MDS PREP
Which of the following is not true about casting gypsum-bonded investment material?
1) Its used for cast metal alloys
2) 50—65% of gypsum changes to form α-hemihydrate
3) The investment material is not heated above 700°C temperature
4) Heating above 700°C causes formation of sulphur dioxide from copper sulphate
Dental Materials Answer: 1
Gypsum-bonded investments are used for gold alloys. The essential ingredients of the dental inlay investment employed with the conventional gold casting alloys are α-hemihydrate of gypsum, quartz or cristobalite. which are allotropic forms of silica.
Ist investments flow contain the α-hemihydrate of gypsum. because greater strength is obtained. This gypsum
product serves as a binder to hold other ingredients together and provide rigidity. The strength of the investment is dependent on the amount of binder present.
A slight expansion takes place between 400°C and approximately 700°C (1292°F). and then a large contraction occurs. This later shrinkage is most likely caused by decomposition and release of sulphur gases, such as sulphur dioxide.
This decomposition not only causes shrinkage but also contaminates the casting with the sulphides of the nonnohlc alloying elements, such as silver and copper. Thus, it is imperative that gypsum investments should not be heated above 700°C (1292°F). however, for gypsum products containing carbon, the maximum temperature should be 650°C (1202°F). In this way, proper ut and uncontaminated alloys are obtained.
What is the role of a primer in the bonding process of resin cements?
1) To clean the tooth surface
2) To increase the surface energy of the tooth structure
3) To reduce the surface tension of the bonding agent
4) To accelerate the setting of the cement
A primer increases the surface energy of the tooth structure, enhancing the ability of the bonding agent to wet and infiltrate the surface.
What is the purity requirement for mercury in dental amalgams according to ANSI/ADA Specification No. 6?
1) It must be at least 99.9% pure.
2) It must be at least 99.0% pure.
3) It must be at least 95.0% pure.
4) It must be at least 90.0% pure.
Mercury that complies with the requirements of the United States Pharmacopoeia also meets requirements for purity in ANSI/ADA Specification No. 6, which is typically 99.9% pure.
What is the typical concentration of camphorquinone used in dental composite resins?
1) 0.1 wt%.
2) 0.2 wt%.
3) 0.5 wt%.
4) 1.0 wt%.
Small quantities of camphorquinone, typically less than 0.2 wt%, are sufficient to initiate the polymerization process when the composite resin is exposed to blue light.
Ability of an orthodontic wire to spring back to Its original shape is evaluated by
1) Brittleness
2) Resilience
3) Tensile strength
4) Toughness
Dental Materials Answer: 2
Resilience is the amount of energy absorbed by a structure when it is stressed to proportional Limit.
The elastic area of the stress-strain graph gives resilience; entire area up to breaking point is a measure of toughness.
The slope of straight Line gives Young’s modulus.
What are the two primary purposes of using a luting agent in the placement of a single crown?
1) To achieve a tight seal and to enhance esthetics
2) To provide insulation against thermal changes and to reduce sensitivity
3) To bond the crown to the tooth and to protect the pulp
4) To disinfect the tooth and to provide a base for the crown
Luting agents are used to create a tight seal between the crown and the tooth and to enhance the retention of the restoration.
What is the primary reason for using zinc polycarboxylate cement as a luting agent?
1) Its high compressive strength
2) Its ability to bond chemically to tooth structure
3) Its ease of manipulation
4) Its insolubility in oral fluids
Zinc polycarboxylate cement is favored as a luting agent for its chemical bonding capability with tooth structure.
Before restoring a tooth with composite, the area is etched. Accidental contamination occurs with saliva, Therefore
1) the area is re-etched before restoration
2) the area is air-dried using an air spray and continue with restoring
3) wipe the saliva with cotton and continue to restore.
4) add excess composie material and cure it
Dental Materials Answer: 1
Etching of tooth surface with 37% phosphoric acid creates microporosities in the enamel and dentin. This is the main method of adhesion of composite resin to tooth surface.
After etching the surface energy of the enamel will increase. This will make it very sticky to any kind of contaminants. If salivary contamination will occur, the salivary proteins will precipitate in the microporosities. This will prevent the adequate penetration of the bonding agent, therby reducing the micotag formation. Hence, complete isolation is essential.