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
Investment Techniques
Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.
a). Brush technique:
The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with mix then the casting ring is applied over the crucible former and the ring is filled under vibration until it is completely filled.
b). vacuum technique:
• The mix in first hand spatulated, and then with the crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration
Two-step investing technique:
The investing procedure is carried out in two steps:
• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.
• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former is removed. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the wax pattern. Any loose particles of investment should beblown off with compressed air should be placed in a humidor if stored overnight.
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
Properties of Acrylic Resins.
- They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
- Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
- Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
- Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
- Good resistance to color change
- Absorbs water and must be kept hydrated (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
- Not resistant to strong oxidizing agents
- Low strength; however, flexible, with good fatigue resistance
- Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners
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 .
Structure of gypsum products
Components
a. Powder (calcium sulfate hemihydrate = CaSO4½H2O)
b. Water (for reaction with powder and dispersing powder)