RDA Restorative and Impression Materials 2 — Questions and Answers

Question 1: What is the setting reaction of amalgam?

Acid-base reaction producing calcium phosphate
Reaction between silver-tin alloy and mercury (trituration) forming gamma-1 and gamma-2 phases (Correct answer)
Polymerization of resin monomers by light activation
Chemical cross-linking of silicone polymers

Correct answer: Reaction between silver-tin alloy and mercury (trituration) forming gamma-1 and gamma-2 phases

Amalgam sets through a reaction between the silver-tin alloy particles and mercury (during trituration/mixing) to form gamma-1 (Ag2Hg3) and gamma-2 (Sn7-8Hg) crystalline phases. Modern non-gamma-2 alloys are stronger and more corrosion-resistant.

Dental amalgam consists of silver-tin alloy (with copper and zinc) mixed with mercury by trituration (mechanical mixing in a capsule). The reaction produces: gamma (unreacted Ag3Sn), gamma-1 (Ag2Hg3 — strongest), and gamma-2 (Sn7-8Hg — weakest, most corrosion-prone). Modern high-copper alloys virtually eliminate gamma-2, improving strength and corrosion resistance. Initial set allows carving; final set takes 24 hours. Amalgam should not be polished until 24 hours post-placement.

Question 2: Which type of impression material is used for final impressions of crown and bridge preparations due to its high dimensional accuracy?

Alginate (irreversible hydrocolloid)
Zinc oxide eugenol (ZOE)
Polyvinyl siloxane (PVS/addition silicone) (Correct answer)
Compound (thermoplastic)

Correct answer: Polyvinyl siloxane (PVS/addition silicone)

Polyvinyl siloxane (PVS, addition-cured silicone) provides excellent dimensional stability, high detail reproduction, and can be poured multiple times, making it the material of choice for crown and bridge final impressions.

Polyvinyl siloxane (addition silicone) is an elastomeric impression material that sets by addition polymerization with no by-products, resulting in minimal dimensional change. It offers: excellent detail reproduction, high tear strength, dimensional stability for days (can be sent to lab without immediate pouring), hydrophobic (moisture compromises the impression — field must be dry), and available in various viscosities (putty/wash technique, monophase, heavy/light body). Alginate is fast but dimensionally unstable. ZOE is rigid (not elastic). Compound is a non-elastic thermoplastic.

Question 3: What is the purpose of bonding agent in composite resin restorations?

To etch the enamel before acid application
To chemically bond the composite resin to acid-etched enamel and/or dentin (Correct answer)
To block dentinal tubules from sensitivity only
To act as a base protecting the pulp

Correct answer: To chemically bond the composite resin to acid-etched enamel and/or dentin

Bonding agents (adhesives) form a chemical and micromechanical bond between composite resin and the acid-etched tooth structure (enamel and/or dentin), creating a durable interface that resists microleakage.

The composite bonding system: 1) Acid etching (37% phosphoric acid) creates microporosities in enamel by dissolving hydroxyapatite; 2) Primer (for total-etch systems) penetrates dentin and creates a hybrid layer; 3) Adhesive/bonding agent (Bis-GMA or urethane dimethacrylate resin) penetrates the etched surface and polymerizes with the composite. Modern systems include total-etch (etch-and-rinse), self-etch, and universal adhesives. Adequate bond strength reduces microleakage, secondary caries, and postoperative sensitivity. Moisture contamination of the bonded surface ruins the bond.

Question 4: What is the purpose of the zinc oxide eugenol (ZOE) temporary restoration?

Permanent pulp protection under final composite restoration
Temporary coverage and sedative effect on the pulp between appointments (Correct answer)
Etching the cavity preparation before bonding
Final luting cement for porcelain-fused-to-metal crowns

Correct answer: Temporary coverage and sedative effect on the pulp between appointments

ZOE temporary restorations provide a sedative (soothing) effect on the pulp due to eugenol's analgesic and anti-inflammatory properties, temporary cavity coverage, and a seal between appointments.

Zinc oxide eugenol (ZOE) is a sedative temporary filling material used between appointments (e.g., during endodontic treatment, waiting for fabrication of permanent restorations, or post-pulp cap). Eugenol is a natural phenol derived from clove oil with analgesic and mild anti-inflammatory properties. Limitations: eugenol inhibits polymerization of some composite resins, so ZOE must not be used under resin-based composites (use zinc oxide non-eugenol or IRM). It is also used as a base/liner under some permanent restorations and as a root canal sealer.

Question 5: When mixing type III gypsum dental stone for a diagnostic model, what does an increased water-to-powder (W:P) ratio result in?

Stronger, denser model with better surface detail
Weaker model with decreased strength and increased porosity (Correct answer)
Faster setting time
Darker final color

Correct answer: Weaker model with decreased strength and increased porosity

Too much water (high W:P ratio) produces excess porosity as water evaporates, resulting in a weaker, softer model with poor surface detail and lower compressive strength.

Gypsum products (plaster, stone, die stone) are mixed with specific W:P ratios: Type II plaster (50 mL/100g), Type III stone (30 mL/100g), Type IV die stone (22 mL/100g). Higher W:P ratios weaken the set material because excess water evaporates leaving pores. Lower W:P ratios (less water) increase strength but decrease workability and setting time. Mixing must be done under vacuum (vacuum spatulation) to minimize air bubbles. Models should be stored properly and not dried in an oven unless specifically designed for rapid setting.

Question 6: Which cement is most appropriate for the final cementation of a porcelain-fused-to-metal (PFM) crown on a vital tooth?

Zinc phosphate cement
Resin-modified glass ionomer (RMGI) cement (Correct answer)
Zinc oxide eugenol (ZOE)
Calcium hydroxide liner

Correct answer: Resin-modified glass ionomer (RMGI) cement

Resin-modified glass ionomer (RMGI) cement is widely used for final crown cementation because it provides good adhesion, fluoride release, low solubility, biocompatibility, and reduced postoperative sensitivity compared to zinc phosphate.

Luting cement options for PFM crowns: Zinc phosphate — strongest compressive strength, acidic mix can cause pulpal irritation, no adhesion, historical standard. RMGI (e.g., RelyX Luting, GC Fuji Plus) — fluoride release, minimal pulpal response, some adhesion, easy cleanup, widely used for vital teeth. Resin cement (e.g., Panavia) — highest bond strength, requires bonding protocol, used for all-ceramic and fiber post cementation. Polycarboxylate — chemically bonds to enamel/dentin, biocompatible. ZOE is only for temporary cementation. Selection depends on retention needs, tooth vitality, and restoration type.

RDA Restorative and Impression Materials 2

What is the setting reaction of amalgam?