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© HAAG‑STREIT AG, 3098 Koeniz, Switzerland − HS‑Doc. no. 1500.7006000.04260 – 26. Edition / 2020 – 03
3.6 Description of models
•
The Applanation Tonometer
AT 900 model R
can be left on the slit lamp perma‑
nently. It is fastened to the microscope on a mounting base and swivelled in front
of the microscope for the examination. The observation of the applanated surface
is conducted monocularly – only through the left eyepiece.
•
The Applanation Tonometer
AT 900 model T
is set on the guide plate over the slit
lamp axis, for tonometry purposes. The instrument can be moved between two
positions on the guide plate for observation through the right or left eyepiece. The
sensor arm with the measuring prism projects from the underside into the beam
path of the microscope and the illumination.
•
The Applanation Tonometer
AT 900 model BQ
has a working position and two
resting positions. To set the correct angle of incidence of the illumination, the to‑
nometer is swivelled and the illumination apparatus is positioned to the left of the
tonometer bracket. In this position, the patient's left and right eyes can be easi‑
ly examined (no 60° position). The applanated surface is observed monocularly
through the right eyepiece of the stereo microscope.
•
The Applanation Tonometer
AT 870
is positioned over the slit lamp's (from other
manufacturer) microscope. The feeler arm projects into the microscope and illumi‑
nation beam path from above. The tonometer is ready for use with a single swivel
movement of the feeler arm.
NOTE!
The instructions for appliances from other manufacturers are to be ob‑
served.
4. Commissioning
WARNING!
It is imperative to read the 'Safety' chapter and to observe its precautions
before operating the equipment.
NOTE!
A smooth operation can only be guaranteed with the use of original
Haag‑Streit measuring prisms and Tonosafe.
5. Operation
WARNING!
Reusable measuring prisms must be cleaned and disinfected after ev‑
ery examination.
5.1 Astigmatism
•
If the cornea is spherical, measurements can be taken on any meridian, but it is
most convenient to measure on the 0° meridian.
•
The choice of meridian is of significance, however, when measuring eyes with cor
‑
neal astigmatism greater than 3 dioptres, since the flattened area is not circular but
elliptical.
•
It has been calculated that, in cases of greater corneal astigmatism, a surface of
7.354 mm
2
(ø 3.06 mm) is applanated if the measuring prism is at an angle of 43°
(mark (A)) to the meridian of the greatest radius.
Example:
If the corneal astigmatism measures
6.5 mm / 30°
8.5 mm / 120°
=
=
52.0 dpt / 30° and
40.0 dpt / 120°
(A) 43°
the 120° graduation mark on the prism is set on the mark (A)
of the prism holder.
If, on the contrary, the measurements are
6.5 mm / 120°
8.5 mm / 30°
=
=
40.0 dpt / 120° and
52.0 dpt / 30°
then the 30° graduation mark is set on the mark (A), meaning that simply the axis
position of the largest radius is placed on the mark (A).
5.2 How pressure is measured
The applanation tonometer functions according to the 'Goldmann method': measure‑
ment of the pressure required to maintain a uniform applanation of the surface of the
cornea.
•
The readings obtained are based on the assumption that a cornea with a 'normal'
thickness is being measured. (a different corneal thickness leads to changes in the
measured IOP) A 'normal' corneal thickness is considered to fall within the range of
530 to 560 Mikrometer.
•
If wrong measuring results are suspected, please conduct a functional verification
in accordance with Section 'Maintenance'.
5.3 Preparing the patient
WARNING!
Measurements without fluorescein can lead to false results.
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