Analysis of the Evaporation to Sublimation Phase Transition during Minimal Invasive Surgery Procedures via Pulsed CO2 lasers
Journal Title: Biomedical Journal of Scientific & Technical Research (BJSTR) - Year 2017, Vol 1, Issue 6
Abstract
Objective This Paper describes a mathematical approach to quantify the phase transition from ablation to evaporation of PMMA irradiated by pulsed CO2 laser devices to be used as reference data for Ultra-Conservative Minimally Invasive Surgery (UCMIS) with commercially available medical lasers. This step is important because it allows forecasting the micro boundary drilling conditions of a laser device implemented in Operating Room (OR) in conjunction to minimally invasive tools. The primary goals of reducing the invasive characters of an operation, and the associated risks of unwanted lateral tissue damage during surgery, are the key objectives of UCMIS protocols. Background Data Currently, the data available on literature do not report any guideline for a generic set-up configuration which produces the smallest ablations using non-Gaussian laser beams. This would help to further improve the overall quality of the UCMIS protocols via endoscopic scalpels to deliver minimal ablative energy. The Author has recently published a study on absolute UCMIS conditions. Methods The equation of the evaporation time has been used to identify the most effective and safest correlation amongst the following five magnitudes: output power W, focal length f, pulse width t1b, beam TEM mode (M2 parameter) and the safest "not-to-exceed” radius of the allowed crater on the irradiated sample. The mathematical validation of this methodology is described and discussed. Results The optimized combination of these 5 magnitudes for a TEM22 laser beam profile has been identified and it can be used for reference of UCMIS procedures using commercially available pulsed CO2 lasers at the wavelength of 10.6 μm. The PMMA ablation temperature and the start of the evaporation phase can happen only starting from 354.5 degC or above. More investigations are needed to validate the whole procedure before any preliminary surgical utilization can be considered. Introduction The Definition of UCMIS and Its Importance Minimally Invasive Surgery (MIS) is a type of surgery aiming to minimize the size of surgical incisions. This type of surgery is performed using thin-needles combined to endoscopes to visually control the surgical operation via several smaller incisions rather than more radical and larger ones. The goal of minimally invasive surgery is to reduce postoperative pain, speed recovery, minimize blood loss, and lessen tissue scarring. The Ultra-Conservative MIS (UCMIS) is a type of MIS which aims to use the physically smallest incisions possible on a human tissue, still allowing the surgical treatment. The quantification of the overall conditions to geometrically reach these smallest possible injuries is mathematically determined by using several laws ofthermodynamic in conjunction to the physics of the laser beam interaction with the biological media being irradiated. Additionally, these minimal sizes are dependent on the type of surgical operation required case by case and on the type of exposed tissue. Reference values for generic UCMIS conditions are: W<0,7 Watt; crater diameter < 0,4 cm; t1b < 0,01s.; evaporated mass per pulse < 0,01 gr. and crater depth < 0,2 cm (all data refer to PMMA) [1]. These concepts are important for both the industry and the surgeons communities: the first one can plan the design of future medical equipment based on the calculated physical limits intrinsically present in each operative procedure, while the latter can better foreseen the boundaries of a given surgical operation during the planning phase of the same and consequently better estimate the associated risks. UCMIS brings lots of new insights into both early thermodynamic and mechanical ablation phenomena associated to the smallest possible thermal injury and avoidable collateral complications [2]. Examples are the treatments, in orthopedic applications, of fine human bone structures, in neurology the micro dissection of nerves and in general surgery the generic treatment of other small anatomical structures as required case by case. In order to define a unified theory which addresses all the complex correlated thermodynamic phenomena taking place during the production of laser beam craters in low-water-content tissues, one key parameter to use as reference all the relevant optical coefficients of the PMMA at 10.6 μm. Currently, the data available on literature do not report any numerical correlation amongst configurations of values which guarantee a safe CO2 laser beam spot size with still surgical relevance. This must be in conjunction to minimal heath ablation with reduced side thermodynamic damage at 10.6 μm for generic combinations of output power in CW, non-Gaussian TEM mode, pulse width and focal length. The transition phase from ablation to evaporation must be discussed in detail [3]. The procedure to obtain these parameters would help to further improve the overall quality of UCMIS protocols via endoscopic scalpels. These use both mechanical focusing heads and fiber optic instrumentation to deliver ablative energy on tissue. Materials and Methods The focused lasers' spot size on any irradiated media is linearly depending on the focal length of the focusing head: this means that the volume of the ablated tissue is minimal if the focal length in use is he shortest possible one keeping the same output power [4]. The mathematical calculations to obtain the best combination of all the user-configurable parameters of a generic non-Gaussian medical laser device used for Ultra Conservative Minimally Invasive Surgery (UCMIS) procedures are here described. The same experimental set-up to validate the correctness of the LCA Algorithm has been published by the same Author several years ago (Appendix) is used in the present study also. The LCA algorithm is still an essential tool to determine the entire time-dependent coefficients linked to CO2 laser beam ablation processes in PMMA. However, for the purposes of this Paper, it becomes also evident that in case of a generic combination of non-minimal laser parameters in output irradiating for a longer period of time, the pure evaporation processes of the irradiated media have here a much more important meaning. In all the other limiting processes related to the minimal onset of the crater creation, the full process of combustion and evaporation cannot be observed; rather one can see the beginning phase of the ablative process only. This is caused by the very short-in-time temperature increase beyond the melting point but still below the evaporation threshold followed by full-blown combustion. This crucial difference has been described by other Authors also [1,2] most of polymers tend to ablate before evaporation. Normally, the heating of a substrate would lead to thermal expansion of a material and an explosivephase transition [1]. However, the high viscosity and cohesive energy of polymers greatly delays thethermal expansion and evaporation during the laser irradiation in both photochemical and photo thermalprocesses. It has been shown that the ablation of polymers starts when the density of broken bonds in the surface layer reaches a certain critical value [1]. The direct scission events, as in photochemicalprocesses, lead to modification of material and consequent reduction of molecular weight and cohesiveenergy of the polymer chain. However, the photochemical processes alone would require very highfluencies to increase the energy density and number of broken bonds [5].
Authors and Affiliations
Franco Canestri
Keywords
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- EP ID EP572924
- DOI 10.26717/BJSTR.2017.01.000493
- Views 164
- Downloads 0
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