Jill Ann Williams

The effect of the disc support system on biaxial tensile strength of a glass ionomer cement

OBJECTIVE This was to examine how varying the type of support, from a complete ring to a series of point supports, affected the biaxial tensile strength of one glass ionomer cement. METHOD Three support diameters from 11.5 to 28.6 mm were achieved using 3 mm ball bearings as point supports equidistantly spaced around the diameter. From 3-30 point supports were used depending on diameter. At the maximum number of point supports for each diameter the support points were 3 mm apart. After 24h storage in water at 37 degrees C the biaxial tensile strength of 1 mm thick glass ionomer restorative cement discs was measured using a loading rate of 1 mm min(-1). The load at break was converted to biaxial tensile strength using the Timoschenko and Woinowsky-Kreiger equation using a Poissons ratio value of 0.30. The mean strength of six specimens tested per support regime was calculated. RESULTS Comparison with the mean result obtained from using a continuous knife-edge support showed there to be no significant difference (unpaired t-test) between the different support systems except in two cases, both being when a four-point support was used. Neither the support diameter nor the number of point supports was crucial. SIGNIFICANCE Results from studies where different systems have been used to support brittle cement discs may be compared.

The effects of adding fluoride compounds to a fluoride-free glass ionomer cement on subsequent fluoride and sodium release.

Studies have shown that ions in a glass ionomer matrix are 1-10% of the amounts present in the original glass. To measure more precisely the release from a cement matrix, known amounts of ions were added to LG30 glass which was fluoride and sodium-free. Cement without additions acted as the control. 1.4-1.6% of each of sodium, calcium and aluminum fluorides were added to three portions of control blend. The sodium and fluoride release into deionised water from five discs of each cements blend was measured for 8 months. This represented complete release for sodium but not for fluoride. Traces of fluoride and sodium in the glass produced low but measurable amounts indicating about a third of the fluoride and substantially all sodium present in LG30 was released. The addition of calcium fluoride had no significant effect on sodium or fluoride release and aluminium fluoride minimal effects. Adding sodium fluoride significantly enhanced release of both ions although fluoride release was less than from a glass containing 5% fluoride. Only small proportions of the additions, 2-5% of the fluoride and 13% of sodium, were released. Sodium and fluoride appeared to be released independently. For LG30 cements additives were poor at supplying extra ions.

Effects of adding sodium and fluoride ions to glass ionomer on its interactions with sodium fluoride solution.

This investigates the effects of the addition of Na and F ions to a glass ionomer cement in which those ions are not inherently present on its interactions with dilute (0.2%) NaF solution. Both the effect of the solution on the cements surface morphology and the effect of the cement on the solution in terms of take up of Na+ and F- and of change in pH are to be investigated. These results are to be compared to previous results obtained with glasses which contained both, one, or neither of the ions as components of their glasses. NaF (1.3% by weight in the mixed cement) was added to the powder components of a glass ionomer based on LG30 glass (which contains Al, Si, Ca, P, and O only). Discs of cement were set in moulds at 37 degrees C for 1 h then stored in water at 37 degrees C for 3 days. Each test disc was then immersed in 10 ml 0.2% NaF solution whereas controls remained immersed in water (N = 3 for test and control). Test and control disc surfaces were assessed both qualitatively by electron microscopy and quantitatively by linear profilometry (Ra values). Potentiometry was used to measure solution pH and Na and F concentrations using a pH electrode and suitable ion selective electrodes both before and after cement immersion. The surface of test specimens was subject considerable disruption with the polysalt cement matrix being removed and residual glass particles being disclosed. The controls showed no such disruption. This effect was reflected in a significant difference of Ra. Such an effect was not shown by test and control surfaces of LG30 but a similar effect was to that shown by LG26 (which contains F as a glass component). Solution pH changed by 1 unit which was much more than the change shown by LG30 or LG26 but is similar to that of AH2 and MP4 cements which both contain Na. The Na and F uptake was much lower than for LG30 whereas that of LG26 was higher than LG30. The Na:F ratio was 0.29:1 compared to 1.26:1 for LG30 (LG26 = 1.01:1, AH2 = 1.02:1, MP4 = 1.04:1). Fluoride addition to a F-free glass ionomer renders it vulnerable to surface disruption by NaF solution showing that fluoride complexes produced in glass dissolution are not necessarily involved in this process. Sodium addition to a Na-free glass ionomer confirms the role of this cement in enhancing pH change in NaF solution. The level of uptake of F- from a NaF solution in much lower than that for the F-free glass ionomer which shows there is no direct relationship between F- uptake and surface disruption. The ratio of Na:F uptake is below 0.3:1, but the pH change is similar to cements where the ratio is close to unity which indicates that F-/OH- interchange is not a significant mechanism even when anion/cation uptake is not balanced.

The glass ionomer cement: the sources of soluble fluoride

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 proportion, 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.

Effect of monovalent ions in glass ionomer cements on their interaction with sodium fluoride solution.

The effects on surface morphology of glass ionomer cements following exposure to 0.2% NaF solution were studied. The effect of cement on the solution was also evaluated. The four cements were chosen to contain Na and F, Na alone, F alone and neither Na nor F to show any interactions produced by having the same ion in both the cement and solution. Four glass ionomer cements were formulated so that they differed only in respect of the glass component. AH2 (a glass used in dental restorative cement) contained both Na and F, MP4 (a glass used in orthopaedic cement) contained Na only, LG26 (a glass used in surgical cement) contained F only and LG30 (an experimental control glass) contained neither F nor Na. Discs of cement were set in moulds at 37 degrees C for 1 h, then matured in water for 3 d. Each test disc was then immersed in 10 ml 0.2% NaF for 24 h at 37 degrees C whereas control discs remained in water. The test and control disc surfaces were assessed qualitatively using electron microscopy and quantitatively by linear profilometry generating roughness values (Ra). Test solution pH was measured before and after cement immersion. Inspection of the electron micrographs showed considerable disruption of AH2 and LG26 test surfaces compared to their controls whereas MP4 and LG30 showed similar surfaces for test and control. Statistical analysis of the Ra values showed that AH2 and LG26 test surfaces were significantly rougher than their controls as well as LG30 and MP4 test surfaces, which were not significantly different from their controls. All NaF solutions show pH increases; those for AH2 and MP4 were significantly higher than those for LG26 and LG30. The F-containing cements were subject to surface disruption whereas F-free cements were not. The Ra values of test surfaces correlated strongly (r = 0.998) with the F uptake of the cements (data from a previous study) but it was not possible to ascribe the causality to this association. The pH changes appear to be influenced by whether or not Na is present in the cement. The resultant pH values are too near to neutral for pH alone to explain the surface disruption observed. In addition, it is concluded that the changes in OH ion concentration are too low to permit F-/OH- interchange as a possible explanation for F uptake by these cements.

The influence of sample dimensions on fluoride ion release from a glass ionomer restorative cement

The fluoride release from a commercial, restorative glass ionomer cement was found to be strongly dependent on sample surface area rather than volume. This was noted for disc, cylindrical- and bar-shaped specimens over periods ranging from 1 day to 3 yr. Release from all shapes of specimen followed the established pattern of an initial non-linear region followed by one where release was proportional to the square root of time. If fluoride levels in the cement matrix of specimens were artificially increased by incorporation during the mixing then the release pattern during the first few months was altered. The initial release increased for some specimen sizes and decreased for others. The dependency on surface area was greatly reduced for several months. By the time a year had elapsed the correlation between fluoride ion release and surface area had been re-established. The influence of additional fluoride during setting can therefore act to perturb the normal release pattern and may in some instances reduce the initial fluoride release. Release should be quoted in terms of, or with measurements of, the surface area of specimens under investigation.

Comparison of ion release from a glass ionomer cement as a function of the method of incorporation of added ions

The addition of potassium and fluoride ions to a water-admixed glass ionomer cement initially free from either ion was accomplished either by addition via the mixing water or by immersing mixed set cement in a solution of the ions in water. In both cases the aqueous concentration of ions was the same. The release of both potassium and fluoride was measured for 500 days. Ion release from immersed specimens was some 20-fold higher than from the other method indicating enhanced concentration of ions inside the cement. This calculation assumes that the ions resided in the aqueous phase. Ions added by mixing appeared to follow the release pattern shown by glass ionomers containing intrinsic fluoride; ions added by immersion did not. As specimens matured the amount which could be taken up by immersion was reduced. Potassium release was still continuing at 500 days whereas fluoride release was complete.

In vitro erosion of 20 commercial glass ionomer cements measured using the lactic acid jet test

The lactic acid jet test has been proposed for incorporation in international standards for dental cements, since results correlate well with in vivo erosion. In the literature, jet test results have been reported on only six glass ionomer cements. This study examines three types of glass ionomer, restorative, luting and metal reinforced, twenty materials from six manufacturers. A wide range of erosion rates was found particularly for restorative cements. The hypotheses put forward by previous workers to explain differences in erosion rates found on their limited range of glass ionomers do not apply to the wider range that we tested.

Kinetics of fluoride release from zinc oxide-based cements.

Considerable attention has been given to the release of the cariostatic fluoride ion from glass-based dental cements (dental silicate and glass ionomer). In these, the total available fluoride content is not precisely known since fluorine is distributed between the cross-linked aqueous salt matrix, partially dissolved glass, and undissolved glass. In analogous cements based on zinc oxide the fluoride is added as highly soluble SnF2. The object of this study is to compare the F- ion release profiles of commercial zinc polycarboxylate and zinc phosphate containing 4.4 and 3.6% SnF2, respectively. Mixed cements were clamped in split ring moulds to produce discs of 10 mm x 1 mm after storage at 37 degrees C for 1 h. Each was weighed and immersed in 10 ml of deionised water. When this changed, at 13 time intervals up to 98 days, the fluoride content was measured using an ion selective electrode. The mean (N = 3) values obtained were expressed cumulatively [F] in micromol F ion/g cement. The total [F] released was 111 for the zinc polycarboxylate and 286 for zinc phosphate compared with total F in the cements of 561 and 464, respectively. When the cumulative [F] was plotted versus t(1/2) close associations were found for both cements. For the polycarboxylate the regression line [F] = 10.6t(1/2) + 9.9 fitted well over the whole 98 days (R = 0.997). For the phosphate a better fit regression line was obtained using results up to 32 days only; [F] = 36.8t(1/2) - 8.4 (R = 0.999). For t > 32 days results increasingly deviated from this line. These results fitted a regression line of the form [F] = 81.7log(e) t - 87.3 (R = 0.9997). Comparisons are made with data from previous authors both for zinc phosphate cement and glass-based cements and with diffusion theory of F ion release. It is concluded that zinc-based cements provide some indications of how glass-based cements may behave over long periods of release and that zinc phosphate is the material of clinical choice for orthodontic cementation if maximal fluoride release is the prime criterion.

Lactic acid jet test : in vitro erosion rates of glass ionomer dental cements containing radiopacifying elements

The lactic acid jet test erosion rates were measured for 13 radiopaque glass ionomer dental materials obtained from a number of manufacturing sources. The erosion rate was compared with that found for the non-radiopaque restorative from the same manufacturer to determine whether the addition of an extra element had affected the resistance to erosion. Six materials were not significantly affected, six showed a significant increase in erosion rate. Only one material showed a reduced erosion rate. Materials containing a high proportion of any additive could show an increased erosion rate. Glass ionomer cements with or without radiopacifying elements had low erosion rates compared with other dental materials.