[Biophysical bases of the effects of holmium laser on articular cartilage and their impact on clinical application technics]. / Biophysikalische Grundlagenuntersuchungen zur Wirkung der Holmium-Laserstrahlung am Knorpelgewebe und deren Konsequenzen für die klinische Applikationstechnik.

The in vitro study presented helps to clarify the biophysical mechanisms and tissue interactions of the holmium laser at the point of impact on the surface of cartilage-bone specimens investigated in different experimental settings. A striking event is the creation of a vapor bubble that opens up access for the laser beam through the fluid medium. This bubble shows a reproducible dynamic behavior function of the laser irradiance and the angle of incidence of the delivery fiber. These determine directly the amplitude of the pressure waves induced when the bubble collapses. Apart from this acoustic effect, which is correlated with epicentric histological features that can hardly be considered biologically relevant, a thermal effect is recognized that is finally responsible for the ablation and tissue damage. It induces typical histological alterations that can be observed along the laser beam axis, with a penetration function mainly of the irradiance but also of the angle of incidence. Nevertheless, at a pulse energy of 1 J and an irradiation angle of 30 degrees, the recorded overall depth of the immediate histological changes was down to 500-600 micrometers. Thus, in realistic working conditions, the damage observed after cartilage sealing with the holmium laser remains within an absolutely acceptable range. This is in agreement with the better results compared to mechanical cartilage debridement that have been reported in previous prospective clinical studies.

Effect of pulse duration on bubble formation and laser-induced pressure waves during holmium laser ablation.

BACKGROUND AND OBJECTIVE: One concern during laser ablation of tissue is the mechanical injury that may be induced in tissue in the vicinity of the ablation site. This injury is primarily due to rapid bubble expansion and collapse or due to laser-induced pressure waves. In this study, the effect of laser pulse duration on the thermodynamics of bubble formation and accompanying acoustic pressure wave generation has been investigated. STUDY DESIGN/MATERIALS AND METHODS: Q-switched holmium:YAG laser pulses (pulse duration 500 ns, pulse energy 14 mJ) and free-running holmium:YAG laser pulses (pulse duration 100-1,100 microseconds, pulse energy 200 mJ) were delivered in water and tissue phantoms via a 200- and 400-microns fiber, respectively. The tissue phantoms consisted of polyacrylamide gels with varying mechanical strengths. Bubble formation was recorded with a fast flash photography setup, while acoustic transients were measured with a needle hydrophone. RESULTS: It was observed that, as the pulse length was increased the bubble shape changed from almost spherical for Q-switched pulses to a more elongated cylinder shape for longer pulse durations. The bubble expansion velocity was larger for shorter pulse durations. Only the Q-switched pulse induced a measurable thermo-elastic expansion wave. All pulses that induced bubble formation generated pressure waves upon collapse of the bubble in gels as well as in water. However, the magnitude of the pressure wave depended strongly on the size and geometry of the induced bubble. CONCLUSION: The magnitude of the collapse pressure wave decreased as laser pulse duration increased. Hence it may be possible to reduce collateral mechanical tissue damage by stretching the holmium laser pulse.

[Photoablation of the cornea with pulsed 2790 nm ErCr:YSGG laser irradiation. Basic studies]. / Zur Photoablation der Hornhaut mit gepulster 2790 nm ErCr:YSGG-Laserstrahlung. Grundlegende Untersuchungen.

The potential of 3 microns solid-state lasers as an alternative to excimer lasers for photoablative corneal surgery was investigated. A Q-switched ErCr:YSGG laser (2790 nm, 200 ns) was used for irradiation of porcine corneas and agar-agar samples. Mechanical tissue effects (stroma, endothelium) were documented by micromorphology. Laser-induced shock-waves were analyzed by piezo-electric transducers. No sharp ablation threshold, as in excimer laser photoablation, could be determined. Energy fluences < 2 J/cm2 led to dehydration of the irradiated samples. Higher fluences are necessary for the evaporation of tissue water to be so vigorous that the tissue matrix is expelled along with the organic matrix. At high fluences, the ablation rate exceeds the absorption depth of the laser radiation (up to 25 microns/pulse). At fluences between 2.5 and 28 J/cm2 the thermal necrosis zone adjacent to the crater was 7 +/- 3 microns. The intensity of the laser-induced acoustic shock waves can peak to some hundred bar. Small gas bubbles up to 1 mm were found in the surrounding area of the ablation crater. Apparently, they were pressed between the collagen lamellas by the explosive force of the ablative process. In deep excisions (> 75%) endothelial defects underneath the beam axis could be documented. Large-area tissue ablation, with a resolution in the range of 1 micron, as necessary in myopia correction, will not be possible with the present generation of ErCr:YSGG lasers. Its high ablation rate makes this laser suitable as a cutting (astigmatism, keratoplasty, vitreous surgery) and drilling (glaucoma) device.

Q-switched CTE:YAG (2.69 microns) laser ablation: basic investigations on soft (corneal) and hard (dental) tissues.

Ablative infrared lasers either show poor transmission in optical fibers (Er:YAG: 2.94 microns; ErCr:YSGG: 2.79 microns or are characterized by potential relevant thermal side effects (Ho:YAG: 2.1 microns). The CTE:YAG laser (Cr,Tm, Er doted YAG) emits radiation at a wavelength of 2.69 microns. Efficiently high optical fiber transmission is accomplished (attenuation: < 8db/m for Low-Hydroxy-Fused-Silica (LHFS): 0.3 ppm). Since the laser can easily be run in the Q-switch mode (pulse duration: 0.5-2.5 microseconds) thermal side effects of tissue interaction were expected to be low. Laser tissue interaction was studied on soft (porcine and human cornea), as well as on hard (human dental) tissue. Histological and micromorphological examinations were performed by light microscopy and scanning electron microscopy. It was found that ablation rates in corneal tissue increased from 5 to 90 microns/pulse with increasing laser fluences (5.5-20 J/cm2). Collateral thermal damage reached as far as 20 +/- 5 microns, and was higher (up to 50 microns) when craters where processed in the contact mode using LHFS-optical fibers. In comparison to soft tissue ablation, hard dental tissue ablation showed very little increase of ablation rate (1-3 microns/pulse) when higher fluences were applied. In dental tissue processing, the ablative effect was accompanied by a luminescence, indicating the presence of plasma. We conclude that the presented CTE:YAG laser can be considered as an effective tool for a variety of laser surgical applications where high power optical fiber delivery is required and where strong thermal side effects are not desired.