LUTING CEMENTS
Over last two decades, the emphasis has been on materials for luting in view of the increase in indirect metal and ceramic restoration, provisional or interim restorations, cosmetic laminate veneers/ or anterior teeth, for retention of restoration like pins and posts.
New International Standards are being developed like:
ISO- International Standard Organization.
ANSI/ ADA – American National Standards Institute/ American Dental Association.
In the early years of this century Zinc Oxide –
Phosphoric acid, Zinc Oxide- Eugenol (Clove oil 85%) and Silicate glass- phosphoric Acid cements were discovered. These were widely used until the 1970s when new luting cements began to be developed.
The introduction of new types of luting cements was prompted by the emphasis on improved biocompatibility and bonding to the tooth that began to develop 20 years ago. New information on pulpal histopathology resulting from particular clinical techniques and materials, as well as the demonstration of marginal leakage involving penetrations of bacteria to the dentin interface and a reduction in retention of restoration, led to the realization that new materials, possessing good wetting and bonding to Enamel and dentin and low toxicity were needed.
For Acceptable Performance luting cements Must have:
1. Adequate resistance to dissolution in the oral cavity
2. Non-Toxic and Non-Irritant to pulp
3. Develop High strength in tension, shear and compression to resist stresses at the restoration / tooth interface.
4. Good working time and setting time.
5. Should be biologically acceptable
6. Should have low viscosity to give a low film thickness
7. Performance, it should be bacteriostatic and Anticariogenic.
uses for luting cements :
For permanent luting:
1. Zinc Phosphate
2. Zinc Polycarboxylate
3. Glass Ionomer
4. Zinc Oxide Eugenol EBA/ Alumino
For temporary luting:
1. Zinc Oxide- Eugenol
2. Non-Eugenol Zinc Oxide
Zinc Phosphate
Zinc Phosphate is the oldest of the luting cements and thus is one that has the longest tract record. It serves as the standard with which newer systems can be compared.
Terms like “Crown and Bridge” and “Zinc Oxyphosphate”.
American Dental Association Specification NO. B Designates them as two types (On the basis of the intended use)
1. Type 1: fine Grained for luting
2. Type 2: For all other purpose
Type 1 or Fine Grain is capable of forming film thickness of 25 micron or less.
Type 2 or Medium Grain allows maximum thickness of 40 micron
Chemistry and all other properties of type 1 and type 2 are same.
Mode of Supply
1. Powder and liquid system
2. Capsules of preproportioned powder and liquid.
A variety of shades are available: Yellow, Grey, Golden brown, Pink and White.
Composition
Powder: Weight (%)
ZnO : 90.2
MgO : 8.2
MgO : 1.4
Bi2O3 : 0.1
BaO; Ba2So4; CaO : 0.1
Liquid:
H3PO4 (Free Acid) : 38.2
H3PO4 (Combined with Al&Zn) : 16.2
Al : 2.5
Zinc : 7.1
H2O : 36.0
Powder:
Principal constituent of powder is Zinc Oxide. The principal modifier is MgO which is usually 10%, it reduces the temperature of calcinations (i.e. it aids in sintering of the Zinc Oxide powder)
Silicone dioxide is an inorganic filler and during manufacturer aids in the calcinations.
Bismuth trioxide imparts a smoothness to the freshly mixed cement mass and in large amounts, it may lengthen the setting time.
TANIN FLUORIDE may be added to provide a source of fluoride ions is some products or St. Fluoride 10%.
Ingredients are heated together at a temperature ranging from 10000C to 13000C for 4 to 8 hours longer, depending on the temperature. Sintering is done to reduce its reactivity.
Calcination results in a fused or sintered mass. The core is then crushed and pulverized to a granular powder, which is sieved to retrieve picked powder particle size. The degree of calcinations, fineness of particle size and composition determine the reactivity of the powder with the liquid.
Liquid:
Produced by adding aluminum and sometimes Zinc to a solution of Ortho Phosphoric Acid.
It is a syrupy fluid and contains 1/3rd water.
The partial neutralization of the Phosphoric Acid by the aluminium and Zinc tempers the reactivity of the liquid and is described as buffering. This reduced rate of reaction helps establish a smooth, nongranular, workable cement mass during mixing procedure.
Amount of water controls the ionization of the liquid and it is an important ingredient since it influences the rate and the type of powder-liquid reaction.
SETTING REACTION
When an excess of powder is brought into contact with the liquid t begin the cement mix, wetting occurs and a chemical reaction is initiated. The surface of Alkaline powder is dissolved by Acid liquid, resulting in exothermic reactions,
The set cement is a hydrated amorphous network of ZnPO4 that surrounds incompletely porus. There is no evidence that MgO present in powder reacts with liquid. Although no crystalline phosphate is involved in the setting process of the cement, there can be subsequent growth of crystalline hopeite in presence of moisture.
Zn(PO4) 2 4H2O
Zinc aluminophosphate gel is formed after the reaction. Thus the set cement is a cored structure consisting of unreacted ZnO particles embedded in a cohesive amorphous matrix of Zinc aluminophosphate gel.
MANIPULATION
An accurately chilled, thick glass slab is used. It helps in dissipating the heat of the reaction. Mixing slab warmth should be low just to effectively cool the cement mass but not be underneath the dew point unless the frozen glass slab is used. A temperature of 180 to 240C is shown when room humidity permits. The moisture compression on a slab cooled below the dew point contaminated the mix, diluting the liquid, and shrinking the setting time.
FROZEN SLAB METHOD: a glass slab is chilled in a refrigerator at 60C or a freezer at -100C. No effort is made to limit moisture from compressing on a glass slab when it is brought to room temperature. A mixture of cement is made on the cold slab by combining the powder until the correct texture is reached. The amount of powder joined with the frozen slab method is 50% to 75% more than with normal procedure. The compressive and tensile sites of the cements prepared by the frozen slab method are not significantly different from those prepared from normal mix. No variation exists in the solubility of the frozen slab and sound mixes.
Advantage of this method is substantial increase in the working time (4 to 11 min) and shorter setting time (20-40% less) of the mix after placement into the mouth.
With normal mix:
The amount of powder that can be mixed into liquid greatly determines the properties of the mixed cement mass.
Recommended P : L = 1.4gm/ 0.5 ml
CARE OF LIQUID
Exposure of liquid to a humid atmosphere will absorb water whereas exposure to dry conditions tends to lose water.
↑ Water → ↑ Reaction → ↓ S.T.
↓ Water → ↓ Reaction → ↑ S.T.
Thus bottle should be kept tightly closed. Polyethylene squeeze bottle do not require removal of a dropper thus eliminates the tendency of gaining or loosing water from the liquid.
MIXING PROCEDURE:
On a cool dry glass slab the powder is dispensed first. The liquid must be dispensed just before mixing. Powder is divided into small increments.
Mixing is done by stainless steel spatula. Powder is gathered in the shape of rectangle about 1mm thick. Next, it is divided into 8 sections. Then each increment is added in same sequence. It allows to control the rate of mixing and setting time. Each increment is mixed for 10 seconds thus total of 1 minute and 15 seconds is recommended. Mixing is done over large area as the reaction is exothermic thus it helps in heat dissipation. If manipulation is done in small area the rise in temperature of the mix speeds the reaction and hinder control over consistency.
Thus initially, two small increments are added then larger increments can be added to saturate the newly forming complex zinc phosphate and finally small increments are added to control the consistency.
Longer is the mixing time within practical limits and greater the pressure of the spatula against the mix, the larger is the setting time. Most effective may control WT and ST is to regulate the temperature of Glass Slab.
CHARACTERISTIC PROPERTIES
Consistency And Film Thickness:
Luting “consistency” or “Inlay Seating” are the two arbitrary terms used.
For retention of Orthodontic Bands “Band-Seating Consistency” term is used.
Retention between Cast Restoration and tooth structure is brought about by “Mechanical Interlocking” between surface irregularities.
Depends upon:
a) W : P Ratio
b) Temperature of the Glass Slab
More the powder to liquid ratio, more will be the consistency.
If the consistency is too thin the pH remains low/ or relatively longer time and thereby unduly jeopardize the pulp. At the same time the strength is reduced and its resistance to solubility also decreases.
If the consistency is too thick, casting may not seat adequately.
Standard consistency may be defined as the powder to liquid ratio needed to obtain a 30 mm disc when subjected 0.5 ml of cement to a 120 gm load applied to parallel surface with the cement between them.
Film Thickness:
It plays an important role in the way casting fits the prepared tooth.
Film thickness adopted by ANSI/ ADA for Type 1 ZnPO4 is 25 mm or less.
The strength of the retention bond may also be influenced by the film thickness.
Ultimate film thickness that a well- mixed, nongranular cement attains depends on:
a) Particle size
b) W : P Ratio
Film thickness varies with the amount of force and the manner in which this force is applied to a casting during cementation.
To clinically check- luting cement should string up from the slab on the spatula about 2-3 cm as the spatula is lifted away from the mass.
VISCOSITY:
The consistency of cement can be quantified by measuring viscosity. The rapid increase in viscosity demonstrates that restorations should be cemented promptly after completion of the mixing to take advantage of the lower viscosity of the cement. Delay in cementation can result in considerably larger values of film thickness and insufficient seating of the restoration.
Viscosity of Zinc Phosphate cement as a function of temperature and time.
(Vermileya; Power; Craig JDR 56; 762; 1977)
SETTING TIME:
Adequate WT is expressed by proper net setting time, which, as determined by ANSI/ ADA specification no 96 and based on an inlay setting consistency is between 2.5 and 8 minutes at a body temperature of 370C
Factors Governing the Rate of Set of Zinc Phosphorous cement-
Controlled by Manufacturer:
a) Powder composition
b) Degree of Powder Calcination →↑Temperature – ↓Reaction
c) Particle size of liquid → Smaller particles, faster reaction
d) Buffering of liquid → Slows the reaction
e) Water content of liquid → Excess water faster reaction
Controlled by Operator:
a) Powder liquid ration
b) Rate of Powder Incorporation
c) Mixing Temperature
d) Manner of spatulation
e) Water contamination or loss of water
STRENGTH:
Compressive strength: 103.5 MPa (15000psi)
Set cement gains 75% of its maximum strength in the 1st hour. Maximum strength is attained on the first day.
– ANSI/ ADA specification no 96 (ISO 9917) stipulates that a standard inlay setting consistency must exhibit a minimum of 24 hours compressive strength of 70 MPa.
– Prolong contact with oral fluids or water gradually reduces its strength. This is because of slow dissolution of cement.
Factors Affecting Strength:
a) P/L Ratio → More the powder added into the liquid, more is the strength.
b) Water content→ Gain or loss of water, reduces the strength.
It has a low TENSILE STRENGTH, thus making it BRITTLE i.e. 5.5 MPa (800psi)
Modules of Elasticity
It is comparatively high. This makes it still and resistant to elastic deformation when used as luting agent for restorations that are subject to high masticatory stresses.
→ 13 GPa (1.9 X 106 Psi)
SOLUBILITY AND DISINTEGRATION:
According to standard method, the solubility and disintegration are distilled water after 23 hours may range from 0.04% to 3.3% for inferior materials.
– The standard limit according to ANSI/ ADA is 0.2%
– Fluoride containing luting cements give the figure of 0.7% to 1.0% because of reaching of fluoride.
– Solubility in Acid (Lactic or Citric) is 20-30 times more.
– According to ANSI/ ADA specification no 96, it allows a maximum rate of erosion of 0.1 mm/ hours when the cement is subjected to lactic acid erosion by Impinging Jet Technique.
– To increase the resistance to solubility and disintegration, higher P/L ratio must be used.
– Any change in water content will increase the solubility and disintegration.
– Premature contact of the incompletely set cement with water results in the dissolution and reaching of the surface. Varnish application prevents that
DIMENSIONAL STABILITY
ZnPo4 cement exhibits shrinkage on hardening. The normal dimensional charge when properly mixed cement is brought into contact with water after it has set is that of slight initial expansion, apparently from water absorption. This expansion is then followed by a slight shrinkage on the order of 0.04% to 0.06% in 7 days. This shrinkage gives rise to the slit at the tooth/ cement and cement/ restoration.
THERMAL PROPERTIES
– It is a good thermal insulator and may be effective in reducing galvanic effects.
– Recent studies have shown that presence of moisture does not have a significant effect of thermal conductivity of the cement.
– Moisture, reduces the potentially good electrical insulating properties of the material.
BIOLOGICAL PROPERTIES
– The pH of Zinc Po4 cement liquid is approximately 2.0
– In the early manipulative stages this increase in pH is relatively rapid, with a standard mix reaching a pH of 4.2 in 3 minutes after mixing is started. At the end of 1 hour this value increases to about 6 and is almost neutral after 48 hours.
– The pH is lower and remains lower for a longer period of time when thin mixes are employed.
– Thus the damage to pulpal tissue from acid attack probably occurs during the first few hours after insertion.
– Studies show that the use of a poorly mixed cement in conjunction with a hyperemic pulp can result in Necrosis.
– Before permanently seating the casting most clinicians recommend the application of one or two coats of suitable cavity varnish.
Some Products:
Permanent Luting:
– Fleck’s Extraordinary : Mizzy, Inc.
– Hy Bond : Shofer Dental Corp.
– Modern Tenacin : L.D. Caulk
– Zinc Cement Improved : Mission White Dental Inc.
ZINC OXIDE EUGENOL AND NON-EUGENOL CEMENT
– When certain kinds of Zinc Oxide are mixed with Eugenol, the mix sets to a hard cement that is compatible with both the hard and soft tissues of the mouth.
Classification on the basis of ADA specification no. 30
1. Type 1 : For temporary cementation
2. Type 2 : Permanent cementation
3. Type 3 : Temporary filling material an thermal insulation
4. Type 4 : Cavity liners
COMPOSITION
Powder W%
ZnO : 69.0
White Rosin : 29.3
Zinc Stearate : 1.0
Zinc Acetate : 0.7
Liquid:
Eugenol : 85.0
Olive Oil : 15.0
(Adapted from: Wallace; Hansen JADA 26, 1933)
Zinc Oxide is the principal ingredient of powder.
White Rosen → Reduces the brittleness of the cement
Zinc Stearate → Accelerator and Plasticizer
Zinc Acetate → Accelerator and improves strength
Olive oil → Plasticizer
It was first used in dentistry nearly 100 years ago. Since then, the list of application for which it is used has grown dramatically.
SETTING REACTION
The setting of ZOE cement is a chelation reaction in which an amorphous, zinc eugenolate is formed
In the first reaction:
Hydrolysis of Zinc Oxide to its hydroxide takes place. Water is essential for the reaction to proceed (Dehydrated) Zinc Oxide does not react with dehydrated eugenol).
In the second reaction:
Typical Acid-Base reaction occurs to form a chelate.
Two molecules of eugenol react with ZnO in the presence of water to from the chelate → Zinc Eugenolate.
Excess Zinc is always used, so the set material consist of a matrix of Amorphous Zinc Eugenolate that bonds the unreacted Zinc Oxide particles together.
The chelate forms an amorphous gel that tends to crystallize imparting strength to the set mass.
Setting time of the cement is in the range of 4-10 minutes.
FACTORS AFFECTING SETTING REACTION
The complete reaction between Zinc Oxide and Eugenol takes place in about 12 hours. This is too slow clinical convenience.
1. Manufacture:
The most active Zinc Oxide powder is formed by decomposing Zinc Salts like Zinc hydroxide and Zinc Carbonate by heating at 3000C.
2. Particle size: Smaller the Zinc Oxide Particle size, faster is the reaction.
3. Accelerators:
Alcohol, Glacial Acetic Acid and small amount of water Accelerates the Reaction.
4. Heat :
High temperature accelerates the reaction
5. Retarders:
Set can be retarded with Glycol and Glycerine.
6. Powder to liquid Ratio:
Higher the ratio, faster is the reaction.
CLASSIFICATION
ADA specification no. 30 has listed and types of Zinc Oxide Eugenol luting cements.
Type I : Temporary cementation
Type II : Permanent cementation
Type III : Temporary Filling Material and Thermal Insulation
Type IV : Cavity liners
PROPERTIES
1. Mechanical Properties:
a) Compressive strength
– It is relatively weak cement.
– It ranges from 3-4 MPa to 50-55 MPa
– Particle size affects the strength
– Smaller the particle size, stronger is the cement
– Maximum strength required for temporary cementation is 35 MPa.
– Minimum of 35 MPa is required for permanent cementation
– 2.5 MPa of minimum is required for Filling Materials and Bases.
– Lining Material require minimum strength of 5 MPa.
– The strength of cement for temperature cementation is selected in relation to the retentive characteristics of the restoration and the expected problems of removal of restoration.
THERMAL PROPERTIES
a) Thermal Conductivity:
It has excellent thermal insulating properties and is approximately same as that of human dentin.
i.e. 3.98 (Cal. Sec-1 cm-2 (0C/cm)-1) X 10-4
b) Coefficient of thermal expansion:
35 X 10-6/ 0C
SOLUBILITY AND DISINTEGRATION:
It is a important property for cements used for permanent cementation.
This is reflected in the maximum specification values for disintegration in 24 hours.
A maximum value of 2.5% is acceptable for provisional cementing material but a value of 1.5% is required for the other cements.
The test used is the amount of disintegration measured by weight loss, which occurs in a disk of cement immersed in distilled water for 24 hours.
– Solubility of set ZOE cement is highest among all the cements.
They disintegrate in oral fluids. This breaks down is due to hydrolysis of the Zinc Eugenolate matrix to form Zinc Hydroxide and Eugenol.
– Solubility can be educed by increasing the powder/ liquid ratio.
– A clinical study of the use of use of various unmodified cements for provisional luting indicated that a cement with a compressive strength of 15-24 MPa was the most appropriate cement based on retention, taste, case of removal and case of cleaning.
– Another clinical study indicated that an unmodified ZOE cement with a compressive strength of .9 Mpa was the most commonly used material for the temporary cementation of complete crown and bridge, without a range of luting cements with compressive strength from 14 to 21 Mpa was found to be diserable.
FILM THICKNESS:
Film thickness of ZOE luting cements are higher than other cements – 20-25 m.
BIOLIOGICAL PROPERTIES
– They are least irritating of all dental luting cements.
– Pulpal response is classified as ‘MILD’
– PH = 6.6 to 3.0
– They inhibit growth of bacteria and have anodyne or soothing effect on the pulp in deep cavity, reducing pain when present.
– It maintains good marginal seal, despite volumetric shrinkage of 0.9%.
– Eugenol is a potential allergen
MANIPULATION
– Powder-liquid ratio is 4% to 6:1 by weight.
– Mix is done on the glass slab on paper pad in a circular motion with a still blade stainless steel spatula. Smaller increments are added in the end to make it more viscous.
– Oil of Orange is used to clean eugenol cements from instruments.
– Cement sets in moisture and heat.
Setting time recommended by ANSI/ ADA according to specification no. 30
Type I : Temporary cement At 370C
Class 1 : Powder-liquid 4-10 minutes
Class 2a : Paste-Paste (Eugenol) 4-10 Minutes
Class 2b : Paste-Paste (Non-Eugenol) 4-10 Minutes
Class 3 : Paste-Paste (Non-setting) –
Type II : Permanent cement
Class 1 : Powder-liquid 2-10 minutes
Type III : Filling Material
Class 1 : Powder-liquid 2-10 minutes
Class 2 : Paste-Paste 2-10 minutes
Type IV: Cavity liners
Class 1 : Powder-liquid 4-10 minutes
Class 2 : Paste-Paste 4-10 minutes
Film thickness recommended according to specification no. 30 is:
For temporary cementation : 40 micron
For permanent cementation : 25 micron
Strength:
Type 1 : 35 Mpa Maximum
Type 2 : 35 Mpa Minimum
Type 3 : 25 Minimum
Type 4 : 5 Minimum
MODIFIED ZINC OXIDE EUGENOL CEMENT
EBA- Alumina modified luting cements:
In order to further improve on the basic Zinc Oxide Eugenol system, many researchers have investigated mixtures of Zinc and other Oxides with various chelating agents. The only system that has received extensive commercial explaitation for luting and lining is that containing ortho-ethoxy benzoic acid.
Non Eugenol cements have also been developed is which fatty acids or low odor phenolic derivatives are used to overcome the smell and taste of Eugenol.
EBA- Alumina modified – used for cementation of inlays and crown and bridge, for temporary filings and as a base or a liner material.
COMPOSITION
Powder: Liquid:
ZnO – 70% EBA – 62.5%
Alumina – 30% Eugenol – 37.5%
PROPERTIES
Working time at room temperature is long because of dependence upon moisture. The setting time ranges from 7-13 minutes.
– Film thickness appears to be in the range of 40-70 mm.
– At cementing consistency, the compressive strength
– At cementing consistency, the compressive strength of these materials is in the range of 55-70 Mpa (8000 to 10000 Psi)
Higher values similar to ZnPo4 cement can be achieved by increasing the powder-liquid ratio.
– Tensile strength is considerable low – 3-6Mpa (500-900 Psi)
– EBA luting cements show viscoelasticity with very low strength and large plastic deformation at slow (0.1 mm/min) rate of deformation and at mouth temperature (370C)
– Resistance to solubility in organic acids appears to be greater than that of the ZnPo4 cement.
– Clinical surveys by Silver and Meyers (1978) of the performance of an EBA-alumina luting cements over 3 years showed only very slightly wore results than for Zinc Phosphate cement and polycarboxylate cement.
– Biologic properties are similar to that of ZnO Eugenol cement
Advantages
a) Easy mixing
b) Long working time
c) Good flow characteristics
d) Low irritation to pulp
e) Strength and film thickness comparable to ZnPO4
Disadvantages
a) Critical proportioning
b) Hydrolytic breakdown in oral cavity
c) Liable to plastic deformation
d) Poor retention than ZnPO4 cement.
These materials are best suited to luting of restoration with good fit and retention where there is no under stress and as a cavity linear/ bases.
Some Products:
Permanent luting:
Fynal – L.D. Caulk
Optow Alumina EBA – Teledyne Getz
Temporary luting:
Type Manufacture
Flow Temperature : ZnO Eugenol Premier ESPE
Freeginal : Eugenol free GC int.
Nogenol : Eugenol free Coe lab.
Temp-Bond : ZnO Eugenol Kerr Manufacturing
limit
Temporary cement : ZnO Eugenol Buffalo Dental
Manual
ZOE 2000 : ZnO Eugenol L.D. Caulk
Zone : Eugenol free Cadco Dental
Products
POLYMER BASED CEMENTS
The majority of the material in this group are poly (methacrylates) of two types.
1. Materials based on Methyl Methacrylate
2. Materials based on Aromatic dimethacrylates of Bis-GMA type
3. The closely related cyfanoacrylates monomers, notably ethyl and isobutyl, have found some limitations for the attachment of facing and for pins cementation.
Synthetic resins luting cements based on methyl methacrylate have been available since 1952 for the use of cementation of inlay, crowns and appliances.
ACRYLIC RESIN CEMENTS
Application:
Acrylic resin luting cements are used for the cementation of restoration, facing and temporary crown.
Composition
Powder is a finely divided methyl methacrylate polymer or copolymer containing benzoyl peroxide as the initiator. Mineral filler and pigments may also be present. The liquid is a methyl methacrylate monomer containing an amine accelerator.
The monomer dissolves and softens the polymer particles and concurrently polymerizes through the action of free radicals from the peroxide amine interaction. The set mass consist of the new polymer matrix uniting the undissolved but smaller original polymer granules.
MANIPULATION
The liquid added to the powder with minimal spatulation to avoid an incorporation. The mix must be used immediately because WT is short. Excess material must be removed at the final set hard stage and not when the material is rubbery, otherwise marginal deficiency will be created.
PROPERTIES
They are comparable to those of the cold-curing acrylic resin filling material. They are stronger and loess soluble than other luting cements.
Compressive strength : 180 Mpa
Tensile strength : 30 Mpa
Film thickness : 10-25 m
Bond strength to enamel : 7.4 Mpa
Enamel bonding is attained by Acid Etch Technique.
Dentin bonding is achieved by DBA, Organo Phosphates, HEMA and META
– Shows high polymerization shrinkage
– Irritating to the pulp
– ST = 4-10 minutes
Main Disadvantage
1. High pulpal irritant
2. Difficulty in removal of excess cement from the margins
3. Short working time
Advantages
1. High strength and toughness
2. Low solubility
Modified Acrylic Resin cement
Adhesive acrylic materials have been formulated by adding an adhesion promoter, 4-methyloxy ethyl trimelletic anhydride (4-META), to the methyl methacrylate monomer as well as an addition polymerization inititor, tributyl boron, that is also believed to aid chemical bording to dentin.
Cement for Metal Crowns, Bridges especially of Base Metal
(Super bond, Parkell)
And for Bonding Amalgam to Dentin and Composites
(Amalgam Bond, Parkell)
Studies have shown high strength for luting cement to oxidized, etched or silica coated casting alloy.
Sheer bond strength to amalgam is significantly less than bond strength to dentin i.e. 20 MPa. Since these materials have only low (<10%) filler content the physical properties are typical of acrylic resins, which is moderate strength with high deformation under load. Although the materials have been widely used for cementation of crown and bridge, there is little clinical data on longevity and the cement are said to be technically sensitive.
DIMETHACRYLATE CEMENTS
Materials of more recent development are usually based on the BIS-GMA system. They are a combination of an aromatic dimethacrylate with other monomers containing various amounts of ceramic filler. They are basically similar to composite restorative materials.They are basically similar to composites. These materials have been supplied as two viscous liquids, two pastes or as powder/ liquid materials.
Application: Used for cementation of crown, bridges, inlays and veneers.
Composition and setting:
May be
a) Chemical (Auto) cured
b) Visible light cured
c) Chemical and light cured
The later dual-cure systems are increasingly favored to achieve the rapid solidification associated with light cure while allowing for full polymerization of the cement in areas of the restoration that cannot be reached efficiently by light irradiation.
In the powder-liquid material, the powder is generally a finely grained divided Borosilicate or Silica glass together with five polymer powder and an organic peroxide initiator. The liquid is a misture of BIS GMA and/ or other dimethacrylate monomers containing amine promotors for polymerization. Some materials contain monomers with potentially adhesive group, such as phosphate or carboxyl, similar to dentin bonding materials. The two paste materials are of similar overall composition but with the monomers and fillers combined into two pastes. In light cured and dual cured materials, light sensitivity such as diketones (e.g. Camphorquinone) and amine promotors are present. On mixing the components, polymerization of the monomers occur, leading to a highly cross linked composite resin structure.