DENTAL CEMENTS AS LUTING AGENTS

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Numerous dental treatments necessitate attachments of indirect restorations and appliances to the teeth by means of a cement. These include metal ,metal- ceramic, and ceramic restorations provisional restorations, laminate veneers for anterior teeth , orthodontic appliances, pins and posts used for retention of restorations .

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Characteristics of Abutment -Prosthesis Interface

When two relatively flat surfaces brought into contact, space exists on microscopic scale, there are peaks and valleys

There are only point contacts along the peaks

Main purpose of cement is to fill this space completely

If cement is not fluid enough voids can develop around deep valleys

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Procedure for cementation of prostheses

To be effective a Type-I cement must be fluid and be able to flow into a continuous film of 25 um thick or less without fragmentation

The cement paste should coat the entire inner surface of the crown and extend slightly beyond the margin. It should fill about half of interior crown volume

Moderate finger pressure should be used to displace excess cement and to seat the crown

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Variables that can facilitate seating of prosthesis include –

Using a cement of lower viscosity Increasing the taper of preparation Decreasing the height of crown

preparation Vibration

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Removal of excess cement Removal of excess cement depends on

properties of cement used – If cement sets to a brittle state and does not

adhere to surrounding surfaces,tooth and prosthesis, it is best removed after it sets . This applies to Zinc phosphate and ZOE cements

For cements which are capable of adhering chemically like GIC, polycarboxylate and resin cement ,coat surrounding surface with petroleum jelly and remove excess as soon as seating is completeds

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Dislodgment of prosthesis

Recurrent caries may occur Disintegration of cement can result from

fracture or erosion of cement In oral environment , cement layer near

the margin can dissolve and erode leaving a space. This space can be susceptible to plaque accumulation and recurrent caries

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Loss of cement at marginal area resulting from exposure to oral fluid

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CHOICE OF LUTING AGENT

An ideal luting agent has-Long working time Adheres well to both tooth structure and cast

alloys Provides a good seal Is nontoxic to the pulp Has adequate strength properties Has low viscosity and solubilityExhibits good working and setting characteristicsAny excess can be easily removed

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ZINC PHOSPHATE CEMENT

It is the oldest of cementation agents It serves as standard by which newer

systems can be compared Zinc phosphate cement is still the luting

agent of choice for otherwise normal, conservatively prepared teeth

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Working time- 5 min Setting time-5-9 min Manipulation-a cool mixing slab should

be employed ,cool slab prolongs the working time

Recommended powder:liquid ratio 1.4 gm powder to 0.5ml liquid

Mixing is initiated by addition of small amount of powder with brisk spatulation

A considerable area of mixing slab should be used ,this reduces the amount of heat generated.

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Two mixes of cement prepared with identical powder liquid ratio. Temp in A is 18 degree C. temp in B is 29.5 degree C

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Physical properties – Compressive strength -104 MPa Modulus of elasticity -13 GPa It is quite stiff and resistant to elastic

deformation even when employed for cementation of restorations that are subjected to high masticatory stress

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Retention Primary bonding occurs by mechanical

interlocking at interfaces and not by chemical interactions

Biologic properties The acidity of cement is quite high at the time

when prosthesis is placed on prepared tooth The pH then increases rapidly but still is only

about 5.5 at 24 hours Thus if underlying dentin is not protected against

infiltration of acid via dentinal tubules pulpal injury may occur

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ZINC POLYCARBOXYLATE CEMENT

It was the first cement system that developed an adhesive bond to tooth structure

This agent is recommended on retentive preparartions when minimal pulp irritation is important e.g. in children with large pulp chambers

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If good bonding to tooth structure is to be achieved ,the cement must be placed on the tooth surface before it loses its glossy appearance. The glossy appearance indicates a sufficient number of free carboxylic acid groups on the surface of mixture that are vital for bonding to tooth structure

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Consistency of polycarboxylate on completion of 30 sec mix

Prolonged mixing or mix is allowed to remain on slab cement becomes dull and consistency becomes s tacky

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Working time –much shorter than for zinc phosphate ,2.5 min

Setting time – ranges from 6-9 min Manipulation- P:L ratios - 1.5 parts of powder to 1 part

of liquid Cool the slab and powder but under no

circumstances should the liquid be cooled Powder is rapidly incorporated into liquid

in large quantities.30 sec mixing time is adequate

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Physical Properties – Compressive strength -55 MPa Modulus of elasticity - 5.1 GPa At manufacturers’ recommended powder:

liquid ratios ,mixed polycarboxylate cement is very viscous .however polycarboxylate have different rheological or flow properties than zinc phosphate , exhibiting thinning with increased shear rate .this means they are capable of forming low film thicknesses despite their viscous appearance

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Bonding to tooth structure –it bonds chemically to tooth structure ,the polyacrylic acid is believed to react via the carboxyl groups with calcium of hydroxyapatite

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Biologic properties- The excellent biocompatibility with the pulp is a major factor in the popularity of this cement system .this is due to larger size of polyacrylic acid which may limit its diffusion through the dentinal tubules

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GLASS IONOMER CEMENT

Glass ionomer is generic name of group of materials that use silicate glass powder and an aqueous solution of polyacrylic acid . It is also referred to as polyalkenoate cement

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TYPES OF GIC

Type-I Luting applications

Type-II Restorative material

Type-III Liner or Base

Type-IV Pit and Fissure Sealant

Type-V Orthodontic Bracket

Type-VI Core build up

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Working time-range is 3-5 min Setting time- 5-9 min Manipulation- The powder is introduced into the liquid in

large increments and spatulated rapidly for 30 to 45 seconds

Recommended P:L ratio is 1.25 to 1.5 g of powder per 1ml of liquid

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Physical properties- Compressive strength -86 MPa Modulus of elasticity-7.3 GPa A disadvantage is that during setting ,

glass ionomer appears particularly susceptible to moisture contamination and should be protected with a foil or resin coat or by leaving a band of cement undisturbed for 10 min

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Biologic Properties- the glass ionomer cement bond adhesively to tooth structure and they inhibit infiltration of oral fluids at the cement tooth interface

In addition because it releases fluoride it may have an anticariogenic effect although this has not been documented clinically

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ZINC OXIDE EUGENOL CEMENT

ZOE is extremely biocompatible and provides an excellent seal .however its physical properties are inferior to those of other cements ,which limits its use

Type-I is used for temporary cementation Type-II is intended for permanent

cementation of restorations

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Type I ZOE Cements –it has a pH of 7 and is biocompatible with pulp .

It seals the cavity well against ingress of oral fluids , hence irritation caused by microleakage is minimized

The strength of a temporary cement must be low to permit removal of the restoration without trauma to the teeth

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Type-II ZOE Cements- The 2-ethoxy benzoic acid (EBA) modifier replaces a portion of eugenol in conventional ZOE cement ,it improves the strength ,but it should be used only in restorations with good inherent retention form where emphasis is on biocompatibility and pulpal protection

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RESIN CEMENTS

Composite resin cements with greatly improved properties were developed for resin retained prosthesis and are extensively used for bonded ceramic technique

Resin cements are available with adhesive properties i.e. they are capable of bonding chemically to dentin

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Bonding is achieved with organophosphonates , HEMA (hydroxy ethyl methacrylate ) or 4-META (4-methacrylethyl trimellitic anhydride )

Resin based cements are irritating to pulp especially if they are not fully polymerized

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RESIN MODIFIED GLASS IONOMER

They were introduced in an attempt to combine some of the desirable properties of glass ionomer i.e. fluoride release and adhesion with the higher strength and low solubility of resins .

These materials are less susceptible to early moisture exposure than glass ionomer

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Currently among the most popular luting agents ,resin modified glass ionomer have low solubility , adhesion and low microleakage also perceived benefit of reduced post cementation sensitivity

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Resin modified glass ionomers should be avoided with all ceramic restorations because they have been associated with fracture which is probably due to their water absorption and expansion

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THANKYOU

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