Effect of intrinsic wetness and regional difference on dentin bond strength
Introduction
The development of new dentin bonding systems has brought about great improvements in adhesive dentistry and changes in the philosophies of cavity preparation. Extensive cavity preparations have been replaced by more conservative techniques, with careful removal of the infected, permanently damaged carious tissue, and preserving the transparent caries-affected dentin [1]. The caries-affected dentin possesses properties that are different from those of normal dentin, such as reduced permeability and decreased intrinsic water content because of the formation of sclerosis in the tubules of the transparent dentin layer [2], [3], [4]. Bonding to caries-affected dentin shows different bonding characteristics compared to bonding to normal dentin. Some adhesive systems produce lower bond strength to caries-affected dentin than to normal dentin [5]. Other systems utilizing higher concentrations of phosphoric acid bond well both to normal and caries-affected dentin [6]. Ideally, the clinical bonding substrates should be caries-affected dentin where possible. However, it is very difficult to preserve the caries-affected dentin because of the irregular shape of the lesion. Consequently, the clinical practitioner may over-cut the dentinal tissue, creating different substrates for bonding within the same cavity (i.e. areas of normal dentin and caries-affected dentin).
Dentin moisture, as well as regional differences, are important factors that may affect dentin bonding [7], [8], [9]. Moreover, adhesive systems using phosphoric acid etchant should be used with the ‘moist technique’ to ensure good bonding between the resin and dentin [10]. Owing to the lack of appropriate measurement methods, limited information exists in the literature comparing regional strengths of bonding systems with difference in permeability, as well as varying degrees of wetness of the dentin substrate. Recently, a new method for testing bond strengths has been developed, permitting the measurement of small bonded areas as small as 0.5 mm2 [11]. The microtensile bond test permits testing of various substrates and regions, such as caries-affected dentin [5], sclerotic dentin and wedge-shaped defects [9], and flat or cavity surfaces [7], [8]. As this new method permits measurement of small areas, it can be used to compare regional bond strengths with different permeability, and degrees of wetness of the dentin substrate.
In deep dentin, close to the pulp, the permeability becomes greater and influence of pulpal pressure and presence of water influence the degree of surface and intrinsic wetness. Our hypothesis was that regional bond strengths would be affected by intrinsic wetness of the dentin substrate, and that an outflow of water from the pulp chamber would occur in more permeable regions and affect bond strengths. Therefore, the purpose of this study was to measure the regional tensile bond strength, under the effect of hydrostatic pressure, or complete desiccation of the dentin substrate, by means of the microtensile method.
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Materials and methods
The teeth used in this study were obtained by protocols that were reviewed and approved by the appropriate institutional review board of the Tokyo Medical and Dental University and with the informed consent of donors. Human caries-free third molars which were extracted for orthodontic reasons and stored frozen were selected for this study. The teeth were randomly divided into three groups of six teeth each for bonding: Group I (no pulpal pressure), Group II (pulpal pressure of 15 cm H2O); and
Results
The means and standard deviations of the micro tensile bond strength (μTBS) of the tested materials and the RDT are presented in Table 3. For Group I, there was no significant regional difference for Liner Bond II (pulp horn=38.3±13.6 MPa, center=42.0±14.0 MPa, periphery=34.2±10.4 MPa; p>0.05). In contrast, although the bond strengths of One Step to the center and periphery regions were greatest (48.1±10.7 and 42.9±16.8 MPa, respectively), the bond strength at the pulp horn region were
Discussion
The wetness of dentin is extremely important when measuring bond strength, especially when testing adhesive materials in vitro with the intention of simulating the in vivo situation [13]. Although several reports have indicated that bond strengths of former dentin adhesive systems were negligible in the presence of water [14], [15], [16], [17], [18] recent studies have advocated that moisture on the dentin surface is essential for successful bonding [19] and that in vitro dentin bonding is
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