8
EW ASCRS NEWS
June 2017
Dr. Koch noted that even in
normal corneas, errors in estimating
total corneal power can be as high as
0.5 D due to variations in the poste-
rior to anterior ratio.
"Then you have corneas for
which this extrapolation creates
much larger errors—atypical cor-
neas, after refractive surgery, after
keratoplasty, ectasia, and the use of
toric IOLs," Dr. Koch said.
Two ways to accurately measure
the posterior cornea are tomogra-
phy and the Cassini (i-Optics, The
Hague, the Netherlands). Tomogra-
phy—using Scheimpflug technology
and OCT devices—measures the pos-
terior cornea on an elevation-based
method. Direct measurement of the
second Purkinje is done with a Cas-
sini, which uses color light-emitting
diodes for anterior corneal curvature
and white LEDs to measure posterior
curvature.
Dr. Koch's research comparing
posterior corneal power with earlier
software versions of the Galilei and
the Cassini show a large range in
corneal power among post-LASIK
and post-PRK corneas.
"We have to do better," Dr.
Koch said. Companies are aware and
making progress.
Formulas' role
The posterior cornea is difficult to
measure because of the small differ-
ence in the refractive index between
the cornea and aqueous. Another
Inaugural continued from page 3
Steinert Refractive Lecture at Refractive Day
