Mikhail Ivanenko

Osteotomy with 80-μs CO2 laser pulses: histological results

Haemostatic and aseptic effects and intricate cut geometry are beneficial aspects of non-contact laser osteotomy. Collateral thermal damage, however, has severely limited the use of conventional lasers. The purpose of this study was to test the side effects on bone after cutting it with short CO2 laser pulses and simultaneous application of a fine air–water spray. The 10.6 μm CO2 laser emitted 80-μs pulses of 46 mJ energy, f=100 Hz, focused to a spot diameter of 130 ìm. Scan rate amounted to 40 mm/s. To approximate live conditions 10 samples of cortical bone and 10 rib segments were prepared immediately after sacrificing of pigs. A reference cut with a bandsaw and three laser cuts with an increasing number of beam passes (4, 16, 64) were performed on each sample. Half of the samples were decalcified in EDTA. The others were embedded in plastic to cut non-decalcified sections. The laser incisions were not accompanied by carbonisation. The incisions with slightly convergent walls were 150 ìm wide. The depths of the cavities increased with the number of the beam passes from approximately 0.5 mm (4 passes) to 3 mm (64 passes). At the border of the incisions two narrow zones of damage were noted: an amorphous intensively stained zone of 1–3 μm width and a wider, also sharply demarcated but faintly stained zone of 7–10 μm. A broader zone of about 50 μm was characterised by empty lacunae and osteocyte damage. These effects were not predictable; intact osteocytes were also observed near to the cut surface. Polarised light microscopy showed no alterations in the inorganic structure of the bone at the cut borders. The histological results indicated only minimal damage to bone ablated at the specified parameters. The described laser procedure might have advantages over mechanical instruments.

Bone Tissue Ablation with sub-µs Pulses of a Q-switch CO2 Laser: Histological Examination of Thermal Side Effects

Abstract.The goal of this study is an in vitro evaluation of thermal side-effects by the application of short sub-µs CO2 laser pulses in combination with an air–water spray on different types of bone tissue. A mechanically Q-switched CO2 laser delivered 300 ns pulses at 9.6 µm wavelength, which were focused down to a spot size of 440 µm on the tissue (a corresponding energy density of 9 J/cm2). Bone samples (blocks from pig femur, rib, or cartilage) were moved through the beam repeatedly until 1–5 mm deep cuts were produced. An air driven water spray was applied to prevent the tissue dehydration. Subsequent visual and histological examinations revealed no carbonisation, melting traces or fissuring of the tissue. An extremely narrow, 2–6 µm thick thermally altered layer was observed at the cut border in compacta and cartilage. No accumulation of the thermal damage occurred with increasing cut depth. Laser incisions in trabecular tissue were accompanied with a 100–200 µm thick zone of thermal necrosis in bone marrow. The difference from compacta and cartilage can be explained considering the particular character of the spreading of the ablation products in the trabecular meshwork. Minor thermal side effects make the Q-switched and probably other short pulsed CO2 laser systems interesting for hard tissue surgery.

Computer-guided CO2-laser osteotomy of the sheep tibia: technical prerequisites and first results.

OBJECTIVE The purpose of this study was to examine for the first time the feasibility of performing complete osteotomy of sheep tibia using a computer-guided CO2-laser osteotome, and to examine bone healing under functional loading. BACKGROUND DATA Bone cutting without aggravating thermal side effects has been demonstrated with scanning CO2-laser osteotomy. Further research is necessary to develop a clinically usable laser osteotome, which may allow new types of bone surgical procedures. MATERIALS AND METHODS The scanning parameters for performing tibial osteotomies were determined in preliminary ex vivo trials. Osteotomies were performed in the mid-diaphysis of sheep tibia using either the prototype laser osteotome (osteoLAS, study group; n = 12), or an oscillating saw (control group; n = 12). Both groups were divided into two subgroups each (n = 6), and the two groups were sacrificed after 4 and 12 wk. Radiographs were taken postoperatively and after 4, 8, and 12 wk to compare the course of bone healing. RESULTS Laser osteotomies of sheep tibia up to a depth of 20 mm were possible without visible thermal damage to the bone. A sequential PC-controlled cut geometry with artificial widening of the osteotomy gap was required for a complete osteotomy. Both clinically and radiologically, the laser and control groups showed undisturbed primary gap healing. Bone healing was similar and undelayed after both laser osteotomy and osteotomy done by mechanical saw. CONCLUSIONS Osteotomy of multi-layered bones with a scanning CO2-laser demonstrates clinical and radiological healing patterns comparable to those seen with osteotomy done by standard mechanical instruments. It is, however, a technically demanding procedure, and complete laser osteotomies of long bones are only reasonable in bones with a diameter <20 mm, which will likely restrict the use of this technique to bones 7-10 mm thick. Through the use of computer guidance, extremely precise osteotomies and sophisticated cut geometries are possible using this technique. For practical applications, precise control of the depth of laser cutting and easier manipulation of the osteotome are required.

Hard-tissue ablation with a mechanically Q-switched CO2 laser

Bone ablation with 400 ns pulses of a mechanically Q- switched CO2 laser is reported. A miniature water spray was used, which alleviates tissue carbonization, even at high laser pulse repetition rates, and increases ablation efficiency. An ablation threshold of less than 2 J/cm2, an optimal energy density of 10 J/cm2, and a corresponding specific ablation energy of 25 - 30 J/mm3 was found for pig thighbone compacta at (lambda) equals 9.57 micrometers , and a beam waist diameter of 0.5 mm.

Laser Osteotomy with Pulsed CO2 Lasers

Non-contact laser osteotomy offers new opportunities in various surgical fields, since it allows very precise pre-programmed incisions with completely free geometry. However laser osteotomy is a demanding task, because bone is a tough composite material, which is at the same time a living tissue and sensitive to temperature increases. Besides thermal side effects, practical laser applicability was limited until now because of very low cutting rates and limited incision depths. We discuss how to overcome these disadvantages by means of an optimal arrangement of thermo-mechanical ablation with a pulsed CO2 laser and with a water-spray as an assisting media. To this arrangement belong optimal pulse duration, irradiance and radiant exposure of the laser pulses, as well as multi-pass cutting procedures. Effective ablation of hard bone tissue with minimal thermal damage is possible with relatively long CO2 laser pulses of 80 µs duration and an average laser power of up to 40 W. To overcome the depth limitation special scanning techniques, which allow deep incisions even in thick multi-layer bones in feasible irradiation times, were developed in our group.

Optimizing laser beam profiles using micro-lens arrays for efficient material processing: applications to solar cells

High power laser sources are used in various production tools for microelectronic products and solar cells, including the applications annealing, lithography, edge isolation as well as dicing and patterning. Besides the right choice of the laser source suitable high performance optics for generating the appropriate beam profile and intensity distribution are of high importance for the right processing speed, quality and yield. For industrial applications equally important is an adequate understanding of the physics of the light-matter interaction behind the process. In advance simulations of the tool performance can minimize technical and financial risk as well as lead times for prototyping and introduction into series production. LIMO has developed its own software founded on the Maxwell equations taking into account all important physical aspects of the laser based process: the light source, the beam shaping optical system and the light-matter interaction. Based on this knowledge together with a unique free-form micro-lens array production technology and patented micro-optics beam shaping designs a number of novel solar cell production tool sub-systems have been built. The basic functionalities, design principles and performance results are presented with a special emphasis on resilience, cost reduction and process reliability.

New monolithic Gaussian-to-Tophat converter with integrated Fourier function and Gaussian-to-Tophat beam splitter

Optical Gaussian-to-tophat converters (g2T) that convert a Gaussian intensity distribution into a tophat profile find growing applications in different laser processing technologies. Usually, such refractive or diffractive g2T converters comprise of two or more optical components. For example, one aspherical component to form a tophat angular distribution followed by a Fourier lens that transforms it into the desired tophat intensity distribution in the focal plane. Here we report an optical design, which combines both optical functions in a single monolithic component. The component is designed and manufactured by LIMO as a free-form profile, providing the square tophat of 100-μm width at the distance of 125 mm. Compared to the traditional g2T-converters it is much more compact, easy to adjust, and less sensitive to alignment errors. In many industrial applications, not a single but multiple tophat foci are desirable for a fast parallel processing. For such applications we have developed a Gaussian-to-Tophat beam splitter. The beam splitting is done by a refractive-diffractive high-order grating with a smooth continuous pitch profile. Thanks to the smooth profile, such a Gaussian-to- Tophat beam splitter demonstrates very high efficiency of above 95% and high homogeneity between the diffraction orders.

Spectral Analysis of the Acoustic Signal During Ablation of Biological Tissue with Pulsed CO2- Lasers

The advantages of laser osteotomy are free cut geometry and minimal thermal damage. Due to the lack of haptic feedback there is need for an alternate feedback method for accurate Laser Osteotomy. Based on the frequency analysis of the acoustic signal, generated by the ablation process, we are developing a feedback system to obtain in situ information on the ablation and for differentiation between different sorts of biological tissue. We used a pulsed slab CO2-laser (wavelength 10.6 µm, pulse length 80 µs) and piezoelectric sensors for sound detection. We studied the correlation of the ablation signal of different kinds of tissue in the frequency domain.

System development and clinical studies with a scanning CO 2 laser osteotome

Non-contact laser osteotomy brings new opportunities in maxillofacial and other surgical fields, since it allows very precise pre-programmed incisions of arbitrary geometries. Laser osteotomy is however difficult, because bone is a tough composite material, which is at the same time sensitive to a temperature increase. Besides thermal side effects, practical laser applicability was limited until now because of very low cutting rates and limited incision depths. We discuss how to overcome these disadvantages by means of an optimal arrangement of thermo-mechanical ablation with a pulsed CO2 laser and with a water-spray as an assisting media. To the arrangement belong optimal duration, intensity and energy density of the laser pulses, as well as a multi-pass cutting procedure. We show that effective ablation of hard tissue with minor thermal damage is possible with relatively long CO2 laser pulses of 80 μs duration and average laser power up to 40 - 50 W. To overcome the depth limit we have developed a special scanning technique, which allows cutting of massive multilayer bones with a feasible rate.

Anisotropic transformation of the beam quality of DPSS lasers for shaping of a narrow line focus for laser crystallization of Si

Advanced laser crystallization of Si for flat panel displays demands a narrow line-shaped light focus with an ultimately high homogeneity. Key element of LIMO line shaping system is an anisotropic quality transformation of a multimode laser beam, which permits a very good homogenization for the long axis and tight focusing with a large depth of focus for the perpendicular high-quality axis. A prototype system has been built with a 90-W 532-nm DPSS laser. It provides a 59-mm long and down to 8 μm (FWHM) narrow focus with a residual inhomogeneity of only 1% (rms). The focus width is adjustable and its shape can be tuned from a quasi-Gauss to a top-hat intensity distribution. The depth of focus at 90% of the peak intensity DOF0.9I varies from 120 μm for a line width of 8 μm to 275 μm for FWHM = 14 μm. The design of longer lines is in progress at LIMO.