Q: IOLMaster axial length measurement: does the intrument make account of a
retinal thickness factor ?
A: The IOLMaster measures
from the anterior corneal vertex (or, to be more precise: from the anterior tear film surface) to the retinal
pigment epithelium along the VISUAL axis. Ultrasound measures from the anterior
corneal vertex to the internal limiting membrane along the OPTICAL axis. As a
result, we have 2 different distances in 2 different directions. All IOL calculation experience, however, is
based on ultrasound axial lengths. So, in order to connect to this vast amount
of experience, optically measured data had to be translated into what high
precision ultrasound would have measured.
Therefore, a calibration curve was deduced from over 600 comparative
measurements using high precision (22 µm reproducibility) simultaneous
segmental immersion ultrasound. This
curve - which also makes allowance for the overrepresentation of the lens in
the axial length of short eyes - is built into the IOLMaster. (We have reported
on these measurements at the 1999 ASCRS meeting in Seattle: W.Haigis, B.Lege: Optical and acoustical
biometry; German papers on this topic are available - a detailed English
version is presently under preparation).
So, the IOLMaster ACTS AS IF
if it were a high precision IMMERSION instrument as far as the absolute values
of the axial lengths are concerned - with all consequences especially with
respect to personalization of IOL constants. IOL constants like A-constants
which work well for contact coupling ultrasound will NOT work well for the
IOLMaster: the IOLMaster needs STRONGER A-CONSTANTS THAN CONTACT ULTRASOUND.
Information regarding IOL
constants to be used with the IOLMaster can be found on the EULIB web site www.augenklinik.uni-wuerzburg.de/eulib . EULIB is a user group on optical biometry (EULIB:
European user group for laser interference biometry) which is not restricted to
Europeans but proud and happy to
welcome colleagues from all over the world. (Wolfgang Haigis, July 19, 2000)
Q: IOLMaster ACD measurement: what does the instrument
measure ?
A: The ACD with the IOLMaster
is measured from the ANTERIOR corneal vertex to the ANTERIOR vertex of the
lens, just like an ultrasound ACD would be measured. In fact, IOLMaster ACDs
are calibrated against immersion ultrasound ACDs. Compared to an ACD measured
ultrasonically in contact coupling, the IOLMaster ACD is likely to be a bit
longer ( 0.1 - 0.2 mm), since it is not affected by a possible globe impression
as might be the case in contact ultrasound. (Wolfgang Haigis, July 19, 2000)
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Last revision: July 19, 2000