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 “CONTROLLEDRELEASE 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
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.
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.
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 Invivo 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.
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.
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 fluoridereleased 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.
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).
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.
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 – F 14 days Type II Glass ionomer Cermet Silver alloy admix Type I Glass ionomer Glass ionomer liners: - Conventional - Light wred
440 200 3350 470
650 800 4040 700
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. 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 200300µg/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 fluoridereleasing 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. 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.
ďƒ 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). ďƒ 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. ďƒ 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.
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 nonfluoride-releasing control liner (Souto and Donly, 1994).
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/
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.
FLUORIDE-RELEASING DENTAL RESTORATIVE MATERIALS Contents INTRODUCTION ANTICARIOGENIC
RELEASED FROM RESTORATIVE MATERIALS INDIVIDUAL FLUORIDE-RELEASING MATERIALS CONCLUSION