More and more people getting cataract surgery are looking to get out of glasses. Cataract surgery has become not only a therapeutic procedure, it has also become a vision correction procedure.
Getting in the way of correcting vision is astigmatism. Uncorrected astigmatism will blur vision and must be corrected at the time of cataract surgery. And this is the role of toric lenses.
But for all the good that toric lenses do, there can still be some problems. Issues calculating the correct power of the lens, rotation of the lens or a change in astigmatism over time can leave you with residual astigmatism and blurry vision after cataract surgery.
But before we get in depth into those problems that can arise with toric lenses and how they can be fixed, let’s first review astigmatism.
How Astigmatism Correction Works
Correcting astigmatism is important during cataract surgery. Just as how astigmatism blurs your vision before surgery and requires glasses or special toric contact lenses to fix, astigmatism will also blur your vision after cataract surgery.
About one-third of all patients going through cataract surgery would benefit from some form of astigmatism correction. Confident you don't have astigmatism? Well, hate to bear bad news, but you may have astigmatism after cataract surgery even if you don't have much beforehand. I'll get into the reasons how shortly.
For small amounts of astigmatism, special incisions called limbal relaxing incisions can be performed. Correcting astigmatism with these incisions is a key benefit of laser cataract surgery.
But for more than small amounts, special toric lenses exist which correct astigmatism.
Toric cataract lens; image © 2018 Jung, N.Y., Lim, D.H., Hwang, S.S. et al., used under CC BY 4.0 / modified from original
Astigmatism not only has a power (how strong the astigmatism is), it also has a direction. (Note: This is an important concept when talking about the problems with toric lenses for cataract surgery). The direction of the astigmatism can be up and down, left and right, or in any direction in between. For cataract surgery, a toric lens is selected which corrects for the power of your astigmatism and then oriented in the exact direction of your astigmatism.
This is how astigmatism correction works during cataract surgery.
But there can be problems…
Calculating The Power Of Astigmatism
Most astigmatism in the eye comes from the cornea.
But NOT all astigmatism comes from the cornea. There can be some astigmatism coming from the rest of the eye. This includes the lens.
So what happens if the astigmatism in your lens happened to be canceling out the astigmatism in your cornea? Before cataract surgery this can mean you don't have astigmatism. (the situation I was mentioning above). But when that lens is removed during cataract surgery, you can suddenly end up with astigmatism afterwards! - the astigmatism coming from the cornea. But in practice this is where surgeons will see this cornea astigmatism and talk about toric lenses…
After cataract surgery, toric lenses are calculated based on the astigmatism from the cornea.
And you would think those calculations would be pretty straightforward and easy. But there are challenges in those calculations:
For one, the cornea has two surfaces: an anterior surface and a posterior surface. And astigmatism can come from BOTH of those surfaces.
Astigmatism coming from the anterior cornea surface is easy. We have great equipment to accurately measure the front of the cornea. (as long as the surface of the eye doesn't have any bad dry eye)
The posterior surface of the cornea is much more challenging. There are a limited number of machines that can even measure the posterior surface. And the measurements just aren't as accurate as what can be done with the front of the cornea.
So, as a result, instead of being able to perfectly calculate how much astigmatism you will need corrected during cataract surgery, the calculations instead have to predict how much astigmatism needs to be corrected.
(and this isn't even talking about astigmatism from the rest of the eye outside of the lens)
As a result Toric Lens calculations aren't 100% accurate
Toric lenses may not be perfect at getting rid of all your astigmatism. While they still work well, you can end up with some residual astigmatism EVEN after getting a toric lens after cataract surgery. With the best calculations, about 20% can still have some visually significant astigmatism remaining.
But having much less astigmatism is still much better than having a lot more astigmatism.
Rotation Of The Toric Lens
Let's say you ended up perfect after cataract surgery. A second problem with toric lenses is that this can change. It’s possible the lens can rotate out of the correct direction of your astigmatism.
The direction of astigmatism is measured in degrees (with 180 degrees covering all the different directions, from left to right to up and down that astigmatism can exist in the eye). After the toric lens is placed within the eye, it is lined up on the exact degree of the astigmatism. This provides the maximum correction of astigmatism.
But this lens may not stay in that same position after surgery. And if this lens rotates out of position, it ends up correcting less astigmatism. For every 10 degrees of rotation out of position, the toric lens loses about 30% of its astigmatism power. This means you end up with remaining astigmatism.
Fortunately this isn’t too common. Lens rotation that requires any sort of treatment happens approximately 2% of the time after cataract surgery. This typically happens very early on during the cataract surgery recovery (such as within the first 24 hours), but may not be detected until about a week out when the vision and prescription is measured. The further you get from the procedure, the much less likely this occurs. Beyond a week or especially a month, the toric lens has settled down into position and is unlikely to rotate more.
Some people will be at a slightly higher risk of this than others. Particularly those who have a high amount of nearsightedness. Those individuals have longer eyes and thus have slightly more space for the lens to rotate.
Long Term Change In Astigmatism
There is one final thing to mention which can leave you with astigmatism even despite having toric lenses with cataract surgery. Your astigmatism will gradually change over time.
Fortunately we aren’t talking about much astigmatism. The amount of change is very small. If you have zero astigmatism today, it can take 20-30 years for you to develop any noticeable amount of astigmatism.
However, it becomes more of an issue if you have borderline amounts of astigmatism after cataract surgery. The amount you have may not cause you any trouble with your vision now. But as it gradually changes and gets worse over time, it can start to effect you more and more.
Of note, this increase in astigmatism is only in one direction (horizontal astigmatism, also known as against-the-rule astigmatism). If you have astigmatism in the opposite direction (vertical astigmatism, also known as with-the-rule astigmatism), this change will actually reduce the amount of astigmatism you have until it eventually flips direction.
Ok, You Have Astigmatism. Now What?
You went through cataract surgery, had a toric lens placed to correct astigmatism and through one of the reasons above you have residual astigmatism left over which is causing your vision to be blurry.
Well, there is hope to correct this astigmatism.
Lens Rotation
If your vision was initially great but then became more blurry, its possible that the toric lens may have rotated out of position. This can be confirmed by your cataract surgeon. They will dilate your eye and look to see where the toric lens is aligned in the eye.
This lens can be rotated back into position, however, it does require a second surgery.
Laser Vision Enhancement
If you have remaining astigmatism and rotating the toric lens wouldn’t improve anything, than another option to correct the astigmatism is through a laser eye surgery procedure such as lasik. This can be a very effective and accurate way to treat any residual astigmatism after cataract surgery.
Most surgeons will want to wait a certain period of time (such as three months) prior to performing the correction. In addition, check with your toric lens surgery package to see if a laser vision enhancement is included; if not, there will be an extra out of pocket expense for this treatment.
See also What To Know About A Lasik Touch Up After Cataract Surgery to learn more
Summary
Toric lenses aren’t perfect. Even despite a routine surgery, you can still end up with blurry vision from residual astigmatism. But despite the problems with toric lenses, they are still very beneficial and essential to getting out of glasses with cataract surgery. After-all, being left with a very small amount of astigmatism after a toric lens is still better than being left with a very large amount after a standard non astigmatism correcting lens.
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