Comprehensive Dental Casting, Wax Patterns, and Die Systems Guide

Fabrication of Fixed Prostheses and the Lost Wax Technique

The fabrication of metal or pressed-ceramic fixed prostheses primarily utilizes two methods: the "lost wax technique" and the "pressing technique." The lost wax casting of metals is a multi-step process involving:

1. Construction of a wax pattern.
2. Coating of the wax model with plaster (investment).
3. Melting the wax out of the resulting mold (burnout).
4. Filling the space with molten metal.

Wax is the chosen material for patterns because it can be conveniently manipulated and precisely shaped. Upon heating, it can be completely eliminated from the mold after investing. Dental waxes are thermoplastic materials that remain solid at room temperature but melt without decomposition to form mobile liquids.

Inlay Casting Wax: Types and Composition

Inlay casting waxes are classified into two main types:

• Type I: A medium-hardness wax generally used with the direct technique for making patterns directly in the oral cavity.
• Type II: A softer wax generally used for the indirect fabrication of castings on a die.

Composition of Inlay Waxes:

• Paraffin: This is usually the main constituent, comprising $40\%$ to $60\%$ of the wax. However, paraffin is likely to flake when trimmed and does not produce a smooth, glossy surface on its own.
• Natural Resins (e.g., Dammar resin): Added to reduce flaking and increase the toughness of the wax.
• Modifyng Agents: Carnauba, Ceresin, and Candelilla wax are added to raise the melting temperature. Beeswax is also used.
• Dyes: Incorporated to provide color contrast against the die material.

Requirements for Dental Waxes

Dental waxes must meet ten specific requirements to ensure successful restoration:

1. Adequate flow at typical forming temperatures.
2. Flow control: Flow must be controlled to maintain stability at the carving/polishing temperatures ($37^\circ C$ for Type I and $25^\circ C$ for Type II).
3. Clean Burnout: It must burn out cleanly with no residual ash to allow gas escape and proper metal entry.
4. Uniformity: The wax should be uniform when softened, possessing no granules or hard spots.
5. Anti-flaking: It should not fragment into flakes or particles when molded after softening.
6. Dimensional Stability.
7. Color Contrast: A color different from the die material is required for visibility.
8. Ease of Carving: It must be easily carved without chipping.
9. Burnishing/Polishing: It should be capable of being burnished and polished.
10. Fracture Property: In the case of an undercut, the wax should fracture rather than deform when removed from the cavity.

Methods of Wax Pattern Construction

There are several techniques for developing a wax pattern:

• Addition Method: Building the pattern by adding wax incrementally.
• Dipping Method: Dipping the die into molten wax.
• CAD/CAM System: Involves taking a digital impression or scanning a cast, digitally designing the pattern, and then milling the pattern from a block of wax.
• Printed Wax Pattern (3D Printing): Uses digital design to expel microdroplets of wax in a layer-by-layer manner.
• Molten Press / Injection Methods: Primarily used for industrial purposes.

Advantages of CAD/CAM and 3D printing:

1. Faster production times.
2. Increased precision and accuracy.
3. Reduced stress between wax layers.

Prerequisites and Die Preparation

Every defect in a wax pattern will appear in the final metal casting. While defects are easily corrected in wax, they are difficult or impossible to fix in metal once formed.

Correction of Defects:
Small die defects (from caries or lost restorations) are blocked out intraorally using Glass Ionomer Cement (GIC). If on the die, they can be blocked with zinc phosphate cement or autopolymerizing resin, provided the defect is at least $1\,mm$ away from the cavity margin.

Die Trimming:
Excess stone is removed, and the die is trimmed apical to the finish line. Over-trimming leads to over-contoured restorations. Advantages include accentuating the finish line, resembling normal root contours, and producing a smooth gingival area.

Ditching:
Ditching involves creating a circumferential groove below the finish line to a depth of $0.3\,mm$. This facilitates the establishment of a good margin.

Marking the Margins:
Margins are marked with a colored pencil that contrasts with the wax. Graphite (lead) pencils must not be used because they can abrade the die, interfere with adaptation verification, and act as an "antiflux," preventing the metal from casting completely at the margins.

Provision of Adequate Cement Space

A space is required between the internal surface of the crown and the tooth preparation (except within $1\,mm$ of the margin) to accommodate the luting agent.

• Ideal space: $20$ to $40\,\mu m$ per wall (total internal diameter increase of $40$ to $80\,\mu m$).
• Narrow space risks: If too narrow, hydraulic pressure from the viscous cement prevents the restoration from seating properly.
• Wide space risks: If too wide, the casting is loose, resistance form is reduced, and the risk of the crown loosening during function increases.

Die Spacers:
These are formulated materials painted onto the die to maintain constant thickness. A $1\,mm$ band immediately adjacent to the margin must remain unpainted.

Factors Affecting Cement Space

Factors include the choice of impression material, die material/technique, investment material, and casting alloy.

Waxing Posterior Teeth: Technique and Anatomy

The construction of the wax pattern follows a strict sequence:

1. Internal surface reproduction.
2. Wax pattern removal and evaluation (using up to $10\times$ magnification).
3. Proximal surfaces.
4. Axial surfaces.
5. Occlusal surfaces.
6. Margin finishing.

Proximal Surfaces and Contact Areas:

• Most posterior contacts are in the occlusal third; however, maxillary first and second molars make contact in the middle third.
• Maxillary premolar/molar contacts are usually toward the buccal, making the lingual embrasure larger.
• Contacts should never be concave. Abnormally large contacts hinder plaque control (periodontal risk); small point contacts are unstable.
• The surface from the contact to the CEJ should be flat or slightly concave to accommodate free gingival tissues.

Axial Surfaces and Emergence Profile:
The "emergence profile" is the tooth surface gingival to the height of contour, adjacent to soft tissues. It must be flat or concave. Convexity or ledges here make plaque removal difficult and cause inflammation or hyperplasia. The height of contour is usually in the gingival third, except on mandibular molars (middle third of the lingual surface).

Occlusal Surfaces:
Nonfunctional cusps (maxillary buccal, mandibular lingual) should overlap horizontally and vertically to prevent cheek/tongue biting. Cusps must follow the Curve of Spee (anteroposterior) and Curve of Wilson (mediolateral).

• Cusp-Marginal Ridge Scheme: Functional cusps contact embrasures (tooth-to-two-teeth).
• Cusp-Fossa Scheme: Functional cusps contact only one tooth (tooth-to-one-tooth). This is superior as it prevents food impaction, directs forces along the long axis, and provides stability via tripod contacts.

Margin Finishing:
Margins must be reflowed over a $1\,mm$ band and refinished immediately before investing to optimize adaptation and minimize cement dissolution.

Connectors:

• Mechanical: Larger for strength.
• Biological: Must be $1\,mm$ above the interproximal soft tissue crest with smooth, archlike cervical configurations.
• Esthetic: Placed slightly lingually in anterior prostheses.

Spruing: Definitions and Requirements

A sprue is a channel through which molten alloy reaches the mold after wax elimination.

Functions of Spruing:

1. Forms a mount for the wax pattern.
2. Creates a channel for wax elimination.
3. Forms a channel for metal entry.
4. Provides a reservoir to compensate for alloy shrinkage during solidification.

Selection Criteria:

• Diameter: Should be approximately the same size as the thickest area of the pattern.
      * $2.6\,mm$ ($10\,gauge$) for molars and metal-ceramics.
      * $2\,mm$ ($12\,gauge$) for premolars and partial coverage.
• Reservoir: An additional amount of wax $1\,mm$ below the pattern to prevent "suck back porosity" by ensuring the sprue solidifies last.
• Position: Placed at the greatest bulk, away from margins or occlusal contacts. Ideal site is the largest non-functional cusp.
• Direction: Ideally at a $45^\circ$ angle to prevent turbulence; never at a right angle.
• Length: Determined by the casting ring. Pattern must be $6\,mm$ from the end of the ring.
      * Gypsum bonded: $6-8\,mm$.
      * Phosphate bonded: $3-4\,mm$.
• Attachment: Must be smooth and unrestricted to reduce porosity.

Spruing Types:

• Direct: Straight connection between the pattern and sprue base.
• Indirect: Uses a connector or "runner bar" (reservoir bar). Used for multiple units or FPDs.
• Materials: Wax (preferred as it melts at the same rate), Plastic (adds rigidity but may block wax escape), and Metal (must be non-rusting; often hollow to increase surface area for attachment).

Investing and Materials

Components of Investment:

1. Refractory material: Silica (Quarz or Cristobalite) for thermal expansion.
2. Binder: Gypsum (Calcium sulfate hemihydrate) or Magnesium Oxide/Ammonium phosphate.
3. Modifiers: Accelerators, retarders, and reducing agents (e.g., carbon).

Types of Investment:

• Gypsum-bonded: For conventional type II, III, and IV gold alloys. Not stable above $1000^\circ C$ (or $1080^\circ C$). Mixed with distilled water.
• Phosphate-bonded: For metal-ceramic alloys. Stable at high temperatures. Mixed with colloidal silica. Carbon-containing (gray) is for high gold/palladium; carbon-free is for base metals to avoid altering alloy composition.
• Silicate-bonded.

Expansion Types:

1. Setting expansion: Occurs during crystal growth.
2. Hygroscopic expansion: Extended setting expansion achieved by adding water (using a wet liner or a $37^\circ C$ water bath for 1 hour).
3. Thermal expansion: Solid-state phase transformations of silica during heating.

Methods to Increase Expansion: Use of two liners, prolonged spatulation, storage in $100\%$ humidity, or lowering the water-powder ratio (gypsum) / increasing special liquid (phosphate).

Burnout and Casting Process

Burnout Temperatures:

• Gypsum-bonded: $650-700^\circ C$.
• Phosphate-bonded: $850-900^\circ C$.

Melting and Heating:
Methods include the blow torch (Gas-air for precious/semi-precious; Gas-oxygen for base metal) and electricity (Induction).
Blowpipe Flame Zones:

1. Mixed zone (colorless).
2. Consumption zone (gas/air mixture).
3. Reducing zone (light blue): The hottest zone used for melting (prevents oxide formation).
4. Oxidizing zone (outer red).

Flux: A reducing agent (e.g., Borax) added to prevent oxides, increase fluidity, and reduce melting temperature.

Recovery:

• Precious alloys: Quenched when the red glow disappears to disintegrate investment; then pickled in warm $HCl$.
• Base metals: Cooled to room temperature and sandblasted with $Al_2O_3$.

Casting Errors

• No Casting: Caused by sprue blockage, cold rings, or low pressure.
• Fins (Frayed Margins): Cracks in investment from too rapid heating or high water-powder ratio.
• Bubbles/Nodules: Trapped air from inadequate vacuum or lack of surfactant (debubblizer).
• Incomplete Casting: Due to insufficient alloy, inadequate burnout, or cold rings.
• Porosity types:
      * Back pressure: Low pressure/venting/investment porosity.
      * Gas inclusion: From blow torch misuse or overheating.
      * Shrink-spot: Metal shrinkage during solidification without flow from reservoirs.
      * Suck-back: Occurs when the sprue cools before the mold.
      * Subsurface: Due to a cold mold.

Cast and Die Systems

Definitions:

• Working Cast: Positive reproduction of prepared teeth, ridges, and the arch.
• Die: Positive reproduction of the prepared tooth for wax pattern creation.

Requirements of Cast/Die Materials: Accurate reproduction, high hardness/mechanical properties, dimensional accuracy, and color contrast.

Removable Die Techniques:

1. Dowel Pin: Positioned parallel to the tooth's long axis. It has a flat side to prevent rotation.
2. Di-lock Tray: A snap-apart plastic tray with internal grooves for reassembly.
3. Pindex System: A reverse drill press that drills parallel holes from the underside of a trimmed cast.
4. DVA/Zeiser Systems: Use base plates and alignment fixtures/drilling devices for precision pin placement.

Materials:

• Gypsum: Types I (Plaster) to V (High strength/High expansion).
• Refractory Dies: Ceramic dies that withstand high temperatures for all-ceramic restorations.
• Flexible Dies: Silicon/polyether consistency for temporary or composite inlays/onlays.
• Digital: 3D printed or Virtual casts (CAD).

Classification of Casting Alloys

By Noble Metal Content:

• High Noble: $\ge 60\%$ (e.g., $AuPd$, $PtAuPdAg$).
• Noble: $\ge 25\%$ (e.g., $AgPd$).
• Base Metal: $< 25\%$ noble (e.g., $NiCr$, $CoCr$).

By Mechanical Properties (Gold):

• Type I: Soft (small class I).
• Type II: Medium (inlays/onlays).
• Type III: Hard (crown/bridge).
• Type IV: Extra Hard (RPD frameworks/long spans).

Comparison: Gold vs. Base Metal

• Gold: Yellow, High density ($18.3\,g/cm^3$), Low melting temp ($935-1150^\circ C$), $1.6\%$ shrinkage, ductile/burnishable.
• Base Metal: Silver, Low density ($7-9\,g/cm^3$), High melting temp ($1150-1400^\circ C$), $2.4\%$ shrinkage, high modulus, non-ductile.