Purchase this article with an account.
AC Fisher, M Batterbury, SB Kaye, S Shah; Harmonic Representation of the General Ellipsotoric Surface: Its Application in Corneal Elevation (Orbscan) Topography . Invest. Ophthalmol. Vis. Sci. 2002;43(13):171.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: To develop a visual representation of corneal elevation topography data based on the formal general ellipsotorus using graphical orthogonal Fourier series descriptors. Principle: This approach seeks to resolve the impasse between the rigour of an analytically authentic corneal shape model and the requirement for a reduced form which is readily understood and utilisable in the clinical environment. Paraxial complex regular astigmatism where the apical toricity is coupled with progressive corneal curvature change along the meridians is well described by the ellipsotorus: viz hemi-meridians are varyingly elliptical and circumferential mires are varyingly toric. In the present model, the rotational (angular) dependence of meridional elliptical eccentricity and the radial dependence of toricity are independently represented by their Fourier series. The true generality of this model allows for the oblique orientation of toric principal axes. Method: A meta data set (n=250) was consolidated from 3 Orbscan databases. Raw image data of anterior elevations were mapped into a graphically-completed apex-centred polar map. Fourier series representations of eccentricity and toricity were constructed, and then parametized using an artificial neural network to classify normal, regular astigmatic and irregular astigmatic corneas. Results: The 3 groups were unequivocally qualitatively discernible from their visual representations. The quantitative delineation of groups was illustrated by identifying clinically manifest keratoconus, in an early trial, with a sensitivity of 0.94 and specificity of 0.82. Conclusion: The model, despite its rather intimidating formal basis, lends itself well to visual interpretation via a Fourier series representation and is easily, and usefully, parametized for descriminant analysis. An exam report layout based on this model is currently in development.
This PDF is available to Subscribers Only