Practical Astigmatism Planning and Analysis: Video Chapters
Toric Implant, Astigmatism correction, toric IOLs, refractive surprise post surgery There has been a lot of talk about toric IOL’s and they have certainly become more popular in ophthalmology in recent times. When do you use toric IOLs in your practice? My approach to treating corneal astigmatism at the time of cataract surgery depends on the amount of preoperative corneal astigmatism. Less than 1 D I will use the phaco incision by placing it on the steep corneal meridian. Between 1-2 D I use limbal relaxing incisions and greater than 2D I use toric IOLs. How do you plan for the most accurate toric IOL? It’s all about the accuracy of the toric IOL calculator used. It must incorporate: • The total corneal power • The axial length of the eye • Personalised IOL constants to calculate the effective lens position • Adjustment of the IOL toricity for the spherical component of the IOL when converting to the corneal plane Which calculator do you use when planning toric IOL selection? Well the Assort Toric calculator encompasses all the concepts that we have discussed, plus others which could be found at www.Assort.com, This calculator is free to use and includes the total corneal power, the Abulafia-Koch adjustment and a long eye adjustment for increased accuracy.
Refractive Surprise, refractive cylinder post cataract surgery, toric IOLs, correcting astigmatism, What is your approach to rotating a misaligned toric IOL? A month postoperatively is a good time because tissue is able to fully heal by then. The rotation is a relatively straight forward procedure - using visco elastic to separate the leaves of the anterior and posterior capsule, and going in through the How do you measure the axis of the toric implant post-operatively? I use a narrow slit lamp beam with the patients pupil fully dilated so that I can visualise the toric markings on the IOL. It is important to line the beam up with both sets of toric markings. Once in theatre I use the Callisto or Verion guiding systems for accuracy of rotation. Is there a minimum amount of misalignment you consider before rotating the toric IOL? I haven’t rotated any implants that are less than 10° of axis because the patients are not really aware of it. The Assort toric IOL surprise calculator tells me how much reduction in refractive cylinder can be gained by rotating the already implanted IOL. Dr Daniel Tognetto published a paper in the JCRS in 2018 that showed that when you are 30° off you have not lost all of the effect you have only lost half the effect. You don’t lose all the affect until you are 45° off, so misalignment is a lot more forgiving than most are stating. How is a toric IOL analysis different to an incisional or LASIK analysis? So when you do astigmatic analyses always make the point that you want to have the same device before and after measuring the astigmatism. – regardless of Whether it is a topographer, an aberrometer or manifest refraction. The toric IOL analysis is the exception because pre-operatively the manifest refraction in a cataract patient is not very useful because it is imprecise, so you have to use corneal values. Post-operatively you are being judged by the manifest refractive cylinder that is left over so you do what we call a hybrid analysis, and this is what you find in the refractive surprise calculator we have at Assort.com which is also free to use. Hybrid analysis involves comparing vectorially the post-operative refractive cylinder to the pre-operative corneal astigmatism after the effect of the phaco incision.
CorT Anterior and Total, corneal topographic astigmatism (CorT), Simulated Keratometry (SIm K) The measurement of corneal astigmatism is most important when planning Toric IOL selection. What is your approach to measuring the most accurate corneal astigmatism parameter? On any patient’s visit you are going to take multiple corneal astigmatism readings, manual keratometry, corneal topography, IOL master, Lenstar. There are a lot of ways of measuring the corneal astigmatism so you take them all to see which is the most representative. Over time I have found the most accurate measure to be corneal topographic astigmatism or CorT where instead of using a 3mm zone with only one ring as is the case for simulated keratometry measures, all the rings of the corneal topographer are used taking a vectorial average of all the measures. How does a doctor calculate the CorT parameter? CorT Anterior or CorT Total which includes the posterior cornea can be calculated by having the iAssort interface with the particular topographer or tomographer you use. The CorT calculation uses the measured data only without any extrapolation.
Vector Planning for LASIK, SMILE and PRK Ocular residual astigmatism (ORA) corneal astigmatism, refractive cylinder Let’s discuss the corneal refractive differences that are often seen in refractive laser surgery and how one can address those differences to optimize the surgical outcome. Well the differences between the corneal and refractive values are prevalent and the more the differences the more possibility of an adverse outcome. So these need to be addressed. Up until now the treatment is at one extreme or another. You either treat by manifest refraction, , or you go to the other extreme and you go for a spherical cornea treating by corneal values alone, but in each of those two cases you are actually not really taking any notice of what is happening at the other level. So what can be done using the technique of vector planning is is to use both the refractive and corneal astigmatism in the treatment para So the idea of treating by a wavefront refraction really is 100% refraction. Or topographically guided is the other extreme that you mentioned, but somewhere in the middle of the of the group is the sweet spot. Yes, so all those different sophistications of wavefront refraction, corneal wavefront. The wavefront refraction is just another refractive element that does not really address the corneal values. Corneal wavefront does not address the refractive values. They are just very fancy ways of looking either at a refraction or topography, but they don’t actually incorporate the two together, which vector planning does. What’s the paradigm behind the vector planning process? Well it is quite simple actually and that is that you have to think about the target astigmatism. With the rule astigmatism is generally more optically favourable than against the rule astigmatism. and an oblique astigmatism really has no particular advantage at all. So when the target is found to be somewhere closer to with the rule, then you might swing a little bit closer to emphasising refraction because it is a favourable outcome. But if the target is unfavourable, either against the rule or oblique, then you might say I don’t really want to leave that kind of astigmatism behind on the cornea, I would rather sphericise the cornea and therefore you might give more emphasis to corneal parameters than refractive. Can Vector Planning be used for all LASIK patients? I use Vector Planning for all patients who require astigmatic treatment. The higher the ocular residual astigmatism the more important Vector Planning becomes to minimise the amount of corneal astigmatism remaining postoperatively and hence optimise visual outcomes.
Combining corneal and refractive parameters leaving less corneal astigmatism postoperatively reduces symptoms of aberrations You discuss vector planning in your book for the treatment of astigmatism. A lot of doctors out there will be asking or thinking at the very least why don’t you sculpt the spectacle refraction onto the cornea if you want to get rid of glasses in laser surgery. And that is a very good question and that is the conventional thought ever since laser surgery has come into being for the correction of astigmatism back in 1992 and the thing that concerns me about that is it is really the optical solution but it actually disregards the corneal shape which is what the patient is left with. Leaving the cornea with less corneal astigmatism than treating by reactive cylinder parameters alone results in better visual outcomes. By reducing the amount of corneal astigmatism remaining using Vector Planning, does this result in increased refractive cylinder postoperatively? Well this is where I like to say you actually get something for nothing. Not only do you get less corneal astigmatism, but there is no penalty of increased refractive cylinder as you might expect. Maria Arbelaez’s study using Vector Planning showed this to be the case and this can be related to the better shape of the cornea when incorporating corneal parameters into the treatment plan and not just refractive cylinder alone. You mentioned a Vector Planning study by Dr Arbelaez. Can you describe this study and the findings? Well Maria Arbelaez from Oman performed a LASIK study on one hundred and sixty eyes with myopic astigmatism. She performed eighty eyes using vector planning, eighty eyes with conventional treatment using manifest refraction and she showed that not only did these corneas with the vector planning have less corneal astigmatism, they actually had better visual outcomes with less refractive cylinder.
What is Irregular Astigmatism, asymmetric, nonorthogonal topography, Topography What is meant by irregular astigmatism. Irregular astigmatism is when the bow tie of a topography map is a bit tweaked. It is either not in a straight line, which means it is non-orthogonal, or there is asymmetry between the two sides. In other words, you have a lot more colour on the topographic map on one side than the other, or you have a combination of the two. How did you envisage treating irregular astigmatism. I know we can treat the regular component of the irregular astigmatism but how do we actually address the highly regular component? Conventional treatments don’t address corneal irregularity. What can be done to address the irregularity is to divide the cornea into two hemidivisions with a corneal astigmatism magnitude and orientation for each of the two halves and with common manifest refraction So in effect you are doing vector planning for the two sides of the cornea separately and developing a TIA or a treatment for the two halves of the cornea separately and in this way you are able to regularize and reduce the astigmatism at the same time. Given you are treating the corneal irregularity one would imagine that there is potential to improve ap patients best corrected visual acuity. Would that be correct? I think that is absolutely true because not only are you going to have a reduction in the need for glasses but you can actually see better. If you are reducing irregularity and you are reducing astigmatism you have less of a tweak on the cornea, you are going to have less distortions and therefore you are going to actually have an improvement in the best corrected vision.
Topographic Disparity, asymmetric, nonorthogonal bow tie, irregular astigmatism, double-angle vector diagram With so many different corneal parameters displayed on the various topographers and tomographers which do you think most accurately describes the corneal irregularity? Well I use the I-S index or inferior/superior index on a daily basis to quickly determine inferior steepening as one indication of possible keratoconus. I also use the topographic disparity or TD. This is an excellent vectorial quantification of irregularity. It looks at the vectorial difference between the superior and the inferior arms of the meridian and magnitude of the astigmatism and so in a perfect world the topographic disparity would be 0, that means you have no irregularity. In some eyes you can go up to 5 and so it is quite a significant range. Every topographer has a different way of measuring irregularity so the TD is a standardized parameter that can be displayed on every topographer. You mentioned that the topographic disparity is available on topographers. How does the doctor access this? The potential for availability is there. You need to connect the topographer or tomographer with the iASSORT software and that gives you topographic disparity and a number of other parameters such as ORA, Ocular Residual Astigmatism. The iAssort software uses all the measured data acquired from the device in the calculations. How does addressing the topographic disparity or TD reflect on visual outcomes? It is like looking through a distorted or wonky window. When you have a clear glass plane you see more clearly than you do through a distorted window and in the same way, if you reduce topographic disparity you reduce irregularity and you are going to have an improvement in your best corrected vision. It is important to note that the TD is related to the ORA so that the greater the TD the higher the ORA
Treatment of Irregular Astigmatism, Asymmetric, non-orthogonal bow tie, Hemidivision, Irregular astigmatism presents a challenge for refractive surgeons. How can this be best treated with LASIK surgery? Well we are really looking for the perfect treatment and I describe in detail the reduction of astigmatism by vector planning in my book. The technique of Vector Planning can be applied to the whole cornea or each half of the cornea using different treatment parameters for the, superior and inferior halves, This aims to regularize the cornea and potentially improve the best corrected vision. Is the regularization of the cornea based on topographic assessment? The regularization part of it is purely topography, actually designing a treatment on the topography to regularize the cornea by treating each half 90° apart. In other words, the two treatments being diametrically vectorially opposite to each other changes the shape but does not change the refraction. It just improves the shape of the cornea and then reducing it This is then followed up in the same ablation by a maximal reduction of astigmatism. You mention there the potential for getting an improvement in best corrected visual acuity and I presume this relates back to reducing the corneal aberrations having reduced the amount of irregularity. Would that be true? Yes I would say that to me as a corneal specialist and a corneal surgeon it makes sense to reduce higher order aberrations. By reducing the amount of irregularity on a cornea you are going to reduce the higher order aberration because it is really the irregularity that creates these higher order aberrations.
Polar plots - representing the eye, Double-angle (Cartesian) plots - a mathematical construct, Astigmatism analysis What are polar and double angle diagrams and they and how can they be used clinically? Well that is a really interesting subject and there has been some conjecture about it. How I like to understand it is a polar diagram is a representation of astigmatism as it appears on an eye, whether it be cylinder or corneal astigmatism. The polar diagram is displayed as a semi-circle that goes from 0 – 180 degrees. Horizontally is against the rule and the vertical direction is with the rule. A double angle diagram is an absolutely essential tool that allows the calculation of vectors using Cartesian co-ordinates. It’s a mathematical construct and does not represent the eye. So how does one construct a double angle vector diagram from a polar diagram? The axis of the astigmatism represented on a polar diagram is doubled but the magnitude remains the same to convert to a double angle vector diagram. So 90 degrees becomes 180 degrees and 180 degrees becomes 360 degrees. What essentially needs to be displayed as part of any astigmatic analysis are polar diagrams depicting the astigmatic treatment or the TIA, the surgically induced astigmatism, the difference vector and the correction index to determine any over or under correction of astigmatism. Display of preoperative or postoperative astigmatism is NOT a vectorial analysis.
The Alpins Method of Astigmatism Analysis, The golf analogy, Target induced astigmatism vector (TIA) Now the Alpins’ Method is being used to analyse astigmatic outcomes in quite a number of papers and the major ophthalmology journals are recommending its use for authors submitting papers that investigate astigmatism. For those wanting to read about the Alpins Method which seminal papers do you recommend? I would say there are five seminal papers. In 1993 was a New Method of Analysing Vectors, January 1997 A New Method of Targeting Vectors December 1997 was Vector Analysis of Astigmatism by Flattening, Steepening and Torque. May 1998 was the Treatment of Irregular Astigmatism and then by invitation from Dr Koch in the January 2001 JCRS was Astigmatism Analysis by the AlpinsMethod which is a demonstration of how to use my method, for both corneal and refractive parameters. They are all available on ResearchGate to download. Can the Alpins Method be used in the analysis of all procedures that look to treat astigmatism? The book highlights the extensive use of the Alpins Method when analysing astigmatism in the peer reviewed journals. The Alpins Method is not restrictive in its application. It can be used for incisional surgery, ablations, limbal relaxing incisions and toric IOL procedures. The user essentially needs three parameters for a basic astigmatism analysis. 1. How much astigmatism is present preoperatively (magnitude and orientation)? 2. How much astigmatism are you intended to treat? 3. How much astigmatism is present postoperatively (magnitude and orientation)? When it comes to the treatment and analysis of astigmatism there are a number of terms being used to describe the same thing which makes the subject confusing. One of the key factors in understanding astigmatism treatment and analysis is using the same terminology so everyone is on the same page. Whether you’re analysing astigmatism for incisions, ablations, limbal relaxing incisions or toric IOLs the Alpins Method uses the same terminology – there is no confusion.