Measurement
with the IOLMaster - Keratometer
1. When do I get faulty or no measurement
results with the keratometer?
You will get faulty or no
measurement results, if the
-
instrument is not properly
focused on the eye,
-
measuring marks are partly
obscured by eyelid or eyelashes,
-
the eye is being closed
during the measurement,
-
strong reflections are
additionally produced at the anterior surface of the IOL with pseudophakic
eyes,
-
the anterior surface of the
cornea is affected by scars, local or other irregularities,
-
the tear film is strongly
impaired.
2. What can I do to nevertheless obtain results
after an error message was displayed?
-
Focus the instrument exactly
onto the light dots (mostly, additionally a fine circle is visible around the
dots)
-
Ask the patient to open the
eye wide, then repeat the measurement.
-
Let the patient close and
open the eye several times to improve the tear film, then quickly start the
measurement.
-
If measuring marks are
obstructed, carefully hold the eyelid.
-
Slightly decentre the
instrument by moving the instrument towards the eye to measure, for instance,
beside local scars.
You should observe however,
that with strongly irregularly formed corneas or after refractive surgery of
the cornea the measured values cannot be used for the calculation without
thorough consideration. In this respect, the built-in keratometer does not
differ from any other manual keratometers or autokeratometers commercially
available.
3.
How do I recognize "appropriate" or "inappropriate"
reflections (risk of erroneous
measurements)?
If you can
see six peripheral circular light dots on the screen, everything is OK. If one
of the dots is irregularly shaped, this might be caused, for instance, by a
local corneal scar. If several dots are deformed, this might be due to a poor
tear film. In this case, tell the patient to shortly close and open her or his
eyes again.
If, for
instance, the top dot is missing, you should consider a hanging down eyelid or
eyelashes.
4. Is it possible to measure pseudophakic eyes
with the keratometer?
This
depends on the intensity of reflections additionally produced at the IOL. If
the additionally visible six light dots are faint, you can measure the eye
without any problems. If they are strong, measurements are impossible.
This
depends on the curvature of the implanted IOL.
If the
additional light dots are so strong, that the measurement is impossible, you
can slightly defocus the IOLMaster (approx. 1...2 mm). In this way,
the additional reflections become so faint that they do not disturb the
measurement. On the other hand, defocusing the instrument by 1 to 2mm will
affect the results only insignificantly.
5. What is to be observed for patients after
refractive surgery (PRK, LASIK, ...)?
In the calculation of the
future IOL at least the measured axial length and the corneal curvatures
(refractive power of cornea) are entered. The changed corneal geometry does not
affect the measurement of the axial length of the eye but the keratometer
values. The formulas presently used for IOL calculation base on the “Gullstrand
Eye“ and thus presuppose defined conditions of the radii of curvature of
anterior and posterior surfaces of the cornea. This “geometry“ however has been
changed by refractive surgery. For this reason, the data obtained with the
keratometer cannot be included unchanged in the IOL calculation.
6. What
should I bear in mind when I compare corneal power (K values) obtained with
keratometers of different manufacturers?
Keratometers/ophthalmometers
always measure the radius of the cornea. On one and the same eye (equal
radius), different diopter values will be displayed due to the different eye
models used by different manufacturers (refer to the table below for corneal
refractive indices). This may result in deviations of 0.5 to 0.8 D that are to
be attributed only to the eye model used, but not to the actual measuring
accuracy.
Influence
of the measurement with different keratometers using different corneal
refractive indices:
|
Corneal
refractive index |
Resulting
deviation of refraction, referred to 1st line |
Topcon, Zeiss |
1.332 |
0 D |
Haag-Streit,
American Optical |
1.336 |
0.5 D |
Rodenstock,
Humphrey |
1.3375 |
0.7 D |
Hoya |
1.338 |
0.8 D |
|
|
|
If you want to compare the results of keratometers of different
manufacturers with the IOLMaster, you will obtain comparable diopter results,
if you enter the same corneal refractive indices in the menu
"Options–>setup–>program settings->Keratometer".
Dr. Wilfried Bissmann
Carl Zeiss
Ophthalmic Instruments Division
Phone +49
3641 64 2058
Fax +49
3641 64 2917
E-mail bissmann@zeiss.de
The telephone hotline +49
3641 64 2030
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Last revision: July 24, 2000