J. Dent. 1992;
Surface pH of resin-modified glass polyalkenoate (ionomer) cements M. J. Woolford and R. G. Chadwick Department
The Dental School, The University,
ABSTRACT The recently developed group of materials known as light-activated, or resin-modified, glass polyalkenoate (ionomer) cements have been produced in response to clinical demands for a command set cavity base material. This study monitored the surface pH of three commercially available resin-modified glass ionomer cements over a 60-min period following either mixing alone or mixing followed by a 30-s exposure to a curing lamp. The results indicate that each material behaves in a unique manner. For all materials and conditions the pH reached after a 60-min period was significantly (P < 0.001) higher than the initial value. Light curing the materials significantly increased (P < 0.01) the surface pH of two of the materials (Baseline VLC and Vitrebond) as compared to the same materials in the uncured state. In the case of XR-Ionomer, however, no significant (P > 0.05) effect of light curing upon the surface pH was apparent. The precise clinical consequences of a low surface pH are unclear but may be an aetiological factor in postoperative pulpal
sensitivity. It is therefore recommended that a sublining of a proprietary calcium hydroxide lining material should be placed routinely beneath these materials and every effort made to ensure effective light curing. KEY WORDS: Glass polyalkenoate J. Dent. 1992)
cements, 19 April
Light activated, Surface pH 1992;
Correspondence should be addressed to: Mr M. J. Woolford, Dental School, University of Dundee, Dundee DDI 4HN, UK
in the early 1970s at the Laboratory of the Government Chemist, London (Wilson and Kent, 1972). Since then, in an endeavour to improve the properties, the basic formulation has been progressively modified. Most recently light-activated resin components have been added to the system resulting in a light-curable material described as a resin-modified glass polyalkenoate cement (Wilson, 1990). This gives the advantages of both command curing and an extended working time. In addition, the setting reaction is said to be more rapid and predictable than conventional materials (Wilson, 1990). This is described as a dual set procedure consisting of an acid-base reaction, between ion-leachable glass and poly(alkenoic acid), together with a light-activated free radical polymerization of the cements resin component (Wilson. 1990). The resultant set material therefore consists of two matrices. a metal polyalkenoate salt and a resin based polymer. These surround the unreacted cores of the glass particles.
@ 1992 Butterworth-Heinemann 0300-57 12/92/060359-06
A possible problem with the dual setting mechanism is that there may be rapid polymerization associated with a slow acid-base reaction resulting in a prolonged period of acid release. This could cause irritation of the dental pulp as this group of materials have largely been marketed as cavity linings or bases for use beneath all types of restorative material. When glass ionomer cements have been used as luting cements for crown and bridgework prolonged release of acid has been implicated as a possible cause of postoperative sensitivity (Smith and Ruse. 1986). It is therefore quite likely that a similar problem may exist for resin-modified polyalkenoate lining cements in the light of previous work which has demonstrated a low surface pH for both conventional glass ionomer luting (Ban et al., 1985) and cavity base (Woolford, 1989) cements. This study examined the relationship of surface pH with time for three different resin-modified glass ionomer cements.
J. Dent. 1992;
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Table 1.The materials used in this study: details of composition from manufacturers’ data
Dentsply Ltd. Weybridge, Surrey, UK
Batch No. 9 10605/06
Powder: radiopaque, fluorine-containing aluminosilicate glass Liquid: polyacrylic acid and photocurable acrylic monomers
Kerr Ltd. Peterborough, UK
Batch No. 9 11 24
Powder: radiopaque, fluorine-containing ionomer glass Liquid: 50% solution of acrylic acid dimethacrylate copolymer
3M Health Care, Loughborough, UK
Batch No. 1155
Powder: light-sensitive fluoroaluminosilicate glass Liquid: acrylic acid copolymer containing pendant methacryloxy groups with hydroxyethyl methacrylate
The chemical composition ofeach ofthe materials used in the study is outlined in Table I. A flat-ended pH electrode (Type CFW-711, Russell pH, Auchtermuchty, UK) (Fig. 1) connected to a microprocessor controlled pH meter (CD660 pH meter, Russell pH, Auchtermuchty, UK) giving readings to f 0.01 of a pH unit was used to monitor continuously the pH of the cements. To facilitate this, the meter was connected to a chart recorder (Rikadenki R-01 Chart Recorder, Mitsui
Fig. 7. Schematic diagram of the apparatus used to monitor the surface pH.
Electronics, Chessington, UK) set to give a direct reading of pH with time. Prior to the measurement of each cement sample, the meter was calibrated with reference to two standardized buffer solutions at pH 7.00 and pH 4.00 respectively, made up from laboratory buffer tablets according to the manufacturer’s instructions (FSA Buffer Tablets, FSA Laboratory Supplies, Loughborough, UK). The glass ionomer cement was mixed according to the manufacturers’ directions and was placed into a central chamber, 1.5 mm deep, containing a temperature probe in its lateral wall. This was connected to the pH meter, thereby ensuring that all pH measurements were temperature compensated. To prevent the setting cement attaching to the functional part of the pH electrode, on subsequent assembly of the apparatus, a circular piece of filter paper (Whatman Qualitative Grade 1 Filter Paper, Whatman Labsales Ltd, Maidstone, UK), 0.18 mm thick and 210 mm in diameter, moistened with 0.05 ml of deionized water (pH = 6.17) was placed over the cement’s surface. This enabled the filter paper to stay in position during the assembly of the apparatus. The pH electrode was then positioned in contact with the paper in a separate chamber containing 0.2 ml of deionized water (pH = 6.17) making every effort to ensure that the functional part of the electrode was coincident with the underlying cement. Adhesive waterproof tape was then applied to seal the chamber. The curing source, a high intensity quartz halogen blue light (Luxor, ICI Dental, Macclesfield, UK), was applied to the inferior surface of the specimen, in close approximation to it, for 30 s. The libreoptic light guide was held securely within the apparatus, at a constant distance from the surface of the cement, by the use of a 1 mm hard plastic spacer. The duration of application of the light source was monitored using a stopwatch, as during preliminary investigations the integral timing circuit in the light unit was found to be inaccurate.
Tab/e II. Mean surface pH values at time zero (t = 0)
The mean values contained in the body of the table are calculated from five indepent observations. Values in parentheses represent the standard deviation.
Table 111.Mean surface pH values at 60 min (t = 60)
The mean values contained in the body of the table are calculated from five independent observations. Values in parentheses represent the standard deviation.
For each material the variation in surface pH with time, over a 60-min period (t = 60) was determined for both the uncured and cured states. All observations were made at intervals of 5 min relative to a predelined reference point termed time zero (t = 0). In the case of the uncured specimens this was the point at which the lowest stable pH reading was achieved. For all the materials studied this was reached within 1 min of the assembly of the apparatus. In the case of the light-cured specimens the equivalent reference point (t = 0) coincided with the cessation of exposure to the curing lamp. All reference times were within 1 min of the commencement of mixing. For each material state five specimens were studied and the mean pH values at intervals of 5 min calculated. Thus for each material a total of 10 specimens were investigated. All procedures were carried out at an ambient temperature of 22.5”C f 2°C.
RESULTS Figs 2 and 3 depict graphically the mean values of surface pH, in the uncured and cured states respectively, as observed over a 60-min period. It can clearly be seen that each material behaves in a unique fashion. For all materials and conditions the pH reached after 60 min was significantly higher (P < 0.001, Students t test) than the initial value. Tables ZZand ZZZcontain the mean surface pH values together with the standard deviations at time zero (Table ZZ)and 60 min (Table ZZZ). Two-way analyses of variance were performed to compare the different materials and treatments at the intervals oft = 0 and t = 60. In case of the data oft = 0 (Table ZZ)the effect of cure upon the surface pH was not significant (P > 0.05). In addition, there was no significant interaction (P > 0.05)
Fig. 2. A graph of surface pH against time for the materials used in the study in the uncured state. Bars represent one standard deviation. +, Baseline VLC; --A--, XR ionomer; U, Vitrebond.
Fig. 3. A graph of surface pH against time for the materials used in the study following light curing. Bars represent one standard deviation. +, Baseline VLC; -.A--, XR ionomer; +, Vitrebond.
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Versus B/U X/U V/U B/C X/C V/C
of the data in Table
of the Tukey analysis
P 0.05) between the values recorded for XR Ionomer.
DISCUSSION Smith and Ruse (1986) monitored the surface pH of a variety of conventional glass ionomer luting agents, post setting, for 60 min and demonstrated that some of these materials were far more acidic than either the zinc phosphate or zinc polycarboxylate cements. They concluded that following cementation the early acidity of the glass ionomer cements may be a contributory factor in the aetiology of pulpal sensitivity. The results of the current work show that the resin-modified glass ionomer cements maintain a low surface pH for at least the first 60 min of setting and that this pH value varies from one material to another. The pH of the cement is a function of the degree of ionization of the acids present and their rates of diffusion.
The flat-ended pH electrode, as used in this study, is a very sensitive instrument to monitor this change (Ban et al., 1985). Resin-modified glass ionomer cements are said to set by a dual set mechanism. This comprises an acid-base reaction and a light-activated free radical polymerization of the materials resin component (Wilson, 1990). To facilitate the acid-base reaction the glasses used in the cements are formulated to be ion leachable on exposure to poly(alkenoic) acids. The concentration of free acid at the surface of the cement is thus dependent upon the rate of ion leaching. It is this ionized free acid that is measured by the pH electrode used in this study. The significantly lower (P < 0.01) pH values (Tables ZZZ and V) recorded for Baseline VLC and Vitrebond at 60 min (t = 60) when uncured, compared to the cured state, could result in pulpal irritation. Both these materials are therefore likely to be less irritant in the cured state. In contrast, the potential pulpal irritancy of XR-Ionomer will remain constant, irrespective of this material’s state of cure, as no significant difference (Z’> 0.05) in surface pH was detected between the values recorded at t = 60 in either state. According to the manufacturers the resinmodified glass ionomer cements have a recommended depth of cure of approximately l-2 mm. Beyond this thickness the materials may not be fully cured for at least the first 12 h (Burkeet al., 1990). Under such circumstances hardening of the cement will be completed by the progression of the acid-base reaction which will take place slowly, and as a consequence the cement’s pH will take longer to rise. This phenomenon is illustrated by the slower pH gains shown by both uncured Vitrebond and Baseline VLC (Fig. 2) when compared to the observations made on these materials following cwring (Fig. 3). This prolonged low pH may contribute to a certain degree of pulpal toxicity (Hume and Mount, 1988). At one time it was thought that the considerable buffering capacity of the dentine (Wang and Hume, 1988) would reduce simple acidic effects. Other workers however, investigating the pulpal response to conventional glass ionomer materials, demonstrated no significant effect of the residual thickness of dentine upon the pulpal inflammatory response (Plant et al., 1988). It is possible therefore, that, given the length of time the surface pH of these materials remains at a low level, they will have some detrimental effect upon the dental pulp. The materials examined in this study have been designed for use as cavity liners/bases and so it is fair to assume that the pulp must be in close proximity to them when used clinically. In the light of the findings of this study it would be prudent to use a calcium hydroxide-containing sublining beneath these materials, especially where the cavity is deep. Two further points must be addressed in relation to the potential for pulpal irritation; first the effect of heat from the curing light (Lloyd et al., 1986) and second the heat of reaction upon curing these resin-containing materials (Bourke et al., 1992). These two points strengthen the recommendation for the use of a sublining beneath this
group of materials. Another potential consequence of the low surface pH of the materials during the first 60 min of setting is the detrimental effect upon the surface microhardness of overlying composite resin (Marshall et al., 1982; Berrong et al., 1989). This effect may be mediated by an interaction between the hydrogen ions of the poly(alkenoic) acid and the tertiary amine components of the resin matrix or alternatively may be a simple plasticizing effect (Marshall et al.. 1982). This may be of some clinical significance where the resin-modified glass ionomer cements are used in the composite laminate technique as described by McLean et al. (1985). Further work is necessary to confirm this interaction and elucidate the mechanisms concerned.
this work it can be concluded
1. All the resin-modified glass ionomer cements studied maintain a low surface pH for the first 60 min of setting. This low surface pH prevails, whether or not light curing has been carried out. In the cases of Baseline VLC and Vitrebond, at 60 min, higher surface pH values were found in the photocured state as compared to the uncured materials. No such difference was demonstrated for XRIonomer. 2. Different formulations of resin-modified glass ionomer cements behave uniquely with regard to acid release from the surface of the cement. 3. It would be prudent to subline the resin-modified glass ionomer liners and bases examined in this study with a proprietary calcium hydroxide lining material.
Acknowledgements The authors would like to acknoweledge the technical assistance of Mrs N. Dicker and the help and encouragement of Professor A. R. Grieve. This study was supported, in part, by the Scottish Hospitals Endowment Research Trust, grant 892.
References Ban S., Fukui H., Mori S. et al. (1985) pH Determinations
on the surface of luting cements. I. A test method. Dent Mater. J. 4, 208-215. Berrong J. M., Cooley R. L. and Duke E. S. (1989) Effect of glass ionomer base on composite resin hardness. Dent. Mater. 5, 38-40. Bourke A. M., Walls A. W. and McCabe J. F. (1992) Lightactivated glass polyalkenoate (ionomer) cements: the setting reaction. J. Dent. 20, 115-120. Burke F. M., Hamlin P. D. and Lynch E. J. (1990) Depth of cure of light-cured glass ionomer cements. Quintessence Znt. 21,977-981. Hume W. R. and Mount G. J. (1988) In vitro studies on the potential for pulpal cytotoxicity of glass ionomer cements. J. Dent Res. 67, 915-918.
Lloyd C. H., Joshi A and McGlynn E. (1986) Temperature rises produced by light sources and composites during curing. Dent. Mater. 2, 170-174. Marshall S. J., Marshall G. W. and Harcourt J. K. (1982) The influence of various cavity bases on the microhardness of composites. Aust. Dent. J. 27,291-295. McLean J. W., Powis D. R., Prosser H. J. et al. (1985) The use of glass ionomer cements in bonding composite resins to dentine. Br. Dent. J. 158,410-414. Plant C. G., Knibbs P. J., Tobias R. S. et 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. Wilson A. D. (1990) Resin-modified glass ionomer cements. Int. J. Prosthodont. 3,425-429. Wilson A. D. and Kent B. E. (1972) A new translucent cement for dentistry. The glass ionomer cement. Br. Dent. J. 132, 133-135. Wong J-D. and Hume W. R. (1988) Diffusion of hydrogen ion and hydroxyl ion from various sources through dentine. Int. Endodont. J. 21, 17-26. Woolford M. J. (1989) The surface pH of glass ionomer cavity lining agents. J. Dent. 17, 295-300.
Book Reviews Biomechanics in Orthodontics. M. R. Marcotte. Pp. 173. 1990. f28.00
London, Wolfe. Softback,
The use of controlled forces to achieve specific tooth movements is at the heart of much day-to-day orthodontic practice. Yet surprisingly few orthodontists relish the study of biomechanics, perhaps because there is a dearth of intelligible material on the subject. This book attempts to fill the gap. It is not an essay in structural mechanics for the amateur bioengineer, nor is it just an orthodontic cookbook. It is instead a practical manual of fixed appliance mechanics written by a clinician for clinicians. The book starts with a chapter on basic mechanics which manages to present the essential principles without becoming excessively technical. There is a clear discussion of the role of moment-to-force ratios in tooth movement and a simple introduction to spring design. The chapter on preliminary bracket alignment gives a good account of sectional canine retraction. There are useful sections on anchorage management, headgear mechanics, overbite reduction and vertical control. The value of the section on en-masse space closure will depend largely on one’s willingness to use the rather startling spring activations which are recommended. In addition to the sections on specific aspects of mechanics, there are two useful chapters which apply the ideas presented to treatment planning and to monitoring treatment progress. The book is pleasantly produced with copious line diagrams and the absence of glossy photographs has had a welcome effect on the price tag. The typography is generally good although the frequent substitution of the Scandinavian ~4for the Greek a is an irritating error. Many readers will greet the allocations of chairside time on page 52 with some disbelief. One hopes that good mechanics is not always so timeconsuming. The Burstone segmented approach predominates, perhaps to excess. The continuous wire methods used by many orthodontists are curtly dismissed without discussion. Even so, many of the ideas presented will in fact carry over into other systems. The book is surprisingly readable, although some careful study will be needed in places. It can happily be recommended to postgraduates and established orthodontists alike. D. C. Tidy
Introduction to Dental Local Anaesthesia. H. Evers and G. Haegerstam. Pp. 96. 1990. London, Wolfe. Hardback, f 20.50. Teachers of local anaesthesia and many dental students will be familiar with the earlier softback version of this book thanks to its free distributinn by a well-known drug company. This hardback edition benefits from an improved print quality and new colour plates, the earlier version using the clinical photographs from the same authors’ Handbook of Dental Local Anaesthesia. This book is true to its title in that it is an introduction to the subject. The text is short and not exhaustive. Introductory sections on anatomy and neurophysiology are followed by descriptions of infiltration and regional block techniques in both jaws. The method described for buccal infiltrations in the maxillary incisor and molar regions is one of subperiosteal injection contrary to the advice in other sections of the text. It is disappointing that only one method of achieving anaesthesia of the inferior alveolar nerve is described. lntraligamental techniques are not mentioned. The text ends with a discussion of the complications. It is unfortunate that such a short book contains so many typographical errors, more than the number of pages (seven alone on page 92). This is not the fault of the authors, as neither’s first language is English. The provision of an index would be another improvement. The strength of this book lies in its illustrations. The diagrams drawn by Poul Buckhoj are superb and the reproduction of the colour clinical photographs demonstrating techniques, first class. Dental students and hygienists being introduced to dental local anaesthesia would benefit from a study of the illustrations; however, more experienced practitioners in search of advanced techniques should look elsewhere. J. G. Meechan