In recent years, ophthalmologists from different countries have gained extensive clinical experience in the use of excimer laser for correction of refractive errors [3, 5, 6, 7, 8, 9, 10, 11]. In Russia, researchers are working to design excimer laser systems and introduce them into clinical practice. Since the late 1980s, Eye Microsurgery, an intersectoral research & technology complex, has created excimer lasers and deployed them into clinical practice. This complex has been doing this in cooperation with laser manufacturer – Physics Instrumentation Center of the General Physics Institute, Russian Academy of Sciences and Optosystems. Excimer laser system MicroScan-PIC was presented in 2000. Along with operations based on LASIK (laser in situ Keratomileusis) and TransPRK (transepithelial photorefractive keratectomy) standard technologies, the laser allows to perform personalized operations based on corneal topography and aberrometry data. Analysis of clinical and functional results of such operations showed that they are of high predictability, stability and safety [12].

Several criteria are used for objective assessment of the quality of excimer laser systems. One of them is the smoothness of corneal surface formed by the excimer laser. A smoother corneal surface facilitates normal course of corneal epithelialization after surgery and minimizes the likelihood of fibroplasia [13]. Quantitative characteristics of smoothness allow to obtain a measurement with interferometric microscopes from Zygo Corporation (USA). A number of investigators have shown that surface smoothness is higher in flying spot scanning lasers than in full-aperture lasers and lasers that use scanning slit and stopped down ablation formation system [14, 15].

Another important criterion is the association of the diameter of the resulting optical zone of full correction with the computed optical zone diameter specified by the manufacturer in the computing program of the laser system. According to O'Donnel et al. [15], the ablation zone diameter obtained during excimer laser surgery using stopped down forming system is usually less than that indicated by the manufacturer. According to the researchers, the greater the optical and transition zone of influence, the better the functional outcome and the lesser the likelihood of regression of postoperative effect. A smaller optical zone can cause very unpleasant sensations in patients in lowered or increased illumination [16].

Another important indicator of the quality of excimer lasers is that there is no significant increase in corneal temperature during surgery. Despite the fact that excimer lasers are referred to as “cold”, they can break molecular bonds in cells and release energy that increases the corneal temperature [17, 18]. The temperature increase cannot cause collagen denaturation but may affect the activity of keratocytes, postoperative healing and development of fibroplasia. Flying spot scanning lasers can significantly reduce possible heating of the cornea thanks to the small beam diameter and scanning algorithm in which the local areas of the corneal tissue, which are at a distance from each other, are ablated. Therefore, the cornea has no enough time to increase its temperature significantly [19, 20].

Efficiency in clinical practice is of course the main indicator of the quality of an excimer laser system. However, based on the criteria given above, the technical features and prospects for the use of the device at the pre-clinical study stage can be identified.

Next-generation device MicroScan-Visum is a modification of the MicroScan-PIC laser system and developed in two models — 300 and 500 Hz scanning frequencies. The aim of our study was to evaluate the surface ablation smoothness formed on polymeric materials and to investigate the thermal processes in a rabbit cornea treated with the studied laser systems during phototherapeutic keratectomy (PTK).

METHODS

An experiment to assess ablation surface smoothness was carried out using polymethylmethacrylate (PMMA) plates. In the control computer of each of the three test excimer lasers — MicroScan-PIC 100 Hz, MicroScan-Visum 300 Hz and MicroScan-Visum 500 Hz — myopia correction surgery was set in 3.0 diopter with the same optical zone diameter 6 mm but different transition zone diameters — 2.3 mm for MicroScan- PIC 100 Hz and 1.9 mm for MicroScan-Visum 300 Hz and MicroScan-Visum 500 Hz (this was due to different excimer laser light spot diameters — 1.15 and 0.95 mm respectively). Ten lenses for each of the lasers was formed on the plates, after which all the lenses were measured using interference microscope New View – 5000 Zygo. The following indicators characterizing the quality of the formed lens surface were identified:

RMS — root mean square deviation of the surface points relative to the average height across the study area;

PV — distance between the highest and lowest points of the study area;

Ra — average deviation of surface points from the middle surface.

Moreover, it was considered that the better the surface smoothness, the smaller the value of these indicators would be.

Thermographic analysis of corneal temperature changes was conducted in 15 gray chinchilla rabbits, more than one year old and with live weight of 2–3 kg. The experiment was approved by the Intercollegiate Ethics Committee (Minutes No 10–12 of 18 October 2012). Each of the three laser systems was used to operate on 5 eyes (one eye per animal). 15 min before the surgery, the rabbits were administered with 2 ml of relanium solution. The surgery was performed under local anesthesia (triple instillation of 1 % inokain solution). The estimated ablation depth during PTK was 52 microns.

Thermal imaging complex Thermo View Ti30 (Raytek, USA) was used to measure the corneal temperature. Imaging was carried out in a room with ambient temperature of 16.7 °C at a distance of 60–70 cm with frequency of 1 Hz and up to 0.1 °C accuracy. On the PC, thermal imager data were converted to thermographic map using a software tool supplied with the device, and the maximum and minimum corneal temperatures during the surgery were determined. Corneal emissivity was set at 0.93 (like water), i. e. it was considered that 0.93 of the total corneal radiation enters the device and the device adds 0.07 when calculating the temperature. For example, if the temperature is 37.3 °C for k = 0.93, then temperature will be equal to 36.1 °C for k = 1 for the same thermogram.

Data was statistically processed using software programs Statistica 6.0 (StatSoft, USA) and Excel 2003. Variation statistics methods were used. The results were presented as arithmetic mean (M) and standard deviation (σ). Student's t-test for independent cases was used to compare the means and evaluate the significance of differences (p = 0.05).

RESULTS

The results of comparative analysis of the quality of ablation surface of lens are presented in tab. 1. Laser system MicroScan-Visum 500 Hz yielded the best smoothness. Fig. 1 and fig. 2 show examples of measurement for MicroScan-PIC 100 Hz and MicroScan-Visum 500 Hz systems.