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Influence of pulselength on mid-infrared laser ablation of hard dental tissue was studied experimentally by applying free-generated Er:YAG laser pulses with pulselengths from 50 ts to 1 ms and different energies to dentin slices in vitro with no spray cooling. The results show that for pulselengths between 50 and 300 ps, dentin ablation turns up abruptly at a well defined fluence value ofapproximately 4 J/cm2, which is independent of the pulselength. Similarly, volumes of the resulting craters as a function of laser fluence at these pulselengths follow nearly the same curve, which corresponds well to a model of laser beam screening by the ejected debris. However, with 1 ms long laser pulses, the ablation turns up at laser fluences above 4 J/cm2 only gradually, reaching differential ablation efficiency similar as in previous cases at fluence values several times higher than the ablation threshold value. An estimate of this "second threshold" fluence and the pulselengths at which it should in general be observed are derived from a simple model of heat diffusion and ablation front dynamics.

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BackgroundPrecise assessment of dental pulp anatomy is of an extreme importance for a successful endodontic treatment. As standard radiographs of teeth provide very limited information on dental pulp anatomy, more capable methods are highly appreciated. One of these is 3D magnetic resonance (MR) microscopy of which diagnostic capabilities in terms of a better dental pulp anatomy assessment were evaluated in the study.Materials and methodsTwenty extracted human teeth were scanned on a 2.35 T MRI system for MR microscopy using the 3D spin-echo method that enabled image acquisition with isotropic resolution of 100 μm. The 3D images were then post processed by ImageJ program (NIH) to obtain advanced volume rendered views of dental pulps.ResultsMR microscopy at 2.35 T provided accurate data on dental pulp anatomy in vitro. The data were presented as a sequence of thin 2D slices through the pulp in various orientations or as volume rendered 3D images reconstructed form arbitrary view-points. Sequential 2D images enabled only an approximate assessment of the pulp, while volume rendered 3D images were more precise in visualization of pulp anatomy and clearly showed pulp diverticles, number of pulp canals and root canal anastomosis.ConclusionsThis in vitro study demonstrated that MR microscopy could provide very accurate 3D visualization of dental pulp anatomy. A possible future application of the method in vivo may be of a great importance for the endodontic treatment.

Optoacoustic measurements were carried out in order to obtain better understanding of the ablation mechanisms during the illumination of hard dental tissue by Er:YAG laser radiation. A broadbard microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of tht laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the har i dental tissue independently of other spikes. This is in agreement with the model of ablation by means of micro explosions. The optoacoustic signal is observed to be approximately linearly related to the ablation efficiency, and is thus demonstrated to be a good measure of the ablation efficiency. The experiments als) show a significant difference in optoacoustic signals obtained during ablation in caries, enamel and dentin.

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Enamel and dentin samples were exposed extraorally to pulsed TEA CO2 lasers with pulse durations of 1 tsec and 0.1 JLsec. The ablation rate is for energy densities above 5 J/cm2 independent of the CO2 laser energy. For 1js long CO2 pulses the ablation rate is 3 pm/pulse for drilling in enamel, and 8 pm/pulse for drilling in dentin. Drilling with 0.1 jis CO2 laser results in lower ablation rates of approximately 1 pm/pulse in enamel, and 4 pm/pulse in dentin. At all experimental energy densities plasma formation is observed, effectively reducing the amount of energy deposition. Compared with these results, experiments with the Er:YAG laser show that 200 ps long Er:YAG laser pulses achieve better ablation in the high energy density range because ablation is not diminished as rapidly by the plasma formation. The Er:YAG laser saturated ablation rates are approximately 60 pm/pulse for drilling in dentin and 40 pm/pulse for drilling in enamel.

The irradiation time of a visible-light-activated composite necessary to achieve full polymerization throughout the material was studied. Curing-time dependence on the thickness of the material was also investigated. To monitor the visible light-activation effect, the free radical concentration was measured as a function of irradiation time. If the composite sample is less than 0.5 mm thick and exposed to light for a time interval recommended by the manufacturer, full radical concentration is indeed created uniformly. This is not the case in thicker samples. Electron paramagnetic resonance (EPR) was used to monitor the concentration of free radicals in the samples. The number of radicals was monitored as a function of irradiation time during which the radicals were generated in samples 0.5, 0.8, 2.0, 3.0 and 5.0 mm thick. An EPR X-band spectro-meter was used to detect the free radical spectra. The number of free radicals per unit mass as a function of irradiation time shows that 60% of the maximum concentration of radicals in a 1 mm sample is reached in 24 s curing time, while in thicker samples it takes hundreds of seconds. On the basis of the experiments, a depth and irradiation time-dependent radical concentration model was developed. This model shows that a 2.0 mm thick sample is cured at the bottom side if irradiated for 60 s. It is proposed that the measure of the degree of polymerization in composite materials should be the polymerization of the bottom layer of the sample which is modelled from the number of free radicals generated in the sample.