The advantages of the self-etching system include complete infiltration of the bonding agent into the demineralized dentin and a reduced number of clinical procedural steps. The self-etching system is attractive in pediatric dentistry because it requires fewer steps and less time than does the total-etching system.

Several authors have reported the results of dentin bond strength tests in noncarious primary teeth.^5–9 Clinically, the restoration of primary teeth primarily involves dentin that is affected by caries. After removing the outer carious dentin or infected dentin, the clinician treats the remaining affected inner dentin.

The dentin of primary and permanent teeth is different in composition and structure. The concentration of calcium and phosphate in peritubular and intertubular dentin is lower in primary teeth than it is in permanent teeth.¹⁰ In addition, tubule density of dentin in primary teeth is lower than that of dentin in permanent teeth.^11,12

After the dentin becomes carious, its structure alters. In the transparent zone of primary and permanent teeth, dentinal tubules become occluded with mineral content and their hardness decreases compared with intact dentinal tubules.^13,14 Nakajima and colleagues¹ found that the tensile bond strength of adhesive bonded to caries-affected dentin of permanent teeth was lower than that of adhesive bonded to intact dentin. The hybrid layer of caries-affected dentin also varied more than did the hybrid layer of intact dentin.¹⁵

To date, the bond strength of adhesive systems to caries-affected dentin of primary teeth has not been reported. Our objectives, therefore, were to determine the microtensile bond strength of two adhesive systems applied to caries-affected dentin compared with intact dentin in primary teeth and to examine the micromorphological structure of the resin/dentin interface.

	MATERIALS AND METHODS

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Microtensile bond strength test. Forty proximal carious and 40 noncarious anterior primary teeth, which had been extracted because of prolonged retention, were stored in saline solution containing 0.2 percent sodium azide at 4 C. They were used within one month of being stored. Inclusion criteria were radiographic verification that the dentinal caries extended no farther than the middle one-third of the dentin thickness, and some parts of the root remained in all teeth. One of us (S.N.) removed the roots and fixed the coronal portions of the teeth on an acrylic bar to facilitate bonding procedures. Figure 1 is a schema of the specimen preparation.

View larger version (15K): [in this window] [in a new window]   Figure 1. Schema showing the specimen preparation for the microtensile bond strength test.

For the noncarious teeth, the researcher used radiographic examination to estimate the level of dentin used for bonding, which was approximately the middle one-third of the dentin. She used 400-grit silicon carbide paper on the circular polisher to prepare a flat dentin surface on the specimens. The teeth then were randomly divided into two groups according to the bonding system used. Table 1 shows the ingredients in the two dentin adhesives.^16,17

In the second group of 20 carious and 20 non-carious teeth, the researcher applied a total-etching adhesive (Single Bond, 3M ESPE, St. Paul, Minn.) according to the manufacturer’s instructions. She etched the tooth specimens for 15 seconds, washed them for 10 seconds and blot-dried them, applied two coats of adhesive, gently blow-dried them for two to five seconds and light-cured the specimens for 10 seconds.

After the bonding process, the researcher applied a hybrid resin-based composite (Z100 Restorative, 3M ESPE) in two increments (each approximately 2 millimeters thick). She then cured each increment for 40 seconds. All specimens then were stored in distilled water at 37 C for 24 hours.

Using a microtome (Accutom-50, Struers, Copenhagen, Denmark), the researcher sectioned the restored specimens into slices that were approximately 0.7 mm thick. She used a superfine diamond bur (Intensiv SA, Lugano-Grancia, Switzerland) to trim and shape the slices to form a curved section with a cross-sectional area of 1 mm². Using a microscope (Measurescope MM-11, Nikon, Tokyo), the researcher measured the remaining dentin thickness (RDT) of the specimens. She then attached the specimens to a Bencor-Multi-T testing machine apparatus (Danville Engineering, San Ramon, Calif.), which was placed on a universal testing machine (Instron 5566, Instron, Canton, Mass.) with a cyanoacrylate adhesive (Zapit, DVA, Corona, Calif.). The researcher set the apparatus to a crosshead speed of 1 mm per minute. Tension was applied to the specimens until they fractured.

After the tensile test, the researcher embedded the specimens in epoxy resin and polished them with a diamond paste of decreasing-sized particles and 0.05-micrometer-sized aluminum oxide particles. To confirm the hardness of the dentin interface, she determined the Knoop hardness of dentin, as described above.^1,15 Using a microhardness tester (FM-700e, Future-Tech, Tokyo) under a load of 50 grams and at a duration of 15 seconds, the researcher measured the dentin specimen at a depth of 50 µm below the bonded surface. The specimens were kept moist during the test. For each specimen, the researcher determined the Knoop hardness number as the mean of three measurements.

Bonding interfaces. We used 10 occlusal carious and 10 noncarious primary teeth to observe bonding interfaces. The researcher randomly divided the teeth into four groups of five teeth each. She prepared flat-surface dentin as described above. In the carious group, the researcher used caries-detecting dye to differentiate infected dentin from noninfected dentin. She removed infected dentin as part of the specimen preparation for the microtensile test.

The researcher bonded the resin-based composite (Z100) to the dentin using the self-etching dental adhesive or the total-etching dental adhesive according to the manufacturers’ instructions. The bonded specimens were longitudinally sectioned perpendicular to the bonded interface and embedded in epoxy resin. The specimens were metallurgically polished as described above. The researcher then subjected the polished specimens to an acid-based challenge by etching them with 10 percent phosphoric acid for five seconds, followed by immersion in 5 percent sodium hypochlorite for five minutes. They then were washed gently with copious amounts of distilled water, left to dry, gold sputter-coated and examined with a scanning electron microscope (JSM-5410LV, JEOL, Tokyo).

With the aid of an image digitizer (SemAfore SEM Image Grabber, version 1.2, JEOL, Sollentuna, Sweden), the researcher measured the hybrid layer thickness at five points equally distanced from each other along the interface of the specimens.

Statistical analysis. We used a t test to analyze the differences in microtensile bond strength between intact and caries-affected dentin within the same adhesive group. The RDT and micro-hardness were compared using analysis of variance and the least-significant difference test. We tested all data using statistical software (SPSS version 10, SPSS, Chicago) at the .05 level of significance.

	RESULTS

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Table 2 shows the mean microtensile bond strengths and the RDT and microhardness measurements. Microtensile bond strengths for the two adhesives bonded to caries-affected dentin were not significantly different from each other. However, when bonded to intact dentin, the total-etching adhesive exhibited lower bond strength than that of the self-etching adhesive. With regard to each bonding system, the bond strengths for the self-etching adhesive were statistically similar when it was bonded to intact dentin or caries-affected dentin (P = .84). However, the bond strength for the total-etching adhesive was statistically significantly lower when it was bonded to intact dentin than when bonded to caries-affected dentin (P = .001).

Figure 2 shows scanning electron micrographs (SEMs) of the resin/dentin interface. The hybrid layer thickness of the total-etching adhesive in intact and caries-affected dentin was approximately 2 to 3 µm. In the self-etching adhesive group, the thickness of the hybrid layers was about 1 µm in both intact and caries-affected dentin. The resin tags of the total-etching adhesive group in intact and caries-affected dentin had funnel shapes, while those of the self-etching adhesive group exhibited a cylindrical shape. In addition, the resin tags of the total-etching adhesive group had more lateral branches compared with those of the self-etching adhesive group.

View larger version (145K): [in this window] [in a new window]   Figure 2. Scanning electron micrographs of the resin/dentin interface of the polished cross-sectional dentin bonded with a total-etching adhesive (Single Bond, 3M ESPE, St. Paul, Minn.) (top) and a self-etching adhesive (Clearfil SE Bond, Kuraray, Osaka, Japan) (bottom). Left: intact dentin. Right: caries-affected dentin. R: Resin. Arrows: Hybrid layer.

	DISCUSSION

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In our study, we used proximal carious dentin of anterior primary teeth, which had been extracted owing to prolonged retention and had some part of the root remaining. For this reason, we should expect the dentin substrate to be similar to the dentin in teeth to be restored in a clinical setting.

RDT values. The results show that the mean RDT values for specimens in the four groups were not statistically different, indicating that all samples were bonded at approximately the same dentin level. In intact dentin, the microtensile bond strength of the self-etching adhesive was higher than that of the total-etching adhesive. A possible explanation may be the more complete resin infiltration of the self-etching adhesive⁴ and the higher intrinsic strength of the bonding agent resulting from the presence of fillers in the self-etching adhesive.¹⁹

Resin/dentin interface. SEMs also revealed the difference in resin/dentin interfaces between the two adhesive systems. We saw a thicker hybrid layer and funnel-shaped resin tags (resulting from the removal of peritubular dentin) in specimens bonded with the total-etching system, in which phosphoric acid is used. However, we observed a thinner hybrid layer and fewer lateral branches of resin tags, which were cylindrically shaped, in specimens bonded with the self-etching system, the result of a milder etching effect. These findings support the results of a study by Prati and colleagues,²⁰ who reported that there was no correlation between the hybrid layer thickness and bond strength.

Bond strengths. We found that bond strengths were significantly higher in caries-affected dentin than in intact dentin when using the total-etching adhesive, but were not significantly different when using the self-etching adhesive. These results differ from those of a study of permanent teeth, in which the bond strength was significantly lower for caries-affected dentin than for intact dentin when using the total-etching adhesive system. The authors of that study²¹ explained that the lower bond strengths were due to a thicker hybrid layer that was not completely infiltrated by the resin and fewer resin tags, resulting from the presence of acidic insoluble whitlockite crystals in dentinal tubules.

The SEM examination of the resin/dentin interface in our study, however, showed no apparent morphological difference in the thickness of the hybrid layer between intact and caries-affected dentin in primary teeth. Moreover, owing to the shorter period of demineralization and remineralization of dentin in primary teeth, the calcium crystal deposits in dentinal tubules of carious dentin in primary teeth seem to have been soluble by the phosphoric acid in the total-etching adhesive and by the acidic primer in the self-etching adhesive. Therefore, the resin tags found in caries-affected dentin were not different from those found in intact dentin.

From this morphological perspective, we might expect the bond strength to caries-affected dentin in primary teeth to be similar to that to intact dentin, because no differences were seen in the hybrid layer and resin tags. In our study, however, the bond strength to caries-affected dentin when using the total-etching adhesive was greater than that to intact dentin. The reason for this is not clear; however, it might be due to the effect of the polyalkenoic acid copolymer, which is a component of the total-etching adhesive.

Nakajima and colleagues²¹ reported that the bond strengths to caries-affected dentin in permanent teeth were lower when this polyalkenoic acid copolymer was removed from the bonding components. Van Meerbeek and colleagues²² found that the polyalkenoic acid copolymer reacted with calcium, forming Capolyalkenoic acid complex. However, it is difficult to explain why polyalkenoic acid copolymer reacts better with caries-affected dentin than with intact dentin in primary teeth.

The SEM examination of specimens in the group treated with the self-etching adhesive revealed no difference between intact and caries-affected dentin with regard to the morphology of the hybrid layer or the number and length of resin tags. This may indicate that the self-etching primer is capable of etching caries-affected dentin as well as it does intact dentin.

Hardness of dentin substrate. The results of this study show that the hardness of caries-affected dentin in primary teeth was lower than that of intact dentin. This is in agreement with the findings of Hosoya and colleagues,¹⁴ and it confirms that we worked on the caries-affected area of dentin. However, it is interesting to note that in this study, the bond strength to caries-affected dentin—which is a softer substrate than intact dentin—was higher than that to the harder substrate of intact dentin. These results may bring into question whether the hardness of the dentin substrate contributes to the interfacial bond strength, and they suggest that the physical properties of infiltrated polymerized resin, as well as those of collagen fiber, may play important roles in the bonding mechanism of the adhesive to caries-affected dentin.

The results of this study clearly indicate that even though the two adhesives achieved different results when bonded to intact primary dentin, they had an equally effective bond strength to caries-affected primary dentin. This suggests that both systems can be used effectively with caries-affected dentin. However, the self-etching system may be of particular interest because it involves no rinsing and is a less sensitive technique, which may make it more suitable for use with pediatric patients.

	CONCLUSION

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In this in vitro study, the self-etching dental adhesive resulted in similar bond strengths to intact and caries-affected dentin. However, the total-etching dental adhesive exhibited lower bond strengths to intact dentin than to caries-affected dentin.