Practical Astigmatism Planning and Analysis: Video Chapters
Regulatory Adoption of Alpins Method, The Alpins Method, ANSI Terminology, The ANSI Study Group used your method for astigmatism analysis but changed your terminology. This confused many doctors. I think it was gratifying back in 2006 when the ANSI Study Group actually adopted my method for reporting by the companies to the FDA on results of their laser when they treat astigmatism. The 2014 October issue of the Journal of Refractive Surgery goes through why the terminology that I chose and developed is the terminology of choice compared to the varied ANSI standard terminology. By adhering to my original terminology everyone is then talking the same language. Which are the journals that recommend the Alpins Method of astigmatism analysis? Astigmatism needs to have the one language, the one terminology, whereas up until I developed my method there were so many techniques of analysing astigmatism, so many different terms. It was very confusing. When I first looked at it I realized there had to be a better way and hence that was really the impetus for me to develop my methods as I have. Adoption of my method was initiated by the Journal of Refractive Surgery and the Journal of Ophthalmology which is the output of the American Academy of Ophthalmology. The Journal of Cataract and Refractive Surgery has also fallen into line and require authors to also use my terminology.
Coupling Concept, Coupling, Coupling Adjustment, Steepening and flattening of corneal meridia With any discussion of astigmatism comes the term coupling. What is coupling? Corneal coupling is the spherical change associated with astigmatic treatment. So with incisions, any incision you place on the steep meridian is going to have an equal and opposite effect on the meridian 90° away and so the spherical equivalent doesn’t change. This is 100% coupling. Ablations don’t affect the meridian 90° away and so induces a change in spherical equivalent. Here there is 0% coupling. Now you published a paper on coupling in the Journal of Cataract and Refractive Surgery in 2014. What have you introduced that has been different to what we have known in the past? Coupling got much more complicated when laser treatments came along so the formulas that were prevailing in the 1990’s for coupling worked fine for incisions. As soon as ablations came into view we had a change in the spherical equivalent and that didn’t work. You wanted to have a formula for coupling that works for incisions and ablations and we changed two things: 1) in the coupling formula we just inverted it instead making it flattening of opposite meridian over flattening of treatment meridian 2) instead of using the whole of the SIA we only used the effective SIA and the intended treatment meridian By doing that we were able to use the coupling ratio for both incisions and ablations. How can your definitions of coupling be used clinically? This is the important thing. We created a new parameter called Coupling Adjustment. The coupling adjustment is for any 1.0 dioptre of astigmatism corrected how much of that translates into a spherical change. This needs to be analysed separately for both incisions and ablations and applied to future treatments.
Mixed Astigmatism - occurs when one principal meridian is short-sighted and the other is long-sighted, Treatment of mixed astigmatism including coupling adjustments for myopic and hyperopic cylinder, Coupling, Mixed astigmatism treatment in refractive laser surgery There are more complicated forms of astigmatism such as mixed astigmatism. Can you just give us a definition of mixed astigmatism? Mixed astigmatism is when you have a greater amount of cylinder magnitude than sphere and the sphere and cylinder are of the opposite sign. Optically in cases of mixed astigmatism there are two focal planes – one in front of the retina and one behind the retina unlike hyperopic or myopic astigmatism where the focal planes lie either in front or behind the retina. How do you go about treating mixed astigmatism? So this is where it gets a little bit more complicated because you need to look at previous treatments so you look at all your plano cylindrical myopic treatments and you look at all your plano cylindrical hyperopic treatments to see how much coupling effect you have, you then combine both of them in to your paradigm for mixed astigmatism keeping in mind that you have a plano cylindrical myopic component of the mixed, you have a plano cylindrical component of the hyperopic but you have no spherical treatment. And the idea of having no spherical treatment, you don’t really need to have a spherical treatment. If you just want to move those two images, one in front of the retina and one behind the retina, then you want to move them on to the retina by differing amounts to get it to the retinal plane and so therefore you do not need to have a spherical treatment and this reduces the amount of ablation treatment that you need to have. Can you give us a simple explanation of why that separate coupling adjustment for myopic and hyperopic cylinder needs to be done. The adjustment is done for those two differing amounts to be able to determine how do you get this image on to the retina from one from in front of, being a myopic, and one from behind the retina being the hyperopic without the spherical treatment. So it goes back to high school type of calculations when you have two trains travelling in opposite directions from different places, at different speeds but you want them to arrive at the same station at the same time at the same place. It is kind of a good analogy of the complexity of these trains like the complexity of mixed astigmatism treatments, one being minus and one being plus.
Alpins Performing Vector Analyses, Vector Analysis of Astigmatism - includes examples in ablative and incisional procedures There will be a lot of interest in these presentations that you are providing, particularly with people wanting to perform their own vector analyses on their own data. Let’s discuss the analysis of different procedures using the Alpins Method. I think it is a good idea for people to take it up themselves because it gives them a deeper understanding of it. So to give you an overview of it, you need to calculate three principle vectors, the SIA, TIA and the Difference Vector. So looking at the TIA first. That is actually the treatment – Target induced astigmatism vector. The astigmatic treatment is either what numbers you put into the laser, the numbers you put into your nomogram for incisional surgery or the astigmatic power of a Toric implant at the corneal plane. That gives you a magnitude and an orientation which is your TIA. Your SIA is the astigmatic change measured by any device. It has to be the same device post-operatively versus pre-operative. So it is cornea versus cornea, it’s refractive versus refractive or it’s hybrid which is for a Toric analysis comparing postop refractive cylinder to corneal astigmatism. So that’s important to use a refractive analysis or a corneal analysis and I think it is effective to use both because the corneal analysis is based on objective values, whereas refractive analysis is more subjective. So you will find the two analyses are parallel but a little bit different and often the answer lies somewhere in between the two. And what does the Difference Vector tells us? So the Difference Vector is the third one. The difference vector is what is left over in the astigmatism, what you have missed your target by. the difference vector is a very objective means of measuring how much astigmatism is left over in absolute terms. But if you want to relate it to the difficulty of the treatment then the difference vector divided by the TIA gives you a relative measure of success known as the Index of Success so that you if you have got a big astigmatic treatment versus a small treatment but the same leftover then obviously the big treatment is more accurate. So if you have no astigmatism remaining postoperatively then the Difference Vector and the Index of Success are both zero. When analysing hundreds or even thousands of cases how is the mean of the various parameters calculated in the Alpins Method? So when you look at a lot of vectors you can do a mean magnitude. But if you want to bring the axis into the equation you want to do a summated vector mean. Also, some people call this a centroid. So the summated vector mean is when you do a head to tail summation of many vectors and you take the end point of the last to the starting point of the first and you divide it by the number of components which gives you the overall effect of any number of treatments. So the summated vector mean is a very useful number when determining trends, especially when you compare it to the original mean magnitude because if the trend is random you are going to have a very small ratio. If the trend is strong you are going to have a larger number closer to one.
ASSORT Surgical Management Software, Ophthalmology software analyzing Cataract and Refractive Laser Surgery using The Alpins Method of Astigmatism analysis In most instances surgeons don’t have the time to create spreadsheets to analyse the data so what’s available to determine their own nomograms and improve on visual results? So what I am going to talk about is the Assort Surgical Management Systems and there is a range of software available that we have for ophthalmology and specifically for cataract and refractive. So we start off with the flagship software, which is the Assort software, and this looks at incorporating the Alpins Method with regards to analysing astigmatism for refractive and corneal procedures, including planning and analysing not just refractive Excimer laser procedures using vector planning, but also looking at Toric IOL’s and planning those and Femto LRI’s as well as mechanical LRI’s. The analyses can be done for single case or for hundreds, or even thousands and do a group analysis which shows the polar diagrams, together with summated vector means as well. There are both numerical and graphical reports available with the Assort software and they can be exported for further statistical analysis. The other software that we have available is iAssort and iAssort specifically interfaces with topographers or tomographers. So some of the key parameters with the iAssort Software is that it analyses one by one case. So unlike the Assort flagship it is not a group analysis it is a one by one case and you get parameters such as the ocular residual astigmatism being calculated for each case, the topographic disparity level we’ve also mentioned in these presentations together with the CorT or corneal topographic astigmatism and the corneal topographic astigmatism can be calculated for anterior cornea as well as total cornea and be used for the Toric IOL calculations, as again has been discussed. The Alpins’ Method of Analysis is also calculated in each of these cases, together with the basic graphics of the analysis so that is with iAssort and topographers and tomographers. The third software that we have is the VectraK software which is a vector calculator. Now the VectraK does not have any graphics involved at all but basically asked the user four questions which relate to the pre-operative astigmatism, whether it’s corneal refractive, the post-operative astigmatism, again corneal refractive, the amount of astigmatism you are trying to treat and the axis of the treatment and it gives you, again the Alpins’ Method, describing the index of success, the correction index, flattening and torque. It can do a group analyses from 100 up to 1000 cases with the VectraK software and can do import and export. So if you have data already on the spreadsheet then the VectraK Software will allow you to import that data from the spreadsheet and export, again for further analyses. Using the software, you can look at how your results have performed. In other words, whether you have over corrected the astigmatism or under corrected the astigmatism using the Alpins’ parameters and can incorporate these in future processes or future procedures with nomogram adjustments. The other calculators that we have available are freely available on the web site, on the Assort.com website, and these include the Assort Toric IOL calculator which shows us not only how to plan for Toric IOL and the Toric IOL that is available worldwide. We have all the key manufacturers of Toric IOL’s come up on the system. We also have refractive surprises. So when you have a refractive surprise with a Toric IOL, what do you do? Can you actually rotate the Toric IOL? Do you need to replace it? Do you need to do Lasik surgery on top? These can be investigated again using the Alpins’ Method. All these calculators on the website are actually free. How can someone purchase the Assort, iAssort, the VectraK software? They can be purchased from the website again. So the other calculators that are freely available is the Femto LRI calculator which allows you to plan and analyse your Femto LRI incisions, as well as a basic vector calculator where you can do the addition or subtraction of two vectors, or astigmatisms. The final one that we have is a Coupling calculator which looks at the effects of coupling or incisions and ablations. I didn’t mention there the Vector Planning calculator, which again is freely available, to look at treating somewhere between refractive and corneal parameters and reducing the amount of corneal astigmatism remaining post-operatively.
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