Removing the central 8-10mm of the epithelium and applying a riboflavin solution (0.1% riboflavin-5-phosphate and 20% dextran T-500) to the corneal surface 30 minutes before irradiation and at 5 minutes intervals during the course of a 30 minute exposure to 370 nm UVA with an irradiance of 3 mWcm-2 (UFalink, Russian Federation)

impregnation of the cornea with a riboflavin 0.1% hypotonic solution is performed by using an iontophoresis device (galvanizator; Potok-1, Russian Federation). The passive electrode (anode) was applied to the inferior part of the cervical vertebrae. The active electrode (cathode), a bath tube (glass or plastic - 10-12 ml) is applied to the open eye,then r the tube is taped to the skin of the orbital margins and filled with riboflavin 0.1%. The current intensity is initially 0.2 mA and then gradually increased to 1.0 mA at 0.2 mA for 1 minute at 10-second intervals increments to determine individual tolerance. The total time that the riboflavin administration is 10 minutes.

Standard surface UVA irradiation (370 nm, 3 mW/cm2) using UFalink device, (Russian Federation) is then applied at a 5-cm distance for 30 minutes with continuing hypotonic riboflavin drops every 2 minutes

Using topography measurements, clinical stabilisation is defined as an increase of no more than 1 diopter of the maximum keratometry value over the preoperative maximum keratometry value

Using optical coherence tomography to estimate the pachymetry changes

Decimal visual acuity assessed on a study later was converted to the logarithm of minimal angle of resolution (logMAR).