Influence of corneal curvature on central and paracentral pachymetry with optical coherence tomography

Optical coherence tomography (OCT) has been found to be valuable to measure the corneal thickness (CT). The purpose of this clinical study was to determine the influence of corneal curvature on central and paracentral pachymetry with slit lamp-adapted OCT using a linear scanning geometry.

In a prospective, comparative observational study, a total of 77 consecutive patients (77 eyes) participated. Central and paracentral (4 mm) CT with slit lamp-adapted OCT (Heidelberg Engineering, Heidelberg, Germany) and with ultrasound (US) pachymetry were performed. The corneal curvature was determined with manual keratometry, and the influence of the corneal curvature on CT with OCT was compared with curvature-independent US values.

The overall mean central CT values were 533 +/- 53 microm (OCT) and 546 +/- 56 microm (US) and paracentrally 562 +/- 55 microm (OCT) and 569 +/- 55 microm (US). The mean central and paracentral differences (P < 0.001) between both methods were 12.7 and 6.4 microm, respectively. The mean corneal curvature was 7.79 +/- 0.30 mm corresponding to a mean corneal power of 43.39 +/- 1.69 diopters. In the central area, there was no influence on the OCT measurements (r = -0.01, P = 0.935), whereas there was a minimal influence (r = 0.131, P = 0.021) on paracentral pachymetry with OCT using a linear scanning module. In paracentral areas, the systematic difference between OCT and US decreased from 8.4 microm in flat corneas (P < 0.001) to 5.0 microm in steeper corneas (P = 0.06).

Slit lamp-adapted OCT allowed noncontact central and paracentral CT measurements. Although there were significant differences between OCT and US measurements, the corneal curvature had only a minimal influence on paracentral pachymetry with OCT using a linear scanning geometry. For more peripheral measurements or in marked alterations of the corneal curvature, correction factors may be needed.