NEET MDS Lessons
Dental Materials
CAD/CAM Restorations
Applications-inlays, onlays, veneers, crowns, bridges, implants, and implant prostheses
Stages of fabrication
CSD-computerized surface digitization
CAD-computer-aided (assisted) design
CAM-computer-aided (assisted) machining
CAE-computer-aided esthetics (currently theoretic)
CAF-computer-aided finishing or polishing (which are currently theoretic steps)
Classification
Chairside or in-office systems
(1) Cerec (Siemens system)-inlays, onlays, veneers
(2) Sopha (Duret system)-inlays, onlays (and Crowns)
Laboratory systems
(1) DentiCAD (Rekow system)-inlay, onlays, veneers, crowns
(2) Cicero (Elephant system)-porcelain fused-to-metal crowns
Materials
a. Feldspathic oorcelains (Vita)
b. Machinable ceramics (Dicor MGC)
c. Metal alloys limited use)
Cementing
- Etching enamel and/or dentin for micromechanical retention
- Bonding agent for retention to etched surface
- Composite as a luting cement for reacting chemically with bonding agent and with silanated surface of restoration
- Silane for bonding to etched ceramic (or metal) restorations and to provide chemical reaction
- Hydrofluoric acid etching to create spaces for micromechanical retention on surface or restoration
Properties
1. Physical properties
a. Thermal expansion coefficient well matched to tooth structure
b. Good resistance to plaque adsorption or retention
2. Chemical properties-not resistant to acids and should be protected from APF
3. Mechanical properties
a. Excellent wear resistance (but may abrade opponent teeth)
b. Some wear of luting cements but self-limiting
c. Excellent toothbrush abrasion
4. Biologic properties-excellent properties
POLYCARBOXYLATE CEMENT
Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material.
Clinical Uses
Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium.
c. Chemical Composition.
(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included.
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous).
d. Properties.
The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength.
e. Setting Reactions:
The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix.
Precautions.
The following precautions should be observed.
o The interior of restorations and tooth surfaces must be free of saliva.
o The mix should be used while it is still glossy, before the onset of cobwebbing.
o The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening.
Tooth Polishing and Cleansing Agents
1. Cleansing-removal of exogenous stains, pellicle, materia alba, and other oral debris without causing undue abrasion to tooth structure
2. Polishing-smoothening surfaces of amalgam, composite, glass ionomers, porcelain, and other restorative materials
Factors influencing cleaning and polishing
- Hardness of abrasive particles versus substrate
- Particle size of abrasive particles
- Pressure applied during procedure
- Temperature of abrasive materials
Structure
Composition
-contain abrasives, such as kaolinite, silicon dioxide, calcined magnesium silicate, diatomaceous silicon dioxide, pumice. Sodium-potassium
-aluminum silicate, or zirconium silicate; some pastes also may contain sodium fluoride or stannous fluoride, but they have never been shown to produce positive effects
Reactions-abrasion for cleansing and polishing
Properties - Mechanical
- Products with pumice and quartz produce more efficient cleansing but also generate greater abrasion of enamel and dentin
-Coarse pumice is the most abrasive
-The abrasion rate of dentin is 5 to 6 times faster than the abrasion rate of enamel, regardless of the product
-Polymeric restorative materials, such as denture bases, denture teeth, composites, PMMA veneers, and composite veneers, can be easily scratched during polishing
-Do not polish cast porcelain restorations (e.g., Dicor) that are externally characterized or the color will be lost
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
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 .
Manipulation
1. Selection-based on strength for models, casts, or dies
2. Mixing
(1)Proportion the water and powder
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds
3. Placement
(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression
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