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Degradation of the Hybrid Zone

January 7th, 2006 · No Comments

Degradation of the hybrid zone

     Ever since Hashimoto in 2000 demonstrated that the hybrid zone was subject to deterioration, a number of additional papers have confirmed this concern. There are a number of potential causes for this unwelcomed problem. The first is related to an incomplete diffusion of the bonding agent into the evacuated spaces. Over time, the exposed and unprotected collagenous fibers undergo organic decomposition.

     Another cause may be related to residual acid from the dentin-bonding agent itself. Finally, it is possible that once the hydroxyl apatite has been removed, the resin-bonding agent will not effectively protect the surfaces collagenous fibers. The relationship between the surface of the collagenous fiber and the resin component of the dentin-bonding agent is strictly physical; not chemical. Regardless of the etiology, there is concern that the once promised condition of long-term clinical survival may not be a guarantee.

     Perhaps the greatest cause of potential degradation of the collagenous structure may be related to the clinician’s skill in placing the dentin-bonding agent. It is imperative that the directions provided by the manufacturer be followed very carefully. Failure to do so will often lead to an inadequate diffusion of the bonding resin into the evacuated spaces surrounding the collagenous structure.

     Clinical symptoms of  biologic degradation include delayed postoperative sensitivity, debonding and eventual secondary caries. Some of the more important citations related to this problem are presented below.

 

1.  Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. In vivo degradation of resin-dentin bonds in humans over 1 to 3 years. J Dent Res. 2000 Jun;79(6):1385-91.


Moreover, under fractographic analysis, the proportion of demineralize dentin at the fractured surface in specimens aged in an oral environment was greater than that in control specimens. Furthermore, degradation of resin composite and the depletion of collagen fibrils was observed among the specimens aged in an oral environment. Analysis of the results of this study indicated that the degradation of resin-dentin bond structures occurs after aging in the oral cavity.

2.  Okuda M, Pereira PN, Nakajima M, Tagami J, Pashley DH. Long-term durability of resin dentin interface: nanoleakage vs. microtensile bond strength. Oper Dent. 2002 May-Jun;27(3):289-96.

The authors speculate that hydrolytic degradation within the hybrid layer gradually increased due to water penetration through nanoleakage channels, resulting in lower bond strengths and interfacial failure after as little as nine months.

3.  Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. Resin-tooth adhesive interfaces after long-term function. Am J Dent. 2001 Aug;14(4):211-5


Morphological changes, such as digestion of the collagen fibrils and deterioration of the bonding resin and hybrid layer, were observed at the interface of the oral environment specimens. The results of this study demonstrated that degradation of resin-dentin bond structures appeared to occur in the human oral environment over time.

4.  Okuda M, Pereira PN, Nakajima M, Tagami J. Relationship between nanoleakage and long-term durability of dentin bonds. Oper Dent. 2001 Sep-Oct;26(5):482-90.

Nanoleakage occurred in both adhesive systems, and bond strengths gradually decreased over time. However, there was no correlation between bond strength and nanoleakage for either adhesive system in this study.

5.  Burrow MF, Satoh M, Tagami J. Dentin bond durability after three years using a dentin bonding agent with and without priming. Dent Mater. 1996 Sep;12(5):302-7.

It was concluded that priming may only be useful to achieve strong bonding in the short term. These results may have significant implications related to clinical longevity of restorations.

6. Wang Y, Spencer P.  Continuing Etching of an All-in-One Adhesive in Wet Dentin Tubules. J Dent Res. 2005 Apr;84(4):350-4

Increased dentin demineralization and loss of adhesive integrity were noted after aqueous storage. The degree of monomer conversion at the interface was consistently greater than conversion within the tubules. Fluid within the tubules may inhibit monomer conversion. The acidic characteristics of this adhesive may be retained and, thus, continue to affect/demineralize the surrounding dentin.

Karl Leinfelder

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