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Instruction Manual Easygraph
“Decentration + spher.”
The direction of
decentration is usually
vertical or
approximately vertical
(as opposed to
horizontal or ap-
proximately horizontal
in the healthy eye)
and depends on the
apex position of the
cone, which is
represented very
clearly in the
combined presentation mode.
The degree of decentration is usually greater than in
the normal eye, exceeding 0.45 mm. However, this
parameter too is subject to variation and not to be
used as an index.
“Regular astigmatism”
Whereas in
astigmatism the axis
runs in a straight line, in
keratoconus it not
infrequently undergoes
a rotation from the
center to the periphery,
thus taking on a spiral
appearance.
“Irregularities”, resp., Irregularities + spherical
equivalent”
In keratoconi of various degrees of severity one often
encounters maps with more or less clear-cut three or
four-leafed shapes. These are indicative of higher-
order corneal aberrations, referred to in the context
of Zernike polynomials as trefoil or four-lobed defect.
Combining these irregularities with the spherical
equivalent often renders them more distinct.
Trefoil images (irregularities)
Three and four-leafed shape (irregularities plus
spherical component)
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