Glass ionomer cement is also referred to as polyalkenoate cement. It was discovered in the year 1969 by Wilson and also reported by Kent in 1971. There are basically nine types of GIC
Type 1 : Used in luting
Type 2 : Used as restoration
Type 3 : Used as liners and base
Type 4 : Used for fissures
Type 5 : Used for orthodontic purpose
Type 6 : Used as core build up material
Type 7 : Fluoride releasing GIC
Type 8 : For atraumatic restoration(ART)
Type 9 : Used for pediatric purpose
The composition of glass ionomer is unique as it has a powder liquid system
Powder composition
Aluminum fluoride
Aluminum oxide
Calcium fluoride
Sodium fluoride
Aluminum phosphate
Silicon dioxide
These components are heated around 1200 degrees and fused to form as one component "glass".
For radiopacity : zinc oxide,Barium is added,
Liquid composition
Copolymer with itaconic acid, maleic acid or tricarboxyllic acid and tartaric acid
Tartaric acid helps to increase working time and reduces the setting time.
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After the discovery of glass ionomer in early 1970's by Wilson and Kent there was a flaw in the cement they were :
1. The Glass ionomer cement took long time to set.
2. The translucency of the glass ionomer cement was also less.
So in the year 1976 it was Wilson and his co workers reported that addition of tartaric acid to the glass ionomer cement. After adding they found out that
1. Fluoride content of the glass ionomer reduced
2. It had more setting time
3. Viscosity of the cement was also reduced.
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I have provided a brief note on the indications and the contraindications for the usage of GIC.
Indications:
Restorative Materials:
Restoration of erosive/abrasive lesions without cavity preparation.
Sealing and filling of occlusal pit and fissures.
Restoration of primary teeth.
Restoration of Class V carious lesions.
Restoration of Class III carious lesions, preferably using lingual approach.
Repair of defective margins in restorations.
Minimal cavity preparations in a proximal lesions through buccal and occlusal approach (tunnel preparations).
Core built up.
Provisional restorations where future veneer crowns are contemplated.
Sealing at root surface for over dentures.
Fast Setting Lining Cements and Bases:
Lining of all types of cavities where a biological seal and cariostatic action are required.
Replacement of carious dentin for the attachment of composite resins using the acid etch technique.
Sealing and filling of occlusal tissue showing early signs of caries.
Luting Cements:
Fine grain versions of the glass Ionomer cement are now available for luting purpose because of their fluoride leach, these cements are particularly useful in patients with high caries incidence. In addition, the translucency of the glass Ionomer cement is of great value where porcelain margins are used for cosmetic reasons.
Contraindications for Use:
1) Class IV carious lesions (or) fractured incisors
2) Lesions involving large areas of labial enamel where esthetics is of major importance.
3) Class II carious lesion where conventional cavities are prepared, for replacement of existing amalgam restorations.
4) Lost cusp areas.
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Dental amalgam is produced by mixing liquid mercury with solid particles of an alloy of silver, tin, copper and sometimes zinc, palladium, indium and selenium.This combination of solid metals is known as the amalgam alloy. It is important to differentiate between dental amalgam and the amalgam alloy that is commercially produced and marketed as small filings, spheroid particles, or a combination of these, suitable for mixing with liquid mercury to produce the dental amalgam.
The freshly mixed mass of amalgam alloy and liquid mercury developed by the dentist has a plasticity that permits it to be conveniently packed or condensed into a prepared tooth cavity. A dental amalgam restoration results. Such amalgam restorations usually are limited to the replacement of tooth tissue in posterior teeth because their silvery gray metallic appearance. Dental amalgam restorations are reasonably easy to insert, are not overly technique-sensitive, maintain anatomic form, have reasonably adequate resistance to fracture, prevent marginal leakage after a period of time in the month, and have a relatively long service life.
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Humankind always has been plagued by the problem of restoring parts of the body lost as a result of accident or disease. Practitioners of dentistry have been confronted with this problem since the beginning of the dental practice, and the means of replacing missing tooth structure by artificial materials contains to account for a large part of dental science. During the period from 1600 to 1840, the foundations for the science of dentistry were established. In 1819 in England dental amalgam was first known as BELL’S PUTTY. M. Traveau is credited with advocating the first form of dental amalgam-silver-mercury paste in 1826, in Paris France. It was presented by Crawcour brothers to the dental profession in America in 1833 as “Royal Mineral Succedaneum” as substitute for gold.
The ensuing chaos in organized dentistry of that time is known historically as the “Amalgam War”. E.Parnly stated that dental amalgam was “ Wholly inapplicable and UNFIT for use in the MOUTH”.
The Crawcour brothers introduced “Silver paste”, the amalgam of silver with mercury in the U.S as a filing material in 1883. The first National Dental Society, the American Society of Dental Surgeons was established in 1840. One of the early actions of the American Society of Dental Surgeons was to forbid its members to use Silver amalgam for restoring lost tooth structure. About the time of this, “war” copper amalgam was introduced in 1844.
A silver-tin-mercury alloy, or amalgam was introduced in 1855 by Elisha Townsend, followed by another formula by J.F.Flogg., Dr.J.Foster Flagg and Dr.G.V.Black studied amalgam, and the research of Black led to the development of the formula in 1896 that has been closely adhered to until recent years.
Dr.G.V.Black’s formula in 1896:
68.50% - silver
25.50% - tin
5% - gold
1% - zinc
Since amalgam is the most widely used single restorative material in dentistry, its overwhelming importance cannot be ignored. It is relatively easy to use, a fact that perhaps invited its use in many unwarranted circumstances and widespread abuse. There is still no adequate, economic alternative for dental amalgam as a restorative material for moderately sired carious lesion in a high-load-bearing area. The combination of reliable long-term performance in load-bearing situations and small cost per unit is unmatched by any other dental restorative material.
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MANUFACTURE OF ALLOY POWDER
Lathe-Cut powder
To make lathe-cut powder, an annealed ingot of alloy is placed in a milling machine or in a lathe and is fed into a cutting tool or bit. The chips removed are often needle like, and some manufacturers reduce the chip size by ball milling.
Homogenizing Anneal
Because of the rapid cooling conditions of from the as-cast state, an ingot of an Ag-Sn alloy has a cored structure and contains non-homogenous grains of varying composition. A homogenizing treatment is performed to re-establish the equilibrium phase relationship. The ingot is placed in an oven and heated at a temperature below the solidus for sufficient time to allow diffusion of atoms to occur and the phases to reach equilibrium. The time of heat treatment may vary, depending on the temperature used and the size of the ingot, but a thermal time of 24 hours at the selected temperature is not unusual.
At the conclusion of the heating cycle, the ingot is brought to room temperature for the succeeding steps in manufacture. The proportion of phases present in the ingot after cooling is affected by the manner in which the ingot is cooled. If the ingot is withdrawn from the heat treatment oven rapidly and quickly quenched, the phase distribution will remain essentially unchanged. On the other hand, if the ingot is permitted to cool very slowly, the proportions of phases will continue to adjust towards the room temperature equilibrium ratio.
For example, in a Ag-Sn alloy, rapid quenching of the ingot results in the maximum amount of Beta phase retained, whereas slow cooling results in the formation of the maximum amount of the Gamma phase.
Particle Treatments
Once the alloy ingot has been reduced to cuttings, it is treated with acids. The exact function of this treatment is not entirely understood, but it is probably related to preferential dissolution of specific components from the alloy. Amalgams made form acid washed powders tend to be more reactive than those made from unwashed powders.
The stress induced into the particle during cutting and ball milling must be relieved or they slowly release over time, causing a change in the alloy, particularly in the amalgamation rate and dimensional change occurring during hardening. The stress relief process involves an annealing cycle at a moderate temperature of 100degrees C for several hours.
Atomized Powder
Melting together the desired elements makes atomized powder. The liquid metal is atomized into fine spherical droplets of metal. If the droplets solidify before hitting a surface, the spherical shape is preserved, and these atomized powders are called “spherical powders”.
Particle Size
The manufacturer controls maximum particle size and the distribution of sizes within an alloy powder. Most significant influence on amalgam properties is the distribution of sizes around the average. For example, very small particles greatly increase the surface area per unit volume of the powder. A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam.
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PROPORTIONS OF ALLOY TO MERCURY
Correct proportioning of alloy and mercury is essential for forming a suitable mass of amalgam for placement in a prepared cavity. Some alloys require mercury – alloy ratios in excess of 1:1, whereas others use ratios of less than 1:1 with the percentage of mercury varying from 43% to 54%.
SIZE OF MIX
Manufacturers commonly supply capsules containing 400, 600, or 800 mg of alloy and the appropriate amount of mercury. For large size cavities capsules containing 1200 mg of ally are also available.
MIXING OF AMALGAM: (TRITURATION)
Trituration of the alloy with the mercury is normally carried out in a mechanical amalgamator. The efficiency of the machine will be influenced by:
1. Speed of the particular unit.
2. The length and type of “throw” of the capsule.
3. The presence of a pestle in the capsule.
4. Length of time of mixing.
Mechanical amalgamators are available in the following speeds:
Low speed: 32-3400 cpm.
Medium speed: 37-3800 cpm.
High speed: 40-4400 cpm.
Over-trituration
Alloy will be hot, hard to remove from the capsule, shiny wet and soft.
It is better to slightly over-triturate than to under triturate an amalgam since:
-Extended trituration may reduce plasticity
-Shorten working time and
-Increase final contraction.
Under-trituration
Alloy will be dry, dull and crumbly; will crumble if dropped from approx 30 cm.
Reduced tritutation may result in:
-Incomplete wetting of the surfaces of the alloy particles by mercury.
-A weak interface between the matrix (gamma1) and the particles.
-Lower strength.
-Increased porosity.
-A rougher surface.
-Increased corrosion.
-Loss of surface finish.
Normal Mix: Shiny appearance separates in a single mass from the capsule.
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DENTAL MERCURY HYGIENE
Recommendations from the World Dental Federation include the following:
1. All personnel involved in the handling of mercury should be alerted, especially during training, to the potential hazard of mercury vapor and the need to observe good mercury hygiene practice.
2. The work place should be well ventilated, with fresh air exchange and outside exhaust. Air filters such as those in air-conditioning systems may act as mercury reservoirs and should be replaced periodically.
3. The surgery atmosphere should be checked periodically for mercury vapor.
4. Mercury should be stored in unbreakable, tightly sealed containers away from any source of heat.
5. Use single use capsules rather than reusable ones or any other methods of dispensing the alloy and mercury.
6. Avoid the need to remove excess mercury before or during packing by selecting an appropriate alloy: mercury ratio.
7. Use an amalgamator with a completely closed activator arm.
8. Mercury and unset amalgam should not be touched by the bare hands.
9. All amalgam scrap and free mercury should be salvaged and stored in a tightly closed container under used radiographic fixer solution.
10. Spilled mercury should be cleaned up immediately and placed in the scrap jar.
11. Do not use ultrasonic amalgam condensers.
12. Skin accidentally contaminated by mercury should be washed thoroughly with soap and water.
13. If a mercury hygiene problem is suspected, personnel should undergo urinalysis to detect mercury levels.
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