NEET MDS Lessons
Dental Materials
CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.
Objectives of casting
1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with awell adjusted torch .
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.
STEPS IN MAKING A CAST RESTORATION
1. TOOTH PREPARATION
2. IMPRESSION
3. DIE PREPARATION
4. WAX PATTERN FABRICATION
5. SPRUING
METALLURGICAL TERMS
a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.
b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.
c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.
d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480o Fahrenheit, and then immersing it in water.
Stages of manipulation
Definitions of intervals
- Mixing interval-length of time of the mixing stage.
- Working interval-length of time of the working stage
- Setting interval-length of time of the setting stage
Definitions of times
- Mixing time-the elapsed time from the onset to the completion of mixing
- Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
- Initial setting time-time at which sufficient reaction has occurred to cause the materials to be resistant to further manipulation
- Final setting time-time at which the material practically is set as defined by its resistance to indentation
[All water-based materials lose their gloss at the time of setting]
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
Components
a. Fillers added to most to control shrinkage
b. Matrix
COMPOSITE RESINS
Reaction
- Free radical polymerization
Monomers + initiator. + accelerators-+ polymer molecules
- Initiators-start polymerization by decomposing and reacting with monomer
- Accelerators-speed up initiator decomposition
- Amines used for accelerating self –curing systems
- Light used for accelerating light-curing systems
Retarders or inhibitors-prevent premature polymerization
Acrylic Appliances
Use - space maintenance or tooth movement for orthodontics and pediatric dentistry
1. Components
a. Powder-PMMA powder. peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)
2. Reaction
PMMA powder makes mixture viscous for manipulation before curing . Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer . New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical
Temporary Filling Materials
Applications / Use
While waiting for lab fabrication of cast restoration
While observing reaction of pulp tissues
Objectives
Provide pulpal protection
Provide medication to reduce pulpal inflammation
Maintain the tooth position with an aesthetic restoration
Classification
Temporary filling cements
Temporary filling resins
Components
Temporary filling cements
1. Zinc oxide-eugenol cement with cotton fibers added
2. Polyme r powder-reinforced zinc oxide eugenol cement
Temporary filling resins
• MMA / PMMA filling materials
• Polyamide filling materials
• BIS-GMA filling materials