Chemo-mechanical characterization of carious dentine using Raman microscopy and Knoop microhardness

Abstract

One of the aims in the clinical operative management of dental carious lesions is to remove selectively the highly infected and structurally denatured dentine tissue, while retaining the deeper, repairable affected and intact, healthy tissues for long-term mechanical strength. The present study examined the correlation of chemical functional groups and the microhardness through the different depths of a carious lesion using Raman spectroscopy and Knoop microhardness testing. The null hypothesis investigated was that there was no correlation between Raman peak ratios (amide I : phosphate _ν1 ) and equivalent Knoop microhardness measurements. Ten freshly extracted human permanent teeth with carious dentine lesions were sectioned and examined using high-resolution Raman microscopy. The ratio of absorbency at the amide I and phosphate bands were calculated from 139 scan points through the depth of the lesions and correlated with 139 juxtaposed Knoop microhardness indentations. The results indicated a high correlation (p < 0.01) between the peak ratio and the equivalent Knoop hardness within carious dentine lesions. This study concluded that Raman spectroscopy can be used as a non-invasive analytical technology for in vitro studies to discriminate the hardness of carious dentine layers using the peak ratio as an alternative to the invasive, mechanical Knoop hardness test.

Keywords: Knoop microhardness; Raman spectroscopy; amide I; dentine caries; phosphate.

Figure 1.

A representative diagram for one of the examined samples shows: (A) enlarged cross section of the tooth with carious dentine after creating the fiducial marker 800 µm from the occlusal pit. (B) the eight examined points under Raman microscopy and Knoop hardness tester, (C) The sound dentine examined point under KHN and Raman microscopy, and a scatter plot with a regression line (R) of the hardness on the peak ratio (amide I : phosphate _v1).

Figure 2.

Representative Raman peaks chart for one of the examined samples after normalization by processing software (WiRe, Renishaw, UK), explaining the methodology of calculation the peak ratio (amide I : phosphate _ν1) in each scan point.

Figure 3.

A scatter plot and a regression line (R) of the microhardness (KHN) vs the peak ratios (amide I : phosphate _ν1) in the 20 carious dentine samples, illustrating the de novo prediction of histological status, including hardness of the tissue across the carious lesion. It shows the difference between reduced mineral (more amide 1), greater than 1 peak ratio, and more mineral (more phosphate _ν1), less than 1 peak ratio.