CDT Crown & Bridge Fabrication 2 — Questions and Answers
Question 1: What is the primary purpose of a proximal contact in a fixed bridge pontic design?
- To prevent food impaction and maintain arch integrity (Correct answer)
- To increase the surface area for porcelain bonding
- To reduce occlusal stress on the abutment teeth
- To improve the esthetic appearance of the restoration
Correct answer: To prevent food impaction and maintain arch integrity
Proximal contacts in bridge pontics prevent food impaction and help maintain the spatial integrity of the dental arch.
Question 2: Which alloy system is most susceptible to green corrosion (verdigris) when used in crown and bridge work?
- High-gold alloys
- Base-metal alloys containing copper (Correct answer)
- Palladium-silver alloys
- Titanium alloys
Correct answer: Base-metal alloys containing copper
Base-metal alloys with high copper content are prone to green corrosion (verdigris) due to copper oxide formation in oral environments.
Question 3: When waxing a full-contour crown, the ideal buccolingual width of the occlusal table on a mandibular molar should be approximately what fraction of the total tooth width?
- One-half
- One-third (Correct answer)
- Two-thirds
- Three-quarters
Correct answer: One-third
The occlusal table should be approximately one-third of the total buccolingual width to reduce lateral forces and preserve periodontium.
Question 4: During the soldering of a fixed partial denture, which flux type is appropriate for high-fusing base-metal alloys?
- Boric acid and alcohol paste
- Fluoride-containing flux (Correct answer)
- Sodium borate (borax) flux
- Zinc chloride flux
Correct answer: Fluoride-containing flux
Fluoride-containing fluxes are required for base-metal alloys because they effectively dissolve the tenacious chromium oxide layer that forms on these alloys.
Question 5: A 'working cast' in crown and bridge fabrication differs from a 'master cast' primarily because the working cast:
- Is poured in a softer stone for easier die trimming
- Has removable dies that can be separated from the cast (Correct answer)
- Includes a full-arch impression for opposing occlusion
- Is fabricated from an alginate impression rather than polyvinyl siloxane
Correct answer: Has removable dies that can be separated from the cast
A working cast features individually removable dies so each preparation can be accessed independently while maintaining the full-arch relationship.
Question 6: Which die spacer application technique best ensures an adequate cement space without compromising marginal fit?
- Apply spacer from margin to occlusal surface uniformly
- Apply spacer stopping 0.5–1 mm short of the finish line (Correct answer)
- Apply spacer only on the occlusal surface of the die
- Apply two coats of spacer over the entire die including the margin
Correct answer: Apply spacer stopping 0.5–1 mm short of the finish line
Die spacer should stop 0.5–1 mm short of the finish line to create cement relief while allowing the crown margin to seat accurately at the preparation margin.
Question 7: When evaluating a metal coping for porcelain-fused-to-metal (PFM) crown, what minimum metal thickness is generally recommended to prevent flexure and porcelain fracture?
- 0.1–0.2 mm
- 0.3–0.5 mm (Correct answer)
- 0.8–1.0 mm
- 1.5–2.0 mm
Correct answer: 0.3–0.5 mm
A minimum coping thickness of 0.3–0.5 mm is recommended to provide adequate rigidity and prevent flexure that could crack the overlying porcelain.
CDT Crown & Bridge Fabrication 2
What is the primary purpose of a proximal contact in a fixed bridge pontic design?