title: Differences in the early biomechanical effects of hyperopic and myopic laser in situ keratomileusis.
abstract: OBJECTIVE
  To compare changes in corneal hysteresis (CH) and the corneal resistance factor (CRF) in myopic and hyperopic laser in situ keratomileusis (LASIK) and evaluate their relationship to the number of photoablative pulses delivered, a surrogate for ablation volume.
  
  
  METHODS
  Cleveland Clinic Cole Eye Institute, Cleveland, Ohio, USA.
  
  
  METHODS
  Preoperative and 1-week postoperative Ocular Response Analyzer measurements in eyes that had femtosecond-assisted LASIK were studied retrospectively. Changes in CH and CRF were compared and tested for correlation with the number of excimer laser pulses.
  
  
  RESULTS
  Thirteen myopic eyes and 11 hyperopic eyes were evaluated. Preoperative corneal thickness, CH, CRF, programmed correction magnitude, flap thickness, and total number of fixed spot-size photoablative pulses were similar in the 2 groups (P>.1). Decreases in CH and CRF were greater after myopic LASIK than after hyperopic LASIK (P<.005), and changes in CRF were correlated with the number of excimer laser pulses in the myopic group only (r = -0.63, P = .02). Regardless of ablation profile, changes in CH were more strongly correlated with preoperative CH values than with attempted ablation volume.
  
  
  CONCLUSIONS
  With comparable flap thickness and attempted ablation volumes, myopic photoablation profiles were associated with greater decreases in CRF and CH than hyperopic profiles. Results indicate that preoperative corneal biomechanical status, ablation volume, and the spatial distribution of ablation are important factors that affect corneal resistance and viscous dissipative properties differently. Preferential tissue removal in the natively thicker paracentral cornea in hyperopia may partially account for the rarity of ectasia after hyperopic LASIK.
authors.affiliation: Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio 44195, USA.
authors.forename: Fabricio Witzel
authors.initials: FW
authors.lastname: de Medeiros
authors.forename: Abhijit
authors.initials: A
authors.lastname: Sinha-Roy
authors.forename: Milton Ruiz
authors.initials: MR
authors.lastname: Alves
authors.forename: Steven E
authors.initials: SE
authors.lastname: Wilson
authors.forename: William J
authors.initials: WJ
authors.lastname: Dupps
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D003315:Cornea / Q000503:physiopathology*; D005260:Female; D006801:Humans; D006956:Hyperopia / Q000503:physiopathology* / Q000601:surgery; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology* / Q000601:surgery; D012189:Retrospective Studies; D013524:Surgical Flaps