On the use of Orbscan II to assess the peripheral corneal thickness in humans : a comparison with ultrasound pachometry measures

Purpose:  To compare the measures of corneal thickness measurements obtained by an optical scanning slit method with those obtained by an ultrasound (US) pachometer, with special interest in the mid-peripheral (2.5 mm from centre) and peripheral (4.5 mm from centre) region of the cornea. Methods:  Three measures of corneal thickness were taken using Orbscan II and then by US pachometry (under topical anaesthesia with benoxinate 0.4%) on 24 adults, aged 20–58 years and with up to 8.5 D of myopia. The full Orbscan topography maps were used to extract single point data along the horizontal corneal meridian for the geometric centre, 2.5 mm from centre (nasal and temporal) and 4.5 mm (nasal and temporal) from centre. No correction factor was used for the Orbscan data. The same set of measures were made with the US pachometer. In all cases, the averages of three (centre) or six (mid-periphery and periphery) readings were taken as the measurements from each cornea. Results:  Orbscan readings on the right eyes averaged 0.576, 0.632 and 0.712 mm for central, mid-peripheral and peripheral sites with average values for emmetropic subjects (<1 DS, n = 12) being marginally higher than for myopic subjects (average – 4.00 DS, n = 12). For US pachometry, the average values were however 0.522, 0.554 and 0.606 mm. Similar results were obtained on left eyes. Combining data from both eyes also showed that the mean difference between Orbscan II and US measures was not constant across the cornea, being 0.055 ± 0.014 mm at the centre, 0.080 ± 0.019 mm at mid-peripheral locations and 0.107 ± 0.046 mm at the peripheral sites. These differences persisted after application of the generally recommended acoustic factor (×0.92) to all of the Orbscan readings. Conclusions:  A single acoustic factor correction cannot be applied to all corneal thickness measures made with an Orbscan II to equate the measures to those made with an US pachometer.