The lens implanted at the time of cataract surgery results in a change in the power of a patient’s eye. Biometry measurements (most importantly, the curvature of the front surface of the cornea) taken prior to cataract surgery predict the power of the cornea. While it is not possible to guarantee this outcome, modern technology allows a certain confidence that the final outcome will be within a particular range.

Assumptions regarding the shape of a cornea are used to extrapolate the corneal power from the shape of the cornea are no longer valid in a patient who has previously had laser vision correction.  Many techniques have evolved and attempted to overcome this to improve the accuracy of this outcome

Reports presented at the Hong Kong International Symposium of Ophthalmology indicated that the accuracy of this outcome approached 80% of cases achieving within 0.5 dioptres of the desired outcome. Other reports aren’t quite as favourable however it is reassuring to know that this accuracy is improving.

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What Is Astigmatism?

Astigmatism is what causes your vision to be blurry, no matter what you’re looking at—meaning it affects your vision at all distances. Medically speaking, astigmatism occurs when the cornea is shaped more like an AFL ball than a soccer ball (i.e., the surface is more curved in one axis than another). There are different types of astigmatism (you may have been told you have regular or irregular astigmatism, or that you have myopic astigmatism, hyperopic astigmatism, or mixed astigmatism). This is a very common occurrence—a study of more than 11,000 people in the U.K. who wear glasses found almost half had significant astigmatism If you include lower amounts of non-significant astigmatism, it is even more common—in fact almost everyone has astigmatism to some degree! The good news for most patients is that this can be corrected at the time of cataract surgery. What is the best way to treat low astigmatism at the time of cataract surgery? Most people need to rely on glasses when they have astigmatism, and that may not be desirable. When you have cataract surgery, your natural lens is being replaced with an artificial lens. Because of technology advances in the past decade or so, we now have the capability of treating astigmatism at the time of your cataract surgery. There are two ways we can correct your astigmatism when you have cataract surgery—we can create little cuts on your cornea (called limbal relaxing incisions or peripheral corneal relaxing incisions) or we can implant a special kind of artificial lens that will correct the astigmatism internally (called a toric lens). For patients with corneal astigmatism above 0.60D, we usually recommend a toric lens since they have been shown to be more accurate at correcting astigmatism and have the most reproducible results. It is important to note, however, that most people with astigmatism will still need spectacles after cataract surgery. What the toric lens provides, however, is a much-reduced need for those spectacles. For some patients who had very low levels of astigmatism before surgery, the toric lens may reduce the astigmatism to a point where it’s not noticeable. To give you an example of how a toric lens can help improve your astigmatism, here are my results for approximately 400 patients who were treated with a toric lens when they had cataract surgery:

The Y Axis looks at astigmatism before and after surgery for different groups of patients; the bigger the green bar, the more astigmatism was present before surgery. The red bar is a measure of astigmatism after surgery. The T values along the X axis indicate the different groups. The lower the “T” number, the lower the level of astigmatism (so, people who received a T2 lens had much less astigmatism than those who received a T9 lens). What patients might find interesting is that people with higher amounts of astigmatism (who received anywhere from a T4 to a T9 lens) had about 70% of their astigmatism eliminated, but patients with much less astigmatism had a smaller percentage of astigmatism eliminated—around 33%. A 2014 study from Moorfields Eye Hospital compared the postoperative vision in people who had peripheral corneal relaxing incisions in one eye and a toric lens in the other eye when they had cataract surgery. The Moorfields study showed patients preferred the toric lens, as the eye receiving the toric lens had the better vision without glasses than the eye that had the extra incisions. If you have astigmatism and need cataract surgery, your surgeon to determine will need to spend more time determining which replacement lens you’ll need than if you did not have astigmatism. You probably will still need to wear glasses after the surgery (but likely not nearly as often as before surgery).

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Toric lenses implanted to replace the cataract have the ability to reduce a patient’s astigmatism.  The main benefit for the patient is that it reduces their dependence on glasses for day to day vision and need for image distorting astigmatism correcting glasses.  I recently attended the International Symposium of Ophthalmology  where there was significant discussion on techniques to achieve the best outcomes with these lenses.

There are two steps to the lens being implanted at the correct orientation.

The first step is determining what is the correct orientation for the lens.  Traditionally, measures of astigmatism have considered only the front corneal surface, ignoring the back surface as a contributor to astigmatism.  Newer techniques now take this into account.

The next step is to facilitate the lens being implanted as close as possible to the advised orientation.  There are currently two systems available facilitate implantation at the advised axis.  The Verion system (Alcon) and Callisto (Carl Zeissi).  There was much discussion on this.  It will be very interesting to see what contribution, if any, these make to reducing patient’s final astigmatism and consequent glasses dependence.

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Keratoconus or its precursor condition, forme fruste keratoconus, are relatively common causes for a patient being unsuitable. A recent report indicated that 2.5% patients presenting to an Australian clinic for laser vision correction had either of these conditions making them unsuitable.  Interestingly, patients with these conditions are over-represented in the population presenting for laser vision correction assessment.

The rate of this condition in the general population in comparison, is only 0.05%

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