Comparison of cutting and coagulation properties of 1.56 and 1.94 µm fiber lasers and a 0.98 µm semiconductor laser

Aim of the study was to compare the cutting and coagulation properties of 1.56 and 1.94 μm fiber lasers with those of a 0.98 μm semiconductor laser. Materials and methods . A comparative study of the biological effects of 1.56 and 1.94 µm lasers and a 0.98 µm semiconductor laser used in a constant, continuous mode was carried out. The cutting properties of the lasers were evaluated on the chicken muscle tissue samples by the width and depth of the ablation zone formed via a linear laser incision at a speed of 2 mm/s, while the coagulation properties were assessed by the width of the lateral coagulation zone. The zones were measured using a surgical microscope and a calibration slide. For statistical analysis, power values of 3, 5, 7, 9, and 11 W were chosen for each laser wavelength. Results. Analysis of the findings confirmed that laser wavelength had a statistically significant effect on the linear dependence between incision parameters and laser power. It was found that the 1.56 μm fiber laser (water absorption) had a greater coagulation ability but a comparable cutting ability compared with the 0.98 μm laser (hemoglobin absorption). When used in the power mode of 7W or higher, the 1.94 µm laser provided superior cutting performance compared with the 0.98 µm semiconductor laser at the same exposure power. Elevating the power in any of the lasers primarily increased the width of the ablation zone, and to a lesser extent – the crater depth and the width of the lateral coagulation zone. Therefore, in comparison with the 0.98 μm semiconductor laser, higher radiation power in the 1.56 and 1.94 μm lasers mainly influences their cutting properties, expanding the width and depth of the ablation zone, and has a smaller effect on their coagulation ability. Conclusion. The findings of the study showed that the 1.56 and 1.94 μm fiber lasers have better coagulation properties in comparison with the 0.98 μm semiconductor laser. was statistically proven that all incision characteristics (width of the lateral coagulation zone, depth and width of the ablation zone) for the 1.56, 1.94, and 0.98 μm lasers depend on the power of laser radiation. The 1.94 µm laser is superior to the 0.98 µm laser in its cutting properties.


INTRODUCTION
In recent years, laser technologies have been rapidly developing with creation of semiconductor and fiber lasers with various wavelengths never used before (0.53, 0.81, 0.97, 1.06, 1.47, 1.56, 1.94 µm) and production of high-power lasers, which expands the scope of their application in different modes (continuous, pulsed, contact, and distance modes). Novel medical equipment expands the range of therapeutic interventions; however, biological effects of the majority of machines rarely receive any empirical evaluation. Therefore, the right operation mode is usually found by trial and error, which makes surgical intervention less predictable and increases the risk of complications. Since the wavelength defines the properties of a laser, a lack of data on the biological effect of different wavelengths prevents practitioners from selecting the right laser device and its operation modes.
In most experimental studies, ablative performance is assessed by measuring the ablation rate, which is the volume of ablated tissue per a unit of time (g/min), while coagulation properties are evaluated by determining the bleeding rate (g/min) and tissue necrosis depth (mm). This method involves the in vivo use of living tissue models (in many cases -blood-perfused porcine kidney), though evaluation results provide only circumstantial evidence on the extent of ablation and coagulation properties [1]. Current Russian and foreign studies on the effects of diode and fiber lasers with various wavelengths (0.81, 0.94, 0.97, 1.47, 1.56 μm) use vein models as samples for comparative experimental analysis, which makes it impossible to evaluate the cutting properties of lasers [2][3][4][5][6][7][8]. There are few papers on semiconductor lasers being used on the cartilaginous tissue models (0.97; 1.56 μm), but they do not discuss the ablative and coagulation characteristics either [9].
The main aspects of laser performance relevant for clinical practice include cutting properties measured by the width and depth of the ablation zone, as well as coagulation properties, or hemostatic effect of the laser, that manifest themselves through tissue blanching along the incision.
The development of new 1.56 and 1.94 μm fiber lasers makes it relevant to study their biological effects prior to using them in clinical practice. The 0.98 μm lasers have been extensively used for various surgical purposes [1], including ENT surgery [10][11][12][13], and, therefore, it is reasonable to use this experience for a comparative analysis of the biological effects of other lasers.
The aim of the study was to compare the cutting and coagulation properties of the 1.56 and 1.94 μm fiber lasers and the 0.98 μm semiconductor laser.

MATERIALS AND METHODS
To assess the biological effects of laser radiation, linear incisions were made on the chicken muscle tissue at a fixed rate. The laser fiber was rigidly fixed with tripods on a support stand at a 60° angle relative to the incision projection. The biological object was placed on a mobile recorder sheet moving uniformly at a speed of 2 mm/s [12]. The width of the ablation crater and the lateral coagulation zone was evaluated using a slide with the graduation of 10 μm and an operating microscope with ×15 magnification. To assess the crater depth, cross sections of the tissue were made relative to the linear incision line, and the parameter was measured using the above-described method. The width and depth of the incision served as indicators of the ablative performance, while the width of the lateral coagulation zone represented the hemostatic properties of the lasers.
The incisions were made with the 1940 and 980/1560 nm lasers (LSP, IRE-Polus, Moscow, Russian Federation) in a continuous contact mode. The procedure was performed with a freshly cleaved optical fiber end with a width of 400 μm after its charring by a short-term contact with a wooden surface (spatula). At power values from 3 to 11 W and interval of 2 W, 3 incisions for each wavelength were made, and the results for each incision were measured in 10 positions (30 measurements in total).
Statistical processing of the data was carried out using the IBM SPSS Statistics software package (version 22). The evaluation methodology included descriptive statistics methods, regression analysis, and multiple regression analysis with a madiating variable. The parameters of the laser incision (width of the lateral coagulation zone, depth and width of the ablation zone) were dependent variables. The regression analysis of each dependent variable was performed separately. The power and wavelength of radiation were regarded as independent variables. Since the experiment involved three lasers with wavelengths of 0.98, 1.56, and 1.94 μm, the qualitative mediating variable "wavelength" was coded with two dummy variables representing the 1.56 and 1.94 μm lasers.

RESULTS AND DISCUSSION
The graphs depicting the dependence of the width of the lateral coagulation zone and the depth and width of the ablation zone on the power and wavelength are shown below (Fig. 1-6).  The statistical analysis of the results was based on multiple regression: where Y is a dependent variable (the coagulation width for the first model; the ablation depth, for the second model; and the ablation width for the third model); P is the laser power; D 1,56 is a dummy variable that equals 1 for the 1.56 μm laser and 0 in other cases; D 1,94 is a dummy variable that equals 1 for the 1.94 μm laser and 0 in other cases.
Thus, the significance of the β 2 , β 4 coefficients indicates the statistical difference between 0.98 μm and 1.56 μm lasers, and the significance of the β 3 , β 5 coefficients indicates the statistical difference between the 0.98 μm and 1.94 μm lasers.
Multiple regression analysis with a mediating variable showed a statistically significant relationship between all the parameters (the width of the coagulation zone, the depth and width of the ablation zone) and the laser power (Table 1, 2). In all but one case, it was established that the laser radiation wavelength (mediating variable) had a statistically significant effect on the linear dependence of laser incision characteristics on the laser power. Only when comparing the 0.98 and 1.56 μm lasers, no statistically significant difference in the effect of the laser power on the ablation depth was revealed.
T a b l e 2

Model
Parameter Coefficient SE р Lower confidence interval limit Statistical analysis of the data obtained showed that the laser power significantly (р < 0.05) influenced the cutting and coagulation properties of all lasers. This was confirmed by the statistical significance of the β 1 coefficient in all the three models (width of the coagulation zone, depth and width of the ablation zone).
The 1.56 μm laser radiation (water absorption) did not differ much from the 0.98 μm radiation (he-Бюллетень сибирской медицины. 2021; 20 (4): 56-62 moglobin absorption) in its cutting properties (the statistical differences in the ablation zone depths are non-significant, p > 0.05), though its coagulation properties were more prominent: compared with the 0.98 μm laser, the 1.56 μm laser created a wider coagulation zone at the same power mode (β 2 differs significantly from 0).
The cutting properties of the 1.94 μm laser followed a different correlation pattern with its power than the 0.98 μm or 1.56 μm lasers (due to a different angle of the regression line, which was confirmed by the fact that β 5 is statistically different from zero in model 2). Unlike the 0.98 μm and 1.56 μm lasers, the power of 3-5 W resulted in a smaller width and depth of the ablation zone, while the power of 9-11 W increased these incision parameters. The 1.94 μm laser was found to have greater coagulation properties than the 0.98 μm laser with hemoglobin absorption.
The research showed that greater laser power primarily increased the ablation zone width (the regression coefficients were 0.1, 0.06, and 0.18 mm/W for the 0.98, 1.56, and 1.94 μm lasers, respectively), to a lesser extent -the ablation crater depth (the regression coefficients were 0.05, 0.06, and 0.11 mm/W, respectively), and to the least extent -the width of the lateral coagulation zone (the regression coefficients were 0.03, 0.05, and 0.07 mm/W, respectively). Thus, the increase in laser power predominantly affected the cutting properties, making the ablation zone deeper and wider, but had a modest effect on the coagulation abilities. Therefore, using greater laser power during surgery will increase the cutting properties of the laser more than the hemostatic ones.
The visual analysis of the regression lines demonstrated that the 1.94 μm laser had less prominent coagulation properties than the 1.56 μm laser, since the former was absorbed by water more easily and had a greater target chromophore absorption rate, as well as a smaller penetration depth. However, the statistical analysis of differences between the 1.94 and 1.56 μm lasers was not performed, as it was not intended by the research design.
Contact laser exposure mainly involves radiation absorption by the area of carbonization and its transmission to the surrounding tissues. Since most of the near-infrared radiation is absorbed by carbon particles, no significant differences in the cutting properties of the tested wavelengths were recorded. All the lasers showed good cutting properties. The difference in the coagulation properties appears to depend on the amount of remaining radiation (unab-sorbed by carbon) penetrating the tissue and can be explained by differences in the wavelength characteristics (different chromophores and tissue absorption coefficient).

CONCLUSION
The research and the resulting data analysis confirmed the statistically significant dependence of all laser incision characteristics (width of the lateral coagulation zone, depth and width of the ablation zone) on the radiation power for the 0.98, 1.56, and 1.94 μm lasers. The 1.56 and 1.94 μm lasers have better coagulation properties compared with the 0.98 μm semiconductor laser. The 1.94 µm laser is superior to the 0.98 µm laser in its cutting properties.