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Corneal astigmatism

THEastigmatism eyepiece can be corneal and/or internal (cristallinien). Young, devoid of cataract patients theastigmatism is mainly corneal. The presence of corneal astigmatism moderate (close to 0.50 D) is also more frequent than the absence of corneal astigmatism! Internal astigmatism of cristallinienne origin can be observed during accommodatifs efforts, or in case of cataract (nuclear shape in particular).

If the corneal astigmatism is not compensated for internally, it causes an eye astigmatism which can be corrected by glasses, lenses or by refractive surgery as needed.

Corneal toricite

Regular corneal astigmatism is caused by a regular strain of the cornea: this geometric feature is named «» toricite« . It's the corneal toricite (geometry), which generates the astigmatism (optical anomaly). When the cornea is horny o-ring, different corneal sections do not have the same curvature.

The o-ring cornea has a variation of curvature, the arched Meridian to the Meridian less arched. By case onregular astigmatismThese meridians are orthogonal and the change in curvature between these meridians is continuous and regular.

Topography of corneal astigmatism

Corneal topography specular is a classical method to characterize the corneal astigmatism. The topography specular use the analysis of the reflection of concentric, circular of Placido mires. The change in curvature of the corneal meridians can be appreciated by circular of the mires of Placido deformation.

corneal toricite obtained by the specular reflection in topography

The reflection of a the front of the cornea corneal focus provides information on the topography of the anterior cornea.

Management linking the main lines of the elliptical reflections of the patterns matches less arched Meridian: one that connects the small axes of the elliptical reflections of the mires corresponds to the arched Meridian.

image placido horny toric astigmatism direct

Reflection of mires of Placido in a strongly o-ring cornea. Note circular of the reflection of the mires deformation.

From this picture, it is possible to predict the existence of a change in curvature between the meridians, which the arched are vertical, and the less arched horizontal. The use of the computerized image analysis to calculate the radius of curvature at each point in certain directions (ex axial mode).

map topographical of corneal astigmatism direct

Topographic map deducted analysis automated deformation of the mires of Placido: the figures represent the radii of curvature in mm.

The topographic map obtained from the analysis of the patterns of Placido allows to quantify the variations of axial curvature. Astigmatism caused by the cornea is direct (or consistent).

The following maps are a responsible strongly o-ring cornea of an astigmatism indirect (reverse or non-compliant).

toricite corneal extreme

Relfets of Placido patterns on the surface of a cornea strongly o-ring, head of an astigmatism reverses.

The resulting axial topographic map shows a camber accentuated next to horizontal meridians:

Topographic map axial astigmatism reverses

Axial topgraphique card: corneal astigmatism reverses (non-compliant). The numbers correspond to the rays of curves measured according to the axial direction (in mm)

 

"' Appearance in ' bowtie ' topography corneal specular (axial mode) is due to both the toricite and the asphericity of the cornea. The physiological corneal Asphericity is of type prolate: the cornea flattens the edges and this caused a progressive "cooling" of the colors.

toricite Asphericity specular topographic aspect

The toricite produces a southern variation of the curvature: If the cornea was purely o-ring, there would be a change in curvature between the meridians, but there wouldn't be many variations along the meridians. The Asphericity prolate causes a gradual reduction of curvature to the edges. The appearance as a result of the conjunction of these geometric properties is represented below: hourglass or "bowtie". More the Asphericity is prolate; the butterfly knot is small.

 

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