Ultra-Conservative Minimally Invasive Surgery (UCMIS) with Pulsed Non-Gaussian CO 2 Laser Beams Focused Through the Shortest Possible Focal Length

Abstract

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 CO₂ 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 τr of polymethyl methacrylate (PMMA) and other time-related parameters. Results: The minimal focal length value f _min=0.013” for a TEM₂₂ laser beam profile can be used for reference in UCMIS procedures using commercially available pulsed CO2 lasers at the wavelength of 10.6 μm. 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.