Validity of pachymetric measurements by manipulating the acoustic factor of Orbscan II.
OBJECTIVE
To assess the validity of pachymetric measurements by examining the constancy of the acoustic factor (AF) of the Orbscan II (Orbtek, Bausch & Lomb, Rochester, NY) after overnight rigid gas-permeable (RGP) contact lens wear.


METHODS
Twenty participants wore CRT (Paragon Vision Sciences, Mesa, AZ) HDS 100 contact lenses on one eye and control lenses on the contralateral eye for one night while sleeping. Another 24 participants wore CRT lenses on both eyes for one night. Central corneal thickness was measured using optical coherence tomography and Orbscan II on the night before lens use, immediately after lens removal on the following morning, and 1, 3, 6, and 12 hours later. By using optical coherence tomography as a reference, the adjusted AF was calculated by using a least squares method over time.


RESULTS
The adjusted AF depended on the corneal thickness in normally hydrated corneas. The adjusted AF and the percentage change of the adjusted AF varied before and after overnight lens wear. There was a strong and significant correlation between the corneal swelling and the percentage change of the adjusted AF (all r at least 0.91, P<0.05).


CONCLUSIONS
The adjusted AF is a variable, not a constant. The AF is a function of the corneal thickness and its alteration with, for example, corneal swelling. The validity of the adjusted Orbscan II pachymetric measures using a single AF is untenable.
Centre for Contact Lens Research, School of Optometry, University of Waterloo, Canada.
Fenghe
F
Lu
Trefford
T
Simpson
Desmond
D
Fonn
Luigina
L
Sorbara
Lyndon
L
Jones
D000328:Adult; D001837:Body Weights and Measures; D003261:Contact Lenses / Q000009:adverse effects; D003315:Cornea / Q000473:pathology*; D015715:Corneal Edema / Q000209:etiology / Q000473:pathology; D003941:Diagnostic Techniques, Ophthalmological / Q000295:instrumentation*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000150:complications; D015203:Reproducibility of Results; D041623:Tomography, Optical Coherence