MDS PREP
What is the preferred method of degassing for powered gold restorations?
1) An alcohol flame
2) An oxygen-acetylene flame
3) A butane flame
4) A nitrogen-hydrogen flame
The preferred method for degassing powered gold restorations is the use of an alcohol flame, specifically one that is a clean blue flame from absolute or 90% ethyl/methyl alcohol. The material is held in the flame until it reaches a dull red glow, which typically takes 15-20 seconds. This process removes any trapped air or volatile components that could compromise the integrity of the restoration.
If a partially set material is seated, the die material obtained from such impression will be
1) Oversized
2) Undersized
3) Retains its dimension due to elasticity of the material
4) Dimensions will be un changed but may leave a rough surface
Dental Materials Answer: 2
If Impression tray comprising of elastomeric impression material is seated too late as elasticity starts to develop, raised seating pressure is applied to overcome the stiffness of impression material.
Because of recovery of excessive elastic deformation there is development of distortion
The die produced in the inaccurate impression is too narrow and too short.
What is the primary advantage of using resin cements over other types of cements?
1) Higher compressive strength
2) Improved adhesion to the tooth structure
3) Lower film thickness requirement
4) Faster setting time
Resin cements are known for their excellent adhesion, providing a strong bond to tooth structure and restorative materials.
What is the maximum allowable level of mercury in the blood according to the given information?
1) 1 µg/L
2) 3 µg/L
3) 5 µg/L
4) 10 µg/L
The maximum allowable level of mercury in the blood is specifically mentioned as 3 µg/L in the provided text.
Maximum shrinkage on firing of ceramic occurs during which stage
1) High bisque
2) Low bisque
3) During condensation
4) None of the above
Dental Materials Answer: 1
Ceramic firing involves a series of processes that transform raw clay
materials into a hard, durable ceramic product. The stages of firing can be
broken down into several steps, but the primary ones are bisque firing and glaze
firing. The bisque firing is done before glazing and is typically the first
firing in the process. The purpose of bisque firing is to remove any remaining
moisture, organic matter, and to harden the ceramic piece to a certain extent,
making it suitable for the application of glazes.
There are two main types of bisque firings:
1. Low bisque: This is a lower temperature firing, usually around 950°C to
1000°C (1742°F to 1832°F). It is used for porous clay bodies and allows for some
shrinkage to occur, but it does not fully vitrify the clay. This stage is often
referred to as the biscuit or bisque stage.
2. High bisque: This is a higher temperature firing, typically between 1000°C to
1150°C (1832°F to 2102°F). It is used for clay bodies that require a higher
degree of vitrification and strength before glazing. At these temperatures, the
clay body experiences significant chemical changes and begins to shrink more
dramatically.
The maximum shrinkage of ceramic occurs during the high bisque stage because the
clay particles undergo a process called vitrification. Vitrification is the
point at which the clay's mineral content starts to melt and fuse together,
creating a glass-like phase within the clay matrix. This results in a stronger,
less porous, and denser material. As the minerals melt and the particles bond,
the overall volume of the material decreases, causing the shrinkage.
The shrinkage rate is influenced by various factors, such as the type of clay,
its composition, water content, and the specific temperature profile of the
firing process. However, it is generally during the high bisque that the most
significant shrinkage occurs. After the high bisque, the subsequent glaze firing
is at a higher temperature, which may cause additional shrinkage, but it is
usually less substantial compared to the initial bisque firing.
What is the range for dimensional change between 5 minutes and 124 hours for an amalgam specimen according to ANSI/ADA Specification No. 1 (ISO 24234)?
1) -5 to +10 ?m/cm
2) -10 to +15 ?m/cm
3) -15 to +20 ?m/cm
4) -20 to +25 ?m/cm
The specification requires that the dimensional change for a cylindrical specimen must fall within the range of -15 to +20 ?m/cm for 5 minutes to 124 hours after setting.
Brinnel hardness number of a dental gold alloy is directly proportional to its:
1) Tensile strength
2) Elongation
3) Modulus of elasticity
4) Modulus of resiLience
Dental Materials Answer: 1
Brinnel and Rockwell tests are cLassified as macrohardness tests and they aie not suitable for brittle materials.
The Knoop and Vickers tests are classified as microhardness tests. Both of these tests employ toads less than 9.8N.
The Shore and the Barcot tests are used for measuring the hardness of rubbers and plastics.
The Bnnnel test is the one of oldest tests used for determining the hardness of materials and is directly related to proportional limit and the ultimate tensile strength of dental gold alloys.
The convenience of the Rockwell test, with direct reading of the depth of the indentation, has lead to its wide usage.
The Knoop hardness test is used to obtain the values for both exceedingly hard and soft materials. The hardness value is independent of the ductility of the material.
The Vickers test is employed for dental costing gold alloys. It is suitable for determining the hardness of brittle materials.
What are the requirements for a good temporary cementing agent?
1) High compressive strength and good thermal conductivity
2) Low compressive strength and good thermal conductivity
3) High compressive strength and low thermal conductivity
4) Low compressive strength and low thermal conductivity
The ideal requirements for temporary cements include promoting the healing process and having a low compressive strength to facilitate removal.