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11. Astigmatism
If the cornea is spherical, measurements may be made along any
meridian, but it is usual to measure along the horizontal 0°
meridian. This is not the case when measurements are made on
eyes affected by corneal astigmatism of greater than 3 diopters,
since the flattened areas will not be circular but elliptical.
It has been calculated that in cases of more severe corneal
astigmatism a surface area of 7,354 mm
2
(ø 3,06 mm) must be
applanated; in this case the measuring prism forms an angle of
43° to the meridian of the maximum radius.
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For example:
For corneal astigmatism of 6.5mm / 30º= 52.0 diopters / 30º and 8.5mm / 120º
= 40.0 diopters / 120º, the 120° prism value will be aligned with the 43° "A"
mark on the prism support.
For corneal astigmatism of 8.5mm / 30º = 40.0 diopters / 30º and 6.5mm / 120º
= 52 diopters / 120º, the 30° prism value will be aligned with the 43° "A" mark.
In other words, align the axial position of the major radius (that is, the axis of a
negative cylinder) with the prism value at the "A" mark on the prism support.
0° marker
43° marker
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