Fundamental Interactions of Lasers with Dental Hard Tissues

By: John D.B. Featherstone and Daniel Fried, Department of Preventative &
Restorative Dental Sciences, UCSF

Introduction

The authors’ studies have demonstrated that treatment of enamel with a carbon dioxide laser can markedly inhibit subsequent caries progression. During irradiation, heat causes carbonate loss from the carbonated hydroxyapatite mineral, converting it into a low solubility hydroxyapatite-like calcium phosphate. The studies show it is possible to produce a laser that can selectively remove carious tissue and protect against future caries within the ablated walls. The purpose of the paper is to review fundamental aspects of hard tissue interactions with particular emphasis on the prevention of progression of dental decay.

Background

Numerous investigators have successfully shown that lasers effectively absorb components of hard tissue

[1, 2]. Laser interactions fall into three major categories, namely, 1) interaction with the mineral, 2) interaction with the protein or 3) interaction with the water. If detection of early decay is of interest, the laser wavelength must be such that the light will scatter in the carious region or have altered fluorescence properties that can be detected by instruments. If caries removal is of interest, the wavelength must be such that there is a major interaction with the components of the decay to lead to its ablation. In case of caries prevention, the laser interaction will most likely need to change the mineral from its acid soluble form to a much less soluble form.

Testing Methods

The paper reviews extensive studies conducted in the laboratory in regards to laser interactions such as absorption coefficients, temperature studies, caries prevention, and scattering/reflection.

Intra-oral Studies Caries Inhibition

Featherstone and coworkers [3] have published a study in which enamel blocks were irradiated in the laboratory and then placed in the mouths of human volunteers for 4 weeks simulating exposure to natural sources of erosion and decay in the mouth. The aim of the study is to use an intraoral model to determine whether caries inhibition due to laser irradiation, similar to our laboratory studies are observed in the human mouth. Samples were assessed by microradiography to compare the mineral loss before and after treatment and derive a net change in mineral value. This study was designed only to show the effect of the laser treatment on inhibition of demineralization and not the potential combined effect of laser and fluoride treatment. Laboratory studies have indicated an additive effect of the two treatment modes on caries inhibition and/or remineralization [4].

Results

Extensive laboratory work has led to the choice of a range of laser conditions that can be used to treat enamel to make it resistant to dissolution by acids in the dental caries process. With a careful choice of laser parameters that are based upon a fundamental knowledge of laser/hard tissue interactions, it is possible to selectively remove carious tissue and protect the preparation from further caries progression. These studies have demonstrated that treatment of enamel by carbon dioxide laser (9.3–9.6 μm) irradiation can markedly inhibit subsequent caries progression.

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