Effect of Acid Etching on Dentin Micromechanical Properties

In composite restorations, the dentin substrate is often etched prior to adhesive application. Etching demineralizes the dentin, exposing the collagen into which the adhesive is expected to infiltrate thus, producing a hybrid layer. The etching process however does not produce a sharp demineralized zone. Instead it produces a top demineralized layer which transitions to the native dentin through a partially demineralized zone.

Objective: To investigate the effect of etching on dentin micromechanical properties using scanning acoustic microscopy (SAM).

Methods: Scanning acoustic microscopy (SAM) has gained recognition as one of the few methods that can be used for non-destructive characterization of the micro-scale elastic properties of materials by measurement of the reflected signal. An unerupted 3^rd molar was collected under an IRB approved protocol. Following sectioning, the mid-coronal dentin region was imaged with SAM before and after etching using two different transducers with 30 and 50 MHz central frequencies. For additional quantitative comparisons, A-scan waveforms received by each transducer, were collected from the same location and were Fourier transformed.

Results: C-scan images were analyzed by comparing the average and standard deviations of grayscale values. The mean C-scan grayscale value was 50 (30MHz) and 28 (50MHz) before etching compared to 39 (30MHz) and 13 (50MHz) post-etching indicating a softened surficial zone. Fourier spectra comparison shows that the peak frequency shifts and the higher frequencies reflect less than the lower frequencies. There are three possible explanations for this effect (1) etching induced surface roughness, (2) viscoelasticity of the demineralized material, and (3) graded surficial zone.

Conclusion: SAM measurements indicate that etching results in a complex surficial zone that combines effects of roughness and graded demineralization. The micromechanical properties of the zones within the etched-dentin influence substantially stress distribution and failure at the adhesive/dentin interface.

NIH/NIDCR DE014392, S10 RR16710, R13 DK069504