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The cornea
Published in Mary E. Shaw, Agnes Lee, Ophthalmic Nursing, 2018
Corneal topography is a non-invasive medical imaging technique for mapping the surface curvature of the cornea. The anterior surface of the cornea, with its tear film, is the major refractive element in the eye and, because of this, even minute changes in its shape can reduce vision. Such vision-altering shape changes include irregular astigmatism induced by trauma and surgery, contact lens warpage and tear film irregularities.
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.
SKILL Biometry
Published in Sam Evans, Patrick Watts, Ophthalmic DOPS and OSATS, 2014
A number of keratometric (K) measurements are made, and, once again, the surgeon should ensure consistency in the readings. Inconsistent results suggest inaccuracy and should not be relied on. In this situation, either repeat the assessment, undertake separate corneal topography or disregard the corneal cylinder and aim for astigmatically neutral surgery. If efforts are made to correct the corneal cylinder during cataract surgery (on axis incisions, opposite clear corneal incisions or limbal-relaxing incisions) then the steep meridian must be identified and the chosen astigmatic corrective techniques employed with this in mind.
Therapeutic non-ectasia applications of cornea cross-linking
Published in Clinical and Experimental Optometry, 2023
Haitham Al-Mahrouqi, Isabella Mei Yan Cheung, Lize Angelo, Tzu-Ying Yu, Akilesh Gokul, Mohammed Ziaei
Refractive procedures with simultaneous CXL have been used to prevent surgery-induced ectasia (iatrogenic ectasia), as well as regression of refractive error in high-risk eyes. The combination of refractive surgery and CXL is also utilised in the management of keratoconus, however, this is outside the scope of this review. Simultaneous CXL was first performed with PRK (often referred to as PRK Xtra) in myopic eyes at high risk of iatrogenic ectasia.100 Such risk factors include young patient age, high attempted refractive correction and abnormal but not ectatic corneal topography.101 Subsequently, simultaneous CXL was also performed with LASIK,102 and Small incision lenticule extraction103 (referred to as LASIK Xtra and SMILE Xtra, respectively).
Comparison of Javal-Schiøtz keratometer, Orbscan IIz and Pentacam topographers in evaluating anterior corneal topography
Published in Clinical and Experimental Optometry, 2023
Jennifer M Turner, Christine Purslow, Paul J Murphy
Ocular surface topography (OST) is a commonly used technique for an accurate and reliable measurement of the ACS.11 Elevation-based corneal topography relies on the measuring instrument being able to produce an accurate representation of the ocular surface profile.2 This depends on the availability of a precise mathematical function for the best interpretation of the observed corneal images. Topographic maps are then rendered to provide a visual picture of the small surface curvature variations seen when comparing normal and abnormal surface shapes. These curvature maps subtract the surface profile from a best-fit sphere, providing the height difference between the two in a colour-coded representation. This allows the viewer to identify areas of discrepancy and observe any patterns of progressive change.
The Association between Fourier Parameters and Clinical Parameters in Myopic Children Undergoing Orthokeratology
Published in Current Eye Research, 2021
Di Wang, Dejia Wen, Bin Zhang, Weiping Lin, Guihua Liu, Bei Du, Fang Lin, Xiaorong Li, Ruihua Wei
Corneal topography is commonly used to evaluate and monitor the corneal shape and refractive status. The corneal topography’s circular nature makes it a perfect candidate for Fourier analysis, which decomposes a complex pattern into the combination of a series of sine waves of different frequencies. The direct current component represents the mean values of the refractive power. The sine wave that runs one cycle per 360 degrees describes the corneal asymmetry. The sine wave that runs two cycles over 360 degrees describes regular astigmatism. Any other pattern would indicate irregular corneal astigmatism.18 Two of the most often used topography system, Pentacam and Tomey, have equipment Fourier analysis capacity. The method has been applied to the assessment of the optical quality of cornea in normal eyes,19–21 eyes with an ocular condition such as keratoconus and pterygium,22,23 and eyes after corneal surgeries,24–26 lens surgeries,27,28 and retinal detachment surgery.29 After orthokeratology, over one-third of eyes had corneal asymmetry beyond the normal range.30 Therefore, it is easy to see why Fourier analysis in orthokeratology.