LAB PROCEDURES IN CPD
- 1. Department of prosthodontic s TOPIC- Lab procedures in Cast Partial Denture. Guided by- Dr. Sonal Pamecha
- 2. Introduction Removable partial denture: the replacement of missing teeth and supporting tissues with a prosthesis designed to be removed by the wearer-GPT. Cast partial denture is a type of partial denture comprising a cast metal framework with acrylic resin prosthetic teeth. Traditional acrylic partial dentures are less durable, retentive, and stronger than cast partial dentures.
- 3. LAB PROCEDURES Care of impression & making casts. Pouring corrected cast impressions Stock and custom impression trays. Record bases & mounting casts. Survey & design. Design transfer, blockout , relief & beading. Duplication and refractory casts. Waxing & spruing. Investing, burnout & casting. Finishing & polishing in framework. Selecting & arranging teeth. Flasking, processig, deflasking & finishing.
- 4. I. CARE OF IMPRESSIONS & MAKING CASTS Some restorations are made to fit patient directly and few are made to fit the cast. If the cast is not accurate the restoration cannot possibly fit patient satisfactorily. Essentials for casts All surfaces to be contacted by the prosthesis should be accurate & free of voids/nodules. Removel of nodules resulting from voids/bubbles in the impression is essential, but hand carving in critical areas is, of course, not acceptable. The cast extensions should include all of the area available for denture support, for example, 3-4 mm beyond the hamular notches on the maxillary cast & 3-4 mm beyond the re tromolar pad on the mandibular cast.
- 5. Alginate impressions 1. Examining the impression. 2. Care of impression. 3. Pouring the cast. 4. Trimming the cast. II. POURING CORRECTED CAST IMPRESSION The 4 techniques for pouring corrected cast impressions are boxing with a: Plaster pumice- mix & wax Beading and boxing with wax Beading with wax & 2 stage pour North Carolina technique
- 6. BEADING & BOXING WITH WAX Beading & boxing the corrected cast impression with wax works well for zinc oxide eugenol paste/ plaster of paris impressions. Beading & boxing with wax & 2 stage pour Beading the corrected cast impression with wax & 2 stage pour procedure is similar to beading & boxing procedure with wax. It can be used for metallic paste or plaster of paris impression.
- 7. III. STOCK & CUSTOM IMPRESSION TRAYS Custom trays are usually made acrylic resin trays. The tray materials are usually not applied directly over the cast. Spacing material such as a wax or tin foil substitue is used so as to provide relief for the impression material when the impression is removed from an undercut. For alginate relief should be one-forth inch ( 0.6 cm) & for elastomeric materials a thickness of one-eighth inch (0.3 cm) is necessary
- 8. STEPS- o Outline of relief is drawn on the cast. o Even layer of wax is applied to cast & painted with tin foil substitute. o Tray formers are lubricated with petroleum jelly. o Fluid resin mix is poured into a lubricating mould. o Acrylic resin in mould must set until it becomes plastic & can be handled.
- 9. o The plastic baseplate is centered over the cast & the entire outline marked is covered. o Plastic baseplate is adapted onto the cast. o Before the material sets some of the excess is added to form the handle. o Tray separated cleanly from cast & wax relief when tin foil substitute is used. o Tray trimmed accordingly. Borders are rounded off and tray is perforated by round bur
- 10. IV. RECORD BASE & MOUNTING CAST RECORD BASE REQUIREMENTS: Same adaptation to basal seat area as finished denture base. Same border form as finished denture base. Sufficient rigidity to withstand occlusal forces. Dimensional stability. Construction that permits use as base for teeth arrangement. Easy, quick, and inexpensive fabrication & desirable color.
- 11. Method of fabrication of record base Autopolymerizing record base ◦ Sprinkle on method ◦ Sprinkle on method with framework ◦ Finger adapted dough method ◦ Wax confined dough method Shellac record base Stabilized shellac record base ◦ Stabilised with zinc oxide eugenol paste ◦ Stabilised with elastomeric impression material ◦ Stabilised with autopolymerising resin. ◦ Vaccum adapted thermoplastic resin base ◦ Wax record base ◦ Visible light cured record base
- 12. V. SURVEY AND DESIGN Principles of surveying- Surveying is the procedure of studying the relative parallelism or lack of parallelism of teeth & associated structure so as to select a path of placement for a restoration that will encounter the least tooth or tissue interference & that will provide adequate and balanced retention. Objectives of surveying: To locate & evaluate hard & soft tissues undercuts on the cast & based on evaluation, select a path of insertion that will make optimal use of favorable undercuts will minimising effect of unfavourable undercut.
- 13. Surveyor Mechanical devices that permits placement of cast in selected fixed position while instrument is used to first analyze & then mark the undercuts on the cast. PARTS OF SURVEYOR:
- 14. VI. DESIGN TRANSFER, BLOCKOUT, RELIEF & BEADING DESIGN TRANSFER If designed diagnostic casts accompany the master cast to lab, metallic portion of design on diagnostic casts will be transferred to master cast. Procedure- examine the master casts to determine acceptability. If acceptable the cast should be examined for nodules that must be removed.
- 15. • Place 3 widely separated marks on the diagnostic casts. • Mark 3 spots on master cast in exactly same place as on the diagnostic cast. • Using the carbon marker in surveyor, mark survey lines on master cast. • Using the diagnostic cast as a guide, draw the metallic portion of design on the master cast with a brown pencil & resin finish lines with blue pencil. • The design on master cast should be same as design on the diagnostic
- 16. Blockout & duplication Preparing cast for duplication. Cast are blocked out to eliminate undercut areas on the master cast that will be crossed by rigid parts of partial denture. All parts of framework, except the retentive clasp tips, are non flexible & will not spring into an undercut. Additional area to be blocked out,which are eliminated for convenience include: Ledges on which clasp patterns will be placed. Beneath connectors to avoid soft tissue impingement. To provide for later attachment of the denture base to the framework.
- 17. Beading Beading on the borders of maxillary major connectors surveys to prevent food particles from collecting beneath the framework & producing discomfort for the patient. Beading is accomplished with a small spoon excavator by scraping the cast along the anterior & posterior borders of major connector. Beading depth should not exceed 1mm. It should fade out as the gingival margins are approached or as prominent area in the midline of the palate.
- 18. Prepared master casts are duplicated to produce an exact copy of the master cast in investment material. This cast is called the refractory cast because it contains ceramic material that permits it to withstand burnout temperature of about 1300 degree F( 704 degree C). The framework pattern is formed on the refractory cast using the wax and plastic. After the framework wax-up is complete, it is invested, burned out, and cast to produce the metal framework. VII. DUPLICATION & REFRACTORY CASTS
- 19. Procedure The master cast must be thoroughly soaked in clear slurry water. A minimum of 30 minutes soaking is usually required. When the color in the entire cast has changed, it is an indication that the cast is thoroughly soaked. If a cast is completely submerged in water, it will require 5 to 8 hours to soak all the way through. Investment cast Most gold alloys are cast into gypsum bound silica investment. Cobalt chromium alloys that solidify at a temperature of about 1300 degree C also use a gypsum binder. These are usually referred to a phosphates bound investments, and they are also used with high fusing gold and other alloys to which porcelain is fused.
- 20. VIII. WAXING & SPRUING After the master cast is duplicated to make a refractory cast, it is necessary to transfer the design to the refractory cast and complete the wax pattern. Waxing the maxillary cast procedure:
- 21. There are four main types of retention: 1. beads or nailheads 2. open retention without relief 3. open retention or mesh with relief, and 4. all metal base with or without tube teeth, without relief.
- 22. Waxing the mandibular cast Study the framework design that was transferred from the master cast. The most critical point of the transfer is the lingual bar. The superior margin of the bar must not be placed closer than 3 mm to the gingival margin of the teeth. Also, it must not be so low that the inferior border will interfere with the frenum when the tongue is raised. The position of the clasps and rests is determined by the rest seat preparations and by the ledge created during the blockout procedure.
- 23. Spruing Spruing for casting of removable partial dentures involves a large quantity of metal, so special attention must be given to points of attachment, directions of the sprues, and the direction of flow of the molten metal. There are two basic types of sprues, single and multiple. A heavy sprue is an additional aid for offsetting this shrinkage. It acts as a reservoir for the casting because its bulk remains molten longer than the rest of the casting and supplies molten metal to those areas which solidify and shrink first.
- 24. Main sprues should always be attached to bulky sections of the wax patterns. Never attempt to feed a thick section through a thin section to reach a heavy section, an additional or auxillary sprue should be used. Mandibular partial dentures are usually sprued through a hole in the centre or the refractor cast because it is easier to attach the sprue leads. Maxillary casts are sprued the same way if the shape of the castings permits, other wise, they are sprued from above.
- 25. IX. INVESTING, BURNOUT & CASTING The investment for RPD consists of two parts the investment cast and on which the pattern is formed and the outer investment surrounding the cast and pattern. Investment must conform to the shape of the pattern and must preserve the configuration of the pattern as a cavity after the pattern is eliminated thorough vaporization. The burnout has three purposes : It drives off moisture in the mold. It vaporizes and thus eliminates the pattern leaving the cavity in the mold. It expands the mold to compensate for contraction of metal on cooling.
- 26. The method of casting will vary widely with the alloy and equipment used. All methods use force to quickly inject the molten metal into the mold cavity.
- 27. X. FINISHING & POLISHING THE FRAMEWORK After the framework is obtained by divesting the sprues are removed although some authorities prefer that the sprue not be removed. Care should be taken while polishing a cast clasp arm. High speeds are preferred compared to lower speeds. The wheels or points and the speed of their rotation should do the cutting. Excessive pressure heats the work, and slows the cutting. A definite sequence for finishing should be adopted and followed for every framework. Clean polishing wheels should be used. Make sure each finishing operation completely removes all scratches left by the preceding one. Each successive finishing step uses a finer abrasive and therefore cuts more and slowly requires more time to accomplish.
- 28. XI. SELECTING & ARRANGING TEETH • The teeth are set in harmony with the opposing dentition. XII. FLASKING,PROCESSI NG,DEFLASKING & FINISHING
- 30. References: Textbook of prosthodontics- Deepak Nallaswamy Veeraiyan- 2nd edition. Textbook of prosthodontics- V Rangarajan- 2nd edition.