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S. Jonuscheit, M.J. Doughty, N.F. Button; Why the Acoustic Correction Factor is Not Suitable for the Comparison of Orbscan and Ultrasound Pachymetry . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1326.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the corneal thickness with scanning–slit (Orbscan) and ultrasound pachymetry at central, mid–peripheral and peripheral sites and compare the outcome of both instruments.
Corneal pachymetry was performed on 50 eyes of 25 healthy subjects. Data was extracted for Orbscan II at the geometrical centre, mid–peripheral locations 2.5 mm to either side of the centre and peripheral locations 4.5 mm from the centre along the horizontal meridian. Ultrasound readings were obtained from the locations used for the Orbscan pachymetry maps, after instillation of a drop of a topical anesthetic (Minims Benoxinate 0.4 %). Three measures were taken at each location and averaged for each cornea.
Orbscan readings averaged 0.578 +/– 0.037 mm (+/– SD), 0.633 +/– 0.036 mm, and 0.718 +/– 0.035 mm for central, mid–peripheral, and peripheral sites. Ultrasound measures were 0.522 +/– 0.033 mm, 0.553 +/– 0.035 mm, and 0.611 +/– 0.055 mm. The mean difference between Orbscan II and ultrasound was not constant for the different corneal locations, being 0.055 +/– 0.014 mm at the centre, 0.080 +/– 0.019 mm at the mid–periphery and 0.107 +/– 0.046 mm at the peripheral sites. These differences were all statistically significant (p < 0.001), and application of the acoustic correction factor of 0.92 did not correct them.
The default acoustic correction factor of 0.92 of the Orbscan II may be suitable for central corneal thickness correction. However, it is clearly not suitable to bring all Orbscan measurements into accordance with ultrasound results. It should therefore be applied with great caution, especially when assessing more peripheral corneal locations.
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