5. Resin-strengthened gypsum products such as ResinRock,' with high strength and low expansion Types of dental stone  2. Resin Smorescuant (Most available)epoxy resins Polyurethane High strength and abrasion resistance More expensive , polymerization shrinkage Detail reproduction is better Prostheses fabricated on resin dies tend to fit more tightly than those made on gypsum ?? Certain impression materials (e.g.polysulfide and hydrocolloid) not compatible with resin  3. Electroplated Dies A. A coat of pure silver or copper powder (on the impression) B. Placed in an electroplating bath C. A layer of pure metal is deposited on the impression then supported with type IV stone or resin  Disadvantages of Electroplated Dies Distortion Technique sensitive Time consuming ( 8 hours ) Can’t be used with polyether and silicon impressions Toxicity ( cyanide solution )  Flexible Die Materials Heavy bodied silicone or polyether impression materials Used to make interim restorations or indirect composite resin inlays or onlays in chairside More rapid setting and easy to remove restoration or inlay.  Selection criteria for cast and die system 1. Dimensionally accurate cast ,strong and resistant to abrasion 2. Easy to section and to trim 3. Compatible with the separating agent and impression 4. Surface detail accurately 5. Available in a color 6. Wettable by the wax 7. The type of restoration needs ( all ceramic stronger die ) Die Materials  Available Die Methods Removable dies : The die is an integral component of the definitive cast and can be lifted from the cast to facilitate access The die separate cleanly and return to place accurately  Solid cast with individual die (multiple-pour technique): 1st pour for the area of the preparation only ( most accurate , Die) 2nd pour is for the entire arch (mounted on an articulator ) 3rd pour for the solid cast ( intact gingival tissues around the prepared teeth, used as a guide in contouring the restorations ) The wax pattern is started on the initial pour (the die) and is then transferred to the articulated cast for refinement of axial contours and occlusal anatomy  Available Die Methods Solid cast with individual die (multiple-pour technique): Advantages: Simple More accurate Disadvantages: Difficult to transfer wax pattern Relieve the stone before seat the pattern Used only with elastomeric impression Alternative die systems : a) The Di·Lok* technique (articulated tray) Size of the tray can make articulation and manipulation awkward and difficult b)The OVA Model System c) the Zeiser model system These systems offer the advantage of allowing for the expansion of stone, which is relieved by the saw cuts. Choice of Definitive Cast-and-Die System Operator preference Method's advantages and disadvantages  The technician preference The techniques for pouring stone dies are similar for most of the popular systems  MOUNTING CASTS ON AN ARTICULATOR The technique for mounting a solid definitive (master) cast is identical. Only that access must be allowed to the area of the base into which the dowels penetrate.  Definitive casts should be mounted with a record made at the occlusal vertical dimension, by using maximum intercuspation (MIP of unprepared teeth. If this is not possible, a kinematic facebow recording is recommended  Problems associated with mounting casts ,with a CR record by using an arbitrary facebow were analyzed by Weinberg. In a 3-mm thick record can create an occlusal discrepancy in the first molar region of 0.2 mm when the arbitrary axis differs from the true hinge axis by 5 mm (a common error). Technique for closed mouth impression Thank You