Corneal Transplantation ~ Calculating IOL Power
There is presently no method that can be used to accurately carry
out IOL power calculations prior to corneal transplantation combined
with cataract removal and intraocular lens implantation.
This is because it is impossible to know the central power of
the donor graft prior to surgery. Simply basing pre-operative calculations
on a "best guess" of post-operative corneal power (such
as 44.0 D) will quite often lead to an unpleasant post-operative
refractive surprise. And sometimes, donor corneal transplant tissue
ends up coming from eyes with prior LASIK. when this occurs, very
high hyperopic errors will result if intraocular lens implantation
is carried out as a primary procedure at the time of corneal transplantation.
What to do?
It is a much better idea to instead carry out corneal
transplantation with cataract removal, but without intraocular lens
implantation. The lens implantation would then be carried out at a
later time, as a secondary procedure.
Below is what we consider to be the optimum method for achieving the
intended post-operative refractive result, when it is desirable to do
both corneal transplantation and cataract removal in the same operative
The host tissue is removed in the usual fashion with a 0.25 mm to 0.50
mm graft-host disparity. An "open sky" continuous tear anterior
capsulotomy is carried out by capsulorhexis and extracapsular cataract
extraction is completed. Residual cortex is removed. The posterior
capsule is polished and the eye is left aphakic. Corneal
transplantation is then completed in the usual manner. We prefer a
combination of interrupted and running 10-0 nylon sutures for the
greatest flexibility in post-operative astigmatism control.
After four to eight months, when the corneal curvature has stabilized,
and corneal astigmatism has been minimized, a careful aphakic
refraction is performed and simulated keratometry by topography is used
to estimate central corneal power. The power of a foldable secondary
intraocular lens (such as the Staar AQ2010V) is calculated by means of the Refractive
Vergence Formula. The intraocular lens is then placed
into the ciliary sulcus, and over the intact posterior capsule, via a
small scleral tunnel.
This approach is based on the aphakic refraction, vertex distance,
lens position (A/C, sulcus, or bag) and corneal power. It is axial
length independent (another potential source of errors) and generally
gives very good results.