Mechanism of dentin hardness measurements using a dentin hardness measuring device with a light-emitting diode.

Significance: Managing caries is imperative in a rapidly aging society. Current diagnoses use qualitative indices. However, a quantitative evaluation of hardness in a clinical setting may lead to more accurate diagnoses. Previously, hardness meter using indenter with light for tooth monitoring (HAMILTOM) was developed to quantitatively measure tooth hardness. Herein, the physical interpretation of dentin hardness measured using HAMILTOM and the dentin hardness measurement mechanism are discussed. Aim: This study evaluates the mechanism of dentin hardness measurements using HAMILTOM physically and compare the invasiveness to dentin by HAMILTOM with those using a dental probe for palpation. Approach: Eleven bovine dentin samples were used to create caries models. HAMILTOM measured the dark areas, and its indentations were observed using scanning electron microscopy. Also, its invasiveness was evaluated by comparing the results with those from dental probe palpation. Results: The indentation areas were smaller than the dark areas in HAMILTOM, which may be due to exuded water from the dentin sample and the elastic recovery of dentin sample. Additionally, the dental probe indentation was deeper than the HAMILTOM indentations. Conclusions: The results demonstrate that the indentation areas were smaller than the dark areas measured by HAMILTOM, which might contain the influence of exuded water and the deformation of dentin sample. Also, HAMILTOM is less invasive than dental probe palpation. In the future, HAMILTOM may become a standard hardness measuring method to diagnose root caries.

Demonstration of an optical dentin hardness measuring device using bovine dentin with different demineralization times.

Significance: The increase in root caries is a serious problem as society ages. Root caries is diagnosed by inspection and palpation, which are qualitative. A method to objectively and quantitatively evaluate the progress of root caries in a clinical setting is strongly desired. The root caries could be diagnosed by measuring hardness because dentin becomes softer as the caries progresses. Vickers hardness has been customarily used as an indicator of tooth hardness. However, this method cannot be used to in vivo teeth because the teeth must be dried prior to measurement to make the indentation. A hardness meter using an indenter with light for tooth monitoring (HAMILTOM) is proposed as an optical device. HAMILTOM could measure hardness of teeth in wet condition as a dark area while applying a load to dentins without drying. Therefore, HAMILTOM may realize hardness measurements of in vivo teeth in a clinical setting quantitatively. Aim: The aim of our study is to demonstrate the optical dentin hardness measuring device HAMILTOM using bovine dentin with different demineralization times and to evaluate the correlation between the dark areas measured by HAMILTOM and the Vickers hardness measured by the Vickers hardness tester. Approach: The samples were 20 bovine dentins. They were demineralized by a lactic acid solution with different times and divided into groups 1 and 2 of 10 samples each. In both groups, the dark areas and Vickers hardness were measured for each sample. Group 1 was used to obtain a calibration curve to calculate Vickers hardness from the dark area. Group 2 was used to validate the calibration curve obtained from the dentin samples of group 1. Results: The areas appearing black without a total internal reflection of the indenter measured by HAMILTOM increased as the demineralization time increased. Additionally, the Vickers hardness of group 2 calculated by the dark areas of group 2 and the calibration curve obtained in group 1 and the Vickers hardness of group 2 measured by the Vickers hardness tester were strongly correlated with a determination coefficient of 0.99. Conclusions: The results demonstrate that HAMILTOM may be a suitable alternative to the conventional method. Unlike the conventional method, which cannot be used for in vivo teeth, HAMILTOM holds potential to quantitatively evaluate the progress of caries in in vivo teeth.