This three-part feature discusses tips for success when getting started with toric IOLs. Part 2 tackles calculating toric IOL sphere and cylinder power; though it can seem daunting when treating astigmatism, the process can be simplified with a few key considerations.
Part 1, published in the last month’s edition of Ophthalmology Times Europe®, looked at patient selection and optimising biometry. In Part 2 of this three-part series, I will provide tips on calculating toric IOL sphere and cylinder power, which can be a great concern to surgeons who use these lenses. I will attempt to simplify the process and explain away certain myths that exist.
There are many toric IOLs on the market. One of the most common questions I am asked is: “What is your go-to lens?” My answer is that all the major brands have good-quality toric lenses that will give you favourable results if you follow the steps I outline.
Hydrophilic vs hydrophobic
Dr Ben LaHood
One of the key considerations is hydrophilic versus hydrophobic designs. Hydrophilic lenses go through a much smaller incision than hydrophobic lenses (1.8 mm) which we know is astigmatically neutral. However, an incision this size requires more specialised equipment, and the hassle-to-benefit ratio favours it being a hassle, in my view.
I advise new surgeons to make their incision size as small as they comfortably can. There is no longer a reason to use a 2.75-mm or even 2.4-mm incision. The sweet spot of incision sizes is 2.2 mm. It provides for a multitude of choices, including hydrophobic lenses.
Hydrophilic lenses have had bad press in the past few years because of fears of opacification, especially if they are exposed to gases inside the eye. In my experience of reviewing these IOLs over many years, I have not seen a single opaque lens, but I accept it does occur on rare occasions.
I think the risks of hydrophilic lens use have been overstated. I avoid them in patients who may undergo procedures with gas in the future, such as an endothelial transplant, but in a standard eye it would not sway my decision.
Certain hydrophilic lenses also have an advantage in terms of ease of rotation. For example, the haptic design of the AT TORBI Toric IOL (Carl Zeiss) is bidirectional and bi-toric, so it can be implanted upside down. This benefits those who are learning because there is no need to worry about the direction of rotation to the targeted axis.
With the standard C-loop haptic design of many toric lenses, if you go slightly past where you intended to place it then you have to bring it back around 180° to get closer to where you want it. Some people call this “the lap of shame” and some call it “the lap of honour.” Either way it can be a frustrating process.
The extra time involved should not be a concern, though. Keep in mind that you are working hard to place it in the right spot for the rest of the patient’s life, so it does not matter if it takes an extra 5 minutes. This is better than having to schedule a second surgery to rotate a misplaced toric IOL.
Another consideration is the cylinder steps that each manufacturer produces. For example, the AcrySOf IQ Toric IOL or Clareon Toric IOL (Alcon) and the Tecnis Toric II (Johnson & Johnson) have 0.75-D steps between most of the cylinder powers, whereas the AT TORBI Toric IOL has a consistent 0.5-D cylinder step.
My colleagues and I recently completed a study evaluating whether the step magnitude made any difference in around 1,000 implanted eyes with two surgeons using 0.5-D steps and two surgeons using 0.75-D steps. We audited our results and found no significant difference between the two in terms of residual refractive error (unpublished data).
This might seem counterintuitive because you would think that smaller steps yield better results. The key is to keep in mind that regardless of whether you are using 0.75-D or 0.5-D steps, you are doing a good job and do not need to change your approach, so do not base your toric choice on the cylinder step magnitude.
In the end, IOL selection will likely come down to familiarity with the platform. There are no features of the major toric IOLs that make me feel that any one is significantly superior to the others.
Whether you are used to a plate haptic or a C-loop haptic, stick with that selection for your early cases and then feel free to experiment once you have some experience. Eventually you will choose based on the predictability of refractive results but, until that time, I urge you to try all the options and see what suits you best.
Calculating toric IOL power
There are a lot of third-party and manufacturer toric IOL calculators online. With very basic maths, anyone can calculate what spherical power an eye needs by simply knowing the shape of the front and the length of the eye.
Calculating a toric lens means calculating two lenses – one for each axis. This is still rather simple but people have put a lot of time and effort into refining the process by taking into account the expected lens position, anticipated tilt, posterior corneal astigmatism, etc., so it makes sense to rely on robust, tested calculators. Each of the main lens manufacturers has a calculator, and those such as the Barrett Toric Calculator are also available.
For beginners, I recommend starting out with your lens manufacturer’s calculator. When people are implanting a spherical lens, they will usually look at multiple formulas and decide on which formula suits the eye. With a toric calculator, you do not have this option unless you want to look at multiple calculators.
I have used the Z CALC Online IOL Calculator (Carl Zeiss) website in conjunction with the Goggin nomogram adjusted anterior keratometry (GNAK) when implanting Zeiss toric IOLs and the Barrett Toric Calculator when implanting Alcon toric IOLs and been very happy with both sets of results.
Historically, toric calculators would use the anterior keratometry values and tell you which toric lens would suit a particular eye to give the best refractive outcome. However, we then rediscovered that posterior corneal astigmatism has an impact on the precision of that calculation.
Therefore, different ideas arose for how to incorporate the posterior cornea into the toric IOL calculation. One of these was the Baylor nomogram by Dr Douglas Koch, which makes an adjustment to the cylinder power of the toric IOL. He reported that, depending on the orientation of the anterior corneal astigmatism, you can adjust the cylinder power for that eye up or down.
The other nomogram was from Dr Michael G. Goggin: he adjusted the input instead of the output. He adjusted the keratometry values themselves, and his nomogram is available online at goggintoric.com. I use this calculator because it is the only toric calculator that has prospective published data proving that it is effective.1
Third-party calculators, such as the Alcon Toric IOL Calculator, the Johnson & Johnson Toric Calculator and the Barrett Toric Calculator, incorporate a posterior corneal adjustment that means that nomogram adjustments are not necessary. If you used one of those calculators and also added the Baylor or the GNAK, in principle you would over-adjust for the posterior cornea because you would adjust for it twice. Now, you can simply use one of those calculators mentioned.
Axis adjustment
What you will find is, when an adjustment is made for the posterior cornea in the calculation of the toric IOL, there is an adjustment made to the power of the cylinder that you are putting in, and you will also notice an adjustment to the axis of implantation. This means that when you take the posterior cornea into consideration as well as the anterior cornea, you are playing with the total corneal power. It is simple vector mathematics that makes sense.
Notably, the steep axis of the total cornea may be quite different from what you have measured on the anterior surface. This will have the biggest impact for a toric lens to treat low-powered oblique astigmatism. A recent study revealed that when you are dealing with an eye with oblique anterior corneal astigmatism, there is no need to adjust the axis of toric IOL implantation from the steep axis of the anterior cornea.2
It can be quite disconcerting to go against the recommendation of online calculators, so I recommend starting with simple with-the-rule or against-the-rule eyes. In some instances, toric IOL calculators can provide various powers with different options.
Some of those options are going to ‘flip the axis’. For example, one cylinder power option will suggest leaving the eye with 0.5 D of astigmatism in the pre-existing axis. Essentially, you underpower the treatment. This could be reasonable, or you could put in the next cylinder magnitude step up and flip the axis of astigmatism but leave that person with only 0.1 D of residual astigmatism.
With glasses, we know that you should not flip the axis, but inside the eye, minimal residual astigmatism regardless of axis is beneficial in terms of visual quality. My advice is to aim for the smallest predicted residual astigmatic error and not to worry at all about flipping the axis of astigmatism.
Age considerations
The other consideration that often arises with targeting minimal residual astigmatism is whether patients should be left with some with-the-rule astigmatism so that they are happy as they age and their cornea changes shape. We know that at a population level, there is more against-the-rule corneal astigmatism in older age and more with-the-rule astigmatism in younger patients. However, we cannot currently predict at an individual level whether an eye will change with time.
I do not personally subscribe to the philosophy of leaving someone with residual with-the-rule astigmatism to ‘future-proof’ their vision. I instead prefer to aim for the best possible unaided visual acuity initially and to deal with any change in the future. This comes down to personal preference.
Alignment
The final myth that I would like to address when it comes to calculating an appropriate toric IOL cylinder power is that you can ‘depower’ a toric IOL cylinder power by leaving it a few degrees off axis. The idea for this comes from our knowledge that if a toric IOL is not aligned correctly to the steep corneal axis, then it has less optical effect to neutralise astigmatism at that axis. However, the cylinder power does not magically disappear; it is still inside the eye.
The best optical result you can achieve is when the cylinder of the toric IOL is aligned with the steep axis of the cornea. Rotating it away from this location to depower it and precisely treat the astigmatism on the steep axis will only add residual astigmatism at another axis. This can be proven mathematically and graphically but the technique is still commonly discussed.
My advice is to calculate a toric IOL with the best cylinder power to treat the astigmatism in the eye and to aim to leave it on that steep corneal axis. It may slightly under- or over-treat the corneal astigmatism but it will ultimately give the best achievable refractive outcome with that IOL.
Surgically induced astigmatism
When inputting your data and plans into toric calculators, you will be asked for a surgically induced astigmatism (SIA) prediction. What you are basically doing is telling the calculator, when it is doing the vector maths and taking into consideration the anterior and posterior corneal astigmatism as well as that of the toric IOL, to add a vector of a certain magnitude acting at the axis of your planned incision. This should seem straightforward enough.
If you were to plot the SIA figures from multiple eyes onto a double-angle vector diagram or a centroid plot, you would obtain roughly 0.1 D. Many surgeons would say that is what you should put into the calculator, a 0.1-D flattening effect at your incision. This estimate does work but the problem is that we do not know where that surgically induced astigmatism is going to act in an individual eye.
When we make a cut into a cornea, we simply do not know enough about corneal biomechanics to be able to predict how it will respond. Classical teaching was that if you make a cut at 180°, you will get flattening on that axis and steepening at 90°. If only life were that simple!
Certainly, this is the case for very large incisions, however, for smaller phaco incisions the orientation of the average flattening effect is extremely unpredictable. The centroid value is reasonable since, when you add the vectors of all the SIA values and average them, a 0.1D flattening effect occurs at the axis of the incision; however, if we are going to guess where this effect occurs for an individual eye, I personally would rather estimate zero. This way, even if the flattening effect occurs at 90° to what I had expected, my outcome is no worse off.
I realise that the difference between zero and 0.1 D is tiny but if you understand that this is an estimate for an individual eye based on an extremely variable axis, then I would hope you would also see that zero makes more sense in terms of eliminating unnecessary variables.
So, we have chosen a patient, taken biometry and calculated a toric IOL. The next step is to perform surgery. In the third and final part of this series on getting started with toric IOLs I will discuss what I consider the key points of the surgical and postoperative periods to achieving successful outcomes and how to deal with problems.
Ben LaHood, MBChB, PGDipOph, FRANZCO
E: benlahood@gmail.com
Dr LaHood is an ophthalmologist specialising in refractive cataract and laser surgery in Adelaide, Australia. His research and teaching focus on the management of astigmatism. He is a consultant to Alcon and Zeiss.
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References
1. LaHood BR, Goggin M, Esterman A. Assessing the likely effect of posterior corneal curvature on toric IOL calculation for IOLs of 2.50 D or greater cylinder power. J Refract Surg. 2017;33:730-734.
2. Ophir SS, LaHood B, Goggin M. Refractive outcome of toric intraocular lens calculation in cases of oblique anterior corneal astigmatism. J Cataract Refract Surg. 2020;46:688-693.
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