Corneal Transplantation
Calculating IOL Power
Currently, there is no method available to accurately perform IOL power calculations before corneal transplantation, which is often performed in conjunction with cataract removal and intraocular lens implantation.
This is because it is impossible to know the central power of the donor graft before 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 comes from eyes that have undergone 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 is the optimum method?
It is a much better idea to carry out corneal transplantation in conjunction 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 session:
The host tissue is removed in the usual fashion, with a graft-host disparity of 0.25 mm to 0.50 mm. 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 optimal 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 error) and generally yields outstanding results.
