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
Root canal sealers
Applications
Cementation of silver cone gutta-percha point
Paste filling material
Types
Zinc oxide-eugenol cement types
Noneugenol cement types
Therapeutic cement types
properties
Physical-radiopacity
Chemical-insolubility
Mechanical-flow; tensile strength
Biologic-inertness
Gingival tissue packs
Application-provide temporary displacement of gingival tissues
Composition-slow setting zinc oxide-eugenol cement mixed with cotton twills for texture and strength
Surgical dressings
1.Application-gingival covering after periodontal surgery
2. Composition-modified zinc oxide-eugenol cement (containing tannic, acid. rosin, and various oils)
Orthodontic cements
Application-cementation of orthodontic bands
Composition-zinc phosphate cement
Manipulation
Zinc phosphate types are routinely mixed with cold or frozen mixing slab to extend the working time
Enamel bonding agent types use acid etching for improved bonding
Band, bracket, or cement removal requires special care
PHYSICAL PROPERTIES OF MATERIALS
Definite and precise terms are used to describe the physical properties of dental materials.
a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.
b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.
c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.
d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.
e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.
f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.
(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.
(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.
g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.
h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.
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
Acrylic Denture Bases
Use - used to support artificial teeth
Classification
a. PMMA/MMA dough systems
b. PMMA/MMA pour resin systems
1. Components
a. Powder-PMMA polymer, peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, and cross-linking agents
2. Reaction
a. Heat (or chemicals) is used as an accelerator to decompose peroxide into free radicals
b. Free radicals initiate polymerization of MMA into PMMA
c. New PMMA is formed as a matrix around residual PMMA powder particles
d. Linear shrinkage is 5% to 7% of monomer on polymerization
3. Manipulation
a. P/L mixed to form dough or fluid resin to fill mold
b. Mold heated to start and control reaction
RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .
DISTORTION OF THE PATTERN
Distortion is dependant on temperature & time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .
PREREQUISITES
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .