LUTING AGENT

I. Introduction

Numerous 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,

orthodontic appliances; and pins and posts used for retention of

restorations.

The term LUTING is often used in textbooks to describe the use of a

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

These different applications make varying demands on manipulative

properties, working and setting times, resistance to mechanical breakdown,

and to dissolution. Thus some materials are better suited to some

applications than others. Because one type of cement is unlikely to perform

satisfactorily under all conditions, specific cements must be selected and

developed for each applications.

II. Basic considerations:

The discrepancies of fit arising during the fabrication process for the

inlay or crown, the preparation of the tooth leaves a rough and debris

covered surfaces. The cement lute then must have the ability to wet the

tooth and restoration, flow into the irregularities on the surfaces it is

joining and fill in and seal the gaps between the restorations and the tooth.

Because an exposed cement line at the restorations margin is

inevitable – especially with todays restorative materials the cement must

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also have adequate resistance to dissolution in the oral environment. It

must also develop an adequately strong bond through mechanic

interlocking and adhesion. High strength in tension, shear, and

compression are required, as well as good fracture toughness to resist

stresses at the restoration tooth interface.

Good manipulation properties including adequate working and

setting time are essential for successful use. The manipulation, including

dispensation of the ingredients, should allow for some margin of error in

practice. The material must be biologically acceptable.

As the literature says the oldest established and most widely used

types of dental cements.

1. Zinc phosphate and

2. Zinc oxide-Eugenol were developed in the late nineteenth century

and early twentieth century. Although they have undergone

considerable technical improvement, in principle they have

remained almost unchanged for 50 years. most clinical techniques

and evaluation criteria are based on long experience with these

materials significant research on new cements has been carried out

only in the last 25 years.

The advent of acrylic resins led to the development of fine grained

cold curing polymethyl methacrylate cements in the Mid 1950’s. Such

materials did not become popular for routine cementation. More recently,

cements based on the BIS-GMA monomer of Bowen have become

available in powder to liquid (filler monomer) form and two paste forms.

All these materials set by polymerization mechanisms, so the handling

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qualities are different and generally less satisfactory than conventional

cements. Another problem with resin cements that has limited their use is

the potential tissue reaction to residual monomers in the set material.

None of the foregoing cements shows significant adhesion to clean

enamel and dentin. The need for good wetting and bonding and low

toxicity led to the development of cements based on the reaction of

polymerize organic acids and metal ions in the mid 1960s by Smith. The

early carboxylate (or polycarboxylate) cements were based on zinc oxide

and an aqueous solution of polyacrylic acid or its co-polymers later work

by Wilson and cowoskers resulted in the glass ionomer cements that utilize

non-leachable glasses rather than zinc oxide. Development in this area still

proceeding.

As a result of the research of the last few years there are now

available cements of four basic types, classified according to the matrix

forming species.

1. Phosphate bonded.

2. Phenolate bonded

3. Carboxylate bonded and

4. Methacrylate (resin) bonded

Within each category are several classes and this multiplicity

together with the choice of several brands of material in each class, has

lead to confusion among clinicians as to which type of cement is most

suitable to a given situation. Although there are national (ADA, ANSI,

BSI, ASA) and international (ISO, FDI) standards for cements, these are of

limited value in predicting clinical durability. Another difficulty is that

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reports in the literature are often based on the testing of one or two brands

of a cement type, and the results are then assumed to apply to all such

cements. It is therefore appropriate to briefly review the characteristics of

the available cements.

III. Types of cement

1. Phosphate based cements.

a. Zinc phosphate cement : It is the oldest of the cementation agents

having a widest range of application and terms is the one that has

the longest track record. It serves as a standard by which newer

systems can be compared. It consists of powder and liquid in two

separate bottles.

Composition and chemistry: the main ingredients of the powder are:

a. Zinc oxide (90%).

b. Magnesium oxide (10%).

The ingredients of powder are sintered at temperature between

1000°C and 1400°C into a cake that is subsequently ground into five

powders. The powder particle size influences setting rate. Generally, the

smaller the particle size, the faster the set of the cement.

The liquid, contain phosphoric acid, water aluminium phosphate and

in some instances, zinc phosphate. The water content of most liquids is

33%±5%. The water controls the ionization of the acid, which in terms

influences the rate of the liquid powder (acid base) reaction.

It is obvious that because water is critical to the reaction, the

composition of the liquid should be preserved to ensure a consistent

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reaction. Changes in composition and reaction rates may occur either

because of self degradation or by water evaporation from the liquid. This

means that changes in the composition can affect the reaction. Self

degradation effects are best detected as a clouding of liquid over time.

Properties: the long persistence of zinc phosphate cements in clinical

practice indicates that reasonable performance is obtained. Although the

properties are far from ideal they are usually regarded as a standard against

which to compare newer cements. The principal reasons for their

satisfactory performance under routine conditions are that they can be

easily manipulated and that they set sharply to a relatively strong mass

from a fluid consistency.

At standard luting consistency the powder to liquid ratio is 2.5 to 3.5

(g per ml). The cementing mix flows readily under pressure to a film

thickness between 20 and 40 mm, which is adequate to seat most types of

restorations as in practice the space between the restoration and the tooth

may range upto as much as 100 mm. The film thickness achieved in

specific clinical situations is a function of the rheology of the cement and

the geometry of the surface being cemented.

At the recommended powder to liquid ratio, the compressive

strength of the set zinc phosphate cement is 80+0110 MPa after 24 hours.

The strength is strongly and almost linearly dependent on powder to liquid

ratio. The tensile strength is much lower than the compressive strength, 5

to 7 MPa and the cement shows brittle characteristics.

The modulus of elasticity (stiffness) is about 13 GPa. According to

the standard method, the solubility and disintegration in distilled water

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after 23 hours may range from 0.04% to 3.3%, for inferior material the

standard limit is 0.2%.

The comparative evaluation a cement solubility under clinical

conditions has shown significant loss, but conflicting results. Dissolution

contributes to marginal leakage around restorations and bacterial

penetration.

At room temperature (21 to 27°C) the working time for most brands

at luting consistency is 3 to 6 minutes the setting time is 5 to 14 achieved

by use of a cold (frozen) mixing slab, which permits upto an approximately

50% increase in the amount of powder, improving both strength and

resistance to dissolution. The cement has been found to contract about

0.5% linear giving rise to slits at the tooth cement and cement restoration

interface.

Retention: Setting of the zinc phosphate cement does not involve any

reaction with surrounding hard tissues or other restorative materials.

Therefore primary bonding occurs by mechanical interlocking at interfaces

and not by chemical interactions.

Prologic effects: the freshly mixed zinc phosphate is highly acidic with a

pH of 1.6 two minutes after mixing. Even after setting at 1 hr the pH may

still be below 4. after 24 hrs the pH reaches 6 to 7.

One material that has a low acid content and incorporates calcium

hydroxide has little effect on the pulp when used as a lining. Very thin

mixes will also lead to etching of the enamel. The etching may assist

mechanical interlocking to the adjacent substrates.

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Advantages and disadvantages:

The main advantages of the zinc phosphate cements are that they

can be mixed easily and that they set sharply to a relatively strong mass

from a fluid consistency. unless the mix is extremely thin (for instance,

with a very low powder to liquid ratio) the set cement has a strength that is

adequate for clinical service, so their manipulation is less critical than with

other cements.

However, then distinct disadvantages include pulp irritation, lack of

antibacterial action, brittleness, lack of adhesion, and solubility in acid

fluids.

Modified zinc phosphate cements

Fluoridated cements: Some phosphate cements contains fluoride in the

form of stannous or other fluorides.

- Which have low strength and higher solubility rate.

- They used in orthodontic bracket cementation.

Copper cements: they come improves oxide (red) or cupric oxide (oxides)

or copper salts added to the zinc oxide powder.

- They have germicidal action.

Silicophosphate cements: These materials that are combination of zinc

phosphate and silicate cements have been available for many years.

The principal applications have been for the cementation of fixed

restorations especially porcelain, because of their translucence.

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- They are germicidal as they contain little amount of mercury

or silver compounds.

Composition and setting: the powder in these materials consists of a

combination of silicate glass and zinc oxide, the silicate glass contains 13

to 25% fluoride. The liquid is similar to silicate liquids containing about

50% H3PO4; 4% Zn and 2% Al the set cement seems likely to consist of

unreacted and zinc oxide particles bonded together by an alumino

phosphate gel containing zinc, Ca, Aluminium and Flouride ions.

Properties: Powder to liquid ratio is 2.1 to 3.2g per ml.

Flow properties of the mix are not as good as per Zinc Phosphate

contents, leading to a higher film thickness in practice.

Compressive strength to set cement is 135 to 175 Mpa better than

Zinc phosphate.

Tensile strength is 7 Mpa.

These materials appear to be tougher and more abrasion resistant

from phosphate cements.

Solubility in distilled water after 7 days is higher than for Zinc

phosphate cements, but under clinical conditions it is less so.

Biological effects: The set cement is much more translucent than the

opaque Zinc phosphate. Thus it has been used for the cementation of

porcelain restorations.

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Because of the acidity of the mix and the prolonged low

PH (410.5) after setting. Hence pulp protection is necessary on all

vital reduced teeth.

Advantages and Disadvantages: The silicophosphate cements have better

strength and toughness properties than the Zinc phospate cements, show

considerable flouride release, transulcence and, under clinical conditions,

lower solubility and better bording.

Disadvantages include less satisfactory mixing and rheologic properties,

leading to higher film thickness in practice and greater potential for pupal

irritation. They are but suited to cementation of orthodontic bands and

restorations on non vital teeth. Inauguration

PHENOLATE – BASED CEMENTS

There are three main types of cements under this classification:

1) The simple Zinc oxide – Eugenol.

2) Reinforced Zinc oxide – Eugenol.

3) EBA cements.

1) Zinc oxide – Eugenol cement:

Compositon and setting: The basic combination of Zinc oxide and

Eugenol finds it principal applications in the temperory filling of teeth, and

as a cavity lining in deep davities.

The powder is Zinc oxide, with additives such as silica,

may be present. Upto 1% Zinc acitate, chloride, sulfate, or other salts

may be present in accelerate the setting.

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The liquid is purified Eugenol in some cases, oil of cloves

(85% Eugenol).

It may contain about 1% of acetic acid or alcohol to

accelerate setting together with small amounts of water.

The cement sits by a chelation reaction between two basic

components involving the formation of Zinc eugenolate. However, the

reaction is reversible, the Zinc Eugenolate being easily hydrolyzed by

moisture Eugenol and Zinc hydroxide. Thus the cement disintegrates

rapidly when exposed to oral conditions.

Properties: The material is easy to mix but requires a long spatulation

time at least 90 seconds.

Because of work nature of binding agent, the compressive

strength is low, ranging from 7 Mpa (luting consistency) to 40 Mpa

(filling consistencyA).

Tensile strength is much lower.

The solubility of the set cement in distilled water is high

when exposed directly to oral conditions, the material maintains good

sealing characteristics despite a volumetric shrinkage of 0.9% and a

thermal expansion of 35x10-6/degree C.

Biologic effects: The presence of Eugenol in the set cement under clinical

conditions appears to lead to an anodyne and abundant effect on the pulp in

deep cavities.

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The seating capacity and antibacterial action appears to

facilitate pulpal healing.

Reinforced Zinc oxide – Eugenol cements

Composition and setting: These materials contain 10 to 40% of finely

divided natural or synthetic resins added to or coated on to the powder

particles. Additional accelerator (Zinc acetate, chloride, or acetic acid)

may be present as well as antimicrobial agents such as thymol or 8-

hydroxyghinoline.

Properties:

Working time is about 5 mts. and setting 7 to 9 mts. Lly to

Zinc phosphate.

Compressive strength – 35 to 55 mpa and

Tensile strength – 4 MPa and

Modulus of elasticity is – 2 to 3000 Gpa.

The mechanical properties are reduced by impression in

water, resulting in loss of Eugenol. This tendency seems less

pronounced with the polymer – reinforced materials.

Biological Effects: There may be irritation to connective tissue.

Advantages and disadvantages:

Advantages – Main advantage is the minimum reaction to the pulp.

Good sealing properties

The strength is adequate as a lining material and for luting

single restorations and retains with good retention form.

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Disadvantages - Main is hydrolic break down when exposed to oral fluids.

The inflammatory reaction is soft tissues and potential

allergic response.

EBA and other chelate cements

In order to further improve on the basic Zinc-oxide

Eugenol system. Many workers have investigated

mixtues of zinc and other oxides with other liquid chelating agents.

Composition and setting: The Zinc oxide contains 20 to 30%

Aluminium oxide or other mineral fillers; polymeric reinforcing agents,

such as polymethyl methacrycate.

- The liquid consists of 50 to 60% EBA with the reminder

Eugenol.

In order to obtain optimal properties it is important to use as high a

powder – liquid ratio as possible i.e., 3.5g per ml.

Properties:

The working and setting times range between 7 and 15 mts. The

film thickness is in the range 40-70mm.

Compressive strength is 50-70 Mpa.

Tensile strength is 6 to 7 Mpa.

Modulus of elasticity – 5 Gpa.

The EBA cements show viscoelastic properties with very

low strength, and large plastic deformation at slow (0.1 mm/ mint) rates

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of deformation and at mouth temperature (37C). This says its retention

values for crown is low than Zinc phosphate cements.

When exposed to moisture, greater oral dissolution occurs

than for other cements.

Advantages and Disadvantages:

Advantages:

The principal advantages of the EBA cements are their easy

mixing.

Long working time.

Good flow and low irritation to the pulp.

Disadvantages:

- Main is critical proportioning.

- Hydrolic break down in oral fluids.

- Liability to plastic deformation.

- Poorer retention than Zinc phospate cement.

POLYCARBOXYLATE – BASED CEMENTS

Zinc polycarboxylate cements: These cements were developed in late

1960’s as an adhesive dental cement in the search for a material that would

combine the strength properties of the phosphate system in the Biologic

acceptability of Zinc oxide Eugenol materials. These materials have gone

through several stages of development since their acception and progress is

continuing.

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Compositon and chemistry: The polycarboxylate cements are liquid

systems.

The liquid is an aqueous solution of polyacrylic acid or a

copolymer of acrylic acid with other unsaturated carboxylic acids, such

as itaconic acid.

The molecular net of the polyacids ranges from 30,00 to

50,000. The acid concentration may vary to some degree from one

cement to another but usually is about 40%.

The composition and manufacturing procedure for the

powder are similar to those of Zinc phosphate cement. The powders

mainly zinc oxide with some Magnesium oxide. Stannic oxide may be

substituted for magnesium oxide. Other oxides, such as bismuth and

Aluminium, can be added. The powder may also contain small

quantities of stannous flourides, which modify setting time and enhance

manipulative properties. It is an important additive because it increases

strength. However, the flouride released from this cement is only a

fraction (15% to 70%) of the amount released from Silicophosphate and

glass ionomer cements.

The setting reaction of this cement involves particle

surface dissolution by the acid that releases zinc magnesium, and tin

ions, which bind to the polymer chain via the carboxyl groups, as given

below. These ions react with carboxyl groups, as 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 disposed. The microstructure resembles that of

Zinc phosphate cement in appearance.

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STRUCTURE OF THE CHEMICAL

The role of carboxylate functional groups in polycarboxylate cements:

A Yielding matrix through cross linking by zinc ions.

B Bonding to tooth structure through Calcium hydroxide).

- 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 polyacid is freeze dried and

subsequently mixed with water or if the conventional aqueous solution

of polyacid is used as the liquid.

Bonding to tooth structure: The outstanding characteristics of this cement

are that it bonds chemically to the tooth structure. The mechanism is not

clear but as shown in above diagrams, the polyacrylic acid is believed to

react via the carboxyl groups with calcium of Hydroxyapitite. (In reference

to GIC, the inorganic component and the homogeneity of enamel are

greater than those of dentin. Thus, the bond strength to enamel is greater

than that to dentin.

Properties: For luting consistency the recommended powder to liquid ratio

is 1.5 1(2t/wt). About the film thickness, the freshly mixed mix is in

spatulation and seating of a restoration, it exhibits shear thinking. Thus,

contrary to the subjective impression that the correct mix for a Zinc

polycarbohydrate cement is much thicker than a luting zinc phosphate mix.

Under presssure mix tends to thicken more quickly than the zinc

polycarboxylate mix.

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One of the most common errors made with

polycarboxylate cements is to make a mix that appears to be a fluid as a

zinc phospate mix; this will result in the use of low powder-to-liquid

ratio with consequent poor properties in the cement. Measuring devices

for these material will ensure correct proportions.

The working time is 2.5 to 3.5 minutes at room

temperature and the setting time is 6 to 9 mts at 37C, the water mix

materials tending to give slightly longer setting times. As with other

cements, working time can be substantially increased by mixing the

material on a cold slab and by refrigerating the powder. The liquid

should not be chilled as this encourages gelation due to hydrogen

bonding.

At cement consistency the compressive strength from 55

to 85 MPa.

Tensile strength 8 to 12 MPa.

In general these cements have somewhat lower compressive

strengths than zinc phosphate cements but are significantly stronger in

tension. The cement gains strength rapidly after the initial setting period;

the strength at 1 hr. is about 80% of the 24 hr. value. These data indicate a

slow continuance of the setting reaction tending towards greater rigidity

and more brittle behaviour. However, the cement remains much less brittle

and is tougher than silicate, since phosphate, or glass ionomer cement,

through less so than a resin cement.

The solubility of the present day cements in distilled water, when

determined by a specification weight loss method, ranges from less than

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0.1% to 0.6%. The latter high value relates particularly to cements that

contain stannous fluoride. Effective fluoride release can be obtained

without substantial effects on the mechanical properties of the cement.

Significant fluoride intake by neighbouring enamel occurs. As with zinc

phosphate cements, the solubility is much higher in organic acid solutions,

especially at lower PH and if the acid has chelating powers.

Few recent clinical studies of solubility gave conflicting results, two

studies conducted by Mitchem and Osborne in 1978 respectively showing

lower results than zinc phosphate and the other the reverse. Both studies

agreed in finding the zinc silicophosphate both the least loss of the cement

tested. In vivo evaluation of marginal leakage showed similar results for

the two types of cement and whose results for an EBA alumina cement.

Thus these all suggests that polycarboxylate cement has adequate clinic

performance.

The polycarboxylate cements display good adhesion to enamel, and

to a lesser extend, to dentin as well as to the various alloys. Adequate

fluidity of the mix and sufficient available carboxyl groups are necessary

for interfacial interaction as well as a surface free of contaminants and void

defects. Bonding to both or alloy surface is reduced if contaminated with

saliva.

Biologic effects: The effect of Zinc polycarboxylate cements on soft and

calcified tissues found to be molding Macrons investigators like Smita

D.C. in 1971. The effect on the pulp is less than Zinc oxide Eugenol. The

general biocompatibility of these materials seems excellent this appears to

be primarily due to the low intrinsic toxicity the mild effect on the pulp and

other tissues is also due to the rapid rise of the PH of the cement towards

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neutrality; localization of the polyacrylic acid and limitation diffusion try

its molecular size and acid ion bonding to dentinal fluid ca and proteins;

and the minimal movement of fluid in the dential tubules in response to the

cement the presence of stannous fluoride does not appear to affect the mild

respnse.

-It gives anticariogenic properties in fluorides containing cements.

Advantages and Disadvantages:

The main advantages of these materials are the low irritancy

adhesion to tooth substance and alloys.

Easy manipulation and strength, solubility and film thickness

properties comparable to those of zinc phosphate cements.

The need for accurate proportioning for optimal properties and thus

more critical manipulation.

The lower compressive strength and greater viscoelasticity from

zinc phosphate cements, the short working time of some materials and

the need for clean surfaces to utilize the adhesion potential.

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 there is danger

that some of the cement may be pulled out from beneath the margins,

leaving a void. The excess should not be removed until the cement

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becomes hard. The outer surface of the prosthesis must be coated carefully

with a separating medium such as petroleum jelly to prevent excess cement

from adhering.

- Care should be taken not to allow the medium to touch the

margin of the prosthesis. Another approach is to start removing excess

cement as soon as seating is completed. The goal of these two method

is to avoid removing the excess during th rubbery stage.

GLASS IONOMER CEMENT:

Type I GIC is designed for cementation of castings.

Compositon and setting: These materials were formulated by bringing

together the silicate and poly acrylate system. Originally the use of silicate

glasses in the zinc polycarboxylate cements was envisaged that the

available material were insufficiently reactive. Wilson and Kent and their

coworkers in 1975 developed glasses that were ion – leachable by aqueous

polyacrylic acid and its acid copolymers. The powder in these materials is

a fine ground calcium aluminium fluoro-silicate glass with a particle sized

of around 40 cm for the filling materials and less than 25cm for the luting

materials. The liquid is a 50% aqueous solution of a polyacrylic – Itaconic

acid or other poly carboxylic acid copolymer containing about 5% tartaric

acid. On mixing the acids react with the glass leaching ca and aluminium

ions from the surface which cross-link the polyacid molecules into a set. A

recent material has the polyacid contained in the powder and the liquid is a

solution of the tartaric acid. This contributes to easier mixing and better

stability.

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Properties: The powder to liquid ratio for luting is about 1:3:1 for the

conventional types of glass ionomes cement. Best results on a chilled seas.

The slow ratio of hardening initially during formation of the calcium

polysalt before al cross linking becomes effective means that the cement is

sensitive to moisture and more soluble during the early stages of its

hardening. The gel can also craze if allowed to dry out. Thus, it is essential

to protect exposed margins until sufficient strength has developed

The setting time is 8 to 9 mts. somewhat shorter than with zinc

phosphate cements.

The film thickness less than 30 cm was also comparable and was

adequate to seat castings satisfactorily.

Over 24 hours the compressive strength increased to 900 to 1400

Mpa.

Tensile strength to 60 to 80 Mpa.

- The modulus of elasticity was about 7 Mpa.

A glass ionomer cement showed superior retention of gold inlays

and onlays compared with a phosphate and a silicophospate cement.

- The solubility of the cements in water was about 1% and this

was increased in artificial saliva and lactic acid.

Good resistance to dissolution was observed under clinical

conditions. However, the initial slow set and moisture sensitivity may

contribute to leakage.

Varnish protection is desirable.

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These cements have potential for adhesion to enamel, dentin and

alloys in a similar manner to the polycarboxylate. In vitro the adhesion is

variable and affected by surface conditions. Slight and variable marginal

leakage in tests of cemented restorations has been reported.

Biologic effects: Evidence fro in vitro testing and clinical experience with

the restorative form of the glass ionomer cements suggest the tissue

response would be similar to the zinc polycarboxylate cements. However,

there is only limited data on the luting cements. Paterson and Watts

observed pulp necrosis in rat molars after application to exposures.

However parmajer et. al. found little pulp irritation from one commercial

cement in cavities in monkey teeth after 3 months. Likewise Reisbick in

1980, in a clinical study, found in slight sensitivity on cementing but no

evidence of hypersensitivity after 6 months. However, some cases o

postoperative sensitivity have been reported and this may be due to

mismanipulation and marginal leakage of bacteria.

Advantages and disadvantages:

Advantages: The glass ionomer cement materials include easy mixing,

high strength and stiffness, leachable fluoride, good resistance to acid

dissolution, and potential adhesive characteristics.

Disadvantages: It includes initial slow setting and moisture sensitivity,

variable adhesive characteristics, radiolucency, and possible pulp

sensitivity.

Precautions should be taken to protect the pulp when cementing

restorations with glass ionomer cements. The priologic considerations take

precedence over other matters, such as the potential for adhesion that

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ensures a strong bond to tooth structure. The smear layer on the cut surface

of the cavity preparation should not be removed but should be left intact to

act as a barrier to the penetration of the tubules by the acid component of

the cement. All deep areas of the preparation should be protected by a thin

layer of a hard setting calcium hydroxide cement.

Methacrylate (Resin) based cements:

A variety of Resin-based comments have now become available

because of the development of the direct filling resin with improved

properties, the acid etch technique for attaching resins to enamel, and

molecules with a potential to bond to dentin conditioned with organic or

inorganic acid.

Acrylic cements:

For many years powder to liquid cold curing acrylic cements have

been available. These materials have been used for the cementation of

restorations, of temporary crowns, and also as core materials. The powder

in these materials is a finely divided methyl-methacrylate polymer or

copolymer containing benzoyl peroxide as initiations. Mineral filler and

pigments may also be present. The liquid is a methyl methacrylate

monomer containing an amine accelerator. The material sits by

polymerization of the monomer, which concurrently dissolves and softens

the polymer particles. The set mass consists of the new polymer matrix

uniting the undissolved but swollen original larger polymer beads or

particles.

These cements are stronger and less soluble than other cements but

display low rigidity and visco-elastic properties. They have no effective

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bond to tooth structure in the presence of moisture and tins permit marginal

leakage although they may show better bonding than other cements to resin

facings and polycarbonate crowns. Pulp reaction on vital dentin from

monomer in the unset material, and residual monomer in the set material

are biologic concerns. Other problems include the short working time and

the difficulty in removing excess materials from margins.

Bis-GMA type cements:

The materials of more recent development are based on the

BISGMA system and thus are combinations of an aromatic dimethacrylate

with other monomers. Such materials have been supplied as two viscous

liquid or two pastes. The material that has been widest explanation and

investigation is a powder to liquid combination.

The powder is a finely divided borosilicate glass of average particle

size of 15mm. The particles are silam treated to improve bonding and

contain an organic peroxide initiator. The liquid is based on the reaction

product of the diglycidyl either of bis-phenol A and methacrylic acid.

Which is diluted with a low viscosity monomer such as ethylene glycol

demethacrylate. An amine Accelerators is also present. On mixing the

polymerization of the monomer mixture occurs, leading to a highly cross

linked composite resin structure. The material is easily mixed to a fluid

consistency and is used in conjunction with an etching solution of 50%

citric acid to clean the tooth surface and promote adaptation and bonding.

The mix rapidly increases in viscosity and working time is short.

When set, the material has higher bending and compressive

strengths than other cements. The modulus of elasticity was found to be

less than for zinc phosphate, but the plastic strain at fracture and toughness

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much higher. Sections of cemented casting, revealed spaces at the tooth

resin interfaces, presumably due to polymerization contraction. Although

bonding was improved by citric acid treatment, it appeared to be

attributable to penetration of resin into the tubules, a phenomenon that has

also been observed by Vongiduklakis and Smith.

Although the strength and resistance to dissolution of this type of

material is superior to any other type of cement, these biologic and

practical questions common to other types of resin cement have limited its

use on vital teeth. Poorer retention for full crowns than for other types of

cement was observed by Chan et al in 1975. These problems also include

short working time, difficulty in seating castings and difficulty in removing

excess material. They may be best suited to long term temporary

concentration of a loose fitting casting when restoration care is delayed.

Factors affecting the clinical performance of cements:

The correct seating of a restoration is important to occlusal function,

esthetics and durability of the cement, especially in relation to securing the

thinnest set cement time between restoration and tooth.

Another factor that influence the material situation is the taper and

marginal geometry of the restoration.

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. As shown is Fig 1

typical prepared surfaces on a microscopic scale are rough that is there are

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

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are only point contacts along the peaks (Fig2). 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 purpose of a

cement is to fill this space completely. On 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 surfaces

and set to a solid from within a few minutes (Fig 3). The solid matter not

only seals the space but also retains the prosthesis. If the third material is

not fluid enough or is incompatible with the surfaces, voids can develop

around deep, narrow valleys (Fig 4) and undermine the effectiveness of the

cement.

Figure 1 Figure 2

Figure 3 Figure 4

Procedure for cementation of prosthesis: to be effective cement must be

fluid and be able to flow into continuous film of 25mm thick or less

without fragmentation. The procedure consists of placing the cements on

the internal surface of the prosthesis and extending slightly over the

margin, seating it on the preparation, and removing the excess cement at an

appropriate time. Cementation of a single crown as an example is described

with (Fig 5a).

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Placement of cement: The cement paste should coat the entire inner

surface of the crown and extend slightly beyond the margin. It should fill

about half of the interior crown volume (Fig 5b). the clinician should make

certain that the occlusal aspect of the tooth preparation is free of voids to

ensure that there is no air entrapment in the critical area during the early

age of the seating.

Seating: The important factors in seating the cemented restoration include

the rheology of the cement, the working time, the final film thickness and

the geometry of the gap through which the excess cement.

They may be suited to longterm temporary cementation of a loose

filling casting when restoration care is delayed must escape. The cement

should have a fluid consistency and along working time. The mix should

also wet tooth and restriction surface readily. In these respects, fluid

hydrophilic materials appear desirable. The flow or rheologic

characteristics of the cement mix are a function of the pressure and gap

size. The correct mixes of zinc phosphate, polycarboxylate, and EBA

cements flow on to low film thickness with moderate pressure under

practical conditions.

The data of Hoard et al using a model full crown die system showed

that the most fluid cement (zinc oxide eugenol) generated least hydraulic

pressures duirng seating followed by polycarboxylate with zinc phosphate

exhibiting greatest peak and residual hydraulic pressure.

Both Eames and associates and Hembree and Coworkers have

confirmed that venting is a satisfactory method of achieving minimal film

thickness under crowns. In addition to venting, provision of a 30mm relief

space or etching away the interior of the casting have been suggested.

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Eames et al found better seating of full crowns using 10 and 20°

convergence angles and recommended the most satisfactory technique for

allowing escape of cement to be a die relief method.

Moderate finger pressure should be used to displace excess cement

and to seat the crown or other prosthesis on the preparation. An

alternatively method is to use a vibrational instrument to facilitate the

seating of the prosthesis without creating excess pressure. After the

marginal gap area is evaluated for closure with an explorer the patient may

be asked to complete the seating by biting on a soft piece of wood which is

static method and a round stick rolling on the crown which is called as

dynamic method. During this stage, the last increment of excess cement is

expelled through the space between the prosthesis and the tooth. As the

prosthesis reaches its final position on the preparation. The space for

expelling the excess cement becomes smaller, making the seating more

difficult (Fig 5c). variable that can facilitate scaling include using a cement

of lower viscosity, increasing the taper and decreasing the height of the

crown preparation (Fig 5d) vibration, and introducing escape vents on the

occlusal aspect of the prosthesis (Fig 5e), increasing the degree of taper can

compromise retention, monomer the escape vents can be filled with gold

foil or cast gold plugs. If the occlusal surface contacts the axial wall of the

tooth during insertion, air pockets may be introduced (Fig 5f).

Removal of Excess cement:

The excess cement aluminates around the marginal area at the

completion of seating. Its removal depends on the properties of the cement

used. If the cement sets to a brittle state and does not adhere to the

surrounding surfaces, the tooth and the prosthesis, it is best removed after it

27

sets. This applies to zinc phosphate, silicophasphate, and ZoE cements. For

glass ionomer cements, polycarboxylate cements and resin based cements

that are potentially capable of adhering both chemically and physically to

the surrounding surfaces the protocol of excess cement removal varies.

One can coat the surrounding surface with a separating medium such as

petroleum jelly, thereby inhibiting the materials adherence to the surfaces,

and remove the excess after the cement sets. Another technique involves

the removal of excess cement as soon as the seating is completed, thus

preventing the material from adhering to the adjacent surfaces.

Post cementation

Aqueous based cements continue to nature over time well after they

have passed the defined setting time. If they are allowed to nature is 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 resistance to dissolution. It is

recommended that coats of varnish or a bonding agent should be placed

around the margin before the patient is discharged.

Mechanism of retention:

A prosthesis can be retained by mechanical or chemical means or a

combination of mechanical 6 mechanical factors.

As we know the retention of crowns, bridges is a function not only

of the mechanical properties of the luting agent but also the design of the

tooth preparation and the restoration. There factors influence the stress

distribution within the interposed cement layer, the efficiency of bonding

of the cement to both of the surfaces being joined, and the durability of the

28

cement that include its long-term resistance to mechanical breakdown and

dissolution.

Analysis of the stress distribution in the restored tooth indicate that

compressive shear and tensile forces are all generated in the cement layer.

Craig and Farah recommended that a cement with a high tensile

strength should be used for the cementation of crowns as shear stresses in

the marginal area can exceed the strength of low strength cements. For the

support of restorations, the tensile strength was again found to be

important, but the most important property was the elastic modulus of the

cements (stiffness). As previously noted, except for the resin cements, zinc

polycarboxylates tend to display the greatest tensile strength, but have

lower modulus and compressive strength than zinc phosphate cements. The

silicophosphate and glass ionomer cements tend to be superior in the latter

property to both these cements, but are better materials of lower tensile

strength, the EBA and resistance to plastic deformation.

Theoretically, chemical bonds can be resist interfacial separation

and thus improve retention. Aqueous cement based on polyacrylic acids to

provide chemical bonding through the use of acrylic acids. Resin based

cements using some specialty functional groups also have exhibited

chemical bonding. Cavity varnish reduces retention for all cements.

Improved mechanical and adhesives retention is obtained for all cements

by careful clearing of the preparation to remove residual temporary cement

and all residues including cutting debris. Such cleansing may include

mechanical treatment (premier slurry) and chemical agents such as

detergent cleaners and EDTA. Such agents have yet to be fully optimized,

29

however, similarly the interior of restorations should be cleansed by sand

blasting or etching.

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 with in 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 (Fig 7). This space can be susceptible to

plaque accumulation and recurrent caries; therefore, the margin should be

protected with a coating (if possible) to allow continues setting of the

cement. There are two basic modes of failure associated with cements.

Cohesive fracture of the cement (Fig 8a) and separation along the interface

(Fig 8 b). 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.

To summuries the factors of retention of fixed prosthesis.

1. The film thickness beneath the prosthesis

should be be thin. It is believed that a thinner film has fewer internal

flows compared with a thicken one.

2. The cement should have high strength values.

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3. The dimensional changes occurring in the

cement during setting should be minimized, hence isolate the cement

immediately after removal of the excess.

4. 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 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

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

the failure often occurs cohesively through the cement itself. The

prosthesis become loose only when the cement fracturer or dissolves.

Fig: Failure modes of the interface, A) Clearage through the cement

layer. This is unlikely because of the dimension of the cement involved. B)

The most likely failure that occurs at the cement prosthesis and cement

tooth interfaces. Remnants of the cement often remain on the opposing

surfaces.

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Summary

It is evident that none of the materials available to use at present is

free from deficiencies in the required clinical characteristics such as

biocompatibility, ease of manipulation, satisfactory sealing and retentive

properties, and long term stability. Thus, a proportion of clinical failure is

inevitable. This incidence can minimized by proper selection and

manipulation of the cement as previously outlined, however, the two

principal modes of failure for the cement lute (namely, dissolution,

including erosion and disintegration, and mechanical breakdown) are both

dependent on the clinical situation as well as on the intrinsic properties of

the cement. Factors within the control of the clinician, such as the desing of

the preparation, the fit of the restoration the manipulation of the cement,

the seating of the restoration and the finishing of the margins are some of

the important determinants of success.

A more rational approach to cement selection manipulation and

cementation procedures can give us improved postoperative results and

greater average longevity of the restoration. However the development of

new and improved cement systems with higher strength and stiffness and

lower oral dissolution is required. Adhesive and anticariogenic properties

are also desirable. More research is needed on cement performance in

clinical practice for both simple and complex restorative procedure to

develop a predicture correlation between laboratory measurements and

clinical performance. Improved laboratory and clinical characterization of

cements should lead us to the goal of an adhesive biocompatible cement

that will last as long as the restoration.

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CONTENTS

1. Introduction

2. Basic Consideration

3. Types of cement

4. Factors affecting the clinical performance of cements

a. Characteristics of abutment prosthesis interface

b. Procedure for concentration of prosthesis

c. Placement of cement

d. Seating

e. Removal of excess cement

f. Post cementation

g. Mechanism of retention

h. Dislodgement of prosthesis

5. Summary

33