title: Differences in the corneal biomechanical effects of surface ablation compared with laser in situ keratomileusis using a microkeratome or femtosecond laser.
abstract: OBJECTIVE
  To compare the effects of different flap creation techniques on the biomechanical properties of the cornea in patients having myopic laser refractive surgery.
  
  
  METHODS
  UCLA Laser Refractive Center of the Jules Stein Eye Institute, Los Angeles, California, USA.
  
  
  METHODS
  In this retrospective case series, eyes that had myopic laser refractive surgery were categorized according to the type of flap creation: mechanical microkeratome (MK) LASIK (n=32), femtosecond laser (FSL) LASIK (n=32), or no flap creation (PRK) (n=33). The preoperative central corneal thickness, intraoperative flap thickness, and planned ablation depth (AD),and the preoperative and postoperative manifest refraction spherical equivalent, corneal hysteresis (CH), and corneal resistance factor (CRF) were recorded.
  
  
  RESULTS
  The mean change in CH (DeltaCH) was 2.2 mm Hg, 1.9 mm Hg, and 2.3 mm Hg in the MK, FSL, and PRK groups, respectively. There were no significant differences in AD, DeltaCH, or DeltaCRF between the 3 groups. The correlation between AD and DeltaCH was significant in all 3 groups. The correlation was strongest in the FSL group (r=0.82, P<.0001) and weaker in the PRK group (r=0.47, P= .006) and MK group (r=0.46, P= .008).
  
  
  CONCLUSIONS
  The biomechanical measures of CH and CRF decreased similarly after PRK and LASIK using laser or mechanical flap creation. However, LASIK using femtosecond laser flap creation caused a significantly more predictable change in corneal biomechanics, which correlated strongly with AD, than the change with PRK and LASIK with microkeratome flap creation.
authors.affiliation: Jules Stein Eye Institute, David Geffen School of Medicine at UCLA, Los Angeles, California 90095, USA. hamilton@jsei.ucla.edu
authors.forename: D Rex
authors.initials: DR
authors.lastname: Hamilton
authors.forename: R Duncan
authors.initials: RD
authors.lastname: Johnson
authors.forename: Nancy
authors.initials: N
authors.lastname: Lee
authors.forename: Nirit
authors.initials: N
authors.lastname: Bourla
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D003315:Cornea / Q000502:physiology*; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D054018:Lasers, Excimer / Q000627:therapeutic use*; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D018903:Photorefractive Keratectomy / Q000379:methods*; D012189:Retrospective Studies; D013524:Surgical Flaps; D014792:Visual Acuity / Q000502:physiology