(Davies etd . 1993; Bapna and Mueller 1994; Marks etd . 2000).
2.1.4 Extrinsic variables
Extrinsic variables or experimental condition can modify the effect of intrinsic factors on quantitative fluoride release. These factors included manipulation, maturation of the cement before immersion in the aqueous solution, sample preparation: size and shape, storage medium composition and pH.
Mixing
In a study on the effect manipulation on fluoride release. Miller at d. (1995) showed that hand mixed glass-ionomer cement released significantly less fluoride than cement prepared by mechanical trituration. Variations in mechanical mixing protocol did not have an affect on cumulative fluoride release from the capsulated glass-ionomer cement. The experiment did not however take account of differences in the formulation of the commercial glass- ionomer cement used in the study (Miller et d . 1995). The study of De Moor (reported above) showed that if commercial glass-ionomer cements of the same formulation were used, the cumulative fluoride release of capsulated glass- ionomer cement was lower than when hand-mixed. This group suggested that the encapsulation of glass-ionomer cement apparently affected the fluoride release process because the rate of set of the cement is generally faster (De Moor et d . 1996). This study did not take into consideration the variation in powder/ liquid ratio in the capsulated glass-ionomer cement when compared to the same material when hand-mixed.
Maturation time
A study of Bapna and Mueller (1994) showed the effect of cement maturation on fluoride release. They showed that when immature cement (2 min after mixing) was immersed in water, the fluoride released was substantially higher than the set cement (15 min after mixing). Davies etoL (1993) proposed that the rate of fluoride release is dependent on the maturity of the commercial glass- ionomer cement at the time of immersion and become less dependent on cement type with age.
A number of variations in maturation times before immersion have been evaluated. The study of Davies etd. (1993) selected a 10 minutes exposure as an immature cement and 24h as mature cement. In the study of De Moor et aL (1996), the specimen was allowed to set for 15 minutes at room temperature before transfer to deionised water. Anstice et d (1997) evaluated cements that were stored at 37°C for 1 hour before transfer to distilled water. The variations in fluoride release are in large part a result of the differing experimental conditions. This may relate to the time schedule for the fluoride determination and the maturation of glass-ionomer cement, that is the time between mixing and exposure to de-ionised water.
Sample dimension
Williams et d . (1999) determined the influence of the sample dimensions on the fluoride release from commercial restorative glass-ionomer cement. This study measured the fluoride release from the specimens of differing shapes, volume and surface area. The results showed that there is a good correlation between fluoride release and specimen surface area (correlation coefficients = 0.984). This result is in agree with Lee etd . (2000) who investigated the fluoride release from the specimens which have similar volume but different surface areas. The
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Study found that the specimens with the larger surface area released more
fluoride at all times during the evaluation.
Surrounding media
Most of the reported measurements of fluoride release from glass-ionomer cement were into water as the surrounding medium. Intra-orally, the fluoride is released into saliva, a complex solution of inorganic and organic salts (Rezk- Lega etal 1991). These may act as buffering agents and generate osmotic effects which will, in turn influence the fluoride release. It has been demonstrated in a number of laboratory studies (el Mallakh and Sarkar 1990; Williams et aL 1997) that less fluoride is released in the artificial saliva than in water.
2.1.5 Fluoride release and uptake
Fluoride release from glass-ionomer cements alone in studies sometimes does not necessarily represent the real amount of fluoride released within the oral environment. Hatibovic-Kofmann and Koch (1991) and Forsten (1991) believed that glass-ionomer cement while releasing fluoride may also take up fluoride from other sources of fluoride such as toothpaste, solutions and gels. An 'in viuo' study by Hatibovic-Kofmann and Koch (1991) investigated the release of the fluoride from three glass-ionomer cements into the saliva of pre school children for a one year period in conjunction with an additional 'in viifd study. The results showed that before placement of the restoration, the concentration of the fluoride in the saliva was 0.04 ppm. After three weeks it had increased to 0.8 ppm (the fluoride release was greatest during the first three weeks). This was about twenty times higher than before the placement of the restoration. The level of the fluoride in the saliva remained 0.3 ppm after one
year. Moreover the results 'in vM showed that glass-ionomer cement could act as a rechargeable slow release fluoride device. This indicates that the regular use of fluoride containing toothpaste can lead to take up of fluoride by the cement. This fluoride may subsequently be slowly released and thus increase the level of fluoride in the oral cavity.
2.1.6 The other species release from glass-ionomer cement
The other species released from glass-ionomer cement depend on the composition of the cement, maturation and pH of the medium (Nicholson etoL 1991; Forss, 1993; Bapna and Mueller, 1994; Williams etd . 1997). The study of Bapna and Mueller (1994) showed that when glass-ionomer cements were exposed to distilled water at both an early stage of setting (15 mins after mixing) and at very early maturation (2 mins after mixing), soluble ions can be eluted from the matrix. Both non-matrix forming ions such as F ■ and Si and matrix-forming such as A1 and Ca^+ have been found. But the storage media for unset cements showed a much higher level of soluble ions than that containing a mature cement. The study of Forss (1993) examined the release of ions from mature cements stored in deionised water for 16 days. These were then exposed to lactic acid (pH 4.0) for 16 days. After that the cements were immersed in deionised water for 122 days. The cement eluted the matrix- forming cations, aluminium and calcium or strontium. The species found in the storage medium were primarily inorganic components of the glass. The study of Williams et d . (1997) found that the low level of silver and copper ions were found in the media when metal containing glass-ionomer cements were used. The amounts released from alloy-containing materials were dependent on the amount of alloy in that formulation. Both these studies indicate that glass as well as matrix may have been dissolved.
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