308 nm Excimer laser ablation of cartilage

This article reports the investigation of the XeCl excimer laser as a cutting-ablating tool for human fibrocartilage and hyaline cartilage. Quantitative measurements were made of tissue ablation rates as a function of fluence in meniscal fibrocartilage and articular hyaline cartilage. A force of 1.47 Newtons was applied to an 800-μm fiber with the laser delivering a range of fluences (40-190 mJ/mm²) firing at a frequency of 5 Hz. To assess the effect of repetition rate on depth per pulse, a set of measurements was made at a constant fluence of 60 mJ/mm², with the repetition rate varying from 10 to 40 Hz. Histologic and morphometric analysis of preserved specimens was performed using light microscopy. The results of these studies revealed that the ablation rate was directly proportional to fluence over the range tested. Fibrocartilage was ablated at a rate 2.56 times faster than hyaline cartilage. Repetition rate had no effect on the penetration per pulse. Adjacent tissue damage was noted to be minimal (10–70 μm). The excimer laser achieved ablation rates adequate for arthroscopic applications. © 1994 Wiley-Liss, Inc.     

Arterial response to excimer and argon laser irradiation in the atherosclerotic swine

Injury associated with laser-induced tissue ablation may be reduced by using pulsed energy delivery at low repetition rates, as opposed to using continuous wave energy delivery. This study was designed to examine the similarities and differences between these two systems as regards the healing process, and to examine whether one is superior to the other. In order to test this postulate, the healing response of normal and atherosclerotic aorta were examined after exposure in vivo to argon and excimer (XeCl 308 nm) laser radiation in hypercholesterolemic swine. Swine were fed hyperlipidemic diets for eight months following balloon denudation of the descending aorta. Following general anaesthetic, the descending aorta was isolated and laser burns were made on both normal and atherosclerotic intima using a continuous wave argon laser delivered through a 50 diameter quartz fibre, and a XeCl excimer laser carried through a 1 mm diameter fibre. Energy levels of 3 to 5 J were applied with the argon laser. The pulse duration for the excimer laser was 30 ns and craters were produced using 10 to 60 pulses at a repetition rate of 20 Hz and an energy density of 2 J cm^–2.Forty-eight hours after laser application, craters created by both lasers were filled with thrombus material. Argon burns were surrounded by thermal and acoustic injury which was not seen with excimer burns. Three weeks after laser application all crater surfaces were reconstituted. Unlike the excimer burns, argon craters demonstrated necrosis well beyond the crater margins and were characterized by multinucleate giant-cell reaction surrounding char debris. By nine weeks both excimer and argon laser burns were covered by fibrous tissue but could be distinguished by the fact that char debris and subjacent tissue injury arose with the argon burns.The results suggest that both lasers can be used to remove focal atherosclerotic plaque from arteries without inducing excessive thrombogenicity. Rapid healing is observed with both; however, damage to surrounding tissue is significantly greater with a continuous energy delivery laser as opposed to pulsed energy delivery.Work supported in part by: Heart and Stroke Foundation of Ontario, Grant-in-Aid No. 5-17     

Acute response of the arterial wall to pulsed laser irradiation

This study was designed to examine the acute response of normal arterial wall to pulsed laser irradiation. Irradiation with an Excimer or a Holmium YAG laser was performed in 15 normal iliac sites of 8 male New Zealand white rabbits. The excimer laser was operated at 308 nm, 25 Hz, 50 mj/mm²/pulse, and 135 nsec/pulse and the Ho:YAG laser was operated at 2.1 μm, 3.5 Hz, 400 mj/ pulse, 250 μsec/pulse. The excimer and Ho:YAG laser were coupled into a multifiber wire-guided catheter of 1.4 and 1.5 mm diameter, respectively. The mean luminal diameter increased similarly from 2.01 ± 0.29 to 2.46 ± 0.27 mm (P < 0.0005) and from 2.09 ± 0.53 to 2.45 ± 0.30 mm (P < 0.005) after excimer and Ho:YAG laser irradiation, respectively. Perforation occurred in 3 of 15 Ho:YAG irradiated sites and 0 of 15 excimer laser irradiated sites. The sites irradiated with excimer or Ho:YAG laser had similar histologic features, consisting of shedding of the endothelium, disorganization of internal elastic lamina, localized necrosis of vascular smooth muscle cells, and fissures in the medial layer. However, the sites irradiated with excimer laser had lower grading scores than those irradiated with the Ho:YAG laser (P<0.05). Irradiation with excimer or Ho:YAG laser of normal arteries results in: (1) vasodilation of the irradiated artery; (2) localized mechanical vascular injury, and (3) Ho:YAG laser induces more severe damage to the arterial wall than excimer. © 1993 Wiley-Liss, Inc.     

Rate-dependent,nonlinear photoablation of ocular tissue at 308 nm

To quantify the dependence on pulse repetition rate of 308 nm laser ablation in ocular tissue and elucidate the photoablation mechanisms involved, 85 full-thickness ab interno sclerostomies were created in six human donor eyes using an 800-μm-diameter quartz optical fiber. A laser pulse duration of 135 ns, fluence of 31 mJ/mm², and a fixed repetition rate between 5 and 40 Hz were used for 38 sclerostomies; the remaining 47 sclerostomies were completed at various laser settings during initial experimentation. Surprisingly, the numbers of pulses required for complete penetration of the optical fiber through the fixed tissue thickness were not constant as expected but decreased nonlinearly with increasing repetition rate. This demonstrates that the 308 nm excimer laser cuts ocular tissue significantly more rapidly per pulse at higher repetition rates. To explain this nonlinearity, we propose a composite ablation mechanism composed of photochemical, thermal, mechanical, and optical effects in varying proportions. © 1994 Wiley-Liss, Inc.     

Smooth excimer laser coronary angioplasty (SELCA) and conventional excimer laser angioplasty: Comparison of vascular injury and smooth muscle cell proliferation

Although the excimer laser, which utilizes ‘non-thermal ablation effects’, has achieved encouraging results in early clinical trials, the long-term results have failed to show any advantage over conventional percutaneous transluminal coronary angioplasty (PTCA). A new system, Smooth Excimer Laser Coronary Angioplasty (SELCA), has been developed to reduce the tissue damage in the vessel wall caused by shock waves and vapour bubbles.SELCA (wavelength 308 nm, pulse duration 115 ns, repetition rate 150 Hz and energy density 50 mJ mm^-2) lowers the amount of shock wave formation and pressure peak amplitude in the surrounding tissue by about eight times when compared to the conventional 308 nm excimer laser (ELCA). In this preclinical evaluation, this new system was compared to ELCA. Fifty New Zealand White rabbits were stimulated by repeated weak DC impulses for a period of 28 days in order to form an atherosclerotic plaque in the right carotid artery. The vessels were excised 3, 7,14 and 28 days after laser irradiation for immunohistochemical analysis. SELCA and ELCA laser treatment lead to a decrease in maximal intimal wall thickness 3 days after intervention (control: 177±4 μm; SELCA: 131±22μm; ELCA: 120 ±33μm). In the period between 3 and 28 days, a moderate increase in intimal wall thickness was observed after SELCA treatment compared to a significant increase after ELCA (28 days after intervention: SELCA: 157±22μm; ELCA: 274 ±28μm). Bromodeoxyuridine (BrdU) was applied 18 and 12 h before excision of the vessels in order to determine the percent of cells undergoing DNA synthesis. The percent of BrdU labelled SMC in the intima (control: 13 ± 2 cells mm_-2) increased in both groups after 3 days (SELCA: 248 ± 107 cells mm^-2; ELCA: 162 ± 41 cells mm^-2) and 7 days (SELCA: 162± 55 cells mm^-2; ELCA: 279 ± 119 cells mm^-2). The present results demonstrate that vascular wall injury and increase in intimal wall thickness following SELCA are reduced in comparison to the results achieved with the conventional technique. Further trials are necessary to assess whether these improvements will lead to more favourable long-term results after excimer laser angioplasty.     

Thermal side effects of fiber-guided XeCl excimer laser drilling of cartilage

We examined thermal effects during ablation of human joint cartilage using two XeCl excimer lasers with pulse durations of ～ 20 ns and 60 ns. An increase in radiant exposure or repetition rate caused a rise in tissue temperature up to 82°C at a 100- μm distance. With increasing distance from the crater edge, the temperature dropped exponentially. Radiant exposures higher than 1.8 J/cm² and repetition rates above 20 Hz lead to a formation of hot gaseous products escaping from the laser crater. When os-teoarthritic cartilage is irradiated, these gases spread inside the tissue causing a temperature rise of > 50°C at a distance of 1 mm from the crater edge. In the contact mode, we found a linear rise of ablation rate with increasing repetition rate both in air or saline. But ablation rates in saline were only half the rates achieved in air. Both phenomenons can be explained by additional thermal effects of excimer lasers working in the range of higher repetition rates and pulse energies. © 1994 Wiley-Liss, Inc.     

Optical properties of human articular tissue as implication for a selective laser application in arthroscopic surgery

Background and Objective: Optical density of normal and pathological hyaline cartilage, meniscus, and synovium is determined using native and laser-irradiated tissue samples in order to examine potentials for a selective laser ablation. Study Design/Materials and Methods: One hundred forty-four autopsy specimens were irradiated in a direct contact mode using a XeCl excimer laser (λ = 308 nm; 20 ns; 40 Hz; 40 ± 2.1 J/mm²; 800 μm fused silica fiber) and a continuous-wave Nd:YAG laser (λ = 1,064 nm; 1 s; 124 ± 5.4 W/mm²; 600 μm fused silica fiber). Transmission spectra were obtained by microspectrophotometry in a spectral range from 250 to 770 nm. Results: In the ultraviolet spectrum analyzed, optical density (OD) is calculated to 0.81 ± 0.05 for native hyaline cartilage, to 1.0 ± 0.07 for meniscal tissue, and to 0.68 ± 0.04 for synovium. With increasing wavelength the OD steadily decreases reaching mean values of 0.06 ± 0.01, 0.13 ± 0.03, and 0.15 ± 0.04 at 750 nm. Compared to normal tissue degeneration of cartilage and meniscus lead to a significant increase in OD with a maximum relative OD of 4.39 and 1.26, respectively (P <.001 and P <.01). In synovitis the OD increases with a maximum ratio of 1.45:1 (P <.01). Following Nd:YAG laser exposition the OD of the coagulated zone exceeded the value of native tissue by a factor of 9.71 for cartilage, 4.71 for meniscus, and 3.04 for synovium (P <.001). Excimer irradiation leads to a 3.38-fold increase in OD for cartilage, 2.23-fold for meniscal tissue, and 1.6-fold for synovium (P <.01). Conclusion: The results presented indicate that a preferential ablation of pathological tissue structures in articular surgery is possible by selecting laser systems with an appropriate spectral emission range. However, thermal laser tissue interaction may lead to severe alterations in optical properties reducing potentials of a preferential or selective laser application. © 1995 Wiley-Liss, Inc.     

Effects of pulse width on erbium: YAG laser photothermal trabecular ablation (LTA)

An erbium (Er):YAG laser can remove trabecular meshwork (TM) by photothermal ablation with minimal contiguous thermal damage. A variable pulse width Er:YAG laser was used to investigate the effect of varying pulse width on ablation of human TM. Trabecular photothermal ablation was performed on tissue obtained from eye bank eyes at pulse widths of 50, 150, and 250μs, with energy held constant at 4 mJ. At this energy, a single laser pulse was sufficient for full-thickness ablation of TM. Laser energy was delivered through a 200-μm diameter optical fiber held in apposition to the tissue sample, which was immersed in physiologic saline. High-speed photography of the resultant steam bubbles also was performed. Light microscopy and scanning electron microscopy of TM ablated at 50 μs revealed the greatest variability in size (0–140 μm) of the full-thickness ablated areas and demonstrated blast effects, tissue shredding and ?10 μm thermal damage. At 150 μs, the full-thickness ablated areas were more consistent in size (115–120 μm), showed no blast effects and 10 to 20 μm thermal damage. At 250 μs, the largest ablations were found (180–220 μm) and showed no blast damage; however, a significant amount of thermal damage (?50 μm) was evident. The steam bubbles produced by the laser energy were largest at 50 μs and did not begin to collapse until well over twice the original pulse interval. At 150 and 250 μs, the steam bubbles were successively smaller and dissipated at the end of the laser pulse. In single pulse Er:YAG photothermal laser trabecular ablation, a pulse width (total energy of 4 mJ) around 150 μs appears to be optimal. The resultant acoustic shock wave from steam bubble formation is smaller, its duration does not exceed the laser pulse width and tissue thermal damage is minimal. © 1993 Wiley-Liss, Inc.     

XeCl laser ablation of atherosclerotic aorta: optical properties and energy pathways.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.     

Excimer laser irradiation of non-calcified and calcified human atheroma (248 and 308nm wavelengths)

In order to develop a system of peripheral arterial angioplasty, we carried out an in vitro study to define the quantitative, thermal and morphological characteristics of human-atheroma ablation by excimer laser. A multigas ‘Sopra’ laser was used. The study was performed by using 248nm, krypton fluoride (KrF), then 308nm, xenon chloride (XeCl) wavelengths. The delivered energy was up to 150 mJ pulse⁻¹, pulse duration was 25ns, and the repetition rate could be adjusted to up to 20Hz. Irradiated tissue segments of the superficial femoral and external iliac arteries were obtained in man during surgical procedures and were both calcified and non-calcified atherosclerotic lesions. Quantitative measurements showed a linear increase of ablated tissue mass depending on the energy delivered. For the same energy, the loss of mass was greater with the 248nm wavelength than with the 308nm. The maximum temperature rise measured at the site of irradiation was 6°C at 248nm and 25°C at 308nm. Histological analysis of the irradiated segments revealed neat and precise ablation without thermal injury of adjacent tissue. At 248nm, this phenomenon was observed for calcified as well as non-calcified atheromas. It is concluded that quantitative, thermal and morphological characteristics of in vitro ablation of calcified and non-calcified human atheroma by excimer laser are compatible to clinical requirements. The results observed at 248nm were experimentally more satisfactory.     

Excimer ablation of human intervertebral disc at 308 nanometers

Excimer laser energy, which has been shown to photoablate tissue at a precisely controllable rate with minimal thermal damage, was applied to human intervertebral disc in an effort to develop a technique for percutaneous discectomy. Cadaveric samples of human disc were used. Excimer laser energy was produced by a XeCl, magnetically switched, long-pulse laser working at 308 nm, 20 Hz. Annulus tissue of approximately 1 mm thickness was placed in contact with the output tip of a 400 microns core diameter quartz fiber, and measurements of ablation rate were made at different radiant exposures. Ablation rates were found to vary linearly with radiant exposure, from 0.7 micron/pulse at 10 mJ/mm2 to 11.0 microns/pulse at 55 mJ/mm2, with a correlation coefficient of 0.984. Threshold radiant exposure, calculated by extrapolation, was found to be about 7 mJ/mm2. Histologic analysis showed a minimum of thermal damage in these specimens, and when ablated with modification to maintain constant fiber-tissue contact, thermal injury was nearly absent, as compared to samples ablated with Nd:YAG through a contact probe. Thermographic analysis, performed using the AGA 782 Digital Thermography system, showed increasing temperature with increasing radiant exposure, with a maximum temperature of 47.2 degrees C at 55 mJ/mm2. In that precise tissue ablation was demonstrated with minimal generated heat, and excimer energy at 308 nm is transmissible through fiber optics, excimer holds great promise for the development of a percutaneous discectomy technique.     

Pulsed Er:YAG- and 308 nm UV-excimer laser: an in vitro and in vivo study of skin-ablative effects

Using a pulsed XeCl excimer laser (308 nm) and a pulsed Er:YAG laser (2,940 nm), we investigated skin ablation as a function of pulse number, radiant energy, and repetition rate. In vitro analysis of lesions performed in freshly excised human skin were consistent with in vivo results obtained from experiments on pig skin. Pulsed 308 nm laser radiation caused considerable nonspecific thermal tissue injury followed by an inflammatory reaction and impaired healing of lesions in vivo. These findings were especially pronounced with higher repetition rates, which would be required for efficient destruction of larger lesions. On the other hand, the 2.94 microns Er:YAG laser radiation produced clean and precise lesions with only minimal adjacent injury. In vivo skin ablation caused intraoperative bleeding with deeper penetration. The Er:YAG laser offers a promising surgical tool for careful removal of superficial epidermal lesions, if higher repetition rates, and an appropriate laser beam delivery system are available for clinical use.     

Studies in fiber guided excimer laser surgery for cutting and drilling bone and meniscus.

Our experiments on transmitting high-power excimer laser pulses through optical fibers and our investigations on excimer laser ablation of hard tissue show the feasibility of using the excimer laser as an additional instrument in general and accident surgery involving minimal invasive surgery. By combining XeCl-excimer lasers and tapered fused silica fibers we obtained output fluences up to 32 J/cm2 and ablation rates of 3 microns/pulse of hard tissue. This enables us to cut bone and cartilage in a period of time which is suitable for clinical operations. Various experiments were carried out on cadavers in order to optimize the parameters of the excimer laser and fibers: e.g., wavelength, pulse duration, energy, repetition rate, fiber core diameter. The surfaces of the cut tissue are comparable to cuts with conventional instruments. No carbonisation was observed. The temperature increase is below 40 degrees C in the tissue surrounding the laser spot. The healing rate of an excimer laser cut is not slower than mechanical treatments; the quality is comparable.     

Experimental study of application of excimer laser to laser angioplasty

The possible application of excimer laser to laser angioplasty was studied. In the first experiment, the ablative effects of excimer laser at wavelengths of 248 nm and 308 nm on the pig myocardium were examined in vitro at an air-tissue interface. Crater depth increased with total delivered energy and energy per pulse. Very clear cuts could be observed by histological examination. There was no evidence of thermal damage at a wavelength of 248 nm, at 10 pps. Above 10 pps, a thin bordering zone of suspicious thermal damage was noted with the wavelengths of 248 nm and 308 nm. Thermal damage increased with pulse repetition rate. In the second experiment, the effects of excimer laser irradiation on blood were examined. Five vials, each of which contained 3 ml of blood, were exposed to 37.5 mJ laser beam at 10 pps in repetition rate for 10, 20, 30, 40, 50 seconds. One vial was left untreated as a control. No change in hematocrit value was observed after excimer laser irradiation. In contrast, the level of plasma free hemoglobin rose progressively with each increased duration of exposure. This result indicates that the lysis of erythrocytes does not occur in the laser-exposed cells. However, the damage to erythrocyte membrane took place as it was evidenced by progressive hemoglobin leakage into plasma. In the third experiment, the excimer laser was coupled to a 400 microns quartz optical fiber and the laser energy transmitted through the fiber was measured. At a wavelength of 308 nm, pulse energies up to 9 mJ were noted at the tip of the fiber. At a wavelength of 248 nm, the fiber tip was destroyed. In the fourth experiment, acute and chronic healing responses of normal canine arteries to excimer laser irradiation were studied in 4 mongrel dogs. The artery healed completely at the 18th day after the excimer laser irradiation. There was no evidence of thrombus formation and intimal hyperplasia in these arteries. The results suggest the applicability of excimer laser to laser angioplasty.     

Effect of force on ablation depth for a XeCl excimer laser beam delivered by an optical fiber in contact with arterial tissue under saline.

The effect of force applied to a 430 micron single fiber, delivering 60 pulses of 308 nm XeCl laser radiation at 20 Hz, on the ablation depth in porcine aortic tissue under saline has been investigated. Energy densities of 8, 15, 25, 28, 31, 37, and 45 mJ/mm2 were used. Force was applied by adding weights from 0 to 10 grams to the fiber. The fiber penetration was monitored by means of a position transducer. At 0 grams, the ablation depth increased linearly with incident energy density, but the fiber did not penetrate the tissue; with any weight added, the fiber penetrated the tissue at energy densities above 15 mJ/mm2. The fiber did not penetrate during the first several pulses, possibly due to gas trapped under the fiber. After these first pulses, a smooth linear advancement of the fiber began, which lasted until the pulse train stopped. The ablation depth increased with increasing energy densities and weights. This effect was largest above 25 mJ/mm2 where the ablation efficiencies (unit mm3/J), with weights added to the fiber, were substantially larger than values found in 308 nm ablation experiments described in the literature, which were conducted with either a focused laser beam or a fiber without additional force. The results imply that in 308 nm excimer laser angioplasty, force must be applied to the beam delivery catheter for efficient recanalization, and that experiments performed with a focused beam or without actual penetration of the fiber do not represent the situation encountered in excimer laser angioplasty.     

Excimer laser ablation of fibrocartilage: an in vitro and in vivo study

To date, lasers have found only limited applications in orthopedics. We employed a 308 nm XeCl excimer laser for ablation of fibrocartilage, in order to investigate the feasibility of excimer laser assisted meniscectomy. Experiments were conducted both in vitro and in vivo. For the in vitro study, human menisci, obtained during surgery and autopsy, were irradiated via a 600 microns core fiber at radiant exposures ranging between 20 mj/mm2 and 80 mj/mm2, at 20 Hz. Ablation rate measurements and histological analysis of the samples were performed. The ablation rates were found to range from 3 microns/pulse to 100 microns/pulse depending on the radiant exposure and/or the applied pressure on the fiber delivery system. Thermographic analysis was also performed during pulsed excimer as well as CW Nd:Yag and CW CO2 laser irradiation. Temperatures were lower for excimer laser (Tmax less than 65 degrees) than CW ND: Yag (Tmax less than 210 degrees) or CW CO2 (Tmax less than 202 degrees) laser. For the in vitro study, medial meniscectomy was performed in 15 rabbits with the excimer laser and a CW Nd:Yag laser in the right and left knee respectively. Excimer laser irradiation was performed at 70 mj/mm2. Nd:Yag irradiation was performed via a 600 microns core fiber at power outputs between 20 to 40 W for 10 and 20 seconds duration. The healing response to injury was investigated by histological analysis of the menisci after 1 day, 1, 2, 4, and 8 weeks following the laser procedure. Excimer laser treated menisci showed less inflammatory reaction and noticeable repair with minimal inflammatory response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fragmentation of biliary stones with a 308 nm excimer laser

The use of a XeCl excimer laser (308 nm) for biliary stone fragmentation is reported. Laser energy is delivered via UV grade fused silica fibers to the target stones immersed in normal saline solution. Sixty biliary calculi--pigment (n = 40), and cholesterol (n = 20)--were fragmented in vitro. The total energy delivered per unit mass of the stone is kept constant. Two energy fluences (80 and 110 mJ/mm2) at two repetition rates (5 and 20 Hz) delivered through fibers of two core sizes (300 and 600 microns) are utilized to study the effect of different laser parameters on the fragmentation process. Although both pigment and cholesterol stones are susceptible to excimer laser fragmentation, higher fragmentation efficiency is obtained for the pigment stones than for the cholesterol stones. Our study suggests that higher energy fluence and larger fiber core size result in higher fragmentation efficiency for pigment stones. Fragmentation thresholds at stone surface for a variety of biliary calculi of known composition were measured. The threshold energy fluence is approximately 3 mJ/mm2 and 17 mJ/mm2 for pigment and cholesterol stones, respectively. Our study indicates that the 308 nm excimer laser may be effective as a laser lithotriptor with low threshold and good efficiency for biliary stone fragmentation.     

Disparate absorption of argon laser radiation by fibrous versus fatty plaque: implications for laser angioplasty

The thermal response of white fibrous atheromatous plaque to argon laser irradiation was compared with the thermal response of yellow fatty plaque and normal aortic wall to the same type of radiation. Samples of normal aorta, fibrous, and fatty plaque were irradiated in air with 3.5 Watts of laser power on a 2 mm spot for 5 and 10 seconds. Heterogeneous foci, each covering normal aorta and either fibrous or fatty plaque, were additionally irradiated with 7 Watts on a 1 cm spot for 30 seconds to 2.5 minutes. Tissue surface temperature was monitored during laser irradiation via a 3-5 micron infrared camera. For the 2 mm spot and 5 second exposure time, argon laser irradiation of normal aorta produced popping and surface tearing at a peak temperature of 145 +/- 10 degrees C. Irradiation of fatty plaque produced popping and crater formation at a peak temperature of 200 +/- 10 degrees C. However, fibrous plaque was nonablatively discolored by the same dose of laser radiation with a peak temperature of only 85 +/- 10 degrees C. Irradiation for 10 seconds caused crater formation and carbonization in the fatty plaque but failed to produce ablation in the fibrous plaque. Irradiation of the heterogeneous foci confirmed the disparity in the temperature attained by these two types of plaque and their degree of damage. Therefore, this study suggests that the ablation threshold for soft atheroma is strongly dependent on the optical properties of the particular type of tissue. Yellow fatty plaque preferentially absorbs argon laser radiation, but white fibrous plaque absorbs this radiation less readily than normal aortic wall.     

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

. 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 τ_P(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. Paper received 15 May 2001; accepted after revision 14 January 2002. Correspondence to: Dr Christian Apel, Department of Conservative Dentistry, Periodontology and Preventive Dentistry, University of Aachen, Pauwelsstr. 30, D-52074 Aachen, Germany. Tel.: +49 241 8089088; Fax: +49 241 8888468; e-mail: capel@post.klinikum.rwth-aachen.de