Topography-based calculation of keratoconus dimensions

Background Keratoconus is a corneal dystrophy which usually develops in the second or third decade of life and shows various speed of progression. Thi s disease may degrade the image-forming properties of the eye even in its e arly stage. The purpose of this study was to support the conventional clini cal qualitative diagnostic methods of keratoconus handling with a topograph y-based algorithm to quantify the "balooning" of the anterior corneal surfa ce. Patients and methods Eighty-eight patients with keratoconus (46 with mild a nd 42 with severe clinical signs) and a control group of 40 normal subjects were included in this study. Topographic height data were calculated from refraction data of a commercially available topographer (TMS-1) using a loc al approximation algorithm. A decomposition of corneal topography height da ta into orthogonal Zernike polynomials was performed to define a asphero-cy lindrical model surface. From the difference of the raw height data and the model surface, the base, height and volume as well as the localization of the protrusion was quantified. Results The height of the corneal protrusion (23 to 71 mu m), the volume of the cone (0.066 to 0.141 mm(3)) and the horizontal dimension (0.67 to 1.32 mm) increased highly significantly. In contrast, comparing the early to th e severe stage of the disease the vertical dimension of the cone (0.64 to 0 .93) changed much less. In the severe stage, the center of the protrusion w as much more decentred (1.33 mm) than in the early stage (0.44) due to a sh ift in the inferior direction. Conclusions The quantification of cone dimensions using corneal topography height data has the potential to assist qualitative clinical graduation in keratoconus independent of the system currently used. The knowledge about t he exact amount of corneal protrusion and position of the cone may improve the assessment of the progression of the disease, thus being helpful for in dication of a penetrating keratoplasty.