Weakening of dentin from cracks resulting from laser irradiation

Cracking of tooth structure is a frequent mechanism of clinical failure necessitating treatment. Some laser conditions, particularly those without sufficient water cooling, may cause surface cracking of dentin. Surface cracks may serve as initiation sites for the onset of catastrophic fracture under mechanical stress, resulting in failure of the dentin. In this study, the hypothesis that laser initiated cracks result in lower bending strength of dentin was tested. Dentin beam specimens were prepared from human molar teeth, 1.1mm×1.1mm×∼9mm, and divided into groups C (control), W (wet), D (dry) of 12 beams each. In groups W and D, the middle of each beam on one surface (buccal) was irradiated with either a Er-YAG or Q-switched Er-YSGG laser and measured under a microscope, noting the dimensions in the irradiated area and immediately adjacent to irradiated area. Each beam was placed in a mechanical testing machine in a four-point bend jig and tested with a monotonically increasing load at a displacement rate of 1mm/min until failure. The bending strengths for groups C, W (Er-YAG laser) and D (Q-switched Er-YSGG laser) were, respectively, 141.6, 114.0, and 90.9MPa. A one-way ANOVA determined a significant difference between groups C and D, p <0.001. Conclusion: The Q-switched Er-YSGG laser without water caused cracks in the surface that significantly decreased the bending strength of dentin.