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K. C. Baker, Y.-L. Chen, J. W. L. Lewis, M. Wang; Optical Impact of Keratoconus Cone: Theoretical Investigation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1840. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The NIH/NEI Collaborative Longitudinal Evaluation of Keratoconus (CLEK) research group has studied keratoconus (KC) patient vision and its impact on the quality of life. Currently, little is known about how the specific KC corneal cone characteristics affect the optical aspects of vision. This study investigates the optical correlations between the irregular cornea surface elevation and the resulting spherical equivalent, cylinder and high-order ocular aberrations.
Using statistical descriptions of KC topography, schematic eye models of various KC conditions are constructed. The cone shape, protruding height and extent, and cone location were independently inspected with 3-dimensional optical eye modeling. The subsequent tilt, defocus, and residual higher-order ocular aberrations were examined for each cone characteristic.
The KC eye's optical performance is changed by the effective cone curvature (second derivative of elevation) in the visual zone. The myopic condition (spherical equivalent, SE) depends largely on the cone location. The cone shape plays a less significant role. SE is the worst for KC cones near the visual axis. SE error reduces as the cone distance increases and for an outlying cone tends to be hyperopic. The cylindrical error, on the other hand, is affected by both the cone location and the shape. When near the visual center, the cone shape plays the major role on the cylinder amplitude. If far away from visual center, the cone location dominates. The meridian pointing to the cone apex corresponds to the less powerful meridian. The power in the perpendicular meridian is always myopic, although the SE could be hyperopic for an outlying cone. For a KC cone in the lower visual zone, 270±30 degree, the resulting astigmatism is against-the-rule. The high-order aberrations strongly relate to the cone dimension. Moderate KC cone volumes produce the greatest, or worst, high-order aberrations. Combining these cone sizes with an irregular shape results in the worst scenario of high-order aberrations, even if the cone is far from center. Finally, the significant KC tilt suggests an important role that has yet to be investigated.
Optical eye modeling provides an effective method in studying human vision. The optical explanation in how the physical KC structure influences the vision is presented.
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