patients who are candidates of PRK will be recruited in this intrasubject randomized clinical trial. Inclusion criteria are myopia or myopic astigmatism with refractive cylinder. Exclusion criteria were unstable refractive error, age less than 21 years old, corneal opacity, keratoconus, keratoconus suspects, residual corneal stromal thickness less than 350 µm, moderate or severe dry eye disease and any other ocular pathology other than refractive error. Complete eye exam will be performed preoperatively including: uncorrected distance visual acuity (UDVA), best spectacle corrected distance visual acuity (CDVA), cyclorefraction, slit lamp biomicroscopy, Goldmann applannation tonometry, dual scheimpflug tomography (Ziemmer), topography in mesopic condition accompanied by pupilometry and iris registration (Topolyzer Vario, Alcon Laboratories, Inc., Fort Worth, TX), and funduscopy with dilated pupil.

Surgical Technique The same surgeon (A.F.) will perform all surgeries. After installation of Tetracaine 1%, periorbital skin will be prepared with povidone iodine 5%. Iris registration will be performed in all eyes before epithelial removal. Ethylalcohol 20.0% into a 9.0 mm well for 20 seconds will be applied on the central cornea, and then the epithelium of the central 9.0 mm area will be removed by hockey knife. In the right eye of each patient PCS compensation will be turned on (PCS-On group) and in the left eye it will be turned off (PCS-Off group). Excimer laser ablation will be performed by Wavelight Allegretto EX500 (Alcon, Fort Worth, TX, USA). Selection of which eye to be first ablated will be done randomly. Randomization will be performed based on a computer software by a Biostatistician which produced the first eye to be operated. Static cyclotorsion compensation will be active in all eyes. The ablation profile will be wavefront-optimized in all cases.

Postoperative Course Postoperatively, chloramphenicol 0.5% eye drops and betamethasone 0.1% eye drops will be prescribed every 6 hours for 10 days. Thereafter, Fluorometholone 0.1% eye drops every 6 hours for 1 month and every 8 hours for the next month will be given. Follow-up examinations were scheduled at 1, 2, 3, 4, 7 and 15 days and 1, 2, 3, and 6 months postoperatively. After contact lens removal each follow-up examination includes the UDVA, manifest refraction, and IOP measurements as well as a slit lamp biomicroscopy. Corneal imaging with Galilei tomography, CDVA and cyclorefraction will be performed at the final follow-up examination 6 months postoperatively.

Inclusion Criteria:

• myopia
• myopic astigmatism

Exclusion Criteria:

• unstable refractive error, age less than 21 years old
• corneal opacity
• keratoconus, keratoconus suspects
• residual corneal stromal thickness less than 350 µm
• moderate or severe dry eye disease
• any other ocular pathology other than refractive error