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Precise ablation of dental hard tissues with ultra-short pulsed lasers. Preliminary exploratory investigation on adequate laser parameters

31/08/2016  |  Tags: dental laser manufacturer, ,

Abstract

This study aimed to evaluate the possibility of introducing ultra-short pulsed lasers (USPL) in restorative dentistry by maintaining the well-known benefits of lasers for caries removal, but also overcoming disadvantages, such as thermal damage of irradiated substrate. USPL ablation of dental hard tissues was investigated in two phases. Phase 1—different wavelengths (355, 532, 1,045, and 1,064 nm), pulse durations (picoseconds and femtoseconds) and irradiation parameters (scanning speed, output power, and pulse repetition rate) were assessed for enamel and dentin. Ablation rate was determined, and the temperature increase measured in real time. Phase 2—the most favorable laser parameters were evaluated to correlate temperature increase to ablation rate and ablation efficiency. The influence of cooling methods (air, air–water spray) on ablation process was further analyzed. All parameters tested provided precise and selective tissue ablation. For all lasers, faster scanning speeds resulted in better interaction and reduced temperature increase. The most adequate results were observed for the 1064-nm ps-laser and the 1045-nm fs-laser. Forced cooling caused moderate changes in temperature increase, but reduced ablation, being considered unnecessary during irradiation with USPL. For dentin, the correlation between temperature increase and ablation efficiency was satisfactory for both pulse durations, while for enamel, the best correlation was observed for fs-laser, independently of the power used. USPL may be suitable for cavity preparation in dentin and enamel, since effective ablation and low temperature increase were observed. If adequate laser parameters are selected, this technique seems to be promising for promoting the laser-assisted, minimally invasive approach.

Introduction

Modern dentistry and its minimally invasive concepts are supported by the development of innovative materials and advanced techniques [1]. The conservative intervention targets reliable sealing of the prepared cavity and preservation of healthy dental structures and was only possible due to the development of adhesive materials [2].

In this context, laser ablation of hard tissue for cavity preparation has attracted several investigators, since it is considered to be safe, reduce pain, and provide more comfortable treatment for patients by significantly decreasing noise and vibrations in comparison with traditional burs [3–8]. The adequate interaction of lasers with dental hard tissue may result in efficient and safe removal of compromised structure, and this condition is only achieved if close attention is paid to the correct choice of laser settings. As opposite to this, if wrong parameters are chosen, side-effects like thermal and mechanical tissue damage may be the consequence. Therefore, selecting the most adequate irradiation conditions is an extremely important step in laser research.

Recently, the influence of pulse duration on the laser ablation process has become a matter for research, following the development of high-tech laser devices that allow the selection of pulse durations in the range of microseconds (1 μs = 1 × 10−6 s), nanoseconds (1 ns = 1 × 10−9 s) and, lately, picoseconds (1 ps = 1 × 10−12 s) and femtoseconds (1 fs = 1 × 10−15 s), the so-called ultra-short pulsed lasers (USPL) [9–13]. The available wavelengths provided by these sources range from the ultraviolet to the infrared region of the electromagnetic spectrum.

High-power pulses with duration shorter than the tissue thermal relaxation time are indicated to avoid thermal denaturation of organic components of the tissue adjacent to irradiated surfaces [14,15]. Longer pulse durations are thought to be responsible for inducing thermomechanical stress and damage to tissue, since thermal energy accumulates and penetrates deeply [16]. Studies on the influence of shorter pulse durations in the ablation process have indicated that not only the energy for the ablation threshold decreases [17], but improvement of the ablation process is also observed due to a complex, non-thermal ablation process that causes the rapid ionization of irradiated tissue. It has been observed that, in this case, reduced residual thermal damage is induced because of the minimization of heat diffusion [16,18].

There are few studies in the literature that assess the use of USPL for the ablation of dental hard tissues. The variety of laser parameters tested lead to different and contradicting outcomes as regards the presence of tissue carbonization, damage to surrounding tissues, and selective laser interaction [11,19–22]. There is still much to be elucidated for this technology to become a clinical reality, starting with the establishment of adequate laser parameters for ablation of dental hard tissues. Based on the above, it has become important to assess the suitability of USPL in Dentistry by evaluating fundamental aspects of laser interaction with dental substrates, such as temperature increase and capacity for cavity preparation.

Materials and methods

With the intention of establishing adequate parameters for cavity preparation in dental hard tissues, the experiments were divided into two phases. In phase 1, different wavelengths and irradiation parameters were evaluated, so that the best results regarding low temperature increase and efficient interaction with dental substrates could be identified. The parameters that showed more suitable results for clinical application were selected and further evaluated to verify the correlation between ablation efficiency and temperature increase. An additional experiment was also conducted to determine the most suitable cooling method for use during irradiation.

Phase 1—Screening

Twenty-five freshly extracted human third molars were sectioned perpendicularly to their long axis, using a diamond bandsaw (E 300, EXAKT GmbH, Norderstedt, Germany), so that the occlusal portion was discarded and slices of approximately 2 mm thickness were obtained. The slices were polished with SiC #800 grinding paper (Struers A/S, Ballerup, Denmark), utilizing a grinding system (CS401, EXAKT GmbH) with water irrigation, until a thickness of 1 mm was achieved. The slices consisted of a round inner portion of dentin surrounded by an external enamel layer of at least 1 mm width. They were kept in distilled water throughout the experiment. For this study, there was no fixed number of samples per group, and cavities were prepared in the samples on demand.

Dental ablation with the picosecond laser at 1,064 nm

The effect of USPL was initially observed by irradiating enamel and dentin with a high-power Nd:YVO4 (neodymium:vanadate) laser in the picosecond (ps) regime at 1,064 nm wavelength (Super Rapid, Lumera Laser GmbH, Kaiserslautern, Germany). The laser source delivered an average power of up to 18 W at a pulse repetition rate of up to 2 MHz. The pulse duration was approximately 10 ps. Beam deflection was performed by a two-axis galvanometer scanner (hurrySCAN14, Scanlab, Puchheim, Germany) at a scanning speed of up to 2,000 mm/s. A motorized z-stage (Kugler Microstep, Kugler GmbH, Salem, Germany) was used to position the samples in focus. The beam was focused by means of an f = 120 mm scan lens (Sill Optics, Wendelstein, Germany), and the resulting focal diameter was 22 μm.

Dental ablation with the picosecond laser at 532 nm

The same Nd:YVO4 laser source was used to generate a 532 nm wavelength beam (visible green radiation) by second harmonic generation (SHG). Under these conditions, the laser source was able to provide an average power of up to 7 W, a surface scanning speed of up to 2,000 mm/s, and a pulse repetition rate of up to 2 MHz. The f  = 100 mm scan lens (Sill Optics, Wendelstein, Germany) was designed for a wavelength of 532 nm, and the resulting spot was 20 μm in diameter.

Dental ablation with the picosecond laser at 355 nm

The laser source was additionally equipped with a second crystal for third harmonic generation (THG) and therefore produced a beam of 355 nm wavelength. dental laser tips,The average power was up to 4 W at 355 nm, the scanning speed up to 2,000 mm/s, and the pulse repetition rate up to 2 MHz. In combination with a 355 nm, the f = 100 mm scan lens (Sill Optics, Wendelstein, Germany) resulted in a focal beam diameter of 20 μm.

Dental ablation with the femtosecond laser at 1,045 nm

For irradiation in the femtosecond (fs) regime, a fiber laser emitting at 1,045 nm wavelength with a bandwidth of 10 nm (FCPA μJewel, IMRA America Inc., Ann Arbor, MI, USA) was used. The laser beam was directed into an optical scanner and focused to a spot of 8.8 μm in diameter by a 5×/0.15 microscope objective (Plan-Neofluar, Carl Zeiss AG, Oberkochen, Germany). This system determined an irradiation field of 5 mm diameter, a scanning speed of up to 400 mm/s, and a transmitted average power of up to 0.3 W at 100 kHz repetition rate. The pulse duration was approximately 500 fs.

Sample irradiation

The samples were positioned on the z-stage and the laser beam focus was individually adjusted for each sample. The laser scanner was configured to irradiate a square with 1 mm sides line by line (interlaced scanning) for both ps- and fs-lasers. The distance between two pulses in the X direction was set to 8 μm, while the distance in the Y direction varied with scanning speed and repetition rate. The target area in square form was irradiated, and one complete surface scan (1 × 1 mm) was considered one repetition. The scan repetitions varied for different samples to evaluate the influence of scan repetitions on cavity formation, since a single repetition would not be able to produce a cavity, but only surface conditioning. Several scan repetitions were evaluated, and when more than 50 repetitions were tested, irradiation consisted in 25 surface scans (repetitions), followed by a 20-s interval, and then by another 25 surface scans. The excess of distilled water was removed from the sample surface with absorbent paper, so that irradiation could be conducted on a moist substrate. No cooling system was used during irradiation in this phase.

The cavities were ablated on both dentin and enamel with different laser parameters, as presented in Table 1. Ablation rate and temperature increase were recorded for each cavity. Cavity topography and profile were also analyzed.

Results

The evaluation of cavities produced with the ultraviolet 355 nm picosecond laser showed that enamel and dentin present different ablation rates and temperature increases for the same laser parameters. For fluence below 5 J/cm² (irradiance of 5 × 1011 W/cm2) and considering the same scanning speed, the ablation rate for dentin is higher than for enamel (Fig. 2a, b). A lower scanning speed caused a higher ablation rate but was also associated with tissue carbonization. Scanning speed of 200 mm/s and maximum fluence of 4.8 J/cm2 (irradiance of 4.8 × 1011 W/cm2) result in an exaggerated temperature increase of up to 80 °C for dentin and 100 °C for enamel, while for a higher scanning speed of 2,000 mm/s, a lower temperature increase was observed (up to 25 °C) for both substrates. For enamel, the ablation process is more intense in the area close to dentin, resulting in a deeper cavity and slight carbonization in this area. For dentin, the same is observed in areas far away from enamel. Both enamel and dentin presented well-defined cavities with sharp cavo-surface edges (Fig. 2c, d).