ORIGINAL ARTICLE In vitro CO 2 9.3-μm short-pulsed laser caries prevention—effects of a newly developed laser irradiation pattern Peter Rechmann 1 & C. Q. Le 1 & R. Kinsel 1 & C. Kerbage 2 & B. M. T. Rechmann 1 Received: 14 June 2019 /Accepted: 22 December 2019 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020 Abstract Caries prevention with different lasers has been investigated in laboratory studies and clinical pilot trials. Objective of this in vitro study was to assess whether 9.3-μm microsecond short-pulsed CO 2 laser irradiation enhances enamel caries resistance without melting, with and without additional fluoride application. Seven groups of enamel, totaling 105 human enamel samples, were irradiated with 2 different carbon dioxide lasers with 2 different energy application systems (original versus spread beam; 9.3 μm wavelength, pulse repetition rate 43 Hz vs 100 Hz, fluence ranges from 1.4 to 3.9 J/cm 2 , pulse duration 3 μs to 18 μs). The laboratory pH-cycling was performed with or without additional fluoride, followed by cross-sectional microhardness testing. To assess caries inhibition, the mean relative mineral loss delta Z (ΔZ) was determined. To evaluate for melting, scanning electron microscopy (SEM) examinations were performed. For the non-laser control groups with additional fluoride use, the relative mineral loss (ΔZ, vol% × μm) ranged between 512 ± 292 and 809 ± 297 (mean ± SD). ΔZ for the laser-irradiated samples with fluoride use ranged between 186 ± 214 and 374 ± 191, averaging a 58% ± 6% mineral loss reduction (ANOVA, P < 0.01 to P < 0.0001). For the non-laser-treated controls without additional fluoride, the mineral loss increased (ΔZ 914 ± 422 to 1224 ± 736). In contrast, the ΔZ for the laser-treated groups without additional fluoride ranged between 463 ± 190 and 594 ± 272 (P < 0.01 to P < 0.001) indicative of 50% ± 2% average reduction in mineral loss. Enhanced caries resistance was achieved by all applied fluences. Using the spread beam resulted in enhanced resistance without enamel melting as seen by SEM. CO 2 9.3-μm short-pulsed laser irradiation with both laser beam configurations resulted in highly significant reduction in enamel mineral loss. Modifying the beam to a more homogenous profile will allow enamel caries resistance even without apparent enamel melting. Keywords CO 2 9.3-μm laser . Microsecond short-pulsed . Galvo element . Improving caries resistance . Enamel melting . Laboratory study . pH-cycling . Fluoride . Cross-sectional microhardness testing Introduction In the early 1970s, laboratory laser research using the conven- tional 10.6-μm laser wavelength produced greater caries re- sistance of tooth enamel after using CO 2 laser irradiation [1–10]. Applying substantially lower energies than reported in most of the early studies, Featherstone and co-workers described in vitro enhancement of enamel caries resistance by short-pulsed CO 2 laser irradiation under particular irradia- tion conditions [9–11]. Dental enamel absorbs the CO 2 laser wavelengths 9.3 and 9.6 μm ten times stronger than the con- ventional 10.6-μm CO 2 laser wavelength [12]. Furthermore, 9.3- and 9.6-μm CO 2 lasers are capable of emitting short pulses in the microsecond (μs) range, while conventional 10.6-μm CO 2 laser releases millisecond pulses or emit contin- uous wave light with potential harmful side effects such as thermal transfer to the dental pulp [13]. In 2003, Rechmann and co-workers [14] showed improved caries resistance around orthodontic brackets using short- pulsed CO 2 9.6-μm laser irradiation [15]. These favorable and significant results were revealed by cross-sectional micro- hardness testing. Consequently, Rechmann et al. performed a controlled, randomized, single blind clinical pilot trial, irradi- ating fissures of second molars with the short-pulsed 9.6-μm * Peter Rechmann Peter.Rechmann@ucsf.edu 1 Department of Preventive and Restorative Dental Sciences, School of Dentistry, University of California at San Francisco, 707 Parnassus Avenue, San Francisco, CA 94143, USA 2 Convergent Dental, 140 Kendrick Street, Bldg C3, Needham, MA 02494, USA Lasers in Medical Science https://doi.org/10.1007/s10103-019-02940-z