All eyes were treated with the StreamLight® PRK software in WaveLight EX500 Excimer Laser (WaveLight®; Alcon Laboratories, Fort Worth, TX, USA). Postoperative emmetropia was targeted in all eyes and refractive correction was adjusted based on full cycloplegic refraction [13 (link), 14 (link)] with no specific nomograms used. After choosing the StreamLight profile, the epithelial ablation depth was determined based on epithelial mapping in a range between 45 and 65 μm (personal communication with Alcon recommends the use of maximum epithelial thickness) while the epithelial optical zone (OZ) in hyperopic corrections is 8.0 mm as a default setting. The stromal ablation OZ was set to the standard 6.5 mm for all eyes. The total ablation zone (which is a composite ablation zone for both the epithelial and stromal circles) was automatically adjusted to 8.9 mm for both the epithelial and stromal ablations to ensure epithelial-stromal ablation matching. Initially, a drop of a preservative-free local anesthetic was instilled followed by sterilizing the periocular skin and eyelashes with 10% povidone-iodine. An eyelid speculum was inserted and gentle wetting of the cornea with Merocel sponge (Medtronic Inc., Minneapolis, MN, USA) soaked with cold balanced salt solution (BSS, Alcon Lab., Fort Worth, TX, USA) followed by gentle drying was performed. Every patient was instructed to maintain their eye fixation on a green intermittent spotlight. The eye-tracker was activated, and laser ablation was focused and centered on the center of the pupil [15 (link)]. Stream excimer laser firing was started to remove the epithelium followed by stromal ablation in a single step. The manufacturer recommends a momentary stop for 10 s on hearing 3 pop sounds marking the transition between epithelial and stromal ablations to cool down the cornea. Mitomycin C (0.02%) [16 (link), 17 (link)] was applied mid-peripherally for 60 s followed by irrigating the stroma copiously with cold BSS. A soft bandage contact lens was applied until complete epithelial regeneration. An additional movie file shows the surgical steps in more detail (see Additional file 1). Postoperative medications included Moxifloxacin 0.5% eye drops 4 times daily for a week, Fluorometholone 0.1% eye drops twice daily for a month, preservative free artificial tears 5 times daily for 3 months and oral non-steroidal anti-inflammatory pills for post-PRK pain. One surgeon (MA) performed all the TE-PRK surgeries in the study.
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