d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Glass ionomer cements: Effect of strontium substitution on esthetics, radiopacity and fluoride release S. Shahid ∗ , U. Hassan, R.W. Billington, R.G. Hill, P. Anderson Dental Physical Sciences Unit, Institute of Dentistry, Barts and London School of Medicine and Dentistry, Queen Mary University of London, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. SrO and SrF2 are widely used to replace CaO and CaF2 in ionomer glasses to produce
Received 25 March 2013
radiopaque glass ionomer cements (GIC). The purpose of this study was to evaluate the
Received in revised form
effects of this substitution on release of ions from GIC as well as its effect on esthetics
18 October 2013
(translucency) and radiopacity.
Accepted 4 December 2013
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− , Sr2+ , Ca2+ , and Al3+
Keywords:
release. Radiopacity and translucency were measured according to BS EN ISO 9917-1:2003.
Glass ionomer cements
Results. Ion release was linear to t1/2 suggesting that this is a diffusion controlled mechanism
Radiopacity
rather than dissolution. The fluoride release from the cements is enhanced where some or
Esthetics
all calcium is replaced by strontium. Radiopacity shows a strong linear correlation with Sr
Strontium
content. All cements were more opaque than the C0.70 0.55 standard but less opaque than
Fluoride release
the C0.70 0.90 standard which is the limit for the ISO requirement for acceptance.
Ion release
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. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Glass ionomer cement (GIC) was developed for use in restorative dentistry through modifications of the dental silicate cement (DSC) [1]. Poly(acrylic acid), which was also being used in zinc polycarboxylate cements, replaced the phosphoric acid of the DSC [2]. 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 [3]. 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
∗ Corresponding author at: Dental Physical Sciences Unit, Francis Bancroft Building, Queen Mary University of London, Mile End, London E1 4NS, UK. Tel.: +44 020 7882 5983; fax: +44 020 7882 7979. E-mail address: [email protected] (S. Shahid). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.12.003
309
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
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 [4] silver-tin amalgam alloy powder [5] and silver sintered to the GIC glass particles [6]. All these produced adequate levels of radiopacity [4] 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 [7] 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 [8], in the mid-1980s patents describing strontium as a radiopacifying glass component were filed [9]. 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 [10]. The fluoride release from GICs is well established and sustained over at least five years [11,12]. The release rate into water and artificial saliva is proportional to t1/2 indicating a diffusion controlled process, whereas into buffered lactic acid, release is proportional to t indicating dissolution control [13,14]. 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 Sr2+ is regarded as particularly relevant as the remineralizing effect of F− is reported to be enhanced by the presence of Sr2+ [15]. 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 SiO2 , 3.0 Al2 O3 , and 1.25 P2 O5 whereas the molar levels of CaO, SrO, CaF2 , SrF2 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. 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 m mesh sieve to give a powder (Table 2), which was used in the subsequent cement formation.
Table 1 – Composition of QMAB glasses (mol%). Glass
SrF2
SrO
CaO
CaF2
QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
0 2 2 2 1 1 1.5 0.5 0.25 0 0
0 3 1.5 0.5 0 1.5 1 2 2.25 2.5 0
3 0 1.5 2.5 3 1.5 1 2 2.25 2.5 5
2 0 0 0 1 1 1.5 0.5 0.25 0 0
F:Sr:Ca 2:0:5 2:5:0 2:3.5:1.5 2:2.5:2.5 2:1:4 2:2.5:2.5 3:2.5:2.5 1:2.5:2.5 0.5:2.5:2.5 0:2.5:2.5 0:0:5
Table 2 – Particle size analysis of QMAB glasses. Glass QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
D90 (m) 34.8 34.9 31.3 30.2 32.7 28.8 31.3 32.0 26.8 28.9 28.5
D50 (m)
D10 (m)
6.25 6.65 6.27 7.80 6.85 6.60 6.79 6.02 5.70 5.02 5.90
0.88 0.95 0.88 0.96 0.98 0.78 0.86 0.82 0.79 0.71 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). 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
Table 3 – Working time (WT) and setting time (ST) in seconds of QMAB cements. Cement QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
WT (s) 72 78 90 60 69 81 72 66 105 135 132
ST (s) 276 294 282 474 279 267 222 294 507 705 546
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
Table 4 – Slopes, intercepts and correlation coefficients for F− release from QMAB cements. Cement
Fluoride content (mol%)
QMAB-1 QMAB-2 QMAB-3 QMAB-4 QMAB-5 QMAB-6 QMAB-7 QMAB-8
Slope values (ppm/sqrt h)
2 2 2 2 2 2 3 1
1.65 3.23 3.92 2.41 1.22 2.50 9.64 1.52
Intercept values (ppm) 5.30 3.89 8.43 5.62 9.60 5.47 10.27 3.18
R2 values 0.99 0.96 0.98 0.97 0.99 0.98 0.97 0.98
Table 5 – Cummulative ion relase (mequiv/g) and change in pH after 7 days. Cement
Ca
Sr
Al
F
pH
QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9
1.05 0.09 0.57 0.65 1.74 0.85 0.97 0.38 0.92
0.02 1.09 0.92 0.26 0.32 0.51 0.47 0.14 0.71
0.29 0.26 0.34 0.25 0.42 0.23 0.44 0.10 0.11
0.56 0.93 1.11 0.75 0.47 0.80 2.61 0.29 0.21
0.7 0.8 0.7 0.8 0.6 0.7 0.8 0.5 0.7
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− , Ca2+ , Sr2+ , Al3+ ) 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 ICPOES (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 C0.70 values of 0.35 and 0.55.
3.
Fig. 1 – Release of fluoride at different time intervals from QMAB4.
Results
The release of F ion is linear when plotted against t1/2 . A plot of release from QMAB4 containing both Ca and Sr and F (see Table 1) is shown as an example in Fig. 1.
Table 6 – Translucency (visual opacity) of cements relative to the C0.70 0.55 standard. Cement QMBA1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 C0.70 0.35 C0.70 0.55 Air a
Opacity at 1 h 1.3 1.3 1.3 1.3 1.3 1.2 0.8 1.0a 0.45
Reference 1.0 by definition.
Opacity at 24 h 1.3 1.2 1.2 1.2 1.2 1.1 0.8 1.0a 0.45
Fig. 2 – Effect of increasing strontium content on slope and intercept.
The slopes, intercepts, and square of the correlation coefficients for all nine formulations are shown in Table 4. The six cements having an F content of 2 mol% (QMAB1–6) show slope increasing with Sr content whereas the intercept shows no obvious trend, see Fig. 2. The effect of increasing F content increases both slope and intercept as shown in Fig. 3. Increasing F content shows what
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
Fig. 3 – Effect of increasing fluoride content on slope and intercept.
appears to be an exponential increase in aluminum release as shown in Fig. 4. The effect of the ion release on the pH of the acetic acid varied only slightly ranging from +0.6 to +0.8 pH units (Table 5). There was no apparent relation to the Sr:Ca ratios. Radiopacity shows a strong linear correlation with Sr content (see Fig. 5). Visual opacity differs little between cements (see Table 6). All are more opaque than the C0.70 0.55 standard but less opaque than the C0.70 0.90 standard which is the limit for the ISO requirement for acceptance.
4.
Discussion
The release of both cations and fluoride anion show a linear relationship with t1/2 rather than with t. This indicates that ion release is diffusion controlled rather than dissolution controlled process. The results found differ from those another
Fig. 4 – Effect of increasing fluorine content on cumulative aluminum release.
311
Fig. 5 – Effect of increasing strontium content on radiopacity of cements.
study on F release into an acidic medium [11] where release kinetics were shown to be linear with respect to time indicating dissolution control [12]. However, in this study acetic acid at pH 4 was used whereas in the study cited above lactic acid at pH 2.7 was used. It should also be noted that lactic acid is a stronger acid than acetic acid (pKa of lactic acid is 3.85 and pKa of acetic acid is 4.74 according to the CRC Handbook of Chemistry and Physics 93rd Edition). The relatively low pH changes observed in this study of 0.5–0.8 pH units contrast with larger changes found in the lactic acid study of 1.8. One particularly interesting aspect of the current study was the effect of partial or total replacement of Ca by Sr on F release. A previous paper [13], reporting on cements containing glasses very similar those in this study, showed no increase in F release for Sr compared to Ca (over a 20 week period into water). Cement with Sr:Ca = 0:1 released 17 mol F/g cement whereas that with Sr:Ca 1.5:3.5 released 15.4 mol/g cement and that with Sr:Ca 3:2 released 16.6. In another study [14] the same authors state: “substituting strontium for calcium has little influence on fluoride release. The small increase observed is due to the increased density of strontium affecting the measured powder:liquid mixing ratio”. However the results appear to be those of their previous publication [13]. In contrast a study, again using similar glasses but as ground glass, acid washed glass, and as cements made with acetic acid, does show slight increase in F release as Ca is replaced with Sr [15]. Interestingly, both in the current study and in this study, the slopes of the regression lines increase with increasing Sr:Ca ratio whereas the intercepts decrease. Intercepts give an indication of the magnitude of the initial “burst” effect. Previous studies have shown this to be related to the availability the monovalent Na ion [16,17]. This species is not present in the glasses used in this study. How Ca could enhance initial F release as compared to Sr, given the much greater solubility of SrF2 is difficult to comprehend from this study. The addition of CaF2 to an F-free glass ionomer does not result in any additional F release [18]. However the ICP analysis of the aluminum release from QMAB 1 and 2 shows it to be considerably higher from QMAB1 (Ca only). There is therefore either the possibility
312
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
of Al “complexes” with F ions and/or the higher solubility of AlF3 (4100 ppm as contrasted to 8 ppm for CaF2 and 390 ppm for SrF2 ) results in the greater “initial burst release” observed. Overall, it should be noted that total F release and slope of the t1/2 release profile are highest for QMAB3 which contains Sr and Ca rather than QMAB2 which contains only Sr. This may again be related the 18% higher release of Al. QMAB5 which has the highest Al release has the highest intercept value for the F release plot vs t1/2 . The release of the three cations Sr2+ , Ca2+ , and Al3+ show similar release profiles to that of F− . Comparison of anion and cation release as a mass balance is not possible as neither silicate nor phosphate ion release was measured. The pH changes found in this study are relatively small as noted in Results. This is probably due to the high ratio of acetic acid solution to cement. Specimens of cement weigh ∼140 mg and the 50 ml of pH 4 acetic acid contains ∼580 mg of acetic acid. This is 9.7 mequiv. of acetic acid whereas only 0.23 mequiv. of cations on average are released from the cement accounting for the relatively small pH changes observed. The addition of Sr to glasses for dental restoratives is designed to enhance radiopacity. In this study, the level of radiopacity is found to increase linearly with Sr content as shown in Fig. 5. The radiopacity of the lowest Sr content GIC is lower than the requirement of ISO standard [20] which requires the radiopacity of 1 mm of cement to be at least equivalent to 1 mm of Al. This cement has only 7.9% Sr in the glass which equates to 5.9% Sr in the cement (Note: 1 mm of Al is approximately equivalent to the radiopacity of 1 mm of dentin [21]). The highest level Sr produces a degree of radiopacity that enables one to distinguish the restoration from enamel. It is however not so high as to give rise to the “Mach Effect” which level causes difficulty detecting incipient caries adjacent to a very radiopaque filling. Most dental restoratives, in which the radiopacity is produced by design rather than being an inherent property of the type of restorative (e.g. amalgam), have levels from 1 to 3.5 mm Al [21]. In the first glass ionomers introduced to the dental market from 1975 to 85 there were no introduced radiopacifying elements present. Subsequently many different formulations were produced often adding the radiopacifying elements as compounds forming secondary fillers to the primary filler of partially dissolved glass particles. A range of the elements found in the cements is reported [3,19]. Most of the additions produced cements that appeared much more visually opaque than the unmodified cements from the same manufacturers. Use of simple measures of opacity such as photographic techniques [21] or visual comparison with opacity standards [20] confirmed these finding, Although a patent [8] reports that addition of strontium fluoride powder to glass ionomer produces radiopacity without appreciably increased visual opacity, no commercial materials have been reported using this additive. The results in this study show that all the materials have translucency values slightly greater than the translucency ISO standard with a C0.70 value of 0.55. However all formulations are much more translucent than the C0.70 0.90 standard which is the opaque limit of the ISO standard. Replacing Ca by Sr completely produces a slight improvement in translucency but only by a trivial 6% and all the mixed
Ca/Sr glasses are similar to the 100% Sr formulation and more translucent than the 100% Ca one. Comparing translucency after 1 h with that at 24 h shows that all five Sr containing cements have opacity reduced by 7% or more (see Table 6) whereas the cement containing Ca and no Sr shows less than 3% reduction in opacity. The changes reported for early Ca containing GICs were also small compared to those found for dental silicate cement [22].
5.
Conclusions
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. The release of fluoride ion and all cations is linear with respect to t1/2 indicating diffusion control rather than dissolution controlled kinetics. This is consistent with the relatively high pH of the acidic immersion medium and the small pH increases observed in the medium.
references
[1] Smith DC. Development of glass-ionomer cement systems. Biomaterials 1998;19:467–78. [2] Atkinson AS, Pearson GJ. The evolution of glass ionomer cements. British Dental Journal 1985;159:335–7. [3] Williams JA, Billington RW. The radiopacity of glass ionomer dental materials. Journal of Oral Rehabilitation 1990;17:245–8. [4] Williams JA, Billington RW, Pearson GJ. Lactic acid jet test: in vitro erosion rates of glass ionomer dental cements containing radiopacifying elements. Biomaterials 1993;14:551–5. [5] McLean JW, Gasser O. Glass-cermet cements. Quintessence International 1985;16:333–43. [6] Wilson AD, Kent BE. Surgical cement. British Patent 1,316,129. [7] Billington RW, Williams JA. US Patent 4,861,808; issue date 1989. [8] Billington RW, Williams JA. US Patent 4,797,431; issue date 1989. [9] Forsten L. Fluoride release of glass ionomers. In: Hunt PR, editor. Glass ionomers: the next generation. PC, Philadelphia: International Symposia in Dentistry; 1994. p. 241–8. [10] Billington RW, Hadley PC, Williams PC, Pearson GJ. Kinetics of fluoride release from zinc oxide-based cements. Biomaterials 2001;22:2507–13. [11] Limanowska-Shaw H, Nicholson JW. Buffering and ion-release by a glass ionomer cement under near neutral and acidic conditions. Biomaterials 2002;23:2783–8. [12] Billington RW, Williams JA, Pearson GJ. Ion processes in glass ionomer cements. Journal of Dentistry 2006;34:544–55. [13] Guida A, Towler MR, Wall JG, Hill RG. Preliminary work on the antibacterial effect of strontium in glass ionomer cements. Journal Materials Science Letters 2003;22:1401–3. [14] Guida A, Hill RG, Towler MR, Eramo S. Fluoride release from model glass ionomer cements. Journal of Materials Science: Materials in Medicine 2002;13(7):645–9.
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[15] Luo J, Billington RW, Pearson GJ. Kinetics of fluoride release from glass components of glass ionomers. Journal of Dentistry 2009;37:495–501. [16] Thanjal NK, Billington RW, Shahid S, Luo J, Hill RG, Pearson GJ. Kinetics of fluoride release from dental glass ionomer restorative cements: the effects of ultrasound, radiant heat, and glass composition. Journal of Materials Science: Materials in Medicine 2010;21:198–295. [17] De Barra E, Griffin S, Henn G, Devlin J, Hatton PV, Brook IM, et al. Fluoride release from glass polyalkenoate (ionomer) cements. Key Engineering Materials 1995;99–100:315–22. [18] Williams JA, Briggs E, Billington RW, Pearson GJ. The effect of adding fluoride compounds to fluoride free glass ionomer
[19]
[20] [21]
[22]
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cement on subsequent fluoride and sodium release. Biomaterials 2003;23(2):191–200. Williams JA, Billington RW, Pearson GJ. The lactic acid jet test: in vitro erosion rates of glass ionomer dental cements containing radiopacifying elements. Biomaterials 1993;14:551–5. ISO 9917-1-2007 Dentistry Water-based Cements. Williams JA, Billington RW. A new technique for measuring the radiopacity of natural tooth substance and restorative materials. Journal of Oral Rehabilitation 1987;14:267–9. Crsip S, Abel G, Wilson AD. The quantitative measurement of the opacity of aesthetic dental filling materials. Journal of Dental Research 1979;58:1585–96.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Glass ionomer cements: Effect of strontium substitution on esthetics, radiopacity and fluoride release S. Shahid ∗ , U. Hassan, R.W. Billington, R.G. Hill, P. Anderson Dental Physical Sciences Unit, Institute of Dentistry, Barts and London School of Medicine and Dentistry, Queen Mary University of London, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. SrO and SrF2 are widely used to replace CaO and CaF2 in ionomer glasses to produce
Received 25 March 2013
radiopaque glass ionomer cements (GIC). The purpose of this study was to evaluate the
Received in revised form
effects of this substitution on release of ions from GIC as well as its effect on esthetics
18 October 2013
(translucency) and radiopacity.
Accepted 4 December 2013
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− , Sr2+ , Ca2+ , and Al3+
Keywords:
release. Radiopacity and translucency were measured according to BS EN ISO 9917-1:2003.
Glass ionomer cements
Results. Ion release was linear to t1/2 suggesting that this is a diffusion controlled mechanism
Radiopacity
rather than dissolution. The fluoride release from the cements is enhanced where some or
Esthetics
all calcium is replaced by strontium. Radiopacity shows a strong linear correlation with Sr
Strontium
content. All cements were more opaque than the C0.70 0.55 standard but less opaque than
Fluoride release
the C0.70 0.90 standard which is the limit for the ISO requirement for acceptance.
Ion release
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. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Glass ionomer cement (GIC) was developed for use in restorative dentistry through modifications of the dental silicate cement (DSC) [1]. Poly(acrylic acid), which was also being used in zinc polycarboxylate cements, replaced the phosphoric acid of the DSC [2]. 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 [3]. 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
∗ Corresponding author at: Dental Physical Sciences Unit, Francis Bancroft Building, Queen Mary University of London, Mile End, London E1 4NS, UK. Tel.: +44 020 7882 5983; fax: +44 020 7882 7979. E-mail address: [email protected] (S. Shahid). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.12.003
309
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
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 [4] silver-tin amalgam alloy powder [5] and silver sintered to the GIC glass particles [6]. All these produced adequate levels of radiopacity [4] 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 [7] 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 [8], in the mid-1980s patents describing strontium as a radiopacifying glass component were filed [9]. 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 [10]. The fluoride release from GICs is well established and sustained over at least five years [11,12]. The release rate into water and artificial saliva is proportional to t1/2 indicating a diffusion controlled process, whereas into buffered lactic acid, release is proportional to t indicating dissolution control [13,14]. 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 Sr2+ is regarded as particularly relevant as the remineralizing effect of F− is reported to be enhanced by the presence of Sr2+ [15]. 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 SiO2 , 3.0 Al2 O3 , and 1.25 P2 O5 whereas the molar levels of CaO, SrO, CaF2 , SrF2 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. 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 m mesh sieve to give a powder (Table 2), which was used in the subsequent cement formation.
Table 1 – Composition of QMAB glasses (mol%). Glass
SrF2
SrO
CaO
CaF2
QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
0 2 2 2 1 1 1.5 0.5 0.25 0 0
0 3 1.5 0.5 0 1.5 1 2 2.25 2.5 0
3 0 1.5 2.5 3 1.5 1 2 2.25 2.5 5
2 0 0 0 1 1 1.5 0.5 0.25 0 0
F:Sr:Ca 2:0:5 2:5:0 2:3.5:1.5 2:2.5:2.5 2:1:4 2:2.5:2.5 3:2.5:2.5 1:2.5:2.5 0.5:2.5:2.5 0:2.5:2.5 0:0:5
Table 2 – Particle size analysis of QMAB glasses. Glass QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
D90 (m) 34.8 34.9 31.3 30.2 32.7 28.8 31.3 32.0 26.8 28.9 28.5
D50 (m)
D10 (m)
6.25 6.65 6.27 7.80 6.85 6.60 6.79 6.02 5.70 5.02 5.90
0.88 0.95 0.88 0.96 0.98 0.78 0.86 0.82 0.79 0.71 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). 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
Table 3 – Working time (WT) and setting time (ST) in seconds of QMAB cements. Cement QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9 QMAB10 QMAB11
WT (s) 72 78 90 60 69 81 72 66 105 135 132
ST (s) 276 294 282 474 279 267 222 294 507 705 546
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Table 4 – Slopes, intercepts and correlation coefficients for F− release from QMAB cements. Cement
Fluoride content (mol%)
QMAB-1 QMAB-2 QMAB-3 QMAB-4 QMAB-5 QMAB-6 QMAB-7 QMAB-8
Slope values (ppm/sqrt h)
2 2 2 2 2 2 3 1
1.65 3.23 3.92 2.41 1.22 2.50 9.64 1.52
Intercept values (ppm) 5.30 3.89 8.43 5.62 9.60 5.47 10.27 3.18
R2 values 0.99 0.96 0.98 0.97 0.99 0.98 0.97 0.98
Table 5 – Cummulative ion relase (mequiv/g) and change in pH after 7 days. Cement
Ca
Sr
Al
F
pH
QMAB1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 QMAB7 QMAB8 QMAB9
1.05 0.09 0.57 0.65 1.74 0.85 0.97 0.38 0.92
0.02 1.09 0.92 0.26 0.32 0.51 0.47 0.14 0.71
0.29 0.26 0.34 0.25 0.42 0.23 0.44 0.10 0.11
0.56 0.93 1.11 0.75 0.47 0.80 2.61 0.29 0.21
0.7 0.8 0.7 0.8 0.6 0.7 0.8 0.5 0.7
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− , Ca2+ , Sr2+ , Al3+ ) 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 ICPOES (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 C0.70 values of 0.35 and 0.55.
3.
Fig. 1 – Release of fluoride at different time intervals from QMAB4.
Results
The release of F ion is linear when plotted against t1/2 . A plot of release from QMAB4 containing both Ca and Sr and F (see Table 1) is shown as an example in Fig. 1.
Table 6 – Translucency (visual opacity) of cements relative to the C0.70 0.55 standard. Cement QMBA1 QMAB2 QMAB3 QMAB4 QMAB5 QMAB6 C0.70 0.35 C0.70 0.55 Air a
Opacity at 1 h 1.3 1.3 1.3 1.3 1.3 1.2 0.8 1.0a 0.45
Reference 1.0 by definition.
Opacity at 24 h 1.3 1.2 1.2 1.2 1.2 1.1 0.8 1.0a 0.45
Fig. 2 – Effect of increasing strontium content on slope and intercept.
The slopes, intercepts, and square of the correlation coefficients for all nine formulations are shown in Table 4. The six cements having an F content of 2 mol% (QMAB1–6) show slope increasing with Sr content whereas the intercept shows no obvious trend, see Fig. 2. The effect of increasing F content increases both slope and intercept as shown in Fig. 3. Increasing F content shows what
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 308–313
Fig. 3 – Effect of increasing fluoride content on slope and intercept.
appears to be an exponential increase in aluminum release as shown in Fig. 4. The effect of the ion release on the pH of the acetic acid varied only slightly ranging from +0.6 to +0.8 pH units (Table 5). There was no apparent relation to the Sr:Ca ratios. Radiopacity shows a strong linear correlation with Sr content (see Fig. 5). Visual opacity differs little between cements (see Table 6). All are more opaque than the C0.70 0.55 standard but less opaque than the C0.70 0.90 standard which is the limit for the ISO requirement for acceptance.
4.
Discussion
The release of both cations and fluoride anion show a linear relationship with t1/2 rather than with t. This indicates that ion release is diffusion controlled rather than dissolution controlled process. The results found differ from those another
Fig. 4 – Effect of increasing fluorine content on cumulative aluminum release.
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Fig. 5 – Effect of increasing strontium content on radiopacity of cements.
study on F release into an acidic medium [11] where release kinetics were shown to be linear with respect to time indicating dissolution control [12]. However, in this study acetic acid at pH 4 was used whereas in the study cited above lactic acid at pH 2.7 was used. It should also be noted that lactic acid is a stronger acid than acetic acid (pKa of lactic acid is 3.85 and pKa of acetic acid is 4.74 according to the CRC Handbook of Chemistry and Physics 93rd Edition). The relatively low pH changes observed in this study of 0.5–0.8 pH units contrast with larger changes found in the lactic acid study of 1.8. One particularly interesting aspect of the current study was the effect of partial or total replacement of Ca by Sr on F release. A previous paper [13], reporting on cements containing glasses very similar those in this study, showed no increase in F release for Sr compared to Ca (over a 20 week period into water). Cement with Sr:Ca = 0:1 released 17 mol F/g cement whereas that with Sr:Ca 1.5:3.5 released 15.4 mol/g cement and that with Sr:Ca 3:2 released 16.6. In another study [14] the same authors state: “substituting strontium for calcium has little influence on fluoride release. The small increase observed is due to the increased density of strontium affecting the measured powder:liquid mixing ratio”. However the results appear to be those of their previous publication [13]. In contrast a study, again using similar glasses but as ground glass, acid washed glass, and as cements made with acetic acid, does show slight increase in F release as Ca is replaced with Sr [15]. Interestingly, both in the current study and in this study, the slopes of the regression lines increase with increasing Sr:Ca ratio whereas the intercepts decrease. Intercepts give an indication of the magnitude of the initial “burst” effect. Previous studies have shown this to be related to the availability the monovalent Na ion [16,17]. This species is not present in the glasses used in this study. How Ca could enhance initial F release as compared to Sr, given the much greater solubility of SrF2 is difficult to comprehend from this study. The addition of CaF2 to an F-free glass ionomer does not result in any additional F release [18]. However the ICP analysis of the aluminum release from QMAB 1 and 2 shows it to be considerably higher from QMAB1 (Ca only). There is therefore either the possibility
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of Al “complexes” with F ions and/or the higher solubility of AlF3 (4100 ppm as contrasted to 8 ppm for CaF2 and 390 ppm for SrF2 ) results in the greater “initial burst release” observed. Overall, it should be noted that total F release and slope of the t1/2 release profile are highest for QMAB3 which contains Sr and Ca rather than QMAB2 which contains only Sr. This may again be related the 18% higher release of Al. QMAB5 which has the highest Al release has the highest intercept value for the F release plot vs t1/2 . The release of the three cations Sr2+ , Ca2+ , and Al3+ show similar release profiles to that of F− . Comparison of anion and cation release as a mass balance is not possible as neither silicate nor phosphate ion release was measured. The pH changes found in this study are relatively small as noted in Results. This is probably due to the high ratio of acetic acid solution to cement. Specimens of cement weigh ∼140 mg and the 50 ml of pH 4 acetic acid contains ∼580 mg of acetic acid. This is 9.7 mequiv. of acetic acid whereas only 0.23 mequiv. of cations on average are released from the cement accounting for the relatively small pH changes observed. The addition of Sr to glasses for dental restoratives is designed to enhance radiopacity. In this study, the level of radiopacity is found to increase linearly with Sr content as shown in Fig. 5. The radiopacity of the lowest Sr content GIC is lower than the requirement of ISO standard [20] which requires the radiopacity of 1 mm of cement to be at least equivalent to 1 mm of Al. This cement has only 7.9% Sr in the glass which equates to 5.9% Sr in the cement (Note: 1 mm of Al is approximately equivalent to the radiopacity of 1 mm of dentin [21]). The highest level Sr produces a degree of radiopacity that enables one to distinguish the restoration from enamel. It is however not so high as to give rise to the “Mach Effect” which level causes difficulty detecting incipient caries adjacent to a very radiopaque filling. Most dental restoratives, in which the radiopacity is produced by design rather than being an inherent property of the type of restorative (e.g. amalgam), have levels from 1 to 3.5 mm Al [21]. In the first glass ionomers introduced to the dental market from 1975 to 85 there were no introduced radiopacifying elements present. Subsequently many different formulations were produced often adding the radiopacifying elements as compounds forming secondary fillers to the primary filler of partially dissolved glass particles. A range of the elements found in the cements is reported [3,19]. Most of the additions produced cements that appeared much more visually opaque than the unmodified cements from the same manufacturers. Use of simple measures of opacity such as photographic techniques [21] or visual comparison with opacity standards [20] confirmed these finding, Although a patent [8] reports that addition of strontium fluoride powder to glass ionomer produces radiopacity without appreciably increased visual opacity, no commercial materials have been reported using this additive. The results in this study show that all the materials have translucency values slightly greater than the translucency ISO standard with a C0.70 value of 0.55. However all formulations are much more translucent than the C0.70 0.90 standard which is the opaque limit of the ISO standard. Replacing Ca by Sr completely produces a slight improvement in translucency but only by a trivial 6% and all the mixed
Ca/Sr glasses are similar to the 100% Sr formulation and more translucent than the 100% Ca one. Comparing translucency after 1 h with that at 24 h shows that all five Sr containing cements have opacity reduced by 7% or more (see Table 6) whereas the cement containing Ca and no Sr shows less than 3% reduction in opacity. The changes reported for early Ca containing GICs were also small compared to those found for dental silicate cement [22].
5.
Conclusions
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. The release of fluoride ion and all cations is linear with respect to t1/2 indicating diffusion control rather than dissolution controlled kinetics. This is consistent with the relatively high pH of the acidic immersion medium and the small pH increases observed in the medium.
references
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