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.

Dr Ben LaHood

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This three-part series on using toric IOLs in cataract surgery is designed to provide some practical tips for success. 

, looked at patient selection and optimising biometry. 

, published in the November issue, covered the calculation of toric IOL sphere and cylinder power. In this final article in the series, I will examine the perioperative period and provide postoperative troubleshooting.

A toric IOL has maximum optical impact when aligned perfectly with the steep axis of the cornea. If it is a few degrees off, there is a decline in effect of astigmatic neutralisation. In general, most surgeons consider alignment within 10° to be accurate enough but in reality, a lot depends on the magnitude of astigmatism.

Misalignment of a high-power toric IOL can leave someone with a lot of residual astigmatism whereas misalignment of a very low cylinder power toric IOL may have a minimal effect. To align a toric IOL accurately, a reference mark must be used. There are many different methods of preoperative marking: ink markers, needle marking and even apps.

I prefer to use an intraoperative digital display system (Verion Digital Marker, Alcon or Callisto Eye, Carl Zeiss), not because these methods are more precise than manual marking but because they are more efficient and speed up the surgery. I do still make a manual mark on the eye in the sitting position prior to surgery as a back-up reference in case of difficulty with the digital system, but rarely need it.

Regarding manual marking, a single mark is adequate to align a Mendez ring. The human eye is incredibly good at halving things, so I will make a small mark inferiorly at the bottom of the cornea to split the cornea in half.

In the worst-case scenario, if your digital marking system fails, you have not pre-marked the eye and you still want to implant a toric IOL, I suggest that you still can safely do so. Simply look at the top of the cornea and identify the vascularised pannus. If you look at the midpoint of that pannus, this will be the midpoint vertically and you could drop a line down from that to halve the cornea.

If you look again at the widest point of the cornea and draw a line horizontally across it, you are going to give yourself an excellent surrogate marker; you could then use a Mendez ring and still be able to mark the axis to implant the toric IOL. You can safely use a toric even in this worst-case scenario.

The surgical technique of toric IOL implantation differs from standard cataract surgery because of the preoperative marking and the rotational placement of the lens. The IOL injection is similar to cataract surgery but I recommend being primed and ready to rotate quite rapidly as the IOL is expanding.

The unfolding of the IOL is a prime time to get it into a good position. Many will suggest, if you are using a C-loop haptic, to put it 5° before where you want it so that you can nudge it into the final resting position when you are doing the next steps of surgery. If you are using a plate haptic IOL, you can put it exactly where you want it because you can nudge it in either direction very easily.

I do not use a capsular tension ring unless necessary due to zonular instability, even in high myopes. I find that toric lenses from the major manufacturers have good rotational stability, and the need to rotate is extremely low.

If you do find that you have over-rotated your toric IOL past its intended position, there is no shame in turning it around again. Take your time. Inject some more viscoelastic into the capsular bag and use two instruments to completely rotate the lens all the way back to where you want it.

If you are wanting to move just a few degrees in the opposite direction, it is possible but be wary of putting too much stress on the capsule. My tip is to slightly deflate the anterior chamber, which will allow easier “backwards” rotation of the IOL within the capsular bag.

A key time that you may encounter a little IOL rotation away from its intended position will be at the end of the case when you take the instruments out of the eye. Sometimes, if the incision has not perfectly sealed, shallowing of the anterior chamber will allow some movement of the toric IOL within the capsule.

I highly recommend being quick with your incision hydration, trying not to let the anterior chamber flatten and keeping things quite firm at the end of the case. This prevents setting up a cycle of IOL rotation, entering the eye, flattening the chamber and further movement, etc. I have noticed a major decrease in my need to hydrate the main incision after injecting the Clareon toric IOL with the AutonoMe injector and this has been helpful in leaving a stable anterior chamber at the conclusion of the case with little risk of early rotation.

If you notice any early rotation after you have finished the case, go back in and put the lens back where it needs to be. This is a lot easier to do while you are still in the operating theatre rather than seeing it the next day and wishing you had taken the extra few minutes.

Postoperative care of a toric IOL patient is identical to that following any other cataract surgery. I recommend seeing the patient at 1 day, 1 week and 6 weeks postoperatively. On Day 1, I start to look for any early and significant rotation. If I notice a lens with significant malrotation, I take the patient directly back to the operating theatre and put the lens in the originally intended position.

If I notice on Day 1 that the lens is off by a few degrees, I am not concerned because the patient may be completely happy and it is fine to wait and see. Also, if the lens has moved a little, it means that it is not particularly stuck to the capsule and is a somewhat mobile. I would rather wait for the capsule to become more condensed so that if I do choose to rotate the lens, it is more likely to stay in that position long term.

At the 6-week postoperative appointment, I audit my results, looking at lens position both in terms of axial position and depth within the eye. This is also when I would perform an accurate subjective refraction.

Even if the patient is happy, I recommend doing a subjective refraction. It will give you so much more information that you can use to improve your future results. I also take more optical biometry measurements for my records. These are handy if I want to consider any kind of toric rotation or lens exchange and provide me with postoperative anterior chamber depth.

If a patient has significant post-op astigmatic refractive error, I follow a structured decision-making tree by asking myself first whether the patient is happy. If the patient is unhappy, I want to do something for them. On the other hand, a happy patient with refractive error may be truly happy as they are or may prefer glasses anyway so do not try to fix things — instead, learn from what may have gone wrong in that case.

If you examine the patient and find the lens is exactly where you intended yet they still have some residual refractive error, you may ask yourself what you have done wrong, but the answer is likely nothing. Either the patient had significant change in their surgically induced astigmatism or their initial biometry was not quite right. This is now a situation that needs a little further investigation before treatment decisions can be made.

You simply need a subjective refraction and measurement of the axis and pseudophakic anterior chamber depth, along with knowledge of the sphere and cylinder power of the implanted toric IOL. Working out whether you can rotate the IOL to fix the residual astigmatism or if an IOL exchange may be needed can seem daunting.

You will need to use an online tool to help you with this, but a good rule of thumb is that if the spherical equivalent of the subjective refraction is close to zero then you should be able to rotate the current IOL into a more optimal position for the patient. Ultimately, the patient will have to decide whether they want to have another surgical procedure so that you can rotate the IOL or whether they can have laser treatment to the cornea.

My preference is to rotate the lens to a better position to get rid of the residual astigmatism if I can, and it is a simple solution. My colleague Dr Michael Goggin and I created an app called Toric Pro.1 Other calculators include astigmatismfix.com and the Barrett RX formula. All these resources can tell you whether you can rotate the lens for a better residual refractive outcome.

There will be some patients who do not want to undergo surgery again and there will be some eyes that you will not want to re-enter. Perhaps they had unstable zonules or it was a difficult operation. For these cases, laser vision correction can be a safe and effective option.

Again, the most important thing in this situation is an accurate subjective refraction as this is what will be used in planning the laser surgery. Most people will know a friendly laser surgeon who is happy to help with such cases.

One of the most important points to make about re-rotating an incorrectly positioned IOL is that it is not appropriate to simply put the IOL at the originally intended position if it is not there already. Once you have made an incision and implanted a toric IOL, you have changed the eye. Surgically induced astigmatism has altered the shape and you now know how deep the toric IOL sits.

You have new information and a new subjective refraction to work with. Do not be tempted to just go in and put the IOL back unless on Day 1 there is a large rotational change from intended. In this scenario, I think it is reasonable to go back and reposition, often with the added security of a capsular tension ring.

If at any stage you do have to go back inside the eye, it can be particularly stressful because you want to get it right this time and because this is not a common procedure. My suggestion, if going back into the eye early, is to free up the lens and take a minimalist approach.

Use a little viscoelastic but do not fill the bag. Rotate the lens to its new position — and I always gently press or tap the IOL against the capsule to maximise contact between optic material and capsule.

If going back into the eye much later, when capsular fibrosis has sandwiched the IOL in place, take your time and be kind to the zonules. I recommend putting a needle between the IOL and the capsule edge before visco-dissecting the two apart.

There are times when an IOL simply cannot be completely removed and haptics must be left inside the bag. It is always good to have a sulcus and anterior chamber IOL option on standby in case there are any complications with this relatively tricky procedure. Also, remember that if you find yourself having difficulty rotating a previously implanted toric IOL, there is no harm in backing out and deciding that the safest course of action is to instead perform laser vision correction surgery.

I would like to encourage surgeons to implant toric IOLs more frequently. The tips and advice I have given in this series of three articles will help you plan for success and to be prepared for the rare occasions you must deal with failure.

Success is all about doing all the small steps well and minimising the chance of error. You will have to be a little more pedantic about reviewing preoperative results and considering postoperative data but the improvement in patient results will be worth it.

Articles

Giving due consideration to preoperative marking and the surgical technique in toric IOL implantation, as well as being diligent about reviewing postoperative results, can improve patient outcomes whilst minimising the chance of errors.

Toric IOLs for beginners: What to consider before and during surgery and in the postoperative period

, published in the last month’s edition of 

, 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.

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.

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.

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.

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.

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.

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.

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. 

2. Ophir SS, LaHood B, Goggin M. Refractive outcome of toric intraocular lens calculation in cases of oblique anterior corneal astigmatism.

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.

Toric IOLs for beginners: Advice on selecting and calculating toric IOL sphere and cylinder power

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

1. Goggin M, van Zyl L, Caputo S, Esterman A. Outcome of adjustment for posterior corneal curvature in toric intraocular lens calculation and selection. 

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. 

3. LaHood BR, Goggin M, Beheregaray S, et al. Comparing total keratometry measurement on the IOLMaster 700 with Goggin nomogram adjusted anterior keratometry. 

This three-part feature discusses tips for success when getting started with toric IOLs. Part 1 looks at patient selection and optimising biometry; Part 2 examines IOL calculation; and Part 3 looks at the perioperative period and postoperative troubleshooting.

Author | Dr Ben LaHood

Articles

Toric IOLs for beginners: What to consider before and during surgery and in the postoperative period

Toric IOLs for beginners: Advice on selecting and calculating toric IOL sphere and cylinder power

Toric IOLs for beginners: Tips for patient selection, preoperative assessment and biometry