Ultra-conservative minimally invasive surgery (UCMIS) with pulsed non-Gaussian CO(2) laser beams focused through the shortest possible focal length.

OBJECTIVE: This article describes a mathematical approach to identifying the absolute shortest laser optical focusing head f(min) associated with the smallest not-to-exceed ablative crater radius and depth to be used during ultra-conservative minimally invasive surgery (UCMIS) procedures with pulsed CO(2) lasers. This value is important because it allows forecasting the micro-boundary ablative conditions of a laser device implemented in the operating room (OR) in conjunction with minimally invasive tools. The primary goals of reducing the invasive character of an operation, and the associated risks of unwanted lateral tissue damage during surgery, are the key objectives of MIS protocols. BACKGROUND DATA: Currently, the data available in the literature do not report any numerical value of a critical focal length and its spot size that produce the smallest ablations. This would help to further improve the overall quality of the MIS protocols via endoscopic scalpels to deliver minimal ablative energy on the irradiated tissue. METHODS: Several mathematical software routines have been used in parallel to handle all the complex numerical calculations needed to extrapolate f(min) by using the crater, the crater depth acceleration, and speed coupled with the relaxation time ?r of polymethyl methacrylate (PMMA) and other time-related parameters. RESULTS: The minimal focal length value f(min)=0.013? for a TEM(22) laser beam profile can be used for reference in UCMIS procedures using commercially available pulsed CO2 lasers at the wavelength of 10.6??m. No laser thermoablation of compact PMMA samples is possible below f(min) while delivering I(min)=7.6?mW on its spot. Gaussian beams showing TEM00 profile need longer focal lengths for the same minimally ablated volume. Suggestions about the calibration requirements for such a lens are presented. More investigations are needed to validate the whole procedure before any preliminary surgical utilization can be considered.

Accurate quantification of the optical absorption coefficient and of the thermal relaxation time for PMMA and for low-water-content media during early ablation with CO2 laser beam at the wavelength of 10.6 µm.

OBJECTIVE: This article describes a new mathematical approach for a more accurate identification of the optical absorption α (per centimeter) and the thermal relaxation time τr (seconds) of dry poly(methyl-methacrylate; PMMA) at λ = 10.6 µm. BACKGROUND: The data available in the literature do not accurately describe the numeric value of α (per centimeter) for PMMA and other biologic media. The relaxation time, the surface threshold time, and the heat incubation time are all reported on Literature in rather inaccurate fashion. METHODS: Several well-polished PMMA blocks (1 × 4 × 4 cm) were perpendicularly exposed to focused CW radiation of CO2 medical laser devices showing a TEM22 mode. The exposure was kept constant at 10-W CW. A theoretic focal length of 0.5 inches has been extrapolated to better study the most relevant threshold conditions for ablation on PMMA. RESULTS: Values of α = 536.9 per centimeter and τr = 818 µsec for the PMMA at 10.6 µm were identified. With a similar approach, the same parameters were determined for low-water-content tissues, along with other key thermodynamic coefficients, such as the energy threshold of ablation. The starting radial speed of ablation of the "horizon" of the crater was found to be equal to 0.57 cm/sec, with a threshold average power density for ablation equal to 11.5 W/cm in CW mode. The computerized diagnostic CT(2) tool proposed by the same author has been also extended to include these preliminary results.

Computerized thermal characterization tool (CT)2 for complete thermodynamic coefficients mapping at the wavelength of 10.6 microm: a PMMA case report.

OBJECTIVE: This paper details a proposed clinical identification tool, the Computerized Thermal Characterization Tool or (CT)(2), designed to precisely quantify and forecast the ablation capabilities of a CO(2) laser beam and to optimize its usage when human tissue is exposed to 10.6 microm wavelength radiation. BACKGROUND: As seen in other studies by the same author, the correct identification of the optical absorption of polymethylmethacrylate (PMMA) allows isolating other key time-dependent coefficients, all described qualitatively rather than quantitatively in the literature, with better accuracy. Tests on other biological media were performed and reported as potential contribution for minimally invasive surgical procedures. METHODS: The laser in use was configured in different combinations amongst the following parameters: transverse electromagnetic modes (TEM(22)), output power, exposure times, and focal lengths. Several PMMA blocks (1 cm x 4 cm x 4 cm) were exposed to the continuous wave radiation of three commercially available CO(2) medical laser devices with a TEM(11) beam profile. RESULTS: The data were used in a computerized simulation to test a priori the thermal behavior of biological media exposed to a CO(2) laser beam. Interestingly, this behavior could be reproduced on a variety of biological and nonbiological media. Threshold injury conditions were reached for the myocardium at 786 W/cm(2) per pulse, for the aorta at 519 W/cm(2) per pulse, and for the PMMA samples at 393 W/cm(2) per pulse. CONCLUSIONS: These values can be used as reference for both minimally invasive surgery and for transmyocardial laser revascularization protocols, combined with the proposed (CT)(2). Further investigations are needed to completely validate the potential clinical utilization.

Optical absorption coefficient, time of thermal relaxation, time of surface threshold, and time of heat incubation for PMMA samples at the CO2 laser-beam wavelength of 10.6 microm.

OBJECTIVE: This paper discusses in detail the mathematical identification of the optical absorption alpha (cm(1)) of Beer's law, a crucial parameter to study the development of laser beam craters into dry poly(methyl methacrylate) (PMMA) samples exposed to steady CO(2) laser beams emitting radiation at lambda = 10.6 microm in continuous- wave (CW) mode. Three additional time-dependent coefficients have been determined as well. In clinical applications, these results are important in order to precisely quantify and forecast the ablation capabilities of the CO(2) laser beam, to optimize its usage in the operating room, and to address the safety issues related to surgical interventions on human tissue. BACKGROUND DATA: Currently, the data available in the literature do not allow the identification of the numerical value of alpha (cm(1)) for PMMA at lambda = 10.6 microm with enough, and therefore satisfactory, accuracy. Additionally, the correct identification of the optical absorption of PMMA would allow the isolation, with better accuracy, of other key time-dependent coefficients, such as relaxation time, surface threshold time, and heat incubation time, which are all described in the literature in a qualitative rather than quantitative fashion. Correct bone cement preparation depends on the value of alpha (cm(1)) of the PMMA in order to avoid unwanted complications in patients during cement removal via laser techniques. METHODS: The laser in use was configured in different combinations with the following parameters: transverse electromagnetic modes (TEMnm), output power (I0), exposure times (te), and focal lengths (fk). Several PMMA blocks (1 cm x 4 cm x 4 cm) were exposed to CW radiation of three commercially available CO(2) medical laser devices showing a TEM11 mode. Each block was exposed to the beam on a horizontal and well-polished surface of each sample. Four focal lengths (2.5", 5", 7.5", and 15.75" [400 mm]) were used to focus the beam on the well-polished and dry surface of the PMMA samples. The resulting dimensions of the craters were measured after each exposure, which has been kept at a 10-Watt CW beam. Exposure time ranged from 0.5 to 2 sec. RESULTS: The value of alpha = 502 (cm(1)) for PMMAat 10.6 microm was identified, matching other results reported in the literature for similar compact media in the absence of water content, such as PMMA. The time of thermal relaxation was 9.358 x 10(4) sec, the time of surface threshold was 9.365 x 10(4) sec, and the time of heat incubation was 3.6 x 10(7) sec (all three for PMMAat 10.6 microm for any exposure). Using the calculated value of alpha, one of the practical clinical recommendations would be, for instance, to reduce or to abolish the utilization of colorant dopants in the preparation of the bone cement mixture and therefore reduce the danger of bone damage possible during the removal of bone cement via laser techniques. Other examples refer to other clinical bone and dental treatments.

Sudden and unpredictable below-surface ablation pattern changes by CO2 laser beams: a qualitative description of five macroscopic cases observed in PMMA with high probability to occur during surgery in low-water-content tissues.

OBJECTIVE: This paper describes five cases of macroscopic irregular CO(2) laser-beam ablation patterns that can generate below-surface complications during surgery. These five cases are related to curved reflected beams, curved craters generation with abnormal superficial thermal damage, and craters that show irregular wall contours. Although these alterations have been observed during irradiation in PMMA samples (polymethilmethacrylate), it is possible that similar unpredictable changes also happen in low-water-content, hard and uniform biological tissues such as compact bone, enamel, and dentin. This fact can predict severe impacts on the quality of the final surgical outcome, especially there where precision surgery techniques are required. A qualitative description about the possible causes of these effects and how to avoid them during surgery have been suggested too. BACKGROUND DATA: In the past decades, daily surgery and research studies have provided useful information about the interaction between medical CO(2) laser beams and animal, human, and other biological tissues. Several mathematical models describe with acceptable accuracy all the ablative properties of the 10.6 microm laser beam. Very few studies describe the presence and address the consequences of the ablative aberrations, which can frequently and randomly happen during laser surgery. The probability that these changes happen in below-surface, therefore invisible, parts of the biologic media under treatment makes the whole matter crucial, even in cases of traditional surgery. Where gross mass removals are considered, the presence of unpredictable and sudden deviations from the expected traditional cone-shaped patterns raise several questions about safety. The continuous need for properly engineered medical laser-beam devices, online laser-beam monitoring, and real-time control becomes mandatory in modern surgery. MATERIALS AND METHODS: The equipment used in this study was provided by the National Cancer Institute of Milan, Milan, Italy, and by the Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel. A large TEM-mode laser-beam device (Valfivre, Italy) and TEM00 laser-beam device (Synrad, USA), both coupled to 2.5- and 5-inch focusing lenses, have been used to irradiate, at 10 Watts nominal output, on the focal spot, several PMMA blocks (3 x 2 x 4 x 2 x 2 cm) up to 10 sec CW. For one set of experiments, a metallic, well-polished mirror was placed against one surface of each sample to simulate possible internal beam reflections caused by generic metallic surgical instruments, such as conventional scalpels or clamps. RESULTS: The experimental evidence of five major and unpredicted changes in the shape of craters produced by CO(2 )laser beams in PMMA are shown in photos and discussed in a qualitative way. Several physical and thermodynamic phenomena are proposed to identify and therefore minimize or avoid the causes of these events. CONCLUSION: The results discussed in this paper show how important it is to constantly and carefully observe both the irradiated tissue's or media's structure and the beam's broadening under surface during the ablation process. Fume attenuation of the incoming laser beam, fume escape paths, internal beam interference phenomena, media's microstructural changes and, perhaps more importantly, their combination can offer good explanations of all the described phenomena. In summary, the presence of fumes in at least two out of the five reported cases plays a key role in aberrant crater generation processes. Therefore, the author recommends paying extra attention to them in order to provide a higher quality outcome in Surgery where CO(2) laser beams are deployed.