Practical Astigmatism Planning and Analysis: Video Chapters
Introduction to the 24 chapters, authored by Prof. Noel Alpins. A compendium of Prof. Alpins work over the past 30 years which includes: The Alpins Method, Vector Planning, Astigmatism Analyses of toric IOL, LASIK and SMILE procedures, Practical astigmatism analysis, Corneal Irregularity, Double-Angle Vector Diagrams, encompass all my scientific innovations in ophthalmology over a series of more than twenty peer reviewed papers. The book is comprised of twenty-four well illustrated chapters that helps deal with the treatment of astigmatism whether it be incisional, ablation, toric implants and, when you get a refractive surprise how you might apply my method to treat and to analyse these surprises.
Astigmatism In The Population, With-the-rule astigmatism, Against-the-rule astigmatism, Oblique astigmatism, Regular astigmatism, Irregular astigmatism, Astigmatism is an unevenness of the cornea where instead of the cornea being round it is more oval so a good way to think about it is with looking at a dessert or a soup spoon. So a soup spoon is where the cornea is round and has no astigmatism. But if you look at a dessert spoon it is more oval than round What are the different types of corneal astigmatism? Well corneal astigmatism is delineated from 0 to 30 and 150 to 180 degrees as against the rule, then you have 30 to 60 and 120 to 150 degrees being oblique, leaving the 60to 120 degrees being with the rule and with the rule is more of a favourable orientation. What percentage of people have a significant amount of astigmatism? Significant astigmatism of 1D or greater can be found in more than 36% of people. Surgically the aim is to leave patients with less than 0.50D of astigmatism.
The surgical correction of astigmatism over the years, Arcuate keratotomies, Transverse keratotomies, Definition of a Vector, Surgically induced asitgmatism, The Alpins Method - Concepts and Terms What got you first interested in the subject of astigmatism? Astigmatism became obvious to us when implantology commenced. I spent a lot of time dividing sutures and doing what I call a dynamic vector analysis to divide sutures to try and minimise and orientate the astigmatism better. Lans in the late 1890’s was the first to demonstrate that incisions could actually change the astigmatism status of the eye Then Naylor in the 1960s discussed the obliquely crossed cylinders, the addition of them for lenses, but then Jaffe and Clayman in 1975 took that one step further rather than talking about lenses they started talking about corneas and incisions and sutures. What is the basic difference between a vector and astigmatism? Is this a vector or is this an astigmatism, is based on can you measure it? If you can measure it, it is an astigmatism but if you can’t measure it you can only calculate it then it is a vector. So when performing astigmatic analysis many are done looking at the magnitude alone. What more does the axis of the astigmatism tell us about how well a procedure corrected astigmatism? It is important to know whether the astigmatism increases or decreases and so certainly magnitude comparisons have a usefulness but if you want to understand the process of how the operation is performed success wise over/under correction then you really need to look at the axis as well or the meridian
Cataract incisions, refractive laser ablations, Astigmatic Keratotomy, Ocular residual astigmatism (ORA), Wavefront and Vector Analysis How does the treatment of astigmatism differ when using incisional procedures compared to laser ablations? Interesting question because during the eighties I was doing Radial Keratotomy and then moved on to doing astigmatic keratotomy, , with incisions you always use the topography or the keratometry to operate at the steepest meridian. So the steepest meridian was the midpoint of your incisions. In 1991 along came the laser and all of a sudden, the whole paradigm changed because when you perform laser surgery with Excimer laser you operate according to the spectacles and the spectacle astigmatism cylinder is often on a different axis, a different magnitude, than the cornea. Professor Alpins, can you describe this difference between refractive cylinder and corneal astigmatism a little more? Differences between corneal astigmatism and refractive cylinder are very common. This can be in magnitude, axis or both. I have termed this difference the ocular residual astigmatism (ORA) and is expressed in dioptres. An ORA greater than 0.75D is significant. Did the introduction of wavefront treatments address this refractive corneal difference or ORA? No as the wavefront aberrometry that is measured is based on refractive parameters and does not consider the refractive corneal differences in astigmatism. It is important to also remember that the second order sphero cylinder measured using aberrometry does not incorporate any cortical component that is measured using manifest refraction.
Non-zero goal, refractive cylinder, corneal astigmatism There appears to be different terminology describing the same principles when it comes to astigmatism treatment and analysis. This is something you encountered along your journey – is this what initiated the Alpins Method to achieve a standardized means of reporting astigmatic outcomes? Incisions were based on the steep corneal meridian and refractive laser surgery purely on the manifest refractive cylinder, which in many cases, did not match the corneal astigmatism in magnitude and/or orientation. I wanted a systematic approach to address refractive, cataract and corneal surgery. One in which the terminology was consistent.
Non-Zero Target, Corneal astigmatism target, Refractive cylinder target, Spherical equivalent You mention throughout your book differences between corneal and refractive parameters, particularly with regards to astigmatism. Let’s discuss this difference and what really the surgeon should be aware of. Well the difference between refraction and topography are prevalent and always have been. There are always going to be measurement differences between refraction and topography because there are so many different ways of measuring it. Even with different topographers and keratometers, they are measuring different parts of the cornea and you are going to get different numbers so when you are looking for a Toric implant you are looking at different corneal values to work out which is the one that most closely represents the refractive cylinder. Now you have termed corneal refractive difference the ocular residual astigmatism or ORA. Does it come down to the magnitude of existing astigmatism or does it happen with all magnitudes? The ocular residual astigmatism is a vector because you calculate it so it is quantified in dioptres and in degrees and it is the vectorial difference between the corneal value of astigmatism and the refractive cylinder taken back to the corneal plane. It can happen with any magnitude. It’s surprising how high an ORA can be, even with a lower amount of astigmatism when the brain perceives there to be quite a lot of refractive cylinder and so there is a misconception that you are going to have a high ORA when you have high corneal astigmatism. But that is a misconception because you can have a high ORA with low corneal astigmatism and high refractive cylinder, or low refractive cylinder and high corneal astigmatism. It works either way. It is the difference between the two. The term ‘non-zero target’ – why would a surgeon be targeting something other than zero when treating astigmatism? Well, this again comes back to the ORA where there is a difference between corneal and refractive astigmatism. The surgeon will either target to reduce the astigmatism zero in the refraction or on the cornea but will not be able to reduce both to zero because of the inherent preoperative differences. So there will be a non-zero amount of astigmatism remaining either on the cornea or in the refraction.
Golf analogy, Alpins Method target induced astigmatism vector (TIA), surgically induced astigmatism vector (SIA), difference vector (DV) Now the Alpins Method has been adopted by the major Ophthalmology Journals, including JCRS, JRS and Ophthalmology. Let’s discuss the Alpins’ Method and what it actually involves. Well the fundamentals of the Alpins’ Method involves the relationship between 3 basic vectors. The TIA is the Target induced astigmatism vector which is actually the astigmatic treatment, what you put into the laser or what you put in to your nomogram for limbal relaxing incisions. Or even the power of the Toric implant at the corneal plane. The SIA is the change that you have measured from pre-op to post-op astigmatism, the Surgically induced astigmatism vector And then the third is what we call the Difference vector or DV and that is an interesting one because it is actually a very objective means of measuring how successful the surgery has been. It is how much you have missed the astigmatic target by and if you have corrected all the targeted astigmatism then the DV will be zero
target induced astigmatism vector (TIA) surgically induced astigmatism vector (SIA) correction index (CI) Alpins Method The Alpins’ Method has become the standard in Ophthalmology, for treating and analysing astigmatism Let’s discuss the basic terminology. The three fundamental vectors are the target induced astigmatism vector or the treatment - TIA, the surgically induced astigmatism vector SIA- the difference between the postop and preop astigmatism in both magnitude and orientation and the difference vector or DV which quantifies how much you missed the target by. When you look at the relationship between these vectors you can work out some indices that gives you a good idea of how well the surgery is performed. First one is really the ratio between the SIA and TIA known as the correction index. If the SIA is greater than the TIA you have over corrected the astigmatism, or if the SIA smaller than the TIA it means you under corrected the astigmatism. The angle of error is the angular difference between the SIA and the TIA, and the magnitude of the error is the magnitude difference between the two. An additional parameter you have mentioned in your book is the flattening index – what does this tell us about the success of the procedure? The flattening index tells us the amount of astigmatism reduced at the intended meridian. If you divide this by the amount of astigmatism you were intending to correct, that is, the TIA, then it should be 1.0 because you have effectively corrected all the astigmatism at that targeted orientation.
Flattening, Steepening and Torque, Incision flattening, steepening, rotation, Corneal astigmatism, flat corneal meridian, steep corneal meridian The terms of flattening, steepening and torque. Please describe those for us as far as incisional surgery goes. The SIA is a composite parameter that includes how much the astigmatism has been reduced by flattening effect or how much it has been increased by steepening at the intended meridian or axis of astigmatic treatment. It also includes torque, how much it has been rotated in a clockwise or anti-clockwise sense. In a practical sense, if you get a keratometer and measure at the intended meridian and then you do the surgery, and measure at that point again with the same keratometer it shows you how much flattening or steepening happened at that axis. So the flattening effect is how much affect the SIA is having at the treatment meridian and what’s happened to the rest of the SIA which is the torsional effect. So SIA is a composite of flattening and torsion How much reduction in effect is achieved when an astigmatic treatment is misaligned? Using vector analysis and not just comparing magnitudes, an astigmatic treatment that is 15 degrees off-axis loses approximately 15% of the flattening effect and 30 degrees off axis results in a 50% loss of effect. It is not until the misalignment is 45 degrees that there is a 100% loss in flattening effect. This was shown in a recent paper by Dr Tognetto in the JCRS.
Ocular Residual Astigmatism (ORA), Vectorial difference between refractive cylinder (at the corneal plane) and corneal astigmatism, refractive cylinder and corneal astigmatism mismatch The differences between corneal and refractive parameters have been discussed throughout the book and termed Ocular Residual Astigmatism or ORA. How prevalent is ORA and what are the normal ranges of this parameter? That’s a good question -we have done two studies on that – one in 1997 which showed an ORA of 0.73 D and the other in 2001 with an ORA of 0.81 dioptres, so it is in that range. However, another study which we did in 2007 on 45 keratoconic eyes the average or mean ORA in that group was 1.34 which is more than 50% more than healthy normal astigmatic eyes, So there is no doubt that in the presence of corneal irregularity such as keratoconus the ORA is increased. Some people have termed ORA as lenticular astigmatism. How do you interpret that? I just think it comes down to terminology again. No doubt the crystalline lens has astigmatism of a greater or lesser degree but that is not the total explanation for differences between corneal and refractive values. So I think ORA is a good term because it measures the difference between the cornea and the refractive cylinder all the way from the spectacles all the way back to the occipital cortex What is the importance of calculating the ORA routinely before surgery? By calculating the ORA the doctor can then predict ahead of time that the patient is going to end up with excess astigmatism remaining and advise them on lower expectations.