NEET MDS Lessons
Dental Materials
Dental Porcelain and PFM Porcelains
Applications/Use
a. Porcelain inlays and jacket crowns
b. PFM crowns and bridges
c. Denture teeth
Terms
PFM-porcelain fused to metal
Fusing-adherence of porcelain particles into a single porcelain mass
Classification
Dental porcelain is manufactured as a powder. When it is heated to a very high temperature in a special oven, it fuses into a homogeneous mass. The heating process is called baking. Upon cooling, the mass is hard and dense. The material is made in a variety of shades to closely match most tooth colors. Baked porcelain has a translucency similar to that of dental enamel, so that porcelain crowns, pontics, and inlays of highly pleasing appearance can be made. Ingredients of porcelain include feldspar, kaolin, silica in the form of quartz, materials which act as fluxes to lower the fusion point, metallic oxide, and binders. Porcelains are classified into high-, medium-, and low-fusing groups, depending upon the temperature at which fusion takes place.
High-Fusing Porcelains. High-fusing porcelains fuse at 2,400o Fahrenheit or over. They are used for the fabrication of full porcelain crowns (jacket crowns).
Medium-Fusing Porcelains. Medium-fusing porcelains fuse between 2,000o and 2,400o Fahrenheit. They are used in the fabrication of inlays, crowns, facings, and pontics. A pontic is the portion of a fixed partial denture, which replaces a missing tooth.
Low-Fusing Porcelains. Low-fusing porcelains fuse between 1,600o and 2,000o Fahrenheit. They are used primarily to correct or modify the contours of previously baked high- or medium-fusing porcelain restorations. Eg for PFM restorations
Structure
Components
a. Large number of oxides but principally silicon oxide, aluminum oxide. and potassium oxide
b. Oxides are supplied by mixing clay, feldspar, and quartz.
Manipulation
Porcelain powders mixed with water and compacted into position for firing
Shrinkage is 30% on firing because of fusing and so must be made oversized and built up by several firing steps
Properties
1. Physical
a. Excellent electrical and thermal insulation
b. Low coefficient of thermal expansion and contraction
c. Good color and translucency; excellent aesthetics
2. Chemical
a. Not resistant to acids (and can be dissolved by contact with APF topical fluoride treatments)
b. Can be acid-etched with phosphoric acid or hydrofluoric acid for providing microll1echanical retention for cements
3. Mechanical
a. Harder than tooth structure and ,will cause opponent wear
b. Can be polished with aluminum oxide pastes
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
PFM Alloys
Applications-substructures for porcelain-fused-to-metal crowns and bridges
Classification
o High-gold alloys
o Palladium-silver alloys
o Nickel-chromium alloys
Structure
Composition
o High-gold alloys are 98% gold. platinum. And palladium
o Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals
Manipulation
o Must have melting temperatures above that of porcelains to be bonded to their surface
o More difficult to cast (see section on chromium alloys)
Properties - Physical
Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain
Chemical-high-gold alloys are immune, but others passivate
Mechanical-high modulus and hardness
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
Finishing and Polishing
Remove oxygen-inhibited layer .Use stones or carbide burs for gross reduction.Use highly fluted carbide burs or special diamonds for fine reduction.Use aluminum oxide strips or disks for finishing. Use fine aluminum oxide finishing pastes. Microfills develop smoothest finish because of small size of filler particles
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 .
Suspension liners
Applications
o Dentin lining under amalgam restorations
o Stimulation of reparative dentin formation
Components
-Calcium hydroxide powder
-Water
-Modifiers
Manipulation
Used as W/P or pastes Paint thin film on dentin → Use forced air for 15 to 30 seconds to dry → Film is thicker (15 µm) than varnishes → Do not use on enamel or cavosurface margins
Properties
Physical
-Electrically insulating barrier
-Too thin to be thermally insulating
Chemical
-High basicity for calcium hydroxide (pH is II)
-Dissolves readily in water and should not be used at exposed cavosurface margins or gaps may form
Mechanical - weak film
Biologic - calcium hydroxide dissolves, diffuses, and stimulates odontoblasts to occlude dentin tubules below cavity preparation