Fluoride release and uptake by glass ionomers

LENNART FORSTEN

Institute of Dentistry, University of Turku, Turku, Finland

Forsten L: Fluoride release and uptake by glass ionomers. ScandJ Dent Res 1991; 99: 241-5.

Abstract - The aim was to study the fiuoride release 1) from 7 and 15-month-old glass ionomerspecimens after treating them wilh fluoride; 2) from fresh compared with matured material; and3) from specimens stored for 29 months in running water. Glass ionomer test specimens whichhad been in running water for first 7 then 15 months were treated with a 50 ppm fluoridesolution after which the specimens were again exposed to running water for first 24 h and then1 wk. The fiuoride release was measured after each of the two periods of time. The fiuoridetreated specimens released more fiuoride than the nontreated ones. This effect was not observedwith composite resin specimens which were studied for comparison. Fluoride release from freshglass ionomer specimens was observed to be 3-10 fold compared to specimens that had maturedfor 3 days. The release of fluoride from specimens that had been in running water for 29 monthswas measured and the results were compared with those of earlier measurements. It was foundthat the release reached a constant level for all tested glass ionomers during the second year.

Key words: dental materials; fluoride release; fluoride uptake; glass ionomers.

Institute of Dentistry, University of Turku, Lemminkaisenkatu 2, 20520 Turku, Finland.

Accepted for publication 23 September 1990.

The anticariogenic effect of glass ionomersdepends on the amount of released fiuoridebut obviously even more on the longevity ofthis release. In an experiment which is stillongoing, glass ionomer speeimens are ex-posed to running water. It was found that thelevel of release stabilized for most productsduring the second year of the experiment

(1)-It has been suggested (2) that fluoride is

not only released by the glass ionomer mate-rial but it can also be taken up and storedin the filling. However, in one in vitro studythis assumption was not eonfirmed (3). In

the present study I have analyzed this prob-lem and compared the glass ionomers witha fiuoride containing composite.

The long-term release is especially impor-tant for inhibiting secondary caries. Usingmodern tooth saving preparation techniques(e.g. tunnel preparation) it is not easy toeompletely control the removal of caries. Ifsome small inclusions of softened dentin areleft, a massive fiuoride effect would be desir-able. Such an effect would probably affectthe metabolism of the bacteria and activatethe remineralization of the softened dentin.

When a cavity is filled with a glass iono-

242 FORSTEN

mer, the cavity fioor and walls are exposedto fluoride from a fresh material. Anotherpurpose of this study was to compare therelease of fiuoride from a fresh glass ionomerwith that from a more mature material.

The third aim was to follow up the long-term release reported earlier (1).

Material and methods

Fluoride uptake - The materials (Table 1) werehandled according to the manufacturers' instruc-tions. A round plastic cup (inside diameter 6.7mm, depth 2.7 mm) was filled with material. Dur-ing setting the surface was covered with a glassplate. After hardening the specimens were protect-ed with petroleum jelly and transferred into waterof room temperature (22±2°C). After storing for24 h, the surface of the specimens was groundlightly against a 600 grit paper and transferredinto running water (1). The specimens were leftin the molds during the experiment. Four sampleswere made of each material. After 7 months inrunning water (experiment I) two samples werestored in 10 ml buffered fluoride solution (50 ppmF- in 1 mM Tris-HCl, pH 7.0) for 1 wk. Theother two samples served as controls and werestored in deionized water. After this period of timethe specimens were exposed to running water for24 h. The fiuoride release in 5 ml deionized waterduring 1 wk was then measured as describedearlier (1). The specimens were then exposed torunning water for 1 wk before measuring the fluo-ride release once more. After 15 months in runningwater (experiment II) the two specimens that hadbeen treated with fluoride before were now storedfor 24 h in the fluoride solution and then exposed

Table

Material

Ketac-FilKetac-SilverFuji Cap IIChemFil II

in capsHeliomolar RO

Material

BatchNo.

S131T024171281

891108329402

examined

Symbol

KFKSFC

CFH

Manufacturer

EspeEspeG.C.DeTrey

DentsplyVivadent

to running water for 24 h before the release during1 wk was measured.

Fiuoride release from fresh specimens - The materialsare listed in Table 1. In this experiment the speci-mens were removed from the mold (diameter 5mm and thickness 2 mm) after setting. Four speci-

PPM4

3-

A 2-

1 -T

T

KF KS

CONTR

FTREATED

H

B 1

HFig. 1. Fluoride release from 7 months old controlspecimens (open bars) and from specimens treatedfor I wk with a fluoride solution (striped bars).Half of range, if observed, is indicated at top ofbars. After fiuoride treatment, the specimens hadbeen in running water for 24 h (A) or 1 wk (B).

FLUORIDE EXCHANGE IN GLASS IONOMERS 243

mens were made of each material. Two of thesewere protected with petroleum jelly and allowedto mature for 3 days in water of room temperature,before grinding and were then held in runningwater for 24 h before measuring the 1 wk fluoriderelease. Immediately after setting the other twospecimens were transferred (5 min after start ofmixing) into a cup (with a lid) containing 5 mldeionized water. The fluoride content of the waterwas measured after 1 wk.

Long-term fluoride release - The test specimens(two of each material) from an earlier study (1),which had now been in running water for 29months, were transferred into 5 ml deionizedwater. The fluoride content of that water was mea-sured after 1 week as described earlier (1).

Results

Fluoride uptake - Both the conventional glassionomer (KF) and the cermet (KS) speci-mens treated with fiuoride released twice asmuch fiuoride as the control specimens afterbeing in running water for 24 h (Fig. IA).

PPM1

KS H

CONTROL

FTREATED

Fig. 2. Fluoride release from 15-month-old controlspecimens (open bars) and from specimens treatedfor 24 h with fluoride solution (striped bars). Afterfiuoride treatment specimens had been in runningwater for 24 h.

The fiuoride treated composite specimens didnot release more fiuoride than the untreatedspecimens. The week in running waterfollowing the first measurement diminishedthe fluoride release (Fig. 1 B). The level offiuoride release 8 months later (Fig. 2) haddecreased by about 50% but treating theglass ionomer samples again doubled the re-lease although the specimens were treated for24 h only. As before, the fiuoride solutionhad no significant effect on the compositespecimens.

Fluoride release from, the fresh specimens - Thefiuoride release from the fresh specimens wasabout three times that from the maturatedones of FC, KF, and KS, but about 10 timeshigher from specimens of CF (Fig. 3).

Long-term fluoride release - In Fig. 4 the val-ues for fiuoride release during 1 week after22 months in running water from an earlierstudy (1) are presented together with thefiuoride release values of the same specimensafter 29 months. There was no differencebetween the 22 months and 29 months re-lease values for the different glass ionomersnor for the amalgam or the composite.

PPM50

4 0 -

30-

2 0 -

10-

0

X , D FRESH0 MATURATED

•Fig. 3. Fluoride release from fresh (open bars) andmaturated (striped bars) specimens.

244 FORSTEN

Discussion

As pointed out earlier (1) this experimentalsetup did not simulate the clinical situationcorrectly. The fiuoride release is expected tobe slower in the mouth than in the experi-ment. This is supported by the observationthat less fiuoride is released in saliva than inwater (4). On the other hand, it is very diffi-cult to design an in vitro study with, forexample, constantly running saliva. Thepresent study gives at least some informationon the long-term release of fiuoride. Thisinformation has to be confirmed by in vivostudies. So far there are no indications inclinical studies or experience contradictingthe present findings. It seems that a constantlevel of release is reached after some time,even when "outside" fiuoride is not avail-able. This level does not differ very muchbetween different CIC products but was defi-nitely higher than the overall release fromamalgam and composite.

If fiuoride is available from outside, thelong-term release level in vivo may still be

higher than in vitro. The present study showsthat fiuoride can be taken up by glass iono-mers. The results also indicate that the fluo-ride became chemically bound since therewas a gradual release of this additional fluo-ride. The composite material did not seemto be able to bind fluoride in the same wayas the glass ionomer material.

In this test the fluoride treatment wascomparatively long and the concentrationquite high. It may seem that the in vivofluoride treatments, e.g. by toothbrushing,are of much shorter duration. However, ithas to be remembered that the fluoride ap-plications increase the fiuoride content ofthe plaque. The concentration may riseeven up to 50 ppm (5) and may be avail-able in the plaque for much longer timesthan the actual treatment time. There arealso fluoride application methods, e.g theuse of fiuoride varnishes, that prolong theapplication time.

A growing interest exists in adding fluo-ride to resin filling and sealing materials.According to the present results, such an

PPM2

1 -T_

II

I1I 1FII MM AA

D 22 MONTHS0 29 MONTHS

Fin FII + a KF KS HFig. 4. Fluoride release during 1 wk from test specimens FII (Fuji II), MM (Miracle Mix), AA (ASPA),ChemFil II (C), Fill (Fuji III), FII-|-Sybraloy (Fll + a), KF (Ketac-Fil), KS (Ketac-Silver), A (FluorAlloy) and H (Heliomolar) after 22 months and 29 months in running water. Half of range is indicatedat top of bars.

FLUORIDE EXCHANGE IN GLASS IONOMERS 245

attempt does not seem very promising. Thefiuoride is one of the main components ofthe glass ionomer and the amount of fiuo-ride is considerable. If fiuoride is added insame amounts to a resin it has to effect theproperties of the material. Furthermore, thepossibility of "recharging" the resin withfiuoride seems doubtful.

As may be expected, fresh glass ionomerseemed to release more fluoride than matu-rated material. The specimen was able toraise the fluoride content of the 5 ml waterup to 15-45 ppm. The size of the free sur-face of the specimen was about the same asthat of the surface of a filling facing theeavity wall. The dentin wall and the inter-face between the dentin and the filling ismoist. Fluoride from the fresh glass iono-mer is therefore expected to be released.The space between filling and tooth ismuch smaller than the 5 ml water amountused in the study. It may therefore be spec-ulated that there will be a quite high con-centration of free fiuoride adjacent to unin-tentionally left soft dentin. Due to themoist environment, the fiuoride is likely topenetrate this dentin and effect the bacteria

as well as activate the remineralization ofthe softened dentin.

Acknowledgments - This study was supported byLinda Gadd's grant from Finska Lakaresallskapet.The materials were provided by their manufac-turers.

References

1. FORSTEN L. Short- and long-term fluoride re-lease from glass ionomers and other fluoride-containing filling materials in vitro. Scand JDent Res 1990; 98: 179-85.

2. WALLS AWG. Glass polyalkenoate (glass-iono-mer) cements: a review. J Dent 1986; 14:231-46.

3. JONES DW, JACKSON G, SUTOW EJ, HALL

GC, JOHNSON J. Fiuoride release and fluo-ride uptake by glass ionomer materials.J Dent Res 1988; 67: Spec/Issue AAbstr No. 672.

4. EL MALLAKH BF, SARKAR NK. Fluoride re-lease from glass-ionomer cements in de-ion-ized water and artificial saliva. Dent Mater1990; 6: 118-22.

5. EKSTRAND J, FEJERSKOV O, SILVERSTONE LM.

Fluoride in dentistry. Copenhagen: Munksgaard,1988: 67.