High-speed Scanning Ablation of Dental Tissues with a 9.3 micron CO2 laser

By: Daniel Nguyen, Michal Staninec, Chulsung Lee, and Daniel Fried, University of
California, San Francisco

Overview

The purpose of the study is to determine whether a 9.3-μm mechanically scanned CO₂ laser operated at high laser pulse repetition rates can safely ablate enamel and dentin without excessive heat accumulation and peripheral thermal damage. To this end, three tests performed on samples derived from non-carious extracted molars were conducted to determine the heat accumulation, adhesive bond strength, and four-point bend measurements.

Background

Several studies have demonstrated that CO₂ lasers operating λ=9.3 and 9.6-μm wavelengths, which are strongly absorbed by hydroxyapatite in dental hard tissues, are ideally suited for the efficient ablation of dental caries and for surface treatments to increase the resistance to acid dissolution. The primary concern when operating lasers at high pulse repetition rates is increased potential for peripheral thermal damage due to heat accumulation from multiple laser pulses delivered in rapid succession. This heat accumulation can be offset by rapid scanning the laser beam over the area of ablation and by use of a water spray. The effect of peripheral thermal damage on adhesion has been studied with a wide range of results on a variety of lasers and post-ablation surface treatments. In a previous study, it was observed that high bond strengths were attainable by using a 9.3-μm CO₂ lasers (1).

Testing Methods Heat Accumulation

For this study, two groups were studied, containing 12 and 16 samples per group. Pulpal temperatures were recorded using micro thermocouples situated at the pulp chamber roof, which were occlusally ablated using a rapid-scanning, water-cooled 9.3 µm CO₂ laser over a two minute time period. The laser was operated at a pulse repetition rate of 300 Hz, and two single pulse energy levels were used, 14 mJ for the first group and 22 mJ for the second.

Shear-bond Test

The adhesive strength of dental composite to laser-treated enamel was determined via a single plane shear-bond test with samples being divided into 3 groups: ablated/non-etched (n=10), ablated/acid-etched samples (n=8) and control samples (n=9) prepared only by 320 grit wet sanding. Bonding resin was applied to all the blocks in two coats, dried, and cured for 10 seconds prior to bonding with composite. The modified single plane shear test assembly (SPSTA) followed the procedure used by Sheth et al. and Watanabe et al. Two aligning plates were used to connect the SPSTA to an Instron testing machine, which recorded measurements in kilograms with the crosshead speed set to 5 mm/ min. When the two plates separated, the force level was recorded.

Four-point Bend Measurements

Beams (1 × 1 × 9 mm) of dentin were used in the dentin mechanical strength study. Two groups were studied, with 10 samples per group. The mechanical strength of facially ablated dentin (n=10) was determined via the four-point bend test and compared to control samples (n=10) prepared with 320 grit wet sandpaper to simulate conventional preparations.

Results

It was found that laser-ablated surfaces were smooth and highly uniform. No visible discoloration or charring indicative of thermal damage was observed on either the enamel or dentin surfaces under both macroscopic and microscopic inspection.

The maximum temperature recorded during ablation remained below the ambient temperature of 21°C for all samples in both groups. Heat accumulation measurements indicated that the mean temperature after laser ablation of tooth samples with a pulse energy of 14 mJ (Group TC20) was 17.6 ± 0.9°C (n=16), with a mean change in temperature of 2.0 ± 0.6°C. With a laser pulse energy of 22 mJ (Group TC30), the mean temperature after ablation was 19 ± 0.9°C (n=12), with a mean change in temperature of 3.2 ± 0.8°C.

The bond strengths achieved for both enamel and dentin after laser irradiation and acid etching were very high, near 30 MPa. The shear-bond testing yielded mean bond strengths of 31.2 ± 2.5 MPa (n=8) for the ablated/ acid-etched samples (Group LESB), 5.2 ± 2.4 MPa (n=10) for ablated/non-etched samples (Group LSB) and 37.0 ± 3.6 MPa (n=9) for the control (Group CSB). ANOVA with Tukey post-test indicated that there was a significant difference between all the groups (P < 0.05).

The four-point bend tests yielded that bending strength was not significantly different for the laser irradiated samples which indicate that the minor thermal effects from the laser does not compromise the mechanical strength.

In conclusion these results suggest that dental hard tissues can be rapidly ablated with a mechanically scanned 9.3 µm CO₂ laser at high pulse repetition rates without excessive heat accumulation in the tooth or peripheral thermal damage that produce no significant reduction in the tissue’s mechanical strength or a large reduction of adhesive strength to restorative materials.

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1. Hedayatollahnajafi S, Staninec S, Watanabe L, Lee C, Fried D. Dentin bond strength after ablation using a CO₂ laser operating at high pulse repetition rates. Lasers in Dentistry VX. 2009; Vol. 7162:F1–F7.