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The Advantages Imparted By Resin Addition To Conventional Glass Ionomer Outweigh The Disadvantages

Glass Ionomer Cements (GIC) is created when ion-leachable calcium alumino-silicate glass powder that contains fluoride reacts with polyalkenoic acid. Recently, the use of GIC has been extensive in dentistry as it can be modified by combining it with several substances and the properties can be further enhanced. GIC cement was initially developed by Wilson and Kent in England in the year 1972 (Graig, 2002). When created earlier, the GIC was not an aesthetic material and did not have any translucency. It was utilized to full only small class V abrasive lesions.

It was slowly modified and can be used in several clinical processes such as luting, as a lining and a base, etc. GIC has certain unique properties that may not be present in any other material. It helps to conserve the tooth structure and it directly bonds with the tooth. The GIC cement directly bonds to the dentine present in the tooth. It helps to remineralise caries and hence can be utilized by preserving the tooth material. The GIC cement slowly releases fluoride over a long-period. This property of GIC can also be utilized in caries prevention and in patient’s having a high-risk of developing caries (Mount, 1998).

The powder present in the resin-modified GIC cement is somewhat similar to the conventional GIC’s. The liquid contains monomers and polymers to ensure the strength of the GIC cement is increased (Graig, 2002). The GIC cement has usually two components, a powder and a liquid. The powder is a ion-leachable alumina-silicate glass. The liquid contains polymers and copolymers of acrylic acid dissolved in water. During the setting reaction Aluminium ions and calcium ions are released by the glass and polymers release acid groups.

The setting reaction takes place slowly and results in the formation of a cross-linked gel matrix. Aluminium ions may be exchanged slowly in the gel matrix which helps to strengthen it. This process occurs very slowly until the final set. The calcium present in the gel matrix may combine with the exposed GIC cement bonds using both diffusion an the adsorption phenomenon (Mount, 1998). When the freshly cut tooth surface is smeared with GIC, the polyalkenoic acid plays a very important role in initiating adhesion. The carboxyl ions present in the acid displaces the phosphate ions present in the apatite crystals.

The Calcium phosphate-polyalkenoate crystalline complex formed at the interface of the tooth surface and the GIC material plays a very important role in bonding. This phenomenon is frequently-known as diffusion based adhesion (Mount, 1998). When the intermediate complex phase is subjected to acid etching, it was more resistant to etching than the other portions of the tooth. When forces were applied to debond the restoration to the tooth, the complex phase formed was left behind. The carboxyl group of the polyacid of the GIC combines with the collagen molecules of the dentin to form a bond.

The strength of this bond may be increased by using conditioning agents such as citric acid and hydrogen peroxide which help to remove the smear layer. However, polyacrylic acid is the best material in removing the smear layer and improving adhesion. Polyacrylic acid helps to remove the smear layer, but does not interfere with the dentinal plugs that block the entry into the dentinal tubules. This helps to prevent the development of hypersensitivity following restoration. This solution can also be utilized remove the smear layer (Mount, 1998). The Cement so formed may have a lot of limitations which may restrict use.

GIC cement is slightly more soluble in the saliva compared to several

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other dental cements especially the resin containing cements. However, GIC cement is more resistant to solubility than other cements such as silicate cements. Another problem of GIC cement is that it may begin to disintegrate in the oral cavity when certain problems such as zerostomia or Sjogern’s syndrome are present. This is because the GIC cement tends to lose its moisture in the oral cavity. In such a circumstance, GIC restorations have to be replaced every few months or maximum on an annual basis.

Frequently, the cement has to be covered by a composite resin lining to ensure that the GIC cement does not face the ruthless environment of the mouth, especially in Sjorgren’s syndrome (Mount, 1998). Resin modified GIC cements helps to make the material more resistant to both excessive solubility in the oral cavity and excessive destruction from unfavourable conditions present in the mouth such as Sjogren’s syndrome (Mount 1998). GIC cement tends to shrink slowly with time. The shrinkage on an average basis (volumetrically) is about 3 %.

This shrinkage develops slowly over a period of time. On the tooth side of the restoration, as the GIC combines with the collagen and the tooth surface to form a bond, the shrinkage may not be much to result in debonding of the restoration from the tooth. The stress relaxation is not much on the tooth side of the restoration. When the cement tends to set slowly, it usually absorbs water and also dissolves slightly more in the saliva. Slowly setting cements overall do not have good mechanical properties but anyway shrink less than the faster setting GIC’s.

A cavity varnish or a Composite Resin may have to be applied to the surface of slow-setting GIC cement in order to protect if from the oral environment during the initial stages following setting. In resin-modified GIC cements, even if the resin content is about 5 %, it would result in reduced amounts of shrinkage at the time of placement. The shrinkage that develops over a period of time occurs much more slowly. The adhesion formed between the tooth surface and the GIC helps to limit shrinkage to some extent (Mount,1998). GIC cements are weak material and lack rigidity.

They are susceptible to fracture compared to other materials such as dental amalgams and composite resins. Those restorations with GIC prepared in high stress bearing areas, having high occlusal load, usually fail within a short period of time. However, research suggests that the resin-modified GIC cements have a better strength compared to the conventional GIC cements. The transverse strength of resin-modified GIC cement is almost twice that of conventional GIC cement (Graig, 2002). They also have higher fracture resistance, almost comparable to micro-filled composite resins.

On an average, the compressive strength of conventional GIC cements is about 70 to 220 MPa, and that for luting GIC cements is about 10 to 150 MPa. The compressive strength for a resin-modified GIC cement is about 110 to 220 MPa. The tensile strength for conventional GIC cements is about 12 to 16 MPa and that for luting GIC cement is about 6 to 15 MPa. The tensile strength of resin -modified GIC cement is about 15 to 16 Moa. The Shear strength of the conventional GIC cement is about 30 to 40 MPa and for the luting GIC cement is about 20 to 25 MPa.

On the other hand, the resin-modified GIC cement has shear strength of about 60 to 70 MPa. During the early days, the GIC cements did not have a good compressive and tensile strengths. However, nowadays due to advancements in the materials, the compressive and tensile strengths of conventional GIC is approaching that of resin-modified GIC and also the microfilled composite resins (Mount, 1998). However, the resin-modified GIC cements should only be utilized in low –stress bearing areas. They can be utilized in patients having a high-caries rate as the GIC cement has anti-cariogenic properties (Graig, 2002).

Many dental practitioners consider using cermets cements or sliver-impregnated GIC cements in load-bearing areas as they would feel that it helps improve the facture toughness of the GIC. However, this is a wrong perception as cermets cements only help to improve the abrasive resistance. Conventional GIC cements have a reasonable amount of resistance to abrasion. They may be susceptible to abrasion during the initial stages following placement (Mount 1998). Self-curing GIC cements may have moderate translucency, but this may take several days to develop.

Self-curing GIC cements tend to be affected by the presence of water during the initial period following setting. Hence, self-curing GIC cements may have to be carefully sealed during placement for at least 24 hours until some amount of translucency is achieved. The technique of placement plays a very important role in the case of conventional GIC. Resin-modified GIC cements show a much better translucency compared to the conventional GIC cements. The translucency is achieved immediately following curing with light. The translucency may slightly worsen over the next few days following placement, but this may not be perceived to the eye.

Following this, the translucency improves again and sometimes it even achieves a greater amount of translucency compared to that obtained following curing (Mount, 1998). HEMA (about 15 to 25 %), certain polymerisable groups (1%) and a photo-initiator are present in the liquid component of the resin-modified GIC cement. The light –activation of the GIC enables polymerization of the resin, and the chemical reaction between the liquid and the powder components of the GIC goes on as in an auto-curing system. The final setting of the resin-modified GIC cement is the same as that of conventional cement.

As HEMA is present in the liquid component of the resin-modified GIC cement, using thinly consistency cements would bring about a higher HEMA content in the final set mixture. A thick mixture would contain 4. 5 % HEMA, whereas thinly-mixed GIC cement would contain about 15 % HEMA. The HEMA is capable of drawing water from the oral environment and degrading. Further, HEMA is released into the dentin. The presence of certain trace elements in the GIC cement brings about an oxidation-reduction reaction and ensures that the HEMA is not left behind. Hence, water is not absorbed from the environment by the GIC cement.

During the setting of the resin-modified GIC cement, two basic reactions occur between the powder and the liquid. The first is the acid-base reaction between the polyalkenoic acid and the glass powder. Two separate matrices are formed, one is a hydrogel of the ionomer salts and the other is a poly-HEMA matrix. When these two matrices are formed, the interactions prevent the acid-base reactions from completing. The HEMA particles will begin to set following activation by light, and will prevent the auto-curing GIC cement from absorbing water almost immediately (Mount, 1998).

This will also ensure that a greater amount of strength is achieved by the restoration almost immediately (Graig, 2002). The acid-base reaction, the light-curing reaction (along with the presence of the photo-initiator) and the oxidation-reduction reactions ensure that adequate cross-linking takes place in the resin-modified GIC cement. The light-curing reaction ensures sufficient and immediate hardening of the GIC mass provided the light activation is performed. The acid-base reaction will continue for a few days to bring about hardening within a few days.

The cross-linking formed in the acid-base reactions and the HEMA matrix will ensure that water is not taken up by the GIC mass. The 5 to 15 % HEMA present in the GIC ensures that water is not immediately taken up by the GIC mass. However, as a sufficient quantity of substances present in the conventional GIC cement is present, the chances of dehydration following the initial setting reaction are still high. Hence, light-cured resin-modified GIC cement should also be protected with a low-viscosity resin sealant (Mount, 1998).

When resin-modified GIC cement is utilized as a base below composite resin restorations, there is no need to etch the GIC cement before inserting the composite resin material. HEMA helps in forming a chemical bond between the GIC cement and the composite resin. Efforts should be made during the etching process to prevent accident etching of the GIC cement. However, etching the GIC cement would not result in an adverse affect. When GIC cement is utilized below amalgam restorations, it is better to use resin-modified GIC cement as it can tolerate higher strengths compared to the conventional GIC cements (Graig, 2002).

Resin-modified GIC cements are frequently utilized below composite resin restorations since the year 1985, so as to lower microleakage. Besides, fluorides released by the GIC would ensure that secondary caries does not develop. The composite resin would ensure that superior aesthetic effects of the restoration would be maintained. Fluorides leached by the GIC would ensure that the restoration has some anti-cariogenic effect. Earlier, conventional GIC cement was utilized below composite restorations, and only mechanical interlocking between both the materials occurred.

The GIC present below the resin material was lost over a period of time. There is no chemical bonding between the GIC and the composite resin restoration and hence, the bond strength is very poor. The use of rein-modified GIC cements present below composite resins helps to improve the bond strength as a chemical bonding would be formed between the monomer present in the GIC and certain substances present in the composite resin (Taher, 2007). The GIC utilized in a laminate or sandwich technique can be used in two fashions, that is open sandwich and closed sandwich technique.

In the open sandwich technique, portions of the GIC are exposed to the oral cavity. The GIC is not only utilised to cover the exposed dentin but is also placed peripherally to form a type of seal. In the closed technique, the GIC covers the dentin and is in turn completely covered by the composite resin restoration. Using conventional GIC cements, the failure rates were 13 to 35 % within 2 years and 75 % within 6 years. The conventional GIC cements placed were capable of degenerating to a greater extent in the saliva and are also susceptible to fracture due to decreased fracture resistance.

Gradually, resin-modified GIC began to replace the conventional GIC under composite restorations. As resin-modified GIC cements have superior properties, they would ensure a longer life, and would also have an anti-cariogenic effect. Studies demonstrated that the resin-modified GIC developed better proprieties and was not much technique-sensitive compared to the conventional GIC cements. In the open-sandwich technique, the marginal seal developed by the resin –modified GIC cement was much better than the composite resin materials and hence is preferred.

The caries rate was much less in the patients using Resin-modified GIC cements compared to the conventional GIC cements. A study was conducted by Dentists in a city in Sweden to study the effect of using resin-modified GIC cements on about 239 restorations. It was found that the 5 % of the restorations had become unacceptable after 3 years (that is about 5 % of all restorations treated with open-sandwich techniques had failed). Tooth fractures developed in about 2. 5 % of the restorations. Minor erosions of the GIC were observed in 4 %. Secondary caries developed only in one of the 239 restorations.

The properties of resin-modified GIC cements is much superior to conventional GIC cement when utilized beneath composite restorations in a sandwich technique. The Resin-modified GIC cements are less susceptible to dissolve and disintegrate in the salvia compared to the conventional GIC cements. It also adapts well to the cavity walls and offers a chemical bonding with the composite resin. The failure rate with resin modified GIC cements was much less compared to that of conventional GIC cements. The sandwich technique can be utilized as an alternative to amalgam restorations especially in those with a high caries index.

The restorations produced have a lower failure rate and has a much longer life. However, the use of resin-modified GIC cements for sandwich technique has not been studied on a long term basis (van Dijken, 1999). The bond strengths formed between resin-modified GIC cements and the composite resin materials was higher compared to that between the conventional GIC and the composite restorations following etch and bond technique. In etch and bond technique, the bond strength formed was about 2. 42 MPa compared to that of 6. 87 to 7. 05 MPa formed between the resin-modified GIC and the composite restorations (Knight, 2006).

Another study conducted in the University of Cardiff by Chadwick et al (2007), demonstrated that resin-modified GIC cement had a much better success rate compared to conventional GIC cements. The failure rates of conventional GIC cements were between 6. 6 to 60 %. The failure rates of resin-modified GIC cements were found to be between 2 to 14 %, suggesting superior mechanical properties. Some amount of evidence is present from past literature that resin-modified GIC cements could also be utilised to a certain extent in small and moderate sized class II restorations (Chadwick, 2007).

A study was conducted to study the surface properties of resin-modified composite resins compared to that of conventional GIC and that of composite resin restorations. The materials were examined following polishing with silicon carbide. The composite resin restorations and the resin-modified GIC cements were more resistant to the effect of foodstuff such as tea, coffee and red wine on the restoration than the conventional GIC. The surface roughness of the composite resin restoration and the resin-modified restoration were much lesser compared to the conventional GIC restoration (Bagheri, 2007).

There are not much of differences in the composition of the resin-modified GIC and the conventional GIC cements. The resin-modified GIC contains a small percentage of monomers and polymers to bring about superior mechanical properties. Resin-modified GIC cements are more resistant to solubility and excessive disintegration from several factors that operate from within the oral cavity. The shrinkage that develops in resin-modified GIC is much slower and less compared to conventional GIC cement. Resin–modified GIC cements have higher transverse strengths, compressive strengths and fracture resistance compared to conventional GIC cements.

Even the tensile strengths and the shear strengths of resin-modified GIC cements are higher than conventional GIC cements. However, resin-modified GIC cements should not be utilized in high stress bearing areas, as they could fail. Resin-modified GIC achieves its final properties almost immediately, following curing with light. Resin-modified cements have better properties when placed below composite and amalgam restorations. Below composite restorations, it forms a chemical bond with the composite. As its strength is higher, it can be utilized below amalgam restorations.

It also helped to reduce the secondary caries rate as they released fluorides over a period of time. The surface properties of Resin-modified GIC are also much better compared to that of conventional GIC, and hence the aesthetic properties would be much better. All these superiorities do imply that Resin-modified GIC should be preferred for use in various situations. However, the Resin-modified GIC should not be utilized to fill large cavities in stress bearing areas, as they are susceptible to fail.

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