Biomechanical perspective on the remineralization of dentin

Abstract

The objective of this article is to critically evaluate the methods that are used to assess outcomes of remineralization of dentin. Currently, the most used assessment methods fall either into quantitative analysis of the mineral content of the remineralized structures or dry measurements of their mechanical properties. Properties obtained from the dehydrated organic dentin matrix may not reflect the true mechanical behavior of the remineralized tissue under physiological and hydrated conditions. Here we seek to clarify the biomechanical aspects of remineralization of dentin, pointing out the effects of hydration and dehydration on the mechanical properties of treated tissues. We also emphasize that a more appropriate endpoint to evaluate the effectiveness of remineralization in dentin should be associated with the recovery of the mechanical properties of the hydrated tissue, which is presumed to correlate well with its overall functionality.

Fig. 1

Structural hierarchical modeling of the dentin matrix. From left to right: SEM image of a fixed, demineralized dentin matrix showing the collagen fibrils surrounding dentinal tubules [Marshall et al., 1997]. In the schematic sequence, on the left, collagen fibrils show the extrafibrillar mineral between fibrils. In the middle, the arrangement of the collagen molecules yields 40-nm gap zones and 27-nm overlap zones resulting in the typical 67-nm periodicity of a collagen fibril. The length of the collagen protein triple helix is 300 nm. On the far right, the intrafibrillar mineral is represented sitting in the gap region between the collagen molecules. Noncollagenous proteins are not represented. Figure not drawn to scale. Modified from Rho et al. [1998].

Fig. 2

Schematic of the mechanical response of remineralized dentin. a Poor remineralization under hydration, suggesting the swelling phenomena of the organic matrix and poor interaction of mineral within collagen fibrils lead to low mechanical properties. b With drying, the collagen fibrils collapse, exerting compressive contacts constraining the unbound mineral, which results in a misleading interpretation of improved mechanical response. c When functionally remineralized, the collagen fibrils are reinforced by intrafibrillar mineral, and therefore the response under loading will be high (similar to normal dentin) even when hydrated.

Fig. 3

Nanomechanical properties of dentin as influenced by hydration and drying. De- and remineralized dentin present striking differences of reduced elastic modulus (a) and nanohardness (b) between the dry and wet tissues. Dentin was demineralized with acetate buffer at pH 5.0 for 8 h and remineralized with a solution containing 1.5 mM calcium and 0.9 mM phosphate for 2 days. Mechanical properties were evaluated using atomic-force-microscopy-based nanoindentation as described by Marshall et al. [2001].