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Alejandra Consejo, D Robert Iskander; Determining the position of limbus corneae from anterior eye surface topography. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1965.
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© ARVO (1962-2015); The Authors (2016-present)
To demarcate the limbal region from a profilometry measurement of an anterior eye surface.
Traditionally, the limbus has been assumed to coincide with the HVID or the, white-to-white (W2W) radius estimated from 2D “en face” intensity images. Recently, a new technology based on the principle of profilometry, which has the potential of measuring the corneo-scleral topography extending the acquired area up to 20 mm diameter far beyond the limbus, has become available. Studying the 3D corneo-scleral topography is possible using this technology. Of interest was whether the 3D anterior eye scleral topography could be utilized to demarcate the limbal area based on local curvature. Raw 3D profilometry data was fit with the 8th radial order Zernike polynomial expansion, from which semi-meridional curvature was calculated. This local curvature was analytically calculated using the first and the second derivative of Zernike polynomials. Changes in local curvature sign were used to demarcate the limbus.
In some of the studied cases no change was observed in the sign of local curvature in the temporal side of the anterior corneo-scleral topography leading to incomplete limbus demarcation. Also, the topography-based estimation of the limbus location did not always correspond to the limbus estimated from the changes apparent in the intensity images (i.e., HVID, W2W), see Fig 1 and 2. It is noticed that the limbus demarcation is specific to individual’s anterior eye, indicating large varieties of scleral topography.
A method of limbus demarcation based on the Zernike polynomial expansion of the total anterior eye topography has been proposed. It was noticed that topography-based limbus demarcation is not a trivial task. There are no standards. Moreover, the results depend on the limbus definition (in our case change in sign of local curvature) as well as the technique used for its estimation. With more advanced technologies becoming readily available, one is able to choose a specific definition of limbus suitable for his or her particular purposes. The proposed topography-based estimation of the limbus location can be particularly attractive for scleral contact lens design and fit.
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