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EW FEATURE
February 2015 IOL calculations
Editors' note: Dr. Barrett has no finan-
cial interests related to this article.
Contact information
Barrett: graham.barrett@uwa.edu.au
a secondary tool to confirm your
primary is giving you the correct
reading," he said. "Warren uses the
analogy of a pilot who doesn't have
just one instrument. The pilot has
other instruments to make sure the
primary instrument is correct." EW
wondered why that is. It's odd and
not obeying the rules you would
expect from an optical surface. My
basic theory is that the normal
cornea tends to be elliptical. The
diameter is wider in the horizontal
meridian. Therefore, that means the
curvature of the posterior cornea has
to be steeper than the vertical. That
means you have against-the-rule
astigmatism because the posterior
cornea is convex. That also explains
why almost all corneas exhibit
against-the-rule behavior," he said.
Using that background, Dr.
Barrett was able to calculate a mea-
surement for the diameter of the
cornea for each patient and calculate
the posterior corneal curvature.
Looking at the evidence
Because the Barrett Toric Calculator
is relatively new—it has only been
online since the summer of 2013—
clinical evidence to support it is just
now reaching peer-reviewed jour-
nals. In a submission that is in press,
Dr. Barrett said his calculator was
found to be within half a diopter
of residual astigmatism 75% of the
time compared with only 33% of the
time for the Alcon AcrySof calculator
(Fort Worth, Texas) or the Holladay
calculator. By adding the Baylor no-
mogram, the accuracy of the Alcon
and Holladay calculators increased
to 50%, he said. "The [Barrett Toric]
theoretical method is doing signifi-
cantly better than actually measur-
ing the posterior cornea," he said.
The Barrett Toric Calculator
is available on the websites of
ASCRS (www.ascrs.org/barrett-toric-
calculator) and APACRS (apacrs.org).
The APACRS website also features
Dr. Barrett's True-K formula and
Universal II formula.
The calculator was also recent-
ly built into the LENSTAR LS900
(Haag-Streit, Koniz, Switzerland).
Dr. Barrett encourages surgeons
to think carefully about other vari-
ables that affect their toric IOL use,
such as selecting one primary tool to
measure the cornea and then always
using secondary tools, a concept
he learned from Warren Hill, MD,
Mesa, Ariz.
"You may have the LENSTAR or
other tool for your primary mea-
surement, but you also have to have
