Method to control depth error when ablating human dentin with numerically controlled picosecond laser: a preliminary study.

A three-axis numerically controlled picosecond laser was used to ablate dentin to investigate the quantitative relationships among the number of additive pulse layers in two-dimensional scans starting from the focal plane, step size along the normal of the focal plane (focal plane normal), and ablation depth error. A method to control the ablation depth error, suitable to control stepping along the focal plane normal, was preliminarily established. Twenty-four freshly removed mandibular first molars were cut transversely along the long axis of the crown and prepared as 48 tooth sample slices with approximately flat surfaces. Forty-two slices were used in the first section. The picosecond laser was 1,064 nm in wavelength, 3 W in power, and 10 kHz in repetition frequency. For a varying number (n = 5-70) of focal plane additive pulse layers (14 groups, three repetitions each), two-dimensional scanning and ablation were performed on the dentin regions of the tooth sample slices, which were fixed on the focal plane. The ablation depth, d, was measured, and the quantitative function between n and d was established. Six slices were used in the second section. The function was used to calculate and set the timing of stepwise increments, and the single-step size along the focal plane normal was d micrometer after ablation of n layers (n = 5-50; 10 groups, six repetitions each). Each sample underwent three-dimensional scanning and ablation to produce 2 × 2-mm square cavities. The difference, e, between the measured cavity depth and theoretical value was calculated, along with the difference, e 1, between the measured average ablation depth of a single-step along the focal plane normal and theoretical value. Values of n and d corresponding to the minimum values of e and e 1, respectively, were obtained. In two-dimensional ablation, d was largest (720.61 μm) when n = 65 and smallest when n = 5 (45.00 μm). Linear regression yielded the quantitative relationship: d = 10.547 × n - 7.5465 (R (2) = 0.9796). During three-dimensional ablation, e 1 was the smallest (0.02 μm) when n = 5 and d = 45.00 μm. The depth error was 1.91 μm when 450.00-μm depth cavities were produced. When ablating dentin with a three-axis picosecond laser scan-ablation device (450 μm, 3 W, 10 kHz), the number of focal plane additive pulse layers and step size along the focal plane normal was positively correlated with the single-layer and total ablation depth errors. By adjusting the timing of stepwise increments along the focal plane normal and single-step size when ablating dentin by using the numerically controlled picosecond laser, the single-step ablation depth error could be controlled at the micrometer level.