Practical Astigmatism Planning and Analysis: Video Chapters

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.

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.

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.

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.