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Symmetry Studies
Published in Marlos A. G. Viana, Vasudevan Lakshminarayanan, Symmetry in Optics and Vision Studies, 2019
Marlos A. G. Viana, Vasudevan Lakshminarayanan
In fitting contact lenses or prior to surgical refractive procedures such as LASIK, the parameter of primary importance is the curvature of the cornea, measured in units of inverse meter, the diopters, by an instrument called the keratometer. In the study of corneal curvature, quantities are labeled by a number of ℓ equally spaced points along a given corneal aperture, as shown in Figure 1.6 where the separation is of 10 degrees, so that V={2πk/36:k=0,…35},
Anterior segment OCT
Published in Pablo Artal, Handbook of Visual Optics, 2017
Corneal topography can be revealed by mapping the corneal thickness (pachymetry), surface elevation, or curvature/power mapping (keratometry) and plays an important role in the assessment of keratorefractive surgical procedures, corneal transplantation, or screening the corneal degenerations and dystrophies (e.g., keratoconus). There are several techniques enabling topographic description of the eye (Mejia-Barbosa and Malacara-Hernandez, 2001, Pinero, 2015). Access to volumetric data sets enables mapping the corneal thickness (corneal pachymetry). Several studies have been performed to show reproducibility of OCT pachymetry and agreement with standard instruments (Wirbelauer et al., 2002, Li et al., 2006, 2008, 2010, Pinero et al., 2008). The comparison of corneal pachymetry for normal subject and patient with keratoconus is presented in Figure 4.18 (Karnowski et al., 2011). The maps were calculated from corresponding 3-D data sets after segmentation of corneal interfaces and correction for light refraction.
Vecto-keratometry: determination of anterior corneal astigmatism in manual keratometers using power vectors
Published in Expert Review of Medical Devices, 2023
Raquel Salvador-Roger, Rosa Vila-Andrés, Vicente Micó, José J. Esteve-Taboada
The value of corneal astigmatism is classically determined by using manual keratometer, which are instruments that allow clinicians to determine the value of anterior corneal surface radius of curvature in millimeters as well as its dioptric power [7]. Different types of keratometers have been developed since the 1700s [2,8], although the first successful keratometer was created by Hermann von Helmholtz in 1853 [9]. Later, Louis Émile Javal and Hjalmar August Schiøtz introduced the Javal-Schiøtz’s ophthalmometer [2,10]. Over the years, manual keratometers have been successfully used and nowadays are one of the most cost-effective and extended clinical instruments to manage corneal astigmatism. Nonetheless, new automatic methods have appeared such as topographers [11,12] and tomographers [13] which provide complete corneal characterization and are set in modern clinical practice, especially in developed countries. Helmholtz’s and Javal’s keratometers use the same principle to measure anterior corneal power. In both cases, a pair of sights are projected over the anterior surface of the cornea and the clinician must find the orientations of the principal meridians and take the sights to coincidence at those two orientations [14]. This is one of the main limitations in conventional keratometry since the axis definition depends on the examiner visual alignment criterion. The examiner needs to determine visually when the sights are together as well as when they are aligned. Consequently, expertise is an extremely essential skill [15] and subjectivity plays an important role too. Therefore, non-experienced and/or untrained clinicians are more prone to measurement errors when evaluating corneal astigmatism.