CDT Crown & Bridge Fabrication 3 — Questions and Answers
Question 1: In a three-unit fixed partial denture, the connector between the retainer and pontic should have a minimum cross-sectional area of approximately:
- 2 mm²
- 4 mm² (Correct answer)
- 6 mm²
- 9 mm²
Correct answer: 4 mm²
A minimum connector cross-sectional area of 4 mm² is generally required for anterior bridges, with 9 mm² recommended for posterior units to resist fracture under occlusal load.
Question 2: Which occlusal scheme is most commonly prescribed for implant-supported crowns to minimize off-axis loading?
- Balanced occlusion
- Group function
- Mutually protected occlusion (canine guidance) (Correct answer)
- Lingualized occlusion
Correct answer: Mutually protected occlusion (canine guidance)
Mutually protected occlusion (canine guidance) is preferred for implant crowns because it limits lateral forces to natural canines, reducing harmful off-axis loading on the implant.
Question 3: The term 'hygienic pontic' refers to a pontic design that:
- Has maximum tissue contact for stability
- Spans the edentulous space without contacting the ridge tissue (Correct answer)
- Is fabricated entirely from ceramic for biocompatibility
- Uses a convex tissue surface to facilitate cleaning
Correct answer: Spans the edentulous space without contacting the ridge tissue
A hygienic (also called sanitary) pontic is designed with a space between the pontic and the ridge so that oral hygiene can be maintained easily beneath it.
Question 4: During porcelain firing, what phenomenon causes the porcelain to shrink as it reaches its fusion temperature?
- Oxidation of metal oxides in the porcelain
- Crystallization of leucite during cooling
- Sintering and particle coalescence as glass flows (Correct answer)
- Dehydration of the porcelain slurry
Correct answer: Sintering and particle coalescence as glass flows
Shrinkage during porcelain firing occurs because sintering causes glass particles to coalesce and fuse, reducing the total volume and eliminating interparticle voids.
Question 5: What is the function of opaque porcelain in a porcelain-fused-to-metal (PFM) restoration?
- To create translucency and depth for natural appearance
- To mask the dark metal coping and bond to the alloy oxide layer (Correct answer)
- To provide the final surface characterization of the crown
- To act as a thermal insulator between metal and body porcelain
Correct answer: To mask the dark metal coping and bond to the alloy oxide layer
Opaque porcelain masks the dark metal coping and chemically bonds to the oxide layer on the alloy surface, establishing the ceramic-metal bond.
Question 6: Which condition would most likely result in a 'rocking' or unstable crown on the die before cementation?
- Insufficient die spacer applied to axial walls
- Internal nodule or positive irregularity inside the crown (Correct answer)
- Excessive taper on the preparation die
- Over-reduction of the occlusal surface on the die
Correct answer: Internal nodule or positive irregularity inside the crown
An internal nodule (positive irregularity) inside the crown prevents uniform seating and causes the crown to rock on the die.
Question 7: For a mandibular posterior three-unit bridge, which pontic design provides the best combination of esthetics and cleanability in a patient with a flat, keratinized ridge?
- Conical pontic
- Ridge-lap pontic
- Modified ridge-lap pontic (Correct answer)
- Ovate pontic
Correct answer: Modified ridge-lap pontic
The modified ridge-lap pontic offers good esthetics by contacting only the buccal aspect of the ridge while leaving the lingual surface convex for easy hygiene access.
CDT Crown & Bridge Fabrication 3
In a three-unit fixed partial denture, the connector between the retainer and pontic should have a minimum cross-sectional area of approximately: