LUTING AGENTS FOR FIXED PROSTHODONTICS

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;

provisional or interim restorations; laminate veneers for anterior teeth;

orthodontic appliances; and pins and posts used for retention of

restorations. The long-term clinical outcome of fixed prosthodontic

treatment depends, in part, on the use of adhesives that can provide an

impervious seal between the restoration and the tooth. Schwartz et al in

1970 found that loss of crown retention was the second leading cause of

failure of traditional crowns and fixed partial dentures. Therefore the

clinical success of these luting agents depends on the cementation

procedure and clinical handling of these materials.

The word ‘luting’ is often used to describe the use of a moldable

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

There are several types of available luting agents, each possessing

unique properties and handling characteristics. No one product is ideal for

every type of restoration; some of them requiring multiple technique

sensitive steps. Although the establishment of optimal resistance and

retention forms are obtained from proper tooth preparation, luting agents

should essentially serve the following purposes:

a) Act as a barrier against microbial and the restoration.

b) Seal the interface between the tooth and the restoration.

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c) Hold the restoration and tooth together through some form of

surface attachment.

This attachment may be mechanical, chemical or a combination of both

methods.

Ideal requirements of luting agents:

i) Should provide a durable bond between dissimilar materials.

ii) Should possess favourable compressive and tensile strengths.

iii) Should have sufficient fracture toughness to prevent

dislodgement as a result of interfacial or cohesive failures.

iv) Should be able to wet the tooth and the restoration.

v) Should exhibit adequate film thickness and viscosity to ensure

complete sealing.

vi) Should be resistant to disintegration in the oral cavity.

vii) Should be tissue compatible.

viii) Should demonstrate adequate working and setting times.

Presently there are cements used for the temporary or permanent

cementation of fixed prosthesis. They are:

1) Zinc phosphate.

2) Zinc oxide-eugenol.

3) Zinc oxide-non-eugenol.

4) Zinc polycarboxylate.

5) Glass ionomer Type I.

6) Resin composite cements or compomers.

7) Resin-modified glass ionomer or hybrid ionomers.

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1) Zinc phosphate cement

ADA specification No. 8 in 1935 defines the properties and requirements for

zinc phosphate cement.

Powder Weight (%)

Zinc oxide (ZnO) principal ingredient. 90.2

Magnesium oxide (MgO) reduces the

temperature of the calcination process.

8.2

Silicon dioxide (SiO2) inactive filler and aids in

the calcination process

1.4

Bismuth trioxide (Bi2O3) imparts a smoothness

to the freshly mixed cement in large amounts it

may also lengthen the setting time.

0.1

Barium oxide (BaO), Barium sulphate

(Ba2SO4)

Calcium oxide (CaO)

0.1

Tannin-fluoride may be added in some commercial products.

The ingredients of the powder are heated and sintered at

temperatures between 1000°C and 1400°C into a calcined mass, that is

subsequently pulverized to a fine powder which is sieved to recover

selected particle sizes.

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Liquid Weight (%)

Phosphoric acid (H3PO4) (free acid) 38.2

Phosphoric acid combined with aluminium and

zinc Al and Zn partially neutralize the acid,

temper its reactivity and act as buffering agents

which helps in establishing a smooth,

nongranular, workable cement mass.

16.2

Aluminium (Al) 2.5

Zinc (Zn) 7.1

Water H2O 36.0

Controls the setting time and mechanical properties

The water content controls the ionization of the acid and influences

the rate of setting reaction. This is important to the clinician because an

uncapped liquid bottle will permit loss of water resulting in retarded set.

Setting reaction

When powder particles are wet by the liquid, phosphoric acid

attacks the surface of the particles and releases zinc ions into the liquid.

The resultant mass yields a hydrated, amorphous network of zinc

aluminophosphate gel on the surface of the remaining portion of the

particle. The set cement is a cored structure consisting primarily of

unreacted zinc oxide particles embedded in a cohesive amorphous matrix

of zincaluminophosphate. In presence of excess moisture formation of

crystalline hopeite (Zn3 (PO4)2 4H2O) takes place.

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Manipulation

a) No definite P/L ratio and maximum amount of powder should be

incorporated into the liquid.

b) A cool mixing slab should be employed. The cool slab prolongs the

working and setting times.

c) The liquid should not be dispensed until the mixing is to be initiated

to prevent loss of water.

d) Mixing is initiated by incorporation of small portions of powder into

the liquid over a wide area to minimize the heat and effectively

dissipate it.

e) Spatulate each increment for 15 seconds before adding another

increment.

f) Completion of the mix usually requires approximately 1 minute 30

seconds.

g) The casting must be seated immediately with a vibratory action

before matrix formation occurs.

h) After the casting has been seated, it should be held under pressure

until the cement sets to minimize air inclusion.

i) The procedure should be carried out in a dry, clean environment.

j) Excessive cement should be removed after it has set and a layer of

varnish should be applied to the margin to decrease the initial

dissolution.

k) Frozen Glass slab Method

In this method a glass slab cooled at 6°C or at –10°C is used.

Around 50% to 75% more amount of powder can be incorporated into the

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liquid. The working and setting times are prolonged with little difference in

physical and mechanical properties.

Properties

Working and setting times

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

viscosity of the mix is low enough to flow readily under pressure to form a

thin film.

Setting time mean that matrix formation has reached a point where

external physical disturbance will not cause permanent dimensional

changes. It is defined as the elapsed time from the start of mixing no longer

penetrates the cement as the needle is lowered onto the surface.

Net setting time is 2.5 to 8.0 minutes at 37°C and 100% humidity it

varies from 5-9 minutes.

Factors influencing the setting time:

Those controlled by manufacturer:

- Powder composition.

- Degree of powder calcinations.

- Particle size.

- Buffering of liquid.

- Water content of liquid.

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Those controlled by the operator and their influence on selected

properties:

Manipulative variables

Compressive strength

Film thickness Solubility Initial

aciditySetting

time

Decreased P/L ratio Decrease Decrease Increase Increase Lengthen

Increased rate of powder

incorporationDecrease Increase Increase Increase Shorten

Increased mixing

temperatureDecrease Increase Increase Increase Shorten

Water contamination Decrease Increase Increase Increase Shorten

Physical properties

When properly manipulated, the set cement exhibits a compressive

strength of 104MPa, diametral tensile strength of 5.5MPa, modulus of

elasticity of 13GPa. Thus it is quite stiff and resistant to elastic deformation

even when it is used for cementation of restorations in high stress-bearing

areas. The strength is influenced by P/L ratio, composition, manner of

mixing and handling of the cement.

Solubility and disintegration

The solubility in water in 24 hours is 0.2%. Solubility depends on

initial exposure to water of the incompletely set cement resulting in

increased dissolution. Greater resistance to solubility can be obtained by

increasing the P/L ratio.

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Consistency and film thickness

Two consistencies are used i.e. luting and base. The luting

consistency is tenacious and provides a mechanical interlocking between

the surface irregularities of the tooth and the restoration. The maximum

film thickness is 29µm. It depends on the consistency and seating pressure.

Viscosity

Viscosity increases with increased P/L ratio, mixing time and higher

temperature. Increased viscosity can result in increased film thickness and

incomplete seating.

Dimensional stability

It exhibits shrinkage on hardening ranging from 0.04% to 0.06% in

7 days.

Thermal and Electrical conductivity

It is an effective thermal insulator and protects against thermal

trauma to the pulp.

Acidity

The acidity of the cement is quite high at the time of cementation of

a prosthesis. Two minutes after the start of mixing the pH is approximately

2. It increases rapidly but still is only about 5.5 at 24 hours.

The pH remains relatively low for long durations.

Zinc phosphate cement does not chemically bond to tooth structure

and provides a retentive seal by mechanical means only. Thus, the taper,

length and surface area of the tooth preparation are critical to its success.

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Microleakage, aggravated by dehydration in oral fluids and an initial low

setting pH may affect its biocompatibility in clinical use.

Applications

Permanent luting of well-fitting, prefabricated and cast posts, metal

inlays, onlays, crowns, FPDs, and aluminous all-ceramic crowns to tooth

structure, amalgam, composite, or glass ionomer core build ups.

2) Zinc-oxide Eugenol and 3) Non-eugenol cements

Composition of Type I luting agent

Powder Weight (%)

Zinc oxide 69.0

White resin reduced brittleness of the set cement 29.3

Zinc state plasticizer 1.0

Zinc acetate improves strength 0.7

Liquid

Eugenol 85.0

Olive oil 15.0

To increase the strength of the cement for luting purposes, two

modifications have been made Type II luting agents:

a) Methyl methacrylate polymer is added to powder (20% by weight)

(Kalzinol).

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b) Alumina (Al2O3) (30% by weight) is added to powder and

ethoxybenzoic acid (EBA) is added to liquids (62.5% ortho EBA by

weight).

The non-eugenol cements contain an aromatic oil and zinc oxide.

Other ingredients may include olive oil, petroleum jelly, oleic acid, and

beeswax.

Setting reaction

The cement sets by chelation reaction to form eugenolate and water.

The presence of moisture is essential for setting to occur.

Manipulation

A paper mixing pad is used. A P/L ratio of 4-6:1 is employed. The

bulk of the powder is incorporated in the initial step, the mix is thoroughly

spatulated, and then a series of smaller amounts is added until the mix is

complete.

Mixing time required is usually 90 seconds. The reinforced cements

are kneaded for 30 seconds and then stopped for 60 seconds to develop a

creamy consistency.

Properties

Setting time ranges from 4 to 10 minutes. For reinforced cements,

since the P/L ratio increases, the setting time decreases.

Setting time depends on composition of powder, particle size, P/L

ratio, accelerator and temperature.

Physical properties

Type I luting cement has a compressive strength of 2.0-14MPa, non-

eugenol cements have values of 2.7-4.8MPa, polymer modified has a

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strength of 37MPa with EBA-alumina having the highest strength 64MPa.

Elastic modulus ranges from 0.22 for Type I, 2.7 for Kalzinol and 5.4

for EBA-alumina.

Solubility and disintegration

Due to the bleaching of eugenol, solubility is high and ranges

between 1.5 to 2.5%. addition of additives decreases the solubility. The

solubility in water (%) in 24 hours for polymer modified cements is 0.08

and EBA-alumina is 0.02-0.04.

Film thickness

Film thickness of polymer modified cement is 2.5µm and EBA

alumina is 25-35µm.

Dimensional stability

Shows a shrinkage of 0.9-2.5% on setting.

Biologic properties

It has a pH of 7-8.

It does not cause any harm to the pulp but due to leaching of

eugenol, it is an irritant. Therefore non-eugenol cements are used for some

patients.

Highly compatible with the pulp and has an obtundant effect.

It also has an antibacterial action.

Its disadvantages such as decreased strength, high solubility, irritant

to soft tissues, poor retention and difficulty in manipulation limit its use for

temporary cementation purposes.

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It should not be used for temporary luting purposes when the

permanent luting agent is likely to be a resin cement as the eugenol inhibits

polymerization.

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Applications

It is used primarily for temporary luting of restorations.

3) Sdadsds

4) Zinc polycarboxylate cement (Zinc polycarboxylate cement)

Composition

Powder Weight (%)

Zinc oxide 85

Magnesium oxide or stannic oxide 10

Stannous fluoride traces of silica dioxide,

bismuth, aluminium and colour pigments.

4-5

The powder is sintered and fused to reduce the reactivity of zinc

oxide.

Liquid

Aqueous solution of polyacrylic acid or copolymers of acrylic acid

in the range of 30-40%.

Molecular weight of 25,000 to 50,000.

Tannic acid and malleic acid 10-45%.

Tartaric acid prevents gelation on storage 5%.

Water settable cements – Here the mixing liquid is water – polyacrylic acid

is frozen, dried, powdered and mixed with the original P/L ratio of these

cements is very high.

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Setting reaction and adhesion to tooth structure:

When powder and liquid are mixed, a fast acid-base reaction occurs

as the powders are rapidly incorporated into a viscous solution of high

molecular weight polyacrylic acid. The powder particles are attacked by

the acid and zinc, magnesium and tin ions are released. Zinc ions react with

the carboxyl group of polyacrylic acid of the same chain and the adjacent

chain to cause cross linking. The calcium ions of the tooth structure react

with the free carboxyl groups of acid to form a metallic ionic bond.

The bond between cement and dentin is 3.4 MPa. Under ideal

conditions the adhesion of polycarboxylate cement to a clean, dry surface

of the tooth is greater than any other cement. The cement adheres better to

a smooth surface than to a rough one. It does not adhere well to gold and

porcelain. The failure is at the cement-metal interface. Cement cannot bond

to the metal in chemically dirty or pickled condition. Surfaces of the metal

have to be sandblasted or electrolytically etched to achieve optimum

bonding. Adhesion with stainless steel is excellent.

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Manipulation

P/L ration is 15:1.

A glass slate is used for mixing to prevent absorption of liquid.

Firstly a meticulously clean surface is essential to intimate contact and

interaction between the cement and the tooth. 10% polyacrylic acid

solution is used to clean the tooth surface for 10-15 seconds followed by

rinsing with water for removal of smear layer. After cleansing, isolate the

tooth to prevent further contamination by oral fluids. Blot the surface

before cementation.

The powder is rapidly incorporated into the liquid in large quantities

for a period of 30 to 60 seconds. Mixing on a cooled glass slab prolongs

the working time. The cement must be placed on the inner surface of

casting and on tooth surface before it loses its glossy appearance. Loss of

gloss indicates decreased availability of carboxyl groups, poor bonding,

poor wettability due to stringiness and increased film thickness causing

incomplete seating of the casting.

Precautions

Do not refrigerate the liquid dispense the liquid just before mixing.

Mixing should be rapid.

Use only on cleaned surfaces.

Use before glossiness disappears.

Removal of excess cement

During setting, the cement passes through a rubbery stage. During

this stage, excess cement should not be pulled away from the margins as it

can leave voids at the interface. Remove excess cement only after it

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becomes hard. Apply petroleum jelly to the outer surfaces of the prosthesis

and soft tissues to prevent cement from adhering to them.

Properties

Setting time is 6-9 minutes.

Working time 2.5 – 3.5 minutes.

Viscosity

The set mix is pseudoplastic in nature. The cement seems viscous,

but during cementation pressure, excess flows out from under the margins

of the restoration.

Physical and Mechanical properties

The compressive strength of 24 hour set cement is 57-99 MPa;

tensile strength is 3.6-6.3MPa and elastic modulus is 4.0-4.7GPa. Bond

strength to dentin is 2.1MPa, to enamel is 3.4-13MPa.

Film thickness of polyacrylate cements is 25-48 µm.

Solubility of cement in water is low (<0.05%) but when it is

exposed to organic acids with a pH of 4.5 or less, the solubility increases

markedly. Reduction of P/L ratio also increases solubility and

disintegration in the oral cavity.

Dimensional stability

They show a linear contraction when setting at 37°C.

Biologic properties

The pH of the cement liquid is 1.7, but the liquid is rapidly

neutralized by the powder. PH increases rapidly as the cement sets.

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Despite the initial acidic nature, these cements produce minimal

irritation to the pulp because of quick neutralization and the lack of tubular

penetration of the sized polyacrylic acid molecules.

This excellent biocompatibility with the pulp is one of the strongest

clinical merits of this cement.

Advantages

Low level of irritation and increased biocompatibility with the pulp.

Adhesion to tooth structure.

Easy manipulation.

Anticariogenic.

Thermal insultor

Hydrophilic and capable of wetting dentinal surfaces.

Disadvantages

If acute proportioning is not done, properties are affected i.e.

solubility and disintegration increases.

Working time is short.

Clean surface is required for adhesion.

Absorbs water and softens into a gel.

Increased solubility in acids.

Difficult to remove flash.

Failures occurs at cement-metal interface. After hardening,

polycarboxylate cements exhibit significantly greater plastic deformation;

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thus the cement is not well suited for use in regions of high masticatory

stress or in the cementation of long-span prosthesis.

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Application

Used for the cementation of single metal units in low stress areas on

sensitive teeth.

5) Type I Glass Ionomer Cement / ASPA or Aluminosilicate polycrylate / Alkeneate

Definition

Akinmade and Nicholson in 1993 defined glass ionomer cement as

“a water based cement wherein, following mixing, the glass powder and

the polyalkenoic acid undergo an acid base setting reaction”.

Mclean and Nicholson defined GIC as “a cement that consists of a

basic glass and an acidic polymer which sets by an acid base reaction

between these components”.

Composition

Powder

The basic component of a glass ionomer cement powder is a

calcium fuoroalumino silicate glass with a formula of:

SiO2-Al2O3-CaF2-Na3 AlF6-AlPO4

The nominal composition of the glass is listed below:

Chemical Weight (%)

SiO2

Al2O3

CaF2

Na3AlF6

AlF3

AlPO4

29.0

16.6

34.3

5.0

5.3

9.8

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The raw materials are fused together to a uniform flass by heating

them to a temperature of 1100°C. The glass is then ground to a powder

having particles in the range of 20 to 50µm. glasses high in silica are

transparent, whereas glasses high in calcium fluoride or alumina are

opaque.

Fluoride is an essential constituent of GIC:

- It lowers the fusion temperature.

- Improves the working characteristics.

- Increases the strength of the set cement.

- Contributes to anticarcinogenic property perhaps the

rationale for using GIC as a luting agent is based on

its ability to release fluoride ions into the underlying

dentin. This helps prevent secondary caries which is

the most cause of failure.

The powder is described as an ion-bleachable glass that is

susceptible to acid attack when the Si/Al atomic ratio is less than 2:1.

Cryolite is added to supplement the flexing action of calcium

fluoride and to increase the translucency.

Aluminium phosphate improves translucency and adds body to the

cement paste. Barium glass may be added to provide radiopacity.

Liquid

The liquid typically is 4.75% solution of 2:1 polyacrylic acid /

itaconic acid copolymer (average molecular weight 10,000) in water. The

acid is a polyelectrolyte, which is a homopolymer or copolymer of

unsaturated carboxylic acid known as alkenoic acids.

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The itaconic acid reduces the viscosity of the liquid and inhibits

gelation caused by intermolecular hydrogen bonding.

Intermolecular hydrogen bonding

Tartaric acid is present in 5%, as an optically active isomer and

serves as an accelerator by facilitating the extraction of ions from the glass

powder. Also tartaric acid prolongs the working time, improves handling

characteristics, enables fluoride contact of glass to be reduced and helps in

the production of bear glasses.

Water is the basic reaction medium and plays a role in hydrating

reaction products, that is metal polyalkenoate salts and silica gel.

Water settable GIC

To extend the working time, one GI formulation consists of freeze

dried acid powder and glass powder in one bottle and liquid components in

another. The chemical reaction procedures in the same way except that

these cements have a longer working time and a shorter setting time.

Setting reaction and adhesion to tooth structure

The cement sets hydroacid base reaction and consists of 2 stages.

The first occurs during the initial 5 minutes when the reaction between the

powder and the liquid forms a silaceous hydrogel. The second stage

requires about 24 hours and occurs when a polysalt matrix completely

surrounds all of the initial reaction products.

When the powder and liquid are mixed the following sequence of

events take place:

i) Polyacid attacks the glass to release calcium, aluminium, sodium

and fluoride ions.

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ii) These ions react with the polyanions to form a salt gel matrix.

iii) The polyacrylic acid chains are cross-linked by Ca++ in the first 3

hours.

iv) Subsequently aluminium ions react for atleast 48 hours.

v) The fluorides and phosphates form insoluble salts and

complexes.

vi) The sodium ions form a silica gel.

vii) Some of the sodium ions replace, the hydrogen ions of the

carboxyl groups and the rest combine with fluorine ions.

viii) The cross-linked phase is hydrated by water.

ix) The unreacted portion of glass particles are sheathed by silica

gel.

x) Thus, the set cement consists of an agglomeration of unreacted

powder particles surrounded a silica gel in an amorphous matrix

of hydrated calcium and aluminium polysalts.

xi) The glass ionomer chemically bonds to enamel and dentin. It

seems that bonding involves an ionic interaction with calcium

and/or phosphate ions from the surface of the tooth structure.

This results in chelation of carboxyl groups of the polyacids with

the calcium in the apatite of the enamel and dentin.

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xii) Role of water in the setting process

Water hydrates the cross-linked matrix, thereby increasing the

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

readily removed by dessication and is called loosely bound water. Also at

this stage the GI readily absorbs moisture into the glossy matrix resulting

in a compromised material. Therefore, any contact with saliva or oral fluids

has to be prevented for the first 24 hours to prevent early disintegration and

dissolution.

As the setting continues the water becomes tightly bound and cannot

be removed. This hydration is critical in yielding a stable gel structure and

building the strength of the cement.

Manipulation

The prepared tooth structure and the inner surface of the casting are

cleaned. The tooth surface is cleaned with a slurry of pumice, rinsed and

then dried but not dehydrated. Undue dessication opens up the dentinal

tubules, enhancing penetration of the acidic liquid.

A glass slab or a paper pad is used for mixing. A plastic spatula

should be used. Use of a metal spatula, causes abrasion by the glass

particles of the metal surfaces resulting in discoloration of the set cement.

P/L ratio for GIC Type I is 1.3 : 1.

The powder is introduced into the liquid in large increments and

spatulated rapidly for 30 to 45 seconds. Encapsulated products typically are

mixed for 10 seconds in a mechanical mixer and dispensed directly. Hand

mixed cements often contain bubbles of larger diameter, which may

contribute to a decrease in strength.

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The cement must be used before it loses its glossy apperance. The

field must be isolated completely. Once the cement has achieved its initial

set (7 minutes), the cement margins should be coated with a varnish.

Precautions:

Tooth should be conditioned.

Should be protected from moisture and drying during setting.

Should be used before loss of glossy appearance.

Flash should be removed only after cement hardens.

Properties :

Setting time of GIC is within 6-8 minutes from the start of mixing.

Physical and mechanical properties.

The 24 hour compressive strength of GIC ranges from 93-226MPa,

tensile strength being 4.2-5.3MPa and elastic modulus of 3.5-6.4 GPa.

Strength increases between 24 hours and 1 year and is significantly

increased by initial protection from moisture. Low values of elastic

modulus make them susceptible for elastic deformation increase of high

masticatory stress.

The bond strength of GIC to dentin is 3-5MPa. The bond strength

can be improved by treatment of the dentin with an acidic leaching agent

followed by an application of a dilute aqueous solution of FeCl3. The GIC

bond well to enamel, stainless steel, tin-oxide plated platinum and gold

alloy.

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Solubility and disintegration

A 24 hours solubility for GIC in H2O is 0.4-1.5% solubility is less in

acidic solutions and also depends on initial exposure to water.

Film thickness for GIC is 22-24µm.

Biologic properties:

i) Resistance to microleakage

The cement bonds adhesively to tooth structure and prevents ingress

of fluids at the interface. This is probably because the coefficient of

thermal expansion of GIC is similar to that of the adjacent tooth structure

particularly the dentin.

ii) Anticariogenic due to the release of fluoride ions.

iii) Post cementation sensitivity

This is related to the pH and the length of time that this acidity

persists. The pH of the mix at 2 minutes after mixing is 2.33 and it

increases upto 5.67 in 24 hours but never reaches neutral pH. Also if the

tooth is excessively dehydrated before cementation, the tubules open up

allowing acids to seen through. If the crown is overfilled, the excessive

hydraulic pressure required to remove excess cement caused sensitivity.

Advantages

i) Chemical adhesion to tooth structure.

ii) Anticariogenic.

iii) Esthetic properties.

iv) Ease of manipulation.

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v) They possess low film thickness and maintain relatively constant

viscosity for a short time after mixing. This results in improved seating

of cast restorations.

Disadvantages

i) Low film thickness can cause inhomogenous distribution of

curing stresses and microcracks resulting in cementation failure.

ii) Low elastic modulus

iii) Susceptibility to moisture attack and subsequent solubility if

exposed to water during the initial setting period.

iv) Early exposure to moisture and saliva decreases the ultimate

strength.

v) Susceptibility to dehydration and cohesive failure due to

microcracks.

vi) Post cementation sensitivity.

Applications

Used as permanent luting agent for cast posts, metal inlays, onlays,

crowns, FPDs and all-ceramic crowns to tooth structure, amalgam,

composite core build ups.

6) Resin Composite Cements

Resin cements are variations of filled BIS-GMA resin and other

methacrylates.

Composition

The early resin cements were primarily poly(methylmethacrylate)

powder with inorganic fillers and methyl methacrylate liquid. The

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composition of most modern resin based cements is similar to that of

composite resin materials. Because most of the 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

include organophosphonates, hydroxyethyl methacrylate (HEMA) and the

4-methacryloxyethyl trimellitic anhydrite (4-META) system.

The phosphonate cements also contain a silanated quartz filler. The

phosphonate is very sensitive to oxygen, so the margins of the casting have

to be protected until setting has occurred. The phosphate end of the

phosphonate reacts with calcium of the tooth or with a metal oxide.

The 4-META cement is formulated with methyl methacrylate

monomer and acrylic resin filler and is catalyzed by tri-butyl borax.

They polymerize through conventional peroxide amine induction

systems (chemically initiated polymerization) or by photoinitiation or a

combination of both (dual-cure systems).

Manipulation

The chemically activated systems are available in powder-liquid

system or as two paste systems. The peroxide initiator is in one component

and the amine actiator is contained in the other. The components are mixed

on a paper pad for 20-30 seconds. The restorations should be promptly

seated and excess cement should be removed immediately.

Light activated systems are single component systems. The time of

exposure to light needed for polymerization of the resin cement is

dependent on the light transmitted through the ceramic restoration. It

should never be less than 40 seconds.

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The dual cure systems are 2-component systems. The part of the

cement that does not come in contact with the light source is cured

chemically.

Adhesion to tooth structure

i) Application of an acid or dentin conditioner to remove the

smear layer and smear plus.

ii) The tubules are opened and widen with demineralization of

the top 2 to 5µm of dentin.

iii) The acid dissolves and extracts the apatite mineral phase

that normally covers the collagen fibres of the dentin matrix and

opens 20 to 30nm channels around the collagen fibres. These channels

provide an opportunity to achieve mechanical retention of

subsequently placed hydrophilic adhesive monomers. If application of

the conditioner exceeds 15 seconds, a deeper demineralized zone

results which resists subsequent resin infiltration. If complete

infiltration of the collagen by the primer does not occur, the collagen

at the deeper demineralized zone will be left unprotected and

subjected to future hydrolysis and final breakdown.

iv) After demineralization, the primer, a wetting agent such as

HEMA is applied.

The agent is bifunctional, in that it is both hydrophilic, which

enables a bond to dentin, and hydrophobic, which enables a bond to the

adhesive. The primer is applied in multiple coats to a moist dental surface.

Multiple coats are required to replace the water in the damp dentin with the

resin monomers and to carry the adhesive material into the tubules. The

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primer is gently dried so as not to disturb the collagen network but to

remove any remaining organic solvents or water.

v) Adhesive resin is then applied to the “primed” surface to

stabilize the primer infiltrated demineralized dentin.

Retention is achieved by the following means:

Infiltration of resin into etched dentin, producing a micromechanical

interlocking with the open tubules forming resin tags; which

underlies the hybrid layer of resin interdiffusion zone.

Adhesion to enamel through the micromechanical interlocking of

resin to the hydroxyapatite crystals and rods of etched enamel.

Adhesion to dentin, involving penetration of hydrophilic monomers

through a collagen layer overlying partially demineralized apatite of

etched dentin.

vi) The use of dentin bonding agents has somewhat

compensated for the polymerization shrinkage evident with all resins.

Properties

Their properties on dependent on compositional differences,

amounts of diluent monomers and filler levels.

Setting time 4-5 minutes at 37°C.

Compressive strength 52-224MPa.

Tensile strength 37-41 MPa.

Elastic modulus 1.2-10.7 GPa.

Bond strength to dentin 11-24 MPa with bonding agent.

Virtually insoluble in oral fluids

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Film thickness 13-20µm.

Exhibits polymerization shrinkage which is an impediment to

complete dentinal adhesion.

They are pulpal irritants due to the presence of bleachable

monomers.

Bond strength in Tension MPa

Substrate Resin cement

Dentin (unetched)

Enamel (etched)

Ni-Cr-Be alloy

Sandblasted

Electrolytically etched

Type IV Gold alloy

Sandblasted

Tin-plated

4%

15.0

24.0

27.4

22.0

25.5

Advantages

a) Resin cements bond chemically to resin composite restorative

materials and to silanated porcelain. They increase the

fracture resistance of ceramic materials that can be etched

and silanated.

b) They demonstrate good bond strengths to sandblasted base

metal alloys, the 4-META resin cements show strong

adhesion as a result of chemical interaction of the resin with

an oxide layer on the metal surface. Noble alloys may be

electroplated with tin to increase the surface area for bonding

and enable a chemical bond with tin oxide.

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c) Most resin adhesives are filled, 50% to 70% by weight, with

glass or silica due to which they exhibit high compressive

strength, resistance to tensile fatigue and virtual insolubility

in the oral environment.

d) Improved marginal wear resistance.

e) Some of the formulations contain ytterbium tori fluoride,

other include a barium fluorosilicate filler and have fluoride

release and cariostatic potential.

f) Offer adequate retention for short, tapered crown

preparations.

Disadvantages

a) High filler content increases viscosity, which reduces flow

and increases film thickness and chances of incomplete

seating of the restoration.

b) polymerization shrinkage.

c) Irritant to the pulp.

Their ability to adhere to multiple substrates high strength,

insolubility and shade matching potential have made them the adhesives of

choice for cementation of the following:

Resin composite inlays and onlays.

All-ceramic inlays and onlays.

Veneers, crowns, FPDs.

Fiber reinforced composite restorations.

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Luting base metal resin bonded bridges (“Maryland”

type).

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7) Resin Modified Glass Ionomers / Hybrid Ionomers

To overcome inherent drawbacks of GIC such as moisture

sensitivity and low early strength, polymerizable functional groups have

been added to the formulations to impart additional curing processes and

allow the bulk of the material to mature through acid-base reaction. This

group of materials are also known as light cured GICs, dual cure GICs

(light cure and acid base reaction), tri-cure GICs (dual cure and chemical

cure), resin ionomers, compomers and hybrid ionomers.

Composition and setting reactions

The powder consists of ion-bleachable glass and initiators for light

or chemical curing or both. The powder blends is formed of glass, tartaric

acid and polyacrylic acid.

The liquid component may have only water or polyacrylic acid

modified with HEMA monomers and methacrylate monomers. They

contain hydroxyl groups that make them water soluble. These are the

simplest form of resin ionomers.

They are mixed in the same way as conventional GICs and remain

workable for 10 or more minutes provided they are not exposed to light.

The reaction is dual-setting once exposed to light.

a) Acid base reaction : Calcium fluoroalumino silicate glass (base) and

polyacrylic acid = calcium and aluminium polysalt hydrogel.

b) Free radical or photochemical polymerization HEMA and

photochemical initiator / activator

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Poly Hema Matrix

Thus two matrices are formed; a metal polyacrylate salt and a

polymer. The initial set is a result of polymerization of HEMA. The acid

base reaction serves only to harden and strengthen the already formed

polymer matrix.

Class I materials

Composition

a) Powder component: Calcium fluoroalumino silicate glass,

polyacrylic acid and tartaric acid.

b) Liquid component (replaces water): Water /HEMA, other

difunctional hydroxydimethacrylates (such as ethyleneglycol

dimethacrylate) and bis-GMA.

c) Initiator / Activator.

Chemically polymerized materials:

Initiator Hydrogen peroxide.

Activator Ascorbic acid.

Co-activator Cupric sulphate.

Light activated materials

Visible light photochemical initiator Camphorquinone

Activator Sodium p-toluenesulphinate

Photoaccelerator ethyl 4-N n-dimethylaminobenzoate.

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The two matrices do not interpenetrate but form separate phases. To

prevent phase separation. Class II materials (Vitrebond) have been

formulated.

In this material, polyacrylic acid (PAA) is replaced by modified

PAAs.

In these modified PAAs, a small percentage of –COOH is converted

to pendant unsaturated groups by a condensation process. They are

condensed with methcrylate polymers to have terminal –COOH and –CH3

groups.

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The Class II materials uses an aqueous solution containing 25% to

45% of modified PAA and 21% to 41% HEMA along with initiator system

of camphorquinone and diphenliodonuim chloride with a glass of the

following percentage composition : SiO2 26.84%, Al2O3 0.80%, P2O5

0.94%, NH4F 3.32%, AlF3 20.66%, Na3AlF6 10.65%, ZnO 20.66%, MgO

2.12%, SrO 12.55%.

On mixing and activation by light, the HEMA polymerizes to form

poly HEMA.

The modified PAA copolymerizes with HEMA; thus poly HEMA

will be chemically linked to the polyacrylate matrix and phase separation

will not occur. Also the modified PAA further polymerizes to form a cross-

linked PAA which increases the strength of the cement.

The matrix of such a cement will contain both ionic and covalent

crosslinks.

Properties

Compomers have both advantages and disadvantages compared to

conventional GICs.

They have improved setting characteristics. There is a longer

working time because HEMA slows the acid-base reaction, and yet, they

set sharpely once the polymerization reaction is initiated by light. They are

also resistant to early contamination by water because of the formation of

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an organic matrix and so do not require protection by varnish. This

combination of properties is clinically appealing.

These cements have compressive and diametral tensile strengths

greater than zinc phosphate, polycarboxylate and GIC but les than resin

composite.

24 hours in MPa

Class I Class II

Compressive strength 94 53-96

Flexural - 25.5

Tensile 21.9-33.9 11.2-12.4

Adhesion (dentine) 47 6.2-11.3

Their adhesion to enamel and dentin, and their fluoride release

pattern is similar to GIC. They also bond to resin composite. They have

cariostatic potential and show resistance to marginal leakage. The biggest

advantage is ease of mixing and use, because multiple bonding steps are

not required. They also have adequately low film thickness (10-22µm).

They have a bond strength to dentin of about 10-12MPa without bonding

agent and 14-20MPa with bonding agent.

A significant disadvantage of the resin ionomers is hydrophilic

nature of polyHEMA which results in increased water resorption and

subsequent plasticity and hygroscopic expansion. Although initial water

sorption may compensate for polymerization shrinkage stress, continual

water sorption has deleterious effects. Potential for substantial dimensional

change contraindicates their use with all-ceramic feldspathic-type

restorations.

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Their use for cementing posts in non-vital teeth is questionable

because of the potential for expansion induced root fracture.

They lack translucency and the presence of free monomers in the

freshly mixed cement presents concerns regarding biocompatibility.

Dimetracrylates may elicit allergic response and therefore careful handling

by dental personnel is required during mixing.

It is known that eugenol containing materials inhibit the cross-

linking of resin adhesives. They should not be used for final cementation

when the luting agent for interim restoration has been eugenol containing

provisional materials.

Applications

Luting metal or porcelain fused-to-metal crowns and FPDs to tooth,

amalgam, composite resin or glass ionomer core build ups.

Summary and 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

preparation retention and resistance forms or ill-fitting, low strength

restorations. Practitioners must be aware of the virtues and shortcomings of

each cement type and select them appropriately.

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