PROSTHO - M5

Impressions

• An imprint or negative likeness

• placing some soft, semi fluid material in the mouth and allowing the material to set

elastic

•The impression materials most frequently used for cast restorations are ______ when removed from the mouth

reversible hydrocolloid

• has seen extensive use as an impression material in the fabrication of cast gold restorations for more than 70 years

• approximately 85% water

• It can lose water by syneresis

syneresis

water seeping from the surface

custom resin trays

•Have been used in elastomeric impression techniques because these materials are more accurate in uniform, thin layers of 2 to 3 mm

dual arch impressions

•Have some significant advantages over full conventional full-arch impression techniques

•Include the use of only one tray, which captures the impression of the prepared tooth, the adjacent and opposing teeth, and the relationship between them in maximal intercuspation without the need for an interocclusal record

•This type of impression tray produces extremely accurate occlusion in the resultant mounted casts

•This technique is best used for a patient with intact, mutually protected Angle class I occlusion

•The restoration should be for a single tooth with intact adjacent and opposing teeth

•There should be no arch-of-closure interference in closure to maximal intercuspation

•The trays are available in reusable metal or disposable plastic

Polysulfide

•An elastomer that is also known as mercaptan, thiokol, or simply as rubber base

•Possesses much greater dimensional stability than those hydrocolloid

•However, the polysulfide polymer does contract as curing occurs

•if maximum accuracy is to be obtained, a polysulfide impression should be poured within approximately 1 hour of removal from the mouth or less

•unpoured polysulfide impressions should never be sent to a laboratory

•Large undercut areas in the interproximal region should be blocked out in the mouth with a soft wax

•Otherwise, the impression may get locked in the mouth and distorted by excessive force that must be used to remove the tray

polysulfide

•Because of the hydrophobic nature of this material special care must be taken to ensure that there is no moisture on the preparation when the impression is made

•Thin layers of moisture on the surface can make the cast slightly larger, and moisture that becomes incorporated during the process can cause folds, creases, and voids in the impression

•Any hemorrhage or fluid seepage in the sulcus will result in voids and bubbles that will obscure the finish line

•Polysulfide is unique among impression materials because it is radiopaque

•If fragment becomes entrapped in the gingival sulcus or in a tissue space beyond a ruptured epithelial attachment, its exact location can be easily determined radiographically

•This property is the result of the presence of lead dioxide in the formula, which unfortunately contributes to its toxicity and tendency to irritate soft tissues when it does become trapped

Condensation Silicone

•Condensation-reaction silicones are so named because of the nature of their polymerization reaction

•They also could be called organo-tin silicones, which is a reference to their catalyst

•When the two are mixed, the materials are cross-linked by a reaction between terminal hydroxyl groups on the polymer and ethyl orthosilicate

base paste

is a liquid silicone polymer with terminal hydroxyl groups, mixed with inert fillers

reactor

a viscous liquid, consist of a cross-linking agent, ethyl silicate, with an organo-tin activator, tin octoate

the elimination of ethyl or methyl alcohol

• The condensation reaction occurs by ________

• responsible for the shrinkage of the material and resultant poor dimensional stability

true

•One of the problems in using condensation silicones has been its limited shelf life

•This is caused by the instability of the alkyl silicates in the presence of organo-tin compounds, which may result in oxidation of the tin

there is a much lower dimensional change in the bulk of impression

•The putty has a silica filler content of 75%, which is more than double that in the wash

•As a result, _____

because they provide reasonable accuracy with delayed pouring and do not require a custom tray

•The putty/ reline condensation silicones have become much more popular with dentists than the double-mix condensation silicones

Polyvinyl Siloxane (PVS)

•The dimensional stability of this group of impression materials is so much better than that of condensation silicone

•Polyvinyl siloxane silicone also is commonly called addition silicone, because of its setting reaction, sometimes vinyl polysiloxane, and even vinyl silicone

•Polyvinyl siloxane is least affected by pouring delays, or by second pours, and it is still accurate, even when poured 1 week after removal from the mouth

•In its unaltered form, PVS is hydrophobic

•Surfactants can be incorporated into the material to make it less hydrophobic and easier to pour

•Casts poured in impressions made with altered hydrophilic PVS exhibit fewer trapped voids than casts poured in an unaltered or conventional PVS

•Hydrophilic PVS materials will continue to be used because they are more convenient

Polyether

•Another type of elastomeric impression material that has become popular in the last 25 years

•The impression material exhibits accuracy on par with, or somewhat superior to, that of other elastomers

•It has excellent dimensional stability even when pouring is delayed for prolonged periods of time

•It is accurate when poured 1 week after removal from the mouth

•Polyether has an affinity for water, making it hydrophilic

•The material is stiff, and undercuts must be blocked out

•It is somewhat brittle

1.Chlorine compounds

2.Combination of synthetic phenolic compounds

3.Glutaraldehydes

4.Iodophors

5.Phenol-alcohol combinations

Five types of chemical materials that can be used for this purpose

(CEREC [chairside economical restoration of esthetic ceramics], Sirona)

• The first commercially available CAD/CAM system for fabricating dental restorations was also introduced in 1987

• This technology uses optical scanning and requires the entire area to be captured in the impression to be coated with a reflective powder

E4D Dentist System (D4D Technologies)

• This system uses laser scanning and requires no reflective powder

• can be connected directly to a milling machine to create the restoration