Thermal Damage Zone Research Articles

Ablation Dynamics in Laser Sclerostomy Ab Externo by Means of Pulsed Lasers in the Mid-Infrared Spectral Range

Sclerostomy ab externo with pulsed laser systems is currently in phase II clinical trials. The authors investigated the ablation dynamics of tissue treated with pulsed laser systems in the mid-infrared range to estimate the extent of thermo-mechanical damage to the sclera and the anterior chamber. Freshly harvested porcine eyes were used. A bare 400-micron fiber in direct contact with tissue was used for fistulization. Polarization light microscopy, fast-flash photography, as well as optical and acoustic transients were performed for analysis. Substantial mechanical tissue deformation and dissections were found during pulsed laser ablation. The mechanical damage range within tissue far exceeds the pure thermal damage zone. Aspheric cavitation bubbles of up to 3 mm in length penetrate the anterior chamber after perforation. The cavitation demonstrates a significantly larger time constant in tissue than in water. Early fistula occlusions due to iris adherences may be attributed to iris trauma caused by cavitation. In response to the findings of this study, the authors propose an automatic feedback system to control the ablation process and minimize secondary ocular tissue effects. With respect to the overall damage zones, a new continuous-wave, mid-infrared diode laser system seems to be superior to pulsed laser systems.

Laser irradiation of bone. I. An in vitro study concerning the effects of the CO2 laser on oral mucosa and subjacent bone.

The purpose of this study was twofold: first, to evaluate the histologic effects of CO2 laser irradiation on biopsies of porcine oral mucosa and underlying bone under conditions that simulate the applications of the laser during gingival surgery; and second, to evaluate the histologic effects on cortical bone following irradiation with increasing energy densities. Specimens consisting of mucosa and underlying bone were subjected to multiple passes of the laser beam in the same line of incision at energy densities ranging from 240 to 1,032 J/cm2. A second group of specimens consisting only of cortical bone was irradiated by a single pass of the laser at energy densities ranging from 40 to 2,062 J/cm2. In both groups the mean depth of ablation, width of surface damage, and widths of the zones of thermal necrosis and thermal damage were determined. Results showed a direct correlation between increasing energy density and/or number of energy beam passes and increasing depths of ablation and widths of surface damage. Further, more than three passes at 1,032 J/cm2 penetrated the mucosal layer to involve underlying bone. The mean depth of ablation for bone specimens following a single pass of the energy beam ranged from 0.02 mm at 160 J/cm2 to a maximum of 0.75 mm at 2,062 J/cm2. Using those energy densities most common to oral soft tissue surgery, the mean depth of ablation in bone specimens ranged from 0.17 mm at 240 J/cm2 to 0.28 mm at 640 J/cm2 to 0.35 mm at 1,032 J/cm2. All specimens regardless of tissue composition, energy density, or number of energy beam passes exhibited a distinct layer of residual carbonized tissue, a zone of thermal necrosis characterized by tissue coagulation, and a zone of tissue exhibiting thermal damage.

LASER TREATMENT OF WARTS AND OTHER EPIDERMAL AND DERMAL LESIONS

The CO2 laser is a versatile and effective tool for the treatment of warts and various other epidermal and dermal lesions where there is no easily targeted chromophore other than water. The development of high peak power, short-pulse, or rapidly scanned resurfacing CO2 lasers has significantly improved the safety and efficacy of using the CO2 laser. Many lesions amenable to CO2 laser vaporization can be treated by other far less expensive treatment modalities, however, and it is the laser surgeon's responsibility to use the CO2 laser only in cases in which it is demonstrably the best treatment option. The pulsed dye laser may replace the CO2 laser for the treatment of recalcitrant warts if the impressive early cure rates reported are borne out over time. Newer laser systems such as the Er:YAG laser with its extremely small zone of thermal damage may supplant the CO2 laser in the treatment of other epidermal and dermal lesions in the future.

Experimental in vivo fenestration of guinea pig cochlea using 2.79 Μm laser radiation

Erbium-YSGG laser systems are promising tools in ear, nose and throat (ENT) surgery. The high absorption in biological tissues, resulting in precise tissue ablation with minimal thermal tissue damage, and the possibility to guide the radiation through optical fibres make the 2.79 Μm wavelength a favourite for microsurgery. In order to simulate the fenestration of the human stapes foot plate required for prosthesis implantation when treating otosclerosis, five guinea pig cochleae were irradiated in vivo until perforation was achieved. The laser-induced temperature rise and pressure transients evoke activity in the inner hair cells that was investigated by micro-iontophoresis. Perforation of the cochlea bone (hole diameter of 350 Μm) can be performed with a few laser pulses and high precision with a thermal damage zone of<100 Μm. The bone ablation rate is 10 ± 2 Μm pulse-1 at a radiant exposure of 12 J cm-2. The functionality of the afferent inner hair cells in the guinea pig cochlea was verified before and after laser treatment using glutamate receptor agonists AMPA and NMDA. For the above selected laser parameters, the induced 15-min enhanced activity was blockable with the specific reversible AMPA and NMDA antagonists CNQX and AP-7. Micro-iontophoresis confirms the reversibility of cochlea functionality after its perforation with Er-YSGG laser pulses. A limit of radiant exposure around 12 J cm-2 is found for safe fenestration. It is demonstrated that the Er-YSGG laser is a precise and safe instrument whilst still using adequate laser parameters. On the other hand, this study demonstrates the potential of uncontrollable and unintended induced damage, resulting from vapour channel formation in the perilymph, if a high laser radiant exposure is applied.

Effects of a rapidly scanned carbon dioxide laser on porcine dermis.

This study systematically examined the effect of varying continuous-wave carbon dioxide laser scanning parameters on the resultant tissue effects. The effects of varying scanning speed, laser power, and laser beam diameter were assessed. Residual thermal damage at the center of the crater was approximately 120 microm independent of dwell time and laser irradiance. However, thermal damage zones along the sides of the ablation crater increased as laser dwell times exceeded 50 msec. This study demonstrates that under appropriate conditions a scanned continuous-wave carbon dioxide laser can ablate tissue with a zone of residual thermal injury less than 200 microm, making it useful for cutaneous surgery and the debridement of burn wounds before skin grafting is performed.

Experimental Erbium: YAG Laser Photoablation of Trabecular Meshwork in Rabbits: An in-vivo Study

Photoablative laser trabecular surgery has been proposed as an outflow-enhancing treatment for open-angle glaucoma. The aim of the study was to investigate the time course of repair response following low-thermal Erbium: YAG-laser trabecular ablation.In 20 anaesthetized rabbits gonioscopically controlled ab-interno photoablation of the ligamenta pectinata and underlying trabecular meshwork (TM) was performed with a single-pulsed (200 μs) Erbium: YAG (2.94 μm) laser. The right eye received 12–15 single laser pulses (2 mJ) delivered through an articulated zirconium fluoride fiberoptic and a 200 μm (core diameter) quartz fiber tip, the left unoperated eye served as control. At time intervals of 30 minutes, 2, 10, 30, and 60 days after laser treatment, eyes were processed for light- and scanning electron microscopy.The applied energy density of 6.4 J cm−2resulted in visible dissection of the ligamenta pectinata and reproducible microperforations of the TM exposing scleral tissue accompanied by blood reflux from the aqueous plexus. The initial ablation zones measured 154±36 μm in depth and 45±6 μm in width. Collateral thermal damage zones were 22±8 μm. At two days post-operative, ablation craters were still blood- and fibrin-filled. The inner surface of the craters were covered with granulocytes. No cellular infiltration of the collateral thermal damage zone was observed. At 10 days post-operative, progressive fibroblastic proliferation was observed, resulting in dense scar tissue formation with anterior synechiae, proliferating capillaries and loss of intertrabecular spaces inside the range of former laser treatment at 60 days post-operative. Trabecular microperforations were closed 60 days after laser treatment in all rabbits. IOP in treated and contralateral eyes did not significantly change its level during whole period of observation. Low-thermal infrared laser energy with minimal thermal damage to collateral structures could not effectively prevent early scarring of trabecular surgery in rabbits.

Diode Laser-Pumped, Frequency-Doubled Neodymium:YAG Laser Peripheral Iridotomy

The solid-state, continuous-wave, frequency-doubled neodymium: yttrrium-aluminum-garnet (Nd:YAG) laser pumped by a diode laser has several advantages, including air cooling, higher electrical to optical efficiency ratios, portability, and the use of standard 110-V AC electrical service. The authors wanted to evaluate the use of the frequency-doubled Nd:YAG laser for peripheral iridotomy and to compare the tissue interactions of this laser with those of the argon laser. The authors developed a diode laser-pumped, solid-state, and portable frequency-doubled Nd:YAG laser with a wavelength of 532 nm. The effects of peripheral iridotomy with the frequency-doubled Nd:YAG laser and the argon laser were evaluated in pig eyes in vitro and in rabbit eyes in vivo. Specimens were prepared for light microscopy and scanning electron microscopy. The frequency-doubled Nd:YAG laser successfully created patent iridotomies in all animal eyes treated. The following parameters were used to create penetrating burns: duration of 0.1 second, spot size of 100 microns, and power of 500 mW. In rabbit eyes, the mean number of pulses (P = .16) and the total energy required (P = .21) for iridotomy were not significantly different for the argon laser compared with the frequency-doubled Nd:YAG laser. Gross and histologic evaluation showed similar thermal effects in iris tissues for both the frequency-doubled Nd:YAG laser and the argon laser. The mean zone of thermal damage was 178 +/- 19 microns for the frequency-doubled Nd:YAG laser and 163 +/- 24 microns for the argon laser (P = .14). Scanning electron microscopy showed less disruption of the surface of the lesion for the frequency-doubled Nd:YAG laser compared with the argon laser. Successful peripheral iridotomy can be performed with the frequency-doubled Nd:YAG laser. Coagulative effects with the frequency-doubled Nd:YAG were similar to those with the argon laser, and the thermal damage zones were comparable in size.

Erbiumr:YAG Laser Sclerectomy With a Sapphire Optical Fiber

Laser sclerectomy may offer advantages to conventional glaucoma filtering surgery by minimizing conjunctival manipulation and subsequent subconjunctival scarring and by providing easier access to difficult locations. It has been theorized that minimizing collateral thermal damage may enhance the success rate and reduce complications associated with laser sclerectomy. The thermal damage induced by the pulsed erbium:yttrium aluminum garnet (Er:YAG) laser is notably less than that of other laser modalities, including neodymium:YAG (1.06 microns), Er:YSGG (2.79 microns), holmium: YAG (2.10 microns), and holmium: YSGG (2.10 microns). A major obstacle to the clinical use of the Er:YAG laser has been the lack of an efficient and reliable delivery system. The single-crystal sapphire optical fiber has an acceptable attenuation rate and favorable characteristics for delivery of the Er:YAG wave-length in a clinical setting. An Er:YAG laser (2.94 microns) focused into a 300-micron, single-crystal sapphire fiber was used to create ab-externo sclerectomies with varying energy levels and pulse rates in each eye of six anesthetized rabbits and six human cadaver eyes. Specimens then underwent histopathologic analysis and determination of the thermal damage zone. For the rabbit sclerectomies, there was a significant positive correlation between energy per pulse and the diameter of the thermal damage zone, which averaged 22.0 +/- 12.7 microns for all energy levels. For the human sclerectomies, a positive correlation existed between the total energy delivered (mJ/pulse x total pulses) and the thermal damage zone, with the mean thermal damage zone, being 25.0 +/- 9.0 microns. The Er:YAG laser with a sapphire optical fiber delivery system is an effective means of creating ab-externo sclerectomies with minimal thermal damage.

Fast and effective skin ablation with an Er:YAG laser: determination of ablation rates and thermal damage zones.

Er:YAG lasers are known to superficially ablate skin and other tissues with minimal thermal coagulation zones. The ablation efficacy and thus the clinical applicability of these lasers, however, was limited due to small beam diameters and repetition rates. Aim of this study was to determine the ablation efficacy and the amount of thermal damage with a new high-power high-repetition-rate Er:YAG laser and to find optimal treatment parameters for skin ablation. In vitro and some in vivo ablation trials on human skin were performed with the Er:YAG laser (MCL 29, Aesculap-Meditec, Heroldsberg, Germany, 2.94 microns, max. 500 mJ per pulse, 250 microseconds pulse length, 3 or 4 mm spot size, repetition rate 7-10 Hz) and evaluated microscopically. The ablation threshold was around 1.6 J/cm2. The ablation rates increased linearly with the fluence, and the above-threshold ablation efficacy was around 2.5 microns per pulse per J/cm2, leading to ablation velocities of 70-100 microns per second and higher. With increasing pulse numbers applied to one tissue spot, the ablation per pulse decreased significantly. The amount of thermal damage was clearly dependent on the number of pulses applied (around 25 microns with < 10 imp., up to 100 microns with 40 imp.), whereas higher fluences increased the coagulation zones only minimally. The in vivo trials confirmed these results: overlapping pulses in the 4 J/cm2-range, applied in a sweeping motion, proved optimal for an efficient skin ablation with a smooth resulting surface and a thermal damage zone not exceeding 50 microns. The high power and the high repetition frequency make this laser a fast and effective tool for skin ablation without increasing the thermal damage, but the ablation remains limited to the superficial dermis, since hemostasis cannot be achieved due to the absence of coagulation.

Laser skin resurfacing.

There has always been interest in looking younger, but recently there seems to have been an explosion of public interest in facial rejuvenation. Physicians have been treating photodamaged skin for many years by removing the epidermis and a variable thickness of dermis with dermabrasion or chemical peels, with the expectation that reepithialization and collagen remodeling will result in a more youthful appearance. With the recent development of short-pulsed high-peak power and rapidly scanned carbon dioxide (CO2) lasers, the ability to remove photodamaged skin in a precise and reproducible manner while leaving behind a narrow zone of thermal damage has been achieved. This development has generated tremendous interest in laser skin resurfacing as a technique to reverse photoaging.

Experimental and clinical results of fiberoptic argon laser stapedotomy.

Since 1992 a small air-cooled opthalmological argon laser (Argus system, 3 W max.) equipped with a fiberoptic microhandpiece has been used for stapedotomy at the Inselspital, Berne. The microhandpiece has been developed especially for otological purposes in our electronic laboratory. In order to measure the effect of argon laser pulses applied through the handpiece to the ear, we performed temperature measurements in a saline-filled inner ear model by using ultrathin (2 microns thick), ultrafast (4 ns) thermosensitive rhodamine-coated polyurethane films. Multiple laser pulses of 1-2.5 W and 0.1 s duration - as used in clinical applications - produced a temperature elevation of about 1 degree C in the liquid of the inner ear model. The local laser effect was then examined histologically on the isolated stapes. The thermal damage zone around the stapedotomy perforation was limited to about 100 microns. In a clinical study we compared the results of argon laser stapedotomy (n = 54) with those of a skeeter microdrill stapedotomy (n = 29). Substantial hearing gains were found in all cases in both groups. In the laser stapedotomy group the mean residual air-bone gap (0.5-2 kHz) was 10 dB or less in all cases but one. Inner ear function remained unchanged except for a 40-dB loss at 4000 Hz in one case. Transient vertigo with nystagmus occurred in one case. Facial nerve dysfunction did not occur in any patient. The most important advantage of the laser found was the absence of mechanical trauma to the stapes. Stapes luxation and a floating footplate were avoided. In contrast, thick footplates were more easily perforated with the skeeter. Use of an argon laser equipped with a fiberoptic microhandpiece and a skeeter microdrill as needed seems particularly advantageous for stapedotomy.

Laser Skin Resurfacing

The zone of thermal damage can be reduced by applying the principle of selective photo thermolysis, 8 which states that selective heating can be achieved by preferential laser light absorption and heat production in the target chromophore with heat being localized to the target by a pulse duration shorter than or equal to the thermal relaxation time (time for the target to cool by 63%) of the chromophore. The thermal relaxation time for the 20 to 30 μm thickness of water-containing tissue that absorbs the CO 2 laser light is less than 1 millisecond. 9 To limit collateral thermal damage, the CO 2 laser has to be pulsed or scanned across the tissue with a tissue dwell time less than or equal to the thermal relaxation time (1 millisecond). In addition, sufficient energy has to be delivered in that short time to achievetissue vaporization with a single pulse or

A new type of CO2 laser avoids power density loss due to absorption and the blooming effect by the CO2 gas environment

The aim of this study was to compare the acute tissue effects of a standard CO2 laser (Ultrapulse 5000) with a new design (Ultrapulse 5000L) that utilizes a different carbon isotope (C13) in the rat uterine horn model. Following laparotomy, measured laser injuries were effected with the Ultrapulse 5000 or Ultrapulse 5000L lasers via a laparoscope using CO2 or air for insufflation. Serial sections of the lesions were, thereafter, obtained to evaluate depth and width of total injury, width of defect and thermal damage zone. When CO2 was used as the insufflating gas, Ultrapulse 5000L laser was associated with significantly deeper lesions compared to the Ultrapulse 5000 system for the two tested pulsed energy levels (P < 0.0001). The width of total injury and thermal damage zone were significantly less with the former laser compared to the latter. The width of the defect was, however, significantly larger with the Ultrapulse 5000L laser for the 200 millijoule pulsed energy level, whereas it was comparable for the 75 millijoule level. When air was used as the insufflation gas, all four parameters of tissue injury were comparable between the two types of laser (P > 0.05). The adverse effects on the CO2 laser beam and the resultant altered tissue effects that occur in a regular CO2 environment are avoided by the use of the Ultrapulse 5000L or an air environment.

New semiconductor laser for vitreoretinal surgery.

We investigated the potential application in vitreoretinal surgery of a CW diode laser with cutting capabilities. A semiconductor CW laser emitting 300 mW optical power at 1.94 microns wavelength was used to perform retinotomies and membrane cutting on rabbit eyes. The device was integrated into a dedicated controller. The laser radiation was delivered through a low attenuation fused silica optical fiber of 200 microns core size, terminating into a 20-gauge endo-ocular handpiece. Histological aspects of threshold lesions obtained on the rabbits' chorioretina were evaluated by light microscopy. We obtained circular and linear full-thickness retinotomies with contact and noncontact procedures using energy of 120 mJ (240 mW x 0.5 s). Using a non-contact procedure, a larger peripheral coagulation halo around the retinotomies was observed, as compared to the contact method. The adjacent zone of thermal damage ranged from 50 to 200 microns. Lower efficacy was obtained on experimentally induced epiretinal membranes, where only superficial ablation was achieved. The CW 2 microns diode laser will have a promising future in vitreoretinal surgery when a higher output irradiance is available.

Thermal collateral damage in porcine corneas after photoablation with free electron laser.

The study describes a quantitative and systematic investigation of the collateral thermal damage during infrared photoablation as a function of wavelength between 2.7 and 6.7 microns. Using the tunable Free Electron Laser at Vanderbilt University, Nashville, Tenn, 60 freshly removed porcine cadaver eyes were irradiated at wavelengths between 2.7 and 6.7 microns, at a fluence of 1.3 J/cm2. For wavelengths where no photoablation occurred, fluence was increased to 3.5 J/cm2; pulse length (macropulse) was 4 microseconds, consisting of a train of micropulses (pulse duration 2 ps at a 2.9 GHz repetition rate). The corneal buttons were removed and stained with hematoxylin and eosin (H&E) and analyzed by histologic micrometry. Two thermal damage zones in the remaining tissue were observed: zone 1 showed superficial carbonization and measured between 2 and 4 microns; beyond, the eosinophilic zone 2 measured between 10 and 100 microns. The extent of zone 2 was inversely related to the absorption spectra of the cornea; it was minimal at the 3- and 6-micrometer water absorption bands and maximal at minimal target absorption. The results correlated well with a model of the ablation process. The study provides a systematic and predictive element for the determination of collateral thermal adverse effects; it does not yet include pulse length variation as a determining factor.

Does the loop electrosurgical excision procedure adversely affect the histopathological interpretation of cervical conization specimens?

To assess the suitability of conized specimens obtained by loop electrosurgical excision procedure for histopathological interpretation. We evaluated the histological pictures of 215 tissue sections obtained by loop electrosurgical excision procedure. These sections came from 32 cases of patients with various degrees of cervical intraepithelial neoplasia. All women included in our study had a satisfactory colposcopy and no cytological or colposcopic evidence of invasive cancer. We quantified the thickness of thermal damage in the tissue sections using a stage-mounted, calibrated grid microscope. At its greatest extent, thermal damage occurring next to incision lines ranged from 160 to 520 mu (mean 262, SE 14 mu). Two different zones of thermal damage were produced by LEEP: the carbonization and coagulation zones. The carbonization zone was located at the outermost layer and was very thin, measuring from 10 to 30 mu in depth. The coagulation zone was adjacent to the carbonization zone, was eosinophilic, and was significantly deeper than the carbonization zone at its points of greatest thickness (150 to 500 mu; p < 0.0001, Student's t-test). The depth of the coagulation zone correlated significantly with the depth of the carbonization zone (p = 0.041, least linear correlation). Tissue distortion was present in 53% (17/32) of the cases, and appeared only in the coagulation zone. Tissue structure from the diseased portions of the conized specimens was generally well preserved. The area of thermal damage was limited and thus did not result in diagnostic problems. We conclude that LEEP is a reliable method for obtaining tissue samples for histopathological examination.

The effect of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue.

A thermal model to predict the effect of laser parameters on the zone of thermal injury produced by laser ablation of biological tissue is presented. The model suggests that the Péclèt number based on the optical penetration depth of laser radiation is the key parameter in determining the resulting zone of thermal injury. We show that the zone of thermal injury is minimized for Péclèt numbers greater than one since the transport of energy via conduction beyond the ablation front is minimized. We also show that for Péclèt numbers less than one, larger zones of thermal damage are unavoidable regardless of the laser pulse duration. The predictions of the model are compared with data available in the literature. Deviations between the model predictions and published data are discussed and the potential effects of the model assumptions, optical scattering, pyrolysis, temporal pulse shape, pulse duration, irradiance and pulse repetition rate are explored.

エキシマレーザーと生体脂肪

Effects of ultraviolet laser irradiation on biological lipid have been studied for the medical application. A suet has been adapted as a biological lipid. Drilling depths of a suet for ArF, KrF and XeCI excimer lasers have been measured. Drilling ability for KrF excimer laser was remarkably better than that for other lasers. In addition, drilling without thermal disturbance was found in the low repetitive irradiation, suggesting that the dominant process on the drilling was the photochemical effect. In ArF excimer laser irradiation, the thermal damage zone was created by the thermal effect due to the nonradiative transition. Denaturation of a suet after excimer laser irradiation is examined from the ultraviolet absorption spectra. The absorption owing to the conjugate double bond in a suet was influenced by ArF and KrF laser irradiations.

Effect of varying laser parameters on pulsed Ho:YAG ablation of bovine knee joint tissues

We investigated the effect of varying laser parameters on ablation of fresh bovine knee joint tissues (fibrocartilage, hyaline cartilage, and bone) with a free-running Ho:YAG laser. Ablation rate was measured in saline for each tissue type as a function of laser fluence (160-950 J/cm2), pulse width (150-450 microseconds, full width at half maximum), and fiber core diameter (400 and 600 microns). A weight that was attached to the fiber end was used to exert a constant pressure of 40 g/mm2 to the tissue underneath to ensure a constant contact between the fiber tip and the tissue throughout the ablation process. All tissues could be efficiently ablated, and the ablation rate increased linearly with the fluence. Change in laser pulse width and fiber core diameter resulted in minor variations in the ablation rate for fibro- and hyaline cartilage. On the other hand, use of longer pulses and/or the larger fiber significantly accelerated bone ablation. Histology analysis revealed that zones of thermal damage in Ho:YAG irradiated bovine knee joint tissues differed by tissue types and ranged between 100 and 400 microns, but were independent of fluence, pulse width, and fiber size within the range tested.

Root resection of endodontically treated teeth by erbium: YAG laser radiation

Root resection was performed on endodontically treated extracted human teeth exposed to pulsed erbium:YAG laser radiation at energy levels between 50 and 90 mJ/pulse in wet and dry fields. The lased surfaces were examined by optical and scanning electron microscopy. The smooth, clean resected root surfaces, devoid of charring (in wet fields), indicate that the erbium:YAG laser has a potential application in endodontic periapical procedures. In addition, researchers have shown that the zone of thermal damage and carbonization following erbium laser exposure on soft tissues and bone is appreciably less compared with other lasers, and therefore its use may result in improved healing and diminished postoperative discomfort.