Surface Processing: An Elegant Way to Enhance the Femtosecond Laser Ablation Rate and Ablation Efficiency on Human Teeth

Optical penetration models for practical prediction of femtosecond laser ablation of dental hard tissue

OBJECTIVES

To develop and practically test high-precision femtosecond laser ablation models for dental hard tissue that are useful for detailed planning of automated laser dental restorative treatment.

METHODS

Analytical models are proposed, derived, and demonstrated for practical calculation of ablation rates, ablation efficiency and ablated morphology of human dental enamel and dentin using femtosecond lasers. The models assume an effective optical attenuation coefficient for the irradiated material. To achieve ablation, it is necessary for the local energy density of the attenuated pulse in the hard tissue to surpass a predefined threshold that signifies the minimum energy density required for material ionization. A 1029 nm, 40 W carbide 275 fs laser was used to ablate sliced adult human teeth and generate the data necessary for testing the models. The volume of material removed, and the shape of the ablated channel were measured using optical profilometry.

RESULTS

The models fit with the measured ablation efficiency curve against laser fluence for both enamel and dentin, correctly capturing the fluence for optimum ablation and the volume of ablated material per pulse. The detailed shapes of a 400-micrometer wide channel and a single-pulse width channel are accurately predicted using the superposition of the analytical result for a single pulse.

CONCLUSIONS

The findings have value for planning automated dental restorative treatment using femtosecond lasers. The measurements and analysis give estimates of the optical properties of enamel and dentin irradiated with an infrared femtosecond laser at above-threshold fluence and the proposed models give insight into the physics of femtosecond laser processing of dental hard tissue.

Effect of Optical Wedge Rotary on Ablation Efficiency of Femtosecond Laser on Dental Hard Tissue and Restorative Materials

Objective: Femtosecond laser (fs-laser) is a novel tooth preparation tool but its ablation efficiency is insufficient. The purpose is to establish a new fs-laser tooth ablation method based on a dual-wedges path ablation system, and explore the efficiency of tooth hard tissue and dental restorative materials ablation. Materials and methods: Extracted third molars, pure titanium, cobalt-chromium alloy, gold alloy, and 3Y-zirconia were prepared into samples. These samples were rotary ablated by an fs-laser with dual-wedges. The wavelength was 1030 nm and the pulse duration was 250 fsec. Laser parameters were set as a repetition frequency of 25 kHz, the power percentages as 50% for dental tissues, and 60% for restorative materials. The optical wedge angle was set as 0°, 20°, 40°, 60°, and 80° for restorative materials, 0°, 20°, 30°, 40°, and 60° for enamel, and 0°, 10°, 20°, 30°, and 40°for dentin. Three times of ablation was processed at each parameter to obtain total 90 ablation microcavities of 6 kinds of materials. The diameter, depth, and volume of microcavities were measured by confocal laser microscopy and plotted against optical-wedge-angle in curves of different materials. One-way analysis of variance (ANOVA) was used to test whether the ablation efficiency between different angles was statistically significant. Results: The ablation efficiency of each material at different optical-wedge-angle was statistically significant (p < 0.05) and tends to be correlated. For dental hard tissue, the enamel ablation efficiency was 208.1 times and dentin ablation efficiency were 65.2 times than before when the wedge angle was 40°. For pure titanium, zirconia, cobalt-chromium, and gold alloys, the ablation efficiencies were 3.1, 10.7, 81.5, and 128.8 times than before when the rotation angle was 80°. Conclusions: The ablation efficiency of dental hard tissues and restorative materials was significantly increased with the increase of laser oblique incidence angle. Clinical Trial Registration number: PKUSSIRB-201949124.

A Novel Method for Adjusting the Taper and Adaption of Automatic Tooth Preparations with a High-Power Femtosecond Laser

This study explored the effect of the light-off delay setting in a robotically controlled femtosecond laser on the taper and adaption of resin tooth preparations. Thirty resin teeth (divided into six equal groups) were studied under different light-off delay conditions. Tapers from six vertical sections of the teeth were measured and compared among the light-off delay groups. The mean taper decreased from 39.268° ± 4.530° to 25.393° ± 5.496° as the light-off delay increased (p < 0.05). The average distance between the occlusal surfaces of the scanned data and the predesigned preparation data decreased from 0.089 ± 0.005 to 0.013 ± 0.030 μm as the light-off delay increased (p < 0.05). The light-off delay of the femtosecond laser is correlated with the taper and adaption of automatic tooth preparations; this setting needs to be considered during automatic tooth preparation.

Physiochemical characteristics: A robust tool to overcome teeth heterogeneity on predicting laser ablation profile

To avoid excessive tissue removal and collateral damage, the high-power density laser is apt for dental surgery also need to have high precision. For high-precision dental surgery with minimal tissue damage, the present work frames a method to predict laser ablation profile based on surface morphology and chemical composition of dentin. The surface morphology and chemical composition were studied on different dentin samples using scanning electron microscope (SEM) and Energy Dispersive X-ray Analysis (EDAX), respectively. The key laser ablation parameters (ω₀ , D_eff , and F_th ) were determined by laser irradiation study using 800 nm, Ti:Sapphire femtosecond laser at processing condition of 100 fs, 10 kHz and 10 mm/s. The dentin samples show a strong linear correlation between physiochemical characteristics and laser ablation parameters. The surface morphology exhibits a negative linear correlation with threshold fluence, whereas the converse is true for chemical composition. The laser ablation parameters of a random dentin sample are derived from the knowledge of linearity data. From the obtained laser ablation parameters, the complete theoretical ablation profile is constructed and validated with experimental ablation profile. Even though the surface morphology of dentin shows high linearity, the concentration of Ca and P can be used as the most feasible probe in clinical settings. Furthermore, the laser ablation rate and ablation efficiency are predicted by the method to optimize the laser processing condition for any specific teeth. The versatility of the method overcomes the problem of heterogeneity on various teeth and simplifies the method of finding optimal laser processing condition for immaculate laser surgery.

Characteristics of Bubble Oscillations During Laser-Activated Irrigation of Root Canals and Method of Improvement

BACKGROUND AND OBJECTIVES

Laser-activated irrigation of dental root canals is being increasingly used as its efficacy has been shown to be superior compared with conventional techniques. The method is based on laser-initiated localized fluid evaporation and subsequent rapid bubble expansions and collapses, inducing microfluid flow throughout the entire volume of the cavity. The irrigation efficacy can be further improved if optimally delayed "SWEEPS" double laser pulses are delivered into the canal. This study aims to show that the irrigation efficacy, as measured by the induced pressure within the canal, is related to the double pulse delay, with the maximal pressure generated at an optimal delay. The second aim is to find a method of determining the optimal delay for different cavity dimensions and/or laser parameters.

STUDY DESIGN/MATERIALS AND METHODS

Experiments were made in transparent models of root canals where Er:YAG laser (λ = 2.94 μm, pulse duration t_p  = 25 or 50 microseconds, and pulse energies up to E_L  = 40 mJ) was used with a combination of cylindrical and conical fiber-tip geometries (diameters 400 and 600 µm). High-speed photography (60,000 fps) and average pressure measurements inside the canal were used for process characterization.

RESULTS

The results show that a pressure amplification of more than 1.5 times occurs if the laser pulse delay approximately coincides with the bubble oscillation time. Correlations between normalized oscillation time and canal diameter for a wide range of laser pulse energies (R²  = 0.96) and between the average pressure within the canal and the bubble oscillation periods (R²  = 0.90) were found. A relationship between the bubble oscillation time and the diameter of the treated cavity was found depending on the bubble oscillation time in an infinite fluid reservoir.

CONCLUSIONS

The bubble oscillation time within a constrained volume can be determined based on the known oscillation time in infinite space, which offers a fast and simple solution for optimization of the laser parameters. These findings enable determination of optimal conditions for shock wave generation, and improvement of root canal irrigation at the same dose of laser energy input, leading to improved treatment efficacy and safety. Lasers Surg. Med. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals, Inc.