title: Constancy of the Orbscan acoustic factor to detect contact lens-induced corneal swelling.
abstract: OBJECTIVE
  The aim of the present study was to compare central and peripheral corneal thickness measurements in corneas with and without contact lens (CL)-induced corneal swelling, in order to establish the constancy of the Orbscan acoustic factor (AF) for reliability in detection of CL-induced corneal swelling.
  
  
  METHODS
  Corneal thickness was measured in five corneal locations (central, superior, inferior, nasal and temporal) using both ultrasonic and Orbscan pachymetry before and after one week of high Dk (lotrafilcon A) and low Dk (etafilcon A) contact lens continuous wear (CW). Lenses were randomly fitted for CW in the right and left eyes of 20 healthy subjects to induce various amounts of corneal swelling.
  
  
  RESULTS
  Etafilcon A contact lenses induced greater corneal swelling than the lotrafilcon A contact lenses (p < 0.05, paired Student t-test). Equal corneal swelling percentages (with and without the application of the acoustic factor) were determined from Orbscan measurements. Application of the 0.92 AF did not permit adjustment to all corneal locations. The correlations between Orbscan with and without the 0.92 AF and ultrasonic pachymetry were higher for central pachymetry (r² > 0.88; p < 0.01) and lower for peripheral pachymetry (r² < 0.58; p < 0.01) with similar trends in corneas with and without CL-induced swelling.
  
  
  CONCLUSIONS
  The acoustic factor proposed by the manufacturer to obtain corneal thickness with the Orbscan compared with those from ultrasonic pachymetry was not valid for all corneal topographic positions. The AF is unnecessary when determining CL-induced corneal swelling.
authors.affiliation: IOBA-Eye Institute and School of Optometry, Department of Physics TAO, University of Valladolid, Valladolid, Spain. raul@ioba.med.uva.es
authors.forename: Raul
authors.initials: R
authors.lastname: Martin
authors.forename: Laura
authors.initials: L
authors.lastname: Nuñez
authors.forename: Jesus
authors.initials: J
authors.lastname: Sastre
authors.forename: Victoria
authors.initials: V
authors.lastname: de Juan
authors.forename: Guadalupe
authors.initials: G
authors.lastname: Rodriguez
mesh_terms: D000293:Adolescent; D000328:Adult; D003262:Contact Lenses, Extended-Wear / Q000009:adverse effects*; D003315:Cornea / Q000000981:diagnostic imaging*; D015715:Corneal Edema / Q000175:diagnosis* / Q000209:etiology; D003941:Diagnostic Techniques, Ophthalmological / Q000295:instrumentation*; D004311:Double-Blind Method; D004867:Equipment Design; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D011446:Prospective Studies; D015203:Reproducibility of Results; D014463:Ultrasonography; D055815:Young Adult