Effect of biomimetic remineralization on the dynamic nanomechanical properties of dentin hybrid layers

Abstract

The mineral and organic phases of mineralized dentin contribute co-operatively to its strength and toughness. This study tested the null hypothesis that there is no difference in nano-dynamic mechanical behavior (complex modulus-E*; loss modulus-E''; storage modulus-E'; in GPa) of dentin hybrid layers (baseline: E*, 3.86 ± 0.24; E'', 0.23 ± 0.05; E', 3.85 ± 0.24) created by an etch-and-rinse adhesive in the presence or absence of biomimetic remineralization after in vitro aging. Using scanning probe microscopy and nano-dynamic mechanical analysis, we demonstrated that biomimetic remineralization restored the nano-dynamic mechanical behavior of heavily remineralized, resin-sparse regions of dentin hybrid layers (E*, 19.73 ± 3.85; E'', 8.75 ± 3.97; E', 16.02 ± 2.58) to those of the mineralized dentin base (E*, 19.20 ± 2.42; E'', 6.57 ± 1.96; E', 17.39 ± 2.0) [p > 0.05]. Conversely, those resin-sparse, water-rich regions degraded in the absence of biomimetic remineralization, with significant decline [p < 0.05] in their complex and storage moduli (E*, 0.83 ± 0.35; E'', 0.88 ± 0.24; E', 0.62 ± 0.32). Intrafibrillar apatite deposition preserves the integrity of resin-sparse regions of hybrid layers by restoring their nanomechanical properties to those exhibited by mineralized dentin.

Figure 1.

Dynamic material behavior of a representative hybrid layer (between arrowheads) in the Baseline group after the resin-bonded specimen was immersed in simulated body fluid for 7 days to maximize water sorption, without remineralization. The specimen was scanned in the hydrated condition. (A) Property map of the complex modulus as determined by nano-DMA. (B) Corresponding property map of the loss modulus. (C) Corresponding property map of the storage modulus. (D) Unstained TEM image of a specimen similar to the one examined by nano-DMA. R, adhesive resin; T, dentinal tubules; M, mineralized dentin base.

Figure 2.

The effect of biomimetic remineralization on the dynamic material behavior of a representative remineralized hybrid layer (between arrowheads) from the Experimental group after in vitro aging in simulated body fluid. The specimen was scanned in the hydrated condition. (A) Property map of the complex modulus as determined by nano-DMA. (B) Corresponding property map of the loss modulus. (C) Corresponding property map of the storage modulus. (D) Unstained TEM image of a specimen similar to the one examined by nano-DMA. R, adhesive resin; T, dentinal tubules; M, mineralized dentin base; s, sparsely mineralized areas corresponding to resin-rich regions of the hybrid layer; h, heavily mineralized areas corresponding to resin-sparse regions of the hybrid layer.

Figure 3.

Dynamic material behavior and ultrastructure of a representative hybrid layer (between arrowheads) from the Control group that was subjected to in vitro aging in simulated body fluid in the absence of biomimetic remineralization. The specimen was scanned in the hydrated condition. Asterisk: Areas with degraded collagen within the hybrid layer. R, adhesive resin; T, dentinal tubules; M, mineralized tissue base. (A) Property map of the complex modulus as determined by nano-DMA. (B) Corresponding property map of the loss modulus. (C) Corresponding property map of the storage modulus. (D) Uranyl-acetate- and lead-citrate-stained TEM image of a specimen similar to the one examined by nano-DMA, showing basal regions within the hybrid layer that are devoid of collagen (asterisk). DM: laboratory-demineralized dentin.

Figure 4.

Box plots of the dynamic mechanical properties obtained by nano-DMA from different regions of the baseline specimens, control specimens, and remineralized specimens. (A) Complex modulus (E*). (B) Loss modulus (E′′). (C) Storage modulus (E′). Each box plot represents the minimum, 25^th percentile, median, 75^th percentile, and maximum of 40 values (10 specimens x 4 scans). Boxes with the same letters are not statistically significant (p > 0.05). Group designations: (1) mineralized intertubular dentin (Baseline group), (2) hybrid layers from baseline water-sorbed specimens that had been stored in simulated body fluid for 7 days (Baseline group), (3) non-degraded regions of hybrid layers from the Control group, (4) degraded regions of hybrid layers from the Control group, (5) sparsely remineralized regions of hybrid layers in specimens from the Experimental group that were subjected to biomimetic remineralization, and (6) heavily remineralized regions of hybrid layers in specimens from the Experimental group that were subjected to biomimetic remineralization.