INTRODUCTION

In the 1940s, dentists observed that secondary caries was rarely

associated with silicate cement restorations.

The fact that fluoride was inherent in the composition of the material

relieved much attention.

This heralded the development and increasing use of “CONTROLLED-

RELEASE THERAPEUTIC MATERIALS” in dentistry.

By the mid-1970s some fluoride releasing amalgams and luting cements

were commercially available in Europe.

By the mid-1980s, a wide variety of fluoride-releasing dental restorative

materials were available to dentists and dental consumers.

Also, the cariostatic effect of fluoride ions on enamel caries had been

demonstrated in many studies.

The effect of fluoride-releasing restorative materials on dentin also began to

receive attention.

In fact researchers have found deeper penetration depths of fluoride in

dentin than enamel on cavity walls adjacent to a variety of fluoride

containing restorative materials.

Addition of fluoride can be achieved by 1) physically incorporating

soluble fluoride salt within the bulk material. 2) by adding virtually insoluble

fluoride-containing minerals as fillers. 3) Another alternative for fluoride

release is chemical in nature and uses monomers with fluorine as the matrix

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former. These monomers release fluorine ions by means of ion exchange in

which hydroxy groups replace fluorine ions that have been released.

A brief not on the anticariogenic action of fluoride:

Fluoride contributes to caries inhibition in the oral environment by

means of both:

1) Physico-chemical mechanism.

2) Biologic mechanism.

Physico Chemical Mechanism – Here, fluoride inhibits demineralization

through the formation of an acid resistant phase. Thereby, enhances

remineralization of carious enamel (non-cavitated).

i.e. fluoride, ions released from the restorative materials become

incorporated in hydroxyapatite crystals of adjacent tooth structure to form a

structure that is slightly more resistant to acid attack “Flourapatite”.

Biologic-Mechanism fluoride interferes with the carbohydrate metabolism

of the acidogenic plaque flora.

In fact, it has been shown that bacterial species fail to thrive in the presence

of fluoride, particularly “streptococcus mutans”.

The fluoride also enters the microorganisms and accumulates

intracellularly. The fluoride ions then induce enzyme inhibition, leading to a

slower rate of acid production.

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Meanwhile, the fluoride increases cell permeability and it can rapidly

diffuse out of the bacterium, contributing again to the fluoride content within

the plaque matrix.

Another way by which fluoride inhibits caries is by reducing the surface

energy of the tooth surface, thereby making the adherence of dental plaque to

tooth surface more difficult.

Coming to individual fluoride materials:

AMALGAM

Fluoride containing amalgams have been shown to have anticaries

properties that is sufficient to inhibit the development of caries in cavity walls.

Studies have shown that the concentration of fluoride in the saliva by

fluoride-releasing amalgams is sufficient to enhance remineralization.

Therefore, fluoride releasing amalgam restorations may have a

favourable effect on initial demineralization in the mouth.

Tviet and Lindh (1980) found that the greatest concentration of fluoride i.e.

about 4000µg/mL in enamel surfaces exposed to fluoride-containing amalgams

were found in the outer 0.05µm of the tissue.

In dentin, the greatest concentrations, i.e. about 9000µg/ml were found

at a depth of 11.5µm.

Most of the fluoride-releasing amalgams like other fluoride containing

dental restorative materials show an initial release that is significant. However,

this release of fluoride decreases to minor amounts after 1 week.

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One study found salivary fluoride concentrations at more than 20 times

baseline concentration for the first few days after placement of restorations.

The release declined exponentially to baseline levels after 30 days. One In-

vivo study has shown that fluoride released from amalgams loaded with soluble

fluoride salts was detectable within the first month and thereafter fluorable was

not released in measurable amounts. Another in vitro study showed fluoride

release can continue as long as 2 years (but at a much lower rate than that for

GIC).

Disadvantage – The leaching of fluoride makes the amalgam more susceptible

corrosion.

GLASS IONOMER CEMENT

Glass ionomer cements are perhaps the best known fluoride-releasing

restorative materials.

Like silicates they have been shown to have anticariogenic properties due to

their significant release of fluoride.

The fluoride which is an essential component of glass ionomers imports the

following functions:

1) It lowers the temperature of fusion.

2) Improves working characteristics.

3) Increases the strength of the set cement.

4) In moderate amounts enhances radiolucency.

5) Contributes to the therapeutic value of the cement.

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These cements have also shown the uptake of fluoride in cavity walls,

enamel and plaque.

FORSTEN (1977) found glass ionomers to release significantly more fluoride

than silicate cement and amalgams (1990).

But studies by Tviet and Gjerdet (1981) showed fluoride-release from

silicate cements was about 5 times greater than from glass ionomers.

The presence of fluoride in GICs has shown to inhibit plaque formation.

Glass Ionomer cermets (sintered silver particles to glass ionomer

powder) and metal powder admix materials have demonstrated fluoride release

and caries inhibition at enamel and dentin restoration margins in vitro.

However, less fluoride is released from cermet than from Admix.

This is because the metal filler particles are not bonded to the cement

matrix. Thus, the filler cement interface become pathways for fluid exchange

this greatly increases the surface area available for leaching of fluoride.

The fluoride release levels of Resin-Modified GICs are comparable to those

of conventional GICs.

Both Resin-Modified GICs and conventional GICs may have “Synergistic

effects” when used with extrinsic fluorides.

The mechanism of this syndergy is thought to be or recharging effect, where

extrinsic fluoride is deposited back into the ionomer. Thus, resupplying the

release from the ionomer into the surrounding environment.

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As in the case with other fluoride-releasing materials, all glass-ionomers

have been shown to have a “Burst-effect”.

Studies by De Schepper and others have shown that commercially

available GICs release the greatest proportion of their total fluoride in the first

24 hours after mixing.

This fluoride-release however, stabilizes after 2 weeks to comparable low

release levels. [i.e., 0.16µg/mm2 to 0.42µg/mm2].

Forsten (1977) found the “Burst Effect” to be true over a period of several days

to 2 weeks.

In a more recent study he found that for all GICs, the fluoride release

eventually reached a constant level of approximately 0.5µg/ml to 1.0µm/ml

(other than cermet) during the 2nd year.

Another study Koch and coworkers (1990) found the fluoride

concentration in unstimulated saliva to decrease by 35% after 3 weeks and

another 30% after 6 weeks.

After 6 weeks, however, the fluoride level in saliva was still 10 times the

baseline concentration.

Fluoride release rates have not been found to be proportional to fluoride

concentrations in Glass ionomer products.

Commercially available cements vary in the amounts of fluoride-

released fluoride release from a silver cermet was found to be significantly

less than the release from a standard GIC throughout a 12 month –period.

The cariostatic effect (in vitro) of the cermet was also significantly less.

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The steady fluoride-release was approximately 1.5µm/mL for standard GICs

and 0.5µg/mL for the cermet.

Both the materials however, had significantly higher in vitro caries

inhibition than composite and amalgam.

This indicated that caries inhibition was fluoride dose dependent even at

these low release levels.

Other factors that may influence the release rates of fluoride in GICs may be:

1. Handling.

2. Powder : Liquid ratio.

3. Maturity of the cement matrix.

4. Application of varnish.

1) Handling Hand-mixed GICs have been shown to release

significantly less fluoride than mechanically triturated GICs (Miller and

Others, 1995).

2) Powder:Liquid Ratio : Studies have shown that cements with

lower powder to liquid ratios demonstrated greater fluoride release.

McKnight-Hanes and Whitford (1992) also found that the release rate of

fluoride in a GIC was inversely proportional to the powder to liquid ratio.

This finding is probably due to the composition, amount and maturity of the

reaction matrix forming within the cement.

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3) Apply of Varnish : Studies showed that the varnishing of disks

made from GIC (greatly) sharply reduced fluoride release.

However, the finishing of the varnished disks produced a significant

increase in the fluoride release of one GIC product.

Likewise, another study found a significant reduction in fluoride release

from GIC restorations covered with a sealant.

Preliminary in vitro research has confirmed that GIC (along with

composites) retain fluoride delivered by dentifrices or topical fluoride

treatments at the material surface and then release this fluoride slowly.

Hence, these mateials act as: “Flouride Reservoirs”.

Table : This table shows the fluoride-release from various glass ionomer formulations.

Mg – F14 days 30 days

Type II Glass ionomer CermetSilver alloy admixType I Glass ionomerGlass ionomer liners:

- Conventional- Light wred

4402003350470

10001200

6508004040700

13001600

COMPOMERS : A relatively new class of fluoride-releasing restorative

materials has been introduced. These are combinations of glass ionomer glass

powder with polymerizable acidified monomer.

A study on the compomer “Dyract” showed the initial release of fluoride to

be 25µg/mL.

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This release rate after 28 days was maintained at 6µg/mL.

However, there is little clinical evidence to support a claim for caries

inhibition.

Composites : Composite resins have also been formulated to release fluoride.

As early as 1970s, some composite resins incorporated fluorides and were

shown to release fluorides.

The release of fluoride from composite resins demonstrated a reduction in

2° caries initiation and even remineralization of adjacent demineralized enamel

when examined in vitro. Studies have detected a fluoride release of 200-

300µg/mm2 from composites to completely inhibit in situ secondary caries.

Donly and Gomez (1994) have also demonstrated the remineralizing effects

of a fluoride-releasing composite.

Although fluoride-releasing composites have consistently demonstrated

recurrent caries inhibition at enamel margins, there are still conflicting results

regarding caries inhibition at dentin margins (Donly 1994).

As with GICs, there may be a “Synergistic effect” between fluoride-

releasing composites and fluoride rinses or fluoridated dentifrices.

i.e when exposed to external fluoride, the materials surface undergoes an

increase in fluoride, which is subsequently released.

ADHESIVE PRIMERS

Kerber and Donly (1993) studied the effect of adding ammonium fluoride to

two different dentin primers.

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Results showed that primers containing fluoride demonstrates significantly

less demineralization from the dentin margins than the primers without

fluoride.

Pit and Fissure Sealant

In 1984, Roberts, Shern and Kennedy evaluated an autopolymerizing pit and

fissure sealant as a vehicle for the slow release of fluoride.

Sodium fluoride was added to the sealant at several concentration (upto

concentrations of 2.5%).

The fluoride release was measured to be 0.3µg/mL for a period from 31

days to 90 days at the highest concentration (i.e. 2.5%).

However, when the authors considered the dilution factor due to average

salivary flow, they concluded that this level of release would be below any

known level of physiologic significance.

In the late 1980s, a fluoride-containing sealant was introduced to the dental

materials market place. The product was evaluate in vitro. It was found to

release fluoride over a 7 day evaluation period, beginning at a level of

3.5µg/mL on the 1st day and declining to a level by 0.41µg/mL on the last 2

days.

This same product was clinically compared to a conventional glass-ionomer

sealant.

It was found that retention of the fluoride releasing resin was much higher

and caries incidence was much lower than the glass ionomer (Rock and others,

1996).

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What could not be resolved in this study was whether this lower incidence

of caries was due to fluoride release or the greater retention of the resin.

In another, in vitro study (Jensen et al, 1990) a fluoride releasing pit and

fissure sealant was found to reduce the amount of enamel demineralization

adjacent to the material, compared with conventional pit and fissure sealants.

Seppa and Forss (1991) found that fissures sealed with a glass ionomer

sealant were more resistant to demineralization than were unsealed controls.

They suggested that the result may be the combined effect of fluoride

release and residual materials in the bottom of the fissures.

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LINERS /BASES AND CAVITY VARNISHES

There are currently half dozen or more fluoride releasing liners on the

market.

Some have been found to significantly reduce lesion areas under amalgam

restorations.

Most of these liners / bases have been found to have a “Burst effect” in the

release of fluoride.

Most studies have shown that the largest proportion of total fluoride release

occurs during the first days or weeks, followed by dramatic reductions in the

rate of release.

Long term release of fluoride varied over a range of 0µg/mL to 7 µg/mL.

Glass ionomer cements have also been used as a liner material under

amalgam restorations.

They have been shown to continue releasing measurable amount of fluoride

in the range of 0.3µg/mL to 1.1µg/mL after 1 year.

Certain in vitro studies have also shown glass ionomer cements to reduce

recurrent caries when placed under amalgam.

A light cured and a chemically cured glass ionomer cement liner were found

to have a similar effect in inhibiting demineralization.

Both demonstrated significantly less demineralization than a non-

fluoride-releasing control liner (Souto and Donly, 1994).

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ZINC POLYCARBOXYLATE CEMENT

The powder of zinc polycarboxylate cements contains small quantities

of stannous fluoride.

The stannous fluoride :

1) Modifies the setting time

2) Enhances manipulative properties.

3) Increases strength.

However, the fluoride released from this cement is only a fraction (15-20%)

of the amount released from (zinc silicophosphate) and glass ionomer cements.

There are not many studies done further regarding the amount/rate of

fluoride release for these cements.

CONCLUSION

From the above, it can be concluded that:

1) All fluoride-containing materials release fluoride in an initial burst and

then reduce exponentially to a much lower steady-state level of release.

2) The steady state release of fluoride is reached after approximately 30

days for most materials.

3) Caries inhibition and remineralization potential have been shown in

vitro by all of these materials when release levels have been equal to or

exceeding approx. 1µg/mL/

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There are few clinical studies that appear to support the proposition that

low levels of fluoride release can inhibit in vivo demineralization and caries

formation.

The ultimate goal of correlating fluoride release with actual caries

inhibition reduction is an objective than can be met by completing clinical

studies on materials that release fluoride.

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FLUORIDE-RELEASING DENTAL RESTORATIVE MATERIALS

Contents

INTRODUCTION

ANTICARIOGENIC ACTION OF FLUORIDE RELEASED FROM RESTORATIVE MATERIALS

INDIVIDUAL FLUORIDE-RELEASING MATERIALS

CONCLUSION

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