This was a retrospective cohort study of patients younger than 40 years who underwent LASIK with MM or a femtosecond laser for the correction of myopia with or without astigmatism between 2008-2014.
A masked investigator performed the preoperative examination that included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA) (Nidek autochart projector CP 670, Nidek, Gamagori, Japan), manifest and cycloplegic refraction, ultrasound corneal pachymetry (DGH 5100 contact pachymeter, DHG Technology Inc, Exton, PA; OcuScan RXP, Alcon Laboratories, Inc, Fort Worth, TX), topography/tomography and keratometry (Dicon CT200, Vismed Inc., San Diego, CA; CSO Construzione Strumenti Oftalmici, Italy), mesopic infrared pupilometry (Colvard Pupillometer, Oasis 78 Medical Inc., Glendora, CA), slit-lamp biomicroscopy, Goldmann tonometry and dilated funduscopy.
Exclusion criteria were unstable refraction, suspicion of keratoconus or other ectatic corneal condition (defined as any localized steepening documented with Placido corneal topography or bowing of the posterior corneal surface detected with corneal tomography), prior ocular surgery, or systemic diseases that could alter refractive or visual outcomes.
The choice of a femtosecond or the MM depended mainly on the preoperative keratometric measures and the pupil size. For keratometric measurements < 41.0 diopters (D) or > 46.0 D and pupil diameter ≥ 7 mm, the flap was always created with a femtosecond laser. In patients suitable for both procedures, the final decision was based on the patient’s preference after being thoroughly informed about both techniques. The patients were allocated in one of the three femtosecond groups depending on device availability in the facilities at the time of surgery.
All patients provided informed consent and the Institutional Review Board approved the study protocol. The study was performed in accordance with the tenets of the Declaration of Helsinki.
Two experienced surgeons (M.A.T. and M.G.G.) performed all the procedures in a private practice setting.
Povidone-iodine solution 5% was applied on the eyelids and conjunctiva before the sterile surgical drape and eyelid rigid speculum were positioned. All surgeries were performed under topical anaesthesia (Lidocaine 2%).
In eyes treated with the MM (group H), the flap was cut with the Hansatome Zero-compression® keratome, (Hansa Research and Development, Miami, FL, USA and commercialized by Bausch & Lomb Corporation) using a 8.5-9.5 mm suction ring, a 120 μm blade and superior hinge.
In the femtosecond groups three platforms were used: a) the 60-kHz IntraLase® laser (group IL), programed for raster pattern photon delivery, bed energy level of 0.90 μJ, side-cut energy of 0.90 μJ, spot separation of 7 μm, side cut angle of 70 degrees, superior hinge angle of 50 degrees, attempted flap depth of 110 μm and flap diameter of 8.5 mm; b) the Wavelight FS200® laser (group F) by Alcon Laboratories, Inc. Fort Worth, TX, USA, programed for raster pattern photon delivery, bed energy level of 0.83 μJ, side-cut energy of 0.80 μJ, spot separation of 8 μm, side cut angle of 70 degrees, superior hinge angle of 90 degrees, attempted flap depth of 120 μm and flap diameter of 9.0 mm; c) the Femto LDV Z6® (group Z) by Ziemer Ophthalmic Systems AG, Port, Switzerland, programed for raster pattern photon delivery, bed energy level of 1.0 μJ, side-cut energy of 0.90 μJ, spot size of 1 μm, side cut angle of 70 degrees, superior hinge angle of 90 degrees, attempted flap depth of 110 μm and flap diameter of 9.0 mm. A suction ring of 9-10 mm was used depending on the corneal curvature according to the manufacturer’s recommendations.
In all groups, once the flap was cut, it was lifted with a spatula and the stromal bed was dried with a sponge. The stromal ablation was performed with the Wavelight Allegretto® excimer laser (WaveLight Laser Technologies AG) programed for spot separation of 0.95 mm, fluence of 200 mJ/cm2, repetition rate of 400 Hz, optical zone of 6-7.5 mm (larger than- or equal to the patient’s mesopic pupillary size) and conventional treatment (non-customized) according to the manufacturer’s recommendations.
After the ablation, the residual stromal bed was gently rinsed with balanced salt solution (BSS®, Alcon Laboratories Inc., Ft. Worth, TX) and the flap was repositioned over the stromal bed. Antibiotic drops (ciprofloxacin 3mg/mL, Oftacilox®, Alcon Cusí, Barcelona, Spain) and non-steroidal anti-inflammatory eyedrops (ketorolac trometamol 5 mg/mL, Acular®, Allergan, Madrid, Spain) were instilled before the speculum was removed.
Ciprofloxacin 3mg/mL and steroid drops (dexamethasone alcohol 1 mg/mL, Maxidex®, Alcon Cusí, Barcelona, Spain) were prescribed four times daily during the first postoperative week and preservative-free artificial tears were applied as needed.
All patients were examined at 1 day, 1 week, 1 and 3 months postoperatively by two experienced masked optometrists who recorded UDVA and CDVA in the same room using the same light adjusted to mesopic conditions. At the 3-month visit, a complete ocular examination was performed, including manifest residual refraction, CDVA and topography.
Statistical analysis was performed with the “Statview SE + Graphics” program (Abacus Concepts Inc., Berkeley, CA, USA) for Macintosh.
Visual acuity measurements were performed in decimal scale (Snellen quotation) but were converted to LogMAR quotation for the statistical analysis using a conversion chart.
The Kolmogorov-Smirnov test was used to test normality and factorial ANOVA was used for multiple comparisons analysis. Intra-group linear regression analysis was performed. Ninety-five percent confidence intervals (CIs) were set up and P values < 0.05 were considered statistically significant.