Glass ionomer cements: Effect of strontium substitution on esthetics, radiopacity and fluoride release

Abstract

Objective

SrO and SrF 2 are widely used to replace CaO and CaF 2 in ionomer glasses to produce radiopaque glass ionomer cements (GIC). The purpose of this study was to evaluate the effects of this substitution on release of ions from GIC as well as its effect on esthetics (translucency) and radiopacity.

Materials and methods

Cements were produced from ionomer glasses with varying content of Sr, Ca and F. The cements were stored in dilute acetic acid (pH 4.0) for up to 7 days at 37 °C. Thereafter, the cements were removed and the solution was tested for F − , Sr 2+ , Ca 2+ , and Al 3+ release. Radiopacity and translucency were measured according to BS EN ISO 9917-1:2003.

Results

Ion release was linear to t 1/2 suggesting that this is a diffusion controlled mechanism rather than dissolution. The fluoride release from the cements is enhanced where some or all calcium is replaced by strontium. Radiopacity shows a strong linear correlation with Sr content. All cements were more opaque than the C 0.70 0.55 standard but less opaque than the C 0.70 0.90 standard which is the limit for the ISO requirement for acceptance.

Significance

This study shows that the replacement of calcium by strontium in a glass ionomer glass produces the expected increase in radiopacity of the cement without adverse effects on visual properties of the cement. The fluoride release from the cements is enhanced where some or all calcium is replaced by strontium.

1

Introduction

Glass ionomer cement (GIC) was developed for use in restorative dentistry through modifications of the dental silicate cement (DSC) . Poly(acrylic acid), which was also being used in zinc polycarboxylate cements, replaced the phosphoric acid of the DSC . Although the earliest GICs had poor physical properties which confined their use to class III and class V cavities, their direct adhesion to tooth and release of cariostatic fluoride ion ensured some popularity and encouraged the development of GICs with better physical properties . In class III and V cavities, secondary (recurrent) caries could normally be detected by visual inspection.

One of the drawbacks of earlier GICs was that they lacked radiopacity which made it difficult to radiographically differentiate any underlying caries (recurrent caries) from the restoration. This was a big disadvantage especially in situations where caries could only be detected radiographically. To overcome this deficiency radiopaque secondary fillers (in addition to residual glass) were added. These included zinc oxide, barium sulphate silver-tin amalgam alloy powder and silver sintered to the GIC glass particles . All these produced adequate levels of radiopacity but produced opaque cements and in the case of metal additions the color was also markedly different to tooth.

The glasses described in the original GIC patents were all calcium fluoroaluminosilicates. Since strontium is the most similar element to calcium, and glasses containing it were already being used in dental composite resin restoratives , in the mid-1980s patents describing strontium as a radiopacifying glass component were filed . Patents were also filed for the use of strontium fluoride as a radiopacifying cement additive which did not adversely affect the opacity or color of the cement .

The fluoride release from GICs is well established and sustained over at least five years . The release rate into water and artificial saliva is proportional to t 1/2 indicating a diffusion controlled process, whereas into buffered lactic acid, release is proportional to t indicating dissolution control .

The objectives of this study are to examine the effect of progressively replacing calcium by strontium in the GIC glass on the release of fluoride and the metal cations into dilute acetic acid of pH 4. The release of F − and Sr 2+ is regarded as particularly relevant as the remineralizing effect of F − is reported to be enhanced by the presence of Sr 2+ . The pH was chosen to represent conditions that can be found in interproximal areas in which bacteria and nutrient such as carbohydrate are present. By using unbuffered acid the potential neutralizing effect of the differing cement compositions could be more easily compared in terms of pH change.

2

Materials and methods

Eleven different glasses (QMAB 1–QMAB 11) were used in this study. All glasses had 4.5 SiO 2 , 3.0 Al 2 O 3 , and 1.25 P 2 O 5 whereas the molar levels of CaO, SrO, CaF 2 , SrF 2 were varied in glasses. Glass series QMAB 1–6 were formulated to look at the effect of Sr substitution on cement properties whereas series QMAB 7–11 were formulated to look at the effects of F on cement properties. The molar levels of the variable components in these glasses is given in Table 1 .

Table 1
Composition of QMAB glasses (mol%).
Glass SrF 2 SrO CaO CaF 2 F:Sr:Ca
QMAB1 0 0 3 2 2:0:5
QMAB2 2 3 0 0 2:5:0
QMAB3 2 1.5 1.5 0 2:3.5:1.5
QMAB4 2 0.5 2.5 0 2:2.5:2.5
QMAB5 1 0 3 1 2:1:4
QMAB6 1 1.5 1.5 1 2:2.5:2.5
QMAB7 1.5 1 1 1.5 3:2.5:2.5
QMAB8 0.5 2 2 0.5 1:2.5:2.5
QMAB9 0.25 2.25 2.25 0.25 0.5:2.5:2.5
QMAB10 0 2.5 2.5 0 0:2.5:2.5
QMAB11 0 0 5 0 0:0:5

All ingredients used in glass making were of pure grade and were sourced from Sigma–Aldrich, UK. The ingredients for each batch were carefully weighed and mixed together in a platinum crucible. The glasses were fired for 2 h at temperatures ranging from 1350 to 1520 °C, depending on the composition. Thereafter, the glass melt was rapidly quenched into a tank of deionised water to produce glass frit. The frit was subsequently dried in a vacuum oven for 1.5 h and then in a Gyro mill (Glen Creston Wembley, London, UK). The resulting glass powders were sieved through a 45 μm mesh sieve to give a powder ( Table 2 ), which was used in the subsequent cement formation.

Table 2
Particle size analysis of QMAB glasses.
Glass D 90 (μm) D 50 (μm) D 10 (μm)
QMAB1 34.8 6.25 0.88
QMAB2 34.9 6.65 0.95
QMAB3 31.3 6.27 0.88
QMAB4 30.2 7.80 0.96
QMAB5 32.7 6.85 0.98
QMAB6 28.8 6.60 0.78
QMAB7 31.3 6.79 0.86
QMAB8 32.0 6.02 0.82
QMAB9 26.8 5.70 0.79
QMAB10 28.9 5.02 0.71
QMAB11 28.5 5.90 0.84

All glass powders were milled to the same particle size and the particle size distribution as measured using Malvern/E particle size analyser (Malvern Instruments Ltd., Worcs, UK).

For cement formation, the glass powders were blended with anhydrous poly(acrylic acid) at a 5:1 ratio and were then mixed with 10% aqueous solution of tartaric acid solution at a powder:liquid ratio of 6:1. Cement mixing was performed at room temperature using a glass slab and stainless steel spatula. The cement formulations were characterized in terms of working and setting time at 21.5 °C using a Wilson oscillating plate rheometer (Linseis L220E) ( Table 3 ).

Table 3
Working time (WT) and setting time (ST) in seconds of QMAB cements.
Cement WT (s) ST (s)
QMAB1 72 276
QMAB2 78 294
QMAB3 90 282
QMAB4 60 474
QMAB5 69 279
QMAB6 81 267
QMAB7 72 222
QMAB8 66 294
QMAB9 105 507
QMAB10 135 705
QMAB11 132 546

For fluoride ion release and cation release, cement mix was packed into molds to produce cylindrical specimens measuring 4 mm (diameter) by 6 mm (height). Soon after packing into molds, the specimens were stored for 1 h in an incubator at 37 °C and 100% relative humidity. Thereafter, the samples were individually placed into a test tube containing 50 ml of dilute acetic acid at pH 4.0 (produced by the addition of 1 ml of glacial acetic acid in 950 ml of deionized water). The test tubes were then stored in an incubator at 37 °C. Seven sets of samples were produced in this manner, with each set containing six cement samples from each glass type. This was done to allow ion release (F − , Ca 2+ , Sr 2+ , Al 3+ ) to be measured daily for 7 days.

Release of fluoride ions was measured using an ion selective electrode Elit 8221 (Nico2000 Ltd.) with an AgCl reference electrode. Measurement of Ca Sr and Al was done using ICP-OES (Vista-Pro).

Radiopacity and translucency of the cements was measured according to specification laid out in BS EN ISO 9917-1:2007 standard for water based cements. Translucency of the cement discs was measured at 1 h and 24 h after mixing. Translucency was measured against ISO Standards with C 0.70 values of 0.35 and 0.55.

2

Materials and methods

Eleven different glasses (QMAB 1–QMAB 11) were used in this study. All glasses had 4.5 SiO 2 , 3.0 Al 2 O 3 , and 1.25 P 2 O 5 whereas the molar levels of CaO, SrO, CaF 2 , SrF 2 were varied in glasses. Glass series QMAB 1–6 were formulated to look at the effect of Sr substitution on cement properties whereas series QMAB 7–11 were formulated to look at the effects of F on cement properties. The molar levels of the variable components in these glasses is given in Table 1 .

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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Glass ionomer cements: Effect of strontium substitution on esthetics, radiopacity and fluoride release

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