Axial
Length Measurement with the Zeiss IOLMaster
1. What
should I bear in mind for taking the measurements?
1.1.
General:
-
Let patients blink prior to
each measurement.
-
Position instrument table, headrest
and instrument properly to avoid tensed up patients and unnecessary eye
movements.
-
When positioning the
instrument to the patient (overview menu), a coarse focus will do.
-
Ask patients to focus well
onto the orange/red fixation point.
-
In axial length measurement,
ask whether the patient sees the fixation point.
1.2. Axial length measurement:
-
Measure cataract patients
with a larger light spot (nearly size of the green circle) (measurement
independent of distance).
-
It is advisable to measure
nuclear cataracts slightly off-axis (shift vertically, not laterally) using a
small laser spot.
-
Patients with an
"error" message (in case of dense cataract/strong opacity of
posterior capsule) and a signal-to-noise ratio between 1.4 and 1.6, often still
yield reliable results displayed next to the graph (if the display shows five
consistent values, they should be entered manually in the calculation table by
clicking (with the left mouse button) at the dot in the graph),
-
Adjustment of the dot in the
graph if necessary in case of double or multiple peaks. (details see user’s
manual)
2. How great are the principle-related
measuring differences compared to ultrasound systems?
Ultrasound instruments measure the distance between the
anterior surface of the cornea and the limiting membrane, whereas the IOLMaster
measures the distance between the anterior cornea (strictly speaking, the
anterior surface of the tear film) and the pigmented epithelium.
A great number of comparative measurements have been
performed regarding these methodical differences. The "correction
factors" resulting from these measurements were integrated in the
device-internal software so that the well-known biometrical formulas with
customized IOL constants for immersion technique can be used without any
problems.
In measurements using the
ultrasound contact technique, additionally the amount of applanation
(shortening of anterior chamber due to the pressure exerted on the cornea by
the ultrasound probe by approx. 100...300 µm) must be considered.
The deviations differ from patient to patient due to the differences in anatomy
and, in the case of ultrasound measurements, they also depend on the examiner.
Partly, however, correction factors have already been integrated in some
ultrasound systems so that on these instruments the values obtained with the
contact technique do not differ in mean from those measured on immersion
instruments. At any rate, the IOLMaster measures the exact distance existing in
the viewing process, whereas ultrasound instruments determine only the
approximated distance. With the IOLMaster the measured lengths are
operator-independent.
Due to appropriate factory-set defaults on the IOLMaster, you can expect to
obtain results that correspond to those measured with the ultrasound immersion
technique.
3. Do I obtain different results of axial
length for ultrasound biometry and optical biometry?
The IOLMaster delivers
measurement results adapted to ultrasound biometry with immersion technique.
Compared to contact
ultrasound instruments, longer axial length results are to be expected with the
IOLMaster.
4. What is to
be taken into account in the case of double or multiple peaks?
If the reflection at the RPE
is strongest, the measurement is correct. If in a series of, for instance, five
measurements, there are 4 neat peaks and a double peak with a shorter
axial-length value, it can be assumed that the limiting membrane causes the
stronger reflection of the double peak thus pretending a too short axial-length
value. In this case, the inappropriate 5th measurement can be
deleted or be corrected following the instructions of the user's manual (by
zooming in and manually shifting the measuring point). After the correction,
you will find out that the value very well fits in the measurement series.
If a too large axial-length
value turns up in the measurement series, it is caused by a strong reflection
at the choroid. In this case, follow the procedure described for too short
values (delete or correct the reading).
In the case of multiple
peaks, proceed analogously. That's why you should always see to it that you
obtain consistent measurement series. An additional look at the peak structure
will avoid erroneous measurements.
5. What is the reason for the generation of
fine structures of peaks or multiple peaks?
So far, the reasons haven not
been investigated sufficiently. In practice, there are reflections – possibly
caused by nystagmus – that are additional to those of the layer reflecting most
strongly, the retinal pigment epithelium (RPE). These reflections are produced
by the inner limiting membrane or/and the choroid.
This results in additional
peaks the origin of which can mostly be clarified only by several measurements.
In the case of double peaks, the peaks may be produced by both the limiting
membrane/RPE and the RPE/choroid.
Additonal reflections may be
produced even by a retinal detachment. In that case you will likely get a wrong
reading.
6. Can I also measure patients with staphyloma
(protrusion of sclera) precisely?
Yes. It is just these eyes that often cannot be
measured reliably and accurately by ultrasound, where the IOLMaster achieves
clearly better results. This is due to the measuring principle that simulates
the normal process of vision – measurement along the visual axis at a small
measuring area.
As staphylomas (posterior) cannot be exactly measured
by means of ultrasound A scans. Additionally, a B scan is necessary, if a
staphyloma is suspected. As a rule, this is not done, thus resulting in
erroneous results. Using the optical method, measurements are automatically
correct. In this way, gravely misfitted IOLs will not occur.
In a lecture at the DGII 2000
(German-speaking Society for Intraocular Lens Implantation) in
Luzern/Switzerland, B. Lege (University Eye Hospital, Würzburg/Germany)
reported a case where a misfit of about 5D was avoided by optical measurement.
Irregular shapes of the fundus of the eye do not only exist, as is often
assumed, with long eyes, but also with short eyes. These, however, are mostly
not diagnosed.
7. What is the reproducibility on the eye?
Better than ± 30 µm, this corresponds to a mean
postoperative error of less than 0.1 D. In clinical use, mean standard
deviations of 23 µm were obtained for 5 successive measurements (Dr. Haigis,
University Eye Hospital, Würzburg/Germany) and 25.6 µm for 20 measurements
(Dr. Vogel, University Eye Hospital, Mainz/Germany). The variability between 4
different examiners was 11.8 µm (Dr. Haigis, University Eye Hospital, Würzburg/Germany)
and 21.5 µm between 5 different examiners (Dr.Vogel, University Eye Hospital,
Mainz/Germany).
Safety criterion:
If the results of
individual measurements differ by more than 100 µm to the mean value, no mean value
will be displayed, and the individual measurements must be checked for consistency.
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