Comparison of Femto LDV Z8 Corneal Lenticule Extraction for Advanced Refractive Correction (CLEAR) and Visumax Small Incision Lenticule Extraction (SMILE)

This observational study aims to compare two laser corrective procedures for treating myopia and myopic astigmatism: a novel refractive technique, the CLEAR procedure, and the established SMILE procedure. Participants will be randomized to undergo CLEAR in one eye and SMILE in the other. The investigators will evaluate and compare the two procedures based on the following aspects: refractive predictability (how accurate the vision correction is) by measuring the proportion of eyes close to the target prescription after surgery, clinical dry eye outcomes, changes in tear proteins, and corneal nerve changes at 1, 3, 6, and 12 months after both surgeries. Metabolomic features of the removed corneal tissue and how precisely the laser is centered during surgery will also be examined. Our hypothesis is that CLEAR provides equally good vision results as SMILE, and explore any extra benefits of CLEAR.

Surgical correction for refractive errors may be performed by a number of procedures. Laser correction involves refractive procedures such as laser assisted in-situ keratomileusis (LASIK), small incision lenticule extraction (SMILE), flap based refractive lenticule extraction (femtosecond lenticule extraction and pseudo-SMILE), surface ablation techniques like photorefractive keratectomy or laser epithelial keratomileusis, while lens-based corrections involve the use of phakic intra-ocular lenses.

SMILE (Visumax) is a flapless refractive procedure which was introduced in 2011 for the treatment of myopia and myopic astigmatism. In this procedure a stromal lenticule is extracted through a small incision (2.0 to 5.0 mm). It involves docking, femtosecond laser application, lenticule dissection from the surrounding stroma, and extraction. Studies have suggested that using a SMILE technique resulted in better ocular surface stability and biomechanical strength when compared with LASIK. SMILE has a steep learning curve. The most common complication encountered during the initial learning curve, is difficult lenticule dissection and extraction with an incidence of up to 16%. Other intraoperative complications include suction loss, an opaque bubble layer, and tearing of the lenticule.

Corneal Lenticule Extraction for Advanced Refractive Correction (CLEAR) by FEMTO LDV Z8 (Ziemer) is a novel technique of flapless refractive surgery which uses low energy femtosecond laser. It creates highly focused laser pulses achieving photodisruption in the low nanojoules range (\<100 nJ). Decreased stromal gas generation and accurate laser focus are the advantages of this low energy concept. The advantages of CLEAR treatment are possibility of recentering the treatment area after having performed the docking, which is not possible with the VisuMax Laser System, multiple corneal incisions for easy removal of the lenticule for the beginners, trusted vacuum with minimal suction losses. Previous study concluded that the mean preoperative sphere, cylinder, and spherical equivalents were -3.05 (D), 0.50 D, and -3.30 D respectively and 1 month after CLEAR the sphere, cylinder and spherical equivalents were 0.05 D, 0.3 D and 0.12 D respectively.

As the investigators do not expect to see a great improvement to the results from the already established SMILE procedure, the investigators aim to demonstrate that CLEAR is just as good in terms of visual outcomes. However, the investigators aim to show that CLEAR has additional benefits, on top of the equal or comparative visual outcomes.

Currently there are no publications comparing the refractive predictability, higher order aberrations, changes in tear neuromediators, tear proteomics, keratocytes and nerve regeneration, corneal lenticule metabolomics, intraoperative laser centration comparing CLEAR and SMILE, hence this study will be unique in the fact that the paired eye approach will act as an internal patient control to assess the objective and subjective outcomes in patients following both these procedures.

Eligibility Criteria:
Inclusion Criteria:

* \>/= 21 years of age and \</= 50 years of age.
* Cycloplegic spherical equivalent of \>-1.00D
* Refractive cylinder -2.00 D or less; anisometropia \<1.00D
* Best spectacle corrected visual acuity (BSCVA) of 6/12 or better in BOTH eyes.
* Spherical or cylindrical error has progressed at -0.50D or less per year from date of baseline measurement.
* Contact lens wearers must have removed contact lenses at least two (2) weeks before the baseline measurement.
* No evidence of irregular astigmatism on corneal topography.
* Available to attend post-operative examinations for a 12-month period.

Exclusion Criteria:

* Progressive or unstable myopia and/or astigmatism.
* Clinical or corneal topographic evidence of keratoconus.
* Patients who want bilateral SMILE procedure
* Patients who want extracted corneal lenticule to be stored for future refractive surgery.
* Residual, recurrent or active ocular disease such as uveitis, severe dry eyes, severe allergic eye disease, glaucoma, visually significant cataract and retinal disease.
* Previous corneal surgery or trauma within the corneal treatment zone.
* Corneal vascularisation within 1mm of the corneal treatment zone.
* Taking systemic medications likely to affect wound healing, such as corticosteroids and antimetabolites.
* Systemically immunocompromised.
* Systemic disease likely to affect wound healing, such as diabetes, connective tissue disease and severe atopy.
* Pregnant or nursing.