Christian Apel

Influence of the water content in dental enamel and dentin on ablation with erbium YAG and erbium YSGG lasers

The theory of the ablation of dental hard tissue with erbium lasers is based on a process of thermomechanical interaction, which is explained by the absorption of the radiation in the water component of the tissue. The abrupt evaporation of the water is the cause of tissue fragments being blasted out of the tooth structure. The aim of this study is to examine the effect of the water contained in dental hard tissues on the efficiency of ablation. 192 specimens of both bovine dental enamel and bovine dentin are irradiated with an Er:YAG and an Er,Cr:YSGG laser. Half of the specimens are dehydrated beforehand. Irradiation is carried out in subgroups: without water spray and with water spray at flow rates of 0.8 and 3 mls. The ablated volume is determined following histological preparation. Only in dentin, and then only with irradiation with the Er:YAG laser, is the water contained in the tissue found to have a significant influence (p < 0.0001) on the ablated volume. The water content has no effect on the efficiency of laser ablation in any of the other test groups. In contrast, the externally supplied water always has a significant influence on the effectiveness of the ablation process.

Structural Changes in Human Dental Enamel after Subablative Erbium Laser Irradiation and Its Potential Use for Caries Prevention

Numerous studies have confirmed the potential of erbium laser irradiation for increasing the acid resistance of dental enamel. The objective of the present paper was to investigate the effect of subablative erbium laser irradiation on the structure and acid resistance of dental enamel by means of confocal laser scanning microscopy (CLSM). To this end, 12 samples of human dental enamel were irradiated with subablative energy densities (Φ) of an Er:YAG (λ = 2.94 µm, Φ = 6 J/cm2) and an Er:YSGG laser (λ = 2.79 µm, Φ = 8 J/cm2). The enamel surfaces of 6 samples were polished prior to irradiation. The remaining 6 samples were left intact (without polishing procedures) and, in the further course of the study, they were subjected to 1-week in situ demineralisation. All irradiated test surfaces were assigned a control surface on the same sample. The changes following laser irradiation and the in situ wearing time were assessed qualitatively using a confocal laser scanning microscope. The irradiation of dental enamel with subablative erbium laser irradiation produces fine cracks in the enamel surface. These cracks act as starting points for acid attack and favour deep demineralisation. These changes reduce or eliminate the positive effect of subablative erbium laser irradiation observed in connection with caries-preventive use. The clinical use of subablative erbium laser irradiation to prevent caries would appear not to make sense under the conditions studied.

The ablation threshold of Er:YAG and Er:YSGG laser radiation in dental enamel.

Abstract.The scientific investigation of fundamental problems plays a decisive role in understanding the mode of action and the consequences of the use of lasers on biological material. One of these fundamental aspects is the investigation of the ablation threshold of various laser wavelengths in dental enamel. Knowledge of the relationships and influencing factors in the laser ablation of hard tooth tissue constitutes the basis for use in patients and the introduction of new indications. The present paper examines the ablation threshold of an Er:YAG laser (λ=2.94 µm) and an Er:YSGG laser (λ=2.79 µm) in human dental enamel. To this end, 130 enamel samples were taken from wisdom teeth and treated with increasing energy densities of 2–40 J/cm2. The sample material was mounted and irradiated on an automated linear micropositioner. Treatment was performed with a pulse duration of τP(FWHM)≈150 µs and a pulse repetition rate of 5 Hz for both wavelengths. The repetition rate of the laser and the feed rate of the micropositioner resulted in overlapping of the single pulses.The surface changes were assessed by means of reflected light and scanning electron microscopy. On the basis of the results, it was possible to identify an energy density range as the ablation threshold for both the Er:YAG and the Er:YSGG laser. With the Er:YAG laser, the transition was found in an energy density range of 9–11 J/cm2. The range for the Er:YSGG laser was slightly higher at 10–14 J/cm2.

Demineralization of Er:YAG and Er,Cr:YSGG Laser-Prepared Enamel Cavities in vitro

The aim of the present study was to establish whether cavity preparation by means of an erbium laser with efficient water cooling is capable of reducing the susceptibility of the prepared dental enamel to demineralization and thus of achieving a potential caries-protective effect in the region of cavity margins. To this end, cavities limited to the enamel were prepared in the crowns of 10 teeth each using an Er:YAG (λ = 2,940 nm) and an Er,Cr:YSGG laser (λ = 2,780 nm). A control cavity prepared conventionally with a diamond drill in the same occlusal zone was assigned to each of these cavities. The specimens were then subjected to a pH-cycling caries model. Analysis was performed by quantitatively measuring the demineralization front under a polarized-light microscope. The results of the study showed that enamel cavities prepared with the erbium lasers used display a statistically significant acceleration of demineralization compared to conventionally prepared cavities (p < 0.01). The Er:YAG laser cavities revealed demineralization to a depth of 133.9 (SD 25.7) µm, while the value observed with the Er,Cr:YSGG laser was 133.8 (SD 25.8) µm. The depth of demineralization in the control groups was only 77.4 (SD 13.8) µm and 79.3 (SD 37.6) µm. No difference could be found between the two lasers (p = 0.98). Based on these in vitro tests, it cannot be assumed that use of the erbium laser for cavity preparation offers any advantages in terms of resistance to secondary caries in clinical practice.

CO2 Laser (10.6 μm) Parameters for Caries Prevention in Dental Enamel

Although CO2 laser irradiation can decrease enamel demineralisation, it has still not been clarified which laser wavelength and which irradiation conditions represent the optimum parameters for application as preventive treatment. The aim of the present explorative study was to find low-fluence CO2 laser (λ = 10.6 μm) parameters resulting in a maximum caries-preventive effect with the least thermal damage. Different laser parameters were systematically evaluated in 3 steps. In the first experiment, 5 fluences of 0.1, 0.3, 0.4, 0.5 and 0.6 J/cm2, combined with high repetition rates and 10 μs pulse duration, were chosen for the experiments. In a second experiment, the influence of different pulse durations (5, 10, 20, 30 and 50 μs) on the demineralisation of dental enamel was assessed. Finally, 3 different irradiation times (2, 5 and 9 s) were tested in a third experiment. In total, 276 bovine enamel blocks were used for the experiments. An 8-day pH-cycling regime was performed after the laser treatment. Demineralisation was assessed by lesion depth measurements with a polarised light microscope, and morphological changes were assessed with a scanning electron microscope. Irradiation with 0.3 J/cm2, 5 μs, 226 Hz for 9 s (2,036 overlapping pulses) increased caries resistance by up to 81% compared to the control and was even significantly better than fluoride application (25%, p < 0.0001). Scanning electron microscopy examination did not reveal any obvious damage caused by the laser irradiation.

Influence of the pulse duration of an Er:YAG laser system on the ablation threshold of dental enamel

Abstract.The present study examines the dependence of the ablation threshold on the duration of the applied laser pulses in the dental enamel of human wisdom teeth. To this end, 600 treatments with the Er:YAG laser (λ=2940 nm) were carried out on a total of 50 extracted teeth. The laser light was coupled into a fluoride glass light guide for this purpose, in order to ensure almost gaussian distribution of the light in a radially symmetrical beam. The beam diameter on the specimen was 610 µm. The radiant exposure on the tooth surface was varied between 2 and 20 J/cm2, while the duration of the pulses applied was changed in four steps from 100 µs to 700 µs. The irradiated tooth surfaces were examined for visible signs of ablation under a reflected-light microscope. The experiments revealed that, when pulses of shorter duration are used, the limit at which ablation sets in is reduced by up to approx. 3 J/cm2. This expands the ablation threshold range of Er:YAG laser radiation to between 6 and 10 J/cm2. In this context, both the pulse duration and the radiant exposure have a statistically significant influence on the ablation threshold (logistic regression, p<0.0001). Although the ablation threshold of the dental enamel can be changed by varying the pulse duration of the Er:YAG laser, no clinical consequences can be expected, as the shift is only slight.

Influence of the spatial beam profile on hard tissue ablation Part I: Multimode emitting Er:YAG lasers

AbstractUniform dosimetry is a prerequisite for reproducible laser applications in research and practice. The light–tissue interaction is dependent on the absorbed energy (J) per unit of time (τ) in the case of pulsed lasers, and on the absorbed power (W) per unit of volume (e.g. mm3) in the case of continuous-wave (cw) lasers, and thus directly dependent on the energy distribution within the laser beam. Consequently, precise knowledge of the spatial beam profile, and of the pulse duration and treatment time, is indispensable. The objective of this paper was a theoretical study of the impact of different mode profiles on energy distribution in the beam. Also examined was the question of the influence of changes in the laser parameters on the mode structure. Three erbium:YAG lasers (λ=2.94 μm) were used for this purpose. The transversal mode structure of the lasers was observed by irradiating thermal paper and verified by means of calculations. The effect induced in the mode profile by changing the pulse energy and pulse repetition rate was investigated. The results of the tests show that changes in the laser parameters result in jumps in the transversal modes and associated energy distributions in the beam. The experiments confirm that simply changing the transversal modes has a substantial effect on the threshold energy required for the ablation of dental enamel (50 mJ with TEM00, 22.6 mJ with TEM31). In practice, inhomogeneity makes it impossible to determine the irradiated area in order to calculate the energy or power density. In addition, the energy distribution in the beam changes as a result of variation of the laser output energy and the pulse repetition rate. Consequently, simply measuring the beam diameter yields a totally incorrect result for the applied flux density when using a beam profile with a relatively high mode.

Rehardening of acid-softened enamel and prevention of enamel softening through CO2 laser irradiation

OBJECTIVES The aims of the present study were to investigate whether irradiation with a CO(2) laser could prevent surface softening (i) in sound and (ii) in already softened enamel in vitro. METHODS 130 human enamel samples were obtained and polished with silicon carbide papers. They were divided into 10 groups (n = 13) receiving 5 different surface treatments: laser irradiation (L), fluoride (AmF/NaF gel) application (F), laser prior to fluoride (LF), fluoride prior to laser (FL), non-treated control (C); and submitted to 2 different procedures: half of the groups was acid-softened before surface treatment and the other half after. Immersion in 1% citric acid was the acid challenge. Surface microhardness (SMH) was measured at baseline, after softening and after treatment. Additionally, fluoride uptake in the enamel was quantified. The data were statistically analysed by two-way repeated measurements ANOVA and post hoc comparisons at 5% significance level. RESULTS When softening was performed either before or after laser treatment, the L group presented at the end of the experiments SMH means that were not significantly different from baseline (p = 0.8432, p = 0.4620). Treatment after softening resulted for all laser groups in statistically significant increase in SMH means as compared to values after softening (p < 0.0001). Enamel fluoride uptake was significantly higher for combined laser-fluoride treatment than in control (p<0.0001). CONCLUSION Irradiation of dental enamel with a CO(2) laser at 0.3J/cm(2) (5 μs, 226 Hz) not only significantly decreased erosive mineral loss (97%) but also rehardened previously softened enamel in vitro.

Influence of the spatial beam profile on hard tissue ablation, Part II: pulse energy and energy density distribution in simple beams

When calculating applied flux densities in practice, the beam profile of a laser is often erroneously assumed to be homogeneous. In addition, there is usually no consistency in the choice of a suitable measuring method for determining the beam diameter. This failure to observe the inhomogeneous intensity distribution within the beam cross-section, combined with the imprecise knowledge of the beam diameter, leads to flux densities being stated that represent mean values at best. The present paper gives definitions for the flux densities of simple, radially symmetrical beam cross-sections, taking the top-hat and Gaussian profiles as examples. In connection with the inhomogeneous energy distribution in the Gaussian beam, a concept of integral and local energy density is discussed, which differs from the customary definition of the energy density as a constant. Also presented are the consequences of the mathematical concepts in terms of measurement, giving particular consideration to the case where the energy density as the measured variable matches the integral energy density. The significance of the integral and local energy density for hard-tissue ablation is described, based on the practical example of the ablation of dental hard substance. The central result is that the integral flux density is directly accessible as a measured variable, while the effect on the tissue is determined by the local flux density. If the form of the beam is known, the integral flux density can be converted into the local flux density.

Prevention of toothbrushing abrasion of acid-softened enamel by CO2 laser irradiation

OBJECTIVE The aim of the present study was to evaluate the effect of CO(2) laser irradiation (10.6μm) at 0.3J/cm(2) (0.5μs; 226Hz) on the resistance of softened enamel to toothbrushing abrasion, in vitro. METHODS Sixty human enamel samples were obtained, polished with silicon carbide papers and randomly divided into five groups (n=12), receiving 5 different surface treatments: laser irradiation (L), fluoride (AmF/NaF gel) application (F), laser prior to fluoride (LF), fluoride prior to laser (FL), non-treated control (C). After surface treatment they were submitted to a 25-day erosive-abrasive cycle in 100ml sprite light (90s) and brushed twice daily with an electric toothbrush. Between the demineralization periods samples were immersed in supersaturated mineral solution. At the end of the experiments enamel surface loss was determined using a contact profilometer and morphological analysis was performed using scanning electron microscopy (SEM). For SEM analysis of demineralization pattern, cross-sectional cuts of cycled samples were prepared. The data were statistically analysed by one-way ANOVA model with subsequent pairwise comparison of treatments. RESULTS Abrasive surface loss was significantly lower in all laser groups compared to both control and fluoride groups (p<0.0001 in all cases). Amongst the laser groups no significant difference was observed. Softened enamel layer underneath lesions was less pronounced in laser-irradiated samples. CONCLUSION Irradiation of dental enamel with a CO(2) laser at 0.3J/cm(2) (5μs, 226Hz) either alone or in combination with amine fluoride gel significantly decreases toothbrushing abrasion of softened-enamel, in vitro.