Demineralized dentin matrix

Root caries can occur when tooth roots are exposed to the oral environment, for example after periodontal surgery or gingival recession. Two stages are distinguished microscopically. First, the dentin mineral is dissolved and bacteria penetrate the tubules. Second, the demineralized dentin matrix is degraded, and bacteria infiltrate the intertubular area (Frank et al., 1989 Frank, 1990 Schiipbach et al., 1989). This sequence of events may indicate that the degradation of the dentin matrix occurs after it has become accessible by the removal of mineral. In an in vitro study, Klont and Ten Cate (1991) confirmed that the dentin matrix cannot be degraded unless it is demineralized. 

Modifications of the organic matrix other than degradation have been studied for their effect on dentin de- or remineralization. Glutardialdehyde cross-linking of matrix in dentin lesions inhibits progressive demineralization (Boonstra et al., 1993). Removal of soluble phosphoproteins promotes calcification of demineralized dentin (Clarkson et al., 1991). 

From the studies summarized above, it is clear that the breakdown of the dentin matrix plays an important role in the pathology of dentin- and root surface caries. In addition, the demineralized dentin can be modified by a number of reactions, with consequences for its degradability. The studies described in this thesis were designed to address the role of degradation and modification of the dentin collagen. 

In both types of lesion, the peritubular matrix appeared to be more resistant to proteolytic activity than the intertubular matrix, which may be due to a compositional difference between the matrices. In this respect, it is interesting to note that Takagi et al. (1990) found the calcification of dentin to be accompanied by the degradation of proteoglycans throughout the intertubular matrix, but not in the peritubular matrix. However, there is no evidence that there is less degradation of the peritubular than the intertubular matrix, when demineralized dentin specimens are exposed to the oral environment (Van Strijp et al., 1992). This discrepancy can be explained by the assumption that a wide variety of enzymes participates in the degradation of the dentin matrix in vivo. 

In the course of dentin caries, both demineralization and reactions with the organic matrix take place. Matrix reactions include proteolysis and covalent modifications. From the introduction (Chapter 2) and the review on discoloration in caries (Chapter 3), it becomes clear that there are still few reports on the effect of matrix modifications on dentin caries. In Chapters 2, 4, and 5, the investigations were aimed at filling the information gap concerning the effect of reactions of dentin matrix on caries. To this end, degradation and modification of dentin were studied in demineralized specimens in vitro. In addition, specimens placed in dentures in situ and caries lesions in extracted teeth were analysed for modifications. 

In Chapter 2, the proteolysis of demineralized organic matrix of bovine dentin promoted demineralization of both erosive and subsurface lesions in vitro, especially in advanced lesions. In contrast to previous investigations, the organic matrix was destroyed enzymatically, not chemically. 

The relationship between the degradation of organic matrix and dentin lesion formation has been studied both in vitro and in situ. Several authors employed matrix destruction to assess the role of the matrix in de-and remineralization. For example, Apostolopoulos and Buonocore (1966) reported facilitated demineralization of dentin at pFl<5.5 after treatment with ethylene diamine. Inaba and coworkers (1996) found that removal of matrix from dentin lesions by hypochlorite promotes remineralization, consistent with a larger crystal surface available for mineral deposition after ashing (McCann and Fath, 1958). Flypochlorite-mediat-ed destruction also increases the permeability of mineralized dentin (Barbosa et ah, 1994). 

THE INFLUENCE OF THE ORGANIC MATRIX ON DEMINERALIZATION OF BOVINE ROOT DENTIN... 

Effect of matrix degradation on the rate of demineralization of incipient erosive lesions in root dentin. Specimens were subjected to daily alternating incubations with HAc pH 5.0 (V,Y) or pH 5.5 ( , ) and either collagenase (TM) or buffer (V,ni (n=5). Values represent mean SD. Levels of significance were calculated according to Student s t test. P<0,05, P<0,01 vs. buffer-treatment. 

In lesions not treated with collagenase, the matrix appeared to be unaffected, since no differences were observed between the matrix in the lesions and the underlying dentin, when the latter had been demineralized during the preparation of fhe specimens for microscopy. 

In a previous investigation, Klont and Ten Cate (1991) showed that dentin must be demineralized before its matrix can be degraded by proteases. In the present study, it was demonstrated that proteolytic degradation of the demineralized matrix enhanced the susceptibility of dentin lesions to acid-dependent demineralization. 

Lesion in bovine dentin with tubules protruding from degraded intertubular matrix (left degraded matrix right intact matrix). Demineralization in 0.1 M acetic acid pH 4.0, with subsequent exposure to bacterial collagenase. Fixed and demineralized with glutar-dialdehyde-acetic acid, post-fixed with osmium tetroxide ultrathin sections stained with uranyl acetate - lead citrate. 

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