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INTRODUCTION

The term cement has been applied to powder / liquid materials which are

mixed to a paste consistency. The word luting is defined as the use of a moldable

substance to seal joints and cement two substances together. Various cements are used

for luting for example zinc phosphate, zinc silicophosphate, zinc polycarboxylate,

glass ionomer, and zinc oxide eugenol and resin cements. The clinical success of

fixed prosthesis is heavily dependant on the cementation process.

For a restoration to accomplish its purpose, it must stay in place on the tooth.

No cements that are compatible with living tooth structure and the biologic

environment of the oral cavity possess adequate adhesive properties to hold a

restoration in place solely through adhesion.

Although the establishment of optimal resistance and retention forms in the

tooth preparation are of primary importance, a dental cement must be used as a barrier

against microbial leakage, sealing the interface between the tooth and restoration and

holding them together through some form of surface attachment.

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PRINCIPLES OF CEMENTATION

Dental treatments necessitate attachment of indirect restorations and

appliances to the teeth by means of a cement. These include metal, resin, metal-resin,

metal ceramic, and ceramic restorations, provisional or interim restorations; laminate

veneers for anterior teeth; orthodontic appliances, and pins and posts used for

retention of restorations. The word luting is often used to describe the use of a

moldable substance to seal a space or to cement two components together;.

The properties of various cements differ from each other. Hence, the choice

cement is mandated to a large degree by the functional and biologic demands of the

particular clinical situation. If optimal performance is to be attained, the physical and

biologic properties, and the handling characteristic, such as the working and setting

times and ease of removing excess materials, must be considered in selecting a

cement for a specific task.

CHARACTERISTICS OF ABUTMENT – PROSTHESIS INTERFACE.

When two relatively flat surfaces are brought into contact, analogous to a

fixed prosthesis being placed on a prepared tooth, a space exists between the

substrates on a microscopic scale.

Typical prepared surfaces on a microscopic scale are rough, that is, there are

peaks and valleys. When two surfaces are placed against each other, there are only

point contacts along the peaks. The areas that are not in contact then become open

space. The space created is substantial in terms of oral fluid flow and bacterial

invasion. One of the main purposes of a cement is to fill this space completely. One

can seal the space by placing a soft material, such as an elastomer, between the two

surfaces that can conform under pressure to the “roughness”.

The current approach is to use the technology of adhesives. Adhesive bonding

involves the placement of a third material, often called a cement that flows within the

rough surface and sets to a solid form within a few minutes. The solid matter not only

seals the space but also retains the prosthesis. Materials used for this application are

classified as Type I cements. If the third material is not fluid enough or is

incompatible with the surfaces, voids can develop around deep, narrow valleys and

undermine the effectiveness of the cement.

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BONDING MECHANISM

Non adhesive luting

Originally the luting agent served primarily to fill the gap and prevent entrance

of fluids. Zinc phosphate for example exhibits no adhesion on the molecular level. It

holds the restoration in place by engaging small irregularities on the surface of both

tooth and the restoration. The nearly parallel opposing walls of a correctly prepared

tooth make it impossible to remove the restoration without shearing or crushing the

minute projections of cement extending into recesses in the surfaces.

Micromechanical bonding

Resin cements have tensile strengths in the range of 30 -40 MPa, which is

approximately five times that of zinc phosphate cement. When used on pitted

surfaces, they can provide effective micromechanical bonding. The tensile strengths

of such bonds can sometimes exceed the cohesive strength of enamel. This allows the

use of less extensive tooth preparation for restorations such as ceramic veneers and

resin bonded fixed partial dentures.

The deep irregularities necessary for micromechanical bonding can be

produced on enamel surfaces by etching with phosphoric acid solution or gel, on

ceramics by etching with hydrofluoric acid and on metals by electrolytic etching,

chemical etching, sandblasting or by incorporating salt crystals into preliminary resin

pattern.

Molecular Adhesion

Molecular Adhesion involves physical forces (bipolar, Vander Waals) and

chemical bonds (ionic and covalent) between molecules of two different substances.

Newer cements, such as polycarboxylate and glass ionomers, possess some adhesive

capabilities, although this is limited by their relatively low cohesive strength. They

still depend primarily on nearly parallel walls in the preparation to retain restorations.

Limited success has been achieved in attempts to develop resin cements and

coupling agents that will exhibit strong, durable molecular adhesion to tooth structure,

base metals and ceramics. Noble metal alloys are not suited for direct molecular

bonding. However, a thin layer of silane can be bonded to a gold alloy with special

equipment (Silicoater, Kulzer, Irvine or Rocatec, ESPE-Premier) to serve as a

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coupling agent by bonding chemically to resin cements. Equally effective is a layer of

tin electroplated onto gold alloy.

By applying a silane coupler to roughened porcelain, shear bond strengths in

excess of the cohesive strength of the porcelain have been achieved. However such

bonds tend to become weaker after thermo cycling in water. At this time, molecular

adhesion should be looked upon only as a way to enhance mechanical and

micromechanical retention and reduce micro leakage, rather than as an independent

bonding mechanism.

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DISLODGEMENT OF PROSTHESIS

Fixed prostheses can debond because of biologic or physical reasons or a

combination of the two. Recurrent caries results from a biologic origin. Disintegration

of the cements can result from fracture or erosion of the cement. For brittle

prostheses, such as glass-ceramic crowns, fracture of the prosthesis also occurs

because of physical factors, including intraoral forces, flaws within the crown

surfaces, and voids within the cement layer.

In the oral environment cementation agents are immersed in an aqueous

solution. In this environment the cement layer near the margin can dissolve and erode

leaving a space. This space can be susceptible to plaque accumulation and recurrent

caries; therefore, the margin should be protected with a coating (if possible) to allow

continuous setting of the cement.

There are two basic modes of failure associated with cements: cohesive

fracture of the cement and separation along the interfaces. Because the cement layer is

the weakest link of the entire assembly, one should favor higher strength cements to

enhance retention and prevent prosthesis dislodgement by providing a firm support

base against applied forces.

Several factors have an influence on the retention of these fixed prostheses.

First, the film thickness beneath the prosthesis should be thin. It is believed

that a thinner film has fewer internal flaws compared with a thicker one.

Second, the cement should have high strength values. Generally, greater forces

are required to dislodge appliances cemented with cements that have higher tensile

strength than with cements of low tensile strength. It is also well established that the

stresses developed during mastication are exceedingly complex. Undoubtedly,

properties other than tensile strength may be involved. These include compressive and

shear strength of the cement, fracture toughness, and film thickness.

Third, the dimensional changes occurring in the cement during setting should

be minimized. Sources include gain or loss of water and differences in the coefficients

of thermal expansion among the tooth, the prosthesis, and the cement.

It is, therefore, important to isolate the cement immediately after removal of

the excess. Fourth, a cement with the potential of chemically bonding to the tooth and

prosthetic surfaces or bond- enhancing intermediate layers may be used to reduce the

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potential of separation at the interface and maximize the effect of the inherent strength

on the retention.

When a mechanical undercut is the mechanism of retention, the failure often

occurs along the interfaces. If chemical bonding is involved, the failure often occurs

cohesively through the cement itself. The prosthesis becomes loose only when the

cement fractures or dissolves.

IDEAL PROPERTIES OF LUTING CEMENT

Described by McLean and Wilson

1. Low viscosity and film thickness

2. Long working time with rapid set at mouth temperature

3. Good resistance to aqueous or acid attack

4. High compressive and tensile strength

5. Resistance to plastic deformation

6. Adhesion to tooth structure and restoration

7. Cariostatic

8. Biologically compatible with pulp

9. Translucency

10. Radio opacity

CHOICE OF LUTING AGENT

An ideal luting agent is one which has a long working time, adheres well to

both tooth structure and cast alloys, provides a good seal, is non toxic to pulp, has

adequate strength properties, is compressible into thin layers, has a low viscosity and

solubility and exhibits good working and setting characteristics. In addition any

excess can be easily removed. Unfortunately, no such product exists.

Zinc phosphate cement

Is probably still the luting agent of choice. Cavity varnish can be used to

protect against pulp irritation from phosphoric acid and appears to have little effect on

the amount of retention of the cemented restoration.

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Zinc polycarboxylate cement

This agent is recommended on retentive preparation when minimal pulp

irritation is important.

Glass ionomer cement

This has become a popular cement for luting cast restoration. It has good

working properties and because of its fluoride content, it may prevent recurrent

caries.

Resin modified glass ionomer cement

Currently among the most popular luting agents, Resin modified glass

ionomer cements have low solubility, adhesion and low micro leakage. The

popularity it mainly due to perceived benefit of reduced post cementation sensitivity.

Adhesive resin

Long-term evaluations of these materials are not yet available, so they cannot

be recommended for routine use. Laboratory testing yields high retention strength

values, but there is concern that stresses caused by polymerization shrinkage,

magnified in thin films, leads to marginal leakage. Adhesive resin may be indicated

when a casting has become displaced through lack of retention.

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ZINC PHOSPHATE CEMENT

Zinc phosphate cement is the oldest of the cementation agents and thus has the

longest track record. It serves as a standard by which newer systems can be compared.

It is a traditional crown and bridge cement used for the alloy restorations. It is

supplied as a powder and liquid, both of which are carefully compounded to react

with one another during mixing to develop a mass of cement possessing desirable

physical properties.

Composition

Powder

The principal ingredient of the zinc phosphate cement is zinc oxide.

Magnesium oxide, silicon dioxide, bismuth trioxide, and other minor ingredients are

used in some products to alter the working characteristics and final properties of the

mixed cement.

Zinc oxide (ZnO) 90.2

Magnesium oxide (MgO) 8.2

Silicon dioxide (SiO2) 1.4

Bismuth trioxide (Bi2O3) 0.1

Miscellaneous (BaO, Ba2So4, CaO) 0.1

The magnesium oxide, usually in quantities of about 10%, are added to the

zinc oxide to reduce the temperature of the calcinations process.

The silicon dioxide is inactive filler in the powder and during manufacture

aids in the calcinations process.

Although bismuth is believed to impart smoothness to the freshly mixed

cement mass, in large amounts it may also lengthen the setting time.

Tannin fluoride may be added to provide a source of fluoride ions in some

products.

The ingredients of the powder are heated together at temperatures ranging

from 1000º to 1300º C for 4 to 8 hours or longer, depending on the temperature.

Calcinations results in a fused or a sintered mass. The mass is then ground and

pulverized to a fine powder, which is sieved to recover selected particle sizes. The

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degree of calcination, fineness of the particle size, and composition determine the

reactivity of the powder with the liquid.

The powder particle size influences the setting rate. Generally the smaller the

particle size, the faster the set of the cement.

Liquid

Adding aluminum and sometimes zinc, or their compounds, to a solution of

orthophosphoric acid, produces zinc phosphate cement liquids. Although the original

acid solution contains about 85% phosphoric acid and is a syrupy fluid, the resulting

cement liquid usually contains about one third water

H3PO4 (free acid) 38.2

H3PO4 (combined with aluminum and zinc) 16.2

Aluminum (Al) 2.5

Zinc (Zn) 7.1

Water (H2O) 36.0

The partial neutralization of phosphoric acid by aluminum and zinc tempers

the reactivity of the liquid and is described as buffering. The reduced rate of the

reaction helps establish a smooth, non-granular, workable cement mass during the

mixing procedure. Both partial neutralizing or buffering and dilution adjust the zinc

phosphate cement liquid so it reacts with its powder to produce a cement mass with

proper setting time and mechanical qualities.

The composition of the liquid should be preserved to ensure a consistent

reaction, as water is critical to the reaction. Changes in composition and reaction rate

may occur either because of self-degradation or by water evaporation from the liquid.

Self-degradation of the liquid is best detected by clouding of the liquid over time.

Setting Reaction

When the powder is mixed with the liquid the phosphoric acid attacks the

surface of the particles and releases zinc ions into the liquid. The aluminum, which

already forms a complex with the phosphoric acid, reacts with zinc and yields a zinc

aluminophosphate gel on the surface of the remaining portion of the particles. Thus

the set cement is a cored structure consisting primarily of unreacted zinc oxide

particles embedded in a cohesive amorphous matrix of zinc aluminophosphate. The

set zinc phosphate cement is amorphous and is extremely porous.

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The surface of alkaline powder is dissolved by the acid liquid, resulting in an

exothermic reaction.

Manipulation

The manner in which the reaction between zinc phosphate cement powder and

liquid is permitted to occur determines to a large extent the working characteristics

and properties of the cement mass. Incorporate the proper amount of powder into the

liquid slowly on a cool slab (about 21 º C) to attain the desired consistency of the

cement.

Powder Liquid Ratio

Reducing the powder liquid ratio can increase working and setting times. This

procedure is however not acceptable means of extending setting time because it

impairs the physical properties and results in a lower initial pH of the cement. The

powder liquid ratio is 1.4gm/0.5ml.

Rate Of Powder Incorporation

Introduction of small quantity of powder into the liquid for the first few

increments increases working and setting times by reducing the amount of heat

generated and permits more powder to be incorporated into the mix.

Care Of The Liquid

When zinc phosphate cement is exposed to a humid atmosphere it will absorb

water, whereas exposure to dry air tends to result in a loss of water. The addition of

water causes more rapid reaction with the powder, resulting in a shorter setting time.

A loss of water from the liquid results in a lengthened setting time. Therefore keep the

bottle tightly closed when not dispensing the material. Polyethylene squeeze bottles

do not require removal of a dropper and therefore eliminate the tendency for gain or

loss of water from the liquid.

Mixing Slab

A properly cooled thick glass slab will dissipate the heat of the reaction. The

mixing slab temperature should be low enough to effectively cool the cement mass

but must not be below the dew point unless the frozen slab technique is used. A

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temperature of 18º to 24º C is indicated when room humidity permits. The moisture

condensation on a slab cooled below dew point contaminates the mix, diluting the

liquid and shortening the setting time. The ability of the mixing slab to be cooled and

yet be free of moisture greatly influences proper control of the reaction rate of zinc

phosphate cement.

Mixing Procedure

By incorporating small portions of the powder into the liquid, minimal heat is

liberated and easily dissipated. The heat of the reaction is most effectively dissipated

when the cement is mixed over a large area of the cooled slab. Use a relatively long

narrow bladed stainless steel spatula to spread the cement across this large area to

control the temperature of the mass and its setting time.

During neutralization of the liquid by the powder, the temperature of the

mixing site is inversely proportional to the time consumed in mixing. Thus a large

volume of the powder is carried to the liquid all at once rather than spatulated over a

large area of the slab for a sufficient time, the temperature at the site of the reaction

becomes higher.

This temperature rise speeds the reaction and hinders control over the

consistency.

During the middle of the mixing period, larger amounts of powder may be

incorporated to further saturate the liquid with the newly forming complex zinc

phosphates. The quantity of the unreacted acid is less at this time because of the prior

neutralization gained from initially adding small increments of powder. The amount

of heat liberated will likewise be less, and it can be dissipated adequately by the

cooled slab.

Finally smaller increments of powder are again incorporated, so the desired

ultimate consistency of the cement is not exceeded.

Thus the mixing procedure begins and ends with small increments, first to

achieve slow neutralization of the liquid with the attendant control of the reaction and

last to gain a critical consistency.

Depending on the product 60 to 90 seconds of mixing appears adequate to

accomplish a proper zinc phosphate cementing mass.

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Contact With Moisture

The area near the cement must be kept dry while the powder and liquid is

mixed, during insertion into the mouth and during hardening. If the cement is allowed

to harden in the presence of saliva some of the phosphoric acid is leaked out and the

surface of the cement will be dull and easily dissolved by oral fluids.

After the cement sets it should not be allowed to dry. Drying of the cement

results in shrinkage and crazing of the surface. A coating of varnish should minimize

dehydration as well as prevent premature contact with oral fluids.

Working Time And Setting Time

Working time is the time measured from the start of the mixing during which

the viscosity (consistency) of the mix is low enough to flow readily under pressure to

form a thin film. Adequate working time is expressed between 2.5 to 8 minutes at a

body temperature of 37˚ C. The first 60 to 90 seconds are consumed by mixing the

powder and liquid.

Setting time is the time elapsed from the start of the mixing until the point of

the needle no longer penetrates the cement as the needle is lowered onto the surface.

Practically, it is the time at which the zinc phosphate cement flash (excess) should be

removed from the margins of the restoration. The setting time can be measured with a

4.5 N (1 pound) Gillmore needle at a temperature of 37º C and relative humidity of

100%. A reasonable setting time for zinc phosphate cement is between 5 to 9 minutes,

as specified in ADA specification no. 8.

Frozen Slab Method

The frozen slab method is a way to substantially increase the working time (4-

11 minutes) of the mix on the slab and shorten the setting time (20 to 40% less) of the

mix after placement into the mouth.

In this method, a glass slab is cooled in a refrigerator at 6º C or in a freezer at

–10ºC .

No attempt is made to prevent moisture from condensing on the slab when it is

brought to room temperature. A mix of cement is made on the cold slab by adding the

powder until the correct consistency is reached. The amount of powder incorporated

with the frozen slab method is 50% to 75% more than with the normal procedures.

The compressive strength and tensile strength prepared by the frozen slab method are

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not significantly different from those prepared for normal mixes, however, because

incorporation of condensed moisture into the mix in the frozen slab method

counteracts the higher powder liquid ratio. This method has been advocated for

cementation of bridges with multiple pins.

Mechanical Interlocking

Whenever an inlay is seated in a prepared cavity the surfaces of both the inlay

and the tooth have slight roughness and serrations into which the cement is forced.

Film thickness is a factor for retention. Thinner the cement better is the cementing

action. Zinc phosphate cements are irritating to the pulp. Although the pH of the

cement approaches neutral at 24 hours. Thinner mixes are more acidic and remain so

for a longer period of time than the standard mixes.

Berk, H. Stanely said that thin mix Zinc phosphate cements have more pulp

response than thick mix because Zinc phosphate cements is pushed into dentinal

tubules and it destroys the odontoblast right in place. The application of a cavity

varnish to a cut tooth structure can act as a barrier to the penetration of the acid.

A recent animal study involving cementation of crowns reported pulp response

to none when a cavity varnish was applied to the teeth prior to cementation of crowns.

With respect to the effect of retention, Fetton showed a coat of varnish to have no

influence in crown retention.

Molta JP said that cavity varnish has been shown to reduce the retention of

cemented pins and decrease tensile bond between two opposed dentinal surface when

Zinc phosphate cement is used for luting.

Characteristics Properties

Physical and biologic properties

Two physical properties of the cement that are relevant to the retention of the

fixed prostheses are the mechanical properties and the solubility’s. The prosthesis can

get dislodged if the underlying cement is stressed beyond its strength. High solubility

can induce loss of the cement needed for the retention and may create plaque retention

sites.

Zinc phosphate cement when properly manipulated exhibits a compressive

strength of 104MPa and a diametral tensile strength of 5.5 MPa.

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Zinc phosphate cement has a modulus of elasticity of approximately 13 GPa.

Thus it is quite stiff and should be resistant to elastic deformation even when it is

employed for cementation of restorations that are subjected to high masticatory stress.

A reduction in the powder liquid ratio of the mix produces a markedly weaker

cement.

A loss or gain in the water content of the liquid reduces the compressive and

tensile strengths of the cement.

Retention

Whenever a casting is seated in the prepared tooth, the surfaces of both the

casting and the tooth structure have slight roughness and irregularities into which the

plastic cement is forced. Such extensions many times act as undercuts in providing

retention of the inlay.

The thickness of the film between the casting and the tooth is also a factor in

the retention. The thinner the film, the better is the cementing action.

Solubility and disintegration

The premature contact of the incompletely set cement with water results in

dissolution and leaching of that surface. Prolonged contact even of well-hardened

cement, with moisture demonstrates that some erosion and extraction of soluble

material does occur from the cement.

Even the filling cement mixes show considerable loss of material in the mouth

over a period of time, indicating that zinc phosphate can be regarded only as a

temporary filling material. Wear abrasion and attack of food decomposition products

accelerate the disintegration of zinc phosphate cements. Greater resistance to

disintegration is achieved by increasing the powder liquid ratio. A thicker mix of

cement exhibits less solubility than a thinner mix.

Dimensional stability

Zinc phosphate cement exhibits shrinkage on hardening. The normal

dimensional change 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 slight shrinkage on the order of 0.04% to 0.06% in

7 days.

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Consistency and film thickness

Two arbitrary consistencies of the cement are used based on their use.

Inlay seating or luting and cement base or filling. A third consistency which

lies midway between inlay seating and the cement base, is band seating consistency

used for retention of orthodontic bands.

The inlay seating consistency is used to retain alloy restorations. Although the

unhardened zinc phosphate cement is somewhat tenacious, the retaining action in its

hardened state is one of mechanical interlocking between the surface irregularities of

the tooth and the restoration.

The film thickness of the zinc phosphate cement greatly determines the

adaptation of the casting to the tooth and also determines the strength of the retention

bond.

The maximum film thickness is 25μ m. the heavier the consistency; the greater

the film thickness and the less complete the seating of the restoration.

The ultimate film thickness that a well-mixed, non-granular cement attains

depends first on the particle size of the powder and second on the concentration of the

liquid.

The film thickness also varies with the amount of force and the manner in

which this force is applied to a casting during cementation.

An increased amount of powder incorporated into the liquid will increase the

consistency of the cement mass.

The operator must frequently test each mass as the end of mixing time

approaches. The final consistency will be fluid, yet will string up from the slab on the

spatula about 2-3cm as the spatula is lifted away from the mass.

A heavy putty like consistency of zinc phosphate cement is used as a thermal

and chemical insulating barrier over thin dentin and a high strength base.

Viscosity

The consistency of cements can be quantified by measuring viscosity. A small

but significant increase in viscosity is seen at higher temperatures. A 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.

Delays in cementation can result in considerably thick film and insufficient seating of

the restoration.

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Acidity

During the formation of zinc phosphate cement, the union of zinc oxide

powder with phosphoric acid liquid is accompanied by a change in pH. In the early

stages the pH increases rapidly, with a standard mix reaching the pH of 4.2 within 3

minutes after mixing has started. At the end of one hour this value increases to about

6 and is nearly neutral at 48 hours.

Investigations have shown that the initial acidity of zinc phosphate cement at

the time of placement into the tooth may excite pulpal response, especially where only

a thin layer of dentin exists, between cement and pulp.

Thermal and electrical conductivity

One of the primary uses of zinc phosphate cement is an insulating base under

metallic restorations.

Applications

Zinc phosphate cement is used most commonly for luting permanent metal

restorations and as abase.

Other applications include cementation of orthodontic bands and the use of

cement as a provisional restoration.

Advantages

1. Adequate strength to maintain the restoration

2. Relatively good manufacturer properties

3. Mixed easily and that they set sharply to a relatively strong mass from a fluid

consistency.

Disadvantages

1. Irritating effect on the pulp

2. Lack of anticariogenic properties

3. Lack of adhesion to the tooth

4. Vulnerability to acid attack

5. Brittleness

6. Solubility in acid fluids.

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Reaction Of Pulp To Cement

The phosphoric acid in Zinc phosphate cement can be the cause of the pulpal

reaction.

The closer it approaches the pulp, the greater is the intensity of the response.

Also the ratio of powder to liquid is important consideration. A thick mix of Zinc

phosphate cement used as a base will generate a moderate localized response, whereas

a thin mix used to cement on a crown that is placed under great pressure by patients

biting on a tongue blade can cause a very severe reaction.

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ZINC SILICOPHOSPHATE CEMENT

They are also called as Zinc silicate, Silicate zinc cement.

Zinc silicophosphate cement is a hybrid resulting from the combination of zinc

phosphate cement and silicate powders.

Types Of Zinc Silicophosphate Cements

According to ADA no –28 (1969) there are three types

Type I – as a cementing media

Type II – temporary posterior filling material

Type III – dual purpose cementing media and temporary posterior filling material.

Properties

Zinc silicophosphate cements (ZSP) consist of mixture of silicate glass, a

small percentage of zinc oxide powder and phosphoric acid.

They are used as luting agents for restorations and orthodontic bands,

intermediate restorations and as die material.

Its strength is somewhat superior to that of zinc phosphate cement, and the

major difference is that Zinc silicophosphate cement appears somewhat translucent

and releases fluoride by virtue of silicate glass.

Clinical observation has shown that silicophosphate is less soluble in the

mouth than zinc phosphate cement. The fluoride content should give some

antocariogenic action. Therefore it is recommended for cementation of restoration in

patients with high caries rate.

The flow property of the mix is not as good as zinc phosphate cement, leading

to higher film thickness. The cement does not bound to tooth structure; hence

retention is by mechanical interlocking.

Esthetically it is superior to the more opaque zinc phosphate cement for

cementation of ceramic restorations.

The use of Zinc silicophosphate cement is declining, as practitioners have

choice of other more esthetically pleasing materials such as resin and glass ionomer

cements.

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Advantages

1. Zinc silicophosphate cements have a better strength and toughness than zinc

phosphate cements

2. Shows considerable fluoride release hence anticariogenic

3. Translucent

4. Under clinical conditions lower solubility and better bonding

5. Best suited to cement of ortho bars and restoration on non-vital teeth.

Disadvantages

1. Less satisfactory mixing

2. Higher film thickness

3. Greater pulpal irritation

Trade Names

Flourathin and Lucent ( type I)

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ZINC POLYCARBOXYLATE CEMENT

In the quest for an adhesive cement that can bond strongly to the tooth

structure, Zinc polycarboxylate cement was the first cement system that developed an

adhesive bond to tooth structure in 1960.

Composition

Zinc polycarboxylate cement or zinc polyacrylate cements are supplied as a

powder and liquid or as a powder that is mixed with water.

The cement powder is essentially zinc oxide and magnesium oxide that have

been sintered and ground to reduce the reactivity of zinc oxide. Stannic acid may be

substituted for magnesium oxide. Other oxides such as bismuth and aluminum can be

added. The powder may also contain small quantities of stannous fluoride, which

modify setting time and enhance manipulative properties. It is an important additive

because it increases strength. However, the fluoride released from this cement is only

a fraction. The cement powder that is mixed with water contains 15 % to 18%

polyacrylic acid coated on the oxide particles.

The liquid is a water solution of polyacrylic acid. Most commercial liquids

are supplied as 32% to 42% solution of polyacrylic acid having molecular weight of

25,000 to 50,000. The manufactures control the viscosity of the cement liquid by

varying the molecular weight of the polymer or by adjusting the pH by adding sodium

hydroxide. Itaconic and tartaric may be present to stabilize the liquid, which can gel

on extended storage.

Setting Reaction

The setting reaction of this cement involves particle surface dissolution by

acid that releases zinc, magnesium, and tin ions, which bind to the polymer chain via

the carboxyl groups. These ions react with carboxyl groups of adjacent polyacid

chains so that a cross-linked salt is formed as the cement sets. The hardened cement

consists of an amorphous gel matrix in which unreacted particles are dispersed. The

microstructure resembles that of zinc phosphate cement in appearance.

Water settable versions of this cement are available. The polyacid is a freeze-

dried powder that is then mixed with the cement powder. The liquid is water or a

weak solution of NaH2PO4. However the setting reaction is the same whether the

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polyacid is freeze dried and subsequently mixed with water or if the conventional

aqueous solution of polyacid is used as the liquid.

Manipulation

Mixing

The cement liquids are quite viscous. The viscosity is a function of the

molecular weight and the concentration of the polyacrylic acid thereby varies.

Generally the powder liquid ratio is 1.5 parts of powder to 1 part of liquid by weight.

The consistency of the mixes is creamy compared with that of zinc phosphate

cements. The mixes cement is pseudoplastic that is the viscosity decreases as the

shear rate increases, or in other terms, the flow increases as spatulation increases or as

force is placed on the material. The correct consistency is found in a mix that is

viscous but that will flow back under its own weight when drawn up with a spatula.

The cement liquid should be mixed on a surface that does not absorb liquid. A

glass slab affords the advantage over paper pads supplied by the manufacturers

because once it is cooled it maintains the temperature longer. The cool slab and

powder provides for longer working time, but under no circumstances should the

liquid be cooled in a refrigerator.

Mix polyacrylate cements within 30 to 60 seconds, with half to all of the

powder incorporated at once to provide the maximum length of working time 2.5 to 6

minutes. Working time can be extended to 10-15 minutes by using a cool slab chilled

to 4˚C.

The liquid should not be dispensed before the time when the mix is to be

made. It loses water to the atmosphere rapidly and this results in marked increase in

viscosity.

Use the mixed cement only as long as it appears glossy on the surface. Once

the surface becomes dull, the cement develops stringiness and the film thickness

becomes too great to seat a casting completely.

If good bonding to tooth structure is to be achieved, the cement must be placed

on the tooth surface before it loses its glossy appearance. The glossy appearance

indicates a sufficient number of free carboxylic acid groups on the surface of the

mixture that are vital for bonding to tooth structure.

22

Surface penetration and retention

Despite the adhesion of the cement to tooth structure, polycarboxylate cements

are not superior to zinc phosphate cement in the retention of cast noble metal

restorations.

A comparable force is required to remove gold inlays cemented either with

zinc phosphate cement or with polycarboxylate cement. Examination of fractured

surfaces shows that failure usually occurs at the cement –tooth interface with zinc

phosphate cement.

In the case of polycarboxylate cements, the failure occurs usually at the

cement metal interface.

The cement does not bond to the metal in the chemically contaminated

condition. Thus it is essential that this contaminated surface on the casting be

removed to improve wettability and the mechanical bond at the cement metal

interface. The surface can be carefully abraded with a small stone, or it can be

sandblasted with high-pressure air and alumina abrasives.

Because this type of cement affords an opportunity to obtain adhesion to tooth

structure, a clean cavity surface is necessary to ensure intimate contact and interaction

between cement and the tooth. A recommended procedure is to apply a 10%

polyacrylic acid solution for 10 to 15 seconds followed by rinsing with water.

Removal of excess cement

During setting the polycarboxylate cement passes through a rubbery stage that

makes the removal of the excess cement quite difficult. The excess cement that has

extruded beyond the margins of the casting should not be removed while the cement

is in this stage, because some of the cement may be pulled out from beneath the

margins leaving a void. The excess should be removed when the cement becomes

hard. The outer surface of the prosthesis should be coated with a separating medium

like petroleum jelly, to prevent excess from adhering.

Another approach is to start removing excess cement as soon as seating is

completed.

23

Properties

Viscosity

The initial viscosity of zinc polycarboxylate cement is higher than zinc

phosphate cements and a delay of 2 minutes in cementation reverses the situation.

Film thickness

When polycarboxylate cements are mixed they appear to be much viscous

than zinc phosphate cement. Since zinc polycarboxylate cement is pseudoplastic

cement it undergoes thinning at an increase shear rate. Clinically, this means that the

action of spatulation and seating with a vibratory action will reduce the viscosity and

yield a film thickness of 25-μ m or less.

Working time and setting time

The working time for polycarboxylate cement is much shorter than phosphate

cement that is 2.5 minutes. Lowering the temperature of the reaction can increase the

working time that may be necessary for fixed bridges. Unfortunately, the temperature

of the cool slab can cause the polyacrylic acid to thicken. The increased viscosity

makes the mixing procedure more difficult. It has been suggested that only the

powder should be refrigerated before mixing.

The setting time ranges from 6 to 9 minutes.

Mechanical properties

1. The compressive strength of polycarboxylate cement is 55 Mpa.

2. The diametrical tensile strength is slightly higher than that of zinc phosphate

cement.

3. Its modulus of elasticity is less than half.

4. Brown stated that an increse in the compressive and tensile strength of

polycarboxylate cement can be obtained with the addtion of stainless steel

powder or fibers .

5. Zinc polycarboxylate cement is not as brittle as zinc phosphate cement.

6. Thus it is more difficult to remove the excess after the cement has set.

24

Solubility

The solubility of the cement in water is low, but when it is exposed to organic

acids with a pH of 4.5 or less, the solubility markedly increases.

Also a reduction in the powder liquid ratio results in significantly higher

solubility and disintegration rate in the oral cavity.

Bond strength

An interesting feature of polyacrylate cement is it’s bonding to enamel and

dentin, which is attributed to the ability of the carboxylate groups in the polymer

molecule to chelate to calcium. The bond strength to enamel has been reported to be

from 3.4 to 13 MPa and to that of dentin is 2.1 MPa. Optimum bonding requires clean

tooth surface. Sand blasting or electrolytic etching of the gold alloy surface is

necessary to achieve optimum bonding.

Dimensional stability

The zinc polyacrylate cement shows a linear contraction when setting at 37 C.

the amount of contraction varies from 1 % for a wet specimen at 1 day to 6 % for a

dry specimen at 14 days. These contractions are more pronounced than those

observed for zinc phosphate cements and start earlier.

Acidity

Zinc polyacrylate cements are slightly more acidic than zinc phosphate

cements when first mixed but the acid is only weakly dissociated, and penetration of

the highly molecular weight polymer molecules toward pulpal tissue is minimal.

Mortiner noted that film thickness is thicker than zinc phosphate cement.

According to Wilson and Paddon the cement remains much less brittle and is

tougher than silicate, zinc phosphate and glass ionomer cement.

Abelson said that the retention of full crown was similar to zinc phosphate.

Applications

Zinc polyacrylate cements are used primarily for luting permanent alloy

restorations and as bases. Theses cements have also been used in orthodontics for

cementation of bands.

25

Advantages

1. Biocompatibility with the pulp is excellent. Postoperative sensitivity is

negligible when used as a luting agent

2. Adhesion to tooth and alloy

3. Easy manipulation.

Disadvantages

1. Need for accurate proportioning required for optimal properties

2. Greater viscoelasticity

3. Shorter working time

4. Low compressive strength

5. More critical manipulation.

Trade Names

Dertelon (Premier dental products)

PCA (S.S. White)

Cermaco (Johnson & Johnson)

26

GLASS IONOMER CEMENT

Glass ionomer is the generic name of a group of materials that use silicate

glass powder and an aqueous solution of polyacrylic acid. The material acquires its

name from its formulation of a glass powder and an ionomeric acid that contains

carboxyl groups. It is also referred to as polyalkeonate cement.

Originally, the cement was designed for the esthetic restoration of anterior

teeth and it was recommended for use in restoring teeth with class III and V cavity

preparations. Also because the cement produces a truly adhesive bond to tooth

structure.

Applications

The use of GIC has broadened to encompass formulations as luting agents,

liners, restorative materials, core build-ups and pit and fissure sealants.

Types Of Glass Ionomer Cement

There are three types based on their formulations and their potential uses

Type I

• Luting applications

• Powder liquid ratio is generally 1.5 : 1

• Grain size 15 m or less

• High early resistance to water contamination

• Radiopaque for easy detection of excess

• Limited extension of working time thru chilling glass slab.

Type II

• Restorative material

• Powder liquid ratio 3:1

• Must protect for 24 hours for best results

• Reduced fluoride content to improve translucency

27

Type III

• Liner and base.

• Powder liquid ratio varies according to use

• Lining requires 1.5:1 for easy

• Base requires 3:1 or greater for strength

• Light activated varieties available

Type IV

Metal modified glass ionomer cement

• Miracle mix

• Cermet cement

Light curable versions of GIC are also available. (HEMA added to liquid)

Hybrid glass ionomer \ resin modified

Composition

The glass ionomer powder is an acid soluble calcium fluroaluminosilicate

glass.

The raw materials are fused to a uniform glass by heating them to a

temperature of 1100˚ C to 1500 ˚C. Lanthanum, strontium, barium or zinc oxide

additions provide radiopacity. The glass is ground into a powder having particles in

the range 20 to 50 μm.

SiO2 29.0

Al2O3 16.6

AlF3 5.3

CaF2 34.3

AlPO4 9.8

Fluoride is an essential constituent of glass ionomer cement. It lowers the

temperature of fusion, increases the strength and improves the working characteristics

of the cement paste.

28

The liquid for GIC was aqueous solutions of polyacrylic acid in a

concentration of about 50 %. The liquid was quite viscous and tended to gel over

time. The acid is form of a copolymer with itaconic, maleic, or tricaboxylic acid.

Theses acids tend to increase the reactivity of the liquid, decreases the viscosity, and

reduce the tendency for gelation.

The copolymeric acids used in modern glass ionomer liquids are more

irregularly arranged than in the homopolymer of acrylic acid. This configuration

reduces hydrogen bonding between acid molecules and thus reduces the degree of

gelling. Tartaric acid present in the liquid improves the handling characteristics and

increases the working time however it shortens the setting time.

One of the glass ionomer formulations consist of freeze dried acid powder and

glass powder in one bottle and water or water with tartaric acid in another bottle as the

liquid component. When the powders are mixed with water, the acid dissolves to

reconstitute the liquid acid. The chemical reaction then proceeds in the same manner

as that demonstrated by the powder liquid system. This is usually done to extend the

working time. These cements have a longer working time with a shorter setting time.

They are referred to as water settable GIC’s or as anhydrous GIC’s.

Simmons and Murray et al say that compressive strength has been found to be

significantly increased with the addition of silver alloy powder.

McLean showed that a simple matrix of metal powder and alumino silicate

glass ionomer powder failed to form a sufficient bond at metal/ polyacrylate interface.

The glass ionomer cement is capable of establishing a bond with the dentin substrate

before development start, but the composite start only after stress is started

Chemistry Of Setting

Glass ionomer cement is an acid base reaction cement as defined by Wilson

and Wygant.

When the powder and liquid are mixed to form a paste, the surface of the glass

particles is attacked by the acid. Calcium, aluminium, sodium and fluorine ions are

leached into the aqueous medium. The polyacrylic acid chains are cross-linked by the

calcium ions and form a solid mass. Within the next 24 hours a new phase forms in

which aluminum ions become bound within the cement mix. This leads to more rigid

cement. Sodium and fluorine ions do not participate in the cross linking of the cement.

Some of the sodium ions may replace the hydrogen ions of carboxylic group, where

29

as the rest combines with fluorine ions, forming sodium fluoride uniformly dispersed

within the set cement. During the maturing process, the cross-linked phase is also

hydrated by the same water used as the medium. The unreacted portion of glass

particles are sheathed by silica gel that develops during removal of the cations from

the surface of the particles. Thus, the set cement consists of an agglomeration of

unreacted powder particles surrounded by a silica gel in an amorphous matrix of

hydrated calcium and aluminum polysalts.

Role Of Water In The Setting Process

Water is a most important constituent of the cement liquid. It serves as the

reaction medium initially, and then it slowly hydrates the cross linked matrix, thereby

increasing the material strength. During the initial reaction period, this water can

readily be removed by desiccation and is called loosely bound water. As the setting

continues, the same water hydrates the matrix and cannot be removed by desiccation

and is then called tightly bound water. This hydration is critical in yielding a stable

gel structure and building the strength of the cement.

If freshly mixed cements are kept from the ambient air, the loosely held water

will slowly become tightly bound water over time. This phenomenon results in

cement that is stronger and less susceptible to moisture.

If the same mixes are exposed to ambient air without any covering, the

surfaces will craze and crack as a result of desiccation. Any contamination by water

that occurs at this stage can cause dissolution of the matrix forming cations and anions

to the surrounding areas. This process results in weak and more soluble cement.

Although the dissolution susceptibility tends to decrease over time, the minimum time

at which the danger of cracking from the exposure to air no longer exists has not been

established. The ionomer cement must be protected against water changes in the

structure during placement and for a few weeks after placement if possible.

Role Of Fluoride

Glass ionomer cements are bioactive. They form permanent adhesive bonds to

dentin and enamel, hence preventing the development of secondary caries.

They also release fluoride over a prolonged period and so can arrest the

progress of caries.

30

Of all dental cements they are the most resistant to erosion in the acidic

stagnation regions of the mouth.

Manipulations

To achieve a long lasting restoration several conditions need to be satisfied

like appropriate cavity surface preparation to achieve the bonding, proper mixing to

obtain a workable mixture.

Surface Preparation

Clean surfaces are essential to promote adhesion. A pumice wash can be used

to remove the smear layer that is produced during cavity preparation. On the other

hand organic acids such as polyacrylic acids of various concentrations can remove the

smear layer but still leave the collagenous tubule plug in place. These plugs inhibit the

penetration of the cement constituents and affect the hydrodynamic fluid pressure

within dentin.

One workable method is to apply a 10 % of polyacrylic acid solution to the

surface for 10 to 15 seconds, followed by a 30 second water rinse.t5he smear layer

will be removed but the tubules remain plugged. This procedure of removing the

smear layer is called conditioning.

The purpose of pumice debridement is to remove the fluoride rich layer

surface that may compromise the surface conditioning process.

After conditioning and rinsing of the preparation, the surface should be dried

but it should not be unduly desiccated. It must remain clean because any further

contamination by saliva or blood impairs bonding of the cement.

Preparation Of The Material

Glass ionomer cements mixed with carboxylic acid liquids have a powder

liquid ratio of 1.3: 1 or 1.35: 1, but it is the range of 1.25 to 1.5 g of powder per 1 ml

of liquid.

The powder and liquid are dispensed on a paper or a glass slab. A cool dry

glass slab may be used to slow down the reaction and extend the working time .the

slab should not be used if the temperature is below dew point, that is, at temperatures

that enhance moisture condensation on the glass slab that can alter the acid water

31

balance needed for a proper reaction. By waiting for a few minutes, the temperature of

the slab will rise sufficiently until water vapor no longer condenses on its surface.

The powder and liquid should not be dispensed onto the slab until just before

the mixing procedure is to be started. Prolonged exposure to the office atmosphere

alters the precise acid water ratio of the liquid. The powder is divided into two equal

portions. The first portion is incorporated into the liquid with a stiff spatula before the

second portion is added. The mixing time is 30 to 60 seconds. At this time the mix

should have a glossy surface. The shiny surface indicates the presence of polyacid that

has not participated in the setting reaction. The residual acid ensures adhesive

bonding to the tooth. If the mixing process is prolonged, a dull surface develops, and

adhesion will not be achieved.

Encapsulated products are typically mixed for 10 seconds in a mechanical

mixer and dispensed directly onto the tooth and restoration.

The cement must be used immediately because the working time after mixing

is about 2 minutes at room temperature. An extension of the working time to 9

minutes can be achieved by mixing on a cool slab, (3˚ C), but because a reduction in

compressive strength and modulus of elasticity is observed, this technique is not

recommended. Do not use the cement once a skin forms on the surface or when the

viscosity increases.

Glass ionomer cements are very sensitive to contact with water during setting.

The filed must be isolated completely. Once the cement has achieved its initial set (7

minutes), coat the cement margins with the coating agents supplied with the cement.

It is important to prevent excess cement from spreading to the tooth structure

or to the prosthesis. This cement is particularly susceptible to attack by water during

setting. Therefore, the accessible margins of the restoration should be coated to

protect the cement from premature exposure to moisture.

Properties

Film thickness

The glass ionomer cement is capable of forming films of 25μm or less.

Working time and setting time

The working time ranges from about 3 to 5 minutes the water settable cements

tend to have somewhat longer working time.

32

The setting time is usually between 5 to 9 minutes. The water added cements

have a more rapid initial set than those that use the polyacid liquid.

Both working time and setting time can be determined by indentation tests.

The oscillating rheometer of Wilson gives more information and is a better

measure of working time. Its dynamic nature is closer to the clinical than is static

indentation test.

Strength

The 24-hour compressive strength of Glass ionomer cements ranges from 90

to 230 MPa and is greater than that of zinc phosphate cement.

Tensile strength is similar to those of zinc phosphate cement.

Glass ionomer cements show brittle failure in diametral compression tests.

The elastic modulus of glass ionomer cements is less than that of zinc

phosphate but more than that of zinc polycarboxylate cement. The rigidity of glass

ionomer cements is improved by the glass particles and the iononic nature of the

bonding between polymer chains.

Bond strength

Glass ionomer cements bond to dentin with values of tensile bond strength

reported between 1 and 3 MPa. The bond strength of glass ionomer cements to dentin

is somewhat lower than that of zinc polyacrylate cement, perhaps because of the

sensitivity of glass ionomer cements to moisture during setting. The bond strength has

been improved by treating the dentin with an acidic conditioner followed by an

application of a dilute aqueous solution of ferric chloride.

Glass ionomer cements bond well to enamel, stainless steel, and tin oxide

plated platinum and gold alloy.

Solubility

The solubility in water for the first 24 hours is high. It is important that the

cement should be protected from any moisture contamination during this period. After

the cement has been allowed to mature fully, it becomes one of the most resistant of

the nonresin cements to solubility and disintegration in the oral cavity.

33

Biologic properties

The glass ionomer cements bond adhesively to tooth structure and they inhibit

infiltration of oral fluids at the cement tooth interface. This particular property plus

the less irritating nature of the acid should reduce the frequency of postoperative

sensitivity.

There are several factors contributing to the irritant nature. One is the pH and

the length of time that this acidity persists.

Another factor may be the viscosity. The pH relate to the thinner mixes used

for cementation and do not apply to the higher powder liquid ratio.

Glass ionomer luting cements may cause prolonged hypersensitivity, varying

form mild to severe, micro leakage has been suggested as an explanation, but a recent

study showed no increase in bacterial counts 56 days after cementation of crowns

with a glass ionomer cements. These cements may be bacteriostatic or bactericidal

because of fluoride release.

Graver says that post-cemented micro leakage is the cause of tooth sensitivity.

Smith D.C. states the cause of post cemented sensitivity as bacterial invasion,

hydraulic pressure, acidity in the early setting stage and wash out of thin mix.

Taywn stated that the higher the powder liquid ratio the greater is the thermal

diffusivity.

Adhesion

Glass ionomer has the property of permanent adhesion to untreated enamel

and dentin under moist conditions of the mouth. It reacts with the smear layer on cut

dentin (more for a filling material than for a luting agent). Glass ionomer also bonds

to other reactive polar substrates such as the base metals.

Bonding is of a chemical rather than a micro mechanical nature. Therefore, no

acid etching or surface roughening procedures is deprecated. About 80% of

maximum bond strength is developed in 15 minutes but strength slowly increases for

several days after that.

Mechanism Of Adhesion To Enamel And Dentine

Chemically, tooth material consists of apatite, which makes up 98% of enamel

and 70% of dentin by weight and collagen, which is found in dentin alone. The bond

34

of glass ionomer cements is better to enamel than to dentine, because bonding to

apatite is the principal mode of adhesion.

Beech proposed that the interaction between apatite and polyacrylic acid

produced polyacrylate ions, which then formed strong ionic bonds with the surface

calcium ions of apatite in enamel and dentine.

Wilson suggested that initially, when the cement paste is applied to tooth

material and is fluid, wetting and initial adhesion is by hydrogen bonding provided by

free carboxyl groups present in the fresh paste. As the cement ages, the hydrogen

bonds are progressively replaced by ionic bonds. The cations coming either from the

cement or the hydroxyapatite. Polymeric polar chains of polyacid are essential for the

achievement of adhesion. Their role is thought to be one of bridging the interface

between the cement and the substrate.

Wilson et al postulated that during absorption polyacrylate entered the molecular

surface of hydroxyapatite, displacing and replacing the surface phosphate. Also

calcium ions are displaced from hydoxypatite along with phosphate during this ionic

exchange. Therefore, an intermediate layer of calcium and aluminium phosphates and

polyacrylates would form at the interface between the cement and apatite.

Chain length is also an important factor in adhesion. The polymer chains

capable of bridging gaps between the cement body and substrate.

Bonding to enamel, which is mostly apatite is due to ionic and polar forces

and bonding to dentine is only to the apatite constituent of the dentine. Therefore, the

adhesion of glass ionomer to dentine is weaker.

Collagen contains both amino and carboxylic acid groups, so adhesion could

be due to hydrogen bonding or cationic bridges.

However, recent absorption studies show that polyacrylic acid and

polyacrylate are not absorbed on collagen.

Cements based on polyacrylic acid appear to bond more strongly than those

based on copolymers of acrylic acid with itaconic or maleic acids. Evidence is only

accumulating that bond strength to tooth substances depends on the nature of the

polyacid used.

If it were proved, then the molecular configuration of the polyacid would

become an important factor in controlling adhesion.

35

Improving Adhesion

When the cement tooth bonds fractures, it is by cohesive failure within the

cement rather than adhesive failure at the interface. Therefore, the strength of the

bond is limited by the cohesive strength of the cement used. The smear layer is

considered to be beneficial. However, salivary contamination of a freshly prepared

dentine surface reduces bond strength, but whether this was because of its water

contact or contamination of the dentin surface is uncertain.

Surface Conditioning

A number of research workers have sought to improve adhesion of glass

ionomer cements. One way that is common to nearly all adhesive technologies is by

pretreatment of the surface.

Mclean and Wilson first used the term surface conditioning for this treatment

in order to differentiate it from acid etching. Surface conditioning is needed in order

to eliminate the wide variation found in the structures of the tooth surfaces following

cutting. Rough tooth surfaces are contraindicated. In general, the smoother the

surface, the stronger is the bond. Good interfacial contact is important for adhesion.

Smoothening is necessary to prevent air entrapment and to minimize sites where

stress concentration could occur.

Fluoride Release

Both enamel and cementum can absorb fluoride. Fluoride is incorporated

within the mineral structure as fluoridated hydroxy apatite. Plentiful fluoride is

released in the early life of the restoration and it gradually decreases over a period.

Fluoride is released for at least 18 months. Thickly mixed cements released

more fluoride because they contain proportionately more glasses and therefore more

fluoride. Not all the fluoride is available for release. It is released as sodium fluoride

and is restricted by the sodium and the calcium content of the glass and not by the

total fluoride content of the glass. Sodium fluoride is released preferentially from the

matrix rather than the filler. The rate of release is proportional to the inverse of the

square root of time.

Aluminum ions are also released, temporarily and ceases once the cement has

fully hardened. Aluminum ions absorbed by enamel confer acid resistance upon the

tooth.

36

Action Of Fluoride In Prevention Of Caries

The anticaries effect can be due to the uptake of fluoride ions by enamel

apatite at hydroxyl sites, and high fluoride level at enamel surfaces increases

resistance to plaque acids. Surface energy of apatite is decreased, therefore, the dental

plaque does not adhere to tooth enamel surfaces.

Reaction Of Cement On Pulp

Several reasons have been postulated as to why Glass ionomer cement does

not have the same damaging effect on the pulp than Zinc phosphate cement.

1. First being the polycarboxylic acid used is much weaker than phosphoric acid.

2. Second, the acid is a polymer, means that it will have a much higher molecular

weight and this will limit diffusion along the dentinal tubules towards the

pulp.

3. Thirdly, there is a strong electrostatic attraction between hydrogen ions and

negatively charged polymer chain and dissociation will less readily take place

than with simple anions.

Applications

Glass ionomer cements are primarily used for permanent cement, as a base,

and as a class V filling material.

The cement has been evaluated as a pit and fissure sealant and an endodontic

sealer.

Glass ionomer cements are being used clinically for cementation of

orthodontic bands because of their ability to minimize decalcification of enamel by

means of fluoride release.

37

HYBRID IONOMER CEMENTS

Self cured and light cured ionomers (or resin modified glass ionomers) are

available for cementation.

Composition

One self-cured hybrid ionomer cement powder contains a radiopaque,

fluroaluminosilicate glass and a micro encapsulated potassium persulfate and ascorbic

acid catalyst system.

The liquid is an aqueous solution of polycarboxylic acid modified with

pendant methacrylate groups. It also contains 2 – hydroxyethylmethacrylate (HEMA)

and tartaric acid.

Another self-cured cement contains a mixture of fluroaluminosilicate and

borosilicate glasses in the powder. Its liquid is a complex monomer containing

carboxylic acid groups that can undergo an acid base reaction with glass and vinyl

groups that can polymerize when chemically activated.

A light cured hybrid ionomer cement contains fluroaluminosilicate glass

powder and a copolymer of acrylic and maleic acids, HEMA, water,

camphoroquinone and an activator in the liquid.

Setting Reaction

Setting of hybrid ionomer cements usually results from an acid base glass

ionomer reaction and self-cured or light cured polymerization of the pendant

methacrylate groups. Some cements are only light cured.

Manipulation

The powder is fluffed before dispensing. The liquid is dispensed by keeping

the vial vertical to the mixing pad. The powder liquid ration is 1.6 g of powder to 1.0

g of liquid, and the powder is incorporated into the liquid within 30 seconds to give a

mousse like consistency. The working time is 2.5 minutes. The cement is applied to a

clean dry tooth that is not desiccated. Some products recommend the use of a

conditioner for enhanced bonding to dentin. No coating agent is needed.

HEMA is known as a contact allergen therefore the use protective gloves and

a no touch technique are recommended.

38

Properties

The compressive and tensile strengths of hybrid ionomer cement are similar to

glass ionomer cements.

The fracture toughness is higher than that of other water based cements but

lower than composite cements.

The bond strength to moist dentin ranges from 10 to 14 MPa and is much

higher than that of most water based cements.

Hybrid ionomer cement have very low solubility when tested by lactic acid

erosion.

Water sorption is higher than resin cements.

Fluoride release is similar to glass ionomer cements. The early pH is about 3.5

and gradually rises.

Applications

Self cured hybrid ionomer cement are indicated for permanent cementation of

porcelain fuse to metal crowns, bridges, metal inlays, on lays, and crowns, post

cementation and luting of orthodontic appliances.

Additional uses include adhesive liners for amalgam, bases, provisional

restorations and cementation of specific ceramic restorations.

39

ZINC OXIDE EUGENOL CEMENT

This material has been used to a wide range applications in dentistry including

its use as an impression material for edentulous arches, a surgical dressing, a bite

registration paste, a temporary filling material, root canal filling, a cementing

medium, and as a temporary relining material for dentures.

ZOE cement is one of the least irritating of all the dental materials and

provides an excellent seal against leakage.

Types

According to ADA specification 30

Type I - ZOE cement –temporary cementation

Type II - ZOE cements –permanent cementation of restorations or appliances

fabricated outside of the mouth

Type III - ZOE cements –temporary restoration and thermal insulating bases

Type IV - ZOE cements – cavity liner

Composition

These materials are dispensed in two separate pastes. One tube contains zinc

oxide and fixed vegetable or mineral oil acts as a plasticizer and aids in off setting the

action of the eugenol as an irritant. The zinc oxide should be finely divided and it

should contain only a slight amount of water. Oil of cloves, which contains 70 % to

85% eugenol, is sometimes used in preference to eugenol because it reduces the

burning sensation experienced by patients when it contacts the soft tissues.

The addition of rosin to the paste in the second tube apparently facilitates the

speed of the reaction, and it yields a smoother, more homogenous product. Canada

balsam and Peru balsam are often used to increase flow and improve mixing

properties. If the mixed paste is too thin or it lacks body before it sets, a filler (such as

wax) or an inert powder (such as kaolin, talc, diatomaceous earth) may be added to

one or both of the original pastes.

There are many soluble salts that may act as accelerators. Chemicals

commonly used are zinc acetate, calcium chloride, primary alcohols and glacial acetic

acid. The accelerator can be incorporated in either one or both pastes.

40

Tube no 1 (base)

Zinc oxide 87

Fixed vegetable or

mineral oil 13

Tube no 2 (catalyst)

Oil of cloves or eugenol 12

Gum or polymerized resin 50

Filler (silica type) 20

Lanolin 3

Resinous balsam 10

Accelerator solution and color 5

Chemistry

The setting mechanism for ZOE material consists of zinc oxide hydrolysis and

a subsequent reaction between zinc hydroxide and eugenol to form a chelate.

Water is needed to initiate the reaction and it is also a by-product of the

reaction. This type of reaction is called autocatalytic. This is the reaction why the

reaction proceeds more rapidly in a humid environment. The setting reaction is

accelerated by the presence of zinc acetate dihydrate, which is more soluble than zinc

hydroxide and which can supply zinc ions more rapidly. Acetic acid is a more catalyst

for setting reaction than is water, because it increases the formation rate of zinc

hydroxide. High atmospheric temperature also accelerates the setting reaction.

The free eugenol cement of the set cement is probably extremely low. It

appears to be much higher than it actually is, because the chelate hydrolyzes readily,

forming free eugenol and zinc ions.

Manipulation

The mixing of the two pastes is generally accomplished on an oil impervious

paper, although a glass-mixing slab can be used. The proper proportion of the two

pastes is generally achieved by squeezing two strips of the paste of the same length,

one from each tube, onto the mixing slab. A flexible stainless steel spatula is

satisfactory for the mixing. The two strips are combined with the first sweep of the

41

spatula, and the mixing is continued for approximately 1 minute until a uniform color

is observed.

Cements intended for final cementation of restorations carry manufacturers

directions and measuring devices that are important to use, because of the deceptive

flow qualities of these cements, adding powder until the operator feels the mix is of

suitable consistency for cementing a restoration will lead to a cement deficient in

powder and a lowered strength in the set cement.

Properties

Setting time

The initial setting time may vary between 3 to 6 minutes.

The final setting time is the time at which the material is hard enough to resist

penetration under a load. It can occur within 10 minutes for type I pastes and 15

minutes for type II. The actual setting time is shorter when the setting occurs in the

mouth.

Film thickness

The film thickness should not be more than 25 μm for cements used for

permanent cementation and not more than 40μ m for cements used for temporary

cementation.

Compressive strength

A maximum value of 35MPa is required for cements intended for temporary

cementation.

A minimum of 35 MPa is required for cements intended for permanent

cementation.

The strength of the cement for temporary cementation is selected in relation to

the retentive characteristics of the restoration and the expected problems of removing

the restoration when the time arrives.

Disintegration

A maximum value of 2.5% is acceptable for provisional cementing materials

but a value of 1.5 % is required for the other cements.

42

Provisional Cementation

On many occasions, cementing a restoration provisionally is advised not that

the patient and dentist can assess its appearance and function over a longer time than a

single visit. However, this trial cementation should be managed cautiously. On one

hand, removing the restoration for definitive cementation may be difficult, even when

temporary ZnOE is used.

To avoid this problem, the provisional cement can be mixed with little

petroleum or silicone grease and applied only to margins of restoration to seal them

while allowing subsequent removal without difficulty.

On the other hand, a provisionally cemented restoration may come loose

during function. If a single unit is displaced, it can be embarrassing or uncomfortable

for the patient. If one abutment of a FPD becomes loose, the consequences can be

more severe.

If the patient does not promptly return for recementation caries may develop

very rapidly. Provisional cementation should not be undertaken unless the patient is

given clear instructions about the objective of the procedures, the intended duration of

the trial cementation and the importance of returning if an abutment loosens.

Temporary Cementation

Unmodified ZOE cements are used as a luting material for provisional

restorations in crown and bridge prosthodontics.

Unmodified cements are available in the compressive strengths of 1.4MPa to

21MPa. Studies proved that luting cements with a compressive strength of 15 to 24

MPa was the most appropriate cement based on retention; taste; ease of removal; ease

of cleaning.

Non-Eugenol Paste

One of the chief disadvantages of the ZOE pastes is the possible stinging or

burning sensation caused by eugenol when it contacts soft tissues. Furthermore the

ZOE reaction is never completed, with the result that the free eugenol may leach out.

Some patients find the taste of eugenol extremely disagreeable and in patients who

wear a surgical pack for several weeks; a chronic gastric disturbance may result.

43

A material similar to ZOE reaction product can be formed by a saponification

reaction to produce an insoluble soap, if the zinc oxide is reacted with a carboxylic

acid. The reaction is

ZnO + 2RCOOH→ (RCOO) 2Zn + H2O

Almost any carboxylic acid reacts with zinc oxide, but only a few such acids

provide compounds of dental interest. Orthoethoxybenzoic acid, (EBA), is used in this

regard.

The carboxylic acid is not necessarily a liquid. Powdered acids can be

dissolved or dispersed in a liquid carrying agent, such as ethyl alcohol.

The non-eugenol cements do not adhere well to preformed metal crowns as the

eugenol containing cements, and they are slower setting.

The non-eugenol cements however do not soften provisional acrylic crowns.

44

RESIN BASED CEMENT

Resin luting cements have been in existence since the 1950’s. The early

formulations were lightly filled methyl methacrylate resins. Because of their high

polymerization shrinkage, tendency for pulpal irritation, penchant for micro leakage

and poor handling characteristics, these resins had only limited use.

However , with the development of composite direct filling resins with

improved properties acceptance to acid etch and potential to bond to dentin, a variety

of resin cements have become available.

ISO 4049

Describes three classes of composites for polymer based filling, restoration and luting

materials

Class 1 – self cured materials

Class 2 – light cured materials

Class 3 – dual cured materials

Requirements Based On ISO 4049

Class 1,2,3: maximum film thickness 50μ m

Class 1,3: minimum-working time 60 seconds

Class 1,3: maximum setting time 10 minutes

Class 2: depth cure 0.5mm (opaque) 1.5mm (others)

Class 1,2, 3: water sorption 40 μg/mm³

Class 1,2,3: solubility 7.5μ g/mm³

Composition

The basic composition of the most modern resin based cements is similar to

that of resin based composite filling material. The resin cement consists of a resin

matrix (bis-GMA or diurethane methacrylate) with inorganic fillers that are bonded to

the matrix via coating with an organosilane coupling agent. The filler particle

provides strength. The fillers are those used in composites (silica or glass particles, 10

to 15μ m in diameter) and the colloidal silica is that used in micro filled resins. The

resin matrix binds them together and bonds them to the tooth structure. Because most

of a prepared tooth surface is dentin, monomers with functional groups that have been

used to induce bonding to dentin are often incorporated in these resin cements. They

45

have organophosphates, hydroxyethyl methacrylate (HEMA), and the 4-

methacyrlethyl-trimellitic anhydride (4-META) system.

Bonding of the cement to enamel can be attained through the acid tech

technique.

Polymerization can be achieved by the conventional peroxide amine induction

system or by light activation. Some cements are autopolymerising for use under light

blocking metallic restorations, while others are either entirely photo cured or dual

cured (light activated) for use under translucent ceramic veneers and inlays. In dual

cured cements, a catalyst is mixed into the cement so that it will eventually harden

within shadowed recesses after a rapid initial hardening is achieved with a curing

light.

Dual cured cements come in a base catalyst form and must be mixed before

use.

Light cured composites are photo initiated in the presence of a

camphoroquinone amine system. They provide a wide selection of shades, tints and

opaquers.

Properties

Resin based cements are virtually insoluble in oral fluids.

They are formulated to provide the handling characteristics required for the

particular application for e.g., cements recommended for cementation of indirect

restorations have a film thickness of 25μ m or less.

With respect to bonding to dentin, the so-called adhesive cements, which

incorporate the phosphonate, HEMA or 4-META adhesion systems, generally

develop reasonably good bond strengths to dentin. Bonding to tooth structure may be

more critical for resin based cements than for some other types of cement, because

they possess no anticariogenicity potential.

These cements differ from restorative composites primarily in their lower filler

content and lower viscosity. Resin cements are virtually insoluble and are much

stronger than conventional cements. It is their high tensile strength that makes them

useful for micromechanically bonding etched ceramic veneers and pitted partial

denture retainers to etched enamel on tooth preparations that would not be retentive

enough to succeed with conventional cements.

46

Biologic Properties

Resin based cements, just like composite cements are irritating to the pulp.

Thus, pulp protection via a calcium hydroxide or glass ionomer liner is important

when one is cementing an indirect restoration that involves bonding to dentin.

Manipulation

The chemically activated versions of theses cements are supplied as two

component systems a powder and a liquid or two pastes.

The peroxide initiator is contained in one component and the amine activator

is contained in the other. The two components are combined by mixing on a treated

paper pad for 20 to 30 seconds. The time of excess removal is critical. If it is done

while the cement is in a rubbery state, the cement may be pulled from beneath the

margin of the restoration, leaving a void that increases the risk of plaque buildup and

secondary caries.

Removal of the excess cement is difficult if it is delayed until the cement has

polymerized. It is best to remove the excess cement immediately after the restoration

is seated.

Light cured cements are single component systems just as are the light cured

filling resins. They are widely used for cementation of porcelain and glass ceramic

restorations and for direct bonding of ceramic orthodontic brackets. The time of

exposure to the light that is needed for polymerization of the resin cement is

dependant on the light transmitted through the ceramic restoration and the layer of

polymeric cement. However the time of exposure to the light should never be less

than 40 seconds.

The dual cure cements are two component systems and require mixing that is

similar to that for the chemically activated systems. The chemical activation is slow

and provides extended working time until the cement is exposed to the curing light, at

which point the cement solidifies rapidly. It then continues to gain strength over an

extended period because of the chemically activated polymerization.

Disadvantages

1. Excessive cement film thickness

2. Marginal leakage because of setting shrinkage

3. Severe pulpal reactions when applied to cut vital dentin

47

4. Dentin bonding agents have been reported to reduce pulpal response,

presumably by sealing the dentinal tubules and reducing micro leakage.

Adhesive resin was found to produce better marginal seal than zinc phosphate

cement.

Composite Resin System

Three types of composite resin materials are available for use in indirect

techniques: microfilled resins, small particle composite resins and hybrid resins. All

show excellent wear resistance, but small particle composite resins and hybrid resins

can be etched to produce micromechanical retention. They can also be silanted to

increase the bond strength further. One manufacturer of a reinforced microfilled resin

inlay/ onlay system provides a special bonding agent to increase the bond strength of

its material.

Resin Bonded Bridges

Theses prosthesis are widely employed as alternatives to metal ceramic

bridges.

In this procedure, the preparation of the abutment teeth is minimal and is

confined to enamel of the lingual surface and proximal surfaces. The tissue surfaces

of the abutments are roughened by electrochemical etching or other means and the

surfaces of the prepared tooth enamel are acid etched to provide mechanical retention

areas for the resin cements.

Glass Ceramic Restorations

These restorations are often translucent and require specific shades of

cementation agent to maximize their esthetic appearance.

Resin cements have been the cementation agents of choice recently for all

ceramic inlays, crowns and bridges because of their ability to reduce fracture of the

ceramic structures. To achieve the best retention, the undersurface of the glass

ceramic restorations is usually etched and a silane coating is applied before

cementation.

48

Resin Metal Bonding

Bonding composites to the metal framework of a bridge and denture acrylic to

a partial denture framework can be improved by the use of silica coating. Presently

there are three methods of applying silica to either noble or base metal alloys.

One method applies pyrogenic silica using a propane flame.

Other method s use heat in an oven or ceramic blasting to coat the restoration

or appliance. Bond strengths of composites to silica coated Au-Pd-Cr-Be alloys from

16 to 22 MPa. Silica coating of noble alloys eliminates the need for tin-plating these

alloys to improve adhesion of composites. The bond strength of denture acrylics to

Ni-Cr-Be alloys range from 7 to 23 MPa when alloy is treated with a silica coating or

primed with adhesive resin cement. Liquid cements based on thiosulfates have

recently become available for treatment of alloys. Recently, metal primers based on

thiophosphate chemistry have been introduced as a treatment for resin metal bonding.

49

COMPOMERS

Compomer is the resin based cement indicated for cementation of cast alloy

crowns and bridges, porcelain fused to metal crown and bridges and gold cast inlays

and onlays.

Cementation of all ceramic crowns, inlays onlays and veneers The cement

should not be used as a core or filling material.

Compomers are also known as poly acid modified composites.

Composition

The cement powder contains strontium aluminum fluorosilicate glass, sodium

fluoride and self and light cured initiators. The liquid contains polymerizable

methacrylate / carboxylic acid monomer, multifunctional acrylate / phosphate

monomer, diacrylate monomer and water.

Setting Reaction

Setting is the result of self and light cured polymerization. Once the cement

comes into contact oral fluids an acid - base reaction may occur. The carboxylic acid

groups contribute to the adhesive capability of the cement.

Manipulation

Dry the tooth to be cemented but do not desiccate. The powder liquid ratio is 2

scoops to 2 drops. Tumble the powder before dispensing. Mix the powder and the

liquid rapidly for 30 seconds. Place the mixed cement in the crown only and then seat

the crown.

A gel state is reached after 1 minute, at which time the excess cement is

removed with floss and a scaler. Light cure the exposed margins to stabilize the

restoration. Setting occurs 3 minutes after start of mix. Once set, compomer cement is

very hard.

Properties

Compomer cement has higher values of retention, bond strength, compressive

strength, flexural strength and fracture toughness. The cement has low solubility and

sustained fluoride release.

50

CEMENTATION PROCEDURE

The permanent cementation of the restoration is the final clinical procedure

that marks the success of our efforts.

Our interest is that the permanent cementation should be performed without

long periods of temporary cementation. Otherwise the patient may be exposed to a

series of unpleasant complications such as separation of the teeth, difficulty in

achieving a satisfactory level of oral hygiene, problems in removal of the restoration,

and the possibility of infiltration because the thickness of the temporary cement is

without doubt greater than the thickness of the permanent cement and is much less

fluid.

In immediate cementations the conditions of the healthy periodontium are

ideal and especially in conditions of complete visibility of the entire preparation,

cases in which the provisional restoration has been constructed properly, the only

practice we follow is one of isolating the area, cleaning the preparation and protecting

the prepared surface of vital teeth.

Isolation

The performance of all luting agents is degraded if the material is

contaminated with water, blood, or saliva. Therefore the restoration and the tooth

must be carefully cleaned and dried after the try in procedure, although excessive

drying of the tooth must be avoided to prevent damage to the odontoblasts. The

casting is best prepared by air- brading the fitting surface with 50µm alumina. This

should be done carefully to avoid abrading the polished surfaces or margins.

Alternative cleaning methods include steam cleaning, ultrasonic and organic solvents.

Before initiation of cement mixing, isolating the area of cementation and

cleaning and drying the tooth is mandatory. However the tooth should never be

excessively desiccated. Over drying the prepared tooth will lead to postoperative

sensitivity.

Saliva Control

Depending on the location of the preparation in the dental arch, several

techniques can be used to create the necessary dry filed of operation. In areas where

only supragingival margins are present, moisture control with a rubber dam is

probably the most appropriate method. However, in most instances a rubber dam

51

cannot be used and absorbent cotton rolls must be placed at the source of the saliva;

an evacuator must be placed where the saliva pools. In the maxillary arch, placing a

single cotton roll in the vestibule immediately buccal to the preparation and a saliva

evacuator in the opposing lingual sulcus is generally sufficient.

When working on a maxillary second or third molar, multiple cotton rolls

must be placed immediately buccal to the preparation and slightly anterior to block

off the parotid duct. If a maxillary roll does not stay in position but slips down, it can

be retained with a finger or the mouth mirror.

An alternative to multiple cotton rolls is placement of one long roll “horseshoe

fashion” in the maxillary and mandibular muccobuccal folds.

The use of moisture absorbent cards is another method for controlling saliva

flow. These cards are pressed paper wafers covered with a reflective foil on one side.

The paper side is placed against the dried buccal tissue and adheres to it. In addition

two cotton rolls should be placed in the maxillary and mandibular vestibules to

control saliva and displace the cheek laterally.

Svedopter and Speejector – for isolation and evacuation of the mandibular

teeth, the metal saliva ejector with attached tongue deflector is excellent. By adding

facial and lingual cotton rolls, excellent tongue control and isolation is provided.

Excessive forces are not necessary to make crowns seat during the phase of

cementation. If the space for the cement has been provided by the use of die spacer, it

is not necessary to exert a great deal force, which can determine a permanent

alteration of the integrity of the marginal fit. It should be kept in mind that the

cementation load should not exceed 5-7 kg.

52

The technique used is known as the brush technique and consists of the

application of a small quantity of cement on the incisal edge of the preparation using a

brush for the application.

The interior of the crown in the area of the margins is painted with a small

quantity of cement, and the crown is placed along its path of insertion.

The insertional technique is as follows: the crown is inserted slowly to about

one half the distances; it is then withdrawn by a few millimeters and is reinserted to

almost the full extent of its length. The process is then repeated. We use a slight up

and down movement along this path to assist the layering of the cement. When the

operator no longer feels any resistance, the crown is pushed to the finish line and thus

53

to its final seating. It is necessary to avoid rotational movements to find the correct

seating position. This can be damaging if porcelain margins are present.

Once the crown has been inserted the patient is provided with an occlusal

support and is asked to close to maintain the position of the crown during the setting

of the cement.

In professional practice we prefer to cement one crown at a time, or at the

most two adjacent crowns.

Once the cement has hardened we follow this procedure: after immersion of

the P.K. Thomas no. 2 waxing instrument in a silicone lubricant we enter the

junctional area and remove the excess cement by following the anatomy of that area.

We prefer to use this instrument because it has a rounded tip and a curvature that are

ideal for following the anatomic contour. We place it against the coronal surface and

54

insert it in the gingival sulcus in the junctional area. By applying light pressure we

follow the junction and remove the cement. The purpose of this cement is this

technique is to remove the cement following the contour without causing scratches in

the area of crown margin. The same procedure is repeated on the lingual surface and

on the interproximal surfaces, and because of the instrument curvature; it results as

being efficient and easy to perform.

Some cements like polycarboxylate or resin, tend to pull away from the

margins if excess removal is performed too early.

Dental floss with a small knot in it can be used to remove any irritating

residual cement interproximally and from the gingival sulcus. The sulcus should

contain no cement. After the excess has been removed. The occlusion can be checked

once more with Mylar shim stock.

55

Cements take at least 24 hours to develop their final strength. Therefore the

patient should be cautioned to chew carefully for a day or two.

Post-Cementation

Aqueous – based cements continue to mature over time well after they have

passed the defined setting time. If they are allowed to mature in an isolated

environment, that is, free of contamination from surrounding moisture and free from

loss of water through evaporation, the cements will acquire additional strength and

become more resistant to dissolution. It is recommended that coats of varnish or a

bonding agent should be placed around the margin before the patient is discharged.

56

LUTING OF VENEERS

All ceramic restorations may be cemented with zinc phosphate, glass ionomer

or dual polymerizing resin cement. The cement comes in four shades (A2, C2, B1 &

B3) permitting some influence on the final shade of translucent restorations. This not

only provides better retention and colour control but it makes the ceramic material

less fragile than if it were cemented with non resin cement.

Clean the prepared tooth with non fluoride pumice and try in the porcelain

veneers. Verify the marginal fit. A drop of water or glycerin will help the veneer stay

in place. The restoration should be internally clean, etched and silaned. Remove any

organic debris with ethanol or acetone. Acid etch the internal surface of the

restoration with hydrofluoric acid (for feldspathic porcelain etching time is 5

minutes). The gel is carefully rinsed under running water (this hydrofluoric acid acts

as an organic solvent and helps to remove any residual investment)

Dry the ceramic with oil free air. The silane coupling agent is applied to

internal surface of restorations. Dispense one drop of silane primer and drop of silane

activator into dappen dish. Stir the liquid in the dish for 10-15 seconds with a brush.

Apply to etched porcelain for 1 min and air dry after it.

These silane coupling agents are organosilones which help to form covalent

bonds (methacrylate group) with the resin when it is polymerized. Alternate to it

titanates and zirconates can also be used as coupling agents.

Etch the enamel surface with 37% phosphoric acid rinsed for 20 seconds and

air dry the tooth.

The bonding agent is then applied to the tooth for 30 seconds with a brush and

compressed air is used for 5-10 seconds to remove the excess adhesive

Polymerize the adhesive for 20 seconds with a light source. Dispense equal

amounts of base and catalyst from dual cure resin. Mix for 10-20 seconds with plastic

mixing stick. Apply a thin layer of cement to the internal surface of the crown. Seat

the crown and remove excess cement from the marginal areas with an explorer and

clean brush. Continue polymerizing for an additional 45-60 seconds, directing the

light from the lingual (through the tooth) so that shrinkage will occur toward the

tooth. Then direct the light from the labial (through the veneer). When light

activation is not utilized, allow 6 minutes for auto polymerization.

57

Once the luting agent is polymerized trim the excess cement and check the

occlusion. Final finishing procedures can be accomplished with porcelain polishing

agents.

58

LUTING OF CERAMIC RESTORATIONS WITH RESIN BASED CEMENTS

The crown should be cleaned, etched and silaned. Remove any organic debris

with ethanol or acetone, followed by placing the restoration in an ultrasonic cleaner.

Further cleaning can be accomplished by applying liquid phosphoric acid etchant. The

crown is silaned with a silane coupling agent. Dispense one drop of silane primer and

one drop of silane activator into a dappen dish. Stir the liquid in the dish for 10 -15

seconds with a brush. Apply it to the internal surface of the crown; avoid application

on the external surface of the crown by covering the outside of the crown with wax.

Rinse the crown and dry it with compressed air.

Clean the tooth preparation with a rubber cup and flour of pumice. Ten wash

and air dry. Etch the enamel for 30 seconds. Rinse and air dry the tooth.

Apply bond adhesive over the entire preparation with a brush. Thin the

bonding agent with compressed air for 15 seconds. Polymerize the adhesive.

Dispense equal amount of base from the syringe and catalyst from the tube.

Mix for 10 -20 seconds with a flat ended plastic mixing stick. Apply a thin layer of

cement to the internal surface of the crown. Seat the crown and remove the excess to

avoid ditching the cement at the margin.

Aim the light cure at the marginal areas from facial, lingual and occlusal

directions for 40 -60 seconds.

Adjust the bulky margins and check for premature contacts. Polish the crown

using porcelain finishing kit.

59

CONCLUSION

Luting agents possess varied, complex chemistries that affect their physical

properties, longevity and suitability in clinical situations. It appears a single adhesive

will not suffice in modern day practice. To date, no adhesive can completely

compensate for the shortcomings of the preparation retention and resistance forms or

ill fitting, low strength restorations. Prosthdontics must be aware of the virtues and

shortcomings of each cement type and select them appropriately.

60

REVIEW OF LITERATURE

Edwin. V (1951) in his study on mechanism of dental structure said that the dental

cements act as a bond by keying action. Roughness of interface between the inlay and

the tooth area involved (pitch or taper between opposing walls of cavity) thickness of

the bond.

John E. Johnston (1954) did a evaluation of an acrylic cement for one year. He

concluded that acrylic cement is more difficult to remove then ZnPO4 from cervically,

a complete dehydrated surface is desirable, ability to with stand expansion and

contraction due to temperature charge is not determined yet if the marginal cement is

removed and before polymerization, leaking will occur.

R.W.Phillips (1968) in his article ZNO and Eugenol cements for permanent

restoration in his conclusion was ZNOE was inferior to ZnPO4 in terms of

compressive strength.

Wendi A. Levine (1969) did an evaluation of film thickness of resin luting agents.

Most of the commercially available resin luting cements have films thin enough to

allow to successful placement of etched cast metal retainer. Restorative resins which

have grater film thickness are unsatisfactory for use as luting agents.

W.A. Richter (1970) did a study on predictability of retentive values of dental

cements. He concluded that by comparing the tensile strength of ZnPO4 ,

hydrophosphate and ZOE are equal and carboxylate is at least one third stronger. In

retentive evaluation the carboxylate, ZnPO4 and Hydrophosphate cements are equal

and ZOE is ½ as retentive.

Oilo G (1978) – The influence of surface roughness on the retentive ability of two

dental luting cements.Two series of brass cones and two series of dentine posts with

varying surface roughness were produced. Maximum roughness value and

arithmetical mean roughness were recorded for each cone. A tensile stress was

applied until the crown and cone separated. The retentive force is relation to retentive

area was measured. The results showed that the retentive ability of both cements

61

increased with increasing surface roughness. The increase in retention was greater for

bras than for dentine.

Dorothy McComb (1982) did a comparison of glass ionomer cement with other

cement of retention of castings. She concluded that G.I. have the greater retentive

strength and Zinc Phosphate has the weakest strength.

Michael L. Myers (1983) conducted a study on marginal leakage of contemporary

cementing agents, he concluded that the least amount of leakage were shown by

ZnPO4, than followed by glass ionomer cement with protective varnish and last is

polycarboxylate.

Gudbrand Oilo (1984) did a clinical study of two luting cements used on student

treated patient. In his 6 to 18 months observation there was no difference in both

ZnPO4 and Polycarboxylate cement, both cement was seen equally suitable as a

luting material.

W.R. Lacefield (1985) did a study of tensile bond strength of a glass ionomer

cement, he concluded that the tensile bond strength of G.I. cement to enamel was

significantly greater than to dentin (etched with phosphoric and citric acid has no

significantly effect on temporary bond strength.

G.L.Button (1985) in his article on surface preparation and shear bond strength of the

casting cement interface accounted that air blasting with 60 μm aluminium oxide

particles provided the surface roughness and topography with the greatest resistance

to shear stress.

Antony H.L. Tjan (1987) did a comparison of effect of various cementation methods

on the retention of prefabricated posts according to his study the post cemented with

composite recorded the greatest retention, then the ZnPO4 and glass ionomer.

C.L. Davidson (1991) made a study on destructive stresses in adhesive luting

cements. He said that nature and magnitude of the stress development, depend greatly

on the formulation and film thickness of the luting cement. The thicker the layer, the

62

faster the stress development in the G.I. and slower in the composite. The

contraction stress has a detrimental effect on the corrosive strength of the glass

ionomer and on the adhesive strength of the composite.

R.E. Kerby (1991) compared physical properties of stainless steel and silver

reinforced G.I. Cement, he suggested that the stainless steel reinforced G.I. cement

possess strength properties that should lead to a stronger, more # resistant restorative

when compared with the presently available one.

De Schepper E.J. (1991). Fluoride release form G.I. Cements .This study compared

the amounts and patterns of fluoride release form 11 commercially available glass

ionomer cements into artificial salvia over an 84 day period. The results indicated

that the materials differed in the amount of fluoride released, and that Miracle Mix

released the highest cumulative total of fluoride over the test period. Along with old

Fuji II, Miracle Mix also released the most fluoride during the last time interval (56 to

84 days). All of the materials released the greatest proportion of their cumulative

total fluoride in the first 24 hours after mixing.

Zakia Fakiha (1992) experimented the rapid mixing of ZnPO4 for F.P.Ds., he

experimented mixing of ZnPO4 rapidly in ordinary and frozen glass and compared

their properties. Mixing in frozen glass showed increase in PH, slightly increase in

film thickness, increase in working time and decrease in setting time compared with

the mixing in ordinary glass slab. So frozen glass slab mixing appear to be

satisfactory better for F.P.Ds.

Chung Moonum (1992) experimented the effect of early water contract on G.I.,

ZnPO4, Polycarboxylate luting agents. He found out that the time between start of

mixing and immersion in water decreased the width of both zones in all cements and

markedly lowered the loss of the surface of regular glass ionomer cement than other.

William W. Bracket (1992) analyzed the performance of a glass luting cement over 5

years in a general practice. In 5 years period 1435 received cast restoration luted with

G.I. cement. The material has demonstrated an exceptionally low rate of secondary

63

caries, excellent retention of castings and acceptable compatibility with the dental

pulp.

Johnson H, Powell .L.V (1993). Evaluation and control of post-cementation pulpal

sensitivity : zinc phosphate and glass ionomer luting cements.Many studies have

documented pulpal sensitivity after crown cementation, but non have determined its

cause. By controlling technique variable in a large scale clinical trial, the authors

evaluated the contribution of the zinc phosphate and glass ionomer luting cements in

causing pulpal sensitivity or necrosis.

James E. Metz, William. W. Brackett (1994) in their study on performance of G.I.

luting cement over 8 years in a general practice reported as follows 1230 cast

restorations luted with G.I. cement were followed up for 8 years. The result showed :-

a. Absence of secondary caries

b. 99% retention of restorations

c. 4 % incidence of irreversible pulpitis

Wu-J.C., Wilson (1994) Optimal cement space for resin luting cements..Stainless

steel dies, simulating 0 to 80 microns of die spacing, were seated into a machined

brass crown with phosphacap, Panavia EX, and C and B metabond. The seating

discrepancy after 60 seconds was measured to determine the optimal cement space for

best seating for each agent. The most complete seating for crowns luted with zinc

phosphate cement was observed when at least 40 microns of cement space was

provided. C and B Metabond and Panavia required 30 microns of die spacing.

64

REFERENCES

1. Antony H. L. Tjan: - Effect of various cementation methods on the

retention of prefabricated - J.P.D. – 1987; 58,3; 309

2. Button G.L. – Surface preparation and bond strength of casting cement

interface. J.P.D –1985; 53; 134-8.

3. Chung Moonum: - The effect of early water contact on G. I. Cements. –

Quint. Int.1992; 23; 209.

4. Dorothy McComb – Retention of castings with G.I. cements – J.P.D.-

1982; 48-3; 285.

5. Davidson C.L.- Destructive stress in luting cements. – J. Dent Res: 1991;

70-5, 880

6. De Schepper E.J. – Fluoride release from G.I. cements – Quint: 1991; 22;

3; 215-

7. Edwin V. - Mechanism of dental structures. – J.P.D. 1951,2, 306-10

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