title: Biomechanical Weakening of Different Re-treatment Options After Small Incision Lenticule Extraction (SMILE).
abstract: OBJECTIVE
  To determine the corneal weakening induced by different re-treatment options after small incision lenticule extraction (SMILE) and investigate the potential of corneal cross-linking (CXL) to reestablish the original corneal stress resistance.
  
  
  METHODS
  A total of 96 freshly enucleated porcine corneas were used. The initial refractive correction was defined to be -11.00 diopters (D) and the required enhancement to be -3.00 D. Three different re-treatment options were analyzed: -3D Re-SMILE, -3D photorefractive keratectomy (PRK) on top of the SMILE cap, and cap-to-flap conversion and -3D excimer ablation on the stromal bed (LASIK). The control condition did not receive any treatment. Subsequently, accelerated CXL (9 mW/cm2, 10 min) was performed in two groups with currently common enhancement techniques: following cap-to-flap conversion (-3D LASIK enhancement) and in controls. Biomechanical properties were measured with stress-strain extensometry ranging from 1.27 to 12.5 N.
  
  
  RESULTS
  The Re-SMILE and PRK enhancement did not significantly reduce the overall elastic modulus of the cornea compared to controls (24.7 ± 2.23 and 22.7 ± 2.61 versus 23.8 ± 3.35 MPa, P ≥ .176), whereas LASIK enhancement did (22.2 ± 3.37 MPa, P = .048). CXL treatment significantly increased the elastic modulus compared to all non-cross-linked conditions (P ≤.001). Refractive surgery decreased the overall elastic modulus by 7%, whereas CXL increased it by 20%.
  
  
  CONCLUSIONS
  In enhancement, the corneal biomechanical integrity is less affected with both Re-SMILE and PRK enhancement. Corneal weakening through laser refractive surgery is small compared to the stiffening effect after CXL. [J Refract Surg. 2017;33(3):193-198.].
authors.forename: Sabine
authors.initials: S
authors.lastname: Kling
authors.forename: Bogdan
authors.initials: B
authors.lastname: Spiru
authors.forename: Farhad
authors.initials: F
authors.lastname: Hafezi
authors.forename: Walter
authors.initials: W
authors.lastname: Sekundo
mesh_terms: D000818:Animals; D003094:Collagen / Q000627:therapeutic use*; D003319:Corneal Stroma / Q000637:transplantation; D048988:Corneal Surgery, Laser / Q000009:adverse effects*; D003432:Cross-Linking Reagents / Q000627:therapeutic use*; D004195:Disease Models, Animal; D054018:Lasers, Excimer / Q000627:therapeutic use; D009216:Myopia / Q000175:diagnosis / Q000503:physiopathology / Q000601:surgery*; D010778:Photochemotherapy / Q000379:methods*; D017319:Photosensitizing Agents / Q000627:therapeutic use*; D011183:Postoperative Complications / Q000188:drug therapy*; D019233:Retreatment; D013524:Surgical Flaps; D013552:Swine
chemical_list: D003432:Cross-Linking Reagents; D017319:Photosensitizing Agents; D003094:Collagen