Determination of corneal volume from anterior topography and topographic pachymetry: application to healthy and keratoconic eyes.

PURPOSE

To describe a method to measure corneal volume from topography and pachymetry, and test its clinical use on a sample of healthy human subjects and a case of circumscribed posterior keratoconus.

METHODS

Corneal curvature (PCT 200 corneal topography system; Optopol Technology SA, Zawiercie, Poland) and ultrasonic topographic pachometry on 25 points (Ophthasonic A-Scan/Pachometer III; Teknar Inc., St Louis, MO, USA) were measured on each of 12 young healthy corneas and one cornea suffering from circumscribed posterior keratoconus. Topography and pachymetry data were used to calculate the coordinates for the corresponding points on the posterior surface of the cornea. TableCurve 3D software (Systat Software Inc., Chicago, IL, USA) was used to fit a surface to those points measured. Integration of the surface fitted to the data points, corresponding to the anterior and posterior corneal surfaces, was used to calculate the volume underneath each of them. Subtraction of volumes underneath anterior and posterior surfaces, taking into account an axial offset equal to the central corneal thickness, rendered corneal volume for the central 6 mm of the cornea.

RESULTS

Central corneal thickness ranged from 520 to 630 mum for the healthy corneas. Corneal volumes for this sample analyzed averaged 18.66 +/- 1.15 mm(3) (range 17.25-20.53 mm(3)). For the posterior keratoconic cornea, the affected area was located at about 1.5-2 mm from the corneal center on the 135 degrees hemimeridian of the right eye, observed through topographic pachymetry. Calculated corneal volume for the central 6 mm was 16.072 mm(3), noticeably lower than those found in the sample without pathology, but within the range for corneas presenting with keratoconus.

CONCLUSIONS

Corneal volume is a useful parameter for characterising dystrophic corneas and can aid in the detection of rare anomalies which are hardly detected with corneal topography and/or central corneal thickness evaluation. A potentially useful measure of corneal volume can be calculated from anterior corneal topography and topographic pachymetry data. Values obtained are in good agreement with previous studies using corneal tomography techniques. The methodology has been shown to have potential for retrospective analysis of data, or where no access is available to tomographical techniques.