Saturday, October 15, 2011

CHAPTER 3 GLASS IONOMER CEMENTS


Introduced in 1971 in England by Wilson and Kent
Powder- Aluminosilicate glass prepared with fluoride fluxes.  Type II restorative material is coarser
than the type I luting agents
Liquid- Aqueous solution of various polyacids, commonly polyacrylic or copolymers of acrylic and
maleic acid or acrylic and itaconic acid; Itaconic acid reduces the viscosity of the liquid; Tartaric
acid improves the working and setting properties 

Powder/liquid ratio is 3 to 1 by weight
The cement bonds to the inorganic component (calcium) by hydrogen bonding followed by metal ion
            bridges
Fluoride is available 1-3 mm from the restoration

Formulations
Hydrous- polyacid in the liquid, highest viscosity, highest pH, fastest initial set, lowest solubility,
     most biocompatible
Anhydrous- polyacid is freeze dried in the powder; just add H20; lowest pH, slowest initial set, some
biocompatability problems
Semihydrous- polyacid in powder and liquid; intermediate viscosity and pH (example Ketac Cem
Maxicap and most encapsulated systems)

Clinical failures result from
-loss of restoration- poor adhesion from too dry a mix
-inadequate isolation or improper preparation of the tooth
-cracking or craze pattern- dehydration during setting phase
-increased opacity- moisture contamination during setting phase

Proposed nomenclature for glass-ionomer dental cements and related materials (Ref. Mclean, Nicholson, Wilson ,Quintes Int Vol. 25, No. 9/1994):
Glass-ionomer cement- A cement that consists of a basic glass and an acidic polymer which sets by
     an acid-base reaction
Glass-ionomer hybrid materials- There are two classes of this material basically separated by their
ability to cure in the dark (acid/base reaction)
                        Resin-modified glass ionomer- Has sufficient acid and base to allow the reaction to take
                                    place within a reasonable time  (Vitremere, Fuji II LC)
                        Polyacid-modified composite resin- Correct ingredients are present (glass & acid) but in
       insufficient amounts to promote acid-base cure in the dark (Variglass, Dyract)







ADVANTAGES
DISADVANTAGES
tooth colored
adheres chemically to enamel & dentin (ionic bond)
CTE similar to tooth
high fluoride release
            anticariogenic & antimicrobial
rechargeable
less shrinkage than composite
biocompatible
no bonding agent required
resistant to staining
technique sensitive
sticky, difficult to sculpt
short working time
delayed set
poor polish
moisture sensitive
weak for stress bearing areas
poor abrasion resistance
low tensile and fracture toughness
only average esthetics

RECOMMENDED USES
            -cervical lesions: Class V restorations in adults
            -high caries risk patients

SETTING REACTION
            Decomposition: acid attacks the glass powder, releasing Ca, Al, Na and F ions
            Migration: acid penetrates the disorganized glass and reacts first with the Ca and Na to form cross-linked
                                    chains that form a polysalt
            Gelation: after surface layer loses almost all of its metallic ions, it forms a silica gel; very sensitive to water
                                    now
            Post-set Hardening: Al becomes the dominant component; Al precipitation continues for over 24 hrs.
            Maturation: setting process continues for about a year; over the first few days, the transparency increases

Resin-Modified Glass Ionomer Cements (RMGIC’s)
RECOMMENDED USES
            Class III and V in adults
            High risk caries patients
            Class I restorations in children
            Sandwich Class II

COMPOSITION
            GIC’s with a small quantity of resin (4.5 – 13%)

SETTING REACTION
            the normal glass ionomer acid-base reaction and a free radical photochemical polymerization process

POLYACID-MODIFIED COMPOSITE RESINS (COMPOMERS)
RECOMMENDED USES
            Class III and V restorations in adults
            Class I and II in children
            Medium caries risk patients??

Compomers exhibit few traditional GIC-like properties.  All must be light cured; therefore material should not be used in areas that the light will not reach.  Etching has been found to increase retention 3X and can reduce microleakage by improving the marginal seal.  They do possess easy handling properties; however, their physical properties are less acceptable than composite. 

No comments:

Post a Comment