Wavelength dependence of pulsed laser ablation of calcified tissue

Thermal damage and the prognostic evaluation of laser ablation of bone tissue-a review

In recent years, an increasing number of scientists have focused on conducting experiments on laser ablation of bone tissue. The purpose of this study was to summarize the prognosis of tissue and the extent of thermal damage in past hard tissue ablation experiments, and review the evidence for the feasibility of laser osteotomy in surgery. An electronic search of PubMed, China National Knowledge Infrastructure (CNKI), and Web of Science (WOS) for relevant English-language articles published through June 2023 was conducted. This review includes 48 literature reports on laser ablation of hard tissues from medical and biological perspectives. It summarizes previous studies in which the ideal ablation rate, depth of ablation, and minimal damage to bone tissue and surrounding soft tissues were achieved by changing the laser type, optimizing the laser parameter settings, or adding adjuvant devices. By observing their post-operative healing and inflammatory response, this review aims to provide a better understanding of pulsed laser ablation of hard tissues. Previous studies suggest that laser osteotomy has yielded encouraging results in bone resection procedures. We believe that low or even no thermal damage can be achieved by experimentally selecting a suitable laser type, optimizing laser parameters such as pulse duration and frequency, or adding additional auxiliary cooling devices. However, the lack of clinical studies makes it difficult to conclusively determine whether laser osteotomy is superior in clinical applications.

Application of pulsed laser ablation (PLA) for the size reduction of non-steroidal anti-inflammatory drugs (NSAIDs)

We studied the application of pulsed laser ablation (PLA) for particle size reduction in non-steroidal anti-inflammatory drugs (NSAIDs). Grinding of the poorly water-soluble NSAID crystallites can considerably increase their solubility and bioavailability, thereby the necessary doses can be reduced significantly. We used tablets of ibuprofen, niflumic acid and meloxicam as targets. Nanosecond laser pulses were applied at various wavelengths (KrF excimer laser, λ = 248 nm, FWHM = 18 ns and Nd:YAG laser, λ₁ = 532 nm/λ₂ = 1064 nm, FWHM = 6 ns) and at various fluences. FTIR and Raman spectra showed that the chemical compositions of the drugs had not changed during ablation at 532 nm and 1064 nm laser wavelengths. The size distribution of the ablated products was established using two types of particle size analyzers (SMPS and OPC) having complementary measuring ranges. The mean size of the drug crystallites decreased from the initial 30–80 µm to the submicron to nanometer range. For a better understanding of the ablation mechanism we made several investigations (SEM, Ellipsometry, Fast photography) and some model calculations. We have established that PLA offers a chemical-free and simple method for the size reduction of poorly water-soluble drugs and a possible new way for pharmaceutical drug preformulation for nasal administration.

Atherectomy using a solid-state laser at 355 nm wavelength

Peripheral arterial disease (PAD), caused by atherosclerotic processes, is allied with an increased risk of ischemic events, limb loss, and death. Recently, the use of a solid-state laser at 355 nm within a hybrid catheter was suggested for that purpose. In this work, short nanosecond pulses of a solid-state laser at 355 nm delivered through a hybrid catheter, composed of optical fibers and a blunt mechanical blade, are used to conduct a pre-clinical study and two clinical cases. The pre-clinical study consisted of an atherosclerotic calcified cadaveric leg and a porcine in vivo trial within the iliac artery, respectively. The clinical cases include chronic total occlusions with a calcified lesion. The occluded cadaveric leg is recanalized successfully and no evidence of thermal necrosis is indicated in the histopathology analysis of the porcine study. No arterial wall damage is demonstrated on the animals' treated arteries and no significant impact on blood count and biochemistry analysis is noted in the animal trial. Successful recanalization of the occluded arteries followed by balloon angioplasty is obtained in both clinical cases. Our work constitutes a proof of concept for using a solid-state pulsed laser at 355 nm in atherectomy.

Holmium: Yag Laser Angioplasty

This experimental study was designed to define the potential value of a mid-infrared holmium laser in the free running mode for angioplasty. Immediately after removal, fresh normal and diseased human cadaveric arteries were irradiated under saline with a Ho:YAG laser (wavelength 2.13 μm). The laser was pulsed at 3 Hz, 250 μs pulse width and fluences of 10 to 40 J/cm2. The laser beam was coupled to ring catheters with multiple low-OH quartz fibers. The tip of the delivery device was held in direct contact with the vessel surface with the laser beam oriented perpendicularly. Ablation of atherosclerotic plaque was accomplished at an ablation threshold of 10 J/cm2. The ablation rate was 2.1 to 8.3 μm/pulse. Removal of calcified plaque was only partially effective. There were marked thermal effects with vacuolizations extending up to 1505 ± 178 μm into the adjacent tissue. Laser light at the mid-infrared wavelength of 2.13 μm is supposed to be attractive as it is readily absorbed in water and can easily be transmitted through optical fibers. However, Q-switching seems to be essential to minimize thermal side effects and to make effective ablation of calcium possible.

In vitro investigation on Ho:YAG laser-assisted bone ablation underwater

Liquid-assisted hard tissue ablation by infrared lasers has extensive clinical application. However, detailed studies are still needed to explore the underlying mechanism. In the present study, the dynamic process of bubble evolution induced by Ho:YAG laser under water without and with bone tissue at different thickness layer were studied, as well as its effects on hard tissue ablation. The results showed that the Ho:YAG laser was capable of ablating hard bone tissue effectively in underwater conditions. The penetration of Ho:YAG laser can be significantly increased up to about 4 mm with the assistance of bubble. The hydrokinetic forces associated with the bubble not only contributed to reducing the thermal injury to peripheral tissue, but also enhanced the ablation efficiency and improve the ablation crater morphology. The data also presented some clues to optimal selection of irradiation parameters and provided additional knowledge of the bubble-assisted hard tissue ablation mechanism.

Maxwell's equations-based dynamic laser-tissue interaction model

Since its invention in the early 1960s, the laser has been used as a tool for surgical, therapeutic, and diagnostic purposes. To achieve maximum effectiveness with the greatest margin of safety it is important to understand the mechanisms of light propagation through tissue and how that light affects living cells. Lasers with novel output characteristics for medical and military applications are too often implemented prior to proper evaluation with respect to tissue optical properties and human safety. Therefore, advances in computational models that describe light propagation and the cellular responses to laser exposure, without the use of animal models, are of considerable interest. Here, a physics-based laser-tissue interaction model was developed to predict the dynamic changes in the spatial and temporal temperature rise during laser exposure to biological tissues. Unlike conventional models, the new approach is grounded on the rigorous electromagnetic theory that accounts for wave interference, polarization, and nonlinearity in propagation using a Maxwell's equations-based technique.

Near-IR imaging of Erbium Laser Ablation with a Water Spray

Near-IR (NIR) imaging can be used to view the formation of ablation craters during laser ablation since the enamel of the tooth is almost completely transparent near 1310-nm(1). Laser ablation craters can be monitored under varying irradiation conditions to assess peripheral thermal and transient-stress induced damage, measure the rate and efficiency of ablation and provide insight into the ablation mechanism. There are fundamental differences in the mechanism of enamel ablation using erbium lasers versus carbon dioxide laser systems due to the nature of the primary absorber and it is necessary to have water present on the tooth surface for efficient ablation at erbium laser wavelengths. In this study, sound human tooth sections of approximately 2-3-mm thickness were irradiated by free running and Q-switched Er:YAG & Er:YSGG lasers under varying conditions with and without a water spray. The incision area in the interior of each sample was imaged using a tungsten-halogen lamp with a band-pass filter centered at 1310-nm combined with an InGaAs area camera with a NIR zoom microscope. Obvious differences in the crater evolution were observed between CO(2) and erbium lasers. Ablation stalled after a few laser pulses without a water spray as anticipated. Efficient ablation was re-initiated by resuming the water spray. Micro-fractures were continuously produced apparently driven along prism lines during multi-pulse ablation. These fractures or fissures appeared to merge together as the crater evolved to form the leading edge of the ablation crater. These observations support the proposed thermo-mechanical mechanisms of erbium laser involving the strong mechanical forces generated by selective absorption by water.

In vitro evaluation of Nd:YAG laser radiation at three different wavelengths (1064, 532, and 355 nm) on calculus removal in comparison with ultrasonic scaling

OBJECTIVE

The aim of this study was to investigate the effectiveness of laser treatment for calculus removal at different wavelengths and compare it with that of ultrasonic scaling (US).

BACKGROUND DATA

Conventional methods of treatment of periodontal tissues can be limited by morphology, root anatomy, and the periodontal pocket. In the search for an effective therapy to achieve a biocompatible root surface, laser treatment appeared in the field of periodontology.

METHODS

Radiation from a Nd:YAG laser operating at the fundamental (1064 nm), second harmonic (532 nm) and third harmonic (355 nm) wavelengths was used for calculus removal. Environment scanning electron microscopy (ESEM) was used for the morphological analysis of the treated surfaces. The wavelength dependence of the ablation thresholds for calculus and cementum was evaluated by applying a photoacoustic technique.

RESULTS

US achieved complete calculus removal, leaving a smooth and regular surface in a more efficient way than laser treatment. ESEM examination and photoacoustic methods confirmed a nonselective removal of material that entails partial cementum ablation of the root surface with the laser treatment.

CONCLUSION

UV radiation at 355 nm can constitute an adjunctive therapy to US, by eliminating toxic agents and the contaminated layer of cementum.

Tissue ablation-rate measurements with a long-pulsed, fibre-deliverable 308�nm excimer laser

This ablation rate study has been carried out with a fibre-deliverable, 308 nm, XeCl laser, producing long pulses of 200 ns as opposed to the usual 10-20 ns. This in-depth study aimed to evaluate the ablation rates for this longer pulse length. The effects on the ablation depth of dentine, enamel, soft tissue and bone were investigated ex vivo. Radiant exposure, number of pulses, pulse repetition rate and spot size were independently varied. For all tissues, ablation depth per pulse was found to increase, initially, linearly with radiant exposure. For both dentine and soft tissue a saturation radiant exposure was determined; thereafter the ablation rate decreased. The depth per pulse increased linearly with repetition rate but decreased logarithmically with both number of pulses and spot size. The ablation depth due to a 200 ns pulse is comparable to that caused by a 10-20 ns pulse but has the advantage of fibre delivery.

High-speed photography of plasma during excimer laser–tissue interaction

During high fluence laser-tissue interaction, ablation of tissue occurs, debris is removed from the ablation site and is then ejected at high velocity. This debris may be observed as a combination of luminous plasma and non-luminous plume, both of which have the potential to shield the ablation site. This study examined the role of ablation debris in shielding the tissue and determined its effects on the ablation rate over a range of laser pulse energies, pulse repetition rates and pulse numbers for dentine; the velocity differences between hard and soft tissues were also examined. High-speed photography was carried out at up to 1 x 10(8) frames per second. A maximum velocity of 2.58 +/- 0.52 x 10(4) m s(-1) was recorded for dentine debris within the first 10 ns following ejection. The maximum duration of tissue shielding due to a single pulse, determined by attenuation of a probe beam, was found to be approximately 7 ms, approximately 80 micros of which was due to luminous plasma and the remainder due to the non-luminous plume.

Revision stapedectomy

During the past decade, the evolution of surgical techniques and philosophy for revision surgery after stapedectomy has accelerated. Lasers, new surgical techniques, and new prostheses now permit the precise identification and reliable correction of the conductive problem, while reducing the risk of postoperative sensorineural hearing loss that plagued nonlaser revision techniques.

Changes in relative light fluence measured during laser heating: implications for optical monitoring and modelling of interstitial laser photocoagulation

Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phantoms, the results demonstrate an unexpected rise in optical power transmitted approximately I cm away from the source during laser exposure at low power (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulation and possibly charring. Similar trends were observed in liver tissue, with a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagulation. At 1.5 W irradiation an additional, later decrease was also seen which was most likely due to tissue charring. Independent spectrophotometric studies in Naphthol Green dye indicate the rise in fluence observed in the heated albumen phantoms may have been primarily due to light exposure causing photobleaching of the absorbing chromophore. and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating changes in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and control of laser thermal therapy via interstitial fluence sensors.

Safety of the ArF193 excimer laser for the removal of dental plaque and calculi: an in vitro histological study

OBJECTIVE

To assay the safety of the ArF excimer laser in the integrity of human pulp elements.

BACKGROUND DATA

The use of lasers in dentistry remains controversial, in spite of their increasing application in medical practice. The main reason for this discrepancy is the frequent report of damage to surrounding tissues and the dental pulp, due to the energy transfer, from the site of laser impact. The progress made on laser technology during the last 10 years, could overcome this obstacle and allow the use of lasers in dentistry.

METHODS AND RESULTS

The present study reports the use of the ArF 193 excimer laser, under conditions of strict control of frequency and fluency, for the ablation of dental carries, plaque, and calculi, by the use of a new, articulated arm. We have tested 10 teeth, extracted for prosthetic reasons, immediately after extraction. Our in vitro results show that the ArF193 excimer laser does not produce any harm to the dental pulp (at least at the photo- or electronic microscopy level), whereas in a matter of seconds, it can be effective in removing all dental deposits. In addition, the use of the flexible articulated arm, makes this treatment comfortable and easier for both the dentist and patient.

CONCLUSION

Under a strict control of laser technology, and the use of the new articulated arm presented, the use of the ArF excimer laser in dentistry is safe and comfortable.

Holmium:YAG lithotripsy yields smaller fragments than lithoclast, pulsed dye laser or electrohydraulic lithotripsy

PURPOSE

The mechanism of lithotripsy differs among electrohydraulic lithotripsy, mechanical lithotripsy, pulsed dye lasers and holmium:YAG lithotripsy. It is postulated that fragment size from each of these lithotrites might also differ. This study tests the hypothesis that holmium:YAG lithotripsy yields the smallest fragments among these lithotrites.

MATERIALS AND METHODS

We tested 3F electrohydraulic lithotripsy, 2 mm. mechanical lithotripsy, 320 microns pulsed dye lasers and 365 microns. holmium:YAG fiber on stones composed of calcium hydrogen phosphate dihydrate, calcium oxalate monohydrate, cystine, magnesium ammonium phosphate and uric acid. Fragments were dessicated and sorted by size. Fragment size distribution was compared among lithotrites for each composition.

RESULTS

Holmium:YAG fragments were significantly smaller on average than fragments from the other lithotrites for all compositions. There were no holmium:YAG fragments greater than 4 mm., whereas there were for the other lithotrites. Holmium:YAG had significantly greater weight of fragments less than 1 mm. compared to the other lithotrites.

CONCLUSIONS

Holmium:YAG yields smaller fragments compared to electrohydraulic lithotripsy, mechanical lithotripsy or pulsed dye lasers. These findings imply that fragments from holmium:YAG lithotripsy are more likely to pass without problem compared to the other lithotrites. Furthermore, the significant difference in fragment size adds evidence that holmium:YAG lithotripsy involves vaporization.

Interaction of holmium laser radiation and cortical bone: ablation and thermal damage in a turbid medium

The ablation of cortical bone by holmium laser radiation is described by experimental values of the ablation rate, the depth of tissue damage, and the tissue temperature. An ablation model is presented on the basis of photon diffusion in a turbid medium. When this model is compared with experimental results for the ablation rate, the penetration depth is determined. The expansion of the laser-induced heat can be explained by a point heat source located in a distance beneath the surface equal to the ablation depth. The accumulation of heat as a function of the repetition rate of the laser leads to a limitation of the repetition rate. In order to avoid traumatic heat accumulation, a maximum repetition rate should not be exceeded.

Lasers in dentistry

Since the development of the ruby laser by Maiman in 1960, there has been great interest among dental practitioners, scientists, and patients to use this tool to make dental treatment more pleasant. Oral soft tissue uses are becoming more common in dental offices. The possible multiple uses of lasers in dentistry, beyond soft tissue surgery and dental composite curing, unfortunately, have not yet been realized clinically. These include replacement of the dental drill with a laser, laser dental decay prevention, and laser decay detection. The essential question is whether a laser can provide equal or improved treatment over conventional care. Safe use of lasers also must be the underlying goal of proposed or future laser therapy. With the availability and future development of different laser wavelengths and methods of pulsing, much interest is developing in this growing field. This article reviews the role of lasers in dentistry since the early 1960s, summarizes some research reports from the last few years, and proposes what the authors feel the future may hold for lasers in dentistry.

Interferometric surface monitoring of biological tissue to study inertially confined ablation

We present results from the application of laser interferometry to the study of short-pulsed laser ablation of biological tissue. The mechanical response of tissue to laser-induced stress is examined under subthreshold conditions to determine its role in initiating the ablation process. A theoretical model is developed to relate this surface displacement to the pressure within the tissue and the mechanical properties of the tissue. In the experiment, a 7.5 ns pulse of 355 nm light was used to irradiate bovine shank bone, human meniscus, and an aqueous dye solution. Interferometric monitoring of the tissue surface was used to determine its motion after laser irradiation. The surface movement of bone was qualitatively consistent with the theoretical predictions of the model. The movement of meniscus and an aqueous dye solution showed additional features that are consistent with the growth and collapse of cavitation bubbles.

Excimer laser coronary angioplasty in The Netherlands: preamble for a randomized study

The immediate outcome of ELCA by XeCl excimer laser radiation is described in 53 patients who were selected to undergo ELCA from December 1990 to September 1991 in two centers that are currently performing ELCA in the Netherlands. Immediate success rates on the basis of visual assessment of the angiogram were as follows. Laser success (> 20% reduction of diameter stenosis after ELCA alone) was observed in 77% of patients, procedural success (< 50% residual stenosis after ELCA with or without adjunctive balloon dilatation [PTCA]) in 91%, and clinical success (procedural success without clinical complications) in 83% of patients. Quantitative coronary angiography by automated contour detection was performed in 31 patients who underwent ELCA in the Thoraxcenter. The minimal luminal diameter (mean +/- SD) of the treated coronary segments increased from 0.77 +/- 0.41 mm to 1.24 +/- 0.25 mm after ELCA and further to 1.67 +/- 0.29 mm after adjunctive PTCA in 25 patients. The present experience is put in perspective of results initially reported by other centers and compared with data from multicenter registries of ELCA. Finally, a short description is given of the design of a prospective, randomized trial of ELCA versus conventional PTCA (AMRO trial).

Excimer, Ho:YAG, and Q-switched Ho:YAG ablation of aorta: a comparison of temperatures and tissue damage in vitro

The adjacent thermal and mechanical tissue damage after normal-mode Ho:YAG (pulse length = 250 mus, lambda = 2.09 mum), Q-switched Ho:YAG (pulse length = 200 ns, lambda= 2.09 mum), and excimer (pulse length = 120 ns, lambda = 308 nm) pulsed laser irradiation of human thoracic aorta samples was studied in vitro. Surface temperatures were monitored during laser irradiation with an IR camera in air or a thermocouple in saline. Histological analysis of the irradiated sites was performed to assess thermal and mechanical damage to tissue surrounding the crater. The ablation of aortic tissue with any of the lasers resulted in a temperature buildup inside the tissue; this effect was most significant for the IR wavelength. Mechanical damage was observed in all cases but was most pronounced for the Q-switched Ho:YAG laser. Excimer ablation in air left behind a smooth surface; however, under saline the result was a much rougher surface.