Activate Axial Length Measurement (ALM) Measurement
- Click on the ALM icon, or
- Press key A, or
- Press the button on the joystick in Overview (OVW) mode.
|| Switching to ALM mode will automatically change the magnification
ratio: a smaller section of the eye becomes visible with the reflection of
the alignment light and a vertical line (1, Fig. 27).
- The patient should look at the red fixation point in the
center. A cross-hair (3, Fig. 27) with a circle in the
middle will appear on the display.
- Fine-align the IOLMaster so that the reflection of the alignment light (2,
Fig. 27) appears within the circle.
Ask the patient if he or she sees the fixation point. If
the patient fails to fixate properly, the visual axis will not be correctly
recognized, which may result in measuring errors.
In the case of poor visual acuity/high ametropia (> 4 D) it is advisable
to measure through the spectacles. If the procedure is followed correctly,
no measuring errors will be produced. Measurements should not be
taken while a patient is wearing contact lenses, as it will result in
- Take the measurement by pressing the button on the joystick.
The corresponding display field next to the video image will
show the measured axial length. The video image will be overlaid
with a red graph similar to those in ultrasonic measuring instruments.
Simultaneously, the axial length and signal-to-noise ratio (SNR)
of the measuring signal will be displayed. This value is a gauge
of the quality of measurement. Measurements with an SNR between
1.6 and 1.9 appear with an exclamation mark (!) after the measured
value and the message
"Borderline value!" will appear. For evaluation
of the SNR see Post-run editing of axial
Readings that deviate from the internally calculated mean value
by more than 50 μm are shown in red and marked "multiple
Borderline value! does not necessarily mean that the reading
is incorrect and must be rejected. It rather means that all axial
length measurements for the eye should be checked for
plausibility and consistency, e.g. according to the usual ultrasonic
biometry criteria. If the "uncertain" values are
determined to concur with the other readings, the readings marked Borderline
value! should also be accepted as valid axial lengths.
The number of
measurements is crucial here. The IOLMaster requires FIVE measurements
to be taken. Until then, the message Measure again will thus
appear. Only after five measurements are recorded will a mean value
be passed on to the IOL calculation and an evaluation enabled.
To obtain consistent results
we recommend checking the individual axial length measurements
and carrying out further measurements if necessary.
With stronger lens opacities, it may be advisable to defocus
the IOLMaster. You may choose a reflection (2, Fig. 27
above) as large as the circle on the display. If measurements
are even now impossible, the IOLMaster can be refocused and the
reflection shifted to the bottom and/or top margin of the circle
on the display by varying the vertical adjustment (turning joystick).
Defocusing and shifting the reflection within the circle will have
no effect on the result, because interferometric axial length
measurement is completely independent of distance.
- For the next measurement of this eye, press the button in the
Up to 20 such measurements per eye may be taken on a single day.
Avoid measurements of eyes with retinal detachment. In
such cases, measuring errors cannot be precluded.
As a rule, the axial length should be viewed together with the values
for corneal refraction and overall refraction, and checked for plausibility.
It is likewise helpful to compare the right and left eyes.
The composite signal is calculated after the fifth measurement.
Initially, the individual signals are displayed in red. After
a delay of about 1 second the composite signal is then displayed
in blue. In addition, the axial length reading determined from
this composite signal will appear. The composite signal will
be re-calculated after each further individual measurement, and
an axial length calculated therefrom. Should a reading deviate
from another by more than 0.05 mm, it will be displayed in red
and the message Multiple peaks will appear. This indicates
that the individual measurements should be scrutinized and the
composite signal may need to be post-run edited, as explained
Editing of Axial Length Measurements.
Until an axial length can be determined from the composite signal,
the word Evaluation! will be displayed below the horizontal line
in the list of measurements. If necessary, the potential individual
measurement errors must be deleted or the composite signal post-run
edited, as the readings obtained will not otherwise be accepted
for the IOL calculation and database for optimisation of the lens
constants. Until the fourth individual measurement has been taken,
the last reading will be highlighted in blue. From the fifth individual
measurement onwards, the composite signal is highlighted in blue.
The blue highlighting can be moved through the table of individual
readings with the aid of the cursor buttons ↑ ↓.
In this way the signal curves of the individual measurements can
be displayed. Deleted individual measurements can be restored with
the shortcut <CTRL> + <Z>. The composite signal can be
displayed by clicking on the composite reading.
Error in the display field denotes readings with an SNR smaller
The following plausibility tests are performed with
the axial length measurement (AL) from the composite signal:
When both eyes have been
measured, the difference in axial lengths between right and left
is also checked. If the latter exceeds 0.3 mm, a message appears
to check the readings once again.
- AL < 22mm (indication of short ocular axis, use corresponding
- AL > 25mm (indication of long ocular axis,
use corresponding biometric formula)
If this warning appears, be
sure to verify that no pathological changes have occurred in
the eye. If necessary, the measurements must be repeated (provided
the maximum of 20 measurements per eye and day has not already
been reached). Only confirm the warning with OK if you
are certain that the readings are plausible. Otherwise, try to
determine what has caused the implausible readings. A reference
to the displayed plausibility test message will be transferred
to the comments box.
The number of measurements of the respective eye taken on
this particular day is displayed in the Mode field of the status
bar next to "ALM". If the count reaches 20 no further
measurements of this eye can be taken on this day. The counter
cannot be reset. Deleted readings (see above) do not affect the
Fig. 28 Video image after axial length measurement
ALM Measurement of Non-Aphakic Eyes
To measure non-aphakic eyes, select the corresponding mode from
the AL Settings menu. This special AL mode is displayed
in the video image field and will be active until you reset it
via the menu. The IOL Master will also be reset to Phakic mode
if you change to the patient's other eye or a new patient.
If the axis length of eyes with phakic implants is to be measured,
and are not listed in the additional AL functions, the following
compensation values according to Dr
Wolfgang Haigis of Wurzburg University Clinic,
Germany, should be used.
Fig. 29 AL settings
Every implant, e.g. a phakic IOL, influences the measurement
of axial length in PCI biometry. If a phakic lens implant is measured
in a normal phakic mode, the result will be slightly elevated. The reading
must be corrected, depending on the material used and the center. thickness.
Sample calculation for a phakic
implant (Acrysof) with a center thickness of 0.2 mm:
23.51 mm Compensation value: -0.03 mm Correct axial length: 23.51 + (-0.03)
= 23.48 mm
Two peaks may appear when measuring pseudophakic eyes
and with certain intraocular lenses. The first peak is a side maximum
of the IOL, while the second peak is produced by the retina.
In this case, manual correction is necessary (see Measuring
errors with pseudophakic eyes). It is expedient to measure
at a number of different points.
Use the psph (pseudophakic) button to calculate secondary piggy-back
IOLs. For this purpose, the ACD should be measured by a method other
than the IOLMaster and the readings thus obtained entered into the