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Change in pH during setting of polyelectrolyte dental cements
122
J. Dent. 1993; 21: 122-l 26
Change in pH during setting of polyelectrolyte dental cements* E. A. Wasson* and J. W. Nicholson*t *Materials Group, Laboratory of the Government Dentistry, King’s College, London, UK
Chemist,
Teddington
and tDental
Biomaterials
Unit, School of
ABSTRACT The change in pH during setting has been studied for live different glass polyalkenoate (ionomer) cements and for two different zinc polycarboxylate cements using a flat-headed combination electrode on both the fresh cement and on a slurry of the set cement. The results show that the pH of the glass ionomers was slightly lower in the early stages of setting than was the pH of the zinc polycarboxylates and also that the pH of glass ionomers rises more slowly. For anhydrous cements (i.e. those formulated from dried polymer) pH was found to rise quicker than for hydrous cements (i.e. those prepared from aqueous solutions of polymer). Previous workers have assumed that anhydrous cements undergo slower rises in pH than hydrous ones. Our results clearly refute this assumption, and also suggest that the reported pulpal irritation associated with the use of anhydrous glass ionomers may be due to something other than low pH. KEY WORDS: J. Dent. 1993; 1992)
Glass polyalkenoate 21:
122-l
(ionomer)
cement,
Zinc polycarboxylate,
26 (Received 22 June 1992;
reviewed
pH changes
24 July 1992;
accepted
3 November
Correspondence should be addressed to: Miss E. Wasson, Materials Group, Laboratory of the Government Chemist, Queens Road, Teddington, Middlesex, lW1 1 OLY.
INTRODUCTION Since the introduction of glass polyalkenoate (ionomer) cements into dentistry, a number of studies have been carried out to investigate the biocompatibility of this class of material (Nicholson et al., 1991). These studies have included looking at the inflammatory response in the teeth of humans (Tobias et al., 1978; Cooper, 1980; Planter al.. 1988). ferrets (Tobias et al.. 1989, 1991) and monkeys (Pameijer and Stklanley, 1988: Pameijer et al.. 1991), and also examining the cytotoxicity of the cement towards cell cultures (Kawahara et al., 1979; Meryon et al., 1983; Hume and Mount, 1988; Miiller et al.. 1990). These studies have shown glass ionomers to exhibit varying degrees of response. 1n viva. these cements are reasonably benign when the layer of dentine is relatively thick or has been left unconditioned. However, where the dentine layer is thin, there is evidence of toxicity towards the pulp, hence calcium hydroxide liners are recommended for use in these cases (Paterson and Watts, 1981). By contrast with restorative cements. glass ionomer luting cements have been reported to cause adverse *@1993 Crown copyright. 0300-5712/93/020122-05
reaction in some patients (Council on Dental Materials, 1984, 1988; Stanley, 1990). Luting cements are formulated at lower powder : liquid ratios than restoratives, and therefore tend to set more slowly. It has been suggested that the slower change in pH in these materials may be the cause of the reported pulpal sensitivity (Smith and Ruse, 1986), especially in areas where the dentine layer is thin or damaged. Smith and Ruse (1986) examined five glass ionomers and concluded it was the time for which the pH was below 3 that determined the extent of the sensitivity experienced by some patients. When glass ionomers first appeared they consisted of an aqueous solution of polymer which was mixed with glass powder immediately prior to clinical use. Since that time, alternative formulations have become available that are designated’anhydrous’or’semihydrous’. In anhydrous glass ionomer cements dried polymer powder is incorporated into the glass powder and the setting reaction achieved by mixing the resulting powder with water. In semihydrous cements, some of the polymer is added as a dry powder to the glass and some is present in aqueous solution. A possible problem arises with these materials. since the minimum dissolution time for desiccated
Wasson
Table 1. Details of the cements Cement
and Nicholson:
used in this study and the p/l
Change in pH during
(g/ml)
setting
of cements
123
ratios of mixing.
Manufacturer
Liquid
DeTrey, Weybridge, Surrey, UK DeTrey UnoDent, Essex, UK Espe, Seefeld, Germany Rugby Labs Inc., New York. USA
Water Water Water Acid Acid*
Luting cement Liner Restorative Restorative Luting cement
3.4/l 3.2/l 7.0/l 3.2/l
1.7/l
.o .O .o .O .o
Davis, Schottlander London, UK Bayer, Leverkusen,
and Davis,
Water
Luting
6.0/l
.O
Germany
Acid
Pulp protection Luting cement
4.6/l 2.3/l
.O .O
Use
Glass ionomers AquaCem Baseline AquaFil Chelon-Fil Super-Dent Zinc polycarboxylates Aquaboxyl Bayer Carboxylate *Super-Dent
cement
is a semihydrous glass ionomer.
poly(acrylic acid) is 18 min. whilst some anhydrous systems are fully set in about 7 min (Stanley. 1990). This suggests that a proportion of the polymer powder may not be incorporated into the matrix during the setting process and may then be gradually dissolved by pulp fluid, thereby prolonging sensitivity. There is certainly some evidence that pulpal sensitivity is more prevalent with anhydrous than with hydrous glass ionomer cements (Tobias et al., 1989). Zinc polycarboxylate cements by contrast are reported not to cause pulpal irritation. These cements are formed by the reaction of a zinc oxide/magnesium oxide mixtur‘e with a polycarboxylic acid and hence have some similarity to glass ionomers. Typically the polyacid solution has an initial pH of 1-1.7. the fresh cement a pH of 3-4 and the fully set cement a pH of around 7 (Williams and Cunningham, 1979). As with glass ionomers, these materials are now available in anhydrous versions. but even with these there have been no reports of adverse pulpal reactions. Prolonged time at low pH is not the only possible reason for the observed pulpal irritation. Anotherexplanation which has been suggested by some researchers is the presence of microorganisms around the restoration (Brannstr8m and Nijborg, 1974: Tobias et al., 1982, 1987). Bacteria may be excluded from the cavity by using germfree animals (Paterson and Watts. 1987) or by sealing the restoration (Tobias et al., 1989, 1991). In the latter case a layer of the cement is removed and the restoration sealed using a zinc oxide/eugenol cement. These studies have shown that, where bacteria were excluded in this manner, pulpal inflammation was low and that there was no microbial infection of the restoration (Tobias et al., 1989). These findings suggest that pH may not be such an important factor in pulpal irritation as previously thought. The present paper describes a study of the changes in pH during setting for a range of commercially available glass ionomer and zinc polycarboxylate cements in order to determine the duration of the period of low pH. The aim has been to compare glass ionomers with zinc polycarboxylates, and anhydrous cements with hydrous ones.
MATERIALS
AND METHOD
Five glass ionomer cements and two zinc polycarboxylate cements were used in this study. The details for each cement are given in Table I, along with the p/l ratio at which the cements were mixed. As glass ionomers set by an acid-base reaction, the change in pH of the cement during setting is an important factor. It is, however, difficult to follow the setting once the cement has become stiff and we have chosen to use the technique of Kent and Wilson (1969) to overcome this difficulty. A flat-ended combination electrode was used to measure the pH changes throughout these experiments. To measure the pH in the early stages of setting the cement was mixed and placed in a waxed brass ring on a glass block. No interface was employed between the electrode and the setting cement to allow a ‘true’ measure of pH to be taken; the electrode was carefully cleaned with demineralized water between each measurement to remove old cement. One sample of cement was used for each reading and the pH electrode allowed to stabilize before readings were taken. The electrode was placed on the surface of the setting cement and the pH recorded every minute for up to 5 min; this value was designated pHa. This method is applicable to cements during the first 5-10 min of their setting (Kent and Wilson, 1969). After that time a set of values, designated pHP. were obtained by crushing samples of the set cement at given time intervals, mixing the powder with a minimum quantity of water to form a slurry, and recording the pH of that slurry (Kent and Wilson, 1969). All measurements were made at 21 f 2°C. The working times for all cements were determined using an oscillating rheometer at 23 f 1 “C (Wilson et al., 1976).
RESULTS AND DISCUSSION The change in pH observed for the five glass ionomers is detailed in Table II. Similarly. the results for the zinc polycarboxylate cements are given in Table 111.Figs 1 and 2 use representative data from these tables to show the
124
J. Dent 1993; 21: No. 2
in pH during setting for glass ionomer cements
Tab/e II. Change
Time (min) 1 2 3 4 5 10 15 20 25 30 60 90 120 6h 24h
Aqua Fil
AquaCem
pIi measurements Baseline Chelon-Fil
Super-Denr
3.45 3.21 3.90* 4.03 4.45 4.43 4.53 4.76 4.83 4.85
1.96 2.1 1 2.37 2.62 2.84 3.98* 4.36 4.50 4.61 4.83 5.09 5.34 5.45
2.59 2.82 2.99 3.23 4.15* 4.31 4.53 4.59 4.73 4.78 4.94 5.16 5.22
3.23* 3.90 4.03 4.3 1 4.53 4.61 4.94 5.22 5.31
1.57 1.99 2.16 2.25 2.32 3.39* 3.67 3.67 4.06 3.95 4.49 4.75 5.1 1
5.21 5.67
5.70 5.93
5.57 5.93
5.73 5.35
6.31 6.65
2.14 2.36
1.57 1.74 2.26 -
*Change from pHa to pHp
Tab/e 111.Change in pH during setting for zinc polycarboxylate cements Time (min)
Aquaboxyl
Bayer (luring)
Bayer (Pulp)
1 2 3 4 5 10 15 20 25 30 60 90 120
2.44 3.01 2.95 3.03* 3.70 5.47 5.59 5.79 5.77 5.93 6.13 6.30 6.44
3.07 3.31 3.38 3.39* 3.40 5.67 5.89 6.31 6.35 6.40 6.52 6.51 6.63
2.93 3.02 3.95 4.07* 6.30 6.46 6.48 6.55 6.60 6.63 6.68 6.72 6.89
6h 24h
6.75 7.08
6.60 6.84
7.00 7.25
*Change from pHa to pHP
comparison of an anhydrous and a hydrous version of both glass ionomer and zinc polycarboxylate cements. Before considering the results in detail, it should be noted that there is some uncertainty about what precisely is meant by the pH within a setting cement. The pH is defined as minus the logarithm of the concentration of hydrogen ions in a dilute aqueous solution, a situation far removed from that within a typical dental cement. However, pH measurement can give an indication of the extent to which each polymer has been neutralized, and hence give some insight into the progress of the setting reaction. The present study has used the extraction method of Kent and Wilson (1969), as previously used in studies on the setting of dental silicate cements. This contrasts with the technique employed by Woolford (1989). which measured surface pH of setting cements. This was done by placing a flat-headed pH electrode onto a piece of damp filter paper that was in contact with a disc
Tab/e IV. Working times determined oscillating rheometer, at 23 f 1 “C
Cement AquaFil AquaCem Baseline Super-Dent Aquaboxyl Bayer (luting) Bayer (pulp)
using the Wilson
Working rime (min) 3.3 3.4 2.6 6.2 3.6 6.0 3.2
f * f + f f f
0.4 0.1 0.1 0.7 0.2 0.7 0.2
of the setting cement. Such a technique gives a lower pH at a given time from mixing. For example, Woolford showed that the pH for Baseline was still below 4 at 60 min; using our technique, we found Baseline to have a pH of 4.94 at 60 min. This difference between surface pH and internal pH is important, since it indicates that the effect on the surrounding tissue of a cement may depend more on the extent to which the unneutralized polyacid can be released, than on the extent of the reaction that the majority of the cement has undergone. Of the cements we have studied, all but two reached pH 3 five minutes after setting. Similarly. most of these cements reached pH 4 ten minutes after setting. Thus the change in pH was found to be similar for all of the glass ionomer cements regardless of powder : liquid ratio or of type. Of the aqueous cements AquaCem started at the lowest pH (1.96) but reached the highest (5.45) after 2 h and underwent a rapid change in pH during the first 5 min. AquaFil appeared, on the basis of the pH change, to be the slowest setting of the commercial cements. However, as the working times show, this was not the case (Table IV); Super-Dent actually had the longest working time at 23°C. These results suggest that there is no clear relationship between pH change and rate of mechanical hardening.
Wasson
and Nicholson:
Change in pH during
setting
of cements
125
b 0
I
I
I
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
110
120
Time
II, 0
10
20
The change in pH during the early stages of setting was much more rapid for all of the zinc polycarboxylate cements examined than for the glass ionomer cements. For example. the pH of zinc polycarhoxylate cements changed by an average of 3.05 between 1 and IO min after mixing. compared to glass ionomers which changed by 1.93 pH units. Neutralization also proceeded further in zinc carboxylates than with glass ionomer cements, reaching pH 7 in 6 h for the Bayer (pulp protection) cement and 24 h for Bayer (Luting) and for Aquaboxyl: glass ionomers do not reach pH 7. This confirms previous findings of Cook (1982). Smith and Ruse (1986) showed that some anhydrous luting cements remained below pH 3 for IO min. and suggested that this may be the cause of sensitivity in these cements. However. our results do not show such a slow rise in pH for the anhydrous cements we have studied. which suggests that prolonged acidity is not always a feature of these cements. Only one of the glass ionomers exhibited a particularly slow change in pH and this was a hydrous luting cement. Smith and Ruse also showed that. in contrast to glass ionomers. the two zinc cement systems (zinc phosphate and zinc polycarboxylate) both undergo very rapid neutralization reaching about pH 2 in 1 min and about pH 3 in 5 min. This rapid rise in pH was taken to be the reason for the mild pulpal response to these materials (Council on Dental Materials, 1988). Of the cements we have examined. two have been studied elsewhere to assess their effect on pulpal inflammation. Tobiaset al. (1989) have examined the reaction of ferret pulp to Aquacem alone and Aquacem that has been surface sealed. They have also studied the reaction of pulp to a semihydrous zinc luting cement (Tobias et al.. 1991). now manufactured as Super-Dent. In both of these studies. where the surface was sealed with a zinc oxide/ eugenol cement, bacteria were shown to be excluded from
I
I
I
I
I
I
40
50
60
70
80
90
Time
(min)
Fig. 1. Plot of pH vs time for the zinc polycarboxylate cements Aquaboxyl (-+-, anhydrous) and Bayer Carboxylate (-•-, hydrous).
30
,,I 100
110
120
(min)
2. Plot of pH vs time for the glass ionomer cements anhydrous) and Bayer Carboxylate (-¤-, Aquacem (-A-, hydrous).
Fig.
the restoration and no pulpal inflammation developed. Our results have shown that, whilst both cements have a low initial pH, Aquacem undergoes rapid neutralization. reaching pH 3 in a little over 5 min. Neutralization in Super-Dent is much slower, reaching pH 3.39 by IO min and approximately pH 4 around 30 min. In comparison with zinc polycarboxylate cements this was very slow and might therefore possibly have led to pulpal inflammation. However. this cement has been shown not to lead to pulpal inflammation provided microleakage is prevented (Tobias et al.. 1991). Hence, it seems that pH as such has little influence on the pulpal response. Instead, susceptibility to pulpal irritation seems to be due to microbial contamination of the cavity. This might be associated with the use of anhydrous luting grades of glass ionomer because of contamination of the water with which such cements are mixed. By contrast, zinc polycarboxylates possibly do not exhibit such contamination because of the bactericidal effect of zinc ions leached out of the setting cement.
CONCLUSION The change in pH during setting of zinc polycarboxylate cements has been shown to be quicker than for glass ionomers. with the initial measured pH of freshly mixed cements also being higher than for glass ionomers. The extent of reaction was also greater. so that at 24 h the zinc polycarboxylates were fully neutralized, while the glass ionomers were still slightly acidic. For glass ionomers. the rate of pH change was slightly greater for the anhydrous cements than for the hydrous cements and. contrary to previous reports, we did not observe an unduly long period of low pH in the setting of anhydrous glass ionomer luting cements. This. together with the the results of Tobias et al. (1989,199l) that sterile
126
J. Dent. 1993; 21: No. 2
glass ionomers do not give rise to pulpal irritation, implies that low pH may not necessarily be the cause of pulpal irritation with these luting cements.
Acknowledgement The work reported in this paper was carried out as part of the ‘Materials Measurement Programme’, a programme of underpinning research financed by the United Kingdom Department of Trade and Industry. References Brtinnstrbm M. and Nyborg H. (1974) Bacteria growth and pulpal changes under inlays cemented with zinc phosphate cement and Epoxylite CBA 9080. J. ProsU~et Dent. 31, 556-565. Cook W. D. (1982) Dental polyelectrolyte cements I. Chemistry of the early stages of the setting reaction. Biornaterials 3, 232-236. Cooper I. R. (1980) The response of the human dental pulp to glass-ionomer cements. ht. Ended. J. 13, 76-88. Council on Dental Materials (1984) Reported sensitivity to glass-ionomer luting cements. J. Am. Dent. Assoc. 109, 467. Council on Dental Materials (1988) Biocompatibility and postoperative sensitivity. J. Am. Dent. Assoc. 116, 767-768. Hume W. R. and Mount G. J. (1988) 111vitro studies on the potential for pulpal cytotoxicity of glass-ionomer cements. J. Dent. Res. 67, 915-918. Kawahara H., Imanishi Y. and Oshima H. (1979) Biological evaluation on glass-ionomer cement. J. Dent. Rex 58, 1080-1086. Kent B. E. and Wilson A. D. (1969) Dental silicate cements: VIII. Acid-base aspects. J. Dent. Res. 48, 412-418. Meryon S. D.. Stephens P. G. and Browne R. M. (1983) A comparison of the in vitro cytotoxicity of two glass-ionomer cements. J. Dent. Res. 62, 769-773. Miiller J.. Bruckner G.. Kraft E. et al. (1990) Reaction of cultured pulp cells to eight different cements based on glass-ionomers. Dent. Mater 6, 172-177. Nicholson J. W.. Braybrook J. H. and Wasson E. A. (1991) The biocompatibility of glass-poly(alkenoate) (glassionomer) cements: a review. J. Biomater. Sci. 2, 277-287.
Pameijer C. H. and Stanley H. R. (1988) Biocompatibility of a glass-ionomer luting agent in primates. Part I. Am. J. Denr. 1, 71-76. Pameijer C. H.. Stanley H. R. and Ecker G. (1991) Biocompatibility of a glass-ionomer luting agent in primates. Part II. Crown cementation. Am. J. Dent. 4, 134-171. Paterson R. C. and Watts A. (1981) The response of the rat molar pulp to glass-ionomer cement. Br. Dent. J. 151, 228-230. Paterson R. C. and Watts A. (1987) Toxicity to the pulp of a glass-ionomer cement. Br. Dent. J. 162, I IO-I 12. Plant C. G., Knibbs P. J., Tobias R. S. er al. (1988) Pulpal response to a glass-ionomer luting cement. Br. Dent J. 165, 54-58. Smith D. C. and Ruse N. D. (1986) Acidity of glass-ionomer cements during setting and its relation to pulp sensitivity. J. Am. Dent. Assoc. 112, 654-657. Stanley H. R. (1990) Pulpal responses to ionomer cementsbiological characteristics. J. Am. Dent. Assoc. 120, 25-29. Tobias R. S.. Browne R. M.. Plant C. G. et al. (1978) Pulpal response to a glass-ionomer cement. Br. Dent. J. 144, 345-350 Tobias R. S., Plant C. G. and Browne R. M. (1982) Reduction in pulpal inflammation beneath surface sealed silicates. Int. Ended. J. 15, 173-180. Tobias R. S.. Plant C. G. and Browne R. M. (1987) A comparative pulpal study of two dental amalgams. Int. Endod.
J. 20, 8- 15.
Tobias R. S.. Plant C. G.. Rippin J. W. et al. (1989) Pulpal response to an anhydrous glass-ionomer luting cement. Endod.
Dent.
Traumatol.
5, 242-252.
Tobias R. S.. Browne R. M.. Plant C. G. et al. (1991) Pulpal response to two semihydrous glass-ionomer luting cements. lnt. Endod. J. 24, 95-107. Williams D. F. and Cunningham J. (1979) Materials in Dentislry. Oxford. Oxford University Press. Wilson A. D., Crisp S. and Ferner A. J. (1976) Reactions in glass-ionomer cements: IV. Effect of chelating co-monomers on setting behaviour. J. Dent Res. 55, 489-495. Woolford M. (1989) The surface pH of glass-ionomer cavity lining agents. J. Dent. 17, 295-300.
Book Review Dental Amalgam-A Health Hazard? P. Horsted-Bindslev, L. Magos, P. Holmstrup and D. Arenholt-Bindslev. Pp. 144. 1991. Copenhagen, Munksgaard, Softback, DKKI 60. Health hazards associated with dental amalgam are a recurring theme. The subject has been widely considered not just in the scientific press and by international groups but it has also received wide coverage in magazine articles for the general public. As a result, patients, especially those with chronic diseases such as multiple sclerosis or myalgic encephalitis, are increasingly questioning its safety and requesting the removal of sound restorations and their replacement by alternative restorative materials. It is therefore essential that dentists and their staff have access to the latest information on the toxicology of amalgam and in particular, mercury. This short text is well written and provides a useful, detailed resource for answering questions relating to health and
the use of dental amalgam. Its broadly based, multiple authorship has ensured adequate coverage of both laboratory and clinical aspects, including mercury metabolism and toxicology, mercury hazards to dental staff and the patient, oral mucosal reactions related to amalgam restorations, mercury hygiene, waste disposal, alternative restorative materials, and a discussion of the current controversy over mercury in amalgam restorations. There is a helpful glossary of terms and extensive references. Colour illustrations are included where appropriate. Although this book may contain little that is new to the well-informed, it nevertheless brings together the many aspects of this important topic and should prove worthwhile for both undergraduates and dental practitioners and their staff. It could be especially useful as a source of sound arguments for reassuring overanxious patients. J. Duxbury
J. Dent. 1993; 21: 122-l 26
Change in pH during setting of polyelectrolyte dental cements* E. A. Wasson* and J. W. Nicholson*t *Materials Group, Laboratory of the Government Dentistry, King’s College, London, UK
Chemist,
Teddington
and tDental
Biomaterials
Unit, School of
ABSTRACT The change in pH during setting has been studied for live different glass polyalkenoate (ionomer) cements and for two different zinc polycarboxylate cements using a flat-headed combination electrode on both the fresh cement and on a slurry of the set cement. The results show that the pH of the glass ionomers was slightly lower in the early stages of setting than was the pH of the zinc polycarboxylates and also that the pH of glass ionomers rises more slowly. For anhydrous cements (i.e. those formulated from dried polymer) pH was found to rise quicker than for hydrous cements (i.e. those prepared from aqueous solutions of polymer). Previous workers have assumed that anhydrous cements undergo slower rises in pH than hydrous ones. Our results clearly refute this assumption, and also suggest that the reported pulpal irritation associated with the use of anhydrous glass ionomers may be due to something other than low pH. KEY WORDS: J. Dent. 1993; 1992)
Glass polyalkenoate 21:
122-l
(ionomer)
cement,
Zinc polycarboxylate,
26 (Received 22 June 1992;
reviewed
pH changes
24 July 1992;
accepted
3 November
Correspondence should be addressed to: Miss E. Wasson, Materials Group, Laboratory of the Government Chemist, Queens Road, Teddington, Middlesex, lW1 1 OLY.
INTRODUCTION Since the introduction of glass polyalkenoate (ionomer) cements into dentistry, a number of studies have been carried out to investigate the biocompatibility of this class of material (Nicholson et al., 1991). These studies have included looking at the inflammatory response in the teeth of humans (Tobias et al., 1978; Cooper, 1980; Planter al.. 1988). ferrets (Tobias et al.. 1989, 1991) and monkeys (Pameijer and Stklanley, 1988: Pameijer et al.. 1991), and also examining the cytotoxicity of the cement towards cell cultures (Kawahara et al., 1979; Meryon et al., 1983; Hume and Mount, 1988; Miiller et al.. 1990). These studies have shown glass ionomers to exhibit varying degrees of response. 1n viva. these cements are reasonably benign when the layer of dentine is relatively thick or has been left unconditioned. However, where the dentine layer is thin, there is evidence of toxicity towards the pulp, hence calcium hydroxide liners are recommended for use in these cases (Paterson and Watts, 1981). By contrast with restorative cements. glass ionomer luting cements have been reported to cause adverse *@1993 Crown copyright. 0300-5712/93/020122-05
reaction in some patients (Council on Dental Materials, 1984, 1988; Stanley, 1990). Luting cements are formulated at lower powder : liquid ratios than restoratives, and therefore tend to set more slowly. It has been suggested that the slower change in pH in these materials may be the cause of the reported pulpal sensitivity (Smith and Ruse, 1986), especially in areas where the dentine layer is thin or damaged. Smith and Ruse (1986) examined five glass ionomers and concluded it was the time for which the pH was below 3 that determined the extent of the sensitivity experienced by some patients. When glass ionomers first appeared they consisted of an aqueous solution of polymer which was mixed with glass powder immediately prior to clinical use. Since that time, alternative formulations have become available that are designated’anhydrous’or’semihydrous’. In anhydrous glass ionomer cements dried polymer powder is incorporated into the glass powder and the setting reaction achieved by mixing the resulting powder with water. In semihydrous cements, some of the polymer is added as a dry powder to the glass and some is present in aqueous solution. A possible problem arises with these materials. since the minimum dissolution time for desiccated
Wasson
Table 1. Details of the cements Cement
and Nicholson:
used in this study and the p/l
Change in pH during
(g/ml)
setting
of cements
123
ratios of mixing.
Manufacturer
Liquid
DeTrey, Weybridge, Surrey, UK DeTrey UnoDent, Essex, UK Espe, Seefeld, Germany Rugby Labs Inc., New York. USA
Water Water Water Acid Acid*
Luting cement Liner Restorative Restorative Luting cement
3.4/l 3.2/l 7.0/l 3.2/l
1.7/l
.o .O .o .O .o
Davis, Schottlander London, UK Bayer, Leverkusen,
and Davis,
Water
Luting
6.0/l
.O
Germany
Acid
Pulp protection Luting cement
4.6/l 2.3/l
.O .O
Use
Glass ionomers AquaCem Baseline AquaFil Chelon-Fil Super-Dent Zinc polycarboxylates Aquaboxyl Bayer Carboxylate *Super-Dent
cement
is a semihydrous glass ionomer.
poly(acrylic acid) is 18 min. whilst some anhydrous systems are fully set in about 7 min (Stanley. 1990). This suggests that a proportion of the polymer powder may not be incorporated into the matrix during the setting process and may then be gradually dissolved by pulp fluid, thereby prolonging sensitivity. There is certainly some evidence that pulpal sensitivity is more prevalent with anhydrous than with hydrous glass ionomer cements (Tobias et al., 1989). Zinc polycarboxylate cements by contrast are reported not to cause pulpal irritation. These cements are formed by the reaction of a zinc oxide/magnesium oxide mixtur‘e with a polycarboxylic acid and hence have some similarity to glass ionomers. Typically the polyacid solution has an initial pH of 1-1.7. the fresh cement a pH of 3-4 and the fully set cement a pH of around 7 (Williams and Cunningham, 1979). As with glass ionomers, these materials are now available in anhydrous versions. but even with these there have been no reports of adverse pulpal reactions. Prolonged time at low pH is not the only possible reason for the observed pulpal irritation. Anotherexplanation which has been suggested by some researchers is the presence of microorganisms around the restoration (Brannstr8m and Nijborg, 1974: Tobias et al., 1982, 1987). Bacteria may be excluded from the cavity by using germfree animals (Paterson and Watts. 1987) or by sealing the restoration (Tobias et al., 1989, 1991). In the latter case a layer of the cement is removed and the restoration sealed using a zinc oxide/eugenol cement. These studies have shown that, where bacteria were excluded in this manner, pulpal inflammation was low and that there was no microbial infection of the restoration (Tobias et al., 1989). These findings suggest that pH may not be such an important factor in pulpal irritation as previously thought. The present paper describes a study of the changes in pH during setting for a range of commercially available glass ionomer and zinc polycarboxylate cements in order to determine the duration of the period of low pH. The aim has been to compare glass ionomers with zinc polycarboxylates, and anhydrous cements with hydrous ones.
MATERIALS
AND METHOD
Five glass ionomer cements and two zinc polycarboxylate cements were used in this study. The details for each cement are given in Table I, along with the p/l ratio at which the cements were mixed. As glass ionomers set by an acid-base reaction, the change in pH of the cement during setting is an important factor. It is, however, difficult to follow the setting once the cement has become stiff and we have chosen to use the technique of Kent and Wilson (1969) to overcome this difficulty. A flat-ended combination electrode was used to measure the pH changes throughout these experiments. To measure the pH in the early stages of setting the cement was mixed and placed in a waxed brass ring on a glass block. No interface was employed between the electrode and the setting cement to allow a ‘true’ measure of pH to be taken; the electrode was carefully cleaned with demineralized water between each measurement to remove old cement. One sample of cement was used for each reading and the pH electrode allowed to stabilize before readings were taken. The electrode was placed on the surface of the setting cement and the pH recorded every minute for up to 5 min; this value was designated pHa. This method is applicable to cements during the first 5-10 min of their setting (Kent and Wilson, 1969). After that time a set of values, designated pHP. were obtained by crushing samples of the set cement at given time intervals, mixing the powder with a minimum quantity of water to form a slurry, and recording the pH of that slurry (Kent and Wilson, 1969). All measurements were made at 21 f 2°C. The working times for all cements were determined using an oscillating rheometer at 23 f 1 “C (Wilson et al., 1976).
RESULTS AND DISCUSSION The change in pH observed for the five glass ionomers is detailed in Table II. Similarly. the results for the zinc polycarboxylate cements are given in Table 111.Figs 1 and 2 use representative data from these tables to show the
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in pH during setting for glass ionomer cements
Tab/e II. Change
Time (min) 1 2 3 4 5 10 15 20 25 30 60 90 120 6h 24h
Aqua Fil
AquaCem
pIi measurements Baseline Chelon-Fil
Super-Denr
3.45 3.21 3.90* 4.03 4.45 4.43 4.53 4.76 4.83 4.85
1.96 2.1 1 2.37 2.62 2.84 3.98* 4.36 4.50 4.61 4.83 5.09 5.34 5.45
2.59 2.82 2.99 3.23 4.15* 4.31 4.53 4.59 4.73 4.78 4.94 5.16 5.22
3.23* 3.90 4.03 4.3 1 4.53 4.61 4.94 5.22 5.31
1.57 1.99 2.16 2.25 2.32 3.39* 3.67 3.67 4.06 3.95 4.49 4.75 5.1 1
5.21 5.67
5.70 5.93
5.57 5.93
5.73 5.35
6.31 6.65
2.14 2.36
1.57 1.74 2.26 -
*Change from pHa to pHp
Tab/e 111.Change in pH during setting for zinc polycarboxylate cements Time (min)
Aquaboxyl
Bayer (luring)
Bayer (Pulp)
1 2 3 4 5 10 15 20 25 30 60 90 120
2.44 3.01 2.95 3.03* 3.70 5.47 5.59 5.79 5.77 5.93 6.13 6.30 6.44
3.07 3.31 3.38 3.39* 3.40 5.67 5.89 6.31 6.35 6.40 6.52 6.51 6.63
2.93 3.02 3.95 4.07* 6.30 6.46 6.48 6.55 6.60 6.63 6.68 6.72 6.89
6h 24h
6.75 7.08
6.60 6.84
7.00 7.25
*Change from pHa to pHP
comparison of an anhydrous and a hydrous version of both glass ionomer and zinc polycarboxylate cements. Before considering the results in detail, it should be noted that there is some uncertainty about what precisely is meant by the pH within a setting cement. The pH is defined as minus the logarithm of the concentration of hydrogen ions in a dilute aqueous solution, a situation far removed from that within a typical dental cement. However, pH measurement can give an indication of the extent to which each polymer has been neutralized, and hence give some insight into the progress of the setting reaction. The present study has used the extraction method of Kent and Wilson (1969), as previously used in studies on the setting of dental silicate cements. This contrasts with the technique employed by Woolford (1989). which measured surface pH of setting cements. This was done by placing a flat-headed pH electrode onto a piece of damp filter paper that was in contact with a disc
Tab/e IV. Working times determined oscillating rheometer, at 23 f 1 “C
Cement AquaFil AquaCem Baseline Super-Dent Aquaboxyl Bayer (luting) Bayer (pulp)
using the Wilson
Working rime (min) 3.3 3.4 2.6 6.2 3.6 6.0 3.2
f * f + f f f
0.4 0.1 0.1 0.7 0.2 0.7 0.2
of the setting cement. Such a technique gives a lower pH at a given time from mixing. For example, Woolford showed that the pH for Baseline was still below 4 at 60 min; using our technique, we found Baseline to have a pH of 4.94 at 60 min. This difference between surface pH and internal pH is important, since it indicates that the effect on the surrounding tissue of a cement may depend more on the extent to which the unneutralized polyacid can be released, than on the extent of the reaction that the majority of the cement has undergone. Of the cements we have studied, all but two reached pH 3 five minutes after setting. Similarly. most of these cements reached pH 4 ten minutes after setting. Thus the change in pH was found to be similar for all of the glass ionomer cements regardless of powder : liquid ratio or of type. Of the aqueous cements AquaCem started at the lowest pH (1.96) but reached the highest (5.45) after 2 h and underwent a rapid change in pH during the first 5 min. AquaFil appeared, on the basis of the pH change, to be the slowest setting of the commercial cements. However, as the working times show, this was not the case (Table IV); Super-Dent actually had the longest working time at 23°C. These results suggest that there is no clear relationship between pH change and rate of mechanical hardening.
Wasson
and Nicholson:
Change in pH during
setting
of cements
125
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The change in pH during the early stages of setting was much more rapid for all of the zinc polycarboxylate cements examined than for the glass ionomer cements. For example. the pH of zinc polycarhoxylate cements changed by an average of 3.05 between 1 and IO min after mixing. compared to glass ionomers which changed by 1.93 pH units. Neutralization also proceeded further in zinc carboxylates than with glass ionomer cements, reaching pH 7 in 6 h for the Bayer (pulp protection) cement and 24 h for Bayer (Luting) and for Aquaboxyl: glass ionomers do not reach pH 7. This confirms previous findings of Cook (1982). Smith and Ruse (1986) showed that some anhydrous luting cements remained below pH 3 for IO min. and suggested that this may be the cause of sensitivity in these cements. However. our results do not show such a slow rise in pH for the anhydrous cements we have studied. which suggests that prolonged acidity is not always a feature of these cements. Only one of the glass ionomers exhibited a particularly slow change in pH and this was a hydrous luting cement. Smith and Ruse also showed that. in contrast to glass ionomers. the two zinc cement systems (zinc phosphate and zinc polycarboxylate) both undergo very rapid neutralization reaching about pH 2 in 1 min and about pH 3 in 5 min. This rapid rise in pH was taken to be the reason for the mild pulpal response to these materials (Council on Dental Materials, 1988). Of the cements we have examined. two have been studied elsewhere to assess their effect on pulpal inflammation. Tobiaset al. (1989) have examined the reaction of ferret pulp to Aquacem alone and Aquacem that has been surface sealed. They have also studied the reaction of pulp to a semihydrous zinc luting cement (Tobias et al.. 1991). now manufactured as Super-Dent. In both of these studies. where the surface was sealed with a zinc oxide/ eugenol cement, bacteria were shown to be excluded from
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50
60
70
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90
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(min)
Fig. 1. Plot of pH vs time for the zinc polycarboxylate cements Aquaboxyl (-+-, anhydrous) and Bayer Carboxylate (-•-, hydrous).
30
,,I 100
110
120
(min)
2. Plot of pH vs time for the glass ionomer cements anhydrous) and Bayer Carboxylate (-¤-, Aquacem (-A-, hydrous).
Fig.
the restoration and no pulpal inflammation developed. Our results have shown that, whilst both cements have a low initial pH, Aquacem undergoes rapid neutralization. reaching pH 3 in a little over 5 min. Neutralization in Super-Dent is much slower, reaching pH 3.39 by IO min and approximately pH 4 around 30 min. In comparison with zinc polycarboxylate cements this was very slow and might therefore possibly have led to pulpal inflammation. However. this cement has been shown not to lead to pulpal inflammation provided microleakage is prevented (Tobias et al.. 1991). Hence, it seems that pH as such has little influence on the pulpal response. Instead, susceptibility to pulpal irritation seems to be due to microbial contamination of the cavity. This might be associated with the use of anhydrous luting grades of glass ionomer because of contamination of the water with which such cements are mixed. By contrast, zinc polycarboxylates possibly do not exhibit such contamination because of the bactericidal effect of zinc ions leached out of the setting cement.
CONCLUSION The change in pH during setting of zinc polycarboxylate cements has been shown to be quicker than for glass ionomers. with the initial measured pH of freshly mixed cements also being higher than for glass ionomers. The extent of reaction was also greater. so that at 24 h the zinc polycarboxylates were fully neutralized, while the glass ionomers were still slightly acidic. For glass ionomers. the rate of pH change was slightly greater for the anhydrous cements than for the hydrous cements and. contrary to previous reports, we did not observe an unduly long period of low pH in the setting of anhydrous glass ionomer luting cements. This. together with the the results of Tobias et al. (1989,199l) that sterile
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glass ionomers do not give rise to pulpal irritation, implies that low pH may not necessarily be the cause of pulpal irritation with these luting cements.
Acknowledgement The work reported in this paper was carried out as part of the ‘Materials Measurement Programme’, a programme of underpinning research financed by the United Kingdom Department of Trade and Industry. References Brtinnstrbm M. and Nyborg H. (1974) Bacteria growth and pulpal changes under inlays cemented with zinc phosphate cement and Epoxylite CBA 9080. J. ProsU~et Dent. 31, 556-565. Cook W. D. (1982) Dental polyelectrolyte cements I. Chemistry of the early stages of the setting reaction. Biornaterials 3, 232-236. Cooper I. R. (1980) The response of the human dental pulp to glass-ionomer cements. ht. Ended. J. 13, 76-88. Council on Dental Materials (1984) Reported sensitivity to glass-ionomer luting cements. J. Am. Dent. Assoc. 109, 467. Council on Dental Materials (1988) Biocompatibility and postoperative sensitivity. J. Am. Dent. Assoc. 116, 767-768. Hume W. R. and Mount G. J. (1988) 111vitro studies on the potential for pulpal cytotoxicity of glass-ionomer cements. J. Dent. Res. 67, 915-918. Kawahara H., Imanishi Y. and Oshima H. (1979) Biological evaluation on glass-ionomer cement. J. Dent. Rex 58, 1080-1086. Kent B. E. and Wilson A. D. (1969) Dental silicate cements: VIII. Acid-base aspects. J. Dent. Res. 48, 412-418. Meryon S. D.. Stephens P. G. and Browne R. M. (1983) A comparison of the in vitro cytotoxicity of two glass-ionomer cements. J. Dent. Res. 62, 769-773. Miiller J.. Bruckner G.. Kraft E. et al. (1990) Reaction of cultured pulp cells to eight different cements based on glass-ionomers. Dent. Mater 6, 172-177. Nicholson J. W.. Braybrook J. H. and Wasson E. A. (1991) The biocompatibility of glass-poly(alkenoate) (glassionomer) cements: a review. J. Biomater. Sci. 2, 277-287.
Pameijer C. H. and Stanley H. R. (1988) Biocompatibility of a glass-ionomer luting agent in primates. Part I. Am. J. Denr. 1, 71-76. Pameijer C. H.. Stanley H. R. and Ecker G. (1991) Biocompatibility of a glass-ionomer luting agent in primates. Part II. Crown cementation. Am. J. Dent. 4, 134-171. Paterson R. C. and Watts A. (1981) The response of the rat molar pulp to glass-ionomer cement. Br. Dent. J. 151, 228-230. Paterson R. C. and Watts A. (1987) Toxicity to the pulp of a glass-ionomer cement. Br. Dent. J. 162, I IO-I 12. Plant C. G., Knibbs P. J., Tobias R. S. er al. (1988) Pulpal response to a glass-ionomer luting cement. Br. Dent J. 165, 54-58. Smith D. C. and Ruse N. D. (1986) Acidity of glass-ionomer cements during setting and its relation to pulp sensitivity. J. Am. Dent. Assoc. 112, 654-657. Stanley H. R. (1990) Pulpal responses to ionomer cementsbiological characteristics. J. Am. Dent. Assoc. 120, 25-29. Tobias R. S.. Browne R. M.. Plant C. G. et al. (1978) Pulpal response to a glass-ionomer cement. Br. Dent. J. 144, 345-350 Tobias R. S., Plant C. G. and Browne R. M. (1982) Reduction in pulpal inflammation beneath surface sealed silicates. Int. Ended. J. 15, 173-180. Tobias R. S.. Plant C. G. and Browne R. M. (1987) A comparative pulpal study of two dental amalgams. Int. Endod.
J. 20, 8- 15.
Tobias R. S.. Plant C. G.. Rippin J. W. et al. (1989) Pulpal response to an anhydrous glass-ionomer luting cement. Endod.
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Tobias R. S.. Browne R. M.. Plant C. G. et al. (1991) Pulpal response to two semihydrous glass-ionomer luting cements. lnt. Endod. J. 24, 95-107. Williams D. F. and Cunningham J. (1979) Materials in Dentislry. Oxford. Oxford University Press. Wilson A. D., Crisp S. and Ferner A. J. (1976) Reactions in glass-ionomer cements: IV. Effect of chelating co-monomers on setting behaviour. J. Dent Res. 55, 489-495. Woolford M. (1989) The surface pH of glass-ionomer cavity lining agents. J. Dent. 17, 295-300.
Book Review Dental Amalgam-A Health Hazard? P. Horsted-Bindslev, L. Magos, P. Holmstrup and D. Arenholt-Bindslev. Pp. 144. 1991. Copenhagen, Munksgaard, Softback, DKKI 60. Health hazards associated with dental amalgam are a recurring theme. The subject has been widely considered not just in the scientific press and by international groups but it has also received wide coverage in magazine articles for the general public. As a result, patients, especially those with chronic diseases such as multiple sclerosis or myalgic encephalitis, are increasingly questioning its safety and requesting the removal of sound restorations and their replacement by alternative restorative materials. It is therefore essential that dentists and their staff have access to the latest information on the toxicology of amalgam and in particular, mercury. This short text is well written and provides a useful, detailed resource for answering questions relating to health and
the use of dental amalgam. Its broadly based, multiple authorship has ensured adequate coverage of both laboratory and clinical aspects, including mercury metabolism and toxicology, mercury hazards to dental staff and the patient, oral mucosal reactions related to amalgam restorations, mercury hygiene, waste disposal, alternative restorative materials, and a discussion of the current controversy over mercury in amalgam restorations. There is a helpful glossary of terms and extensive references. Colour illustrations are included where appropriate. Although this book may contain little that is new to the well-informed, it nevertheless brings together the many aspects of this important topic and should prove worthwhile for both undergraduates and dental practitioners and their staff. It could be especially useful as a source of sound arguments for reassuring overanxious patients. J. Duxbury