Matjaz Lukac

Thermomechanical laser ablation of hard dental tissues: an overview of effects, regimes, and models

Derivation and predictions of the basic analytical model of thermo-mechanical laser ablation, treating the limit case of constant laser intensity, 1D geometry, negligible heat diffusion and no debris screening, is summarized first as a reference point for further discussion. Each of the above requirements is then omitted in turn, to analyze additional effects observed under various experimental conditions. Scattering and absorption of the laser radiation in ejected debris is treated using a model that allows the debris extinction coefficient to vary dynamically during the ablation process, resulting in influence of pulse duration on the fluence dependence of predicted ablation crater depths. Next, the influence of heat diffusion on ablation efficiency and amount of thermal side effects is analyzed in a semi-quantitative way, leading to rule-of-thumb formulas that predict the ablation regime for a general ablation process from laser pulse fluence and duration as well as optical and thermal properties of the treated tissue. Influence of the laser beam profile on ablation crater depth and shape is demonstrated and discussed for the case of Gaussian beam profile. In the end, fiber-tip contact ablation in the presence of water spray is discussed as a counter-example of experimentally observed effects that - to our best knowledge - are still beyond the reach of quantitative understanding.

Quantitative investigation of thermal damage in Er:YAG laser skin resurfacing

Feasibility of deep coagulation of skin with superficially absorbed Er:YAG lasers is investigated using a numerical model. Unlike most previous models, the skin is treated as a two-component material: water trapped in spherical cavities inside an infinite elastic medium. In describing the interaction of mid-IR laser light with skin, thermodynamic behavior of pressurized hot water and steam is combined with elastic response of the surrounding medium. A one-dimensional treatment of heat diffusion and the Arrhenius model of the protein denaturation process are also included in the model. Temperature evolution, profile and coagulation depth are analyzed as a function of the pulse duration, number of applied pulses and repetition frequency of the sequence. The results indicate that the depth of coagulated layer, which amounts to 15 - 40 micrometer with a single free-generated Er:YAG laser pulse, can be extended up to 200 micrometer with no surface ablation by using a repetitive pulse sequence of suitable single-pulse fluence, repetition frequency and duration.

Influence of water spray on Er:YAG ablation of hard dental tissues

Influence of water-spray cooling on the efficiency of erbium laser ablation of hard dental tissues is investigated experimentally. Consecutive pulses from a dental Er:YAG laser system are applied to dentin and enamel surface in vitro. Depths and diameters of the resulting craters, obtained with and without the use of a pressurized air/water spray, are compared at single-pulse fluences from 10 to 140 J/cm2. Similar measurements are performed also with direct drilling of enamel with a fiber tip. The results show that direct interaction of the cooling water with the laser radiation increases the ablation efficiency for enamel, especially in fiber-tip drilling.

Effects of pulsed CO2 and Er:YAG lasers on enamel and dentin

Enamel and dentin samples were exposed extraorally to pulsed TEA CO2 lasers with pulse durations of 1 microsecond(s) ec and 0.1 microsecond(s) ec. The ablation rate is for energy densities above 5 J/cm2 independent of the CO2 laser energy. For 1 microsecond(s) long CO2 pulses the ablation rate is 3 micrometers /pulse for drilling in enamel, and 8 micrometers /pulse for drilling in dentin. Drilling with 0.1 microsecond(s) CO2 laser results in lower ablation rates of approximately 1 micrometers /pulse in enamel, and 4 micrometers /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 microsecond(s) 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 micrometers /pulse for drilling in dentin and 40 micrometers /pulse for drilling in enamel.

Fiber-tip drilling of hard dental tissues with Er:YAG laser

Specifics of fiber-tip ablation of hard dental tissues with mid-infrared laser radiation are investigated in vitro. Sequences of free-generated Er:YAG laser pulses are applied to fresh human dentin and enamel slices at a low repetition rate and the resulting craters inspected and measured by optical microscopy. Influence of the laser pulse fluence (up to 55 J/cm2), number of pulses in the sequence (1 to 20), and the gap between the fiber tip and the tissue surface (0 to 1.0 mm), is determined quantitatively. Additionally, the effect of the optical quality of the fiber tip on the ablation efficiency is assessed qualitatively. The results help us identify the optimum working regime for the dental fiber-tip handpieces in both tissues. Additionally, they provide new clues for understanding the process of mid-infrared ablation of hard biological tissues, especially in presence of the water spray, which interacts directly with the laser radiation.

Heat diffusion and ablation front dynamics in Er:YAG laser skin resurfacing

Influence of pulse energy, duration and beam cross-section on the outcome of Er:YAG laser ablation of skin is interpreted on the basis of an analytical model of heat diffusion and ablation front dynamics. Derived expressions enable us to identify different ablation regimes in terms of ablation efficiency and depth of thermally affected tissue layer for any thermally driven laser ablation process. Influence of laser wavelength is also discussed, focusing on a comparison between Er:YAG and carbon-dioxide laser skin resurfacing. Preliminary experimental and clinical evidence in agreement with the model is also presented.

Optoacoustic studies of Er:YAG laser ablation in hard dental tissue

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 broadband microphone was used to detect laser generated acoustic waves in the ambient air. Correlation analysis of the laser pulse spikes and the response of the optoacoustic probe indicates that each laser spike ablates the hard 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 also show a significant difference in optoacoustic signals obtained during ablation in caries, enamel, and dentin.

Influence of hole burning on laser pumping dynamics and efficiency in Yb:Er:phosphate glasses

Measured population dynamics of erbium metastable level in a Yb:Er:phosphate glass following the excitation with a Nd:glass laser is reproduced by a theoretical model based on rate equations. To our knowledge, this is the first model to include frequency hole burning of inhomogeneously broadened pumping transition of ytterbium ion. The model explains also the previously observed dependence of pumping efficiency on Nd:glass laser pulse length.

Ablative and Thermal Effects of Er:YAG Laser on Human Tissue

The Er:YAG laser is known as a premier tool to precisely ablate superficial skin layers in dermatology while inducing minimal thermal damage to the surrounding tissue. This is due to its wavelength of 2.94 mm which corresponds to the peak of the water molecule absorption spectrum, resulting in an extremely shallow skin penetration depth (around 3 µm). In some applications however, controlled heating of the deeper skin layers is beneficial. In aesthetic surgery, for example, coagulation of deeper collagen fibers is assumed to attribute to removal of wrinkles and a more youthful skin appearance.

Debris screening and heat diffusion in Er:YAG drilling of hard dental tissues

A systematical investigation of Er:YAG laser drilling of both human dentin and enamel with pulselengths between 50 microsecond(s) and 1.2 ms is presented. At the shorter pulselengths, the influence of heat diffusion is negligible. Consequently, ablation of tissue starts abruptly at a well-defined fluence value, which is independent of the pulselength. Depths and volumes of the resulting craters show a quasi-logarithmical dependence on the applied laser fluence. An improved analytical model of the laser beam screening by the ejected debris is developed, which fits well to the experimental data. With longer laser pulses, ablation turns up only gradually, with differential ablation efficiency slowly increasing with applied laser fluence. Such double-threshold behavior is explained by a simple model of dynamical interplay between the ablation front and heat-diffusion wave. The model enables us to derive expressions for pulselength and laser fluence ranges in which such behavior should be expected for any combination of tissue and laser properties. Also, it predicts qualitatively the amount of thermal side effects in a general laser ablation process.