Haag-Streit Perkins MK3 Manual De Instrucciones página 17

5.4 Instruction to the patient
Make sure that the measuring arm of the tonometer can move freely and is not
hindered by the patient's eyelashes, cheek, etc.
Figure 4
5.5 Astigmatism
If the cornea is spherical, measurements can be made on any meridian, but it is most
convenient to do it on the 0° meridian.
This is not so when eyes with higher corneal astigmatism than 3 dioptres are examined,
as the flattened areas are not circular but elliptic.
It has been calculated that, in cases of larger corneal astigmatisms, a surface of
7.354 mm2 (Ø 3.06 mm) is to be applanated, when the measuring prism is at an angle
of 43° to the meridian of the greatest radius.
For example: If the corneal astigmatism amounts to
6.5mm/30° = 52.0D/30° and 8.5mm/120° = 40.0D/120°
The graduation value 120° of the prism is set at the 43° mark of the prism holder.
If there is a corneal astigmatism of 8.5mm/30° = 40.0D/30° and
6.5mm/120° = 52.0D/120°
The graduation value 30° is set at the 43° mark.
In other words, set the axial position of the greatest radius, that is the axis of a minus
cylinder, on the prism graduation mark on the prism holder.
Instruct the patient to look straight
ahead or slightly upward and if
necessary use a fixation light for this
purpose.
It is recommended that patients are
repeatedly asked to keep their eyes
wide open during the examination. If
required, the lids of the examined eye
may be held open by the examiner's
fingers provided that no pressure is
applied to the eye.
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