Total Corneal Astigmatism For Toric Iol By Giacomo Savini, Md - OCULUS Pentacam Interpretation Manual

20 Total corneal astigmatism for toric IOL
20 Total corneal astigmatism for toric IOL
by Giacomo Savini, MD
The influence of posterior corneal astigmatism on total corneal astigmatism (TCA) has been
[45, 46, 47]. In the great majority
established by different studies based on Scheimpflug imaging
of eyes, the steepest corneal meridian of the posterior corneal surface has a vertical direction and
produces an against-the-rule astigmatism (ATRA). In contrast to the anterior corneal surface, which
is contact with air and has a positive refractive power, the posterior corneal surface is contact with
aqueous and has a negative refractive power. For this reason a vertically aligned steep meridian on
the posterior corneal surface generates an ATRA rather than a with-the-rule astigmatism (WTRA).
The average magnitude of posterior corneal astigmatism is around 0.50 D, which should be added
to anterior ATRA and subtracted from anterior WTRA to get the total corneal astigmatism (TCA).
However, the average 0.5 D value is not fixed, but is proportional to the amount of anterior corneal
astigmatism, so that a greater contribution can be expected with increasing values of anterior
corneal astigmatism.
On the other hand, ophthalmologists are used to dealing with keratometric astigmatism (KA) and
not anterior corneal astigmatism. There is a subtle difference: both are calculated from the radii of
the anterior corneal surface, but the anterior corneal astigmatism is based on the 1.376 refractive
index of cornea, whereas the KA is based on the 1.3375 keratometric index, which was developed to
take into account the influence of the posterior corneal surface even when this cannot be measured.
For this reason, the difference between KA and TCA is, on average, 0.25 D and not 0.50 D. Some
differences in the axis of the steepest meridian may be expected too.
The influence of posterior corneal astigmatism on the refractive outcome of toric IOL implantation
should therefore be predictable. However this has been demonstrated only recently [48]. Using the
Pentacam® TCRP (i.e. the TCA) to calculate the required cylinder of toric IOLs we were able to lower
the error in refractive astigmatism after cataract surgery as compared to calculations based on KA.
In eyes with WTRA, KA generated an overcorrection of 0.59 ±0.34 D, which dropped to 0.13 ±0.42 D
when using TCA. In eyes with ATRA, KA generated an undercorrection of 0.32 ±0.42 D, which dropped
to 0.07 ±0.59 D when using TCA. Minor and clinically non-significant differences were observed in
the torsional component of astigmatism, meaning that differences in axis orientation between KA
and TCA are less important than differences in magnitude.
TCA measurements by the Pentacam® are shown in the power distribution display as TCRP. For this
study we relied on 3 mm measurements centered on the pupil, considering the whole zone inside
the ring. This setting provided us with the best results; future studies with larger samples may yet
suggest different settings however.
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