research | primer into the fluid-filled dentinal tubules. These resin tags are generally under-cured, soft, and flexible. In ad- dition, the interface is prone to nano-leakage because of gap formation between tags and dentine due to incom- plete penetration of adhesive.3, 7 In the 1990s, erbium lasers were introduced for prepara- tion of hard tissue as an alternative to rotary instruments. Er,Cr:YSGG laser (emitting at a wavelength of 2.79 µm) is an effective tool for removal of dental hard tissues.14, 15 This wavelength is absorbed by the hydroxyapatite and water. The hydroxyl radicals and water in hydroxyapatite crystals receive most of the laser energy. By water evapo- ration in the tooth mineral components, a large volumetric expansion occurs.1, 2 Next, micro-explosions occur that remove the hard tissue from the irradiated regions.16 It has minimal side effects on the sound tooth structure.1 Den- tine conditioning with laser has advantages. As reported in some studies, the laser settings can be adjusted to physi- cally etch the dentine surface. Power, frequency, and other parameters can be adjusted to prevent smear layer for- mation on the dentine surface. Laser does not cause den- tine demineralisation. It does not have the risk of over-etch- ing or over-/under-drying. The erbium laser-treated dentine is dehydrated prior to priming and bonding; thus, the resin tags are more likely to be long and strong.1, 17, 18 Of studies on the effect of different laser parameters on dentine morphology, no study investigated the effect of various durations of Er,Cr:YSGG laser irradiation on den- tine surface morphology. Dentine irradiated with Er,Cr:YSGG laser shows a microscopically rough surface without de- mineralisation,19, 20 open dentinal tubules,21–23 no smear layer, and satisfactory sterilisation of the cavity.24 These characteristics are considered as an advantage of laser preparation if composite resins are to be applied as the filling materials.25 The Er,Cr:YSGG laser setting used in this study included 4.5 W average power, 1,500 W peak power, 0.09 J energy per pulse, 50 Hz frequency, 8.57 J/cm2 energy density, 80 % water and 60 % air, pulse duration of 60 µs, and distance of 2 mm above the surface. The energy density used in our study was not within the ablation range. Only dentine surface was etched and conditioned for the bond- ing process. Five, 10, and 20 s of laser irradiation caused different amounts of smear layer. The applied Er,Cr:YSGG laser setting with 40 s of duration caused a scaly-like appearance on the surface with less homogenous and less regular surface creating a micro-retentive pattern on dentine without heat injury or melting, which is favour- able for bonding process. The dentine surface showed no smear layer; dentinal tubules were open; and the sub- surface was not demineralised. Open tubules and ab- sence of smear layer are additional factors that enhance bonding to laser-treated dentine.14 This can be explained by micro-explosions at the tissue surface, resulting from the sudden boiling of water within the tissue (thermo- mechanical ablation).26 The results obtained from this study can be used in further studies to evaluate the com- posite bond strength with different bonding systems. Conclusion Forty seconds of laser irradiation with the aforemen- tioned parameters eliminated the smear layer from the dentine surface, and the obtained surface had micro- retentive pattern on dentine and open tubules without heat injury or melting and demineralisation which was suitable morphology for bond to composite resin. Laser irradiation for less than 40 s could not completely remove the smear layer from the surface. Each one of these sur- faces could have optimum bonding with composite by applying different adhesives systems which should be investigated in further studies. Editorial note: This article was originally published by Springer International in Lasers in Dental Science (Mahdi- siar, F., Mirzaei, A., Fallah, A. et al. Laser Dent Sci [2018] 2:213. https://doi.org/10.1007/s41547-018-0038-z). It is reprinted here (with editing changes) with permission. Also, the authors declare that they have no conflict of interest. In addition, this article does not contain any studies with human participants or animals performed by any of the authors. This article was done on extracted human third molars, and it does not include any human participant. For this type of study, formal consent is not required. about Iran-based Dr Alireza Mirzaei is an internationally published dentist. He is Assistant Professor at the Bone and Joint Reconstruction Research Center of the Shafa Orthopedic Hospital of the Iran University of Medical Sciences in Tehran in Iran. contact Dr Alireza Mirzaei Hamadan Medical Science University Daneshgah-e-Bu Ali Sina, Hamadan, Iran Phone: +98 81 32520182 alirezamirzaee56@gmail.com roots 4 2019 25