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)
COMPOSITE RESINS
Applications / Use
- Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
- Low-stress posterior restorations (small class I, II)
- Veneers
- Cores for cast restorations
- Cements for porcelain restorations
- Cements for acid-etched Maryland bridges
- Repair systems for composites or porcelains
Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)
Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network
Mercury bioactivity
- Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
- Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
- Half life for mercury elimination from body is 55 days .-
- mercury toxicity is <50 µm / m3 on average per 40-hour work week.
- Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
- Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam
Classification
Rigid impression materials
(1) Plaster
(2) Compound
(3) Zinc oxide-eugenol
Flexible hydrocolloid impression materials
(I) Agar-agar (reversible hydrocolloid)
(2) Alginate (irreversible hydrocolloid)
Flexible, elastomeric, or rubber impression materials
(1) Polysulfide rubber (mercaptan rubber)
(2) Silicone rubber (condensation silicone)
(3) Polyether rubber
(4) Polyvinyl siloxane (addition silicone)
Components
a. Fillers added to most to control shrinkage
b. Matrix
Applications/Use
- Load -bearing restorations for posterior teeth (class I, II)
- Pinned restorations
- Buildups or cores for cast restorations
- Retrograde canal filling material
(1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.
(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.
(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature
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 .