NEET MDS Lessons
Dental Materials
Principles of cutting, polishing, and surface cleaning
- Surface mechanics for materials
Cutting-requires highest possible hardness materials to produce cutting
Finishing-requires highest possible hardness materials to produce finishing, except at margins of restorations where tooth structure may be inadvertently affected
Polishing- requires materials with Mohs ./ hardness that is 1 to 2 units above that of substrate
Debriding-requires materials with Mohs hardness that is less than or equal to that of substrate to prevent scratching
- Factors affecting cutting, polishing. and surface cleaning
- Applied pressure
- Particle size of abrasive
- Hardness of abrasive
- Hardness of substrate
- Precautions
- During cutting heat will build up and change the mechanical behavior of the substrate from brittle to ductile and encourage smearing
- Instruments may transfer debris onto the cut surface from their own surfaces during cutting, polishing, or cleaning operations (this is important for cleaning implant surfaces)
Impression Material
|
Materials |
Type |
Reaction |
Composition |
Manipulation |
Initial setting time
|
|
Plaster |
Rigid |
Chemical |
Calcuim sulfate hemihydrate, water |
Mix P/L in bowl |
3-5 min
|
|
Compound |
Rigid |
Physical |
Resins, wax, stearic acid, and fillers |
Soften by heating
|
Variable (sets on cooling) |
|
Zinc oxide-eugonel |
Rigid |
Chemical |
Zinc oxide powder, oils, eugenol, and resin |
Mix pastes on pad
|
3-5 min
|
|
Agar-agar |
Flexible |
Physical |
12-15% agar, borax, potassium sulfate, and 85% water |
Mix P/L in bowl
|
Variable (sets on cooling)
|
|
alginate |
Flexible |
Chemical |
Sodium alginate, calcium sulfate, retarders, and 85% water |
Mix P/L in bowl
|
4-5 min
|
|
Polysulfide |
Flexible |
Chemical |
Low MW mercaptan polymer, fillers, lead dioxide, copper hydroxide, or peroxides |
Mix pastes on pad
|
5-7 min
|
|
Silicone |
Flexible |
Chemical |
Hydroxyl functional dimethyl siloxane, fillers, tin octoate, and orthoethyl silicate |
Mix pastes on pad
|
4.5 min
|
|
Polyether |
Flexible |
Chemical |
Aromatic sulfonic acid ester and polyether with ethylene imine groups |
Mix pastes on pad
|
2-4 min
|
|
Polyvinyl siloxane |
Flexible |
Chemical |
Vinyl silicone, filler, chloroplatinic acid, low MW silicone, and filler |
Mix putty or use two-component mixing gun
|
4-5 min
|
Applications
a. Dentulous impressions for casts for prosthodontics
b. Dentulous impressions for pedodontic appliances
c. Dentulous impressions for study models for orthodontics
d. Edentulous impressions for casts for denture construction
ACID ETCH TECHNIQUE
Cavities requiring added retention (to hold firmly) are treated with an acid etching technique. This technique improves the seal of the composite resin to the cavity wall. The enamel adjacent to the margins of the preparation is slightly decalcified with a 40 to 50 percent phosphoric acid solution. This etched enamel enhances the mechanical retention of the composite resin. In addition, the acid etch technique is used to splint unstable teeth to adjacent teeth. The acid is left on the cut tooth structure only 15 seconds, in accordance with the directions for one common commercial brand. The area is then flushed with water for a minimum of 30 seconds to remove the decalcified material. Etched tooth structure will have a chalky appearance.
Dental Solders
Applications-bridges and orthodontic appliances
Terms
Soldering -joining operation using filler metal that melts below 500° C
Brazing -joining operation using filler metal that melts above 500°C
Welding-melting and alloying of pieces to be joined
Fluxing
-Oxidative cleaning of area to be soldered
- Oxygen scavenging to prevent oxidation of alloy being soldered
16- 650 -- 650 fineness solder to be used with 16-karat alloys; fineness refers to the gold content
Classification
a. Gold solders-bridges
b. Silver solders-gold-substitute bridges and orthodontic alloys
Structure of gold solders
Composition-lower gold content than of alloys being soldered
Manipulation-solder must melt below melting temperature of alloy
Properties
1. Physical-similar to alloys being joined
2. Chemical-more prone to chemical and electrochemical corrosion
3. Mechanical-similar to alloy being joined
4. Biologic-similar to alloys being joined
DISTORTION OF THE PATTERN
Distortion is dependant on temperature & time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .
PREREQUISITES
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .
Composition of Acrylic Resins.
· Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments
· Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)