Biomaterials 23 (2002) 2191–2200

The glass ionomer cement: the sources of soluble fluoride

J.A. Williamsa,*, R.W. Billingtonb, G.J. Pearsonb

aDepartment of Biomaterials, Eastman Dental Institute for Oral Health Care Sciences, 256, Gray’s Inn Road, London, WC1X 8LD, UKbDepartment of Biomaterials in Relation to Dentistry, Queen Mary & Westfield College, Mile End Road, London, E1 4NS, UK

Received 16 March 2001; accepted 12 October 2001

Abstract

This study aimed to investigate certain processes of fluoride production which enable glass ionomer cements to leach

fluoride. Two fluoroaluminosilicate glasses, G338 and LG26 were used. The free and total fluoride which could be dissolved

from the glasses was measured, before and after acetic acid washing. Both glasses contained appreciable amounts of soluble fluoride

prior to any acid treatment. The latter process reduced the amount to some 75% of the original levels. Replacing the customary

polymeric acid with propionic acid produced a cement which disintegrated in water allowing the amount of fluoride generated by the

cement forming process to be measured. Cement production increased soluble fluoride by a further 3%. Both glasses behaved

similarly when undergoing the various processes. G338 produced significantly greater quantities of fluoride, of the order of 10,

compared with LG26 although containing only three times the amount of fluoride in the glass formula. A substantial propor-

tion, over half, of the total fluoride was complexed especially after contact with cement and when G338 was used. During the

period of the experiment, 21 days, total fluoride release did not seem to depend on the square root of time.r 2002 Elsevier Science Ltd.

All rights reserved.

Keywords: Fluoride release; Glasses; Cements

1. Introduction

Fluoride (F) release from glass ionomer cements (gics)and their predecessors, the silicate and silicophosphatecements, has been investigated for several decadesfollowing the observation of a reduced incidence ofsecondary caries in association with these materials.Leachable fluoride had not been intentionally includedto improve the suitability of the cement for dental usebut occurred as a constituent of one of the severalchemicals used in the manufacture of the glass. Each gicmay use a specialised glass and differ also in the typeand amount of polymeric acid and tartaric acidconstituents. Silicate and silicophosphate cements com-bine a glass with an aqueous solution of, mainly,phosphoric acid. Measurements of fluoride releasefound all three types of cement to release similarquantities of fluoride for a number of years with partof the release profile showing a linear relationship with

the square root of time of cement immersion [1–3].Fluoride release from gics was found to be related to thespecimen surface area but not to specimen volume [4]and a number of investigations have attempted tocharacterise release by mathematical equations [5,6]. Allthese studies have indicated that release of fluoride fromthe cement is controlled by a diffusion process.However, such experiments have measured only thefinal stage in the process of fluoride migration, namelythe movement of fluoride leaching from the outersurface of the cement into a surrounding aqueousmedium. It is not clear how, and from where, thefluoride ion arrives at the cement surface. All intrinsicfluoride, i.e. fluoride which has not been taken up froman outside source, must come originally from the glass inthe cement. A restorative cement typically containsabout 75% glass which in turn may contain 13–25%fluoride ion. The cements may, therefore, containamounts of the order of 150mgF/g. It has been saidthat the leachable fluoride is produced by acid attackupon the glass during the cement forming process [7–9]but the means by which fluoride initially associated withthe glass becomes fluoride ultimately released from the

*Corresponding author. Tel.: +44-20-7915-1133; fax: +44-20-7915-

1133.

E-mail address: j.williams@eastman.ucl.ac.uk (J.A. Williams).

0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 2 - 9 6 1 2 ( 0 1 ) 0 0 3 5 2 - 0

outer surface of a cement is poorly understood and theaim was to investigate aspects of this process.Apart from work carried out by Cranfield [10] none of

the investigations of fluoride release have measured theamount of water-soluble fluoride present in the glassitself prior to any cement formation. This then was theinitial objective. Secondly, it was proposed to measurethe amount of fluoride released from the glass into wateracidified to the same degree as it would have been if thepolymeric and tartaric acids present in a specimen ofcommercial gic were available. Thirdly, since glassesmay have some form of surface treatment involving acidwashing [9,11] prior to use as a cement, it was proposedto measure soluble fluoride after this process. Finally,the intention was to measure the ultimate amount offluoride which could be released from a cement madefrom this glass. That is, to measure the amount offluoride created by the cement forming process, ratherthan measure the slow diffusion of F to the surface ofthe cement and thence into water over a lengthy period.In order to do this the cement needed to be disintegratedand this was accomplished by replacing the polymericacid solution normally used for a gic with propionic acidsolution. The latter chemical is equivalent to a singleunpolymerised unit of polyacrylic acid. Cements madein this way, although capable of setting, disperse inwater when shaken.

2. Materials and methods

2.1. MaterialsFglass

Two types were used, G338 and LG26. G338 glass,formulated by the Laboratory of the GovernmentChemist (Teddington, Middlesex, UK), forms hardsetting cements when blended with aqueous solutionsof polyacrylic and tartaric acids [12]. It had a fluoridecontent of ca. 18.6% [9].LG26 (Limerick University, Eire) was a sodium-free

glass previously used in a study by Hadley et al. [13] andcontained 5.2% F. While formulated for surgicalcementation it could be used at a restorative consistencywhen mixed with polymeric acid solution.Portions of both glasses were treated with an aqueous

solution of 5% acetic acid (Analar grade. BDH/Merck,Poole, UK) for 1 h before filtering through hardenedfilter paper (S & S grade 595, Aldrich, Gillingham,Dorset, UK) and washing with deionised water toremove residual ions and drying. The absence ofcloudiness in the filtrate was taken as evidence ofretention of fine particles by the filter paper. Aftertreatment the glass was designated TRE.Initial experiments showed that both glasses in the ‘as

received’ state (RAW) gave cements of working time

insufficient to allow handling. Therefore, cements weremade only from TRE glass.

2.2. MaterialsFcement

An aqueous solution of 52.6% propionic acid(99.5+% grade, Aldrich, Gillingham, Dorset, UK)and 1.7% tartaric acid (BDH/Merck, Poole, UK) wasmade in distilled water (LIQ). When mixed with TREglass at a powder/liquid ratio of 2.8/1.0, the final cementcomposition was calculated as containing 73.7% glass,14.3% acids and 12.0% water, similar percentages tothose given by manufacturer’s data for ChemFil IIcement (Table 1).

2.3. Method

Portions of RAW glass (0.120–0.180 g) wereweighed and added to 20ml portions of distilledwater (water) in plastic vials. These were immedi-ately shaken and stored at 371C. After 24 h, a 2mlportion of liquid was removed for analysis and the vialshaken. This was repeated at 7, 14 and 21 days. Thesame process was repeated using TRE glass. Thisprocess was then carried out using RAW andTRE glasses where distilled water was replaced bywater containing 0.014m propionic acid/tartaric acid(AcW). This molarity was equivalent to the acidityproduced by dispersing 0.15 g cement in 20ml waterassuming that no acid had been consumed in formingthe cement.Finally, portions of TRE glass were weighed and

mixed with LIQ to give a cement. Sufficient amount wasmade to fill four metal ring moulds of 10mm diameterand 1mm height from one mix. Three mouldswere clamped between acetate sheet and metal platesand placed in an environment of high humidity (>80%)at 371C. The fourth was pressed between acetatesheet and metal plates for 5min before transferringthe friable disc from the mould into a weighedvial containing 20ml distilled water. The vial wasshaken to disperse the cement and re-weighed to findthe weight of cement added. The remaining discs werelikewise treated after 60min, 24 h and 7 days. After 7days the disc was set sufficiently hard to require

Table 1

Formulation of cement

Experimental Chemfil IIa

Glass 73.7 72.9

Acidic component 14.3 14.3

Propionic &

tartaric acids

Polyacrylic &

tartaric acids

Water 12.2 12.8

aFrom manufacturer’s data.

J.A. Williams et al. / Biomaterials 23 (2002) 2191–22002192

dispersion using a glass rod. The vials were maintainedat 371C from which 2ml portions of liquid wereremoved after 1, 7, 14 and 21 days. The whole schemeis summarised in Fig. 1.Six specimens were prepared for each glass/liquid and

cement/water combination. Water without the additionof glass or cement acted as blanks.Fluoride ion concentration was measured using a

calibrated fluoride ion electrode system (Orion 801meter, 913500 pH and 9409SC F electrodes, OrionResearch, Cambridge, MA, USA ). Standards contained0.01–1000mg/l F in decade steps. Both free (FF) andtotal fluoride (TF) were measured, the former beforeadding buffer and the latter following addition of anequal volume of total ionic strength adjustment buffer(TISAB) IV. Samples and standards were treated in thesame manner.This gave a measure of the cumulative fluoride

ion found in the liquid for contact periods of 1–21days. Since the cement was disintegrated no surface areacould be accurately measured and furthermore the glasswas present in powder form. Fluoride measurementswere therefore calculated as mgF/g glass or cement,after subtraction of blank values. Since the cementcontained only 73.7% glass the amount of fluoride‘released’ from the cement was adjusted to be equivalentto 100%.

3. Results

3.1. Analysis of the acetic acid filtrate after treatment of

glass

This was also analysed for F content. The filtrate fromboth glasses contained 5mg/l FF. TF was higher withG338 glass producing 19mg/l and LG26 giving 28mg/l.The addition of an equal volume of TISAB IV was notsufficient to raise the pH to the level of 5.0–5.5 requiredand at least three aliquots of buffer were needed. Thismay have introduced additional errors.

3.2. Analysis of sample solutions

Blank solutions contained 0.02mgF/l or less with nosignificant difference between FF and TF.All other solutions, water and AcW which had been in

contact with RAW and TRE glass and cementcontained both FF and TF with TF always greaterthan FF.

3.3. G338 RAW glass (Table 2)

1.8–3.4mg/g FF were found in water with nosignificant change (t-test, po0:05) resulting from in-creasing contact time. TF (see Fig. 2) was higher than

Fig. 1. Schematic diagram of experimental protocol.

J.A. Williams et al. / Biomaterials 23 (2002) 2191–2200 2193

FF at all time intervals, significantly so (po0:05) after21 days. TF increased as contact time increased andreached 5.6mg/g after 21days.In AcW FF was 1.3–2.1mg/g. There was no

significant change in FF resulting from the increasedacidity present. TF was significantly or highly signifi-cantly (po0:01) greater than FF. However, only theincrease after 21 days from the 5.6mg/g found in waterto 11.0mg/g in AcW was significant. The increasebetween 1 and 14 days in AcW was significant.In neither liquid was TF release proportional to

(time)1/2.

3.4. G338 TRE glass (Table 2)

In water FF ranged from 1.2 to 3.0mg/g, generallyslightly less than from RAW/water but not significantlyso. Like the RAW/water combination there was littleincrease between 1 and 21days contact time. There wasless TF, 75% compared to that found for the RAW/water combination and this reduction was significant orhighly significant. The maximum was 4.2mg/g after 21days and only the increase between 1 and 21 days wassignificant. Although TF increased with contact time(Fig. 3), there was no dependence on (time)1/2.

In general, when AcW was used in combination withTRE both FF and TF were significantly higher thanwhen water was used. FF reached a maximum of 6mg/gmaximum at 7 days before decreasing between 7 and 21days. TF did not significantly change with contact time;after an initial peak of 10.0mg/g it remained relativelysteady and was 7.8mg/g after 21 days.

3.5. G338 cement (Table 2)

As can be seen in Fig. 4 only 0.2–0.7mg FF/g wasproduced regardless of cement maturity or contact timesof 1–21 days. Even when adjusted to the same weightbasis, less FF was found to have been produced fromcement compared to RAW or TRE glass in contact withwater or AcW.TF was some 20 times higher than FF with all four

cement maturation times producing similar amounts.TF from the cements was higher than TF from TRE/

water, RAW/water and TRE/AcW but slightly less thanRAW/AcW. After 21 days TF was 8–9mg/g.Since the cement maturity was not a factor the results

at each sampling period for the four maturation times(5min to 7 days) were pooled and the mean calculated.On this basis TF increased as contact time progressed

Table 2

Free and total fluoride release from G338 as mg/g and (standard deviation)

Time (days) 1 7 14 21

Combination Free or total (F/T)

RAW/Wa F 1.80 (1.04) 1.62 (0.52) 3.53 (0.93) 2.13 (0.84)

T 3.67 (0.46) 5.49 (2.16) 5.67 (0.71) 5.58 (1.14)

RAW/AcWa F 2.11 (0.84) 1.88 (0.53) 1.31 (0.80) 0.36 (0.04)

T 7.08 (2.82) 7.78 (2.93) 6.76 (2.28) 10.71 (1.28)

TRE/Wa F 1.15 (0.16) 2.97 (2.48) 1.27 (0.51) 1.23 (0.68)

T 1.87 (0.60) 3.19 (0.92) 3.59 (1.25) 4.18 (0.52)

TRE/AcWa F 4.84 (0.61) 6.13 (3.25) 5.40 (1.90) 1.45 (0.82)

T 10.05 (3.59) 6.77 (2.19) 6.48 (1.92) 7.75 (3.09)

Actual release from cement

CEM/5min F 0.35 (0.02) 0.20 (0.04) 0.25 (0.06) 0.29 (0.07)

T 3.90 (0.25) 4.42 (0.37) 4.70 (0.90) 6.41 (2.75)

CEM/60min F 0.27 (0.05) 0.21 (0.02) 0.38 (0.23) 0.24 (0.07)

T 3.34 (0.06) 4.10 (0.68) 6.41 (4.19) 6.97 (3.77)

CEM//1day F 0.43 (0.31) 0.68 (1.01) 0.33 (0.17) 0.23 (0.03)

T 7.96 (1.00) 5.38 (3.32) 6.02 (2.29) 6.64 (3.58)

CEM/7day F 0.60 (0.86) 0.59 (0.02) 0.44 (0.26) 0.34 (0.24)

T 3.01 (2.22) 5.50 (3.62) 5.05 (6.32) 5.97 (4.03)

Adjusted for weight in cement (100/73.7)

CEM/5min F 0.48 0.27 0.33 0.39

T 5.20 5.89 6.27 8.55

CEM/60min F 0.36 0.28 0.51 0.32

T 4.45 5.47 8.55 9.20

CEM/1day F 0.57 0.91 0.40 0.31

T 10.61 7.17 8.16 8.85

CEM/7day F 0.80 0.79 0.59 0.45

T 4.01 7.33 6.73 7.96

Pooled mean F 0.55 0.56 0.46 0.38

T 6.07 6.46 7.43 8.64

J.A. Williams et al. / Biomaterials 23 (2002) 2191–22002194

although the increase between 1 and 21 days wasnot significant. TF was not related to (time)1/2.

3.6. LG26 RAW glass (Table 3)

When shaken with water FF was very low, approxi-mately 0.1mg/g. This remained relatively constantthroughout the 21 day period. It was significantly lessthan FF from G338 RAW.TF at o0.5mg/g was also significantly lower than

that for G338. TF increased steadily as contact timeprogressed (Fig. 2) reaching a maximum of 0.45mg/gafter 21 days, the increase between 14 and 21 days beingsignificant at 95% level.The substitution of AcW for water produced a highly

significant increase in FF especially during the first 24 hwhen it was 0.36mg/g. FF continued to rise to amaximum of 0.72mg/g after 7 day and then decreased,until by 21 day the amount was 0.10mg/g and notsignificantly different to RAW glass.TF was also higher than when water was used (Fig. 2).

After 24 h 1.25mg TF/g was released and 2.1mg/g after21 days. The general effect of AcW was to significantly

increase both FF and TF (po0:01) compared to theglass in contact with water.

3.7. LG26 TRE glass (Table 3)

In water FF levels were close to zero. TF (Fig. 3)was higher than FF and was not significantly differentfrom levels produced by the RAW LG26/water combi-nation. The amount increased steadily with time toreach 0.43mg/g after 21 day. Only between 1 and 14days was the increase significant.When AcW was used FF was not significantly

different to when water was used, apart from ahigh, and possibly spurious, result at 7 day. TF inAcW (Fig. 3) showed a sharp increase during the first24 h of contact giving a level of 0.91mg/g. Thereafter itrose to reach 1.37mg/g after 21 day. There was nosignificant difference between RAW/AcW and TRE/AcW LG26.Fluoride release from LG26 glass was always very

much less than that produced by G338 glass (Fig. 5).

Fig. 2. Total cumulative fluoride released from RAW G338 and LG26

glasses.

Fig. 3. Total cumulative fluoride released from TRE G338 and LG26

glasses.

J.A. Williams et al. / Biomaterials 23 (2002) 2191–2200 2195

3.8. LG26 cement (Table 3)

Within experimental variation all four cementsreleased similar levels of FF, with maturity of cementprior to dissolution having no significant effect. Thelevels were not dissimilar to that of TRE/AcW. Theamount decreased as the contact time increased and fortwo of the cements (60min and 1 day maturity) thisdecrease was significant between 1 and 21day.TF increased significantly as the length of contact

with water increased to reach 1–2mg/g after 21 days butlike G338 was not dependent on (time)1/2. Althoughthere were some significant differences caused by thematurity of the cement these were not uniform. Onceadjusted on a weight basis TF was higher than forRAW/water and TRE/water and similar to RAW/AcWand TRE/AcW.

4. Discussion

The amounts of glass and cement (approximately0.15 g) and water in which they were dispersed were

comparable to other studies measuring fluoride releasefrom monolithic gics, that is where fluoride is releasedvia diffusion related processes. There are some varia-tions between these studies. For example, amountsequivalent to 0.11 g in 20ml [1], 0.12 g in 20ml [2] and0.02 g in 20ml [6] have been used (assuming the densityof cement to be 2.0 g/cm3). With glass andcement present as particles in the present study,not as blocks of material, little or no diffusion is likelyto occur and therefore build-up of fluoride ion isunlikely to supress fluoride release (as can happenin a diffusion related process). The limiting factorhere is probably the solubility of the fluoride com-pounds. Nevertheless, the experiment was terminated at21days where there was still a considerable volume ofwater remaining. Another consideration was the diffi-culty of excluding stray particles if the liquid level weretoo low.It was clear from the results that the glasses

themselves contained soluble fluoride and that eachprocess undergone by them altered the level of solublefluoride. In addition the process might also affect theratio of free to total fluoride. The effect that eachprocess has had is discussed below under separateheadings.

4.1. The presence of fluoride before cement formation

occurs

Far from relying upon the cement formation toproduce the soluble fluoride later released from cementboth G338 (in particular) and LG26 glass powderscontained soluble fluoride before any treatment orcement formation occurred. The origin of this solublefluoride is as yet undetermined but one possibility as asource is the milling process used to reduce the glass to aparticle size suitable for cement use. This processnormally involves crushing larger particles of glass witha dense, refractory medium such as quartz pebbles,steatite balls or zirconia cylinders under dry conditions.As a result the surface may be left coated with a fine,reactive, dust, some or all of which could dissolve in anaqueous medium.Although the G338 glass composition contained

about three times as much fluoride than LG26, 18.6%compared with 5.2% F, the amount found in water after21 days contact with RAW glass was higher by a factorof ten. It is not known whether this higher fluoriderelease into water is a consequence of physical orchemical effects. G338 may contain more fine materialof the type suggested above or it may be attributable todifferences in chemical composition. Both glassescontain aluminium and calcium but G338 glass contains8.2% sodium oxide (Na2O) equivalent to 6% sodiumwhereas LG26 is essentially sodium–free. Sodiumfluoride (NaF) is readily soluble in water up to a level

Fig. 4. Total cumulative fluoride released from G338 and LG26

cements.

J.A. Williams et al. / Biomaterials 23 (2002) 2191–22002196

of 40 g/l and it may be that G338 contains fluoride in amore water-soluble form. The finding that G338 lost lessfluoride during the acid treatment process than didLG26 may possibly mean that more has been left forsubsequent dissolution, or possibly that the nature ofthe fluoride has been altered by the acid treatmentprocess.Only a proportion of F appeared as FF and this

proportion remained essentially constant suggesting thata complexation process is present. In the RAW/watercombination, species which could affect the free/totalratio can only have come from the glass itself. With TFincreasing with time it seems unlikely that fluoride wasprecipitated as calcium fluoride (CaF) from the calciumion which both glasses contained, G338 with 10.7%CaO and LG26 with 29% CaO. Both glasses containedphosphate ions. Analysis showed other species suchas MgO were present at very low levels of o0.05%.G338 contained 31% Al2O3 and 28.1% SiO2 andLG26 24% Al2O3 and 36% SiO2. Although there maybe interactions in the silicate matrix since G338has the higher proportion of Al/Si and also a higherratio of FF/TF aluminium would be the suspectedcomplexant.

4.2. The susceptibility of raw glass to acidic water

(AcW)

Contact with acidic water (0.014m) had no significanteffect on FF from either glass but did increase TF(Fig. 2). This indicates that fluoride is capable ofdissolving to a greater extent in this particular mildlyacidic solution. If so, fluoride might therefore be moresoluble in the oral environment than in laboratorystudies in water. LG26 was more affected than G338glass with an increase of about 450% in TF whenchanging from W to AcW, compared to an increase of200% for G338. Why LG26 should apparently be moreaffected by the acidic solution is not clear but thisapparently large increase may be a reflection of the lowlevels of fluoride released from LG26 when in contactwith water. Whereas RAW G338 is increased from 5.6to 10.7mg/g by changing from water to AcW, RAWLG26 is increased from only 0.5mg/g to 2.1mg/g.The ratio of TF measured after the RAW glass had

been in contact with AcW for 21days compared to thepercentage originally present in the glasses was (1.1%/18.6%)=5.9 for G338 and (0.2%/5.2%)=3.8 for LG26.A study by Hatton & Brooks [14] showed that to

Table 3

Free and total fluoride release from LG26 as mg/g and (standard deviation)

Time (days) 1 7 14 21

Combination Free or total (F/T)

RAW/Wa F 0.09 (0.07) 0.04 (0.15) 0.09 (0.02) 0.00 (0.04)

T 0.13 (0.24) 0.23 (0.08) 0.16 (0.04) 0.47 (0.07)

RAW/AcWa F 0.36 (0.04) 0.72 (0.26) 0.32 (0.02) 0.10 (0.22)

T 1.24 (0.24) 1.49 (0.46) 1.06 (0.07) 2.09 (3.01)

TRE/Wa F 0.18 (0.09) 0.03 (0.02) 0.01 (0.01) 0.00 (0.03)

T 0.05 (0.05) 0.17 (0.02) 0.27 (0.12) 0.45 (0.46)

TRE/AcWa F 0.04 (0.04) 0.49 (0.22) 0.01 (0.05) 0.06 (0.06)

T 0.92 (0.15) 1.09 (0.81) 0.75 (0.27) 1.37 (0.45)

Actual release from cement

CEM/5min F 0.38 (0.05) 0.48 (0.06) 0.08 (0.02) 0.07 (0.02)

T 0.96 (0.11) 1.19 (0.07) 1.28 (0.24) 1.58 (0.22)

CEM/60min F 0.29 (0.12) 0.66 (0.12) 0.10 (0.04) 0.09 (0.05)

T 1.07 (0.15) 1.07 (0.63) 1.70 (0.21) 1.86 (0.36)

CEM/1 day F 0.30 (0.30) 0.17 (0.04) 0.03 (0.03) 0.03 (0.03)

T 0.56 (0.07) 0.61 (0.18) 1.09 (0.84) 0.37 (0.37)

CEM/7 day F 0.19 (0.02) 0.04 (0.04) 0.02 (0.02) 0.01 (0.01)

T 0.45 (0.15) 0.72 (0.48) 0.40 (0.41) 0.96 (0.14)

Adjusted for weight in cement (100/73.7)

CEM/5min F 0.51 0.64 0.10 0.09

T 1.28 1.59 1.71 2.11

CEM/60min F 0.39 0.92 0.13 0.12

T 1.43 1.43 2.27 2.48

CEM/1 day F 0.40 0.23 0.04 0.04

T 0.76 0.81 1.45 0.49

CEM/7 day F 0.25 0.05 0.03 0.01

T 0.60 0.96 0.53 1.28

Pooled mean F 0.39 0.46 0.08 0.06

T 1.1 1.20 1.49 1.59

J.A. Williams et al. / Biomaterials 23 (2002) 2191–2200 2197

produce elements other than fluoride in the matrixbetween 1.4% and 9.3% of the glass was consumed.These values are not dissimilar to those found for TF inthis study.The effects of increasing pH which might increase

solubility is discussed further in the next section. Thepresence of polymeric acids did not seem to affect thefree/total F ratio and it is therefore assumed that neitheris involved in the complexation process.

4.3. The effect of acetic acid washing

This has been shown to increase the working time ofcements compared to those made from untreated glass.How it does this is not clear since Wasson & Nicholson[9] have shown that while the concentration of ions inthe acid solution used for washing the glass increased asthe acetic acid concentration increased from 2.5% to10%, the working time did not continuously increasebut passed through a maximum. However, the studyindicated that ions from glasses are more soluble inincreasingly acidic solutions. The present study foundthat treating both glasses with 5% (0.83M) acetic acidfor 60min removed fluoride from the glass and added itto the acid washing solution. Although LG26 glasscontained only a third of the fluoride of G338, slightly

more fluoride ion was removed, producing acidic filtratesolutions containing approximately 1.4mm (LG26) and1.0mm (G338) TF. With similar amounts of fluoride lostfrom the glass regardless of the amount of fluorideinitially contained therein these figures are either ameasure of the solubility of fluoride in the acid aqueousmedium or the amount of soluble glass present on thesurface of the glass particles. It is a similar effect to thatmentioned in a previous section and it appears that theLG26 glass is more vulnerable to acid attack than isG338. Using a similar acid concentration to that of thepresent study, Wasson & Nicholson [9] found G338 lost6.4mm Al, 2.7mm Ca, 4.2mm Si, 4.7mm Na and 0.3mm

P, amounts similar in quantity to the fluoride loss foundabove. In that study soluble ions remaining in the glassbefore and after the process were not measured.After acid treatment less TF was produced from both

G338 and LG26 glass, regardless of whether they weresubsequently in contact with water or AcW. The leastaffected combination (LG26/water) was also that whichreleased the least (0.5mg/g TF). The ratio of TFproduced from TRE/RAW glasses was 75% (G338/water), 95% (LG26/water), 72% (G338/AcW) and 65%(LG26/AcW).FF remained essentially unchanged regardless of acid

treatment.The acetic acid treatment reduced the amount of

soluble fluoride. However, the reduction was smallcompared to that already available for dissolution. Atworst some 65% of fluoride originally present was left inor on the glass prior to cement formation.

4.4. The susceptibility of the TRE glass to acidic water

(AcW)

As with RAW glasses, contact with this liquidincreased TF but had less effect on FF when comparedto water. G338 glass produced more TF than LG26.Therefore, although acid washing had reduced solublefluoride from both glasses the effect of contact withAcW was generally the same as with RAW glass. Somecompounds, such as calcium fluoride, are more solublein dilute acids compared to water [15] and this mayaccount for the increased fluoride in AcW.

4.5. Comparison of glasses and cements

Allowing for the fact that the cement is 75% glassgives an adjusted value of TF from G338 cement of 8.0–9.3mg/g after 21 days. This is higher, but notsignificantly so, than 7.8mg/g TF from the TRE/AcWacombination and lower than the 10.7mg/g for RAW/AcW. Both these latter values were significantly higherthan the 4.2mg/g found for TRE/water. LG26 cementalso adjusted for weight, produced a maximum TF of2.5mg/g glass compared with 1.4mg/g from the TRE/

Fig. 5. Free and total fluoride released after 21 days for glasses and

cement (pooled means).

J.A. Williams et al. / Biomaterials 23 (2002) 2191–22002198

AcWa combination.This also seems to show that LG26is more vunerable to acid attack. This is more than the1mg/g TF released from discs of LG26 cementformulated as a conventional gic [13] but in the studyby Hadley et al. the fluoride release was measured for 97days and may not have been complete.The percentage increase in TF caused by the process

of cement formation is similar (2.4–2.8%) for bothglasses in spite of the differing levels of fluorideoriginally present in the bulk glass (Table 4). Thisincrease from both G338 and LG26 could be producedby the dissolution of only 0.1 mm glass from particles ofdiameter 10 mm (see Table 5).Once adjusted for weight G338 cement generally

produced more TF and less FF than glass. This alteredthe FF/TF ratio and may indicate that the cementforming process has increased the degree of fluoridecomplexation. Although this did not occur with eitherTRE glass with AcW it must be remembered that duringthe cement forming process the glass is in contact, forhowever brief a time, with 5m acid, a much higherconcentration than that in AcW. A 5m solution issimilar to a 35% polyacrylic acid solution often used forgics. The glass may be attacked more severely orpreferentially compared to when the 0.014m AcW isused. More ions which can complex F may be produced.Neither cement maturity prior to contact with water nor

the presence of polymeric/tartaric acid solution affectedthe extent of complexation.The effect of cement formation was to increase

slightly the amount of TF produced compared to thatfrom TRE/water combination. Thus, it may be con-cluded that some acid attack has occurred duringcement formation even though this was only a minorsource of F.

4.6. The effect of time

The finding that cement maturity had little effect onthe amount of fluoride produced indicates a lack ofcontinuing reaction between the acidic liquid and theglass. This may also occur when a silicate cement ismade since this also invokes the attack by a non-polymeric acidic molecule on the glass. It may not be thecase for polymeric acid-based gics where properties suchas compressive strength may change with time [16,17].Since only four time intervals were taken definiteconclusions about the effects of time on fluoride releasecan be only tentative. Considering TF, the typenormally measured in fluoride release studies, it appearsthat where glass powder or cement disintegrates in liquidno strong correlation exists between TF and (time)1/2.The best fit plots were given by Ln/Ln relationships withboth RAW/water and TRE/water having slopes of theorder of 0.3–0.4. In AcW the slopes were reduced to 0.14and this was the value found for cement. The cementplots appear to have a higher degree of correlation withLn time than do those where glass was used but furtherwork would be needed to ascertain this. Cranfield &Kuhn [18] also reported results where the relationshipwas given by a log–log plot and concluded that the(time)1/2 relationship was a special case.No initial burst of fluoride was seen with in the

present study, unlike that generally seen with apolymeric acid-based gic. However, silicate cementshave also been shown [1] not to have an initial burst offluoride so that different types of fluoride release areexhibited by different materials and this may relate tothe type of reaction used to form the cement [1,2].Alternatively, since neither the glass nor the cementremained as monolithic specimens it may be assumedthat the dependence on (time)1/2 seen previously forcements is due to processes such as diffusion orpermeation rather than fluoride release itself. Bothglasses when formulated as conventional gics have beenshown to release fluoride at a rate dependent on (time)1/2

[2,13] Since no measurements were made at timeintervals of o1day it is not known whether the fluorideis released immediately on contact with liquid oruniformly over 24 h.Improvements in the method, such as continuous

stirring of the sample solutions, could be made forfuture work, which would assist in accuracy particularly

Table 5

Calculation of the amount of glass which needs to be dissolved to

produce the increase due to cement formation

Glass G338 LG26

% fluoride present in RAW glass 18.6 5.2

TF g/g initially present in RAW glass 0.186 0.052

TF g/g after acid treatment 0.175 0.050

For R0=5mm, R= 4.90mm 4.94mm

For a glass containing N particles/g of radius R0 from which a uniform

layer is removed leaving the particles of radius R: Assume both glasshave the same particle size and density d, therefore initial weightW 0 =

V0d and final weight W ¼ Vd:V0 ¼ N4=3p(R0)

3 and V ¼ N4=3pðRÞ3 and W0=W ¼ V0=V ¼ðR0Þ

3=ðRÞ3

For R0 = 5mm, W0/W = 125/(R)3.

Table 4

Calculation of total fluoride produced by cement formation

mg TF present/g

G338 LG26

Present in RAW glass initially 186 52

Released from TRE glass into water (21day) 4.2 0.4

Released from cement into water (21day) 8.6 1.6

Inc=(Cement-TRE)a 4.4 1.2

% increase (Inc/RAW) 2.4 2.8

aThe extra amount of F produced from cement formation.

J.A. Williams et al. / Biomaterials 23 (2002) 2191–2200 2199

where only low levels of fluoride are to be measured.However it has been seen that continuously stirredsolutions are slow to settle once stirring ceases and thiswould prevent sampling unless the suspensions of glassin water were centrifuged to prevent particulatesremaining in the liquid samples. Complete re-dispersionafter sampling would then be required. This procedurewould limit the number of sample solutions underevaluation.

4.7. Complexation of fluoride ion

While TF increased steadily with time FF remainedgenerally constant for both RAW and TRE glasses. Oneexplanation is that soluble F is accompanied by otherwater soluble ions which complex F either as it isreleased in the ionic state, or prior to release such thatthe F is released as a complexed ion. The process ofcement formation can also produce ions capable ofcomplexing F so that these ions are likely to be thosesoluble in acids such as aluminum, silica and phosphate[7]. Some F may be removed from the system byprecipitation as calcium fluoride (CaF) and this com-pound although sparingly soluble in water is moresoluble in acids [15]. If calcium were released moreslowly than fluoride ion, precipitation could explain adecrease of TF with time. This would also occur forcompounds such as AlF3 and AlPO4 which are barelysoluble in either water or acids. The high initial FF/TFfound for the G338 TRE/AcW combination might beexplained if complexing ions are slower than fluorideions to move out of the TRE glass. There is a somewhatsimilar effect seen with RAW G338/AcW.

5. Conclusions

It was found that

1. Fluoride containing glasses had considerable quan-tities of water soluble fluoride present before anytreatment to alter surface properties is given andbefore the cement forming process took place. Asmuch fluoride was released from the glass alone asfrom cements made from such glasses. Thus, acidattack on glass during the cement forming process didnot account for all the fluoride available

2. Acid treatment to modify properties such as theworking time of a cement removed ion species fromthe surface and reduced soluble fluoride for bothglasses examined.

3. The presence of mildly acidic water increased solublefluoride.

4. The cement forming process only slightly increasedsoluble fluoride; for both G338 and LG26 glass thiswas about a 3% increase.

5. Fluoride was complexed to a greater extent whenreleased from cement than from glass.

6. During the duration of the experiment the relation-ship TF proportional to (time)1/2 did not describe theprocess of fluoride solubility for powder dispersed inliquid either before or after cement formation.

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