NDEB - National Dental Examining Board Restorative Dentistry and Materials Questions and Answers — Questions and Answers
Question 1: A dentist is restoring a deep Class I occlusal preparation in a mandibular molar using a bulk-fill composite resin. Which factor associated with the cavity's geometry creates the highest polymerization shrinkage stress?
- Proximity of the preparation to the dental pulp
- The use of a high-intensity LED curing light
- The high ratio of bonded to unbonded surfaces (Correct answer)
- The thickness of the adhesive layer applied
Correct answer: The high ratio of bonded to unbonded surfaces
The Configuration Factor, or C-Factor, describes the ratio of bonded to unbonded surfaces of a restoration. A Class I preparation is a box with five bonded surfaces (mesial, distal, buccal, lingual, pulpal) and only one unbonded surface (occlusal). This high C-Factor of approximately 5 limits the resin's ability to flow and relieve stress during polymerization, leading to high internal stresses that can cause marginal gaps, debonding, and post-operative sensitivity.
Question 2: When cementing a lithium disilicate crown, which of the following is the mandatory surface treatment for the intaglio surface of the restoration to achieve a strong adhesive bond with a resin cement?
- Etching with 5% hydrofluoric acid (Correct answer)
- Sandblasting with 50-micron aluminum oxide
- Etching with 37% phosphoric acid
- Applying a metal/zirconia primer
Correct answer: Etching with 5% hydrofluoric acid
Lithium disilicate is a glass-ceramic material. To create micromechanical retention for adhesive bonding, its internal (intaglio) surface must be etched with hydrofluoric (HF) acid. This acid selectively dissolves the glassy matrix, creating a highly porous and retentive surface. This is followed by the application of a silane coupling agent to chemically link the ceramic to the resin cement. Phosphoric acid is used for tooth structure, and sandblasting is the primary treatment for non-glass ceramics like zirconia.
Question 3: Which of the following components was significantly increased in modern high-copper dental amalgams with the primary goal of eliminating the corrosion-prone Gamma-2 (γ2) phase?
- Zinc (Zn)
- Silver (Ag)
- Tin (Sn)
- Copper (Cu) (Correct answer)
Correct answer: Copper (Cu)
In traditional low-copper amalgams, the reaction between mercury and tin formed the weak and corrosion-susceptible Gamma-2 (Sn₇₋₈Hg) phase, which led to marginal breakdown and creep. In high-copper amalgams (containing 12% or more copper), the copper preferentially reacts with the tin to form the Cu₆Sn₅ (eta) phase. This reaction prevents the formation of the Gamma-2 phase, resulting in an amalgam restoration with superior longevity, lower creep, and better marginal integrity.
Question 4: A clinician is using a total-etch (etch-and-rinse) adhesive system for a Class II composite restoration. What is the primary function of the phosphoric acid etching step on dentin?
- To modify and incorporate the smear layer into the hybrid layer.
- To chemically bond with the calcium in hydroxyapatite.
- To disinfect the dentin surface before bonding.
- To remove the smear layer and demineralize the intertubular dentin. (Correct answer)
Correct answer: To remove the smear layer and demineralize the intertubular dentin.
In total-etch systems, 30-40% phosphoric acid is applied to both enamel and dentin. On dentin, its purpose is to completely remove the smear layer created during preparation and to demineralize the superficial dentin (3-5 micrometers). This exposes a network of collagen fibrils within the intertubular dentin, which the subsequent primer and adhesive will penetrate to form the hybrid layer, the primary mechanism for micromechanical bonding.
Question 5: An 8-year-old patient with high caries risk requires a restoration on the facial surface of a partially erupted maxillary first molar where moisture control is challenging. Which material offers the most significant therapeutic and practical advantages in this scenario?
- Packable composite resin
- Conventional glass ionomer cement (Correct answer)
- Dental amalgam
- Flowable composite resin
Correct answer: Conventional glass ionomer cement
Conventional glass ionomer cement (GIC) is the ideal choice in this clinical situation for several reasons. First, it chemically bonds to tooth structure and is more tolerant of a slightly moist environment than resin composites, which require strict isolation. Second, and most importantly for a high-risk patient, GIC releases fluoride, which can help inhibit recurrent caries and promote remineralization of adjacent tooth structure. The other materials lack these combined advantages.
Question 6: Which of the following restorative materials exhibits a coefficient of thermal expansion (CTE) that is most dissimilar to that of natural tooth structure, posing the greatest risk of percolation and marginal gap formation?
- Dental amalgam
- Microhybrid composite resin
- Unfilled acrylic resin (Correct answer)
- Feldspathic porcelain
Correct answer: Unfilled acrylic resin
The coefficient of thermal expansion (CTE) measures how much a material expands and contracts with temperature changes. A large mismatch between the restorative material and tooth structure (enamel CTE ≈ 11.4 ppm/°C) can lead to stress, marginal leakage (percolation), and debonding. Unfilled acrylic resin has a very high CTE (70-90 ppm/°C), making it the most dissimilar. Adding inorganic fillers to create composite resins significantly lowers the CTE (to 25-40 ppm/°C), bringing it closer to tooth structure. Amalgam and porcelain also have CTEs that are more compatible than unfilled resin.
NDEB - National Dental Examining Board Restorative Dentistry and Materials Questions and Answers
A dentist is restoring a deep Class I occlusal preparation in a mandibular molar using a bulk-fill composite resin.
Which factor associated with the cavity's geometry creates the highest polymerization shrinkage stress?