1: Answers and Justifications to Review Questions and Case Studies

Chapter 1

1. b. The FDA is the regulatory agency, whereas the ADA develops standards and administers those standards and guidelines. The AADR is a professional organization for dental researchers. The OSHA mandates the practice of standard (universal) precautions for blood-borne pathogens.

2. c. It is not within the scope of dental hygiene practice to choose the restorative material for the patient. This is the responsibility of the dentist. Understanding how dental materials behave, educating the patient, and assessing the patient are all patient care responsibilities of the hygienist.

3. e. Gingival third caries, anterior and posterior and both lingual and facial, are defined as Class V.

4. c. The degradation of teeth and dental materials, biocompatibility, biting forces, esthetic demands of the patient, and temperature changes are all restrictions that limit the use of dental materials.

5. d. When the temperature changes in the oral cavity, no dental material expands and contracts by exactly the same amount as the natural tooth structure. Over time, this expansion and contraction may cause leakage around the restoration and tooth sensitivity.

6. c. The periodontium supports the tooth in a stable condition and gives feedback on the forces being placed on the teeth. The gingival tissues seal out undesirable agents and attach to the teeth, forming a barrier. The pulp serves to respond to thermal stimuli, as it contains the nerve cells.

7. a. When a significant amount of tooth structure is missing from a particular tooth, a crown will encircle and support the remaining tooth structure. A pontic is the “false tooth” on a bridge, whereas an implant replaces the entire tooth, including the root. A fixed partial denture replaces teeth that may be missing in an arch and is cemented in place.

8. a. It is true that the cavity preparation design helps to secure the restoration in place. But because gold castings such as an inlay are cemented in place and must be first seated without cement as a “try-in,” the walls must diverge so that it can be removed and then cemented. The converging walls of an amalgam preparation, as well as the added undercut areas, help to retain it in the cavity prep.

9. b. Technically, the only terms that would be interchangeable are study model and diagnostic cast. This replica is used to study the size and position of the oral tissues. If a restoration is constructed on the replica, it is called a cast.

10. d. Amalgam, composites, and glass ionomers are all direct restorative materials. They are placed directly into the cavity preparation once all the decay is removed. Porcelain materials must be fabricated in a dental laboratory because they are fired in an oven at high temperatures. Temporary restorations are usually made for the patient to be used between appointments while indirect restorations are being fabricated.

11. c. Class III devices are the most regulated and require premarket approval. Class I is the least regulated. There is no Class IV.

12. a. Calculus is visible on the distal surface of #2 but not others. Do not confuse the crestal bone with calculus. The teeth appear radiographically correct and no distortion has occurred during exposure.

Chapter 2

1. d. Window glass has strong atomic bonds but short-range order similar to that of liquids. The consistent spatial relationship between molecules is 5 to 10 neighbors apart. The short-range order is the reason why glass sometimes flows; an example is a window in a very old house where the bottom half of the window may be slightly thicker than the top half.

2. b. Metallic bonds are formed when the electrons are shared by all the atoms. Covalent bonds result when two atoms share a pair of electrons. Ionic bonds occur when an electron is “given up” by one atom and then accepted by another. Secondary bonds involve the uneven distribution of electrons. Partial charges result.

3. a. The valence electrons in a metallic bond are shared between atoms. Metals have distinct properties, which are the result of their atomic bond. The “negative cloud” and the “positive core” result in many weak, multidirectional primary bonds. In turn, this results in a strong material. Secondary bonds are formed when a partial charge results from an uneven distribution of electrons around an atom or molecule. Covalent bonds result when two atoms share a pair of electrons. Ionic bonds occur when an electron is “given up” by one atom and then accepted by another.

4. b. Translucent materials are those that allow some light to pass through, such as the incisal third area of anterior teeth. Dental ceramics can be matched in translucency and color to replace the natural tooth structure. A ceramic dental restorative material is neither transparent (all light passes through) nor opaque (no light passes through). Brittleness is a disadvantage of ceramics as a dental restorative material.

5. c. The cross-linking not only adds to the strength, but the more cross-links that are present, the more rigid and stiff the material becomes. Polymers will stretch and recoil, similar to the movements of a “Slinky,” if they have no or minimal cross-links.

6. d. Tin foil is a pure metal that has been compressed into a very thin film. The other materials listed are composed of two or more different materials. The resulting material has improved properties compared with those of either individual material.

7. a. A permanent dipole is a bond that has a permanent partial charge. Permanent dipoles form weak secondary bonds. The bonds in a fluctuating dipole are the result of an intermittent, uneven distribution of electrons around the atoms or molecules. Hydrogen bonds are “special” permanent dipoles because of the single electron. Primary bonds are strong bonds between atoms that involve the transfer or sharing of electrons.

8. a. In a metal, each positive core is surrounded by an electron cloud, and the core “feels” little difference when an atom slides by another atom. Since the surroundings (atomic bonding) do not change, neither do the properties. Ceramics would fracture when bent because of the ionic bonding and the resulting electron repulsion if a negative ion slides “one over” to another negative ion. Polymers may bend easily or may fracture depending on the bonding between the chains.

9. b. A composite is a material that is made from two or more different solid materials. A colloid is a mixture of gases, liquids, or solids at a microscopic level and is a suspension of one material in another. A solution is one material completely dissolved in another, whereas an emulsion is a type of colloid composed of two liquids that do not blend together to form one liquid. Their stability when mixed together is usually temporary.

10. c. A covalent bond results when two atoms share a pair of electrons and is one of three types of primary bonds. An ionic bond results when an atom gives up an electron and another accepts it. A metallic bond is a primary bond in which valence electrons are shared, but by all the atoms in the object, not just two. Secondary bonds are the result of partial charges from an uneven distribution of electrons around an atom or molecule.

Chapter 3

1. c. Good wetting indicates that the adhesive is in very close contact with the adherend, forming a low-contact angle. Poor wetting “stays as a drop,” and a high-contact angle is formed.

2. d. Tension is a pulling or stretching stress, which many times is referred to as tensile stress. Compression is a crushing or pushing force. Torsion is a twisting stress, and shear is a sliding stress.

3. c. The higher the modulus of elasticity, the stiffer is the material. The modulus is a measure of flexibility. A rubber band has a low modulus; a mouth mirror has a high modulus. Resilience is the ability of a material to absorb energy without deforming, whereas toughness is the ability of a material to absorb energy up to and including failure or fracture.

4. c. Nearly all materials will contract (sometimes ever so slightly) when cooled and expand when heated. Measuring this change in size (volume) in relation to the change in temperature is called the coefficient of thermal expansion. In dentistry, this property holds importance because, ideally, the tooth structure and the restorative material should expand and contract the same amount.

5. b. Remember that you cannot have stress (load) without strain (change in length). They occur together. When they are proportional, as illustrated on a graph, it is a straight line. As we increase the load (by adding more monkey charms), eventually, the spring will not go back to the original length. This stress is no longer proportional to the strain. We have reached the yield strength on the graph, and the line begins to curve.

6. a. The modulus of elasticity is defined as the stress divided by the strain in the linear portion of the stress–strain graph.

7. a. Other examples of physical properties are color and thermal conductivity. Strength and stiffness are mechanical properties, and setting reactions are chemical properties.

8. d. Torsion is a twisting stress, such as turning a doorknob. Shear stress occurs when parts of an object slide past each other. Tension is a pulling stress, and bending is a combination of compression, tension, and shear stresses.

9. c. Solubility is calculated as the amount of material that dissolves in a given time. Viscosity is the ability of a material to flow. Water sorption is the ability of a material to absorb water. Wetting is the interaction of a liquid with a surface.

10. b. Mechanical properties include stress, strain, resilience, toughness, fatigue, and elasticity, to name a few. Physical properties include thermal conductivity, heat capacity, vapor pressure, viscosity, hardness, and many more. Chemical properties describe setting reactions and decay and degradation. Biologic properties are the effects materials have on living tissue.

11. d. Thermal conductivity is the rate of heat flow through a material. It is measured as heat flow over time. Heat capacity is a measure of the amount of thermal energy a material can hoard (like a microwave trivet), whereas heat of fusion is the amount of energy required to melt a material. The coefficient of thermal expansion is a measure of change in volume in relation to the temperature change.

12. b. The definitions of these four properties are described in #11 above. The composite resin restorative material expands and contracts at a different rate than enamel and dentin. Over time, a gap is created between the tooth and restoration when it contracts (from cold beverage) and then closes when the soup is eaten. This process is called percolation and may result in microleakage, tooth sensitivity, and recurrent decay.

13. a. Those liquids that act as solvents and evaporate readily have a high vapor pressure. Materials with low vapor pressure do not evaporate quickly. All liquids have some degree of vapor pressure. A liquid’s vapor pressure is a constant and does not vary. Therefore, it cannot be “intermittent.”

14. b. Compression is defined as “a pushing or crushing stress.” The pressing of the plastic mass of amalgam by the amalgam condenser instrument is an example of compression.

15. a. Solubility is a measure of a materials’ ability to dissolve in water. The dental cement has a low solubility and serves the patient well by not dissolving for a long period of time in the patient’s mouth. Cements have the most demanding requirements of any material.

16. c. Stress concentration occurs near voids, pits, and cracks. Stress concentration may cause breaks and fractures at a much lower stress than if they were not present.

Chapter 4

1. a. The sticky material, or the bonding material that sticks the stamp to the envelope, is called the adhesive. The stamp and the envelope are referred to as adherends.

2. c. If the bracket breaks off cleanly between the bonding material and the enamel surface, it is called an adhesive failure. When the fracture or failure occurs within the bonding material, it is termed a cohesive failure. Residual bonding material would be present on both the bracket and the tooth surface.

3. The numbers for the correct sequence are 2, 5, 7, 1, 3, 6, 8, and 4. The tooth surface must first be cleaned and then rinsed and dried before the etchant can be placed. Next, the etchant is placed for 15 to 30 seconds. This, too, must be thoroughly rinsed and gently dried. The bonding resin is then applied, and it flows into the enamel micropores. The restorative material is applied last and bonds to the previously placed bonding resin.

4. c. A 37% concentration of orthophosphoric acid is commonly used. Lower concentrations have been tested but tend to be too weak. Higher concentrations are too aggressive.

5. d. The enamel rods are arranged less regularly than on permanent teeth. The surface must be etched longer to assure that the bonding (resin) material will be retained.

6. a. The micropores formed by the etchant are too small to be seen by the naked eye, so this bonding process is known as micromechanical bonding. Secondary, rather than primary, bonds are formed because the unset bonding resin flows around the decalcified collagen fibers and sets, thus entangling the two molecular chains together without primary bonds. Secondary bonds promote this entanglement.

7. b. This layer of “dentin debris” not only remains on the surface but also extends into the dentinal tubules. It is similar to sticky, sappy pinewood saw dust, and it adheres to the newly cut surface. After the smear layer is removed with etchant, a primer layer is applied and flows into the open tubules of etched dentin. The adhesive (layer) is a low-viscosity resin and sets similarly to enamel-bonding resin.

8. b. The adhesion/bonding mechanism reduces microleakage, which decreases the likelihood of recurrent decay. Bonding agents seal off the space between the tooth and the restoration. This protects the interface area not only from microleakage but also from postoperative sensitivity.

9. d. The hybrid layer is a combination of the resin adhesive and decalcified dentin. The dentin becomes decalcified during the etching process. A bond is formed when the collagen fibers (the organic component of the dentin) are surrounded by and become embedded in the resin adhesive material. Enamel tissue is not a part of the hybrid layer, and the smear layer is actually the “dentin debris” left by the dental bur.

10. c. It is a current belief that the smear layer is created when dentin tissue is cut during tooth preparation (see #7 above). Composites do not irritate the pulp; the microleakage of composites causes the irritation. Etching of dentin does not irritate the pulp tissue, and the smear layer does not need to be preserved.

Chapter 5

1. d. The coefficient of thermal expansion for restorative resins can be 2 to 10 times greater than tooth structure. Repeated expansion and contraction can open and close the gaps at the margin of the restoration. Other problems with resins include polymerization shrinkage and lack of abrasion resistance.

2. c. The silane coupling agent coats the filler particles of the composite and is chemically compatible with both the filler and the resin phases. It also transfers the stress from the weaker resin phase to the stronger filler phase. The matrix is composed of a soft resin, which has little strength and wears easily. The filler particles are made of “engineered” glass and add strength to the restoration. Polymers are another name for plastics, and adhesives are materials that will stick to a flat surface or bond two flat surfaces together. (These are discussed in Chapter 4, Adhesive Materials.)

3. b. Because of the very small particle size, microfills finish and polish better than any other type of composite. They have, however, a high coefficient of thermal expansion, low strength, and a low percentage of filler (40–50%).

4. c. With chemical-cure materials, two pastes are mixed and set by a chemical reaction. This limits the operator’s working time. There is also a possibility of incorporating air bubbles while mixing. Many contain more matrix material and less filler than light-cure products.

5. a. Depth of cure is the amount or increment of composite that can be successfully cured by a light source. Incremental addition refers to adding and curing small amounts of composite material in the cavity preparation.

6. d. The smaller the particle size of the filler, the better will be the finish and polish of the surface of the restoration. Smaller particles are less likely to break down and fall out of the resin matrix, thus leaving a roughened surface to accumulate stain and debris. The size of the filler particle does not affect the polymerization reaction, the technique for adding increments, or the etching time.

7. c. A decreased filler content decreases the viscosity of the material and, therefore, increases the “flowability.” This makes placement easier for the dentist. Flowable composites are usually placed under a hybrid composite material. Condensable composites aid in placement by inhibiting filler particles from sliding by one another. This makes them feel stiffer and thicker than the typical composites. Hybrid composites contain several particle sizes to create a material that has superior strength and that polishes well.

8. b. Hybrid composites are used in restorations undergoing moderate stress in which strength and wear resistance are more important than surface luster. For low-stress areas, high surface luster, and Class V restorations, microfill composites are used.

9. d. Preventive resin restorations combine a pit-and-fissure sealant and a composite restoration. The composite is placed first, with any remaining pits and fissures being covered by the sealant material. Compomers are a mix of glass ionomer and composite materials. Flowable composites will “flow” into the preparation because of lower viscosity. Condensable composites contain filler particles that inhibit them from sliding past each other for a “stiffer, thicker” feel. This consistency allows for easier placement.

10. d. Abrasion resistance for hybrid composites is very good compared to the other composite types. Abrasion resistance is poor for the older, macrofilled composites.

11. a. Thermal expansion is poor for microfill composites. For hybrid composites, thermal expansion is considered to be good.

12. b. Hybrid composites have 75–80% filler by weight. Microfills have 40–50%, and macrofills have 70–75%. Flowable composites are 50% filler by weight.

13. c. Light-activated composites are supplied as a single paste, with no mixing required. Polymerization does not begin until the material is exposed to a very bright light. They are popular among dentists because they will “set on demand” (dentist has control of setting time). The chemically activated materials are usually a two-paste system that has to be mixed together to have the polymerization reaction begin. Once mixing begins, the reaction takes place until a set (hard) product forms.

14. c. Compomers bond and set like composite resins, but initially release fluoride-like glass ionomers. Flowable composites are those that have a lower viscosity due to decreased filler content. Condensable composites are those that have a “thicker, stiffer feel,” which possibly makes condensation easier. Preventive resin restorations are a combination of pit and fissure sealants and composite restorations; the composite is placed first, then the rest of the area is sealed.

15. d. Microfill composites have filler particles sized from 0.03 to 0.5 μm, low strength, good abrasion resistance, and have very good polishability. Macrofill composites have 10- to 25-μm particle sizes, fair strength, poor abrasion resistance, and poor polishability. Hybrid composites range in particle size from 0.5 to 1 μm, good strength, very good abrasion resistance, and good polishability.

Chapter 6

1. d. Remember that the dental amalgam is composed of mercury and amalgam alloy, approximately 50% each. It was mentioned that silver (Ag) comprises 65% of the alloy, but this is 65% of the 50% of the alloy component of the dental amalgam. This reasoning is also true for copper (Cu) and tin (Sn). Aluminum (Al) is not found in dental amalgam.

2. c. Amalgam has a high compressive strength, but tensile and shear strengths are comparatively low. Bending strength is a combination of tensile and compression strengths and is low as well.

3. c. Tin–mercury comprises the gamma-two (γ2) phase of the setting reaction of low-copper (traditional) dental amalgams. “Gamma” is used to designate the silver–tin (Ag–Sn) phase. “Gamma-one” is used to designate the silver–mercury phase (Ag–Hg). There is no tin–tin compound in the setting reaction of amalgam.

4. b. Silver causes setting expansion and increases the strength and corrosion resistance. Zinc will minimize oxidation. Tin will reduce strength and corrosion resistance and will maximize oxidation.

5. b. Lowering the mercury content increases the strength and results in less marginal breakdown. Increasing the mercury content decreases the strength and results in more marginal breakdown.

6. b. The marginal seal of corrosion products that forms over time, between the inner surface of the tooth and the restoration (interface), is the feature that makes amalgam a successful restorative material. The price and the ease of use are advantages, but they do not account for its success as a restorative material. Finishing and polishing can extend the service of the restoration, but the seal at the interface is the most important feature contributing to its success.

7. c. The proper trituration technique is controlled by the dentist, whereas the composition of the alloy, rate of the setting reaction, particle shape, and particle size of the alloy are controlled by the manufacturer.

8. f. An amalgam restoration that has been finished and polished reduces the ability for plaque to adhere, resists tarnish and corrosion, and is more likely to have continuous margins with tooth structure. Voids in the restoration are the result of the condensation technique of the clinician. They are not related to finishing and polishing.

9. b. In the high-copper amalgam reaction, tin reacts with copper rather than with mercury. This eliminates the undesirable gamma-two phase. Tin reacts with silver to form the gamma phase, an Ag–Sn compound. Tin reacts with mercury in the low-copper (traditional) setting reaction to form gamma-two. Zinc, if present, reduces oxidation of the other metals in the alloy.

10. a. The life expectancy of an amalgam restoration is indirectly related to the size of the restoration. As the restoration increases in size, so do the internal stresses, thus decreasing the life expectancy.

11. a. Amalgams using lathe-cut alloy particles require more force during condensation because these particles are rough and do not slide past each other easily. In contrast, spherical particles have a soft, “mushy” feel, and a condenser may push through the mass easily. The blended or admixed alloy particles would have an in-between consistency or “feel,” that of spherical or lathe-cut.

12. b. Overtriturated amalgam is difficult to condense, exhibits a shortened working time, and tends to crumble. Undertriturated amalgam has a mushy-grainy feel and is difficult to properly condense. It may also appear soupy. Properly triturated amalgam is cohesive, smooth, and plastic. The mercury–alloy ratio is set and proportioned by the manufacturer and is usually not the cause of a poor consistency and decreased setting time.

13. d. Amalgam is a cost-effective restorative material that provides good service over a reasonably long time. It is used for amalgam cores, tooth fractures, and cingulum pits on anterior teeth. It is commonly used for Class I, II, V, and VI caries. It is seldom used for Class III and IV caries because the “silver” color can be seen in the interproximal and facial areas.

14. c. One-tenth of 1% (0.1) of the population has a true allergy to mercury. Therefore, for the vast majority of patients, mercury toxicity is not a problem.

15. b. When properly placed, direct gold, or gold foil, is a long-lasting restoration. It is placed by condensing it into the cavity preparation. Its cost is reasonable, but the cost of labor to place it can be expensive. A disadvantage is the lack of strength when compared to other metallic restorations.

Chapter 7

1. d. Glass ionomer is the most common cement at this time that is supplied in capsule form and mixed in the amalgamator.

2. e. Glass ionomer and ZOE cements are used for caries control. Glass ionomer cements leach fluoride that inhibits caries. ZOE has an obtundent effect on the pulp; this is an advantage when caries are deep. Other cements do not have these advantages.

3. a. Only glass ionomer cements release fluoride. ZOE does, however, protect and soothe the pulp, but it is weak and lacks long-term strength compared to other cements.

4. c. In addition to its lack of strength, ZOE is the most soluble compared to glass ionomer, zinc phosphate, and polycarboxylate. Glass ionomer is the least soluble, with zinc phosphate and polycarboxylate ranking second and third, respectively, in their ability to resist solubility.

5. b. Glass ionomer must be mixed within 1 minute to achieve the correct consistency. It must also possess a shiny appearance on luting to maintain maximum adhesion.

6. a. In oral fluids, cements are much more soluble than gold alloy, porcelain, or porcelain-fused-to-metal restorations.

7. d. Because zinc phosphate produces an exothermic reaction during manipulation, it is important to release this heat during mixing. This is done by spreading the mix over the entire surface of the slab so that the heat can dissipate. This technique is not necessary for ZOE, polycarboxylate, or glass ionomer.

8. c. Calcium hydroxide promotes the formation of secondary dentin. Silicates, temporary cements, and composite cements do not promote the formation of secondary dentin.

9. b. Fluoride is added to the raw materials of certain cements to make manufacturing of the glass powder easier. An added clinical benefit is the leaching and incorporation of fluoride into dentin and enamel, which serves as a deterrent to secondary decay.

10. d. Composite cement would be the luting agent of choice because it directly bonds the ceramic restoration to tooth structure. It is assumed that a dentinal bonding system is used before the cement is applied. Composite cements are also called resin cements. Zinc phosphate and polycarboxylate are more opaque and tend to “defeat the purpose” of the esthetic, translucent properties of the ceramic. ZOE is also opaque but can only serve as a temporary luting agent due to its low strength and high solubility.

11. a. The advantages of polycarboxylate cement are its ability to bond to tooth structure (one of the first) and its biocompatibility. It is usually used as a luting cement and an intermediate base. Its disadvantages are lack of strength and moderate solubility.

12. g. Zinc oxide and eugenol cement serves many purposes for the dental patient. Formulations of ZOE are used for luting temporary crowns, temporary restorations, and intermediate bases under permanent restorations. It is soluble and lacks strength, so it is not recommended as a permanent restorative material or a luting agent for permanent crowns.

Chapter 8

1. b. A major advantage of alginate impression material is the ease of use. It is also fairly inexpensive. It is very much affected by the gain or loss of water. It should be poured as soon as possible because of its lack of stability.

2. b. Elastomeric impression materials, or elastomers, are flexible and may be deformed and then return to their original shape. Thermoplastic impression materials may be elastic or inelastic. They set by a physical change when cooled. Inelastic materials cannot be stretched; they also are rigid. Resins, by definition, are rigid polymers.

3. d. An increase in the water temperature will increase the rate of setting or shorten the gelation time. The mix is still usable regardless of water temperature as long as there is adequate working time.

4. d. “Aqueous” refers to water. Therefore, the impression material must be water based. In dentistry, there are two such materials: reversible and irreversible hydrocolloid. Irreversible is the only water-based choice. Polysulfides and addition silicones are elastomers. Impression compound is much like wax. ZOE impression material is an inelastic material and contains no water in its composition.

5. b. The brown paste in polysulfide impression material is the accelerator; the white paste is the base. Retarders are chemicals added to materials to decrease the setting rate. Fillers are added to the pastes to control the viscosity.

6. d. Dental impression compound is a thermoplastic material. It can be heated and softened repeatedly. Alginate is an example of an irreversible material. Once it has set, it cannot be resoftened. The other terms, chemoplastic and hydroelastic, are not used to describe impression materials.

7. b. ZOE impression paste sets into a hard, brittle mass. This makes it inelastic (or not elastic). Polysulfide and addition silicone are elastomers (they are elastic and will flex). Alginate also sets into an elastic material.

8. d. The hydrocolloids, alginate and agar, set by changing from a sol to a gel. This setting process is called gelation. Gypsum products (Chapter 9) set by a crystallization process, and elastomeric impression materials set by polymerization. In dentistry, curing is another term for polymerization.

9. b. The use of agar impression material does require special equipment, but the impression material is very reasonably priced. The detail reproduction is excellent, and the impression is easy to pour compared to elastomeric impression material.

10. c. The shrinkage and exudation of water is syneresis. Imbibition occurs when the impression absorbs water, such as during long exposure to disinfecting solutions. Gelation is the term given to the setting process of hydrocolloid material. Hysteresis refers to a material’s characteristic of having a melting temperature different from its gelling temperature.

11. a. Addition silicones are the most popular of the elastomeric impression materials, but cost is not the reason. They are expensive. The ease of use and the excellent characteristics and properties account for their popularity.

12. c. The accurate fit of the custom tray to the patient’s oral tissues requires an existing model. Therefore, an alginate impression is usually taken and poured with a gypsum product (stone or plaster), and the custom tray is then made on the study model.

13. b. Alginate would be the impression material of choice for these impressions. They are poured in dental stone and used for diagnosis and treatment planning. The accuracy of alginate is adequate for this purpose but not for final impressions. Agar and elastomeric impression materials are final impression materials, which means that casts and dies are made from these impressions on which restorations are fabricated. Dental impression compound is used for the fabrication of dentures.

14. c. With a triple tray, three “records” can be obtained: the prepared tooth, the bite registration, and the impression of the opposing tooth. It is considered a special use tray. A custom tray is made on a model of the patient’s arch with an acrylic resin. It is custom-made by a dental professional. A stock tray is an “off the shelf” tray and can be purchased in a variety of shapes, sizes, and materials. Bite registration trays, another special use tray, record the occlusal surfaces of both arches.

15. a. ZOE, alginate, and addition silicone impression materials all set by a chemical reaction. Wax used as an impression material, agar, and impression compound set by a physical change and will solidify or gel when cooled. These are said to be thermoplastic materials.

Chapter 9

1. a. The amount of water added to a gypsum product is directly related to its strength. The higher the W/P ratio, as in plaster, the softer and weaker is the resulting gypsum product.

2. d. Potassium sulfate is a common accelerator for gypsum products, as is borax, a retarder. Oleic acid, glycerin, and ethyl alcohol are not used to change the setting time of gypsum products.

3. d. High-strength stone may also be called die stone, improved stone, Type IV gypsum, Densite, or modified alpha-hemihydrate. Plaster is referred to as beta-hemihydrate or Type II gypsum. Dental stone may also be called Hydrocal, Type III gypsum, or alpha-hemihydrate.

4. b. As a standard setting time for gypsum products, 30 to 45 minutes for a final set is given. The Gillmore needle (or fingernail) should not leave indentations and can be used to determine a final setting time.

5. b. For 50 g of stone, 14 to 15 mL of water is required. For 100 g of stone, 28 to 30 mL of water is used. For 50 g of improved stone, 10 to 12 mL of water is used. For 100 g of plaster, 45 to 50 mL of water is needed.

6. c. Study models are used for observation, diagnosis, and treatment planning. Restorations or appliances are not made on them, as they are usually constructed from weaker gypsum products. Casts are replicas that are usually constructed with improved stone, and appliances and restorations are fabricated on them. A die is a working replica of a single tooth and is usually part of a cast.

7. a. Increasing the setting time would mean that it takes more time for a product to set. Thus, decreasing the setting time results in a product that sets faster, requiring less time to set. Manipulating the setting time does not enhance the properties of a gypsum product; in fact, it can be deleterious to them.

8. c. Because plaster requires the most water for a correct mix (proper W/P ratio), it would be the weakest, softest, and most porous of the three common gypsum products. Improved stone would be the most dense and the strongest, and dental stone would fall between plaster and improved stone.

9. b. Dry strength refers to the strength when excess water is not present. This may be two or three times the wet strength. The wet strength is measured at the final set (30–45 minutes). The initial strength occurs when loss of gloss is present and is an indication of the working time.

10. c. It is recommended to first add the water and then the powder to the mixing bowl. As a result, less air is incorporated into the mix. “Eyeballing” is not recommended because the W/P ratio will not be correct and an inferior product will result. Adding water to the powder or adding powder and water to the bowl simultaneously tend to increase the porosity of the resulting mix.

11. d. Final setting time can be determined by attempting to penetrate the material with a dull instrument or a fingernail. Initial setting time is said to occur when the material loses its shine, or “loss of gloss.” Final setting time takes approximately 30 to 45 minutes. The change from wet to dry strength usually occurs after 8 hours.

12. a. When setting time is increased, it takes the material longer to set. A decreased setting time yields a faster setting material or takes less time to set. If the setting reaction is decreased, the material will take longer to set, and if it is increased, it will take less time to set.

Chapter 10

1. c. The gypsum investment material that comprises the mold must be expanded to compensate exactly for the shrinkage of the metal casting. Otherwise, the cast restoration will not fit the preparation of the patient’s tooth.

2. c. The mold (investment) must expand and provide compensation for the metal contraction during casting. The main, obvious purpose of the investment, however, is to create a mold from the wax pattern in the casting process.

3. c. Porcelain has outstanding esthetic properties, such as translucency and life-like appearance. It is very brittle, however, and may fracture when subjected to high occlusal forces. It is recommended for veneers and low-stress crowns for anterior teeth, such as maxillary lateral incisors.

4. b. The “crown” portion of a dental bridge is called a retainer. Joined to the retainers are the artificial, replacement teeth called pontics. The prepared teeth onto which the bridge is cemented are called abutments.

5. e. Inlay wax is used for casting procedures. It leaves no residue in the burnout process and has a higher melting temperature. It is also harder than other dental waxes. Sticky wax is also a hard wax that melts at a higher temperature, but this wax is used as an “attachment medium,” such as attaching a sprue to a pattern. Boxing wax is used to pour impressions, and baseplate wax is used in the fabrication of dentures.

6. c. Burnout during casting is done to eliminate the wax pattern and create a mold space for the molten metal. Investment covers the wax pattern that has been mounted within the casting ring with a sprue. The crucible becomes heated when the solid casting metal is melted and turned into liquid, but it is an adjunct to the melting of the metal. Shrinkage compensation is necessary for the mold; it must expand the exact amount that the metal will shrink during casting.

7. a. Once the pattern is waxed, the sprue is gently attached with inlay or sticky wax. It is then placed into the crucible former (also called the sprue base). Next, a gypsum investment is usually vacuum mixed and poured into the casting ring that contains the wax pattern (investing process). After the casting ring is “burned out,” the metal is melted and forced into the casting ring (casting).

8. c. Thirty-three percent of 1,000 parts is 333. Fineness is described in parts per 1,000; hence, 33% gold alloy would be expressed as 333 fine or 8 carat. A 50% gold alloy would be 500 fine or 12 carat.

9. b. Changing porcelain powder to a solid is called sintering. The powder is not melted, so the shape of the restoration is maintained. Burnishing involves pushing the metal toward the tooth to close gaps between the casting and the tooth. Investing is covering a wax pattern mounted in a casting ring with a mixed gypsum product (investment).

10. a. It is true that castable glass is one type of an all-ceramic restoration. All-ceramic restorations are not superior to ceramometal restorations in the aspect of strength and that makes the second statement false.

11. c. Amalgam, gold foil, and composite restorations are direct restorations, which mean that they are constructed directly in the oral cavity. It usually involves placing the restorative material “directly” into the cavity preparation. Other restorations, such as inlays, onlays, crowns, and bridges, are constructed outside of the oral cavity, in a dental laboratory. Hence, they are referred to as “indirect” restorations. The term “fixed” is used to designate that it is not removable because it is cemented, or luted, in place. A fixed restoration is not removable by the patient.

12. b. A veneer is a thin layer of material that covers another material. In dentistry, it is a restoration that is placed on the facial surface of anterior teeth to cover, or “veneer,” esthetic problems. A buildup (core) is when the crown of a tooth is broken down to the point of needing to be “rebuilt” to hold the final restoration. A coping is the metal substructure that supports the porcelain layers in a ceramometal restoration. A pontic is a replacement tooth.

13. d. A provisional crown is the appropriate, professional name (especially used by prosthodontists) for a temporary crown.

14. c. Precious metals include the noble metals and silver. The noble metals are gold, platinum, and palladium. Nonprecious metals are those that do not contain any noble elements. High-noble metals contain 60% or more gold and other noble elements.

15. a. Bonding porcelain to metal in a ceramometal restoration provides a precise fit to the prepared tooth structure. The disadvantage is the opacity, because of the metal. There is no difference in the patient’s ability to remove plaque whether it be an all ceramic or ceramometal crown.

Chapter 11

1. b. Once the denture teeth are set in the wax rims, it is called a denture setup. The other terms are not used to describe this step of denture fabrication and are listed as distracters.

2. a. A thin layer of material is ground away. The monomer is applied to dissolve some of the set material. One end of the dissolved polymer remains embedded in the acrylic denture resin. New material is then mixed and applied, and the new polymer chains entangle with the old dissolved chains. Applying monomer to the surface does not affect the setting reaction, finishing and polishing, or color of the denture.

3. d. The major difference between heat-activated and chemically activated resin systems is that no chemical activator is present in the liquid of the heat-activated resins. Heat-activated resins have more strength than chemically activated resins, and there is less, not more, inhibitor in the heat-activated liquid.

4. a. Today’s denture teeth are usually acrylic rather than porcelain. Porcelain teeth are, however, harder than acrylic teeth and cause excessive wear to the natural opposing teeth.

5. c. Partial denture frameworks usually include clasps, connectors, and mesh. The denture base and teeth are not considered to be part of the framework, although they are a part of the removable partial denture.

6. b. Toughness is the ability of a material to absorb energy beyond the yield point, up to the failure point. Resilience is the ability to absorb energy and not become deformed until it reaches the yield point. Fatigue is the failure of an object after being stressed repeatedly for a long time. Creep is the small change in shape that results when an object is under continuous compression.

7. b. A maxillary denture is much easier to wear than a mandibular denture because of the extension of the borders of the denture (which helps to create the seal) and the larger surface-bearing area. Saliva helps to achieve the seal and improve the suction.

8. d.

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Feb 11, 2020 | Posted by in Dental Materials | Comments Off on 1: Answers and Justifications to Review Questions and Case Studies
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