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 »  Home  »  Endodontic Articles 11  »  Micro-tensile bond strengths of bonding agents to pulpal floor dentine
Micro-tensile bond strengths of bonding agents to pulpal floor dentine
Introduction - Materials and methods.

K. Kijsamanmith, S.Timpawat, C.Harnirattisai & H. H. Messer
Dental Section, Donjedi Hospital, Supanburi Province, Bangkok, Thailand.
Department of Operative Dentistry, Faculty of Dentistry, Mahidol University, Bangkok, Thailand.
School of Dental Science, The University of Melbourne, Melbourne, Australia.

Many core build-up materials have been introduced for the restoration of root-filled teeth including amalgam and resin composites. Increased emphasis is being placed on conservation of natural tooth structure, and the use of Bonding technologies permits a more conservative approach to post and core restorations (Leinfelder 1993). An advantage of Bonding, coupled with composite core buildup, is the high bond strength to tooth structure and increased resistance to fracture (Hernandez et al. 1994). Based on the clinical properties of bonded materials, adhesive systems should provide good retention and reduce microleakage of the resin composite core.
Bonding to dentine usually involves the cut surface of coronal dentine with a at surface, smear layer and smear plugs within dentinal tubules. On the other hand, pulpal floor dentine is a complex biological structure, including primary dentine, regular and irregular secondary dentine (Berkovitz et al. 1992). In addition, the pulpal floor is generally not contacted by cutting instruments during endodontic access and should be largely free of smear layer. Several studies have reported the micro-tensile bond strengths obtained from human and bovine dentine (Tanumiharja et al. 2000, Bouillaguet et al. 2001). However, only one recent study (Belli et al. 2001) has been undertaken to investigate whether pulpal floor dentine should be a good site for Bonding.
When the obturated root canal system is exposed to the oral environment, penetration of microorganisms from coronal direction potentially contributes to failure of root canal treatment. Many authors have stated that obturation alone is unable to provide a thorough seal if the tooth is not appropriately restored within a short time (Swartz et al. 1983, Saunders & Saunders 1994). The quality of the permanent coronal restoration is a crucial factor for the overall prognosis of the tooth after obturation (Ray & Trope1995). Coronal leakage is particularly significant in multirooted teeth, where accessory canals may be present in the furcation area (Vertucci & Anthony 1986). These canals may allow inflammatory changes to occur in the periodontal tissues because of a direct spread of microorganisms from the pulp chamber (Gutmann1978).
When composite resin is used to restore the access cavity of a molar or as a core material, it is very important to achieve a good bond to pulpal floor dentine, both to enhance retention and to maximize the coronal seal. Achievement of the bond between adhesive resin and dentine depends on the penetration of the primer and adhesive resin into the conditioned dentine surface in order to create micromechanical interlocking between the dentine collagen and resin to form a hybrid layer (Nakabayashi et al. 1982). Numerous commercial Bonding systems are available, with two recommended simplified approaches for good hybridization and adequate Bonding. The first is the ‘total-etch’ technique followed by the application of a ‘one-bottle’ solution containing the primer and adhesive resin. The second is the self-etching-priming technique followed by the application of an adhesive resin to the conditioned dentine surface.
The purpose of this study was to investigate the characteristics of the pulpal floor surface and the effectiveness of Bonding of two agents to pulpal floor dentine. One group was a ‘one-bottle’ adhesive system (Prime & Bond NT; Dentsply DeTrey, Milford, DE, USA) and the other group was a self-etching-priming adhesive system (Clearl SE Bond; Kuraray, Osaka, Japan). A bond testing procedure, known as the micro-tensile bond strength test using very small surface areas, was introduced by Sano et al. (1994). This technique allows the testing of very small cross-sectional areas of dentine-resin specimens and develops a uniform stress distribution during testing so that most bond failures occur interfacially. Furthermore, this technique allows the use of smaller, more uniform dentine samples such as pulpal floor dentine. The null hypothesis of the study was that bonding to pulpal floor dentine was not influenced by the Bonding system used.

Materials and methods.
Thirty-six extracted human noncarious first and second mandibular molars with fully developed apices, extracted for periodontal reasons, were used in this study. No data were obtained on patient age. The teeth were stored in 0.1% thymol solution until use, and the teeth were cut horizontally 3 mm above the cemento-enamel junction with a slow-speed diamond saw (Accutom- 50; Struers, Copenhagen, Denmark) to expose the pulp chamber. To avoid touching pulpal floor dentine, pulp tissue was removed carefully with a spoon excavator. The specimens were rinsed with distilled water to remove debris, then air-dried with a triple syringe.
Teeth used for Bonding were randomly allocated to two groups of 14 teeth each. Each group was assigned to one dentine adhesive system, and a dual cure resin composite (FluoroCore; LD Caulk, Milford, DE, USA) was used as the core material in the pulp chamber (Table 1). The pulpal floor dentine was prepared and bonded according to the manufacturers’ instructions (Table 2). After Bonding, equal quantities of base paste and catalyst of FluoroCore were hand-mixed until a uniform shade was obtained within 30-45 s and placed in the pulp chamber as a core restoration. The first two or three increments were placed and light-cured for 40 s, then a large increment was added and allowed to cure chemically for 7-8 min followed by visible light curing for 40 s. The teeth were kept in tap water for 24 h at 37 8C.
Each tooth was sectioned vertically into thin slabs of 0.7 mm thickness over the floor of the pulp chambers with a slow-speed diamond saw under copious water spray. Each tooth yielded two or three bar-shaped specimens. These sections were then shaped to forma gentle curve, with the narrowest portion at the bonded interface and standardized to produce a bonded surface area of1.0x0.2 mm2 using a super-fine diamond bur (Intensiv SA; Swiss Dental Products, Zurich, Switzerland)with a high-speed handpiece under copious air-water spray. The thickness andwidth of the bonded area of each specimen were checked before testing using a digital micrometer (Mitutoyo Corp., Tokyo, Japan). The specimens were then attached to a Bencor-Multi-T-testing apparatus (Danville Engineering, Danville, CA, USA) with a cyanoacrylate adhesive (Zap-It; DVA, Corona, CA, USA) and stressed in tension using a universal testing machine (Instron 5566 series 5000; Instron Corporation, London, UK) at a cross-head speed of1mm min_1. The mean micro-tensile bond strength for each tooth (based on two to three samples per tooth) was calculated. The tensile bond strength of 14 teeth was calculated as the maximum load at failure divided by the bonded cross-sectional area, expressed in MPa.

Table 1. Materials, manufacturers, and system compositions.

Materials, manufacturers, and system compositions

Table 2. Bonding procedures.

Bonding procedures

Statistical analysis.
The mean bond strengths for the two groups were compared using Student’s unpaired t-test and a P-value <0.05 was considered to be statistically significant.

SEM observations.
1 Normal and etched pulpal floor dentine: Eight teeth were used to examine the pulpal floor, including two control teeth with no surface preparation other than initial cleaning as described above. Three teeth were employed for each of the two Bonding techniques. In group A (‘one-bottle’ technique), 34% phosphoric acid gel (Dentsply Caulk) was applied to the dentine surface for15 s and rinsed with distilled water for10 s. In group B, a self-etching primer (Kuraray) was applied for 20 s and rinsed with acetone for10 s. After rinsing, the treated surfaces were air-dried, gold-sputter-coated and examined using a scanning electron microscope ( JSM- 5410 LV; JEOL,Tokyo, Japan). 2 Failure mode: The fractured surfaces from the bond testing were air dried, mounted on aluminum stubs and observed using SEM at x75 magnification to determine the mode of failure after micro-tensile testing.