Fixed Prosthodontic Technology: Wax Patterns, Casting, CAD/CAM, Metal-Ceramic and All-Ceramic Restorations
Fixed Prosthodontic Technology
This domain covers the laboratory chain from die to seated restoration: wax pattern design, sprueing, investment, burnout, casting, ceramic application, soldering, and finishing. Questions test whether you can name the cause of a defect and the corrective change in technique, not just describe a procedure.
Core rules
- Wax patterns: the sprue should attach to the thickest part of the pattern, be 8–10 gauge for single units, and be directed to avoid impinging on the investment wall. A reservoir (crucible former) feeds solidification shrinkage.
- Burnout: typically 650–700 °C for base-metal and about 480–650 °C for noble alloys; under-burnout leaves carbon residue causing gas porosity and poor casting.
- Casting defects: shrinkage porosity is irregular and located away from the sprue in the last-cast thick region; gas porosity is spherical and often near the surface or sprue; back-pressure porosity appears at sharp corners from trapped air; incomplete casting suggests insufficient casting force, cold mould or thin sprue.
- Metal-ceramic: the alloy's coefficient of thermal expansion should slightly exceed the porcelain's, so the porcelain stays in compression on cooling. A thin oxide layer provides bonding, but excessive oxide weakens it. Framework should be at least 0.3–0.5 mm thick with a 0.2 mm metal-ceramic junction.
- All-ceramics: zirconia is milled or pressed in the green/white state and sintered, shrinking about 20–25%, which the CAM software compensates. Veneering porcelain should be supported by adequate core thickness.
- Soldering: use a soldering index, keep the gap small, apply anti-flux beyond the joint, and never overheat the parent metal.
Worked example
A single-unit base-metal crown shows a cluster of small spherical voids on the occlusal surface, and the alloy is bright rather than dull. Reasoning: round voids plus a bright surface mean trapped gas, not solidification shrinkage. Common causes are a short sprue with the end too close to the mould wall, insufficient vacuum or venting during mixing, and a mould that was too cold so the alloy froze before air escaped. The fix is to reposition the sprue, use vacuum mixing, and raise casting temperature - not to lengthen the burnout time.
Traps that cost marks
Naming shrinkage porosity as gas porosity; forgetting that porcelain is stronger in compression than tension; and assuming CAD/CAM eliminates fit error - sintering distortion and margin chipping still occur.