DENTAL-2375; No. of Pages 5
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique Beata Czarnecka a , Anna Kruszelnicki a , Anthony Kao a , Marta Strykowska a , John W. Nicholson b,∗ a
Department of Biomaterials and Experimental Dentistry, University of Medical Sciences, ul Bukowska 70, ´ Poland 60-812 Poznan, b School of Sport, Health & Applied Science, St Mary’s University College, Twickenham, Middlesex TW1 4SX, United Kingdom
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To study the interfaces between model cavities prepared in teeth and four glass
Received 23 May 2013
ionomer cements (two conventional and two resin-modified).
Received in revised form
Methods. Ten non-cavitated molars and premolars were used and, in each, two 3 mm deep
9 October 2013
slot preparations were created on opposing sides of the tooth. The teeth were conditioned as
Accepted 21 May 2014
appropriate, then restored using the open sandwich technique, using a conventional glass
Available online xxx
ionomer (Fuji IX, Ketac Molar) or resin modified glass ionomer (Fuji II LC or N100), followed
Keywords:
resin and cut parallel to the long axis through both restorations, using a low speed diamond
Glass-ionomer
wheel saw. Samples were evaluated using a metallographic light microscope (100×). Three
by completion with composite resin. The teeth were then embedded in a transparent acrylic
Bonding
areas were assessed: the axial wall, the axial gingival line angle and the cavo-surface line
Adhesion
angle. Bonding was categorized as inadequate or adequate based on the appearance and
Tooth fracture
inadequate bonding was further studied and classified. Data were analysed statistically using the McNamara analysis. Results. The majority of materials failed to make adequate contact with the axial wall, and there were also flaws at the axial/gingival line angle in several samples. By contrast, the cavosurface line angle was generally soundly filled and the materials showed intimate contact with the tooth surface in this region. The most serious inadequacy, though, was not lack of intimate contact and/or adhesive bond, but the presence of perpendicular cracks in 30% of the Fuji II LC samples which extended into the underlying dentin. Significance. The problems of placement and dentin cracking experienced with these materials demonstrate that adhesive bond strength alone cannot be used as the criterion of success for restorative materials. In fact good adhesion can, in certain cases, promote cracking of the dentin due to stresses within the material, an outcome which is undesirable. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. E-mail address: [email protected] (J.W. Nicholson). http://dx.doi.org/10.1016/j.dental.2014.05.008 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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1.
Introduction
Table 1 – Glass ionomer cements used. Brand
Glass ionomer (GI) cements were first reported by Wilson and Kent in 1972 [1] and have since become widely used in clinical dentistry [2]. They have many desirable properties, in particular the ability to form satisfactory adhesive bonds with enamel and dentin [3], and to release of fluoride in a sustained way over a prolonged periods of time [4]. As far as bonding is concerned, they have the additional advantages that no extra preparation of the tooth structure is required for consistent adhesion [5], and that the bond becomes more durable with time, due to the formation of an ion-exchange layer between the tooth and the cement [6]. Despite the favorable qualities of conventional glass ionomers, they have some drawbacks. In particular, the slow setting reaction causes the material to be hydrolytically unstable in the early stages of hardening [6], making it sensitive to early water uptake or loss, depending on circumstances. This may affect the mechanical properties of the material and certainly affects the appearance, which may become chalky due to the formation of micro-cracks in the surface [2]. To overcome these problems, the resin-modified glass ionomers (RMGICs) have been developed [7]. These combine the components of conventional glass ionomers with the monomer 2-hydroxyethyl methacrylate (HEMA), together with appropriate polymerization initiators, so that the material sets by a combination of acid–base reaction and addition polymerization. The majority of brands of resin-modified glass ionomers are light-cured, which gives them the advantage that the clinician has considerable control over the setting reaction, much of which cannot occur until the dental curing lamp is switched on. Resin-modified glass ionomers retain many of the advantages of conventional glass ionomers, in particular their adhesion to the tooth surface [7] and their ability to release fluoride [8]. However, the monomer HEMA is potentially damaging and its presence reduces the biocompatibility of these materials relative to conventional glass ionomers [9]. Unpolymerized HEMA is known to be released by these materials [10] and HEMA has been shown to be able to penetrate the dentin to reach the pulp [11], where it proves to be cytotoxic [12]. HEMA monomer is also a well-known allergen [9]. Conventional and resin modified glass ionomer cements have been employed clinically in the so-called open sandwich technique for restoring Class II cavities [13–16]. In this technique, glass ionomer cement, or RMGIC, is used for the gingival portion of the restoration, and composite resin is used to complete the repair [15,16]. Clinical results with this technique are good and the use of a glass ionomer (of either type) has been found to significantly reduce the marginal micro-leakage at the cemento-enamel junction compared with the use of other materials, such as composite resin [13,15]. Glass-ionomers of both types have been tested for their adhesion to the tooth surface [17,18]. Materials have been tested in shear bond mode [17–20] and, more recently, in microtensile mode [21–23]. At relatively short times, bond strengths have been found to be acceptable, even without surface pre-treatment [2]. This leads to the suggestion that forming such adhesive bonds is straightforward, and the
Manufacturer
Fuji IX Fuji II LC Ketac Molar N100
GC (Japan) GC (Japan) 3M ESPE (USA & Germany) 3M ESPE (USA & Germany)
Type Conventional RMGIC Conventional RMGIC (nano-ionomer)
expectation that the resulting interfaces should be sound and free from flaws. This hypothesis was the original one under test in the current study. Since the open sandwich technique was pioneered, new versions of glass ionomers, both conventional and resinmodified, have been launched by manufacturers. The current study was initially undertaken with a view to examining the performance in vitro of some of these newer materials in the open sandwich technique, with particular emphasis on their bonding and the interfacial region with the tooth. However, our work has led to some new observations, not previously reported, that raise important questions about the issue of adhesive bonding by glass-ionomers.
2.
Materials and methods
Ten non-cavitated molars and premolars, extracted for orthodontic reasons, were used in this study. Teeth were stored prior to use in physiological saline to which small amounts of thymol had been added for 120 days at a temperature of 8 ◦ C. Materials used in the study are listed in Table 1, and consisted of two conventional and two resin-modified glass ionomers. One of the materials, N100, is described as “nanoionomer” because it contains nanometer sized filler. It is prepared and dispensed using a double tubed dispenser. Fuji II LC is a capsulated material, and was mixed using a vibratory mixer for 10 s prior to extrusion from the capsule. In each of the ten teeth, two slot preparations were created on opposing sides of the tooth (see Fig. 1), using a fissure bur (ISO 806 314 107 524 012, Lot D-4362 at 300 000 RPM), diamond
Fig. 1 – Slot preparation on opposing sides of the tooth. A = axial wall, B = axial/gingival line angle, C = cavo-surface line angle.
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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round bur (ISO 806 314 001 524 016, Lot D-5113 at 280 000 RPM) and a high speed rose bur (ISO 500 314 001 001 016, Lot D434040 at 280 000 RPM). The depth of each preparation was 3 mm, (the gingival wall of the box is designed so it extends 1 mm below the cervical line of the tooth, into the root). Following cavity preparation, the teeth were stored in saline solution at a temperature of 8 ◦ C for 7 days. The ten teeth were divided, at random, into two groups: Group A (prepared using the two 3M ESPE products) and Group B (the GC products). In each group, one slot in a particular tooth was filled with conventional glass ionomer cement (Ketac Molar or Fuji IX) and the other slot in the same tooth was filled with resin modified glass ionomer (N100 or Fuji II LC). Materials used are listed in Table 1. Having prepared the tooth, the material was placed in a thin layer along the axial wall and gingival floor of the cavity preparation, after which the filling of the cavity was completed by placement of composite material. In this way, four different sample groups were compared for their ability to bind to tooth structure. The cavities designated for conventional glass ionomer samples were conditioned for 10 s prior to restoration with 20% polyacrylic acid, followed by a 10 s water rinse and drying with cotton pellets. The glass ionomer cements were mixed according to the manufacturer’s instructions, in a 1:1 powder to liquid ratio and condensed into the cavity to a depth of 1 mm on both axial and gingival walls. The cement was allowed to set for 5 min after which the cavity was etched, primed, bonded then restored with composite resin (Filtek Z250, ex 3M; shade A2). The resin modified glass ionomer slot for Group A (N100) was primed with Ketac N100 primer for 15 s, air thinned for 10 s and light cured for 10 s. The resin modified glass ionomer was dispensed using the Ketac Clicker dispenser and mixed for 20 s, followed by placement into the cavity (using the dispenser provided by 3M), along axial and pulpal walls to a thickness of 1 mm, followed by light curing. For group B, the dentin was conditioned with 20% polyacrylic acid for 10 s, rinsed with water for 10 s and cotton dried. The resin-modified glass ionomer (Fuji II LC) was mixed on a glass slab and placed using a plugger, followed by light curing for 20 s. Lastly, the cavity was etched, prime and bonded and restored with composite resin as for Group A. Following restoration, each group was stored in saline solution in a tightly sealed container for 1 week, at a temperature of 37 ◦ C. The teeth were then embedded in a transparent acrylic resin (Duracryl) and were cut parallel to the long axis of the tooth, through both restorations, using a low speed diamond wheel saw (model 650). Each tooth yielded two slices, which were polished prior to microscopic evaluation. Samples were then evaluated using a metallographic light microscope (Nikon Eclipse LV 100) under a magnification of 100×. In each tooth, there were three areas of observation: the axial wall (A), the axial gingival line angle (B) and the cavo-surface line angle (C) (see Fig. 1). In the initial assessment, bonding was categorized as inadequate or adequate based on the appearance of the region between the dentin and glass ionomer cement. Inadequate bonding was further studied, and details of the inadequacies recorded. Data were further analysed statistically using the McNamara analysis.
Table 2 – A comparison of adequate (A) and inadequate (I) bonding sites of the four materials tested. Material
Adequate (out of 30)
Inadequate (out of 30)
Fuji IX GIC Fuji II RMGIC Ketac Molar GIC N100 RMGIC
18/30 = 60% 21/30 = 70% 14/30 = 47% 15/30 = 50%
12/30 = 40% 9/30 = 30% 16/30 = 53% 15/30 = 50%
3.
Results
Each restored slice was observed at three points, giving a total of thirty observations for each material used. Bonding was characterized initially as either adequate or inadequate and the results show that Fuji II LC had the highest proportion of adequate bonded specimens, whereas Ketac Molar had the least (see Table 2). Further evaluation of the inadequately bonded samples is given in Table 3 and several distinct types of inadequate bonding were identified. These were: complete detachment of cement from dentin, perpendicular crack through the glass ionomer only, detachment plus perpendicular crack into the glass ionomer and finally crack through the glass ionomer into the dentin. In evaluating the inadequate bonding associated with Fuji IX, it was found that over half (58%) of the inadequate samples showed detachment from the dentin. With Fuji II LC, the inadequacies tended to consist of cracks into dentin. These comprised 56% of inadequate samples. Additionally, 33% of inadequate samples showed transverse cracks within the material. In these specimens, no detachment failures were observed. For Ketac Molar, the most common inadequacies observed were detachment plus transverse cracking of the cement (63% of inadequate samples). In no sample did the crack penetrate into the dentin, meaning that the cracks did not exceed a maximum of 2 mm in length, and were typically shorter than this. Finally, with N100, most of the inadequacies were observed as detachments of the cement from the dentin (53%). Again, no cracks penetrated the dentin with this material. In terms of the geometry of the restoration, most of the inadequate bonding occurred on the axial wall (Table 4), whereas at the axial/gingival line angle and the cavo-surface line angle, bonding appeared more satisfactory and was generally classified as adequate.
4.
Discussion
The fact that glass ionomer cements generally bond well to both dentin and enamel has been recognized for many years [17]. Indeed, this adhesion was considered so important that when resin-modified versions of these materials were first launched, almost immediately a paper appeared confirming their adhesion to dentin [18]. In these early papers, bonding was typically determined by measuring bond strength of cement samples loaded in shear [17–19]. It still is widely used as a test method in reporting the bonding properties of modern cement formulations [20].
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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Table 3 – Inadequate bonding of material to tooth structure according to location and type of inadequacy. Material
Detachment (interfacial failure)
Fuji IX Fuji II LC Ketac Molar N100
7/12 = 58% 0 6/16 = 38% 8/15 = 53%
Transverse crack within glass ionomer
Detach + T-crack
Crack into dentin
3/12 = 25% 3/9 = 33% 0 4/15 = 27%
2/12 = 17% 1/9 = 11% 10/16 = 63% 3/15 = 20%
0 5/9 = 56% 0 0
Table 4 – Number of adequate (A) and inadequate (I) bonding sites according to location within the cavity preparation. Location
Axial wall Axial/gingival line-angle Cavo-surface line-angle
Fuji IX
Fuji II LC
Ketac Molar
N100
A
I
A
I
A
I
A
I
4 5 9
6 5 1
4 7 10
6 3 0
1 4 9
9 6 1
3 6 6
7 4 4
More recently, studies have been reported in which the test regime has been changed to the determination of microtensile bond strength [21–23]. The principal advantage of this mode of testing is that the surface area of attachment is much smaller than in conventional shear bond evaluation. Results are therefore less likely to be affected by the presence of flaws and irregularities at the cement–dentin interface. Using microsized bonding areas reduces the likelihood of such weakening occurring. However, none of these testing regimes examines a realistic bonding situation. Real glass ionomer restorations fill prepared cavities in teeth. The material therefore has to be able to be forced fully into the cavity, occupying the whole space and coming into intimate contact with flat surfaces in three dimensions. Shear or microtensile bond tests only evaluate the latter aspect: the ability to come into contact with relatively flat surfaces and there to form a strong adhesive bond. In the present study we have considered a model that more accurately reflects the conditions in which bonds must form in clinical application. When we have done that, we find that problems of inadequate bonding are apparent mainly at the axial wall. These must occur because of difficulties in fully condensing the cement, meaning that it is not pushed properly to the back of the cavity. This problem has not been reported previously, and appears to arise from problems in technique of placement of a relatively thin layer (1 mm) of the glass ionomer cement, even less than would be placed as a base. There is also a difficulty caused by adhesion of the unset cement to the instruments being used. By contrast, published accounts of bonding of the material Fuji IX have shown that, in the hands of experienced clinicians, it can be placed in intimate contact with the complete surface of a prepared cavity in a carious tooth [6]. However, most of these studies employed glass ionomer cement as a full restoration, not as a base. In the present study, the thinness of the base was necessary the cavities were relatively small and space was necessary for placement of resin composite. This situation is common in class V cavities (for example, abrasion and erosion) at the neck of the tooth. In our clinical experience, it is easier to place the cement in one bulk portion, filling the whole cavity, than to place a thin layer of glass ionomer at the base of the cavity and leaving the remainder of the cavity material free [24]. By contrast with the problems at the axial wall, bonding at the cavo-surface angle was found to be good. This shows that
all of the materials examined can occupy this type of confined space, either by flowing in on application, or as the result of condensing into the corners during placement. The emphasis on bond strength in the published literature has overshadowed consideration of other aspects of the performance of these materials. The finding that Fuji II LC is capable of causing cracks to develop in the dentin in a proportion of the samples (30%) is extremely important. In no instance was this material found to detach from the dentin surface in the course of these experiments. Its adhesion is excellent. Indeed, this is partly the cause of the problems observed. When stresses develop, either due to polymerization contraction or swelling due to water sorption, both of which are known to occur with this material [25], rather than the adhesive bond failing, or cracks developing in the material, these stresses can cause cracks in dentin. This is of considerable clinical importance, and requires further study. To date, clinical results with Fuji II LC have been good [26,27], and it may be that our experimental study exaggerates the problem of dentinal cracking, which may not occur so frequently under clinical conditions. Nonetheless, the fact that excellent adhesion can lead to this type of serious failure is important and should be noted by the research community. Our findings show that these materials cannot be evaluated safely on the basis of adhesive bond strength alone. Other factors (polymerization contraction, swelling in moisture, mechanisms of stress transfer) must be taken into consideration, and it is essential that materials do not have such good bond strengths that their dimensional stresses cause tooth fracture in vivo.
5.
Conclusions
This study has shown that the use of glass ionomer materials, both conventional and resin-modified, for the repair of model cavities using sandwich technique is very satisfactory in the majority of cases. However it has been shown to be capable of leading to problems of adhesion. In a number of instances, there were problems in establishing contact between the material and the furthest axial wall of the cavity, which may arise from problems in handling the placement of a thin layer of unset cement. In addition, for one of the resin-modified glass ionomers (Fuji II LC), in a small but important proportion of the samples (16%), the material caused a perpendicular
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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crack to develop in the adjacent dentin. This has serious implications for clinical application and demonstrates that adhesive bond strength alone cannot be used as the criterion of success for an adhesive restorative material. Other factors such as ease of condensation and dimensional stability are important and need to be considered as well.
references
[1] Wilson AD, Kent BE. A new translucent cement for dentistry: the glass ionomer cement. Br Dent J 1972;15:133–5. [2] Mount GJ. Color atlas of glass ionomer cements. London: Dunitz; 2002. [3] Akinmade AO, Nicholson JW. Glass-ionomer cements as adhesives. Part I. Fundamental aspects and their clinical relevance. J Mater Sci Mater Med 1993;4:93–101. [4] Forsten L. Short and long term fluoride release from glass ionomers and other materials. Scand J Dent Res 1991;99:241–5. [5] Mount GJ. Glass-ionomer cements; past, present and future. Oper Dent 1994;19:82–90. [6] Ngo H, Mount GJ, Peters MCRB. A study of glass-ionomer cement and its interface with enamel and dentin using a low-temperature, high-resolution scanning electron microscopic technique. Quintessence Int 1997;28:63–9. [7] Mitra SB. Adhesion to dentine and physical properties of a light-cured glass-ionomer liner/base. J Dent Res 1991;70:70–4. [8] Forss H. The release of fluoride and other elements from light cured glass ionomers in neutral and acidic conditions. J Dent Res 1993;72:1257–62. [9] Czarnecka B, Nicholson JW. The biocompatibility of resin-modified glass-ionomer cements for dentistry. Dent Mater 2008;24:1702–8. [10] Palmer G, Anstice HM, Pearson GJ. The effect of curing regime on the release of hydroxyethyl methacrylate (HEMA) from resin-modified glass-ionomer cements. J Dent 1999;27:303–11. [11] Hamid A, Hume WR. Diffusion of resin monomers through human carious dentin in vitro. Endod Dent Traumatol 1997;13:1–5. [12] Kan KC, Messer LB, Messer HH. Variability in cytotoxicity and fluoride release of resin-modified glass-ionomer cements. J Dent Res 1997;76:1502–7. [13] Hembree JH. Microleakage at the gingival margin of class II composite restorations with glass-ionomer liner. J Prosthet Dent 1989;61:28–30.
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[14] Prati C. Early marginal microleakage in class II resin composite restorations. Dent Mater 1989;5:392–8. [15] Dietrich T, Losche AC, Losche GM, Roulet JF. Marginal adaptation of direct composite and sandwich restorations in Class II cavities with cervical margins in dentin. J Dent 1999;27:119–28. [16] Arora V, Kundabala M, Parolia A, Thomas MS, Pai V. Comparison of the shear bond strength of RMGIC to a resin composite using different adhesive systems: an in vitro study. J Conserv Dent 2010;13:80–3. [17] Powis DR, Folleras T, Merson SA, Wilson AD. Improved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 1982;61:1416–22. [18] Mitra SB. Adhesion to dentin and physical properties of a light-cured glass-ionomer liner/base. J Dent Res 1991;70:72–4. [19] Lin A, McIntyre NS, Davidson RD. Studies on the adhesion of glass-ionomer cements to dentin. J Dent Res 1992;71:1836–41. [20] Carvalho T-S, van Ameronga W-E, de Gee A, Bonecker M, Sampaio F-C. Shear bond strengths of three glass ionomer cements to enamel and dentine. Med Oral Patol Oral Cir Bucal 2011;161:e406–10. [21] Tanumiharja M, Burrow MF, Tyas MJ. Microtensile bond strengths of glass ionomer (polyalkenoate) cements to dentine using four conditioners. J Dent 2000;28:361–6. [22] Jordehi AY, Ghasemi A, Zadeh MM, Fekrazad R. Evaluation of microtensile bond strength of glass ionomer cements to dentin after conditioning with Er, Cr:YSGG laser. Photomed Laser Surg 2007;25:405–6. [23] Fragkou S, Nikolaidis A, Tsiantou D, Achilias D, Kotsanos N. Tensile bond characteristics between composite resin and resin-modified glass-ionomer restoratives used in the open-sandwich technique. Eur Arch Paediatr Dent 2013;14:239–45. [24] Czarnecka B, Limanowska-Shaw H, Nicholson JW. Microscopic evaluation of the interface between glass-ionomer cements and tooth structures prepared using conventional instruments and the atraumatic restorative treatment (ART) technique. Quintessence Int 2006;37:557–64. [25] Attin T, Buchalla W, Klelbassa AM, Helwig E. Curing shrinkage and volumetric changes of resin-modified glass ionomer restorative materials. Dent Mater 1995;11:359–62. [26] Daou MH, Tavernier B, Meyer JM. Two-year clinical evaluation of three restorative materials in primary molars. J Clin Pediatr Dent 2009;34:54–8. [27] Perdigao J, Dutra-Correa M, Saraceni SHC, Ciaramicoli MT, Kiyan VH. Randomized clinical trial of two resin-modified glass-ionomer materials: 1 year results. Oper Dent 2012;37:591–601.
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique Beata Czarnecka a , Anna Kruszelnicki a , Anthony Kao a , Marta Strykowska a , John W. Nicholson b,∗ a
Department of Biomaterials and Experimental Dentistry, University of Medical Sciences, ul Bukowska 70, ´ Poland 60-812 Poznan, b School of Sport, Health & Applied Science, St Mary’s University College, Twickenham, Middlesex TW1 4SX, United Kingdom
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To study the interfaces between model cavities prepared in teeth and four glass
Received 23 May 2013
ionomer cements (two conventional and two resin-modified).
Received in revised form
Methods. Ten non-cavitated molars and premolars were used and, in each, two 3 mm deep
9 October 2013
slot preparations were created on opposing sides of the tooth. The teeth were conditioned as
Accepted 21 May 2014
appropriate, then restored using the open sandwich technique, using a conventional glass
Available online xxx
ionomer (Fuji IX, Ketac Molar) or resin modified glass ionomer (Fuji II LC or N100), followed
Keywords:
resin and cut parallel to the long axis through both restorations, using a low speed diamond
Glass-ionomer
wheel saw. Samples were evaluated using a metallographic light microscope (100×). Three
by completion with composite resin. The teeth were then embedded in a transparent acrylic
Bonding
areas were assessed: the axial wall, the axial gingival line angle and the cavo-surface line
Adhesion
angle. Bonding was categorized as inadequate or adequate based on the appearance and
Tooth fracture
inadequate bonding was further studied and classified. Data were analysed statistically using the McNamara analysis. Results. The majority of materials failed to make adequate contact with the axial wall, and there were also flaws at the axial/gingival line angle in several samples. By contrast, the cavosurface line angle was generally soundly filled and the materials showed intimate contact with the tooth surface in this region. The most serious inadequacy, though, was not lack of intimate contact and/or adhesive bond, but the presence of perpendicular cracks in 30% of the Fuji II LC samples which extended into the underlying dentin. Significance. The problems of placement and dentin cracking experienced with these materials demonstrate that adhesive bond strength alone cannot be used as the criterion of success for restorative materials. In fact good adhesion can, in certain cases, promote cracking of the dentin due to stresses within the material, an outcome which is undesirable. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. E-mail address: [email protected] (J.W. Nicholson). http://dx.doi.org/10.1016/j.dental.2014.05.008 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
DENTAL-2375; No. of Pages 5
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ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx–xxx
1.
Introduction
Table 1 – Glass ionomer cements used. Brand
Glass ionomer (GI) cements were first reported by Wilson and Kent in 1972 [1] and have since become widely used in clinical dentistry [2]. They have many desirable properties, in particular the ability to form satisfactory adhesive bonds with enamel and dentin [3], and to release of fluoride in a sustained way over a prolonged periods of time [4]. As far as bonding is concerned, they have the additional advantages that no extra preparation of the tooth structure is required for consistent adhesion [5], and that the bond becomes more durable with time, due to the formation of an ion-exchange layer between the tooth and the cement [6]. Despite the favorable qualities of conventional glass ionomers, they have some drawbacks. In particular, the slow setting reaction causes the material to be hydrolytically unstable in the early stages of hardening [6], making it sensitive to early water uptake or loss, depending on circumstances. This may affect the mechanical properties of the material and certainly affects the appearance, which may become chalky due to the formation of micro-cracks in the surface [2]. To overcome these problems, the resin-modified glass ionomers (RMGICs) have been developed [7]. These combine the components of conventional glass ionomers with the monomer 2-hydroxyethyl methacrylate (HEMA), together with appropriate polymerization initiators, so that the material sets by a combination of acid–base reaction and addition polymerization. The majority of brands of resin-modified glass ionomers are light-cured, which gives them the advantage that the clinician has considerable control over the setting reaction, much of which cannot occur until the dental curing lamp is switched on. Resin-modified glass ionomers retain many of the advantages of conventional glass ionomers, in particular their adhesion to the tooth surface [7] and their ability to release fluoride [8]. However, the monomer HEMA is potentially damaging and its presence reduces the biocompatibility of these materials relative to conventional glass ionomers [9]. Unpolymerized HEMA is known to be released by these materials [10] and HEMA has been shown to be able to penetrate the dentin to reach the pulp [11], where it proves to be cytotoxic [12]. HEMA monomer is also a well-known allergen [9]. Conventional and resin modified glass ionomer cements have been employed clinically in the so-called open sandwich technique for restoring Class II cavities [13–16]. In this technique, glass ionomer cement, or RMGIC, is used for the gingival portion of the restoration, and composite resin is used to complete the repair [15,16]. Clinical results with this technique are good and the use of a glass ionomer (of either type) has been found to significantly reduce the marginal micro-leakage at the cemento-enamel junction compared with the use of other materials, such as composite resin [13,15]. Glass-ionomers of both types have been tested for their adhesion to the tooth surface [17,18]. Materials have been tested in shear bond mode [17–20] and, more recently, in microtensile mode [21–23]. At relatively short times, bond strengths have been found to be acceptable, even without surface pre-treatment [2]. This leads to the suggestion that forming such adhesive bonds is straightforward, and the
Manufacturer
Fuji IX Fuji II LC Ketac Molar N100
GC (Japan) GC (Japan) 3M ESPE (USA & Germany) 3M ESPE (USA & Germany)
Type Conventional RMGIC Conventional RMGIC (nano-ionomer)
expectation that the resulting interfaces should be sound and free from flaws. This hypothesis was the original one under test in the current study. Since the open sandwich technique was pioneered, new versions of glass ionomers, both conventional and resinmodified, have been launched by manufacturers. The current study was initially undertaken with a view to examining the performance in vitro of some of these newer materials in the open sandwich technique, with particular emphasis on their bonding and the interfacial region with the tooth. However, our work has led to some new observations, not previously reported, that raise important questions about the issue of adhesive bonding by glass-ionomers.
2.
Materials and methods
Ten non-cavitated molars and premolars, extracted for orthodontic reasons, were used in this study. Teeth were stored prior to use in physiological saline to which small amounts of thymol had been added for 120 days at a temperature of 8 ◦ C. Materials used in the study are listed in Table 1, and consisted of two conventional and two resin-modified glass ionomers. One of the materials, N100, is described as “nanoionomer” because it contains nanometer sized filler. It is prepared and dispensed using a double tubed dispenser. Fuji II LC is a capsulated material, and was mixed using a vibratory mixer for 10 s prior to extrusion from the capsule. In each of the ten teeth, two slot preparations were created on opposing sides of the tooth (see Fig. 1), using a fissure bur (ISO 806 314 107 524 012, Lot D-4362 at 300 000 RPM), diamond
Fig. 1 – Slot preparation on opposing sides of the tooth. A = axial wall, B = axial/gingival line angle, C = cavo-surface line angle.
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round bur (ISO 806 314 001 524 016, Lot D-5113 at 280 000 RPM) and a high speed rose bur (ISO 500 314 001 001 016, Lot D434040 at 280 000 RPM). The depth of each preparation was 3 mm, (the gingival wall of the box is designed so it extends 1 mm below the cervical line of the tooth, into the root). Following cavity preparation, the teeth were stored in saline solution at a temperature of 8 ◦ C for 7 days. The ten teeth were divided, at random, into two groups: Group A (prepared using the two 3M ESPE products) and Group B (the GC products). In each group, one slot in a particular tooth was filled with conventional glass ionomer cement (Ketac Molar or Fuji IX) and the other slot in the same tooth was filled with resin modified glass ionomer (N100 or Fuji II LC). Materials used are listed in Table 1. Having prepared the tooth, the material was placed in a thin layer along the axial wall and gingival floor of the cavity preparation, after which the filling of the cavity was completed by placement of composite material. In this way, four different sample groups were compared for their ability to bind to tooth structure. The cavities designated for conventional glass ionomer samples were conditioned for 10 s prior to restoration with 20% polyacrylic acid, followed by a 10 s water rinse and drying with cotton pellets. The glass ionomer cements were mixed according to the manufacturer’s instructions, in a 1:1 powder to liquid ratio and condensed into the cavity to a depth of 1 mm on both axial and gingival walls. The cement was allowed to set for 5 min after which the cavity was etched, primed, bonded then restored with composite resin (Filtek Z250, ex 3M; shade A2). The resin modified glass ionomer slot for Group A (N100) was primed with Ketac N100 primer for 15 s, air thinned for 10 s and light cured for 10 s. The resin modified glass ionomer was dispensed using the Ketac Clicker dispenser and mixed for 20 s, followed by placement into the cavity (using the dispenser provided by 3M), along axial and pulpal walls to a thickness of 1 mm, followed by light curing. For group B, the dentin was conditioned with 20% polyacrylic acid for 10 s, rinsed with water for 10 s and cotton dried. The resin-modified glass ionomer (Fuji II LC) was mixed on a glass slab and placed using a plugger, followed by light curing for 20 s. Lastly, the cavity was etched, prime and bonded and restored with composite resin as for Group A. Following restoration, each group was stored in saline solution in a tightly sealed container for 1 week, at a temperature of 37 ◦ C. The teeth were then embedded in a transparent acrylic resin (Duracryl) and were cut parallel to the long axis of the tooth, through both restorations, using a low speed diamond wheel saw (model 650). Each tooth yielded two slices, which were polished prior to microscopic evaluation. Samples were then evaluated using a metallographic light microscope (Nikon Eclipse LV 100) under a magnification of 100×. In each tooth, there were three areas of observation: the axial wall (A), the axial gingival line angle (B) and the cavo-surface line angle (C) (see Fig. 1). In the initial assessment, bonding was categorized as inadequate or adequate based on the appearance of the region between the dentin and glass ionomer cement. Inadequate bonding was further studied, and details of the inadequacies recorded. Data were further analysed statistically using the McNamara analysis.
Table 2 – A comparison of adequate (A) and inadequate (I) bonding sites of the four materials tested. Material
Adequate (out of 30)
Inadequate (out of 30)
Fuji IX GIC Fuji II RMGIC Ketac Molar GIC N100 RMGIC
18/30 = 60% 21/30 = 70% 14/30 = 47% 15/30 = 50%
12/30 = 40% 9/30 = 30% 16/30 = 53% 15/30 = 50%
3.
Results
Each restored slice was observed at three points, giving a total of thirty observations for each material used. Bonding was characterized initially as either adequate or inadequate and the results show that Fuji II LC had the highest proportion of adequate bonded specimens, whereas Ketac Molar had the least (see Table 2). Further evaluation of the inadequately bonded samples is given in Table 3 and several distinct types of inadequate bonding were identified. These were: complete detachment of cement from dentin, perpendicular crack through the glass ionomer only, detachment plus perpendicular crack into the glass ionomer and finally crack through the glass ionomer into the dentin. In evaluating the inadequate bonding associated with Fuji IX, it was found that over half (58%) of the inadequate samples showed detachment from the dentin. With Fuji II LC, the inadequacies tended to consist of cracks into dentin. These comprised 56% of inadequate samples. Additionally, 33% of inadequate samples showed transverse cracks within the material. In these specimens, no detachment failures were observed. For Ketac Molar, the most common inadequacies observed were detachment plus transverse cracking of the cement (63% of inadequate samples). In no sample did the crack penetrate into the dentin, meaning that the cracks did not exceed a maximum of 2 mm in length, and were typically shorter than this. Finally, with N100, most of the inadequacies were observed as detachments of the cement from the dentin (53%). Again, no cracks penetrated the dentin with this material. In terms of the geometry of the restoration, most of the inadequate bonding occurred on the axial wall (Table 4), whereas at the axial/gingival line angle and the cavo-surface line angle, bonding appeared more satisfactory and was generally classified as adequate.
4.
Discussion
The fact that glass ionomer cements generally bond well to both dentin and enamel has been recognized for many years [17]. Indeed, this adhesion was considered so important that when resin-modified versions of these materials were first launched, almost immediately a paper appeared confirming their adhesion to dentin [18]. In these early papers, bonding was typically determined by measuring bond strength of cement samples loaded in shear [17–19]. It still is widely used as a test method in reporting the bonding properties of modern cement formulations [20].
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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Table 3 – Inadequate bonding of material to tooth structure according to location and type of inadequacy. Material
Detachment (interfacial failure)
Fuji IX Fuji II LC Ketac Molar N100
7/12 = 58% 0 6/16 = 38% 8/15 = 53%
Transverse crack within glass ionomer
Detach + T-crack
Crack into dentin
3/12 = 25% 3/9 = 33% 0 4/15 = 27%
2/12 = 17% 1/9 = 11% 10/16 = 63% 3/15 = 20%
0 5/9 = 56% 0 0
Table 4 – Number of adequate (A) and inadequate (I) bonding sites according to location within the cavity preparation. Location
Axial wall Axial/gingival line-angle Cavo-surface line-angle
Fuji IX
Fuji II LC
Ketac Molar
N100
A
I
A
I
A
I
A
I
4 5 9
6 5 1
4 7 10
6 3 0
1 4 9
9 6 1
3 6 6
7 4 4
More recently, studies have been reported in which the test regime has been changed to the determination of microtensile bond strength [21–23]. The principal advantage of this mode of testing is that the surface area of attachment is much smaller than in conventional shear bond evaluation. Results are therefore less likely to be affected by the presence of flaws and irregularities at the cement–dentin interface. Using microsized bonding areas reduces the likelihood of such weakening occurring. However, none of these testing regimes examines a realistic bonding situation. Real glass ionomer restorations fill prepared cavities in teeth. The material therefore has to be able to be forced fully into the cavity, occupying the whole space and coming into intimate contact with flat surfaces in three dimensions. Shear or microtensile bond tests only evaluate the latter aspect: the ability to come into contact with relatively flat surfaces and there to form a strong adhesive bond. In the present study we have considered a model that more accurately reflects the conditions in which bonds must form in clinical application. When we have done that, we find that problems of inadequate bonding are apparent mainly at the axial wall. These must occur because of difficulties in fully condensing the cement, meaning that it is not pushed properly to the back of the cavity. This problem has not been reported previously, and appears to arise from problems in technique of placement of a relatively thin layer (1 mm) of the glass ionomer cement, even less than would be placed as a base. There is also a difficulty caused by adhesion of the unset cement to the instruments being used. By contrast, published accounts of bonding of the material Fuji IX have shown that, in the hands of experienced clinicians, it can be placed in intimate contact with the complete surface of a prepared cavity in a carious tooth [6]. However, most of these studies employed glass ionomer cement as a full restoration, not as a base. In the present study, the thinness of the base was necessary the cavities were relatively small and space was necessary for placement of resin composite. This situation is common in class V cavities (for example, abrasion and erosion) at the neck of the tooth. In our clinical experience, it is easier to place the cement in one bulk portion, filling the whole cavity, than to place a thin layer of glass ionomer at the base of the cavity and leaving the remainder of the cavity material free [24]. By contrast with the problems at the axial wall, bonding at the cavo-surface angle was found to be good. This shows that
all of the materials examined can occupy this type of confined space, either by flowing in on application, or as the result of condensing into the corners during placement. The emphasis on bond strength in the published literature has overshadowed consideration of other aspects of the performance of these materials. The finding that Fuji II LC is capable of causing cracks to develop in the dentin in a proportion of the samples (30%) is extremely important. In no instance was this material found to detach from the dentin surface in the course of these experiments. Its adhesion is excellent. Indeed, this is partly the cause of the problems observed. When stresses develop, either due to polymerization contraction or swelling due to water sorption, both of which are known to occur with this material [25], rather than the adhesive bond failing, or cracks developing in the material, these stresses can cause cracks in dentin. This is of considerable clinical importance, and requires further study. To date, clinical results with Fuji II LC have been good [26,27], and it may be that our experimental study exaggerates the problem of dentinal cracking, which may not occur so frequently under clinical conditions. Nonetheless, the fact that excellent adhesion can lead to this type of serious failure is important and should be noted by the research community. Our findings show that these materials cannot be evaluated safely on the basis of adhesive bond strength alone. Other factors (polymerization contraction, swelling in moisture, mechanisms of stress transfer) must be taken into consideration, and it is essential that materials do not have such good bond strengths that their dimensional stresses cause tooth fracture in vivo.
5.
Conclusions
This study has shown that the use of glass ionomer materials, both conventional and resin-modified, for the repair of model cavities using sandwich technique is very satisfactory in the majority of cases. However it has been shown to be capable of leading to problems of adhesion. In a number of instances, there were problems in establishing contact between the material and the furthest axial wall of the cavity, which may arise from problems in handling the placement of a thin layer of unset cement. In addition, for one of the resin-modified glass ionomers (Fuji II LC), in a small but important proportion of the samples (16%), the material caused a perpendicular
Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
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crack to develop in the adjacent dentin. This has serious implications for clinical application and demonstrates that adhesive bond strength alone cannot be used as the criterion of success for an adhesive restorative material. Other factors such as ease of condensation and dimensional stability are important and need to be considered as well.
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Please cite this article in press as: Czarnecka B, et al. Adhesion of resin-modified glass-ionomer cements may affect the integrity of tooth structure in the open sandwich technique. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.05.008
