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In vivo Fluid Movement through Dentin Adhesives in Endodontically Treated Teeth
1 Department of Restorative Dentistry and Dental Materials, University of Siena, Italy; Correspondence: * corresponding author, kfctay{at}netvigator.com
Fluid transudation through simplified dentin adhesives can occur in bonded vital crown dentin, since these adhesives behave as permeable membranes after polymerization. The effect of adhesive permeability in endodontically treated teeth is unknown. This study examined the hypothesis that in vivo fluid movement through simplified adhesives occurs when they are applied to root canals. Dowel spaces were prepared in endodontically treated teeth with single root canals. Six adhesives were applied to the intra-radicular dentin of canal walls. Impressions were obtained with polyvinyl siloxane, and replicas were fabricated with the use of polyether impression material. Replica hemisections were gold-coated for SEM examination. Fluid transudation was evident on the adhesive surfaces of all simplified total-etch and self-etch adhesives. Conversely, most of the specimens bonded with the control three-step total-etch adhesive were devoid of fluid droplets. Permeability of simplified adhesives results in water movement, even in root-treated dentin. This may adversely affect the coupling of auto-/dual-cured resin cements.
Key Words: adhesive permeability • simplified adhesives • root canal • fluid transudation
The development of dentin adhesives has reached a point where a genuine technological breakthrough cannot be expected without the input of paradigms from other scientific disciplines (Tay and Pashley, 2002). As existing bonding strategies are modified to create adhesives that are simpler and faster (Perdigão et al., 2003), compromises have to be made when these adhesives are formulated with a reduced number of bonding steps (Van Meerbeek et al., 2003). An immediate consequence of adhesive simplification is a sacrifice of the universality of the multi-bottle adhesives, with most of the simplified versions capable of bonding only to light-cured composites (Sanares et al., 2001; Pfeifer et al., 2003). Although the adhesion of auto-/dual-cured composites may be improved with the adjunctive use of ternary catalysts that offset the acid-base incompatibility between acidic methacrylate monomers and tertiary amines (Suh et al., 2003), the bonding efficacy of both simplified total-etch (Tay et al., 2003b) and simplified self-etch adhesives (Tay et al., 2003a) to auto-/dual-cured composites/resin cements is hampered by the intrinsic permeability of these adhesives to the water that results from their increase in hydrophilicity (Carvalho et al., 2004; Chersoni et al., 2004a). Transudation of dentin fluid through simplified dentin adhesives has been shown to occur in vivo in bonded vital crown dentin (Chersoni et al., 2004b; Tay et al., 2004), since these adhesives behave as permeable membranes after polymerization (Tay et al., 2002a). This may result in the entrapment of water blisters between the adhesive surface and slow-setting resin composites/cements. The water blisters may act as stress raisers and account for the apparent incompatibility of these adhesives with the bonding of indirect restorations, even in the absence of the true adhesive incompatibility derived from adverse acid-base reactions (Tay et al., 2003a). Although a positive pulpal pressure is absent in endodontically treated teeth, an increase in radicular permeability may follow reduction in root dentin thickness and removal of sealers that penetrated the dentinal tubules during the preparation of spaces for the cementation of endodontic posts (Fogel et al., 1988; Guignes et al., 1996). This is particularly so when total-etch adhesives and the aggressive type of self-etch adhesive (Tay and Pashley, 2001), that completely remove or dissolve the smear layers, are used for bonding to root canals (Tao et al., 1991). Since vital teeth and endodontically treated teeth do not differ significantly in their moisture content (Papa et al., 1994), the effect of adhesive permeability on bonding to root canals with conventional or simplified total-etch and self-etch adhesives is unknown. Thus, the objective of this in vivo study was to examine, by a novel impression replica technique (Chersoni et al., 2004), the permeability of adhesive-bonded intra-radicular dentin to fluid movement. The null hypothesis tested was that there is no difference among the 4 different classes of dentin adhesives (i.e., three-step total-etch, two-step total-etch, two-step self-etch, and one-step self-etch) in preventing fluid movement across bonded intra-radicular dentin.
Twenty-eight endodontically treated teeth with single root canals that required dowel retention as part of the restorative treatment plan were selected for the study. The root canals were previously obturated with vertically compacted warm gutta-percha and a root canal sealer (CRCS, Colténe AG, Altstätten, Switzerland). The patients were between 26 and 47 yrs of age, with a mean age of 36. The subjects’ informed consent was obtained under an in vivo protocol reviewed and approved by an ethics committee from the University of Bologna. Preparation of dowel spaces and the subsequent bonding procedures were performed with the use of rubber dam isolation and an endodontic microscope (Karl Kaps, base model, Asslar, Wetzlar, Germany). The coronal part of each root filling was removed, with a root canal filling of 5–6 mm remaining, and a post space was created for the insertion of a size 2 DT Light-Post (RTD, Grenoble, France), a radiopaque glass fiber post, by use of the corresponding drills. The residual thickness of remaining dentin around the dowel space of each tooth was evaluated radiographically to be at least 1.5 mm. Debridement was performed with distilled water under the microscope to ensure that the post space was free of cutting debris, and that the coronal portion of the remaining gutta-percha could be identified from the base of the post space. Prior to the bonding procedure, each post space was air-dried with a triple syringe equipped with a Stropko irrigator (SybronEndo, Orange, CA, USA) and further dried with 4 paper points to ensure that the post space was free from residual moisture (Hosoya et al., 2000).
Experimental Design Since the experimental protocol required a polymerized adhesive surface for impression-taking, the uncured Pre-Bond resin in All-Bond 2, that was to be placed over the primed dentin, was replaced with a mixture of Pre-Bond and D/E resin, so that a curable adhesive layer could be obtained (B.I. Suh, personal communication). For the 3 total-etch adhesives, the post spaces were blot-dried with paper points after the phosphoric acid was rinsed, to provide a moist substrate for the wet-bonding technique. Although the one-step self-etch adhesives were not recommended for the coupling of auto-cured composites, they were used with the understanding that the incompatibilities associated with these 2 adhesives were apparent in nature, and were attributed to the permeability of the adhesives instead of to adverse acid-base interactions (Tay et al., 2003a). To ensure that subsequent restorative treatments were performed in the best interest of the subjects, we placed an additional thin coat of Scotchbond Multi-Purpose bonding resin (3M ESPE) over these adhesives and light-cured them after the experimental impression-taking protocol. This additional step converts these adhesives from one-step to two-step self-etch adhesives, making certain that they are compatible with auto-/dual-cured resin cements. To ensure optimal curing, we applied each adhesive to the post spaces with a microbrush, and light-cured it via a slightly undersized, non-bonding, light-transmitting plastic post (Luminex, Dentatus AB, Hägersten, Sweden), that was inserted into the post space and transilluminated from the top by means of a LED light-curing unit (L.E. Demetron 1, Sybron-Kerr, Orange, CA, USA) for 40 sec at an output of 800 mW/cm2. Since the uncured adhesive caused by oxygen inhibition must be removed prior to impression-taking, a second coat of the adhesive was applied. Removal of the adhesive oxygen inhibition layer was performed by means of acetone and a microbrush.
The bonded post space was air-dried for 10 sec by insertion of the Stropko irrigator into the root canal. An impression was immediately taken with a polyvinyl siloxane impression material (Affinis light-body, Colténe AG). The material was injected into the post space and distributed by means of a spiral, followed by the insertion of the previously used plastic post as an anchor. After 4 min, the polymerized impression material was removed and de-gassed for 24 hrs. Positive replicas were then fabricated with a polyether impression material (Permadyne Garant, 3M ESPE), with the polyvinyl siloxane negative replica used as a mold, according to the polyether replica technique reported by Chersoni et al.(2004a). Since there is no chemical reaction between polyether and polyvinyl siloxane, this replica technique has been shown to be effective in replicating water exudation from dentin hybrid layers. The polyether replicas were cut into hemisections by means of a sharp razor blade, coated with gold/palladium, and examined under a scanning electron microscope (SEM; Model 5400, JEOL, Tokyo, Japan) at 5–10 kV. Five SEM images of each of the 4 post spaces in each group were taken randomly from different areas of the replicas (N = 20). The number of fluid droplets/1000 µm2 of polymerized adhesive was recorded by two co-authors who did not participate in bonding and were unaware of the group designations. Data from the 6 groups were statistically analyzed by Kruskal-Wallis ANOVA and Dunn’s multiple-comparison tests at
Controls
Fluid droplets could be identified from the control unbonded dowel spaces with intact smear layers (Fig. 1A  ) and after acid-etching (Fig. 1B). All-Bond 2, the control three-step total-etch adhesive, exhibited no fluid transudation across the polymerized adhesive in 3 of the specimens examined (Fig. 1C), and minimal fluid droplets were observed only in the fourth specimen (Fig. 1D).
By contrast, ample fluid droplets were seen along the polymerized adhesive surfaces in the simplified two-step total-etch adhesives Single Bond (Fig. 2A ) and One-Step Plus (Fig. 2B).
Fluid droplets along the adhesive surface were also observed in the intra-radicular dentin bonded with the non-rinsing, two-step self-etch adhesive Tyrian SPE/One-Step Plus (Fig. 3A ). Similarly, fluid droplets were seen in specimens bonded with the simplified one-step self-etch adhesives Xeno III (Fig. 3B) and One-Up Bond F (Fig. 3C). Based on morphologic examination, the sizes of the fluid droplets were smaller in the self-etch adhesives.
Data on the distribution of fluid droplets are summarized in the Table . All Bond 2 exhibited significantly fewer fluid droplets than the other 5 experimental groups (P < 0.05). There were no differences in the numbers of fluid droplets/1000 µm2 among the other 5 groups (Fig. 3D ).
The results of this in vivo study warrant a rejection of the null hypothesis that there is no difference among the different classes of dentin adhesives in preventing fluid movement across bonded intra-radicular dentin. The observation of fluid droplets in bonded root-treated dentin was similar to our previous observations of dentinal fluid transudation from vital bonded deep dentin (Chersoni et al., 2004b; Tay et al., 2004), although, in the latter, there was a propensity for more profuse fluid droplets to be formed.
Since root canal sealers can penetrate only 35–80 µm into the dentinal tubules, even with complete smear layer removal (Kouvas et al., 1998), residual dentinal tubules are likely to be devoid of sealers when dentin thickness is reduced after post space preparation. Rinsing with water during the course of a total-etch technique probably results in the retention of substantial amounts of water within the widened tubular entrance created by acid-etching, that may not be completely removed via the use of paper points. This may account for the occurrence of larger fluid droplets in the simplified total-etch adhesives (Figs. 2A
Previous fluid transport studies reported the presence of an inferior seal along the coronal two-thirds of the root-filled canals after post space preparation, when compared with intact root fillings (Abramovitz et al., 2001; Wu et al., 2003). Presumably, the use of dentin adhesives would improve the seal of this critical region, minimize coronal leakage, and improve the prognosis of endodontic therapy. However, the in vivo observation of fluid transudation through simplified adhesives in endodontically treated teeth suggests that a hermetic seal of the coronal third of root canals is unlikely to be achieved with the use of either simplified total-etch or self-etch adhesives. Theoretically, the use of a two-step self-etch adhesive should minimize water movement through the adhesive (Carvalho et al., 2004). However, the system we examined utilizes a fairly permeable adhesive (One-Step Plus; Fig. 2B Although the use of bonded posts has been clinically successful, a recent SEM study of endodontically treated teeth extracted after clinical service reported that subclinical gap formation and adhesive failures occurred along the resin-dentin interfaces, in spite of the absence of clinical failure (Mannocci et al., 2003). Unfavorable cavity configuration factors in post spaces have been cited as the major reason for low bond strengths and marginal gap formation (Morris et al., 2001; Bouillaguet et al., 2003). Slow-setting resin cements have been recommended to offset the development of high polymerization shrinkage stresses along root canal walls (Goracci et al., 2004). The extended setting time for these resin cements, which is perceived to be beneficial in reducing shrinkage stresses, may be offset by the use of simplified total-etch or self-etch adhesives, since this prolongs the time for water diffusion through the adhesives, creating fluid blisters along the adhesive-cement interface that act as stress-raisers and precipitate bond failure (Carvalho et al., 2004). In the event that coronal leakage occurs during functional stresses, leaching of resin components from these highly hydrophilic adhesives may further contribute to the degradation of the bonds between the adhesive and intra-radicular dentin. Although simplified acidic adhesives may be rendered compatible with auto-/dual-cured resin cements via the use of ternary catalysts (Suh et al., 2003), they must be used with caution for bonding to root canals, in light of the fluid movement that occurs through these adhesives under in vivo conditions. The use of conventional, less permeable adhesives appears to be a more rational alternative.
We thank Paolo Ferrieri for technical assistance with the SEM. This study was based on the work performed by Stefano Chersoni in partial fulfillment of the requirements for the degree of Doctor of Philosophy, the University of Siena, Italy. The adhesives examined were generous donations from 3M ESPE, Bisco, Inc., Dentsply DeTrey, and Tokuyama Corp. There was no remuneration paid directly to any of the investigators in the form of consultant’s fees or other arrangements. This study was supported by grant 60% UNIBO 2002 and 2003, Italy, by grants DE 014911 and DE 015306 from the National Institute of Dental and Craniofacial Research (PI, David Pashley), and by grant 10204604/07840/08004/324/01 from the Faculty of Dentistry, University of Hong Kong. The authors are grateful to Michelle Barnes and Zinna Pang for secretarial support. Received for publication March 8, 2004. Revision received July 23, 2004. Accepted for publication January 2, 2005.
Journal of Dental Research, Vol. 84, No. 3,
223-227 (2005)
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ABSTRACT
INTRODUCTION
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