Objectives: The objective of this study was to evaluate the potential effects on underlying dental hard tissues of a high pulse rate carbon dioxide (CO2) laser that was designed for soft tissue surgery.Methods: Eighteen extracted human teeth were sectioned longitudinally, cleaned, and varnished, leaving nine exposed windows on each: six on the coronal surface (enamel) and three on the root surface (cementum, dentin). The CO2 irradiation conditions used were: wave length 10.6μm; 1.2–2.6J/cm2 fluence per pulse; repetition rate 120–1000Hz; 100–200ms pulse duration; and cumulative fluences ranging from 14 to 2200J/cm2. Each window was irradiated with a 0.3mm beam diameter at one of nine power settings for 0.1, 0.5, or 1.0s. The pulp chamber temperature was measured with a microthermocouple. The irradiated teeth were evaluated by Polarized Light Microscopy (PLM) and Scanning Electron Microscopy (SEM).Results: The pulp chamber temperature rise ranged from 0.5 to 19° C depending on the location of the window and distance to pulp chamber. SEM revealed crystal fusion in both enamel and dentin at all cumulative fluences. At cumulative fluences of 40J/cm2, 200 pulses/second and higher, measurable tissue loss was observed with PLM both in dentin and enamel.Conclusions: These results indicate there are threshold conditions above which pulsed CO2 laser light used for soft tissue surgery may cause detrimental changes to underlying oral hard tissue and to the pulp.

Time-resolved infrared (IR) radiometry was used to measure surface temperatures during pulsed Er:YSGG (l=2.79 mm) and Er:YAG (l=2.94 mm) laser irradiation of dental enamel. Scanning electron microscopy (SEM) was used to determine the melting and vaporization thresholds and to characterize other changes in the surface morphology. The magnitude and temporal evolution of the surface temperature during multiplepulse irradiation of the tissue was dependent on the wavelength, fluence, and pre-exposure to laser pulses. Radiometry and SEM micrographs indicate that ablation is initiated at temperatures well below the melting and vaporization temperatures of the carbonated hydroxyapatite mineral component (1200 °C). Ablation occurred at lower surface temperatures and at lower fluences for Er:YAG than for Er:YSGG laser irradiation: 400 °C vs. 800 °C and above 7 J/cm2 vs. 18 J/cm2, respectively. However, the measured surface temperatures were higher at l=2.79 mm than at l=2.94 mm during low fluence irradiation (<7 J/cm2). Spatially dependent absorption in the enamel matrix is proposed to explain this apparent contradiction.

Several studies in our laboratories have demonstrated that CO2 laser treatment of dental enamel can inhibit subsequent caries-like progression from 10 - 85% compared to controls. The roles of several of the laser parameters in the observed inhibition are still unknown. The aim of the present study was to examine the roles of pulse duration and repetition rate by the use of in vitro caries experiments, single and multiple pulse temperature measurements, and SEM observations, each following CO2 laser irradiation of dental enamel. We used pulse durations of 100 and 500 microsecond(s) at wavelengths of 9.3, 9.6, 10.3 and 10.6 micrometers , fluences of 2.5 or 5 J/cm2 per pulse, and 25 pulses per treatment window on human enamel. To study repetition rate we used 26 pulses, wavelength of 9.3 micrometers , at 1, 10, 25 or 40 Hz, with a fluence of 2.5 or 3.5 J/cm2 per pulse. A complex relationship among pulse duration, wavelength and fluence was demonstrated. Increasing the repetition rate improved the inhibition up to a plateau at 25 Hz. Optimum caries inhibition in enamel appears to be achieved by pretreatments that produce surface temperatures in the range of 800 - 1000 degree(s)C. Optimization of pulse duration, wavelength and repetition rate should allow for effective clinical caries inhibition with short treatment times and minimal change to surface topography of enamel.

Scanning electron microscopy was used to compare the morphology, integrity and distribution of bacterial cells in a test plaque grown on the surface of enamel with that of the cell sediment plaque routinely used in a short-term intraoral caries model. Cultures of S. mutans IB-1600 or S. sobrinus 6715-13 were grown in complex media supplemented with either 2.0% sucrose (glucan plaque) or 0.2% glucose (non-glucan plaque). Cell sediment (CS) plaque was prepared by centrifuging the cultures after incubation, recovering the cell sediment, and spreading it on Metricel membrane filter paper. Surface grown (SG) plaque was prepared by suspending saliva-coated bovine enamel in the culture medium, incubating, and recovering the enamel assembly with bacterial accumulations. Cell morphology and integrity, as well as the appearance of glucan-like material produced by the cells, was similar in both CS and SG test plaques. The cell distribution however, varied in the SG plaque from extremes of all cells to all glucan, whereas the cell sediment plaque was more uniform in cell distribution. A highly standardized test plaque minimizes variability in the intraoral caries model. These findings support the contention that the bacterial cells in a cell sediment plaque are similar in morphology, integrity and glucan production to surface grown plaque, and have the added advantage of uniform distribution, which makes the cell sediment plaque more appropriate for intraoral caries model studies.

Studies of the effects of carbon dioxide (CO 2 ) lasers on dental enamel have demonstrated that surface changes can be produced at low fluences (< 10 J/cm2) if wavelengths are used which are efficiently absorbed by the hard tissues. In this study, scanning electron microscopy (SEM) was used to characterize the wavelength dependence of surface changes in dental enamel after exposure to an extensive range of CO 2 laser conditions. Bovine and human enamel were irradiated by a tunable, pulsed CO 2 laser (9.3, 9.6, 10.3, 10.6 μm), with 5, 25, or 100 pulses, at absorbed fluences of 2, 5, 10, or 20 J/cm2, and pulse widths of 50, 100, 200, 500 us. SEM micrographs revealed evidence of melting, crystal fusion, and exfoliation in a wavelength-dependent manner. Crystal fusion occurred at absorbed fluences as low as 5 J/cm 2 per pulse at 9.3, 9.6, and 10.3 μm, in contrast to no crystal fusion at 10.6 pm (≤ 20 J/cm 2 ). Longer pulses at constant fluence conditions decreased the extent of surface melting and crystal fusion. The total number of laser pulses delivered to the tissue did not significantly affect surface changes as long as a minimum of 5 to 10 pulses was used. Within the four easily accessible wavelengths of the CO 2 laser, there are dramatic differences in the observed surface changes of dental hard tissue.

The formation of calcium (Ca) fluoride (CaF2) on bovine enamel blocks during clinically relevant treatment times using neutral fluoride (F) solutions (0.26 mol/l F) with and without 0.1 mol/l lactate was investigated. Uncoated and pellicle-coated blocks were evaluated for alkali-soluble (1 mol/l KOH, three consecutive 24-hour treatments) Ca, PO4, and F after treatment by the F solutions for 0, 5, 15, 30, and 60 min. There was an overall time-related increase in F recovery, while Ca tended to remain at baseline levels. Less F was recovered from the pellicle-coated blocks. The addition of lactate to the F treatment solution did not result in an overall increase in alkali-soluble F recovery, but did result in the formation of cuboidal shaped crystals which closely approached the morphology of pure CaF2. A 1:2 stoichiometric ratio Ca:2F (mol:mol) was not established based on chemical analyses. The ultrastructural and elemental composition of surface deposits on the samples, as determined using scanning electron microscopy, X-ray diffraction, and energy-dispersive spectroscopy, established the presence of CaF2 after 24-hour F treatments; however, it was not possible to directly demonstrate the formation of CaF2 after clinically relevant treatment times.

Human and bovine enamel samples were irradiated with a pulsed CO2 laser at (lambda) equals 9.3, 9.6, 10.3, and 10.6 micrometers with 5 - 10 J/cm2 pulses of 50 - 500 microsecond(s) duration in order to determine the required energy densities needed to fuse the enamel surface. The resulting temperature rise at the sample surface ranged from 500 to 1500 degree(s)C, as measured by a HgCdZnTe detector. The temperature was significantly higher at 9.3 and 9.6 micrometers than at 10.3 and 10.6 micrometers for the same absorbed energy. Scanning electron micrographs of the irradiated enamel revealed surface changes that were consistent with the surface temperature observations. The temperature rise at the ceiling of the pulse chamber determines the risk of pulpal necrosis. This temperature was measured using thermocouples and a thermal imaging camera for different pulse repetition rates and number of pulses. These results indicate that the more efficient absorption at (lambda) equals 9.3 and 9.6 micrometers may be used to fuse enamel at lower laser energies, therefore requiring less energy, significantly reducing the risk of pulpal necrosis during laser treatment.

The intra-oral enamel demineralization test (IEDT) was introduced by Brudevold et al. (1984). This caries model involves human subjects wearing palatal appliances each holding eight bovine enamel blocks covered by a bacterial cell layer prepared by the harvesting of cultures of Streptococcus mutans (test plaque). The original model used the iodide permeability test for assessment of the extent of demineralization of bovine enamel blocks resulting from acid production by the test plaque after dietary substrate challenge. The IEDT model has been expanded and improved by us in the following ways: (1) Based on encouraging findings from an in vitro study (Zero et al., 1990), the surface microhardness test has been adopted to measure the extent of demineralization occurring at three sites on the enamel blocks corresponding to an area over which the effective plaque thickness is 0.5, 1.5, and 2.5 mm; (2) intra-oral pH of the test plaque is measured by means of a Beetrode miniature pH electrode at baseline, then at five, 10, 15, 30, and 45 min after the start of a test; (3) plaque samples are collected at the end of a test and analyzed for organic acid content by means of HPLC; (4) the bacterial test challenge has been expanded to include different cariogenic bacteria which are grown under various growth conditions. The improved model has the capability of studying fundamental aspects of the caries process, namely, the relationships among dietary substrate challenge, plaque pH change, plaque organic acid profiles, microbial virulence properties, and enamel demineralization. Furthermore, the model has the potential for use in more applied research on caries-preventive agents such as fluoride.

Modifications to the standard operating settings for accelerating voltage, condenser lens current, scan rate, working distance and tilt on the conventional scanning electron microscope (SEM) enabled non-metal coated dental hard tissues and synthetic apatite pellets to be viewed free of charging effects. Well-resolved images at magnifications as high as 35,000x were achieved using accelerating voltages less than 5 kV. The methodology detailed here allowed for serial SEM examination of the same sample at various points during an experimental procedure, and may be applied to other sample types. The procedure is non-destructive to the sample and requires no physical modification to the microscope.

Previous studies by the authors have shown that carbon dioxide (CO2) laser light has marked effects on dental hard tissues and that these effects are wavelength-dependent. The aim of the present study was to determine whether treatment by CO2 laser of caries-like lesions in human enamel would inhibit subsequent lesion progression. Nine groups of 10 teeth each with preformed caries-like lesions were treated with/without CO2 laser (9.32 micrometers , 15 mJ or 25 mJ per pulse) by a pulsed laser (100-200 nsec) for either 200 or 400 pulses. Preformed lesions were then treated with acidulated phosphate fluoride for 5 minutes with control groups with no fluoride treatment. Teeth were subjected to a subsequent pH cycling challenge to determine the protection against lesion progression. Low energy laser treatment coupled with fluoride treatment entirely inhibited subsequent lesion progression in this model system.