NEET MDS Lessons
Dental Materials
Properties of Amalgam.
The most important physical properties of amalgam are
- Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)
- Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
- Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.
Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)
- Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
- Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer
- Because of low tensile strength, enamel support is needed at margins
- Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
- Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
- Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
- Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
- Dental amalgam is very resistant to abrasion
Physical reaction-cooling causes reversible hardening
Chemical reaction-irreversible reaction during setting
Lost Wax Process
The lost wax casting process is widely used as it offers asymmetrical casting withnvery fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:
1 . Create a wax pattern of the missing tooth / rim
2 . Sprue the wax pattern
3 . Invest the wax pattern
4 . Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 . Clean the cast.
7 . Remove sprue from the cast
8 . Finish and polish the casting on the die .
The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out ("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process .
Wax pattern removal:
Sprue former can be used to remove the pattern. If not the pattern is removed with a sharp probe. Then the sprue former is attached to it. The pattern should be removed directly in line with the principle axis of the tooth or the prepared cavity. Any rotation of the pattern will distort it. Hollow sprue pin is advisable because of its greater retention to the pattern.
I . Procedure for single casting :
A 2.5 mm sprue former is recommended
for molar crowns 2.0 mm for premolars & partial coverage crowns .
II . Procedure for multiple casting :
Each unit is joined to a runner bar .
A single sprue feeds the runner bar
4 . SPRUE FORMER DIRECTION
Sprue Should be directed away from the delicate parts of the pattern
It should not be at right angles to a flat surface .(leads to turbulance porosity .)
Ideal angulation is 45 degrees .
5 . SPRUE FORMER LENGTH
Depends on the length of casting ring .. Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue former should be no longer than 2 cm. The pattern should be placed as close to the centre of the ring as possible.
Significance
Short Sprue Length:
The gases cannot be adequately vented to permit the molten alloy to fill the ring completelyleading to Back Pressure Porosity.
Long Sprue Length:
Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.
Top of wax should be adjusted for :
6 mm for gypsum bonded investments .
3 -4 mm for phosphate bonded investments .
TYPES OF SPRUES
I . - Wax . II . Solid
- Plastic . Hollow
- Metal .
Denture Cleansers
Use - for removal of soft debris by light brushing and then rinsing of denture; hard deposits require professional repolishing
a. Alkaline perborates-do not remove bad stains; may harm liners .
b. Alkaline peroxides-harmful to denture liners
c. Alkaline hypochlorites-may cause bleaching, corrode base-metal alloys, and leave residual taste on appliance
d. Dilute acids-may corrode base-metal alloys
e. Abrasive powders and creams-can abrade denture surfaces
Denture cleaning Method
a. Full dentures without soft liners-immerse denture in solution of one part 5% sodium hypochlorite in three parts of water
b. Full or partial dentures without soft-liners immerse denture in solution of 1 teaspoon of hypochlorite with 2 teaspoons of glassy phosphate in a half of a glass of water
c. Lined dentures -- clean any soft liner with a cotton swab and cold water while cleaning the denture with a soft brush
Properties
1. Chemical-can swell plastic surfaces or corrode metal frameworks
2. Mechanical-can scratch the surfaces of denture bases or denture teeth
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)
Introduction
The science of dental materials involves a study of the composition and properties of materials and the way in which they interact with the environment in which they are placed
Selection of Dental materials
The process of materials selection should ideally follow a logical sequence involving
(1) analysis of the problem,
(2) consideration of requirements,
(3) consideration of available materials and their properties, leading to
(4) choice of material.
Evaluation of the success or failure of a material may be used to influence future decisions on materials selection.