Fixed Prosthodontics Quiz

Lost Wax Technique

• The lost wax technique is used in fixed prosthodontics to create a metal framework for a dental restoration.

Waxing Up

• Waxing up is the process of rebuilding a prepared tooth on a die using wax.
• Wax is used because it can be conveniently manipulated and precisely shaped by heating, and it can be completely eliminated from the mold after investing.
• Inlay casting wax is a type of wax used in forming the pattern for cast restorations.

General Requirements of Wax

• The wax should have the following properties:
  • Color: should be suitable for the specific application
  • Flow: should be able to flow easily
  • Carving: should be able to be carved accurately
  • Addition: should be able to be added to without distorting the pattern
  • Residue: should leave minimal residue after removal
  • Compensation: should compensate for metal shrinkage
  • Strength and rigidity: should have sufficient strength and rigidity after cooling

Step-by-Step Process

• Modification of the master cast
• Addition of wax layers
  • Dipping the first layer, followed by anatomical buildup of the wax pattern by addition
• Evaluation
  • Axial contours
  • Occlusal morphology
  • Margins

Removable of Wax Pattern

• Remove the pattern from the die using extreme caution not to distort it

Spruing

• A sprue is a channel in the investment material through which molten wax can escape during the burnout process, and molten metal can reach the mold cavity.
• Requirements for a sprue:
  • Allow molten wax to escape from the mold
  • Enable molten metal to flow into the mold
  • Molten metal inside the sprue must remain molten slightly longer than metal that had filled the mold
• Types of sprue formers:
  • Wax
  • Plastic
  • Metal

Properties of Sprue

• Diameter: should be thicker than the thickest portion of the pattern
• Location: should be attached to the bulkiest non-critical part of the pattern, away from margins and occlusal contacts
• Angulation: should be at an angle (45°) to allow the incoming molten metal to flow freely to all portions of the mold
• Length: should be according to the investment used, with a minimum of 6-8 mm for gypsum-bonded investment and 3-4 mm for phosphate-bonded investment
• Venting: small auxiliary sprues or vents improve casting of thin patterns and help gases escape during casting

Casting

• Done using a centrifugal casting machine or air pressure casting machine
• Molten metal is cast into the mold

Casting Defects

• Can be attributed to poor casting technique and failure to adhere to casting laws
• Types of defects:
  • Rough casting
  • Large nodules
  • Small nodules
  • Fins
  • Incomplete casting
  • Marginal discrepancy

Cleaning the Casting

• After solidification, the ring is removed and quenched in water
• Divesting: removal of the casting from the investment mold
• Pickling: process of removing oxides and tarnish from the surface of the casting using an acid

Porcelain Build-up

• Ceramic layers are built up on a metal coping
• Types of porcelain:
  • Opaque porcelain
  • Body porcelain
  • Incisal porcelain