NEET MDS Lessons
Dental Materials
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
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)
Investment Materials
Investment is mold-making material
Applications
a. Mold-making materials for casting alloys
b. Mold-making materials for denture production
Classification
a. Gypsum-bonded investments (based on gypsum products for matrix)
b. Phosphate-bonded investments
c. Silicate-bonded investments
Components
a. Liquid-water or other reactant starts formation of matrix binder by reacting with reactant powder
b. Powder-reactant powder, filler, or modifiers
Manipulation
a. P/L mixed and placed in container around wax pattern
b. After setting, the investment is heated to eliminate the wax pattern in preparation for casting
Casting of glass or ceramic
A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .
Properties of Acrylic Resins.
- They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
- Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
- Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
- Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
- Good resistance to color change
- Absorbs water and must be kept hydrated (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
- Not resistant to strong oxidizing agents
- Low strength; however, flexible, with good fatigue resistance
- Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners