In Dr. John H. Purk and colleagues’ February JADA article, "In Vivo Versus in Vitro Microtensile Bond Strength of Axial Versus Gingival Cavity Preparation Walls in Class II Resin-Based Composite Restorations," an ingenious technique was used that allowed comparison of 125 microtensile testing of specimens bonded in vivo and in vitro from the axial and gingival walls of restorations. The results showed that the bonding in vivo was much weaker than that in vitro, and that the bonding to the gingival walls was weaker than to the axial walls of cavity preparations.
The discussion of the results focuses on several aspects affecting the wetness of the dentin, which, undoubtedly, will affect the results. Little attention is paid to the difference in structure of the dentin in the two locations, although the authors indicate that differences in the density of the tubules may be part of the explanation, especially because it affects the wetness of the dentin in vivo.
What about the difference in the branching of the tubules? Penetration and polymerization of the resin into the branches of the tubules clearly will affect the bond strength between the dentin and the resin, and this branching differs in the two locations studied. A relatively detailed description of the branching of dentinal tubules in human teeth can be found in the Archives of Oral Biology.1
It is quite possible that the differences in the bonding values between axial and gingival walls may be due to differences in the structure of the dentin in the two locations. I disagree with the authors when they suggest that less intertubular dentin at the gingival floor than on the axial wall may explain the reduced bond strength.
First of all, the dentin at the gingival floor is more superficial than that at the axial wall, as judged from Figure 1 in the article. Therefore, it has relatively more intertubular matrix than the dentin in the axial wall samples.
Second, since peripheral dentin has relatively few tubules, adhesive penetration into the tubules and their branches are less prominent than in axial dentin, and that may explain the reduced bond strength.
The authors refer to an unpublished scanning electron microscopy, or SEM, study of the dentin substrate (J. Purk, D.D.S., Ph.D., unpublished data, 2001). This approach may confirm that my suggestion is correct, provided the adhesive penetrates and polymerizes within the dentin tubules and their branches. I believe this explanation is more feasible than the difference in tubule direction, which has been shown to affect the bond strength at the dentin-resin interface, as suggested by the authors.
The characterization of the structure of dentin sample is an essential part of studies of the adhesive strength between resin and dentin. The authors must be complimented on adding the SEM component to the study. This approach should be a prerequisite for any study of the adhesive strengths between resin and dentin, because dentin is distinctly different in various locations within the tooth.
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