title: The use of the Ocular Response Analyser to determine corneal hysteresis in eyes before and after excimer laser refractive surgery.
abstract: OBJECTIVE
  To compare corneal biomechanical parameters and two measures of intraocular pressure (IOP) in eyes before and after excimer laser refractive surgery, with the Ocular Response Analyser (ORA).
  
  
  METHODS
  Eighty normal eyes of 41 patients undergoing excimer laser refractive surgery in Birmingham, U.K. were recruited into three groups: Laser Assisted-Epithelial Keratomileusis (LASEK) (Myopes), Laser Assisted in Situ Keratomileusis (LASIK) (myopes) and LASIK (hyperopes). The preop and 3 months postop Goldmann correlated IOP (IOPg), corneal compensated IOP (IOPcc), corneal hysteresis (CH), and corneal resistance factor (CRF) were measured by the ORA. Central corneal thickness (CCT) was measured using ultrasonic pachymeter. The differences of the changes in IOPg, IOPcc, CH, CRF and CCT between the three groups were estimated. A General Linear Model was selected to investigate the influence of gender, age, initial conditions (CH, CRF, CCT, IOPcc and IOPg) and changes in CCT on the measured IOP.
  
  
  RESULTS
  The differences between the mean IOPg, CH and CRF after refractive surgery were statistically significant for all three groups. The hyperopic LASIK group had a significantly smaller change compared to the other groups (which had no statistical significance). The preop IOPg, preop CH and gender were significant predictors of the changes in measured pressure and biomechanical parameters after surgery in the myopic groups only.
  
  
  CONCLUSIONS
  CH and CRF were found to decrease after both myopic and hyperopic refractive surgery. CH and CRF measurement may prove important tools to clarify the role of corneal biomechanics for refractive surgery.
authors.affiliation: Heart of England Foundation Trust, Solihull, UK; Ophthalmic Research Group, Aston University, Birmingham, UK.
authors.forename: Sunil
authors.initials: S
authors.lastname: Shah
authors.forename: Mohammad
authors.initials: M
authors.lastname: Laiquzzaman
authors.forename: Ian
authors.initials: I
authors.lastname: Yeung
authors.forename: Xueliang
authors.initials: X
authors.lastname: Pan
authors.forename: Cynthia
authors.initials: C
authors.lastname: Roberts