Study design and patients
This is a prospective clinical observational study including 200 eyes of 100 consecutive patients with mild to moderate myopia (- 2.0 D to -6.0 D spherical equivalent) and mild (0.0 D to -2.5 D) astigmatism who underwent conventional PRK or tPRK at the Augenlaserzentrum Neu-Ulm, Germany between January and December 2019. After comprehensive information about risks and benefits of the two surgical techniques, patients were free to choose one of the two procedures. All patients provided a signed informed consent form for data collection, evaluation and publication. The research has been carried in accordance with the Declaration of Helsinki. All methods were performed in accordance with the relevant guidelines and regulations, although ethical approval was not required and deemed unnecessary according to national regulations of the Bavarian Medical Association (Bayerische Landesärztekammer (BLÄK) Ethikomission (ethics committee), Mühlbaurstraße 16, 81677 München), which states that study projects with anonymized data are not subject to consultation. All patients provided a signed informed consent form for data collection and consent to participate in this study with data evaluation and publication.
All patients qualified for surface ablation according to the German standards for surface ablation of the German Committee of Refractive Surgery (Kommission Refraktive Chirurgie) .
Exclusion criteria were unstable refraction, severe ocular surface disease, corneal epithelial pathology, keratoconus, any posterior segment pathology or any previous intraocular or corneal surgery. All patients were advised to discontinue contact lens wear for a minimum of 3 weeks prior to preoperative examination and treatment.
Pre- and postoperative assessments
Preoperatively, all patients underwent a complete eye examination including uncorrected distance visual acuity (UCDVA) and best corrected distance visual acuity (BCDVA) assessment, manifest refraction, autokeratometry, intraocular pressure measurement, and slit lamp examination to evaluate the anterior segment and the fundus. Visual acuity (at 5 m, decimal) and manifest refraction measurements as well as corneal topography assessment by Allegro Topolyzer-Vario (WaveLight, Erlangen, Germany) and Scheimpflug tomography examination by Allegro Oculyzer ІІ (WaveLight, Erlangen, Germany) were performed by one experienced optometrist.
Postoperatively, slit lamp examinations were performed by one ophthalmologist at day 1, 2, 3, 4, 7, and 6 weeks after surgery. Monocular UCDVA was measured on day 4, 7 and 6 weeks postoperatively. Manifest refraction and binocular BCDVA were assessed at 6 weeks after surgery. Moreover, patients evaluated their subjective maximum pain intensity level within the first four days after surgery on a numeric pain rating scale (0-15) which was completed by the patients themselves. Accordingly, pain could be rated as low (1-5), high (6-10) or very high (11-15). Wound healing was followed during daily slit lamp examinations until day 4.
All surgical procedures were performed bilaterally at the same day, with the right eye being treated first by one single surgeon (HCG). The wavefront-optimized (WFO) ablation profile was planned using the standard planning software including WaveLight nomograms and was based on manifest refraction. In StreamLightTM procedures, a newly calculated shot matrix enables an equal ablation of the epithelium over the treatment zone (7mm for myopia and 9 mm for mixed myopic astigmatism).
In both groups standard wavefront optimization laser ablation profiles with a refractive ablation zone of 6.5 mm in all cases were applied and the use of Mitomycin C was avoided according to the recommendations of the German Committee of Refractive Surgery .
The standard preoperative procedure for both procedures was the same. Topical proparacaine hydrochloride 0.5% (Alcaine; Alcon Laboratories, Inc., Fort Worth, TX, USA) eye drops were instilled twice directly before surgery. In the PRK group, de-epithelialization was performed with a 9 mm rotating brush (Amoils Rotary Epithelial Scrubber, Innovative Excimer Solutions, Inc., Toronto, Canada) and remaining epithelial cells were removed using a dry PVA eye spear. Subsequently, PRK laser ablation was carried out followed by 1 minute of corneal and conjunctival cooling with ice-cold BSS administered with a syringe. In the tPRK group, conjunctiva and cornea were pre-cooled with ice-cold BSS from a syringe for 30 seconds. After removing excessive liquid from the conjunctiva and cornea with a dry PVA eye spear, de-epithelialization was performed using the StreamLightTM PTK mode for 22 - 30 seconds depending on preexisting astigmatism. StreamLightTM allows to individually determine the epithelial ablation depth between 45 µm and 65 µm after epithelial mapping. In our daily practice we usually work with an epithelial ablation depth of 55 µm. After the PTK mode, the cornea was checked and any remaining epithelial cells were removed mechanically with a dry PVA eye spear. After an interruption of around 10 seconds to cool down the cornea, PRK laser ablation was applied followed by 1-minute cooling of the cornea and conjunctiva with ice-cold BSS administered with a syringe. Following both laser procedures preservative-free Ofloxacin and corticosteroid eye drops were instilled, a pre-cooled soft bandage contact lens (Acuvue ®; Johnson and Johnson Vision Care, Inc., Jacksonville, USA) was placed on the cornea and preservative-free eye drops containing 0,15 % sodium hyaluronate were applied. The soft bandage contact lens was removed 4 days after surgery in both groups. Patients were advised to continue three times daily with preservative-free corticosteroid eye drops for 4 weeks and to regularly use preservative-free 0,15 % sodium hyaluronate eye drops at least 5 times per day.
Results are expressed as mean ± standard deviation. Data were analyzed using Statview 5.01 software for Windows (Abacus Concepts, Inc., Berkeley, California). For determining statistical significance between both groups, the non-parametric Mann-Whitney U test and the t-Test for independent samples were performed (p < 0.05 considered statistically significant).