AGD Prosthodontics: Fixed and Removable 5 — Questions and Answers
Question 1: Which of the following is the MOST important property of a die stone used for crown fabrication?
- High expansion to compensate for metal shrinkage
- Low setting expansion and high compressive strength (Correct answer)
- Rapid setting time under 5 minutes
- High porosity to absorb excess moisture
Correct answer: Low setting expansion and high compressive strength
Die stone requires minimal setting expansion (to maintain accuracy) and high compressive strength to withstand waxing and die trimming without fracture.
Question 2: A patient in centric occlusion shows heavy contact on the mesial incline of an upper molar cusp and the distal incline of a lower molar cusp. This type of premature contact is classified as:
- A working-side interference
- A balancing-side interference
- A DURD (distal upper, mesial lower) contact in centric relation (Correct answer)
- A protrusive interference
Correct answer: A DURD (distal upper, mesial lower) contact in centric relation
A premature contact on the distal incline of an upper cusp (DUML) or mesial lower sliding distally from CR to CO is characterized as a DURD/CR discrepancy contact.
Question 3: All-ceramic crowns are considered contraindicated in which clinical situation?
- Anterior esthetic restorations in low-caries-risk patients
- Patients with documented bruxism and heavy posterior occlusal forces (Correct answer)
- Teeth requiring a full-coverage restoration after root canal therapy
- Patients who have a documented allergy to metal alloys
Correct answer: Patients with documented bruxism and heavy posterior occlusal forces
Patients with bruxism generate extreme occlusal forces that can fracture all-ceramic restorations, particularly in the posterior region where forces are greatest.
Question 4: The major connector for a maxillary removable partial denture should:
- Cover as much palatal tissue as possible to maximize support
- Be rigid and positioned at least 6 mm from the gingival margins (Correct answer)
- Extend to within 1 mm of the gingival margin for stability
- Always be a palatal strap design regardless of tooth loss pattern
Correct answer: Be rigid and positioned at least 6 mm from the gingival margins
A rigid major connector is essential for force distribution, and placement at least 6 mm from gingival margins prevents tissue irritation and gingival inflammation.
Question 5: The primary advantage of using a custom impression tray over a stock tray for a crown impression is:
- Custom trays are less expensive to fabricate
- Custom trays provide uniform thickness of impression material around the prepared tooth (Correct answer)
- Stock trays cannot be used with polyvinyl siloxane materials
- Custom trays set faster than stock trays
Correct answer: Custom trays provide uniform thickness of impression material around the prepared tooth
Uniform material thickness in a custom tray reduces dimensional distortion and shrinkage variation, resulting in a more accurate impression of the preparation and margins.
Question 6: During try-in of a wax denture, the patient's upper lip appears flat and unsupported with insufficient tooth display at rest. The appropriate correction is to:
- Increase the vertical dimension of occlusion
- Move the anterior teeth labially or increase their incisal length (Correct answer)
- Reduce the labial flange thickness
- Reposition the posterior teeth buccally
Correct answer: Move the anterior teeth labially or increase their incisal length
Insufficient labial tooth display and poor lip support are corrected by repositioning the maxillary anterior teeth labially and/or increasing incisal length at the wax try-in stage.
Question 7: Which material property is most responsible for the clinical failure of zinc phosphate cement as a luting agent in high-moisture or pulpally challenged teeth?
- Low film thickness compared to other cements
- High initial acidity causing pulp irritation and solubility in oral fluids (Correct answer)
- Inability to bond to dentin surfaces
- Excessive compressive strength leading to tooth fracture
Correct answer: High initial acidity causing pulp irritation and solubility in oral fluids
Zinc phosphate cement has a low initial pH (~3.5), which can irritate vital pulps, and it is relatively soluble in oral fluids over time, leading to marginal leakage.
AGD Prosthodontics: Fixed and Removable 5
Which of the following is the MOST important property of a die stone used for crown fabrication?