An assessment of regional differences in corneal thickness in normal human eyes, using the Orbscan II or ultrasound pachymetry

Agreement and reliability in measuring central corneal thickness with a rotating Scheimpflug–Placido system and ultrasound pachymetry

“…It is widely used for its good repeatability, low cost, portability and relative ease of use [12][13][14][15]. However, USP shows some limits as the difficulties in centration and alignment that can rise variability of measurement.…”

Discrepancy between central and midperipheral corneal thickness measurements obtained with slit-scanning pachymetry and noncontact specular microscopy

“…The net differences were assessed by the Bland-Altman method using the raw data and the data adjusted with a 0.92 acoustic factor or with calculated acoustic-factor values. 11,12 …”

“…[15][16][17] Our results confirmed this but indicated a fundamental difference between how the slit-scanning device generates peripheral corneal thickness measurements in contrast to the simpler optical principles of specular reflection microscopy or even ultrasound pachymetry. [11][12][13][14] There are 2 main issues. First, peripheral thickness measurements with the slit-scanning device are substantially higher than those obtained with noncontact specular microscopy and ultrasound pachymetry.…”

“…Such off-center pachymetry with the slit-scanning device raises an additional issue that has received little attention: the apparent discrepancy between slit-scanning and ultrasound pachymetry does not appear to be constant across the cornea in that the difference is reported to be greater with more peripheral measurements. 11 This discrepancy has been confirmed in a diverse group of subjects, [12][13][14] further arguing that a single acoustic factor cannot be applied to align Orbscan measurements with those from ultrasound pachymetry, especially because the reason for the difference in CCT measurements between a contact method (ultrasound) and a noncontact optical method (slit scanning) is unknown.…”

“…Specular microscopy was followed by slit-scanning pachymetry (Orbscan II), with particular attention to aligning the instrument to include the limbus on both sides to obtain a symmetrical set of scan data across the entire horizontal meridian of the cornea. 12 After the quality of the scan acquisition was checked on the display of the numerical pachymetry map, the following data were selected: zonal thickness measurements (averaged over a 2.0 mm diameter circle) at the central corneal region (at intersection between horizontal and vertical meridians) and 2.0 mm zones with the centers located 3.0 mm from the central point along the horizontal meridian for both the nasal location and the temporal location. A printout of the selected zonal pachymetry was then generated.…”