This three-part series on using toric IOLs in cataract surgery will provide some practical tips and advice to help familiarise readers with this technology. It is aimed at cataract surgeons who would like to get started with using toric IOLs, however, it should also be useful for anyone wishing to optimise their toric IOL outcomes or feel more comfortable with treating astigmatism.

Incorporating toric IOLs into the surgical repertoire is a lot easier than it is perceived to be. It is 99% planning for success and 1% dealing with failure. Even the failures are not catastrophic but can be assessed and corrected with the use of some basic and accessible tools. Most cases are extremely straightforward without the need for a lot of detailed or complex knowledge.

Dr Ben LaHood

If we can leave a patient with less postoperative residual refractive astigmatism than what they have due to preoperative keratometric astigmatism, then a toric IOL is indicated. Implanting and aligning a toric IOL is no harder than non-toric cataract surgery and you can use what is already in your tool belt.

In the first of this three-part series, I will guide the reader through my step-by-step approach for patient selection, preoperative assessment and biometry. In the articles to follow, I will review surgical technique, preoperative marking and postoperative assessment.

Patient selection pearls

The first step to incorporating toric IOL implantation is selecting the right patient. Regardless of the level of astigmatism a patient has, it is important to completely treat it. Residual astigmatism of low magnitude can give a little depth of focus but can come at a cost to visual quality and there are many far more elegant solutions to give a patient a wonderful, clear depth of focus without a range of blur.

I do not like the idea of ‘debulking’ high-magnitude astigmatism. Whether you leave a previously highly astigmatic eye with low-level residual astigmatism or do not treat an eye with low-magnitude astigmatism, you are still not allowing that person to see to their full, unaided potential.

If you are new to implanting toric IOLs, you should not start with a patient who is a pedantic perfectionist. Ideally, the patient has a good, healthy eye—potentially a hyperopic patient—because they will appreciate the improvement in their vision to a greater degree. It is also beneficial if the patient has a desire to be spectacle free afterwards so that they have good early outcomes. A good patient experience, that is, a patient who is happy with what you have done, which will boost your confidence.

Astigmatism levels

It is best to begin with treating lower levels of astigmatism. Between 1 D and 2 D of corneal astigmatism is a ‘sweet spot’ to start. People often ask what my lower threshold is for using a toric IOL. I explain that if a patient can have less residual astigmatism post-operatively than they have pre-operatively, then I use a toric IOL.

This depends on the orientation of the steep axis of very-low-level astigmatism. We also need to consider the availability of different cylinder powers in toric IOLs worldwide.

Remember also that there is a difference in magnitude between astigmatism at the corneal and IOL planes, so some lower levels of astigmatism can be treated reasonably accurately with a 1.5 D cylinder power toric IOL. My advice is to make the calculation for every patient as though they will receive a toric IOL and then decide based on what is available in your region.

Corneal topography

One should select patients with a good ocular surface and regular cornea. The best keratometry measurements possible need to be obtained to provide consistent measurements with a good view of what you are doing.

When you are starting out, corneal topography reassures you that the corneas are healthy. When you treat patients with irregular corneas, even with a perfect operation, the outcome is unlikely to be as good as that for a normal, healthy eye.

Selecting a patient with a healthy tear film who does not have dry eye symptoms will make early toric IOL cases a lot easier, and you will feel better about your biometry. Also, avoid anything that temporarily alters the shape of the eye, including eyes that have had recent pterygia operated on or that have active conditions, or patients who wear contact lenses.

Co-existing conditions

I also recommend avoiding patients who have other pathologies that will degrade their vision, including severe glaucoma or macular degeneration. After you have implanted a handful of patients, you will not need to limit yourself by this patient selection guide, but it is best in the early stages of your learning curve.

The patient should have very good capsular support in the eye. A patient who has pseudoexfoliation or who has had any kind of trauma to their zonules should not be selected because you are going to be rotating a lens within the capsule, and if you have not done this before you may not be as gentle with your force vectors as you will become.

Other considerations

I recommend selecting a patient who has good overall health, which admittedly can be difficult to find in cataract patients. Implanting toric IOLs will initially add minutes to the surgery time, and patients will need to be able to withstand an additional 10 minutes on the table, which will allow you to recheck the axis as you need to.

When planning your surgical schedule for the day, do not overwhelm yourself with complicated cases and then implant a toric IOL last. Start out by scheduling straightforward cases and then adding a toric IOL patient to that list to give yourself plenty of time.

Preoperative assessment

Consenting a patient for toric IOL surgery will initially seem like you are simply telling the patient about a lot of added risks without any added benefits. However, this is only because patients generally expect that you will be treating the entirety of their refractive error by default, so explaining that you have upgraded from a non-toric to toric IOL may not impress them greatly. Offering a non-toric option, which will provide inferior results, should only be an option if there are financial implications for the patient.

I inform them that I am putting a special lens in their eye and that it has to be perfectly aligned to be effective. Although I reassure the patient that it is unusual to have to re-rotate a toric IOL, I tell them there is a small chance that I may have to go back at a later stage and correct the position of the lens if it moves.

Biometry tips

Biometry is obviously a key part of toric IOL decision making and calculation. My approach to measurements is to use the same device consistently. I use optical biometry (IOLMaster 700; Carl Zeiss), which allows me to measure axial length even through a very dense cataract.

Using measurements from the same machine allows me to audit my results so that I can review my outcomes and hone my techniques based on what I see. You cannot improve your outcomes unless you know what they are.

An often-overlooked point is that unless you have got a staff member who is very invested in your results, who will also be seeing patients post-operatively and understands the vital importance of biometry measurements, it is best to do these measurements yourself. It allows you to look at the quality of the reflections and removes the guess work.

I also use an auto-keratometer (Marco; Nidek) to corroborate my keratometry values, axis and magnitude of astigmatism. The results will not be exactly the same because they are measuring at different distances from the centre of the cornea, and it is a living biological surface with water on the surface. I usually expect them to be within about 0.5 D in magnitude and within about 10° in axis.

However, do not be afraid to repeat your biometry if needed. When the difference between the optical biometry and the auto-keratometer measurements is larger than expected, I repeat both.

In most cases a spurious result is caused by the patient blinking or something of that nature, but if I am constantly getting abnormal results, then I perform corneal topography. If I still cannot see anything, and I am still not getting consistent results, nine times out of 10 the corneal tear film is affected by dry eye and I will treat that.

If dry eye treatment does not provide consistent results, then I explain to the patient that the exact power is variable, and I use the lens that I think is going to yield the most correct result. I also explain that the patient has a higher-than-average risk of needing an enhancement of some sort post-operatively.

Tear-film quality

It is important to optimise tear-film quality and observe those reflections from the biometry device. If you are getting smudged or irregular reflections and you think the tear-film quality is poor, do not just put drops on the cornea and remeasure. Always remember you are measuring a very thin tear film that overlies your corneal shape: if you put a big drop of water on the cornea and then measure it, you will measure a lovely, smooth surface with great reflections, but you are not measuring the true shape.

Therefore, if you do have problems with the tear film, that person really needs to have their eye treated properly. By far the most common cause of a poor tear film is evaporative dry eye and Meibomian gland dysfunction. I would usually treat this person over the course of a couple of weeks and then have them return for another biometry measurement.

I do not feel it is necessary to perform a lot of diagnostic tests on the tear film prior to surgery. Instead, if I am unhappy with the appearance of the tear film and accuracy of biometry, I treat for Meibomian gland dysfunction as a default option.

Correct patient alignment

Another tip for obtaining accurate toric IOL measurements is to ensure that the patient’s head is level. I line the patient up with their chin in the machine, and before I do any measurements, I swing the camera from one eye to the other. I should not have to move the device up or down.

Most people’s heads are symmetrical, and you should be able to go from one eye to the other without moving the machine vertically. This will give you a good indication that the head is level and that whatever axis you are measuring will be true.

Measuring total keratometry and corneal astigmatism

You can now measure total keratometric power and total corneal astigmatism, incorporating information about the front and back surfaces of the cornea. Historically the anterior corneal surface was measured, and it was assumed that the back surface was consistent with this. Research has shown that it is more precise to include some type of adjustment for posterior corneal astigmatism in your toric lens calculations.1

Whether you include a population estimate of posterior corneal astigmatism or an actual physical measurement of an individual eye’s posterior cornea, the results will be roughly the same.2,3 If you are starting to do toric IOL surgery and you do not have access to total keratometry, it will not hold you back; you can still employ a method that uses an estimation of posterior cornea. For me, that is the Goggin nomogram adjusted anterior keratometry,4 but there is also the widely used and studied Barrett Universal II Formula.

The freely available Barrett Toric Calculator is also excellent and has an in-built adjustment for the posterior cornea. Newer versions of the Barrett Suite of formulas allow for measurement of individual posterior corneal astigmatism as well, so there are many options depending on whether you want to keep things simple or delve a little deeper.

If you are just starting with toric IOLs, I would advise using one of the manufacturers’ online toric calculators. These incorporate a population-based estimate of posterior corneal astigmatism.

Corneal tomography also provides reassurance as to the steep corneal axis and proves the absence of any irregularity, but not having a corneal tomography device should not hold you back from implanting toric IOLs. Having a corneal tomography measurement has quite honestly never made me implant a different lens power or at a different axis.

My only warning here is to consider whether the patient will wear a rigid gas permeable contact lens after surgery because of their irregular cornea. If they will, then I would not advise implanting a toric IOL. This would be something that you may wish to discuss with the patient’s optometrist. Overall, do not be held back by the limits of available technology.

Conclusion

When starting with toric IOLs, just as for any new procedure, it is completely reasonable to cherry-pick the simplest cases with the highest likelihood of success. Selecting the optimal patient and eye will give you confidence to build up to more complex cases. Making sure that your biometry is as accurate as possible also increases the odds of both you and your patient being happy with the outcome.

In the next article of this series, I will discuss toric IOL selection and calculation.

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. Goggin M, van Zyl L, Caputo S, Esterman A. Outcome of adjustment for posterior corneal curvature in toric intraocular lens calculation and selection. J Cataract Refract Surg. 2016;42:1441-1448.

2. Ferreira TB, Ribeiro P, Ribeiro FJ, O’Neill JG. Comparison of methodologies using estimated or measured values of total corneal astigmatism for toric intraocular lens power calculation. J Refract Surg. 2017;33:794-800.

3. LaHood BR, Goggin M, Beheregaray S, et al. Comparing total keratometry measurement on the IOLMaster 700 with Goggin nomogram adjusted anterior keratometry. J Refract Surg. 2018;34:521-526. www.goggintoric.com.

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