Toric IOLs can provide the added benefit of addressing astigmatism at the time of cataract surgery. They are available in a wide range of powers for both the spherical and cylindrical component and there are even some multi-focal toric IOL designs.
Step 1: Measure (or determine) the total corneal astigmatism
The first step in planning for a toric IOL is to measure the pre-operative corneal power (keratometry) and perform corneal mapping. In the topography shown below, this patient has 0.78 diopters of astigmatism measured in the anterior cornea. Because this is topography, it only measures the outer (anterior) surface of the cornea. In the top, middle panel the steep axis reads 37° with a magnitude of 0.78 D. When we look at the refraction in the bottom, middle panel, the total astigmatism in the refraction is more, about 1.5 diopters when looking at the 3.00 mm zone. This is a negative cylinder axis of 122°, which corresponds to a positive cylinder axis of 32° (122 minus 90). And since 32° is reasonably close to the 37° from the topography, there is good congruence.
Even more data can be gleaned from analysis of dual-scheimpflug tomography (which is different than simple topography). The tomography takes many scans of the cornea (in little “slices”) which can then be reconstructed to make a three-dimensional model of the cornea with far more information.
In this case, the tomography shows a total corneal astigmatism (including both anterior and posterior corneal surfaces together) of 1.23 diopters at an axis of 35° — again this is very congruent with the previous measurements. But why does the tomography measure 1.23 diopters while the topography measures only 0.78 diopters?
Dr. Doug Koch‘s group from Baylor University has been instrumental in elucidating the answer to that question. And it has to do with taking into account the posterior corneal surface and the astigmatism that it induces. For most eyes that have against-the-rule astigmatism like our patient here, you will need to add about 0.5 diopters to the anterior corneal astigmatism value to accurately take into account the posterior surface. This is a good estimation for surgeons who do not have access to a costly tomographer.
In our case 0.78 diopters measured on the anterior surface + 0.5 diopters = 1.28 diopters of total estimated astigmatism. When we compare this to our actual measured total corneal astigmatism from the dual-scheimpflug tomographer (1.23 diopters), we see that they are essentially the same.
Step 2: Perform the spherical IOL power calculation
Now it’s time for the second step which is to perform the spherical IOL power calculation and in this case we prefer the accuracy of the Artificial-Intelligence based Ladas Super Formula 2.0 which is accessible free at www.IOLcalc.com
The printout shows that an IOL power of +22.5 would give just about perfect plano for our patient. Since our patient is a life-long hyperope, aiming for perfect zero is preferable to aiming for mild residual myopia.
Step 3: Use the Toric IOL Calculators to make a surgical plan
The toric IOL calculators are validated tools which will help you make a surgical plan for the cataract procedure and determine which specific IOL will produce the best results.
If you are able to make your incision astigmatically neutral, that is best. But even if you are unable to achieve this, as long as you place your incision on the steep axis, you will not change or shift the overall astigmatic axis of the cornea. In the video below, we are careful to place our diamond keratome incision directly on the steep axis. From previous experience, the incision I make with the diamond will produce very little, if any, astigmatic effect.
Now we are ready to do cataract surgery for this patient’s left eye. We will use a toric IOL in a spherical power of +22.5 with the lowest toric power of +1.0 at the corneal plane. And we will align this IOL at the 35° axis.
Here is the video of the surgery for this specific patient.
The patient did well and achieved much better vision, though the patient is not yet completely satisfied. I suspect this is because the other eye still has a cataract as well as a significant refractive error.