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Background and objective: Femtosecond laser (fs-laser) is a novel tooth preparation tool. After cavities were prepared by fs-laser, their axial-wall taper is closely related to the retention force and success of restorations, while the taper after preparation by fs-laser is not clear yet. This study aimed to explore the value of the taper after fs-laser tooth preparation and the effect of laser fluence, scanning line spacing, and scanning layers on taper. Materials and methods: Twenty cavities with a width of 0.5mm were prepared by an fs-laser both in enamel and dentin to obtain the axial-wall taper (No. PKUSSIRB-201949124). To study the effects of three parameters of fs-laser, five different laser fluence were set as 1.56, 3.13, 4.69, 6.25, and 7.81 J/cm² of five cavities. Scanning line spacing was set as 6, 12, 25, 30, 40, and 50 lm of six cavities and the scanning layers were 5, 10, 25, 50, 75, 100, 200, 300, and 400, respectively, of nine cavities. Laser pulses irradiated perpendicular to the tooth and the scanning path was controlled via a vibratory mirror. The taper was measured with a laser confocal microscope. Curves of the taper versus different parameters were drawn. Results: There was a significant difference in taper between enamel and dentin. Overall, the taper decreased gradually with the increase of laser fluence. The optimal taper obtained was 20.53°- 0.28° in dentin and 38.71°- 0.46° in enamel when the corresponding fluence was 6.25 J/cm². In comparison, the scanning line spacing has little effect on the taper. The scanning layers significantly affect the taper, which decreased dramatically as scanning layers increased within 75 layers and does not change considerably as it was beyond 100 layers. Conclusions: Reasonable selection of laser fluence, scanning line spacing, and the number of scanning layers can optimize the cavity taper by fs-laser. [ABSTRACT FROM AUTHOR]

Objective: Herein, a feasible strategy based on a femtosecond laser (fs-laser) was provided to decelerate dentin wear, and the effect of wear resistance improvement and its potential mechanism were explored. Background: Tooth wear is a common phenomenon that exists throughout life. While once dentin is exposed, it wears away very quickly. Decelerating tooth wear, especially dentin wear, is an important issue, but there is no ideal treatment. Materials and methods: Sixteen third molar dentin samples were randomly divided into the amalgam (N1 = 8) and polymer infiltrated ceramic network (PICN) (N2 = 8) groups. One half of the sample was used as the experimental object and the other for the blank self-control. Array microcavities were fabricated on the experimental parts of all the samples with an fs-laser and then filled with the corresponding materials. The experimental and control parts of all the samples were subjected to 3600 cycles of sliding wear tests with titanium balls. The coefficients of friction (COFs) of every friction pair and the temperature rise were recorded in real time. The wear volume and depth were measured by a laser confocal microscopy. Statistical differences of wear volume and depth between the experimental and self-control parts were calculated by paired t-test. The wear mode was observed with scanning electron microscopy (SEM). Results: The COFs in the two experimental groups were significantly decreased. The maximum temperature rise on the dentin surface was 2.0C. The wear resistance in the amalgam and PICN groups was 4.48 and 3.53 times higher than blank dentin, respectively ( p < 0.001). The SEM images showed fewer plough grooves and cracks in dentin after microtexturing. Conclusions: This method could significantly improve tooth wear resistance by reducing the COFs between the friction pairs and reducing the dentin’s ploughing effect and provide new treatment ideas for slowing the loss of severely worn dentin. [ABSTRACT FROM AUTHOR]

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 3Yzirconia 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. [ABSTRACT FROM AUTHOR]