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BACKGROUND AND OBJECTIVES The purpose of this ex vivo study is to investigate whether it is possible to pre-determine and set the optimal separation times for the SWEEPS Er:YAG laser pulses pair during laser-assisted irrigation of endodontic root canals based on known lateral dimensions of the endodontic access cavities of different types of teeth. STUDY DESIGN/MATERIALS AND METHODS As the optimal SWEEPS laser pulse pair separation for enhanced shockwave generation depends on the life-cycle of a single-pulse bubble, measurements of the oscillation time T B of the Er:YAG laser-generated bubble were made in 23 different endodontic access cavities of different types of teeth progressively widened in three different steps, into larger cavities, for a total of 69 cavities of different shapes and sizes. Different fiber-tip geometries (flat and radial), laser pulse energies (10 mJ and 20 mJ) and depth of fiber-tip insertion (2 mm and 4 mm) were also investigated. The obtained data were then analyzed using the reported relationship between the bubble oscillation time and the diameter of a cylindrically shaped cavity. RESULTS A good fit to the relation analogue for ideal cylindrical cavities was found by taking the characteristic diameter of the access cavity to be represented by the cavity diameter either in the mesiodistal (D min ) or buccolingual (D max ) direction, or alternatively by the average of the two diameters (D ave ). The best fit was obtained for D min (R 2 = 0.73) followed in order by D ave (R 2 = 0.71) and D max (R 2 = 0.63). CONCLUSION In spite of the endodontic cavities being non-cylindrical and of varied shape and size, the bubble oscillation time T B and the corresponding optimal SWEEPS separation time can be well predicted using a single characteristic dimension of the access cavity. This finding enables a simple and practical method for determining optimal conditions for shock wave generation and enhanced photodynamic streaming in differently shaped and sized root canals, leading to improved treatment efficacy and safety of root canal irrigation. Lasers Surg. Med. 2020. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Background and objectives Laser-activated irrigation of dental root canals is being increasingly used as its efficacy has been shown to be superior compared with conventional techniques. The method is based on laser-initiated localized fluid evaporation and subsequent rapid bubble expansions and collapses, inducing microfluid flow throughout the entire volume of the cavity. The irrigation efficacy can be further improved if optimally delayed "SWEEPS" double laser pulses are delivered into the canal. This study aims to show that the irrigation efficacy, as measured by the induced pressure within the canal, is related to the double pulse delay, with the maximal pressure generated at an optimal delay. The second aim is to find a method of determining the optimal delay for different cavity dimensions and/or laser parameters. Study design/materials and methods Experiments were made in transparent models of root canals where Er:YAG laser (λ = 2.94 μm, pulse duration tp = 25 or 50 microseconds, and pulse energies up to EL = 40 mJ) was used with a combination of cylindrical and conical fiber-tip geometries (diameters 400 and 600 µm). High-speed photography (60,000 fps) and average pressure measurements inside the canal were used for process characterization. Results The results show that a pressure amplification of more than 1.5 times occurs if the laser pulse delay approximately coincides with the bubble oscillation time. Correlations between normalized oscillation time and canal diameter for a wide range of laser pulse energies (R2 = 0.96) and between the average pressure within the canal and the bubble oscillation periods (R2 = 0.90) were found. A relationship between the bubble oscillation time and the diameter of the treated cavity was found depending on the bubble oscillation time in an infinite fluid reservoir. Conclusions The bubble oscillation time within a constrained volume can be determined based on the known oscillation time in infinite space, which offers a fast and simple solution for optimization of the laser parameters. These findings enable determination of optimal conditions for shock wave generation, and improvement of root canal irrigation at the same dose of laser energy input, leading to improved treatment efficacy and safety. Lasers Surg. Med. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals, Inc.

Background and objectives To compare temporal rates of debris removal from an artificial root canal for three laser-assisted irrigation modalities single-pulse super short pulse (SSP), and two dual-pulse X-SWEEPS and AutoSWEEPS, and for two fiber-tip (FT) geometries flat and radial, and to evaluate the dependence of the debris flushing rate on the delay between the SWEEPS laser pulse pair. Study design/materials and methods Laser-assisted irrigation was performed with a pulsed Er:YAG laser operating in single-pulse SSP and dual-pulse SWEEPS laser modalities. The laser energy was delivered to the water-filled model access cavity through a FT with either a flat or radial ending. The X-SWEEPS modality delivered pairs of laser pulses separated by a fixed adjustable delay, while with the AutoSWEEPS modality the delay was automatically and repeatedly swept between 200 and 600 microseconds. The debris removal rate was determined with the use of a digital camera by measuring the rate at which a simulated debris was being flushed out of the artificial root canal. Results The simulated debris removal rate of the AutoSWEEPS modality is almost three times higher compared with that of the SSP modality. Further, the flat FT outperforms the radial FT by a factor of more than five in the case of SSP, and by more than 10 with AutoSWEEPS. The X-SWEEPS flushing rate exhibits strong dependence on the delay between the SWEEPS pulse pair, with the highest removal rate measured to be more than seven times higher in comparison with SSP. Conclusion Dual-pulse laser irrigation modalities (AutoSWEEPS and X-SWEEPS) exhibit significantly higher simulated debris removal rates in comparison with the standard single-pulse SSP laser-assisted irrigation. As opposed to the previously reported dependence of pressure generation on FT geometry, the flat FT's simulated debris removal rate significantly outperforms the radial FT. © 2020 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

According to the standard Arrhenius relation, tissue damage is linearly dependent on the duration of exposure to elevated temperatures and exponentially dependent on the temperature itself. However, recently published measurements of damage threshold temperatures at extremely short exposure times (commonly present during laser treatments) exhibit a shift to temperatures that are higher than what would normally be expected from a single-process Arrhenius model. A novel variable heat shock (VHS) response model was developed that takes into account the observed deviation from the single-process Arrhenius relation, by assuming that the cell viability can be described as the combined effect of two biochemical processes that dominate cell survival characteristics at very short and very long exposure times. The potential implications of the VHS model are explored theoretically through an example of non-ablative laser resurfacing. The VHS model shows that under the appropriate conditions, very high temperature heat shocks can be generated within the superficial epithelium tissue layer without causing irreversible tissue damage. A mechanism of action for tissue regeneration by means of non-ablative resurfacing with the Er:YAG laser is proposed, which involves indirect triggering of tissue regeneration through intense heat shock to the epithelia, in addition to the tissue regeneration mechanism by means of direct thermal injury to deeper lying connective tissues.

The study presents an assessment of mechanism of action and a pilot clinical study of efficacy and safety of the Er:YAG laser for the treatment of stress urinary incontinence (SUI). The subject of this study is a treatment of SUI with a 2940 nm Er:YAG laser, operating in a special SMOOTH mode designed to increase temperature of the vaginal mucosa up to maximally 60–65 °C without ablating the epidermis. Numerical modelling of the temperature distribution within mucosa tissue following an irradiation with the SMOOTH mode Er:YAG laser was performed in order to determine the appropriate range of laser parameters. The laser treatment parameters were further confirmed by measuring in vivo temperatures of the vaginal mucosa using a thermal camera. To investigate the clinical efficacy and safety of the SMOOTH mode Er:YAG laser SUI treatment, a pilot clinical study was performed. The study recruited 31 female patients suffering from SUI. Follow-ups were scheduled at 1, 2, and 6 months post treatment. ICIQ-UI questionnaires were collected as a primary trial endpoint. Secondary endpoints included perineometry and residual urine volume measurements at baseline and all follow-ups. Thermal camera measurements have shown the optimal increase in temperature of the vaginal mucosa following treatment of SUI with a SMOOTH mode Er:YAG laser. Primary endpoint, the change in ICIQ-UI score, showed clinically relevant and statistically significant improvement after all follow-ups compared to baseline scores. There was also improvement in the secondary endpoints. Only mild and transient adverse events and no serious adverse events were reported. The results indicate that non-ablative Er:YAG laser therapy is a promising minimally invasive non-surgical option for treating women with SUI symptoms.

Laser-enhanced irrigation of complex root canals appears to be a very promising technique to improve the outcome of root canal treatment. This applies, in particular, if the technique can be effective at very low laser energies in irrigating not only the main canal but also the small lateral canals. This is important in order to avoid potential undesirable effects at higher laser energies such as temperature increase, dentin ablation, or extrusion of irrigating solution beyond the apical foramen. An improved understanding of the role of laser parameters, such as laser wavelength and pulse duration, in irrigation of lateral canals is therefore desired in order to optimize treatment efficacy. The influence of laser wavelength and pulse duration on cavitation phenomena was studied using shadow photography and a method of measuring fluid flow in lateral canals based on tracking of movements of small air bubbles naturally forming in liquid as a result of laser agitation. A simulated model of a root canal including a narrow lateral canal designed to represent typical root canal morphology was used for the water flow measurements. The following three laser wavelengths with relatively high absorption in water were studied: Er:YAG (2.94 μm), Er,Cr:YSGG (2.73 μm), and Nd:YAP (1.34 μm). Among the three wavelengths studied, the Er:YAG laser wavelength was found to be the most effective in formation of cavitation bubbles and in generating fluid motions within narrow lateral canals. A comparison between the shadow photography and fluid motion data indicates that it is the bubble’s radius and not the bubble’s volume that predominantly influences the fluid motion within lateral canals. Based on the results of our study, it appears that effective minimally invasive laser-assisted irrigation can be performed with low Er:YAG laser pulse energies below 10 mJ.

Characteristics of thermo-mechanical laser ablation process are investigated using an original numerical model. In contrast with previous models, it is based on a microscopic physical model of the micro-explosion process, which combines thermodynamic behavior of tissue water with elastic response of the solid tissue components. Diffusion of dissipated heat is treated in one dimension, and the amount of thermal damage is assessed using the Arrhenius model of the protein denaturation kinetics. Influence of the pulse fluence and duration on temperature profile development, ablation threshold, and depth of thermal damage is analyzed for the case of Er:YAG laser irradiation of human skin. Influence of mechanical properties on the ablation threshold of soft tissue is predicted theoretically for the first time. In addition, feasibility of deep tissue coagulation with a repetitively pulsed Er:YAG laser is indicated from the model.

A novel optical transmission method was used for measuring transient temperature dynamics inside an Er:YAG laser crystal during optical pumping with flashlamp pulses of different duration and spectral composition. Time-resolved measurements of the crystal gain at 2.94 /spl mu/m were also performed for the same set of pumping conditions. The results are compared with predictions of a theoretical model for gain and temperature dynamics during optical pumping of Er:YAG. A good spatial resolution of the implemented temperature measurement method was utilized also to determine the lateral temperature profiles inside the laser rod under various pumping conditions.

Erbium lasers are by now an accepted tool for performing ablative medical procedures, especially when minimal invasiveness is desired. Ideally, a minimally invasive laser cutting procedure should be fast and precise, and with minimal pain and thermal side effects. All these characteristics are significantly influenced by laser pulse duration, albeit not in the same manner. For example, high cutting efficacy and low heat deposition are characteristics of short pulses, while vibrations and ejected debris screening are less pronounced at longer pulse durations. We report on a study of ablation characteristics on dental enamel and cementum, of a chopped-pulse Er:YAG [quantum square pulse (QSP)] mode, which was designed to reduce debris screening during an ablation process. It is shown that in comparison to other studied standard Er:YAG and Er,Cr:YSGG laser pulse duration modes, the QSP mode exhibits the highest ablation drilling efficacy with lowest heat deposition and reduced vibrations, demonstrating that debris screening has a considerable influence on the ablation process. By measuring single-pulse ablation depths, we also show that tissue desiccation during the consecutive delivery of laser pulses leads to a significant reduction of the intrinsic ablation efficacy that cannot be fully restored under clinical settings by rehydrating the tooth using an external water spray.

Background and Objective: An expression describing the ablation crater shape as a function of lateral fluence distribution has been derived in a recent paper by Ostertag et al. [Lasers Surg Med 1997; 21:384‐394]. Study Design/Materials and Methods: The ablation model presented therein is improved by taking into account the influence of the ablation front inclination on the ablation dynamics. Results: The resulting crater profiles deviate from the previously predicted ones (which reflect the Gaussian fluence distribution of the impinging laser beam) progressively with increasing pulse fluence. Conclusion: The ablation front inclination must be taken into account to predict the ablation crater shapes and volumes correctly. Lasers Surg. Med. 24:55‐60, 1999. © 1999 Wiley-Liss, Inc.