Dentin is the tissue underlying the enamel that forms the bulk of the tooth. The dentin matrix is formed by about 45–50 vol % mineral in the form of a carbonated hydroxyapatite; 30–35 vol % of organic matter, mostly as type I collagen with associated noncollagenous proteins; with the remainder being oral fluid (Marshall et al., 1997
). In dental caries, oral bacteria metabolize carbohydrates, generating acids that diffuse into enamel and further into dentin, dissolving mineral, and eventually destroying the structural matrix. Apatite in dentin has a much smaller crystallite size, higher carbonate content and is more susceptible to acidic dissolution than enamel apatite. Hence, once the carious process enters dentin, the demineralization rate is accelerated. Moreover, the high organic content in dentin makes its remineralization a much more complex process than remineralization of enamel. Thus, a better understanding of the steps leading to remineralization of dentin is desirable.
It has been well documented that in dentin the apatite occurs in two specific regions, within the fibrils (intrafibrillar mineral) and between fibrils (extrafibrillar mineral) (Katz and Li, 1973
; Katz et al., 1989
; Landis, 1996
). Primarily the intrafibrillar mineral (Kinney et al., 2003
), has been suggested to be crucial for the normal mechanical properties of the tissue. Therefore, a critical aspect in treating carious dentin is not only to replace the lost mineral, but principally to provide the tight association of the re-grown mineral with the demineralized matrix (Bertassoni et al., 2009
), thus enabling the recovery of the mechanical properties of the tissue.
Various in vitro studies have evaluated different saliva formulations (Hara et al., 2008
), the effect of ozone and sodium hypochlorite (Zaura et al., 2007
), CPP-ACP (Rahiotis and Vougiouklakis, 2007
), nano-β-tricalcium phosphate (Shibata et al., 2008
), bioactive glass (Vollenweider et al., 2007
) and continuous mineral formation with the constant solution composition approach (Koutsoukos and Nancollas, 1981
; Tomson et al., 1977
); all reporting the reincorporation of mineral within the demineralized dentin matrix. However, to date, evidence of a successful remineralization has been based mostly upon evaluations of how much mineral the tissue has regained, without evaluation of mineral matrix binding and recovery of the mechanical properties of the tissue, particularly in near physiologic conditions of hydration.
Recent investigations suggest that recovery of mechanical properties of the hydrated dentin matrix provides insight into mineral matrix binding and occurs when there is an effective re-association of mineral with the organic matrix, a result termed functional remineralization (Bertassoni et al., 2009
). On the other hand, little mechanical recovery occurs when mineral content is recovered, but the mineral is poorly attached to the organic matrix. Accordingly, this occurs because the unbound mineral, even in high concentration, floats in water while the collagen becomes highly deformable under load, thus yielding very low properties. However, if the mineral is bound and reinforces the collagen structure then, even in water, the properties are similar to normal dentin.
The objective of this investigation was to define the conditions that facilitate an improved association of mineral with the demineralized dentin matrix, thus allowing for an improved surface mechanical recovery. The hypothesis tested was that surface remineralization is facilitated by controlled and continuous mineral growth