Wax Pattern Handout PDF

Wax Patterns Fabrication of metal or pressed-ceramic fixed prostheses → “Lost wax technique” “Pressing technique” Lost wax casting of metals involves: 1234- Construction of wax pattern Coating of a wax model with plaster Melting the wax out of the resulting mold Filling the space with molten metal W...

Wax Patterns Fabrication of metal or pressed-ceramic fixed prostheses → “Lost wax technique” “Pressing technique” Lost wax casting of metals involves: 1234- Construction of wax pattern Coating of a wax model with plaster Melting the wax out of the resulting mold Filling the space with molten metal Wax is used to make the patterns because it can be conveniently manipulated and precisely shaped. With heating, it can be completely eliminated from the mold after investing. Dental waxes Waxes are thermoplastic materials which are solids at room temperature but melt without decomposition to form mobile liquids. Materials Science: Inlay casting wax: Types of inlay casting wax: ▪ Type I: a medium-hardness wax (generally used with the direct technique for making patterns in the oral cavity) ▪ Type II: a softer wax (generally used for the indirect fabrication of castings) Composition: Paraffin: is usually the main constituent (40% to 60%), is likely to flake when it is trimmed, and it does not produce a smooth, glossy surface. Thus, natural resins& other waxes must be added as modifying agents Natural resin: Dammar resin (to reduce flaking & increases toughness) Page 1 of 9 Carnauba, Ceresin, Candelilla wax (to raise the melting temperature) or Beeswax Dyes: to provide color contrasts Requirements: 1. Adequate flow at typical forming temperatures. 2. Flow requirements are necessary to control the stability of the wax once it has reached the temperature (37°C for type I waxes, 25°C for type II waxes) at which it is carved, burnished, and polished. 3. Burn out cleanly to allow the escape of gases and the complete entry of molten alloy, there can be no residual ash. 4. The wax should be uniform when softened with no granules on the surface and no hard spots within the surface when the wax is softened. 5. The wax should not fragment into flakes or similar surface particles when it is molded after softening. Such flakiness is likely to be present in paraffin wax, so modifiers must be added to minimize this effect. 6. Dimensionally stable 7. Contrast color than die material 8. Easily carved without chipping 9. Can be burnished and polished 10. Should fracture rather than deform upon removal from cavity in case of undercut Method of wax pattern construction  Addition method  Dipping Method  CAD/CAM System Either digital impression or scanning of the cast → Digital designing of the wax pattern → milling of wax pattern from block of wax.  Printed Wax Pattern (3D Printing) Either digital impression or scanning of the cast → Digital designing of the wax pattern → 3D printer expels microdroplets of wax, developing a wax pattern in a layer-by-layer manner. Page 2 of 9 Advantages of CAD/CAM and 3D printing: 1. Faster 2. More precise and accurate 3. Less stress between wax layers  Molten Press Method  Injection Method Molten press and injection method are used for industrial purposes. Prerequisites: It must be understood that every defect or void in the wax will appear in the casting. Most defects can be corrected easily in wax but not in a metal casting. Compensating for an error in waxing technique is typically impossible once the metal casting has been formed. A. Correction of defects. Small defects on the die like dimples due to caries or loss of previous restorations Normally blocked out intraoral by glass ionomer cement (GIC) Can be blocked on the die as long as the defect does not extend to within 1 mm of the cavity margin. Blocked out using zinc phosphate cement, autopolymerizing resin. B. Die trimming: Remove most of the excess stone then trim the die apical to the finish line of the preparation. Over trimming will result in over contoured restoration Advantages of die trimming: Accentuate the finish line. Resembling the normal contour of the natural root for proper cervical contouring of the wax pattern. Produce smooth area gingival to the finish line. Page 3 of 9 C. Ditching: Ditching of the die below the finish line to 0.3 mm. Ditching is a circumferential groove that facilitates the establishment of good margin i.e. proper accentuation for the finish line. D. Marking the Margins The margins should be marked in order to locate it during wax carving. It should be delineated with a colored pencil. The color should contrast with that of the wax that will be used. Graphite (lead) pencil shouldn’t be used: o it can abrade the die o its darker color can interfere with efforts to verify that the wax has been properly adapted at the margin o traces of the graphite can prevent complete casting of the margins (act as antiflux). E. Provision of adequate cement space A space should exist between the internal surface of the casting (crown) and the prepared surface of the tooth everywhere except immediately adjacent to the margin (1 mm). The space provides room for the luting agent (a material that on hardening fills the space and binds the tooth and crown together) and allows complete seating of the restoration during cementation Adequate cement space is 20 to 40 um for each wall. So, the internal diameter of the wall of the crown would be 40 to 80 um. 1mm band around the margin shouldn’t be given any luting space. If the luting agent space is too narrow, the restoration will not seat properly during cementation because of hydraulic pressure that develops when the viscous mass of the luting agent (cement) cannot escape through the narrow gap between crown and preparation as the restoration is seated. Conversely, if the luting agent space is too wide, the casting is loose on the tooth, the resistance form is reduced, and the position of the crown Page 4 of 9 is difficult to maintain accurately during evaluation and occlusal adjustment. In addition, the risk that the crown will loosen during function increases considerably, adversely affecting its longevity. Die spacer: This material is applied to the die to increase the cement space between the axial walls of the prepared tooth and the restoration. It is formulated to maintain constant thickness when painted on the die. A band of about 1 mm immediately adjacent to the preparation margin must be left unpainted for adequate marginal adaptation. Factors affect the size of the cement space 1. 2. 3. 4. The choice of impression material Die material & technique Investment Casting alloy Increasing Space for Luting Agent Reduction of Space for Luting Agent Use of die spacers Use of a solid cast with individual stone dies Use of resin or electroplated dies Use of an internal (initial) layer of soft wax in the wax pattern Use of alloys with a higher melting temperature range Increased thermal and polymerization shrinkage of the impression material Reduced thermal and polymerization shrinkage of the impression material Increased expansion of the investment mold Reduced expansion of the investment Removal of metal from the fitting surface by grinding, airborne-particle abrasion, etching with aqua regia, or electrochemical milling Page 5 of 9 Wax Pattern construction The finished wax patterns should be an accurately shaped anatomic replica of the original teeth that meets all functional dynamic requirements. Information needed to shape the restoration correctly is derived from: - the contours of the unprepared tooth surface adjacent tooth surfaces the opposing occlusal surfaces reproduction of mandibular movement in the dental laboratory Advantages of electric wax instruments: o enable precise control of the wax temperature o minimize carbon buildup, which easily results from overheating a waxing instrument in a Bunsen flame. Disadvantages of electric wax instruments: o because the instrument remains hot, it is not possible to draw solidifying wax in the required direction. Page 6 of 9 Waxing Posterior Teeth The following sequence is recommended for waxing posterior teeth: 1. 2. 3. 4. 5. 6. Internal surface Wax pattern removal and evaluation Proximal surfaces Axial surfaces Occlusal surfaces Margin finishing A) Internal surface The objective of the first waxing step is a perfectly adapted reproduction of the prepared tooth surfaces. B) Wax Pattern Removal Careful evaluation of the pattern, preferably under low-power (up to 10 times) magnification, is crucial to obtain a good casting. C) Proximal Surfaces Contact Areas Reference is made to contacts between the contralateral teeth and knowledge of anatomic form. Most posterior contact areas are located in the occlusal third of the crown. However, the maxillary first and second molars make contact in the middle third. Between maxillary premolars and molars, the contact areas are usually toward the buccal surface (which makes the lingual embrasure larger than the buccal). Under no circumstances should a proximal contact area in the restoration be concave. Page 7 of 9 Abnormally large proximal contact areas make plaque control more difficult, which can lead to periodontal disease. Very small (point) contacts may be unstable and cause drifting. Deficient contacts can also lead to food impaction; although this is not a direct cause of chronic periodontal disease, it can be very uncomfortable and painful for the patient. The proximal surfaces of natural teeth from the contact area to the cementoenamel junction tend to be flat or slightly concave from the contact area to the cementoenamel junction and any restoration must reproduce this feature to provide adequate room for the free gingival tissues of adjacent teeth surfaces. The cervical contour of the restoration should be continuous, with the contour of the unprepared tooth structure immediately cervical to the preparation margin. D) Axial Surfaces The buccal and lingual surfaces should be shaped to follow the contours of the adjacent teeth. The location of the height of contour is generally located in the gingival third of most teeth, although on mandibular molars it is usually in the middle third of the lingual surface. The tooth surface gingival to its height of contour immediately adjacent to the gingival soft tissues, sometimes called the emergence profile, is usually flat or concave. Emergence profile: The tooth surface gingival to its height of contour immediately adjacent to the gingival soft tissues. It is usually flat or concave. The creation of convexity in this area or a shelf or ledge makes bacterial plaque removal more difficult and cause inflammatory and hyperplastic changes in the marginal gingiva. Page 8 of 9 E) Occlusal Surfaces The cusps and ridges of the occlusal surfaces should be shaped to allow even contact with the opposing teeth while the teeth are stabilized and forces are directed along their long axes. Nonfunctional cusps (buccal cusps of the maxillary teeth, lingual cusps of the mandibular teeth) should overlap vertically and horizontally, preventing accidental biting of the cheek or tongue and keeping food on the occlusal surface. Cusp Height and Location The cusps should be positioned so that they follow the anteroposterior curve (curve of Spee) and the mediolateral curve (curve of Wilson). Adjacent marginal ridges should be of the same height. Occlusal contacts in excursive movements must also be evaluated. If there is unwanted contact, grooves can be created in the cuspal ridges to allow the passage of opposing cusps Occlusal Scheme a) Cusp-marginal ridge The buccal cusps of the mandibular premolars and the mesiobuccal cusps of the mandibular molars (functional cusps) contact the embrasures between the maxillary teeth (tooth-to-two teeth) b) cusp-fossa The mandibular functional cusps contact farther distally into the fossa of the maxillary tooth and contact only one tooth (tooth-to-one tooth). The lingual functional cusps of the maxillary teeth contact the fossae of the mandibular teeth in both schemes. Page 9 of 9 Advantages of cusp-fossa arrangement over the cusp–marginal ridge: 1. Food impaction is prevented 2. Centric relation closure forces are nearer the long axes of the teeth 3. Improved stability results from the tripod contacts for each functional cusp F) Margin Finishing The margins must be reflowed over a band approximately 1 mm wide and refinished immediately before the wax pattern is invested to optimize the adaptation of the wax pattern (and the cast restoration) to the die→ Minimizing dissolution of the luting agent Facilitating plaque control Waxing Connectors The connectors that join the separate components of a fixed dental prosthesis are created in wax just before the margins are finalized. Mechanically: the connectors should be as large as possible to ensure optimal strength. Biologically: the connectors should not impinge on the gingival tissues and should be at least 1 mm above the crest of the interproximal soft tissue. Embrasure form gingival to the connectors must enable optimal plaque control. The cervical aspect of the connector must be shaped to a smooth, archlike configuration. Esthetically: In esthetic areas, connectors should be hidden behind the esthetic ceramic veneer. Therefore, connectors are often placed slightly lingually when connectors are waxed for anterior prostheses. Page 10 of 9