Dentine is quite a variable tissue. Within the tooth itself the dentine approaching the dentino-enamel junction is more highly mineralized and the area occupied by the tubules is less than that of dentine adjacent to the pulp. In addition to this, dentine should be considered as a dynamic tissue that changes due to ageing, in response to caries and restoration placement. Most changes relate to occlusion of tubules and also an increase in the mineralization of the dentine. The implication of this is that the dentine becomes slightly more difficult to etch and exposure of collagen fibrils can also be reduced, hence there is a potential for the bond to be somewhat tenuous. This is particularly the case for the highly sclerosed dentine of non-carious cervical lesions (NCCLs). Laboratory studies indicate that the hybrid layer of the dentine surface of NCCLs is thinner than that of normal dentine [29, 63]. In addition, it seems that some bonding systems do not adhere as well to this surface and show a slightly decreased bond strength [25, 35, 79].
A considerable amount of work has also been done looking at the variation of the bond to caries-affected dentine. Some of the early studies used artificial caries like lesions. However, this does not reproduce the situation that occurs in the oral cavity since caries is a process of demineralization and remineralization associated with the damage of the supporting collagen matrix [48, 53]. The increased thickness of the hybrid layer is mainly because the dentine is already partially demineralized from the caries and the action of the acid etch is therefore somewhat greater. This provides a clear basis for not etching for longer than that recommended by the manufacturer. In addition, the water content of caries-affected dentine is believed to be greater than normal dentine. This too will also have an effect on the ability of the resins to penetrate to the full depth of the demineralized dentine. In the case of caries-affected dentine treated with chemo-mechanical caries removal solutions, there appear to be no adverse effects on the bond with a DBA .
However, the bond to radicular and pulp chamber dentine does seem to vary quite a lot depending on the DBA used. This perhaps provides a strong case for being careful with the selection of a DBA for these regions of the tooth. It is believed that it may be necessary to use different DBAs for different regions of the tooth, or a system needs to be selected where it has been shown to provide a reliable bond to all parts of the tooth. Another alternative is the use of GIC restorative materials when then is a deep cavity on the radicular surface of a tooth, as it is known that a reliable bond can be achieved and moisture control is not such a problem [25, 35, 79].
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