An Important Note:
Now and then, there are reports on different K readings on the IOL Master as compared to other keratometers. Also, discrepancies in IOL powers are reported comparing IOL Master results with those obtained on ultrasound equipment even if the same input data was used.
Very often, the described problems can be traced down to keratometer indices and/or to peculiarities of corneal power implementations in the IOL formulas of the respective instruments.
To help clarify this situation, the following link takes you to an important article by Dr. Wolfgang Haigis, PhD, The Users Group for Laser Interference Biometry, at the University of Würzburg in Germany.
K key, the Space bar (available only while in the axial length measuring mode), or the the six-light icon, pictured above, activates the Automated Keratometry, or [KER] mode.
In short:
The best measurements are obtained as follows: place a drop of artificial
tears in each eye, have the patient blink several times, and then do all
measurements with both eyes open as WIDE as possible. Remember to tell the
patient to blink between each measurement. Take as many measurements as
needed until good reproducibility has been established. Those measurements
that show wide variability should be individually deleted. Typically, a set
of good automated keratometry measurements will all be within 0.25 D in each
meridian. When done properly, this feature of the IOL Master is highly
accurate. Use the Space bar to go to the next measurement mode.
In more detail:
In isolated cases, with prior software versions, errors occurred in patients
with drooping eyelids or eyelashes (causing a partial measurement), in the
presence of a poor pre-corneal poor tear film, or with bad focusing. In
software Version V 3.0, an enhanced image evaluation algorithm has been
added that greatly lessens the possibility of measuring errors. If there are
any faulty measuring points, these are displayed. An small graphic animation
reminds the operator to have the patient close their eyelid before the
measurement. A good tear film is required if optimum results are to be
obtained.
Tell the patient to fixate on the yellow light.
As illustrated in the graphic above, align the instrument so that the
six peripheral measuring points are symmetrical, within the circular cross
hairs, and appear optimally focused. The central point usually is not focused and is not analyzed for keratometer measurement.
Note:
Be sure that all six peripheral points are visible, and located in the field
between the two auxiliary circles on the display. Again, remember to place a
drop of artificial tears in each eye, have the patient blink several times,
and do all measurements with both eyes open as WIDE as possible. This will
improve the reflectivity of the cornea. The measuring points should be
circular, or ellipsoid.
If the measuring points are irregular, as might be seen with a corneal scar, accurate measurements may not be possible.
Start the measurement using:
Pushbutton on the joystick or..
The foot switch.
Five measurements are taken within a period of 0.5 seconds, and an average
value is displayed. The completion of the measurement is indicated by a
short acoustic signal.
The corneal curvature (in mm, or diopters) of the principal meridians is
displayed with the corresponding axis. If the cornea is spherical, only one
radius, or one refractive power value will be displayed.
On the display, a blue progress
bar indicates the process of central corneal
computation.
The keratometer measurements may be repeated if desired.
If you take several measurements of the corneal curvature in "one
measure" mode, the previously measured values will be overwritten in the display.
To restore the last (just overwritten) results, simply press shortcut key
CTRL +Z (single-step "UNDO" function).
Note:
The "UNDO" function itself cannot be undone.
Multiple keratometer measurements:
If on the Options - Setup / Program Settings you choose
the "one measure" option, only one measurement result will be displayed (the result of five internal single measurements).
If on the Options - Setup / Program Settings you choose
the "list of measures" option, three measurements results will be displayed, obtained each through five internal single measurements.
As many measurement as necessary may be made, but only the last three
measurements will be displayed.
As mentioned previously, you may delete the last measurement by pressing
CTRL + Z.
To delete one of the three displayed
measurements, select the measurement to be deleted and press the
DEL key. Confirm the delete action with
"YES".
If the results of the last three measurements differ by an average value of
greater than 0.5 D, or 0.08 mm to 0.1 mm (depending on n) has been exceeded,
the display shows the message
"Evaluation", which indicates that you need to check for accuracy. If this occurs, check the pre-corneal tear film of the
eye to be examined. If needed, add artificial tears, have the patient blink,
open their eyes WIDE, and then repeat the measurement. Any erroneous
measurements should be deleted. Keratometry values displayed in the
"Evaluation" state will not be accepted for ACD measurements, IOL power
calculations, or the database optimization of IOL constants.
The last result will be highlighted in
blue by default, and will be
used for IOL power calculations. If you wish to use another value, simply
highlight that result.
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light.
Note:
Be sure that all six peripheral points are visible, and located in the field
between the two auxiliary circles on the display. Again, remember to place a
drop of artificial tears in each eye, have the patient blink several times,
and do all measurements with both eyes open as WIDE as possible. This will
improve the reflectivity of the cornea. The measuring points should be
circular, or ellipsoid.
If the measuring points are irregular, as might be seen with a corneal scar, accurate measurements may not be possible.
The corneal curvature (in mm, or diopters) of the principal meridians is
displayed with the corresponding axis. If the cornea is spherical, only one
radius, or one refractive power value will be displayed.
