Laboratory Procedure for Making a Conventional RPD Framework construction 1.Transfer the design to the master cast A. Transfer the orientation of the diagnostic cast to the master cast. Use spoon excavator or suitable bur depth not more than 0.5 mm. B. Draw the survey line. C. Indicate and measure the retentive undercut( 0.01 inch). D. Draw the design on the master cast. Metallic outline drawn in brown Rest, tripod, undercut cross lines, terminal end of the clasp in red Finish lines in blue. Treated with a surface sealer. The sealer is a mixture of acetone, diethyl phthalate, and cellulose acetate. Impermeable seal that protects the drawn design. Multiple layer should not be applied Design should mimic diagnostic cast. Dry for 5 mins and block out. 2.Beading of the master cast Beading is scribing the maxillary cast with a sharp instrument or bur in desired area to provide seal between the finished prosthesis and the soft tissues. 3. Wax block-out and relief procedure It is the elimination of the undesirable undercut areas on the cast to be used in the fabrication of a RPD. A. Parallel block-out: It is the placement of a small amount of wax, under the survey line of the proximal surface of an abutment tooth. It should be extended to the gingival margin. Also used in the deep interproximal spaces that may be crossed by minor connector. It is contoured in a predetermined relation to the prescribed path of insertion.
All tooth-supported removable partial dentures should be blocked out parallel to the path of insertion. Tooth-tissue-supported removable partial dentures may be blocked out in a similar manner, or they May be given a tapered block-out to allow increased freedom of movement in function Metallic block out instrument B. Shaped block-out: It is prepared on buccal and lingual surfaces to facilitate accurate placement of wax patterns of clasp arm. Buccal or lingual surface to faciliate accurate placement of plastic or wax pattern It must be trimmed flush with the survey line If stone is trimmed off the final frame will not fit in the mouth C. Arbitrary block-out: It is used in the areas of the cast that is not engaged by the metal framework such as the labial surfaces of mandibular anterior teeth. D. Relief: It is the procedure of placing wax sheet in certain areas of master cast to be duplicated in the refractory cast. To provide space between certain parts of metal framework and the oral structures. Relief is given in edentulous areas for adaptation of denture base and to form internal finish line (generally not needed except for unhealed ridges. The dentist should specify if relief is needed) A properly formed internal finish line permits formation of a butt joint between the framework and the acrylic resin denture base. This allows a smooth transition from metal to resin and minimizes the likelihood of trauma to the adjacent soft tissues. One thickness of baseplate wax makes an ideal relief pad. RATIONALE TO NOT USE RELIEF: No relief eliminates one step and chances of error. No need for conventional internal finish line. Practically 1mm relief should be there for acrylic to flow
4. Duplication of the master cast (+ pouring the refractory cast): By means of duplicating flask and reversible hydrocollid material (agar) or silicone to prepare a mold of master cast. Cast is placed on the base of the duplicating flask. 1 4 inch clearance in all direction Use modelling clay to fix the cast and close the body of the flask. Secure the junction between the base and body of the flask The cast should be immersed in slurry water before duplication gently remove the master cast from the duplicating flask after complete setting of agar agar. Pour the investment material in the mould cavity, and allow it to set. Remove the refractory cast from the mold cavity after setting by cutting the agar away. Allow the cast to dry in oven. Immediately after removal from the oven, immerse the cast in molten beeswax. This makes the refractory cast surface smooth and dense. ADVANTAGES OF DUPLICATION: Preserve the original cast Eliminating the danger of fracture or abrading the surface of the original master cast Forming the wax or plastic pattern The metal framework is cast against the surface of refractory cast 5. Wax-up: To contour wax and performed wax patterns to form a pattern for RPD framework.
6. Spruing: Used to attach wax performed forms to the framework wax pattern to provide a channel or hole through which molten metal flow into the mold of the framework wax pattern. Sprues should attach to the bulky points of the mold pattern. The point of attachment should be flared out. Use few sprues of large diameter rather than several smaller sprues. An opening to remove wax and introduce molten metal in sprue. Also acts as a reservoir. Keep all sprues as short and direct as possible. 7. Investing: Select investments that is compatible alloys IDEAL INVESTMENT: Expand to compensate metal shrinkage not decompose at high temperature Strengthed at various temperature Porous to allow the escape of gases Consistent casting temperatures Easy to manipulate Smooth castings Easy to break out Inexpensive Blockout philosophies and techniques are similar regardless of the chosen alloy. In contrast, duplication materials and techniques are alloy specific. Refractory cast expansion determines the ultimate fit of the framework. The process of covering or enveloping the RPD framework wax pattern with a suitable investing material before casting. Type of investing material used for cobalt chromium alloy is phosphate bonded investment.
Gypsum-bonded It s mixed with water Composed of a mixture of silica (SiO2) and calcium sulphate hemihydrates (gypsum product) Phosphate-bonded Three types of gypsum bonded investments can be identified as follows: Type 1 thermal expansion type; for casting inlays and crowns Type 2 hygroscopic expansion type; for casting inlays and crowns. Type 3 casting complete and partial dentures Powder contains silica, magnesium oxide and ammonium Phosphate. Can be mixed with water or colloidal silica Two types of phosphate-bonded investment can be identified as follows: Type 1 for inlays, crowns and other fixed restorations. Silica-bonded* Type 2 for partial dentures and other casts. removable restorations Consist of powdered quartz or cristobalite which is bonded together with silica gel On heating, the silica gel turns into silica so that the completed mould is a tightly packed mass of silica particles The use of colloidal solution of silica instead of water for mixing with the powder has the dual effect of increasing the setting expansion and strengthening the set material. On heating the investment prior to casting, mould enlargement occurs by both thermal expansion and inversion of the silica. Thermal expansion is greater for the colloidal silica-mixed materials than for the water-mixed materials. Mechanical mixing under vacuum removes air bubbles created during mixing and evacuates any potentially harmful gases produced by the chemical reaction of the highheat investments. After investing the pattern, it is allowed to set for an appropriate period- approximately 1 hour for most gypsum and phosphate bonded investment. Excessive vibration should be avaoided as heavy particles will get accumulated at the bottom and free water accumulation adjacent on wax pattern may occur. Allowed to set for 1 hr.
8. Burn out: It drives off moister in the mold and vaporizes (thus, eliminates) the wax pattern. It also expands the mold to compensate for contraction of metal on cooling. For gypsum bonded investments, temperature can be either 468 degrees Celsius for the hygroscopic technique or 6500C for the thermal expansion technique. With phosphate bonded investments, the maximum temperature setting may range from 7000C to 8700C, depending upon the alloy selected. During burnout, some of the melted wax is absorbed by the investment and residual carbon produced by the ignition of the liquid wax become trapped in the pores investment If burnout is not done immediately place the invested ring in 100 % humidity (humidor). It is also advisable to begin the burn out procedure while the mold is still wet. This process is facilitated by placing the ring with the sprue hole down over a slot in a ceramic tray in the burnout furnace. Water trapped in the pores of the investment reduces the absorption of wax and as the water vaporizes, it flushes wax from the mold. Co-cr alloy is a commonly used investment material for casting of partial dentures.this metal doesnot have adequate ductility for adjustment purposes but are light in weight, strong and corrosion resistant. superheating the alloy before casting can increase the ductility of the alloy. Superheating the alloy can cause the metal to react with the mold and a close adhering green scale is formed. This green scale is very hard to be removed and requires a lot of machining. 9. Casting: Injection the molten metal quickly into the mold using force by two ways: Centrifugal force( commonly used). Air pressure. 10. Finishing and polishing: It is the procedure of smoothening the metal framework with progressively finer abrasive agents to remove scratches and rough areas and to give a high luster. High speed lathe, curbid bur, carborundum disc, rubber wheels, cones, polishing whee, rag wheel and high gloss compound.
Clinical steps in RPD construction 1. Diagnosis and treatment planning 2. Primary impression (alginate) 3. Jaw relation 4. Surveying and design 4. Tooth preparation (after design) 5. Final impression (ZnOE or elastomers) 6. Framework try-in 7. Wax try-in 8. Delivery Complete steps for RPD construction (Clinical + Lab) 1. Diagnosis and treatment planning 2. Primary impression using alginate 3. Preliminary cast (LAB) 4. Jaw relation 5. Surveying and design 6. Tooth preparation 7. One stage selected pressure, McLean s, Hindle s tecniques for final impression* 8. Framework construction (LAB) Transfer the design to the master cast Beading of the master cast Wax block-out and relief procedure Duplication of the master cast (+ pouring the refractory cast) Wax-up of refractory cast Spruing Investing Burn out Casting Finishing and polishing 9. Framework try-in in master cast (LAB) 11. Framework try-in on patient 12. Altered cast technique for secondary impression~ 13. Wax try-in 14. Delivery *The techniques in #7 are done BEFORE framework construction ~This technique is done AFTER framework construction NB: You don t do BOTH techniques. If you are using Mclean/Hindle s you need to take the impression before framework construction. If you are using altered cast technique it should be done AFTER framework construction.