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How the pressure is measured
The Applanation Tonometer functions according to the 'Gold-
mann method': The measuring of the pressure required to
maintain a uniform applanation of the surface of the cornea.
The cornea is applanated with a measuring prism of or-
ganic glass which is fitted in a ring shaped holder at the end
of the feeler arm. The contact surface of the measuring prism
has a diameter of 7.0 mm. It is planar with rounded edges to
obviate any damage to the cornea.
The measuring prism is brought into contact with the eye
of the patient by moving the slit lamp forwards. The measuring
drum is turned to increase pressure on the eye until a conti-
nuously regular surface of 3.06 mm ø = 7.354 mm
applanated.
Position of the
Force
measuring drum
9.8 mN
The intraocular pressure in mm Hg is found by multiplying
the drum reading by ten.
According to ISO 1000 the derived unit of force is the N
(Newton) and its decimal sub-multiple the mN (milli Newton).
The conversion of mm Hg into the SI pressure unit kPa
(Kilo Pascal) can be made on the comparison scale (10).
The measurement of the flattened surface is made directly
on the cornea. The built-in duplication system in the measuring
prism divides the image and displaces the two semi circular
halves from each other by 3.06 mm.
Before measuring, it will be necessary to anesthetise the
cornea locally, to place a strip of fluorescein paper in the con-
junctival sac and switch on the blue filter of the slit lamp.
The inner border of the ring represents the line of demarcati-
on between the cornea flattened by applanation and the cor-
nea not flattened.
The major advantage of applanation tonometry is the small
bulbus deformation which amounts to only 0.56 mm
lues found by this method of tonometry are only slightly influ-
enced by scleral-rigidity and radius of corneal curvature, the
intraocular tension increases by about 2.5% only.
The principle of applanation tonometry is simple. The ca-
reful construction of the apparatus is a guarantee of its conti-
nued correct performance.
Nevertheless, in order to obtain exact results, the method
of use indicated must be strictly followed.
The readings obtained are based on the assumption that a
cornea with a 'normal' thickness is being measured, alteration of
the corneal thickness leads to changes in the measured IOP. A
'normal' corneal thickness is considered to be within the area
of 530 to 560 microns.
10
2
has been
Pressure
.33 kPa = 0 mm Hg
Zauberformel?
der gemessenen Tonometrie-Messwerte im Falle einer verän-
derten Hornhautdicke.
sich mittels einschlägiger Literatur stets auf dem Laufenden
halten, insbesondere was die verschiedenen Formeln zur An-
passung gemessener Tonometrie-Messwerte anbelangt, wenn
. The va-
3
dünne oder dicke Hornhäute gemessen werden.
Formule magique?
adapter les valeurs de mesure tonométriques en cas d'altération
de l'épaisseur de cornée.
se tenir informés en lisant les publications spécialisées à ce
sujet, notamment en ce qui concerne les diverses formules
applicables pour adapter les valeurs de mesure tonométriques
mesurées, lorsque les mesures sont effectuées sur des cor-
nées minces ou épaisses.
Magic formula?
of the measured tonometry reading against altered corneal
thickness.
themselves informed, through the current literature, with regard
to the various formulae for adjusting measured tonometry rea-
dings in the event of thin or thick corneas being measured.
1500.7006000.04091
Es gibt keine allgemein anerkannte Formel zur Anpassung
Die Benutzer sollten sich dieser Tatsache bewusst sein und
Il n'existe pas de formule universellement reconnue pour
Les utilisateurs doivent garder ces deux faits à l'esprit et
There is no universally agreed formula for the adjustment
The users should bear these two facts in mind and keep
© Haag-Streit AG, CH-3098 Koeniz, Switzerland
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