Different Types of Restorative Materials and Luting Cements Are Currently

GLASS IONOMER MATERIALS

Definition: aluminosilicate glass and water soluble acidic polymer of poly acrylic acid COMPOSITION AND TYPES HYDROUS

Powder: aluminosilicate glass fillers Contains (alumina,quartz,fluorite,aluminum fluorid, aluminum phosphate and sodium fluoride. Lanthanum added for radiopacities)

The mixture is heated—quenched—milled— glass powder

COMPOSITION AND TYPES HYDROUS

Liquid 50%aqueous solution of polyacrylic acid It contains various acids such as: Itaconic, tartaric , maleic and tricarballic acids to 1-increase its reactivity 2-decrease its viscosity 3-tartaric acid added as accelerator to shorten setting time

COMPOSITION AND TYPES

Anhydrous The liquid could be freez or vacuum dried and added to the powder This is to overcome the problem of gelation that occur for the liquid The liquid in this case is water or water and tartaric acid MODIFICATIONS OF GIC

Fast setting glass ionomer By reduction of calcium % Admixture GIC powder+amalgam alloy powder Improved strength and decrease solubility Disadv—metal particles not bonded to the set material---increase wear

MODIFICATIONS OF GIC Ceramic metalGIC or cermet Silver metal sintered to glass Improved abrasion resistance Discolor the tooth Used in core build up, base and amalgam repair High viscosityGIC High powder content High strength properties High wear resistance resin-modified glass ionomer Incorporation of resin into the GIC improvements in structure and properties. nano-ionomer  is a type of resin-modified glass ionomer that have nano-sized fillers  to improve the strength, optical properties and abrasion resistance of GIC. CLASSIFICATION

Type I : luting Type II: restorative

type II1:esthetic restoration type II2:reinforced restorative admixture and cermet Type III: liner and base

SETTING REACTION Phase I(ion leaching phase) release of ca and Al from glass surface mix---shiny and glossy free carboxylic ions for chemical adhesion with tooth structure Phase II: hydrogel phase(initial set) Rigid and opaque Phase III:POLY SALT GEL(FINAL SET) Cross linking of AL ions lead to final set and hardening

ADVANTAGES

Adhesion to tooth structure

ADVANTAGES Fluorid release and recharge 1-fluoride burst 2-decline within the first week 3-stablize after 2-3months Clinical significant -Caries protection(anticariogenic) -remineralization N:B---application of topical fluorid ---- recharging---fluorida reservior

ADVANTAGES Fluoride cycle

ADVANTAGES Biocompatibility 1-anticariogenic---decrease bacterial adhesion ---bacteriostatic effect (mainly S.m) 2-sealing potential---chemical adhesion 3-pulpal and soft tissue response—favorable As poly acrylic acid—weak, high molecular weight

ADVANTAGES Dimensional stability 1- low setting contraction 3% by volume 2-Controlled shrinkage stress due ion enriched layer 3-coeff of thermal expansion and contraction close to tooth structure GOOD THERMAL INSLUTING CAPACITY

RADIOPACITY

DIS ADVANTAGES 1)POOR STRENGTH PROPERTIES---brittle 2)LOW ABRASION RESISTANCE----increase surface roughness 3)SOLUBILITY AND DISENTIGRATION—low PH 4)MOISTURE SENSTIVITY  Hydration----loss of ca and Al ions Decrease adhesive potintial and decrease strength  Dehydration-----poor esthetic as it increase opacity microcrackes---staining

4)QUESTIONABLE ESTHETIC

INDICATIONS

Class IIIand class V Root caries Core build up Caries control restoration Pit and fissure sealant Pediatric and geriatric restoration Liner and base ART

CONTRAINDICATIONS

Stress bearing areas If esthetic is of prime concern

CAVITY DESIGN 1)Adhesive potential ----no necessary retentive feature 2) Cariostatic mechanism----no extension beyond elimination of carious defect STEPS OF APPLICATION OF GIC Selection of GIC type Isolation of operatory field Conditioning of tooth substrate Matrix application (if needed) Proportioning of powder:liquid ratio mixing packing Maintenance of water balance Finishing and polishing

SELECTION OF GIC TYPE

There is three main properties affect selection 1-The requirement of amount of fluoride release Conventional glass ionomer 2-Esthetics resin modified glass ionomer 3-Function high filled viscous glass ionomer FIELD ISOLATION

As glass ionomer is very sensitive to water intake during setting Proper isolation of operative field by using : 1-rubber dam 2-cotton rolls 3- saliva ejector 4-retraction cord

APPLICATION OF LINER Very deep Cavities lined with calcium hydroxide for pulp protection before application of GIC CONDITIONING THE TOOTH SURFACE

WITH low concentration polyacrylic acid 10% for 10 seconds------rinsing This will remove smear layer but retain smear plugs This will increase surface energy------increase wettability and adaptation SELECTION OF MATRIX BAND PROPORTIONING, MIXING AND PLACEMENT

Proportioning according to manufacture instruction  The powder and liquid available in two forms 1-encapsulated for mechanical mixing 2-separatly in two bottles for hand mixing

PROPORTIONING, MIXING AND PLACEMENT

Mixing Manually done on cool clean glass slab with teflon coated instrument Packing In bulk or injected inside prepared cavity Coating using water proof sealant Light activated resin bond,vaseline or varnish Finishing and polishing

RESIN MODIFIED GLASS IONOMER

Definition hybrid material of traditional GIC with a small amount of resin Setting reaction: 1-acid base reaction which is mainly set reaction 2-polymarization which occur first

 Advantage:  fast setting  Immediate stabilization of water balance  Improved esthetics and strength  Resin facilitate bond to composite( in sandwich technique )

Disadvantage 1-discolaration after time of making restoration 2-less flourid release COMPOMER

Contain major ingredient of glass ionomer and composite resin except water to prevent premature setting in its container Light activated Release fluorid but not as glass ionomere Good intial esthetic but not stable by time Ionomer modified resin Compoite contain glass ionomer fillers but not contain poly acrylic acid .Giomer is a type of fluoride containing composites that comprise pre-reacted glass ionomer particles into the resin in addition to the fluoro-alumino-silicate filler particles,

CHAPTER (5) GLASS IONOMER

CHAPTER OUTLINE:

1- COMPOSITION AND STRUCTURE: A) THE GLASS POWDER. B) THE POLYACID LIQUID. C) COMPOSITIONAL FORMS: 1. Polyacid-mixable cements. 2. Water-mixable cements. 3. Mixed cements. 2-ADVANTAGES AND DISADVANTAGES 3- TYPES OF GLASS IONOMER CEMENTS: A) CONVENTIONAL GLASS IONOMER CEMENTS: -Types. B) RESIN-MODIFIED GLASS IONOMER CEMENTS: -Types. C) POLYACID-MODIFIED RESIN COMPOSITE: -Clinical applications. 4- PROPERTIES OF GLASS IONOMER CEMENTS: a) Hydration and Dehydration. b) Biocompatibility. c) Setting Shrinkage. d) Bonding to Tooth Structure. e) Wear Properties. f) Strength Properties. g) Fluoride Release/Uptake. 5- CLINICAL INDICATIONS AND CONTRA-INDICATIONS. 6- CAVITY DESIGN. 7- MANIPULATION: a) Dispensing and Mixing. b) Placement Technique.

Objectives of chapter (5)

In the end of this chapter, the student will be able to recognize:

1- Composition of structure of glass ionomer.

2- Advantages and disadvantages.

3- Types of glass ionomer cements.

4- Properties of glass ionomer.

5- Indications and contraindications.

6- Cavity design for glass ionomer.

7- Manipulation of glass ionomer cements.

ifferent types of restorative materials and luting cements are Dcurrently used in daily dental practice. The most common are amalgam, composite resins, glass ionomers, dental casting alloys, and ceramics. Each material possesses advantages and disadvantages. Glass-ionomers are considered the most biologically accepted restorative materials. They can be used in a wide range of clinical applications. They also have an anticariogenic potential produced by incorporated fluorine, good biocompatibility, good chemical adhesion to the tooth structure, well-balanced physical properties, and good manipulability.

ISO standard defines glass-ionomer cement as a polyalkenoate cement. However, the term glass-ionomer cement has been more widely accepted by dental professionals and describes this material more accurately.

A glass is an amorphous non-crystalline structure, while ionomer means ionizable or containing ions. Glass Ionomer is a material that is formed of ionizable silicate glass powder and polyacrylic acid co-polymers. It is also called polyalkenoate cement as the utilized polyacids have unsaturated double bonds. It could be defined as a water-based material, which is formed as a product of an acid-base reaction between the basic calcium-fluoro-aluminosilicate glass powder and an aqueous solution of polyacid. A water- based material is a material that precipitated from an aqueous reaction whose reaction medium is water.

COMPOSITION AND STRUCTURE A) THE GLASS POWDER The powder is an acid soluble calcium-fluoro-alumino-silicate glass; the Fl is not incorporated in the skeletal structure of the glass. For this reason, it can diffuse freely in and out of the glass core, depending on the concentration of the ion in the environment surrounding the cement,`

B) THE POLYACID LIQUID It is an aqueous solution of polyacrylic acids co-polymers (a polymer composed of more than one type of monomers). The acids that are co-polymerized with polyacrylic acid to form the polyacid liquid are mainly Maleic, Itaconic and Tartaric acids.

C) COMPOSITIONAL FORMS According to the presentation of the polyacid liquid, the cements are classified into three types:

1. Polyacid-mixable cements: The Polyacid is presented as an aqueous solution forming the liquid of the cement. These cements are more viscous upon mixing and their liquid may gel by time due to hydrogen bonding. In a luting consistency they are less irritating than the water settable cements in virtue of their higher viscosity and higher initial pH.

2. Water-mixable cements: The Polyacid is freeze-dried and added to the powder. The liquid in this case is either distilled water or an aqueous solution of tartaric acid in which the freeze-dried polyacid powder dissolves upon mixing to reconstitute the polyacid liquid. These cements are less viscous upon mixing, have a longer working time with a shorter setting time and an extended shelf life. This form is called water-settable and called anhydrous which implies the absence of water from the cement, while this is not the case because water is a mandatory ingredient without which the reaction can never occur.

3. Mixed cements: These contain part of the polyacid as freez-dried particles incorporated with the cement powder and the other in an aqueous solution form presented as the cement liquid. These cements have intermediate properties between the previous two types.

ADVANTAGES 1. Cariostatic property as a result of sustained release of fluoride that leads to inhibition of caries producing microorganisms. Also, the ionic exchange of fluoride with enamel will decrease the surface tension of enamel decreasing the adhesion of food debris to it and increases its resistance to dissolution. 2. Adhesive potential due to chemical bonding. 3. Low setting contraction. 4. Biocompatibility as a result of its weak acid content, its high molecular weight. 5. Thermal insulating capacity. 6. Low coefficient of thermal expansion. 7. Satisfactory optical properties. 8. Multiple clinical applications. 9. Ease of manipulation and reasonable cost.

DISADVANTAGES 1. Poor mechanical properties; its low wear resistance preclude its use at stress-bearing areas. 2

2. Very sensitive to hydration and desiccation after setting, time dependant degradation as well as surface cracks and voids. 3. Short working time and long setting time. But this problem solved by the development of photo-polymerizable glass- ionomers.

TYPES OF GLASS IONOMER CEMENTS A) CONVENTIONAL GLASS-IONOMER CEMENTS The first glass-ionomer cement developed by Wilson and Kent was a product of an acid-base reaction between basic fluoroaluminosilicate glass powder and polycarboxylic acid in the presence of water. The nature of the set cement comprised an organic-inorganic complex with high molecular weight. Therefore, glass-ionomer cement can be defined as a water-based material that hardens following an acid-base reaction between fluoroaluminosilicate glass powder and an aqueous solution of polyacid.

Types of conventional GICs: 1. Glass-ionomers for direct restoration: The first glass-ionomer cement developed for filling was called Alumino Silicate Poly-Acrylate (ASPA). Currently, many glass- ionomer products are available for restorative purposes. Although resin composites shows superior mechanical properties and better esthetics than glass-ionomer cements, composite resins require bonding agents to adhere to tooth structure. Glass-ionomer cements chemically bond to tooth structure, offer easy handling, show a coefficient of thermal expansion near to that of the tooth, and have a potential effect of remineralization. Glass-ionomers are widely used for pedodontic applications and for the restoration of Class III and Class V cavities. They are not recommended for permanent filling of occlusal surfaces in adults where there is excessive load because of poor strength properties and abrasion resistance.

2. Metal-reinforced glass ionomers: Metals are added to fillers of glass ionomer in interest of producing reinforcing glass-ionomer cements. The powder contains fluoroaluminosilicate glass and a silver alloy or the glass is sintered with silver. The latter product is called a cermet (ceramic, or glass, and metal). Due to the admixing of metals, these materials show poor esthetics than other glass-ionomer cements. They have appropriate strength, easy to manipulate, and are sufficiently radiopaque. These properties, added to their adhesive ability to the tooth structure make cermet cements appropriate for core buildup. Metal-reinforced

glass-ionomers also have been proposed as a temporary posterior restorative material.

3. Highly viscous glass-ionomers: The highly viscous glass-ionomer cements were designed as an alternative to amalgam for posterior preventive restorations. These glass-ionomers are particularly useful for the atraumatic restorative treatment (ART) technique. This is a procedure based on excavating carious dentin using hand instruments only and restoring the tooth with adhesive filling materials. Due to their manipulative and mechanical characteristics, highly viscous glass-ionomers can be used for intermediate restorations, replacing amalgam, and for core buildup procedures. They are applied as glass-ionomer preventive restorations to fill cavities with proper sealing.

4. Low-viscosity glass ionomers: The low-viscosity glass-ionomers have been developed as liners, fissure protection materials and sealing materials for hypersensitive cervical area. The fluoride released from glass-ionomers strengthens the tooth. Such materials are designed with low powder-liquid ratios and are highly flowable. The low powder: liquid ratio material is expected to cause gradual dissolution from the occlusal surface when they are applied as fissure protection materials. Low-viscosity glass-ionomer cements should be suitable as fissure protection materials during the eruption period of the teeth.

5. Bases and liners: These materials are used for the "sandwich" technique in which they are applied as a dentin substitute and composite resin is applied as an enamel substitute. Thus the advantages of both glass- ionomer and resin composite may be combined. The composite resin shows superior physical and esthetic properties, making it suitable for surface application. The glass-ionomers show a coefficient of thermal expansion matching to that of the tooth structure, good bonding to dentin, and good biocompatibility, making them suitable for application as a dentin substitute.

6. Luting cements: The glass-ionomer cements for luting are widely used for cementing metal inlays, crowns, and bridges. They are considered the most suitable luting cements, because of their ease of manipulation, bonding ability, fluoride release, and low solubility in the oral environment. Their physical properties are well balanced compared to those of zinc phosphate cement, polycarboxylate 4

cement, and resin cement.

B) RESIN-MODIFIED GLASS-IONOMER CEMENTS Disadvantages of conventional glass-ionomer cements compared to composite resin are their inferior mechanical properties as tensile strength and fracture toughness. These properties need to be improved in order to widen the range of clinical applications. Resin addition to glass-ionomer cements was designed to produce favorable physical properties similar to those of resin composites and resin cements while retaining the basic features of the conventional glass-ionomer cement. This goal was achieved by incorporating water-soluble resin monomers into an aqueous solution of polyacrylic acid. In this way the system undergoes polymerization of the resin monomer while the acid-base reaction continues simultaneously. The resulting resin-modified glass-ionomer cements exhibit many advantages of both resin cements and glass-ionomer cements. The resin-modified glass-ionomer cement is defined as a material that undergoes both polymerization reaction and acid-base reaction.

The basic composition of the cement liquid is polycarboxylic acid, water, and 2-hydroxyethyl-methacrylate (HEMA). It may also contain a small amount of cross-linking material. The composition and structure of the fluoroaluminosilicate glass for resin-modified glass-ionomer cements are basically similar to those of conventional glass-ionomer cements.

One of the main disadvantages of conventional glass-ionomer cement is that when it comes in contact with water during the early stage of setting, the setting reaction is inhibited, damaging the surface of the cement. Water sensitivity could be reduced by incorporating photo-polymerization, which promotes faster setting, into the setting reaction. Rapid setting is also an advantage for color stability.

Types of resin-modified GICs: 1. Restorative materials: Conventional glass-ionomer cement has a disadvantage is that it cannot be polished immediately after placement. This clinical caution is required to prevent deterioration of the material’s physical properties caused by water sensitivity during the initial stage of the setting. The introduction of resin-modified glass-ionomer cement has made the material significantly less sensitive to water. There are four

major improvements in the resin-modified glass-ionomer cement; decreased water sensitivity, improved mechanical properties, manipulation and translucency. The incorporation of monomers to the cement liquid increases the refractive index of the liquid.

2. Base and liner: The first clinical application of resin-modified glass-lonomer cement was as a base and liner. Conventional glass-ionomer products also have been successfully used for these purposes. However, the base and liner applications are usually followed by restorative or temporary filling procedures. Thus, the quick set with photo polymerization of the resin-modified glass-ionomer cement is more favorable for the requirements for these applications.

3. Fissure protection: Glass-ionomer cements for pit and fissure protection offer several advantages as long-term fluoride release. Their chemical bonding ability to tooth structure without acid etching, are allowing their use on partially erupted teeth with difficult access. Their retention rates were not as high as those of resin sealants, and they require prevention of moisture contamination in the early stages of setting.

4. Luting: The bond strength of glass-ionomer cement for luting is not as high as that of resin cement. Failure often occurs as cohesive fractures within the cement. There are many resin-modified glass- ionomers available that contain a monomer component in the liquid to strengthen the matrix of the cured material. Also, the bond strength of the material to the tooth structure has been improved with the incorporation of monomers.

C) POLYACID-MODIFIED RESIN COMPOSITES An attempt was made to polymerize an acid monomer in the presence of fluoroaluminosilicate glass. This attempt led to the development of a compound that releases fluoride slowly in the oral environment; it is called a compomer. The compomer shows physical properties quite similar to those of a composite resin. At the same time, the acid monomer, which has been polymerized, exhibits acidity when contact with water from saliva and reacts with the basic glass, which contains fluoride. The nature and physical properties of the materials can be modified consecutively between the two extremes. Therefore, several products claim to have the nature of both conventional glass-ionomer cements and composite resin, 6

making the classification of such materials complicated.

The compomer is one of the materials in this category. It basically has a similar nature and similar physical properties to resin composites. According to the manufacturers, it also has the ability to release fluoride and undergoes an acid-base reaction between the acidic monomer and basic glass filler in the presence of water in the saliva. However, the compomer does not contain water and does not self-adhere to the tooth structure, which differentiates it from resin-modified glass-ionomer. The compomer is mainly a resin composite with fluoride-releasing potential.

The material usually contains fluoroaluminosilicate glass powder as filler to release fluoride. Metal fluoride also is included in some materials for the same purpose. The fluoroaluminosilicate glass contains strontium or some other metal to make the material radiopaque. The compomer's original composition contained the acidic monomer in its matrix, although the other part of the matrix is similar to that of composite resin.

Clinical applications: The main clinical application for compomers is restorative filling, because they are not adhesive and require a separate bonding agent. Compomers possess better mechanical properties and easier manipulation than glass-ionomer filling materials and their flowability in the cavity is better than that of resin composite. However, the need for application of a bonding agent prior to filling is considered a disadvantage, and the mechanical properties of compomers are inferior to those of resin composites. Presently, the compomer can be classified as an intermediate material between the glass ionomer for filling and the resin composite.

PROPERTIES OF GLASS IONOMER CEMENTS A) HYDRATION AND DEHYDRATION During the initial setting reaction the restoration can be adversely affected by both moisture contamination and dehydration.

Hydration causes: 1. Dissolution of polysalts (Ca2+ and Al3+ ions) with loss of adhesive potentials. 2. Decrease strength. 3. Loss of translucency. 4. Disintegration of the restoration. 5. The weakened surface will erode.

In the early stages of setting, there is a reasonable quantity of water present and this can be lost if the restoration is not sealed.

Dehydration causes: 1. Surface micro-cracks. 2. Increased opacity. 3. Increased susceptibility to staining and microleakage. 4. Poor esthetic. 5. Weakened restoration. Maintenance of the water balance is therefore essential for the development of the full esthetic properties. Following the initial set of the cement and removal of the matrix band, the restoration should be covered immediately with a layer of a single component, low viscosity, light activated resin bond or varnish to provide a water proof seal. As the bond is an unfilled resin, it will wear off from the surface quite rapidly and therefore, will not interfere with the subsequent fluoride release. The light initiated resin-modified glass ionomer are resistant to water uptake. However it is a good idea to seal a newly placed restoration simply to eliminate surface scratches and roughness following contouring.

B) BIOCOMPATIBILITY Resistance to plaque: It has been shown that bacterial plaque cannot be present on the surface of glass ionomer. Streptococcus mutans is the major pathogen found in dental plaque, and it is thought that it is unable to present in the presence of fluoride. Thus, the response of all soft tissues to glass ionomer restorations is favorable.

Pulp response to glass ionomer: Although glass ionomer is an acid-containing restorative, yet it is considered to be biologically compatible to tooth tissues due to: 1. Compositional polyacid is weak and has high molecular weight which limits its diffusion through dentinal tubules to the pulp. 2. Glass ionomer is subjected to only a minimal temperature rise during setting in comparison to other materials. The main problem in restorative dentistry is microleakage between a restoration and the cavity wall. If bacteria and their toxins are able to penetrate down the interface, there will be rapid inflammatory response in the pulp. Rapid recovery is also possible as long as the bacteria can be eliminated. Because of the ion exchange adhesion, which will prevent microleakage, glass ionomer is of considerable value in isolation of an active carious lesion. So, it was 8

concluded that glass ionomer works because of an effective seal rather than through bacterial action. Traditional glass ionomer maintained a low surface pH for at least the first 60 minutes of setting. This may dictates application of calcium hydroxide liner in deep preparations before application of glass ionomer.

C) SETTING SHRINKAGE Although GIC undergo more setting contraction than do composites; yet, the setting stresses developed in GIC is much lower (2 MPa) than composites (15-18 MPa). This is due to the rubbery stage through which the cement passes during setting, which allow the cement to flow at the free surfaces to compensate for these stresses. Moreover, the low elastic modulus of the cement allows elastic yielding to occur that also relieves a part of these stresses.

Hygroscopic expansion that occurs due to water sorption serves to counteract the setting shrinkage either partially or totally. The GIC restoration is allowed to contact water after 15 min, at which the setting stresses decrease immediately because GIC is capable of absorbing water rapidly. However, the setting stresses increase again as the setting reaction proceeds to maturation. The net setting stresses is then the resultant of maturation stresses as opposed by hygroscopic expansion.

D) BONDING TO TOOTH STRUCTURE Glass ionomer can adhere chemically to tooth structure or metals without an intermediate adhesive. The mechanism is not yet fully understood but it primarily involves an interaction between the carboxyl group in the polymer acid and the metal ions either through an ionic reaction or by chelation. The bond with enamel is always higher than that to dentin, probably because of the higher mineral content of enamel and its more homogenous morphology.

Bond to mineralized tissues: Adhesion is initiated by polyalkenoic acid when freshly mixed material contacts the tooth surface. Phosphate ions are displaced from apatite by carboxyl groups of the polyalkenoic acid, each phosphate ion taking a calcium ion with it to retain electric equilibrium. Therefore, it appears that chemical bonding is achieved by a calcium phosphate-polyalkenoate crystalline structure acting as an interface between enamel or dentin and the set material.

Bond to collagen: Adhesion to the organic component of dentin may also occur through either hydrogen bonding or metallic ion bridging between the carboxyl groups on the polyacids and the collagen molecules of dentin.

E) WEAR PROPERTIES Directly after placement, the GIC has very low wear properties, which is believed to be due to the low initial strength properties. If the cement is given enough time to build-up sufficient strength properties, the wear resistance is improved significantly and reaches an acceptable level. However this may require weeks and sometimes months due to maturation of the material. The wear resistance was found to be at least doubled or trippled for most GICs after 3-4 months. The wear of the GIC is increased under acidic environments where the acids and wear mechanism acts simultaneously, as during the mastication of acidic food. The acid attacks the cement surface and enhances its dissolution but only at a pH less than 5.

F) STRENGTH PROPERTIES GIC is a fairly hard, brittle material with high compressive strength but low tensile strength and wear resistance and therefore are not typically used in stress-bearing areas. The low strength characteristics of the GIC are mainly due to the mismatch between the mechanical properties of the polyacid matrix and the glass filler and the presence of numerous voids in the set cement. Stresses accumulate at the interface between the core and the matrix or at the voids causing a crack that propagates under cyclic loading ending ultimately in fracture of the restoration. The strength properties improves by time due to material maturation.

G) FLUORIDE RELEASE/UPTAKE Fluorides in the GIC were added originally to decrease the fusion temperature of the glass and to improve the handling properties and increase the strength and translucency.

After setting, GICs undergo ion release and water sorption, which results in hydrolytic degradation that depends upon the pH of the surrounding environment. When the material is fully set Fl is located in the glass core in the form of Alminium fluoride complexes in the matrix. The amount of released Fl is not dependant on the Fl content of the cement but on the amount of Na that will maintain the cement neutrality after the Fl release but generally the more the 10

powder:liquid ratio the more the Fl release. At neutral conditions, only the Fl- present in the matrix is released. At acidic pH, Fl release increase due to the increased erosion.

The Fluoride release is characterized by two phases: a) Short term phase and b) Long term phase. A short-term high Fl release during the post-setting maturation stage in the first few days, then a long-term low Fl release through-out the cement life as a result of an equilibrium diffusion process. The salivary Fl level increases after GIC application, but returns to normal levels within a few weeks.

The released Fl is mostly in the form of NaF which is not a matrix forming salt; therefore, the Fl release is not accompanied by any weakening effects or loss of the material properties.

The long-term Fl release is a two way dynamic process that occur through an ion exchange process depends upon the Fl concentration gradient in the external environment in a way to maintain diffusion equilibrium. In the presence of an inverse Fl concentration as during tooth brushing with a fluoride toothpaste or rinsing with a Fl mouthwash, the GIC restoration is capable of absorbing Fl from the external environment, to re-charge its Fl level and to release it later when the Fl concentration drops again in the external environment.

The initial low pH of GIC after application may induce superficial dissolution of the adjacent apatite crystals and redeposition of CaFl salts instead. However, this occur only at the demineralization front and not at deeper sites.

There is no sound evidence to support significant Fl diffusion from the GIC restoration to the enamel walls of the cavity that is bonded to the restoration. However, the dentin walls demonstrate higher Fl uptake owing to the greater porosity, higher water content, smaller crystalline size and the presence of a well defined organic phase that have a higher binding capacity and can take-up a higher amount of Fl than the inorganic phase. Fl bound to the organic phase of dentin enhances dentin mineralization.

CLINICAL INDICATIONS: 1. Class V erosion or abrasion lesions: Erosion lesions are generally two types; V shaped notch or the more saucer shaped lesion. The V-shaped lesion can be restored with no tooth preparation because a margin thickness of at least 1-mm can be obtained.

One mm is the maximum bulk required in case of brittle cement such as glass ionomer. 2. Class III cavities. 3. Pits and fissures sealant: The use of glass ionomer as sealant increased as less viscous formulations produced (light cured) with better physical properties such as wear resistance. 4. Class II restorations in primary teeth. 5. Luting cements. 6. Liner: Glass ionomer used as a liner for composite resin restoration (sandwich technique). Its ability to release fluoride and adhere to dentin and its biocompatibility can be combined with the esthetic appearance and mechanical properties of the composite resin. 7. Bases under direct and indirect esthetic restorations: Under inlays in stress bearings as Class I and II. 8. Cores and build-ups: Metal reinforced glass ionomers are used for this purpose. 9. Root surface caries in class V cavities. 10. As a retrograde filling material after root canal treatment and after vital root amputation.

CONTRA-INDICATIONS: The low tensile strength, brittleness and low resistance to wear should preclude the use of glass ionomer in high stree bearing ares as Class I and II restorations. Recently, metal reinforced glass ionomer exhibits improved wear resistance compared to conventional glass ionomer, but the strength was found to be inadequate for use in areas of high stress.

CAVITY DESIGN: Cavity preparation and design is not as demanding or important with glass ionomer cement as it is with other restoratives. The adhesive potential of the cement precludes any necessity for retentive features, while its potent cariostatic influence similarly precludes any extension beyond elimination of caries. Deep cavities need first to be lined with calcium hydroxide for pulp protection before application of glass ionomer cement.

MANIPULATION: A) DISPENSING AND MIXING: Glass ionomer cements are available commercially in two forms: 1. Encapsulated for mechanical mixing. 2. Powder and liquid supplied separately for hand mixing.

The capsule provides a consistent and satisfactory powder/liquid ratio and ensures optimum physical properties. In addition, it acts as a syringe for placement of the mixed material into the cavity. The powder/liquid ratio is significant in hand mixing. For convenience, the mixed material should be transferred to a disposable syringe for accurate and positive placement into the cavity.

B) PLACEMENT TECHNIQUE: The powder/liquid ratio is very important and the use of capsulated material is recommended. 1. The cavity requires conditioning prior to placement of the cement. The preferred conditioner is 10-15% polyacrylic acid for 10-15 seconds. 2. The cavity should be washed thoroughly and dried lightly without dehydration. 3. Complete isolation is recommended, but if the cavity is contaminated then reconditioning is needed. 4. If the cement is hand mixed, it is better to be transferred into a syringe for placement into the cavity. This is because it improves adaptation and reduces porosity. 5. Then apply a matrix for the final cement placement. 6. Allow the cement to set. 7. Immediately after the removal of the matrix band, the cement should be covered with a layer of sealant. 8. While the sealant is still liquid, trim the excess cement with a sharp blade or a slowly rotating bur, without inducing stresses. 9. If the sealant has been disturbed during contouring then apply a second layer and trim the excess. 10. Contouring and polishing should always be performed under an air/water spray, using very fine diamonds at the beginning and then finishing using aluminum oxide discs. Contouring and polishing should not be carried out nearly after a week from placement, at which the physical properties have achieved a reasonable level and translucency will not be lost. Most light cured glass ionomers can be contoured and finished immediately after light curing.