M. Yoshiyama

Adhesion testing of dentin bonding agents: A review

Adhesion testing of dentin bonding agents was reviewed starting with the adhesion substrate, dentin, the variables involved in etching, priming and bonding, storage variables and testing variables. Several recent reports attempting to standardize many of these variables were discussed. Recent advances in the development of new bonding systems have resulted in bond strengths on the order of 20-30 MPa. At these high bond strengths, most of the bond failure modes have been cohesive in dentin. As this precludes measurement of interfacial bond strength, new testing methods must be developed. One such new method, a microtensile method, was described along with preliminary results that have been obtained. The last decade has produced major advances in dentin bonding. The next decade should prove to be even more exciting.

Scanning Electron Microscopic Characterization of Sensitive vs. Insensitive Human Radicular Dentin

Our aim was to investigate the structural changes of dentinal tubules in specimens obtained from both hypersensitive and naturally desensitized areas in wedge-shaped defects on the same exposed cervical dentin surface of a hypersensitive tooth. A new biopsy technique that makes use of a hollow, cylindrical diamond bur was designed so that specimens from exposed root dentin of vital teeth could be obtained. Twenty-two dentin biopsy pairs were divided into two groups; one was prepared for scanning electron microscopy (SEM) and the other for microradiography (MR). Small hypersensitive areas were identified by a scratch test on exposed human dentin in vivo. SEM observation of the dentin biopsies showed that the orifices of many dentinal tubules in hypersensitive areas were open and that membranous structures appeared on the walls of dentinal tubules. In naturally desensitized areas on the same dentin surface, most of the dentinal tubules were obturated with rhombohedral crystals of all sizes; membranous structures were not observed in these tubules. These results showed that hypersensitivity occurred on the exposed dentin when most of the tubular orifices were open.

Regional bond strengths of self-etching/self-priming adhesive systems

The purpose of this study was to measure the regional tensile bond strengths (TBS) at various portions of human tooth enamel and dentin, and to observe the resin-dentin interfaces of two commercially available self-etching/self-priming adhesive systems by scanning electron microscopy (SEM). Twelve extracted human cuspid teeth were used to measure TBS and four additional teeth were used for scanning electron microscopy (SEM). Outer enamel and dentin were removed from the labial tooth surfaces to form a long cavity preparation into middle dentin extending from the mid-crown to the apex of the root within the same tooth. Either Clearfil Liner Bond 2 (LB 2, Kuraray) or Fluoro Bond (FB, Shofu) was bonded to the surfaces, and covered with resin-composite. The resin-bonded teeth were serially sliced at right angles to the long axis of the tooth, and the bonded surfaces were trimmed to give a bonded cross-sectional surface area of 1 mm2 for TBS tests. LB 2 and FB showed significantly higher TBS in coronal, cervical and middle root dentin than that in enamel and apical root dentin. SEM showed that the thickness of the hybrid layer of both systems was about 1.0 microm in coronal, cervical and middle root dentin, and less than 0.5 microm in apical root dentin. These results suggested that the self-etching/self-priming systems produce good adhesion in coronal, cervical and middle root dentin by creating thin hybrid and transitional layers, but bonding to enamel and apical root dentin should be improved.

Regional bond strengths of resins to human root dentine

OBJECTIVES The demand for restoration of root dentine defects such as cervical erosion and root caries has significantly increased recently, but there is limited information on the performance of the adhesive resins to radicular dentine. The purpose of this study was to measure the regional tensile bond strength (TBS) of composite bonded to human root and coronal dentine, and to observe the interface between resin and root dentine by SEM. METHODS Human extracted cuspid teeth were used to measure TBS with a new microtensile bond test. Enamel and cementum were removed from the labial tooth surfaces to form a long cavity preparation into dentine from the mid-crown to the apex of the root within the same tooth. All Bond 2 (all etch technique) or Imperva Bond (no etch) was bonded to dentine surfaces and covered with Protect Liner resin composite. The resin-bonded teeth were serially sliced into 16 sections at right angles to the long axis of the tooth, and the bonded surfaces were trimmed to give a bonded surface area of 1 mm2 for TBS testing. RESULTS All Bond 2 bond strengths to coronal and apical dentine showed high values (23.5 MPa) but the bond strength was significantly lower on cervical root dentine. Imperva Bond produced a relatively high tensile bond strength to all regional areas. SEM showed that the thickness of the resin-infiltrated layer formed by All Bond 2 in root dentine was less than coronal areas. Resin infiltration with Imperva Bond was always less than 0.5 micron. The results suggest that high TBS values can be obtained with minimal resin infiltration in root dentine.

In vitro study on the dimensional changes of human dentine after demineralization

Dentine rods measuring approximately 0.7 x 0.7 x 5.0 mm were prepared from dentine of extracted human third molars stored in saline containing 0.5% sodium azide at 4 degrees C until used. Forty specimens were demineralized in 10% citric acid plus 3% ferric chloride (w/w) solution for 8 h, then assigned to four groups (A, B, C and D) of 10 specimens each. Groups A and B were used to investigate volumetric changes after air-drying and further immersion in either water, an aqueous solution of 50% 2-hydroxymethylmethacrylate (HEMA) or 100% HEMA, followed by air-drying. Groups C and D were used to investigate the ability of 100% HEMA or 100% ethylene glycol to prevent shrinkage of demineralized dentine during exposure to air. Demineralization caused a small, non-significant (1.9%) reduction in dentine volume. Air-drying further reduced the volume by 65.6%. When demineralized, shrunken specimens were immersed in water for 24 h, they recovered their original demineralized volume. Immersion in 100% HEMA did not re-expand demineralized shrunken dentine. Specimens immersed in 50% HEMA yielded a 50% volume shrinkage when exposed to air for 24 h. Both 100% HEMA and 100% ethylene glycol were effective in preventing shrinkage of demineralized dentine. The technique used provided useful information about maximal dimensional changes that may occur at a microscopic level during adhesive dental restorative procedures.

Dimensional changes of demineralized human dentine during preparation for scanning electron microscopy

Dentine rods measuring approx. 0.7 x 0.7 x 5.0mm were prepared from the crowns of extracted human third molars. The specimens were demineralized in 0.5 M EDTA (pH 7.0) for 3 days and their volume measured with a digital micrometer under a dissecting microscope. The specimens were randomly assigned to experimental groups and were chemically dehydrated in acetone. Next they were dried using either hexamethyldisilazane, Peldri II, or critical-point drying techniques. The dimensions of the specimens were measured again after each step and the changes in volume were expressed as a percentage of the original demineralized volume. The effects of fixing the specimens in 10% buffered formalin before dehydration with acetone were also investigated for every drying procedure. Dehydration in acetone caused a small but significant reduction in the volume of demineralized formalin-fixed specimens but unfixed specimens did not change significantly. In general, all three drying procedures caused some shrinkage in demineralized dentine specimens. Unfixed specimens exhibited a volumetric shrinkage of approx. 15-20% after drying with any of the methods. Fixed specimens shrank more than unfixed specimens after drying (25-35%). Regardless of the drying technique, the specimens shrank a further 10-20% when measured in the vacuum chamber of the scanning electron microscope. Among the three drying techniques employed, hexamethyldisilazane seems to be a very useful alternative to critical-point drying for the preparation of dentine specimens for scanning electron microscopy.

Duration of Dentinal Tubule Occlusion Formed by Calcium Phosphate Precipitation Method: In vitro Evaluation Using Synthetic Saliva

The use of a calcium phosphate precipitation method occluded dentin tubules with apatitic mineral and, thus, showed good potential for the treatment of dentin hypersensitivity. The aim of this study was to elucidate the occluding behavior of the precipitate in the oral environment. Dentin disks treated by the calcium phosphate precipitation method, and disks treated with potassium oxalate, NaF, and SrCl2 solutions, were immersed in synthetic saliva, which was regularly replenished so that ionic concentration would be maintained. Treatment of dentin disks by the calcium phosphate precipitation method immediately reduced dentin permeability to 6 ± 8%. When the disk was immersed in synthetic saliva, dentin permeability remained low, even seven days after immersion. Scanning electron microscopic observation showed no distinct boundary line between the precipitate and intertubular dentin; this indicated further mineralization on the precipitate. Potassium oxalate treatment also reduced the dentin permeability to 8 ± 3%. However, the dentin permeability gradually but steadily increased with immersion time, reaching 39 ± 14% at seven days. To elucidate the mechanism underlying dentin permeability changes in synthetic saliva, we immersed the precipitates, i.e., apatitic mineral and calcium oxalate, in a fixed volume of synthetic saliva. When calcium oxalate was immersed in synthetic saliva, there was a large concentration of oxalate ions, indicating dissolution of the calcium oxalate; this phenomenon was ascribed to the increase in dentin permeability. In contrast, calcium and phosphate ions decreased when apatitic powder, the precipitate formed by the calcium phosphate precipitation method, was immersed in synthetic saliva. The decrease in the calcium and phosphate ions in synthetic saliva indicated further precipitation of calcium phosphate on the apatitic precipitate. We concluded, therefore, that the calcium phosphate precipitation method would have a continuous effect in reducing dentin permeability in the oral environment.

Effects of Fluoride on the Calcium Phosphate Precipitation Method for Dentinal Tubule Occlusion

Use of the calcium phosphate precipitation (CPP) method makes possible the occlusion of dentinal tubules to approximately 10 to 15 pn from the dentinal surface, and thus shows good potential for the treatment of dentin hypersensitivity. The precipitate formed in the dentinal tubules by the CPP method is, however, not apatite [HAP; Ca10(PO4)6(OH)2], a component of tooth and bone, but dicaldum phosphate dihydrate (DCPD; CaHPO 4·2H2O). Since fluoride enhances the conversion of DCPD to HAP, we evaluated the effects of fluoride on the texture of the precipitate formed by the CPP method and on its capacity to occlude dentinal tubules in this in vitro study. CPP solution (1.0 mol/L CaHPO4·2H 2O dissolved in 2.0 mol/L H3PO4) was applied to a dentin disk and was subsequently neutralized with a post-treatment solution (1 mol/L NaOH, from 0 to 0.1 mol/L NaF). Scanning electron microscopy revealed that the precipitate occluded dentinal tubules to a depth of approximately 10 to 15 μm from the dentinal surface, regardless of the NaF concentration (from 0 to 0.1 mol/L) in the post-treatment solution. Also, dentin permeability was reduced to 15% by the CPP treatment regardless of the NaF concentration. The Ca/P molar ratio of the precipitate, measured by x-ray micro-analysis, was higher (1.25 ± 0.04) in the presence of NaF than in its absence (1.03 ± 0.01). For further identification of the precipitate formed in the dentinal tubules, the same procedure was used in glass tubes (diameter, 1 mm), so that a larger amount of precipitate would be obtained. Powder x-ray diffraction analysis and Fourier transform infrared (FT-IR) measurement revealed that the precipitate formed by the CPP method gradually changed from DCPD to apatitic as the NaF concentration increased. We concluded that the addition of NaF to the post-treatment solution with the CPP method was desirable, since it led to more apatitic precipitate formation and did not lessen the occluding capacity of the precipitate.

Occlusion of Dentinal Tubules with Calcium Phosphate Using Acidic Calcium Phosphate Solution Followed by Neutralization

The occlusion of dentinal tubules with calcium phosphate, by a calcium phosphate precipitation method (CPP method), was investigated in vitro for evaluation of the potential value of this method for the treatment of dentin hypersensitivity.The method consists of treating the dentinal surface with a CPP solution, i.e., an acidic solution that contains both calcium and phosphate, followed by neutralization with basic post-treatment solution. The CPP solutions used in this study ([Ca] = 0.2 - 1.0 mol/L, [PO 4] = 0.2 - 4.0 mol/L) were prepared by dissolving Ca(OH)2 or CaHPO 4·2H2O in H3PO4 or HCl, and 1 mol/L NaOH solution was used for the post-treatment solution. Sections of human dentin disks treated by the CPP method were observed by scanning electron microscopy, and the precipitate in the dentinal tubules was subjected to x-ray micro-analysis. After treatment by the CPP method, dentinal tubules were occluded to a distance of approximately 15 μm from the surface, and the precipitate showed a Ca/P molar ratio of 1.03 ± 0.01. To obtain a larger amount of precipitated mineral for further analysis, we used the same procedure with glass tubes (inside diameter, 1 mm). Powder x-ray diffraction analysis and FT-IR measurement revealed that the precipitate was dicalcium phosphate dihydrate (CaHPO4·2H2O). The instant precipitation of calcium phosphate mineral in the dentinal tubules demonstrated the potential value of the CPP method for the occlusion of dentinal tubules; this occlusion may be useful for the treatment of dentin hypersensitivity.

A Quantitative Comparison of Selected Bacteria in Human Carious Dentine by Microscopic Counts

The levels of selected cariogenic and obligately anaerobic bacteria have been compared in carious dentine taken from fissures, smooth surfaces, and root surfaces. The numbers of infected dentinal tubules were determined by immunohistological staining using species-specific antisera against selected bacteria. Selective localization was observed; mutans streptococci were the predominant bacteria in dentine from fissures and smooth surface coronal caries, but not from root surface caries. The proportion of mutans streptococci was higher in the shallow and middle layers of dentine from fissures and smooth surface coronal carious lesions than from the deep layers. In root surface caries, Actinomyces spp. were the predominant bacteria, and were present at higher levels in the deep layer of root lesions than in the shallow and middle layers. The proportion of Lactobacillus spp. was relatively low despite its high detection frequency in all layers of the three types of carious lesion. Immunohistological staining with species-specific antisera was able to reveal the distribution and the localization of various bacteria in carious dentine.