NEET MDS Lessons
Dental Materials
Glass Ionomer Cements
Applications
a. Class V restorations-resin-modified glass ionomers for geriatric dentistry
b. Class II restorations-resin-modified glass ionomers, metal-modified glass ionomers in pediatric dentistry
c. Class III restorations-resin-modified glass ionomers
d. permanent cementing of inlays, crowns, bridges, and/or orthodontic band/brackets. In addition, it can be used as a cavity liner and as a base.
Classification by composition
a. Glass ionomer-limited use
b. Metal-modified glass ionomer-limited use
c. Resin-modified glass ionomer-popular use
Components
a. Powder-aluminosilicate glass
b. Liquid-water solution of copolymers (or acrylic acid with maleic, tartaric, or itaconic acids) and water-soluble monomers (e.g., HEMA)
Reaction (may involve several reactions and stages of setting)
a. Glass ionomer reaction (acid-base reaction of polyacid and ions released from aluminosilicate glass particles)
- Calcium, aluminum, fluoride, and other ions released by outside of powder particle dissolving in acidic liquid
- Calcium ions initially cross-link acid functional copolymer molecules
- Calcium cross-links are replaced in 24 to 48 hours by aluminum ion cross-links, with increased hardening of system
- If there are no other reactants in the cement (e.g., resin modification), then protection from saliva is required during the first 24 hours
b. Polymerization reaction (polymerization of double bonds from water-soluble monomers and/or pendant groups on copolymer to form cross-linked matrix)
- Polymerization reaction can be initiated with chemical (self-curing) or light-curing steps
- Cross-linked polymer matrix ultimately interpenetrates glass ionomer matrix
Manipulation
a. Mixing-powder and liquid components may be manually mixed or may be precapsulated for mechanical mixing
b. Placement-mixture is normally syringed into place
c. Finishing-can be immediate if system is resin-modified (but otherwise must be delayed 24 to 72 hours until aluminum ion replacement reaction is complete)
d. Sealing-sealer is applied to smoothen the surface (and to protect against moisture affecting the glass ionomer reaction)
Properties
1. Physical
-Good thermal and electrical insulation
-Better radiopacity than most composites
-Linear coefficient of thermal expansion and contraction is closer to tooth structure than for composites (but is less well matched for resin-modified systems)
-Aesthetics of resin-modified systems are competitive with composites
2. Chemical
-Reactive acid side groups of copolymer molecules may produce chemical bonding to tooth structure
-Fluoride ions are released
(1) Rapid release at first due to excess fluoride ions in matrix
(2) Slow release after 7 to 30 days because of slow diffusion of fluoride ions out of aluminosilicate particles
-Solubility resistance of resin-modified systems is close to that of composites
3. Mechanical properties
-Compressive strength of resin-modified systems is much better than that of traditional glass ionomers but not quite as strong as composites
- Glass ionomers are more brittle than composites
4. Biologic properties
- Ingredients are biologically kind to the pulp
- Fluoride ion release discourages secondary canes
COMPOSITE RESINS
Reaction
- Free radical polymerization
Monomers + initiator. + accelerators-+ polymer molecules
- Initiators-start polymerization by decomposing and reacting with monomer
- Accelerators-speed up initiator decomposition
- Amines used for accelerating self –curing systems
- Light used for accelerating light-curing systems
Retarders or inhibitors-prevent premature polymerization
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
FLUXING
To prevent oxidation of gold alloys during melting always use a reducing flux .
Boric acid & borax are used .
ACID ETCH TECHNIQUE
Cavities requiring added retention (to hold firmly) are treated with an acid etching technique. This technique improves the seal of the composite resin to the cavity wall. The enamel adjacent to the margins of the preparation is slightly decalcified with a 40 to 50 percent phosphoric acid solution. This etched enamel enhances the mechanical retention of the composite resin. In addition, the acid etch technique is used to splint unstable teeth to adjacent teeth. The acid is left on the cut tooth structure only 15 seconds, in accordance with the directions for one common commercial brand. The area is then flushed with water for a minimum of 30 seconds to remove the decalcified material. Etched tooth structure will have a chalky appearance.
Manipulation
Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions. Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant
Classification
Rigid impression materials
(1) Plaster
(2) Compound
(3) Zinc oxide-eugenol
Flexible hydrocolloid impression materials
(I) Agar-agar (reversible hydrocolloid)
(2) Alginate (irreversible hydrocolloid)
Flexible, elastomeric, or rubber impression materials
(1) Polysulfide rubber (mercaptan rubber)
(2) Silicone rubber (condensation silicone)
(3) Polyether rubber
(4) Polyvinyl siloxane (addition silicone)