title: Assessing corneal hysteresis using the Ocular Response Analyzer.
abstract: OBJECTIVE
  An examination of studies that have assessed corneal biomechanical performance using the Ocular Response Analyzer (ORA: Reichert Ophthalmic Instruments, Depew, NY) raises some questions regarding the influence of measurement variables and the interpretation of the findings obtained with this instrument. This analysis of those questions describes additional factors which do or may contribute to the assessment of corneal hysteresis (CH).
  
  
  METHODS
  Using key words CH and ORA, English language articles relevant to this analysis were selected after a PubMed search with the addition of some articles referenced in the selected publications.
  
  
  RESULTS
  Corneal thickness, the level of edema, intraocular pressure, and corneal temperature as well as the area, location, rate, duration, and sequence of corneal unloading and loading may need to be considered as significant variables when assessing CH.
  
  
  CONCLUSIONS
  CH values may be specific to measurement method and conditions rather than representing an unequivocal corneal property. Consideration of the uncontrolled variables involved may help explain some of the findings obtained with the ORA. That a CH measurement might vary with the sequence of unloading and loading suggests that the ORA CH finding may not represent the CH, but instead represents a hysteresis value better described as central, applanation-derived hysteresis, which is based on a very short unloading/loading sequence. The potential for the ORA to contribute to improved clinical management appears to be considerable but so does the need for better understanding and further development of its functions and applications.
authors.affiliation: School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia. c.mcmonnies@unsw.edu.au
authors.forename: Charles W
authors.initials: CW
authors.lastname: McMonnies
mesh_terms: D001831:Body Temperature; D003315:Cornea / Q000033:anatomy & histology* / Q000502:physiology*; D003941:Diagnostic Techniques, Ophthalmological / Q000295:instrumentation*; D006801:Humans; D007429:Intraocular Pressure; D014783:Viscosity