NEET MDS Lessons
Dental Materials
Wax elimination (burnout):
Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.
• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
precious alloys.
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .
WAX BURNOUT AND HEATING THE RING
After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.
The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.
Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.
Hygroscopic Low-Heat Technique.
After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.
So this technique obtains its compensation expansion from three sources:
(1) The 37º C water bath expands the wax pattern
(2) The warm water entering the investment mold from the top adds some hygroscopic expansion
(3) The thermal expansion at 500' C provides the needed thermal expansion.
High-Heat Thermal Expansion Technique.
After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent
This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern. Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.
Cement Bases
Applications
• Thermal insulation below a restoration
• Mechanical protection where there is inadequate dentin to support amalgam condensation pressures
Types
• Zinc phosphate cement bases
• Polycarboxylate cement bases
• Glass ionomer cement bases (self-curing and light-curing)
Components
o Reactive powder (chemically basic)
o Reactive liquid (chemically acidic)
Reaction
o Acid-base reaction that forms salts or cross linked matrix
o Reaction may be exothermic
Manipulation-consistency for basing includes more powders, which improves all of the cement properties
Properties
Physical-excellent thermal and electrical insulation
Chemical-much more resistant to dissolution than cement liners
Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure
Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios
Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
Light-cured hybrid glass ionomer cements are the strongest
Zinc oxide-eugenol cements are the weakest
Biologic (see section on luting cements for details)
Zinc oxide-eugenol cements are obtundent to the pulp
Polycarboxylate and glass ionomer cements are kind to the pulp
Manipulation
Mixing
o P/L types mixed in bowl (plaster and alginate)
o Thermoplastic materials not mixed (compound and agar-agar)
o Paste/paste types hand mixed on pad (zinc oxide-eugenol, polysulfide rubber, silicone rubber, polyether rubber. and poly-vinylsiloxane)
o Paste/paste mixed through a nozzle on an auto-mixing gun (poly-vinylsiloxane)
Placement
o Mixed material carried in tray to mouth (full arch tray, quadrant tray. or triple tray)
o Materials set in mouth more quickly because of higher temperature
Removal - rapid removal of impression encourages deformation to take place elastically rather than permanently (elastic deformation requires about 20 minutes)
Cleaning and disinfection of impressions
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 .
ZINC OXIDE AND EUGENOL
This material is used for many dental purposes ranging from temporary restorative material to pulp capping. The material is composed of a powder that is basically zinc oxide and a liquid that is called eugenol.
Chemical Composition.
The powder must contain between 70 and 100 percent zinc oxide. The manufacturer may add hydrogenated resins to increase strength and zinc acetate to hasten the set.
Eugenol is usually derived from oil of cloves. The oil of cloves contains more eugenol (82 percent) Eugenol is an obtundent (pain-relieving agent). It is a clear liquid that gradually changes to amber when exposed to light.
Physical Properties.
This material relieves pain, makes tissue less sensitive to pain, is slightly antiseptic, and is low in thermal conductivity. It provides a good marginal seal when placed in tooth cavities. The crushing strength (compression strength) of pure zinc oxide and eugenol is about 2,000 psi, which is low in comparison to other cements. The addition of hydrogenated resin increases the crushing strength to 5,000 psi.
CLINICAL USES OF ZINC OXIDE AND EUGENOL
Treatment Restoration. It helps prevent pulpal irritation in carious teeth, lost restorations, advanced caries, or pulpitis. This dental material also exerts a palliative effect on the pulp.
Temporary Cementing Medium. Zinc oxide and eugenol is used as a temporary cementing medium for crowns, inlays, and fixed partial dentures.
Intermediate Base. Zinc oxide and eugenol is used as an intermediate base. This material provides insulation between metallic restorations and vital tooth structure. Because of the low crushing strength, its use is sometimes contraindicated.
Surgical Packing or Dressing. The surgical dressing applied and adapted over the gingival area after a gingivectomy. This dressing protects the area and makes the tissue less sensitive.
Solution Liners (Varnishes)
Applications
o Enamel and dentin lining for amalgam restorations
o Enamel and dentin lining for cast restorations that are used with non adhesive cements
o Coating over materials that are moisture sensitive during setting
Components of copal resin varnish
o 90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o 10% dissolved copal resin
Reaction
Varnish sets physically by drying → Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)
Manipulation
Apply thin coat over dentin. enamel. And margins of the cavity preparation → Dry lightly with air for 5 seconds Apply a second thin coat → Final thickness is 1 to 5 µ.m
Properties
o Physical
Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion
o Chemical
Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion
o Mechanical
Very weak and brittle film that has limited lifetime
Film adheres to smear layer
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