EyeWorld India December 2018 Issue

not enough information. There’s much more information if you use a topographer or tomographer. Dr. Weikert: [I do not.] They can be challenging to measure, and they have more of a learn- ing curve as compared to auto- Ks. Even though our calculation formulas were developed using manual Ks, once we optimize our lens constants, auto-Ks work well. When you look at manual Ks com- pared to auto-Ks, they measure different zones of the cornea, so you would expect some disagree- ment between them. Attendee: What are your experiences with surgeon-specific biometry like the Hill-RBF? Any tips for transitioning from your go-to calculations to the program? Dr. Miller: The Hill-RBF is my primary formula now. What attracted me was Warren Hill, MD’s data showing that 90% of his patients are within 0.5 D of emmetropia after surgery. That’s pretty compelling and a reasona- ble reason to use it. Like anything, I don’t immediately transition. I go through a phased transition, and I’m still doing that. I advise surgeons to use their comfort zone formulas for a while and be look- ing at the other one; then they can eventually flip over and plan off the new one, while looking back at the old formulas to make sure they don’t have things that are com- pletely nonsensical. Dr. Weikert: Any time you have something new, you want to continue to do what you’re com- fortable with, what you have expe- rience with, then you can compare those results to the new method. When we do our printouts, we have three formulas. We use the Holladay 1, Barrett, and Hill-RBF. I get those for every patient and I compare them. You often find that in a certain type of eye you might lean toward one formula over another. YES connect co-editors: When planning for a toric IOL, do you rely more on the topographer (Placido disc) or the biometer for the axis of astigmatism? What about for the amount of astigma- tism? Dr. Miller: I use tomography devices, primarily the Pentacam, although we have a Galilei and occasionally I’ll look at that. I look at the Pentacam axial map for both the amount of astigmatism, which I get from the Sim K values, and the axis, which I get from the Sim K axis. I do look at the total cornea. If the Sim K axis is 45 but the peripheral cornea is clearly against the rule, more like 180, I bias my toric axis toward the 180. For weird corneas, I’ll split the difference. Often there’s a differ- ence between the anterior cornea and the total cornea, and a purist would say you should go with the total cornea, but my comfort level isn’t totally there yet. I take the total, but I swing the axis a little bit toward the anterior. For the power, I tend to bias toward the total cornea. Dr. Weikert: I would say I don’t defer to one device all the time. We bring in multiple meth- Views from Asia-Paci c CHAN Wing Kwong, MBBS(S’pore), MMed(Ophth), FRCS(Edin), FRCOphth, FAMS Eye & Retina Surgeons One Orchard Boulevard #13-01/02/03/04/05/05A, Singapore 248649 Tel. no. +65-6738-2000 Fax no. +65-6738-2111 wkchan@me.com T his article discusses three issues. The use of corneal topographers and the utility of axial and tangential curvature maps; the use of IOL formulas; and the correction of astigmatism in cataract surgery. The use of corneal topography has become a “must do” investigation in the preoperative evaluation of cataract surgery cases. This is particularly true for the evaluation of patients for presbyopia-correcting IOLs and the correction of preexisting corneal astigmatism. As to whether an axial or tangential map is better, there is no best answer. It depends on what you are looking for. The information acquired is the same, it is just how the information is processed and presented. The short answer for me is that axial maps are best for day-to-day general screening and surgical planning for astigmatism correction. Tangential maps give you more details about the shape of the cornea at a specific point of interest but can generate a lot of noise and confusion, so probably best left to specific applications such as deciding if there is a subtle ectatic disorder of the cornea or evaluating the potential impact of a corneal scar. If you can only use one topographer, it should be one that gives you the picture of the entire cornea, i.e. the anterior and posterior corneal surfaces. So it should be a topographer that is either a Schiempflug or a slit scanning based system. Placido-based systems do not give you any information on the posterior corneal surface, which we now recognize to have an important impact on the correction of astigmatism and the selection and use of toric IOLs. Schiempflug or slit scanning based systems also have the added advantage of being able to provide other useful parameters such as corneal thickness and anterior chamber depth. As for IOL formulas, one should use a fourth generation formula that tries to predict the effective lens position more accurately. I use the Barrett Universal II formula and the Barrett Toric Calculator. The genius of the Barrett Toric Calculator is that it has the ability to take in keratometric readings from various optical biometers, corneal topographers, manual keratometry, and automated keratometry. It then integrates all these inputs and derives a calculated K reading for all the values entered. This calculated K has noticeably improved the accuracy and predictability of toric IOL surgery. It removes, or at least reduces, the dilemma surgeons sometimes face as to which is the correct K reading to use when the readings obtained from the various devices do not correlate when planning for a toric IOL. Finally, the issue of astigmatism correction in cataract surgery. For those of us in Asia where toric IOLs are available, they have now been proved in studies to be the best method to correct preexisting astigmatism in cataract patients. Limbal relaxing incisions and astigmatic keratotomy (be it femtosecond laser or incision based) are simply not comparable in terms of accuracy, predictability, and stability. Laser refractive surgery is also very effective, but this is necessarily a postoperative solution with additional time and cost implications. Certainly, in places where toric IOLs are not available or where cost is a concern, some form of an incisional corneal procedure or a pair of spectacles is still perfectly acceptable. Editors’ note: Dr. Chan declared no relevant financial interests. Preoperative diagnostics – from page 47 48 EWAP CATARACT/IOL December 2018