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

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

Early and late healing responses of normal canine artery to excimer laser irradiation

Acute in vitro histologic studies have shown that the pulsed xenon chloride excimer laser causes precise microablation without the surrounding thermal tissue injury associated with frequently used continuous-wave lasers such as the argon, carbon dioxide, and neodymium:yttrium aluminum garnet lasers. However, the in vivo healing response of artery wall to excimer laser injury is not known. Accordingly, a xenon chloride excimer laser (308 nm, 40 nsec pulse width, 39 mJ/mm2/pulse) was transmitted via a 600 micron fused silica fiber to create 420 craters of varying depths (30 to 270 micron) in 21 normal canine femoral and carotid arteries. At 2 hours, 2 days, 10 days, and 42 days after excimer laser ablation, the artery segments were perfusion fixed in situ and analyzed by light, scanning, and transmission electron microscopy. At 2 hours, craters were covered by a carpet of platelets and entrapped red blood cells. Fibrin and exposed collagen fibers were seen at the crater base. There was a sharp demarcation of the crater-artery wall interface without lateral laser tissue injury. At 2 days, adherent platelets persisted with thrombus covering the base of the craters. Early healing responses were present, consisting of polymorphonucleated leukocytes and new endothelial cells, which extended over the crater rims. At 10 days, no thrombi were seen, and healing continued with almost complete reendothelialization. Macrophages, fibroblasts, fibrin, and entrapped red blood cells were present below the reendothelialized surface. At 42 days, healing was complete with obliteration of the craters by fibrointimal ingrowth. The surface was completely covered by a smooth monolayer of axially aligned endothelial cells. There were no aneurysms or surface hyperplastic responses. These favorable healing responses in normal canine arteries suggest that pulsed lasers with high tissue absorption coefficients, such as the xenon chloride excimer laser, may be suitable energy sources for clinical laser angioplasty procedures. However, further studies in atherosclerotic animals are required before human clinical responses can be accurately predicted.     

Potential use of holmium lasers for angioplasty: evaluation of a new solid-state laser for ablation of atherosclerotic plaque

Tissue effects of the mid-IR Holmium laser (emitting at a wave-length of 2130 nm) were evaluated. This wavelength is attractive because it combines high water absorption and easy transmission through standard optical fibres. The laser was pulsed with pulse durations in the range of 100 microseconds and repetition rates between 2 and 6 Hertz. For all experiments a repetition rate of 2 Hertz was used. The laser beam was coupled into waterfree quartz fibers with core diameters of 200 and 800 microns with an efficiency of 70 and 80%, respectively. Ablation of atherosclerotic plaque has been performed at an ablation threshold of 10J/cm2 for the 800 microns and 40J/cm2 for the 200 microns fibre. Removal of calcified plaque was possible. Ablation efficiency increased in a non-linear fashion with increasing pulse energies. The ablation rate per pulse was approximately 2 mm at energy fluences of 1000J/cm2 for the 200 microns fibre and 1.25 mm at energy fluences of 70J/cm2 for the 800 microns fibre; a further increase in energy densities did not result in higher ablation rates. On macroscopic examination only very limited thermal injury was found in crater adjacent tissue structures. Crater edges were even and did not reveal signs of crater charring or debris in the crater lumen. However, the histologic specimens revealed zones of thermal damage extending 100 up to 1000 microns lateral into adjacent tissue. Thermal damage increased with increasing radiant exposures and depended on the medium used.     

Intravasular ultrasound can be used to evaluate pulsed laser ablation of arterial tissue

Background and Objective: Intravascular ultrasound (IVUS) has been used successfully to detect intravascular lesions. This study evaluates the ability of IVUS to detect acoustic damage to the arterial wall following high power, pulsed laser ablation. Study Design/Materials and Methods: Arterial ablation and disruption were performed in necropsy bovine aorta with a Ho:YAG laser using energy ranging from 140–720 mJ/pulse at 5 Hz. Laser energy was delivered with 2 mm diameter, multifiber over-the-wire catheters. A 20-MHz IVUS catheter was used to image the arterial damage prior to tissue fixation and morphometry. Results: IVUS images revealed ablation craters surrounded by high acoustically backscattering zones. By histology, the arteries revealed ablation craters lined with thermal coagulation surrounded by a region of dissection and vacuolization. The depth and width of the highly backscattering zones on IVUS images correlated strongly with the corresponding morphometric measurements of tissue dissection (r = 0.92, P = 0.0001 and r = 0.80, P = 0.0001, respectively). Morphometric measurements of the ablation crater depth correlated strongly with laser energy (r = 0.90, P= 0.0001), whereas crater width was not correlated with laser energy (r = 0.27, P = 0.09). Conclusion: This study demonstrates that IVUS can detect and measure the extent of arterial damage following pulsed laser ablation. This may provide a means of detecting the extent of tissue disruption and help develop approaches to reduce or prevent extensive tissue damage. © 1995 Wiley-Liss, Inc.     

In vitro Alexandrite laser angioplasty: A study on fibre conduction and tissue effects

Pulsed ultra-violet excimer laser radiation is capable of tissue ablation with only minimal thermal injury of adjacent tissue structures. Since difficult fibre optic coupling of energy was observed, alternative Q-switched laser sources capable of ablation of atherosclerotic plaque are under current investigation. To evaluate tissue effects of Alexandrite laser radiation, 160 arterial segments with macroscopic evidence of atherosclerotic disease were treated. The laser light was transmitted via silica based quartz fibres with different diameters. Using the Q-switched Alexandrite laser at the fundamental wavelength (748 nm) with a pulse duration of 300 ns the energy density threshold for tissue ablation was found to be in the range of 63 to 126 J cm^–2 using a 300m fibre. On macroscopic examination only limited thermal and acoustic injury was found in crater adjacent tissue structures. Crater edges were even and did not reveal signs of crater charring or debris in the crater lumen. However, the histological cross-sections revealed thermal injury extending from 100 up to 200m lateral into adjacent tissue. The crater margins revealed fissuring as a result of shock wave injury. Thermal damage was most evident if irradiation of atherosclerotic tissue was performed in blood.     

Defining parameters for peripheral laser angioplasty

In vitro studies are reported using a Pulsed Dye laser at wavelengths of 440, 480, 504, 560 and 590 nm, to vaporise multiple samples of yellow, fibrous and calcified plaque. The threshold for crater production at 440 nm was 5 mJ/pulse and at 590 nm 65 mJ/pulse. Crater depth was significantly deeper at the short wavelengths (440, 480 and 504 nm) than at the longer (560 and 590 nm). Light microscopy confirmed the absence of thermal damage associated with continuous wave lasers. Electron microscopy revealed smooth-contoured craters and no disruption of subcellular elements at the crater margin. Samples of thrombus- and atheroma-occluded human femoral artery were successfully recanalised at the 480 nm wavelength with atraumatic spherical-tipped and modified spherical tipped optical fibres. The advantages of pulsed laser energy in peripheral vessel recanalisation are discussed.     

Acute effects of a copper vapour laser on atheroma

The acute histological effects of a commercially available copper vapour laser (CVL) on normal and atherosclerotic arterial wall were studied. The multi-cyclic CVL produces a quasi continuous wave output at 511 nm (green) and 578 nm (yellow) [green/yellow ratio 21]. Tissue craters were produced in segments of normal and atherosclerotic human femoral artery using a maximum of 8 J of laser energy delivered with a bare 1 mm quartz fibre in contact and perpendicular to the tissue. Crater dimensions and ablation volumes were determined histologically using an optical graticule. Ablation of atheroma was almost three times more efficient than ablation of normal arterial wall (p<0.001). A narrow zone of vacuolization and coagulative thermal damage lined the crater margins suggesting a predominantly photothermal ablative mechanism. In conclusion, selective ablation of atheroma using low power copper vapour laser light is possible. The CVL is an attractive alternative to the argon ion laser because of its low running costs and applicability to other medical disciplines. Therefore, its potential application in laser angioplasty merits further study, preferably using a modified optical fibretip delivery system.     

Er:YAG laser ablation of cerebellar and cerebral tissue

. With the availability of suitable fibres, the Er:YAG laser has become an indispensable tool for invasive neurosurgical applications as a source of precise ablation. The aim of this study was to investigate the ablative effects of the Er:YAG laser on brain tissue. The response of neuronal tissue to 2.94 μm Er:YAG laser irradiation was investigated on excised rat brain specimens. Ablation craters were created in cerebral and cerebellar tissues using 0.3, 0.5 and 1.0 J single pulses of 150 μs duration. The corresponding average irradiances were 37.7 J/cm², 62.9 J/cm² and 125.8 J/cm², respectively. Craters were checked qualitatively, crater dimensions were measured and compared, and volume of ablated tissue was estimated. Laser-induced crater dimensions were found to be significantly different at different energy levels applied. Moreover, dimensions of craters on cerebral and cerebellar tissues were significantly different in terms of dimensions. We observed that with the Er:YAG laser ablation craters were created with practically no thermal damage to adjacent tissues. The differences observed in the response of cerebral and cerebellar cortical tissues were dependent on the anatomical and chemical differences. Paper received 3 August 1999; accepted after revision 26 June 2000.     

Cell vitality in cartilage tissue culture following excimer laser radiation: An in vitro examination

Excimer laser is used for cartilage debridement, although the resulting cell damage is yet unclear. For examination of cartilage survival after treatment, we used short-term tissue cultures of human joint cartilage. Specimens were treated with a XeCl-Excimer laser using different laser parameters, pulse energies, and repetition rates. Following treatment, discs were cultured for 8 days prior to examination. In contrast to the 20 μm damage zone as instant visible effect in histomorphologic examinations, we found a 0.3 mm zone in which ～ 50% of cartilage cells had morphological signs of damage on light microscopic examinations. Autoradiography revealed that cartilage cells in an 0.5–0.7 mm area surrounding the laser craters had no collagen synthesis. This examination indicates that cell damage of excimer laser is higher than expected from prior studies. © 1993 Wiley-Liss, Inc.     

Pulsed ultraviolet lasers and the potential for safe laser angioplasty

Endoscopic laser ablation of atheroma using continuous wave lasers is limited by imprecise control of thermal ablation, resulting in a crater that expands in width and depth, with thermal damage to adjacent normal tissue. We compared the gross and histologic effects of pulsed 308 mm excimer irradiation to continuous-wave Nd:YAG and Argon Ion laser irradiation, and pulsed 1,060 nm, 532 nm, 355 nm, and 266 nm laser irradiation in 205 atherosclerotic aortic segments. In contrast to the continuous-wave Nd: YAG, Argon Ion, and pulsed 1,060 nm, 532 nm, and 355 nm laser irradiation, which produced gross and histologic evidence of uncontrolled ablation, the 308 nm and 266 nm pulsed lasers induced incisions that conformed precisely to the beam configuration without gross evidence of thermal injury. The incision edges from these two lasers were histologically smooth and comparable to a scalpel incision. Our histologic findings suggest that rapid, precise endoscopic ablation of vascular and nonvascular tissue can be performed at these shorter pulsed wavelengths with very high precision with relatively little damage or risk to adjacent tissue.     

Factors influencing the ablation of atherosclerotic plaque with the argon laser

This paper describes the ablative effect of argon laser light, delivered fibre optically in vitro, on 234 segments of atherosclerotic human aorta. Variables such as energy density, type of atheroma and immersion media were taken into account. All irradiated specimens were subsequently submitted to histological examination and crater volumes in mm³ were derived from micrometer measurements made at light microscopy. Results showed: (1) a linear relationship between energy dose and crater volume in fibrous atheroma; (2) significantly greater surrounding tissue damage in the higher energy dose groups; (3) a lower dose response in calcified tissue than in fatty streaks or fibrous atheroma; (4) immersion of tissue in blood during ablation resulted in a significantly greater dose response than immersion in plasma or saline, and the corresponding surrounding tissue damage was greatest under blood. Thus, argon laser light is both effective and predictable in response when ablating atheromatous tissue, and the efficiency of the process depends on the immersion medium. The degree of surrounding tissue damage depends on the energy dose.     

Laser-assisted microsurgical anastomosis

A low power carbon dioxide laser was used to perform 212 end-to-end laser-assisted microvascular anastomoses (LAMA) of femoral arteries (mean diameter, 1.2 mm) in Sprague-Dawley rats. Eighty-two conventional microvascular suture anastomoses (CMSA) utilizing 10-0 monofilament interrupted sutures were done for comparison of techniques and wound healing. The mean duration of each anastomosis procedure was 16 minutes for the LAMA repairs, compared to an average of 27 minutes for the CMSA repairs (P less than 0.05). All anastomoses were patent at the completion of the procedure. Each laser-assisted anastomosis required an average of seven intermittent laser exposures of 0.1 to 0.3 seconds each with approximately 80 mW of CO2 (wavelength = 10.6 micron) radiation at a spot size of 150 micron. A patency rate of 95% was obtained on the LAMA vessels (202 of 212) compared to 96% for the CMSA repairs (79 of 82). A total of 14 aneurysms were noted in the LAMA group (7%) compared to 11 in the CMSA (13%). All aneurysms were in patent vessels. Histological analysis indicates that the progression of wound healing of LAMA and CMSA anastomoses follows similar paths chronologically and morphologically with increased scar tissue formation around the suture. Scanning electron microscopy confirms the comparable luminal healing of the LAMA and CMSA vessels, with complete reendothelialization occurring by 3 weeks postoperatively. The tensile strength of the LAMA repair, although low immediately after operation, is comparable to that of the intact artery at 21 days. These findings suggest that a low energy carbon dioxide microsurgical laser has potential beneficial clinical application for anastomosis of small vessels.     

The pulsed dye laser and atherosclerotic vascular disease

The use of a pulsed dye laser to ablate atheromatous tissue obtained from post-mortem human aortic specimens is reported. Laser energy was delivered with a 600 micron quartz fibre, at a wavelength of 504 nm and a pulse length of 1 microseconds. Pulse energy was varied from 30-140 mJ, producing peak pulse powers of the order of 100 kW. With these parameters the laser ablated fatty, fibrous and calcified plaques. At this wavelength atheroma is vaporized but there is minimal damage to normal vessel wall, due to preferential absorption of the laser light. Light microscopy shows that by microsecond pulsing, thermal damage to surrounding tissues associated with continuous wave lasers is avoided. Transmission electron micrographs reveal a sharp demarcation between a laser crater and the adjacent vessel wall with little ultrastructural disruption. Scanning electron micrographs show the crater walls to be smooth. The pulsed dye laser may therefore be effective in the treatment of occlusive peripheral vascular disease without undue risk of vessel perforation.     

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

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

Comparing an optical parametric oscillator (OPO) as a viable alternative for mid-infrared tissue ablation with a free electron laser (FEL)

Beneficial medical laser ablation removes material efficiently with minimal collateral damage. A Mark-III free electron laser (FEL), at a wavelength of 6.45?μm has demonstrated minimal damage and high ablation yield in ocular and neural tissues. While this wavelength has shown promise for surgical applications, further advances are limited by the high overhead for FEL use. Alternative mid-infrared sources are needed for further development. We compared the FEL with a 5-μs pulse duration with a Q-switched ZGP-OPO with a 100-ns pulse duration at mid-infrared wavelengths. There were no differences in the ablation threshold of water and mouse dermis with these two sources in spite of the difference in their pulse structures. There was a significant difference in crater depth between the ZGP:OPO and the FEL. At 6.1?μm, the OPO craters are eight times the depth of the FEL craters. The OPO craters at 6.45 and 6.73?μm were six and five times the depth of the FEL craters, respectively. Bright-field (pump-probe) images showed the classic ablation mechanism from formation of a plume through collapse and recoil. The crater formation, ejection, and collapse phases occurred on a faster time-scale with the OPO than with the FEL. This research showed that a ZGP-OPO laser could be a viable alternative to FEL for clinical applications.     

Fracture Healing in Rabbits after Osteotomy Using the CO2 Laser

Fracture healing was studied in 28 rabbits after their femurs had been osteotomized using the carbon dioxide laser. Twenty rabbits whose femurs were osteotomized by means of a Gigli saw served as the control group. Fracture healing was initially delayed in the laser cut femurs, yet after 60 days no significant difference between the groups could be detected. The initial delay was caused by thermal damage to the laser cut bone edges. Further fragmentation of the damaged bone occurred during the immediate postoperative period.     

Fracture healing in rabbits after osteotomy using the CO2 laser.

Fracture healing was studied in 28 rabbits after their femurs had been osteomized using the carbon dioxide laser. Twenty rabbits whose femurs were osteotomized by means of a Gigli saw served as the control group. Fracture healing was initially delayed in the laser cut femurs, yet after 60 days no significant difference between the groups could be detected. The initial delay was caused by thermal damage to the laser cut bone edges. Further fragmentation of the damaged bone occurred during the immediate postoperative period.     

Comparing the effects of root surface scaling with ultrasound instruments and Er,Cr:YSGG laser

There are several studies done to show the comparability of laser scaling and root planing with routine methods. The most suitable wavelengths for such an application are reported as 2,940 and 2,780 nm. The superficial interactions of the current wavelength with root surface is investigated in this study to compare the crater and crack formation during the procedures between ultrasound and Er,Cr:yttrium-scandium-gallium garnet (YSGG; 2,780 nm) laser-treated teeth. Thirty human teeth with calculus on their root surface, which were extracted because of the severe periodontitis, were selected for this interventional in vitro study. Calculus area were divided into two equal parts: One of them was prepared for Er,Cr:YSGG (Biolase, Waterlase, USA) laser irradiation and the other one for ultrasound treatment (Dentsply cavitron, DENTSPLY International, USA). The Er,Cr:YSGG laser was specified as follows: pulse energy = 50 mJ, power = 1 W, wavelength = 2,780 nm, pulse repetition rate = 20 pulse per seconds, tip length = 6 mm, and tip diameter = 600 microm. Nineteen of the laser samples (95%) and eight of ultrasound ones (40%) out of 20 samples in each group had craters showing a statistical significant difference (p < 0.001). The number and depth of these craters are also evaluated. Thirteen samples of the laser cases (65%) and all samples of the ultrasound group showed cracks with significant differences (p = 0.008). In addition, the number and width of cracks in both groups are reported. As a general conclusion, the laser-treated samples show more craters but less cracks.     

Laser-assisted microvascular anastomoses: angiographic and anatomopathologic studies on growing microvascular anastomoses: preliminary report

A low-powered carbon dioxide laser was used to perform end-to-end anastomoses of growing femoral arteries (mean diameter 1.6 mm) in miniature swine. Five microvascular conventional anastomoses were performed with 10-0 monofilament interrupted sutures. Nine laser-assisted vascular anastomoses were performed. The mean duration of each anastomosis was 30 +/- 3 minutes for the conventional anastomosis and 20 +/- 2 minutes for the laser-assisted vascular anastomosis (p less than 0.05). All anastomoses were patent at the completion of the procedure. Each laser-assisted anastomosis required an average of eight laser pulses of 2 to 4 seconds. After 13 weeks the external diameters of the conventional anastomoses were 1.8 mm (+15%), while the laser-assisted anastomoses averaged 3.0 mm (+81%) (p less than 0.05). All nine laser-assisted anastomoses were patent, functional, and free of stenosis compared with one out of five conventional anastomoses. Histologically the laser induced minimal or no fibrosis, allowing normal physiologic healing and growth patterns. Electron microscopy confirmed that the integrity of the arterial layers had been restored. These findings suggest that a low-energy carbon dioxide laser has potential clinical application for anastomosis of small growing vessels.     

Milliwatt carbon dioxide laser and hepatic surgery in mice: surgical technique and pathology

The milliwatt carbon dioxide laser was used to induce focal lesions and to perform wedge resections in the livers of 75 strain A mice. The procedures were feasible and well tolerated by the mice, with only one postoperative death in the wedge resection group in an early experiment. The hepatic lesions produced by the laser were characterized histologically by an inner area of vaporization, an intermediate area of coagulation necrosis, and an outer rim of cells with variable damage. The lesions healed by fibroblastic proliferation and scar formation with no hepatocytic contribution. The small vessel and bile ductule sealing effect of CO2 laser, together with the sound healing of laser-induced wounds, highlights the usefulness of this modality in liver surgery in general, and suggests its particular application in the treatment of liver trauma and a variety of hepatic focal lesions, neoplastic or otherwise.