Far-ultraviolet laser ablation of the cornea: photoacoustic studies

A comprehensive review and outlook on the experimental techniques to investigate the complex dynamics of pulsed laser ablation in liquid for nanoparticle synthesis

Pulsed laser ablation in liquid (PLAL) has been established as one of the most efficient and impactful methods for producing pure and ligand-free nanoparticles (NPs). PLAL has successfully been utilized for the synthesis of metal NPs, semiconductor NPs, ceramic NPs, and even nanocomposites. A variety of NPs, including core-shell, nanocubes, nanorods, and many other complex structures, can be synthesized using PLAL. The versatility associated with PLAL has led to the synthesis of NPs that have found applications in the field of biomedicine, sensing technology, energy harvesting, and various industries. Despite all the aforementioned advantages, there has been an ambiguity in terms of conditions/parameters for the nanoparticle synthesis as reported by various research groups. This has led to a perception that PLAL provides little or no control over the properties of the synthesized NPs. The properties of the NPs are reliant on transient dynamics caused due to a high-intensity laser's interaction with the target material. To understand the process of nanoparticle synthesis and to control the properties of NPs, it is critical to understand the various processes that occur during PLAL. The investigation of PLAL is essential for understanding the dynamical processes involved. However, the investigation techniques employed to probe PLAL present their own set of difficulties, as high temporal as well as spatial resolution is a prerequisite to probe PLAL. Hence, the purpose of this Review is to understand the dynamical processes of PLAL and gain an insight into the various investigation techniques and their data interpretation. In addition to the current challenges, some ways of overcoming these challenges are also presented. The benefits of concurrent investigations with special emphasis on the simultaneous investigation by multiple techniques are summarized, and furthermore, a few examples are also provided to help the readers understand how the simultaneous investigation works.

The Cosine Similarity Technique: A new method for smart EXCIMER laser control

PURPOSE

To introduce additional steps towards smart laser control in eye surgery, with the use of the cosine similarity technique to analyze the spectra of organic polymers obtained using non-contact photoacoustic spectroscopy (NCPAS).

METHODS

The experiments were performed with two organic polymers: polyethylene and polyamide. A 193 nm excimer laser was used for photoablation at a repetition rate of 200Hz. The resulting acoustic signal of the ablation process was recorded by a capacitor microphone and then preamplified and digitized. For each specimen, four measurements with 1000 single pulses were taken. The cosine similarity technique was then used to compare the spectra of the polymers. The performance of the discrimination technique was evaluated by receiver operating characteristic analysis.

RESULTS

It was possible to correctly recognize a material with a probability of approximately 98% using the cosine similarity technique at a laser repetition rate and recording rate of 200 Hz, which represents the acoustic signal of one laser pulse.

CONCLUSIONS

The determination of materials with the cosine similarity method (CSM) is a fast, precise and promising approach towards smart laser control. Additional steps could include the design of a database containing generic spectra, using higher repetition rates, and the combination of NCPAS results with the position of the laser beam.

The influence of Ho:YAG laser irradiation on intervertebral disc cells

BACKGROUND AND OBJECTIVE

Various types of laser have been reported for percutaneous laser disc decompression (PLDD). The aim of this study was to understand the effects on intervertebral disc cells following Ho:YAG laser irradiation, using a three-dimensional culture model, and consider appropriate irradiation conditions.

STUDY DESIGN/MATERIALS AND METHODS

Intervertebral discs from the lumbar spine were obtained from 36 female Japanese white rabbits and processed to obtain isolated cells in three-dimensional cultures. Photoacoustic and photothermal effects were investigated by irradiating three-dimensional cultures with Ho:YAG laser at 27 or 54 J. Residual cell counts after irradiation were estimated based on DNA content according to fluorometric assay. Lactate dehydrogenase levels were also investigated as a marker of damage to cell plasma membranes. Finally, proteoglycan synthesis was measured by rapid filtration assay of (35) S incorporation, as an index of matrix synthesis.

RESULTS

Residual cell count tended to be higher in the 27-J group. Plasma membrane damage was higher and remained high longer after irradiation in the 54-J group. Proteoglycan synthesis was higher in the 27-J group than in the 54-J group, with some conditions (e.g., 90 mJ/pulse condition) showing marked activation of proteoglycan synthesis maintained for a long time after irradiation.

CONCLUSIONS

Three-dimensional culture models of intervertebral disc cells are useful for clarifying relationships between cell reactions and photoacoustic and photothermal effects after laser irradiation. Total energy is closely related to optimization of irradiation conditions, which may allow optimization of cytoprotection and promotion of matrix synthesis in clinical practice.

Theory concerning the ablation of corneal tissue with large-area, 193-nm excimer laser beams

Excimer laser beams (193 nm) of uniform fluence were studied to find out why they produce corneal ablations deeper at the edge than the center. Ablation depth profiles were taken of porcine corneas, including five dehydrated samples. Hydrated corneas and polymethyl methacrylate were ablated with and without central masks. Ablation plumes were photographed. Hydrated porcine corneas showed patterns of central underablation. As the incident beam increased, the crater exhibited increasingly shallower central ablation while maintaining nearly constant depth at the edges. Dehydrated corneas did not vary significantly. Masks did not alter the depth or shape of craters near ablation edges, but depth adjacent to the images of the masks was more than twice that with no mask. Depth adjacent to the mask image was nearly the same as at the edge of the zone. The rate of change in depth with position was nearly equal in both areas. Maximum plume density was centered over the entire ablation with and without the mask. Redeposition of plume particles is not the major cause of central underablation. Propagating transverse energy from the absorption of photons by peptide bonds increases pressure on excited components within the irradiated area, increasing recombination, which raises the ablation threshold.

Rhegmatogenous Retinal Detachment After LASIK for Myopia

PURPOSE

To report the characteristics and incidence of rhegmatogenous retinal detachment in myopic eyes after LASIK.

METHODS

The medical records of 49 patients with rhegmatogenous retinal detachment after LASIK were reviewed. The incidence of rhegmatogenous retinal detachment after LASIK was determined and potential risk factors were evaluated.

RESULTS

LASIK was performed on 59,424 eyes with spherical equivalent refraction (SE) ranging from -0.75 to -26.50 diopters (D) (mean: -6.10 +/- 3.5 D). Forty-nine eyes developed rhegmatogenous retinal detachment between 1.5 and 76 months (mean: 27.3 +/- 21.7 months) after LASIK. The mean preoperative refractive error in these eyes was -8.6 +/- 3.9 D. Mean age of these patients was 38.2 +/- 11.2 years. Thirty-five (71.4%) patients were male. The cumulative incidence of rhegmatogenous retinal detachment was 0.082% (95% confidence interval [CI]: 0.061-0.109), and the yearly incidence was 0.032% (95% CI: 0.023-0.042) after LASIK. The most frequent location of the retinal breaks was the superior temporal quadrant (22.7%). Male sex, older age, and higher preoperative myopia were significantly related to the incidence of rhegmatogenous retinal detachment after LASIK (P<.001).

CONCLUSIONS

Based on the results of this study, following the treatment of high-risk peripheral retinal lesions, LASIK did not appear to be an additional risk factor for the development of rhegmatogenous retinal detachment after LASIK in our patients; however, patients should be informed of the possibility of this complication as a consequence of myopia. Patients who are male, older in age, and have high myopia preoperatively may be at increased risk.

Retinal Phlebitis After LASIK

PURPOSE

To report a case of retinal phlebitis with cystoid macular edema in both eyes 8 weeks after LASIK.

METHODS

A 30-year-old woman underwent bilateral myopic LASIK. Eight weeks postoperatively, the patient experienced blurred vision in the left and right eyes. Fundus examination showed focal whitish patches in the parafoveal and juxtafoveal areas and lack of foveal reflex in both eyes. A diagnosis of retinal phlebitis with cystoid macular edema was made, which was treated with oral corticosteroids with tapering dose.

RESULTS

Visual acuity returned to normal and the whitish fundus patches decreased in number and size in both eyes.

CONCLUSIONS

Surgeons should be aware of potential risks and retinal complications associated with LASIK.

Ultraviolet Corneal Photoablation

PURPOSE

The purpose of this article is to discuss missing information on the basic physical and biological processes of ultraviolet corneal photoablation and to evaluate its potential clinical implications.

METHODS

A physical description of ultraviolet laser corneal ablation that includes photothermal, photochemical, and radiative processes is proposed.

RESULTS

Unresolved issues include the nature of the primary ablation process, the tissue and biological effects of the photothermal and photochemical components of the interaction, and the static and dynamic absorption process.

CONCLUSIONS

A better understanding of the basic physics and biology of ultraviolet corneal photoablation may help us better understand, predict, and perhaps minimize the effect of tissue hydration, plume formation, and other factors that affect the predictability of ablation and the induced tissue damage.

Measurement of the surface temperature of the cornea during ArF excimer laser ablation by thermal radiometry with a 15-nanosecond time response

BACKGROUND AND OBJECTIVE

The purpose of this work was to develop a temperature measurement system with a nanosecond time response to monitor the transient temperature of the corneal surface during laser refractive surgery.

MATERIALS AND METHODS

Thermal radiation from the surface of the porcine cornea during ArF excimer laser irradiation was measured using a photovoltaic HgCdTe detector with a response bandwidth of 150 MHz.

RESULTS

Maximum thermal radiation occurred at 31 +/- 4 nanoseconds, which was longer than the time response of the measurement system. The temperature derived from the detected signal reached over 100 degrees C at a fluence of 80 mJ/cm(2), which was the ablation threshold, and reached 240 degrees C at a fluence of 180 mJ/cm(2).

CONCLUSION

The present system of temperature measurement with a time response of 15.7 nanoseconds revealed that the transient surface temperature of the cornea during ablation is much higher than that previously reported.

Retinal pathology occurring after excimer laser surgery or phakic intraocular lens implantation: evaluation of possible relationship

The increasing number of patients undergoing refractive surgery has led to an awareness of the potential retinal complications of these procedures. The purpose of this review is to summarize the reports of retinal pathology and myopic maculopathy that have occurred after excimer refractive surgery or implantation of phakic intraocular lenses, and to evaluate theoretical pathogenetic mechanisms. We found it reasonable to conclude that retinal detachments and macular hemorrhages are not caused by laser surgery, but are rather characteristic of the natural history in the myopic eye. However, although there is no clear-cut evidence for a cause-and-effect relationship between excimer laser surgery and retinal pathology, it is very important to inform patients that refractive surgery only corrects the refractive aspect of myopia, and that the myopia itself still has the potential for serious complications.

Stress wave amplitudes during laser surgery of the cornea

PURPOSE

To determine the stress wave amplitudes generated during photoablation of the cornea using an argon fluoride excimer laser.

DESIGN

Experimental study using porcine eyes.

METHODS

Profiles of the stress wave amplitudes and enucleated human eyes along the axis of symmetry of porcine eyes and enucleated human eyes were measured using a miniature piezoelectric transducer. The ablation parameters, fluence, and ablation diameters were varied within the range of clinical application.

MAIN OUTCOME MEASURES

Stress wave amplitudes generated during photoablation.

RESULTS

The stress waves pass through a pressure focus located in the posterior lens and anterior vitreous, where amplitudes of up to 100 atm were measured with a 6-mm or larger ablation zone. Posterior to this focus, the stress wave amplitudes rapidly decrease to less than 10 atm at the retinal site. Small diameter excimer laser spots (< or =1.5 mm) produce a declining stress wave with no pressure focus at the lens and anterior vitreous.

CONCLUSIONS

Stress waves may be potentially hazardous to anterior structures of the human eye, including the corneal endothelium, lens and anterior vitreous face. They peak at the lens and vitreous with a broad beam, but not with small spot laser ablation. At posterior retinal and subretinal structures, they may be considered harmless.

Comparison of pulsed CO2 laser ablation at 10.6 microm and 9.5 microm

BACKGROUND AND OBJECTIVE

The pulsed CO2 laser has received attention because of its successful application to dermatologic surgery and burn debridement surgery. Despite impressive results, tissue removal using pulsed CO2 laser irradiation has not been optimized. We examined the ablation processes by performing mass removal and thermal injury experiments at wavelengths where tissue water is the primary absorber (10.6 microm), and where water and collagen have comparable absorption (9.5 microm).

STUDY DESIGN/MATERIALS AND METHODS

Samples of porcine reticular dermis were irradiated with 180-ns laser pulses at either wavelength. Tissue removal was measured using a digital balance. Thermal injury was assessed using a microscope with a calibrated reticle after hematoxylin and eosin staining.

RESULTS

Tissue removal using 10.6-microm radiation resulted in a heat of ablation of 3,740 J/g, an ablation threshold of 1.15 J/cm2, and a zone of thermal injury of 53 microm. By contrast, tissue removal using 9.5-microm radiation resulted in a heat of ablation of 3,330 J/g, an ablation threshold of 1.47 J/cm2, and a zone of thermal injury of 34 microm. The differences in ablation threshold and thermal injury were statistically significant.

CONCLUSION

Pulsed CO2 laser irradiation at 9.5 microm removes tissue more efficiently and with a smaller zone of thermal injury than at 10.6 microm.

Soft-tissue effects of the holmium:YAG laser: an ultrastructural study on oral mucosa

BACKGROUND AND OBJECTIVE

The specifics of the ablation mechanism of the holmium:YAG laser remain largely unexplored. Following laser exposure to the oral mucosa of rats, the ultrastructural damage profile obtaining to varying degrees in blood vessels, erythrocytes, nerves, and muscle cells was examined. An attempt was made to relate the cytoplasmatic alterations to the tissue ablation modes of midinfrared lasers described in the literature.

STUDY DESIGN/MATERIALS AND METHODS

The biological effects of a new pulsed holmium:YAG laser (lambda = 2,120 nm) on the oral mucosa of rats were examined by light and transmission electron microscopy. Laser incisions reaching into the muscle layer were made on different sites of the tongue of white rats. Laser energy (400 mJ, 2.5 microseconds pulse, 2 Hz) was delivered to the target via 400 microns nylon fibers.

RESULTS

The fine-structural morphology of the sublingual mucosa after laser surgery of the epithelial surface revealed no carbonization layer but a 150-micron-wide zone of lacunar structures extending to the lamina propria. In the muscle cells there is partial decomposition of the cell contents resulting in the development of electron optically empty spaces within the cortical cytoplasm underneath the intact plasma membrane of the muscle cell. The organelles within the cell remain ultrastructurally intact.

CONCLUSION

These features support the assumption of an additional nonthermal holmium:YAG laser-tissue interaction.

Intraocular measurements of pressure transients induced by excimer laser ablation of the cornea

BACKGROUND AND OBJECTIVE

The evolution of pressure waves induced by argon-fluoride laser ablation of the cornea in the typical operative conditions of clinical laser keratectomy has been studied experimentally and analyzed.

MATERIALS AND METHODS

Freshly enucleated porcine eyes were irradiated at a laser fluence of 180 mJ/cm2 with various spot diameters in the range 1-6.5 mm. Pressure transients were detected by a fast rise time needle hydrophone inserted into the eyeball from the posterior pole and moved along the eye optical axis toward the cornea.

RESULTS

Pressure peaks as high as 90 bar and 50 ns pulse duration (FWHM) were measured in the anterior chamber. Observation of the pulse shape evolution during propagation put in evidence the onset of a marked rarefaction phase following the compressional spike, with intense negative peaks (up to -40 bar) located at increasing distances from the corneal surface for increasing spot diameters.

CONCLUSIONS

This behavior was explained by means of simplified models describing pressure pulse generation and diffraction effects occurring during its propagation. Implications to clinical procedures, as possible damages due to tissue stretching and cavitation formation, are also discussed.

Relative Mydriasis After Photorefractive Keratectomy

PURPOSE

To report the incidence of anisocoria after unilateral excimer laser photorefractive keratectomy (PRK) for myopia and subsequent corticosteroid therapy in a retrospective and prospective study and to explore possible etiologies.

METHODS

The horizontal pupil diameter was determined in 6 patients (6 eyes) at 21.8 +/- 12.6 months after unilateral wide-field excimer laser PRK (retrospective group) as well as in 8 consecutive patients (8 eyes) before and 3.4 +/- 2.9 months after unilateral PRK (prospective group). The Schwind-Keratom wide-field excimer laser was used. Measurements were done in an examination room using Rosenbaum card comparison pupillometry and with a Goldmann perimeter at 31.5 asb. In the prospective group, the effect of fitting a hard contact lens of zero diopter power and the application of 0.1% pilocarpine were evaluated.

RESULTS

Relative mydriasis was present in all treated eyes and the difference in pupil diameter between the two eyes measured 0.25 to 1.75 mm (retrospective group: +0.56 +/- 0.82 mm; prospective group: +0.72 +/- 0.29 mm). At the time of pupil measurement, the retrospective group had a significantly longer mean postoperative follow-up (21 mo) than the prospective group (3.4 mo) and significantly more eyes still received topical corticosteroid treatment (retrospective group, 1 of 6 eyes; prospective group, 7 of 8 eyes). The amount of anisocoria did not correlate with the applied laser energy, ablation depth, or refractive change, but showed a negative correlation with increasing time after PRK. Neither hard contact lens fitting nor pilocarpine 0.1% reduced the amount of anisocoria significantly.

CONCLUSION

Unilateral PRK with wide-field excimer laser ablation and subsequent application of topical corticosteroids regularly resulted in a relative pupillary mydriasis. Neither an altered corneal profile nor parasympathetic denervation is responsible for this. Weakening of the pupillary sphincter of the treated eye may cause this phenomenon.

Measurement of tissue absorption coefficients by use of interferometric photothermal spectroscopy

We describe a spectroscopic technique called interferometric photothermal spectroscopy (IPTS) that can measure the absorption coefficient of pulsed laser radiation in nonscattering tissue samples. The technique is suitable for measuring effective absorption coefficients from 10(3) to 10(5) cm(-1). IPTS is particularly attractive because it requires minimal disturbance of the sample. These features indicate potential use for in vivo measurements of tissue absorption coefficients. To validate the technique, the absorption coefficient of pulsed Q-switched Er:YSGG (2.79-microm) radiation in pure water was measured to be 5200 (+/-500) cm(-1) when IPTS was used, in agreement with other published values. IPTS was also used to measure the absorption coefficient of pulsed ArF excimer laser radiation (193 nm) in bovine corneal stroma (in vitro), giving a value of 1.9 (+/-0.4) x 10(4) cm(-1).

Dynamic 193-nm optical properties of water

Previous assumptions that water is not a 193-nm chromophore during ArF excimer laser tissue ablation are based on room-temperature data and ignore spectroscopic literature that suggests a strong temperature dependence of far-ultraviolet water absorption. By the use of a Q-switched Er:YAG laser as a pump source and an ArF excimer laser as a probe source, thermal generation and relaxation of 193-nm water absorption were characterized under nonequilibrium high-temperature and high-pressure conditions. At volumetric energy densities as small as 2 kJ/cm(3) relative to room temperature, the 193-nm absorption coefficient of water was measured to increase by more than 5 orders of magnitude. These results are consistent with the hypothesis that the absorption of 193-nm radiation by water may play a role in ArF excimer laser ablation of tissue.

Thermodynamic response of soft biological tissues to pulsed infrared-laser irradiation

The physical mechanisms that achieve tissue removal through the delivery of short pulses of high-intensity infrared laser radiation, in a process known as laser ablation, remain obscure. The thermodynamic response of biological tissue to pulsed infrared laser irradiation was investigated by measuring and analyzing the stress transients generated by Q-sw Er:YSGG (lambda = 2.79 microns) and TEA CO2 (lambda = 10.6 microns) laser irradiation of porcine dermis using thin-film piezoelectric transducers. For radiant exposures that do not produce material removal, the stress transients are consistent with thermal expansion of the tissue samples. The temporal structure of the stress transients generated at the threshold radiant exposure for ablation indicates that the onset of material removal is delayed with respect to irradiation. Once material removal is achieved, the magnitude of the peak compressive stress and its variation with radiant exposure are consistent with a model that considers this process as an explosive event occurring after the laser pulse. This mechanism is different from ArF- and KrF-excimer laser ablation where absorption of ultraviolet radiation by the collagenous tissue matrix leads to tissue decomposition during irradiation and results in material removal via rapid surface vaporization. It appears that under the conditions examined in this study, explosive boiling of tissue water is the process that mediates the ablation event. This study provides evidence that the dynamics and mechanism of tissue ablation processes can be altered by targeting tissue water rather than the tissue structural matrix.

Keratocyte-Populated Collagen Gel as an In Vitro Model of Excimer Laser Keratectomy

BACKGROUND

To develop an in vitro model to study the effects of excimer laser keratectomy on corneal stromal cells, we evaluated two types of collagen gel populated with keratocytes.

METHODS

Keratocyte-populated collagen gels were prepared with type I collagen in 6-well plates or in culture plate inserts, the bottom of which consisted of a nitrocellulose membrane, contained within 6-well plates. The gels were ablated by the 193-nm excimer laser, set to ablate 50, 100, or 200 microns deep, and was observed under a phase-contrast microscope for 2 days.

RESULTS

Keratocytes cultured in collagen gel developed cytoplasmic processes and formed networks of interconnected cells. Cells within the ablated area in the 6-well plates began to lose their cytoplasmic processes and became round approximately 3 hours after excimer laser ablation. These cellular changes were more prominent in the gels ablated to a depth of 200 microns. Cells outside of the ablation zones in the 6-well plates and the culture plate inserts remained intact.

CONCLUSIONS

These results suggest the use of keratocyte-populated collagen gel as an in vitro model of cellular response to excimer laser keratectomy and also suggest that gel prepared in culture plate inserts is the preferred method.

Study of corneal ablation with picosecond laser pulses at 211 nm and 263 nm

BACKGROUND AND OBJECTIVE

Corneal ablation has been studied by picosecond laser pulses in the far-UV region.

STUDY DESIGN/MATERIALS AND METHODS

Laser pulses of 25 ps duration at 211 nm and 263 nm wavelengths and a 1 kHz repetition rate have been used to ablate human and rabbit corneas. The dependence of the etch rate on laser fluence has been measured at both wavelengths. The collateral tissue damage has been investigated by electron microscopy.

RESULTS

The ablation threshold for human cornea is determined to be about 3.0 mJ/cm2 at 211 nm, while the thresholds for rabbit cornea are about 2.3 mJ/cm2 at 211 nm and 8.0 mJ/cm2 at 263 nm. The slopes of the ablation curves and the dimensions of the damage zones have also been determined.

CONCLUSION

We compare these results to the existing data on corneal ablation by nanosecond UV pulses and discuss the deficiency of the photochemical model. Experimental results are analyzed in terms of a model that features plasma ablation assisted by chromophore absorption.

Study of photoablation of rabbit corneas by Er:YAG laser

BACKGROUND AND OBJECTIVE

This work studied the ablation mechanisms of rabbit corneas by the Erbium:YAG laser. The occurrence of thermal and mechanical damages in the tissue as a function of the laser fluence was also investigated.

STUDY DESIGN/MATERIALS AND METHODS

The experiments were performed both on enucleated eyes and in vivo. An ultrafast imaging technique was used to investigate the dynamic evolution of the ablation. The treated samples underwent histological and ultrastructural study.

RESULTS

A single high fluence laser shot led to the complete removal of the epithelium by a photomechanical effect. In eyes whose epithelium was manually removed, high fluence pulses resulted in evident tears in the stroma, whereas low fluence pulses led to few microns deep incisions, characterized by limited mechanical and thermal damages.

CONCLUSION

The photomechanical action plays a significant role in the ablation of the cornea by Erbium laser. Precise control of the fluence is required to avoid cracking phenomena in the stroma.

The thermodynamic response of soft biological tissues to pulsed ultraviolet laser irradiation

The physical mechanisms that enable short pulses of high-intensity ultraviolet laser radiation to remove tissue, in a process known as laser ablation, remain obscure. The thermodynamic response of biological tissue to pulsed laser irradiation was investigated by measuring and subsequently analyzing the stress transients generated by pulsed argon fluorine (ArF, lambda = 193 nm) and krypton fluorine (KrF, lambda = 248 nm) excimer laser irradiation of porcine dermis using thin-film piezoelectric transducers. For radiant exposures that do not cause material removal, the stress transients are consistent with rapid thermal expansion of the tissue. At the threshold radiant exposure for ablation, the peak stress amplitude generated by 248 nm irradiation is more than an order of magnitude larger than that produced by 193 nm irradiation. For radiant exposures where material removal is achieved, the temporal structure of the stress transient indicates that the onset of material removal occurs during irradiation. In this regime, the variation of the peak compressive stress with radiant exposure is consistent with laser-induced rapid surface vaporization. For 193 nm irradiation, ionization of the ablated material occurs at even greater radiant exposures and is accompanied by a change in the variation of peak stress with radiant exposure consistent with a plasma-mediated ablation process. These results suggest that absorption of ultraviolet laser radiation by the extracellular matrix of tissue leads to decomposition of tissue on the time scale of the laser pulse. The difference in volumetric energy density at ablation threshold between the two wavelengths indicates that the larger stresses generated by 248 nm irradiation may facilitate the onset of material removal. However, once material removal is achieved, the stress measurements demonstrate that energy not directly responsible for target decomposition contributes to increasing the specific energy of the plume (and plasma, when present), which drives the gas dynamic expansion of ablated material. This provides direct evidence that ultraviolet laser ablation of soft biological tissues is a surface-mediated process and not explosive in nature.

Infrared laser soft tissue ablation versus ultraviolet excimer laser. Experimental introduction of the Hol:YAG-laser in oral surgery

The in vivo tissue ablation characteristics of a pulsed infrared laser (Hol:YAG, lambda = 2120 nm) and a pulsed excimer laser (XeCl, lambda = 308 nm) were studied in an animal model. Laser energy was delivered via nylon fibers for the Hol:YAG laser and via quartz fibers for the excimer laser. Laser incisions were made under precise reproducible conditions on the sublingual side of the tongue and the gingiva of white rats. Laser surgery was done at two different energy output settings for the Hol:YAG laser and at one setting for the excimer laser. Histologic studies revealed tissue defects with clean contours for both laser types with small zones of necrosis of the adjacent tissue (Hol:YAG: 180 microns to 640 microns; excimer: 40 microns to 160 microns) and without carbonization. Both laser types function on the principal of photoablation and permit excellent control of tissue ablation. Wound healing was studied over a 10-day period and showed complete wound closure by re-epithelialization. The in vivo tissue ablation characteristics and the surgical reliability of the two lasers are compared and discussed with respect to oral and periodontal surgery.

Dynamic optical properties of collagen-based tissue during ArF excimer laser ablation

Various attenuation mechanisms affecting the absorption of ArF excimer laser light in collagenous tissues have been studied. Temporal distortion of the laser pulse reflected from the cornea has been observed over a range of incident pulse fluences including the ablation threshold. Reflected pulse shortening begins near the ablation threshold and advances with increasing fluence. The measurement of laser light scattered 30 degrees off specular axis from collagen gel targets indicates that the reflected-pulse distortion is partially a result of scattering. Collagen film transmission studies show an increase in 193-nm light transmission in ablation conditions. These nonlinear attenuation mechanisms may impact significantly on the photoablation process.

Pump/probe transmission measurements of corneal tissue during excimer laser ablation

Transient changes in the transmission of a 355 nm probe pulse through corneal tissue during 193 nm ArF laser ablation have been examined. A significant decrease in collimated transmission of the probe beam was observed for time delays between 10 ns and 1 ms after the 193 nm laser pulse. At 10 ns delay the collimated probe transmission was 70% of the preablation level. Minimum collimated transmission (40%) was observed at 30 microseconds delay. Transmitted probe examination by both diode array and integrating sphere measurements indicate the observed attenuation is due to scattering of the incident probe beam and not due to absorption. The significant scattering at nanosecond delay times suggests onset of the ablation process during the ArF pulse. Scattering therefore may affect the deposition of the 193 nm radiation in the ablation target.

Pulpal effects of argon: fluoride excimer laser irradiation and acid-etching of rat molar enamel

The reaction of enamel, dentine and pulpal tissues to exposure from a laser beam has been shown to depend on the type of laser medium used. The objective of this study was to examine the pulpal response in rat molars after external enamel surface treatment with either an Ar:F excimer laser or acid-gel application. Maxillary right molar occlusal surfaces in 22 animals were irradiated (energy density = 45.0 J/cm2). Maxillary left molar occlusal surfaces were treated with 37 per cent phosphoric acid for 30 s. Untreated mandibular right molars served as controls. At two postoperative time periods (1 and 6 weeks), molars were removed, sectioned, stained (H&E) and scored. Data analysis indicated no significant difference between Ar:F irradiation and controls at 1 week. Treatment with laser or acid-etching left a similar degree of pathosis at 1 and 6 weeks. Although the Ar:F excimer laser produced a more exaggerated pulp response than controls at 6 weeks, tissue vitality was maintained. The Ar:F excimer laser may be useful for ablating vital tooth structure since pulpal tissue in rat molars exhibited no damage in response to low-power irradiation.

Excimer laser corneal ablation: absence of a significant "incubation" effect

Pulse-to-pulse consistency of excimer laser etching of cornea has been examined via two noncontact techniques: photoacoustic probe beam deflection, and time-resolved excimer pulse reflectometry. These methods clearly document the incubation phenomenon accompanying excimer laser ablation of polymethyl-methacrylate and the absence of the effect during polyimide ablation. In comparison, results for corneal ablation indicate consistent tissue etching over a train of pulses. Consequently, incubation appears to have negligible impact on corneal ablation.

Laser-induced photoacoustic injury of skin: effect of inertial confinement

Argon-fluoride (ArF) excimer laser-induced acoustic injury was confirmed by ablating the stratum corneum (s.c.) inertially confined by water in vivo. Hairless rats were irradiated through a quartz chamber with flowing distilled water or air and a 2.5 mm aperture. The laser was adjusted to deliver 150 mJ/cm2 at the skin surface for both conditions. Partial and complete ablation of the s.c. was achieved with 12 and 24 pulses, respectively. Immediate damage was assessed by the transmission electron microscopy. Partial ablation of the s.c. through air produced no damage, whereas partial ablation through water damaged skin to a mean depth of 114.5 +/- 8.8 microns (+/- SD). Full thickness ablation of the s.c. through air and water produced damage zones measuring 192.2 +/- 16.2 and 293.0 +/- 71.6 microns, respectively (P less than 0.05). The increased depth of damage in the presence of inertial confinement provided by the layer of water strongly supports a photoacoustic mechanism of damage. The damage induced by partial ablation of the s.c. provides evidence that photochemical injury is not a significant factor in the damage at a depth because the retained s.c. acts as a partial barrier to diffusion of photochemical products. Combined with our previous studies, these experiments demonstrate that pressure transients are responsible for the deep damage seen with 193 nm ablation and that photoacoustic effects must be considered when using short-pulse, high-peak power lasers.

193 nm excimer laser ablation of bone

The argon fluoride excimer laser is investigated as a cutting-ablating tool for bone surgery. Quantitative measurements are presented for various fluences of laser energy and number of pulses. Histological data are presented that demonstrate the minimal damage to the surrounding material from the laser interaction. Comparisons are made for non-decalcified and decalcified bone. The differences observed, as a function of decalcification in the fluences required for specified depth penetration, are noted and a possible explanation is suggested.

Cell selectivity to laser-induced photoacoustic injury of skin

Cell selectivity to photoacoustic injury induced by argon-fluoride excimer laser (193 nm) was studied. Rats were irradiated through air or water and a 2.5 mm aperture. The laser was adjusted to deliver 150 mJ/cm2 at the skin surface with 12 and 24 pulses. Immediate damage was assessed by transmission electron microscopy. Cell selectivity was observed in dermis and epidermis. Fibroblasts showed alteration of nuclear chromatin and cytoplasmic organelles, while some of the migratory cells adjacent to fibroblasts did not. Similar difference of damage was observed between keratinocytes and Langerhans cells in epidermis. Considering the relationship between cells and their microenvironment in tissue, this selectivity may be due to the difference of acoustical coupling of propagation of acoustic waves rather than to differential sensitivity of the cells to damage.

Pulsed CO2 laser ablation of tissue: effect of mechanical properties

The ablation rate of guinea pig skin and bovine aorta, myocardium, and liver by a CO2 laser emitting 2-microseconds-long pulses was quantified. Ablation efficiency was found to be strongly dependent upon the ultimate tensile strength of the tissue; the ablation efficiency of liver is seven times that of skin. Gluteraldehyde cross linking of skin, which is known to greatly increase tissue stiffness but not significantly affect ultimate tensile strength, did not change the ablation rate. The water content of the tissues, which largely determines the optical and thermal properties, was measured and found to vary only slightly. The results demonstrate that tissue mechanical properties are important in the interpretation and modeling of pulsed laser ablation of tissue and that variations in these mechanical properties can lead to drastically different cutting rates for different tissues.

Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage

The thermal damage caused by 2.94-micron Er:YAG laser ablation of skin, cornea, aorta, and bone was quantified. The zone of residual thermal damage produced by normal-spiking-mode pulses (pulse duration approximately 200 microseconds) and Q-switched pulses (pulse duration approximately 90 ns) was compared. Normal-spiking-mode pulses typically leave 10-50 microns of collagen damage at the smooth wall of the incisions; however, at the highest fluences (approximately 80J/cm2) tears were produced in cornea and aorta and as much as 100 microns of damaged collagen is found at the incision edge. Q-switched pulses caused less thermal damage, typically 5-10 microns of damage in all tissues.