Requirements of Dental Cements for Lining Applications

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Requirements of DentalCements for Lining

ApplicationsSAJID KHAN

1

st

yr BDS


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CONTENTS

1. Introduction2.Why are Liners

required?3. Requirements of Liners

4. Purpose of Lining Thermal Barrier

Chemical Barrier

Electrical Barrier

Strength & Flow

Radiopacity &Compatibility

5. References


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Introduction

The American Dental Associationand the International StandardsOrganization (ISO) have teamed upto classify dental cements according

to their properties and their intendeduses in dentistry.

Type I

Luting agents thatincludepermanent andtemporarycements.

Type II

Restorativeapplications.

Type III

Liner or baseapplications.


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Why Are Liners Required?

Certain filling materials are not suitable forplacing directly into a freshly prepared cavity.In such circumstances, a layer of cavity liningmaterial is placed prior to the placement offilling


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The requirements of cavity lining

material chosen for any specificapplication depend on:

1. The depth of the cavity (determines thethickness of residual dentin between the baseof the cavity and the dentinal pulp)

2. The type of the filling material (being used torestore the tooth)


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Purpose of the Cavity LiningIs to act as a barrier between the filling materialand the dentin which by virtue of dentinal tubuleshas direct access to the sensitive pulp.

A lining is expected to provide Thermal Barrier Chemical Barrier

Electrical Barrier

In addition, the cavity lining must have sufficientmechanical strength and provide a firm and rigidbase to support the filling above it.


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Thermal Barrier

The insulating properties of

cements are characterizedby its virtue of thermalconductivity or thermaldiffusivity


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The lining is expected to form a

thermal insulating barrier to in orderto protect the pulp from sudden

intolerable changes in temperature.


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Thermally insulating cavity lining is

particularly required when a metallicfilling is used.

The thermaldiffusivity value foramalgam is 40 timesgreater than that fordentin.


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In deep cavities, there is danger of

thermal shock to the pulp when thepatient takes hot or cold food.

A layer of insulatingcavity lining materialof sufficient thicknesshelps to prevent this.


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In thin cavities the use of cavity

varnish is used beneath the metallicfillings

For amalgam restorations in shallow cavities,

the use of thick layer of lining may reduce thethickness of the overlying amalgam to such anextent that it becomes weakened and liable tofracture.

Hence, such shallow cavities are lined withvarnish.


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VarnishA solution of a natural or synthetic resin in a volatilesolvent.

It is painted onto the cavity and the solvent evaporates toleave a thin layer of resin.

The thin layer of resin helps to seal the ends of thedentinal tubules.

Although varnish does not provide an adequate thermalprotection in deep cavities. However, the resin in thevarnish swells in the presence of water and may establisha marginal seal around the metallic restoration.


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Another potential cause of thermalinjury to the pulp is through the

considerable amount of heat liberated

by certain filling materials duringsetting.

Like acrylic resin, can give a

temperature of 10C or more forsmall cavity while some light

activated Composite materialscan show a transient increase of15C for an average sized cavity.


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One complicating factor is that many

of the lining cements them selves setby an exothermic reactionE.g. Zinc PhosphateCement


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Chemical Barrier

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Cavity lining materials may be

required to form a protective barrier

against potential chemical irritants

present in some filling materials.

Phosphoric acid in Silicate and Zinc

phosphate cements and acrylic monomersin some resin based materials are suchpotential irritants


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It is now accepted that the dentin and

pulp are able to survive contact withquite powerful acids ( 37 % phosphoric

acid) providing that access to the pulp

is effectively sealed at the end of thecourse of the treatment.


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Electrical Barrier


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When two dissimilar metals

are placed adjacent to oropposing each other (e.g.

amalgam/gold) it is

possible to set a galvaniccell which not only

accelerates corrosion but

can cause pain.

The use of electrically insulating cavity

lining helps to prevent such activity.


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Unfortunately, most of the liningmaterials used are either water basedor contain polar Organometalliccompounds.They are not, therefore, idealelectrical insulators


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Varnishes can be used to provide someelectrical resistance

Varnishes are

sometimes paintedonto the surfaces ofthe metallicrestorations giving

temporary relief to thesymptoms of thegalvanic pain


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Strength & Flow


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Setting of Lining before placement of

FillingMost of the cavity lining materials are supplied as two componentswhich are mixed together, initiating a setting reaction.

The setting characteristics should allow sufficient time for mixingand placing the material in the cavity followed by rapid setting inorder that the filling material can be placed with out too muchdelay.

The cavity lining material should remain intact during theplacement of the filling material.

When amalgam filling is being placed it is important to ensure that

the lining is fully set or there is every possibility that the materialwill undergo considerable flow due to the high pressures used.

If the lining material has set at the time of amalgam condensationthere is little chance of flow.

The strength of the set lining material should be sufficient to resistfracture.


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The integrity of the cavity lining

material depends on several factors:1. The degree of the set achieved at the time the filling

material is placed.

2. The strength of the set material and its thickness.

3. The type of the cavity.

4. The pressure exerted during the placement of the filling

material.

5. The degree of support from the surrounding structures.

6. The choice of correct operative techniques.

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Radiopacity & Compatibility

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Radiopacity

Cavity lining material should ideally beradiopaque in order that the dentist can observethem on the radiographs, thereby aiding thediagnosis of caries around the filling.

A radiolucent lining material may hinder the

early detection of such lesions.

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CompatibilityThe lining material must be compatible with thefilling materials.

The contents of the lining materials should not haveany effect on the setting characteristics or propertiesof the filling

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References

Applied Dental Materials(ninth edition)John F. McCabe and Angus W.G Walls

Images and figures credited to www.google.com

Documents

Requirements of Dental Cements for Lining Applications