EyeWorld India March 2013 Issue

March 2013 9 EWAP FEATURE Views from Asia-Pacific Arup CHAKRABARTI, MD Senior Consultant, Cataract and Glaucoma Services, Chakrabarti Eye Care Centre No. 102, Kochulloor, Trivandrum 695011, Kerala, India Tel. no. +91-471-2555530 Fax no. +91-471-2558530 arupeye@gmail.com I n this era of refractive cataract surgery, our goal is to reduce the patient’s dependence on glasses in the postoperative period, if not eliminate it altogether. This has come about due to our ability to more accurately determine IOL power (Optical Biometry: IOLMaster and Lenstar) and to the incorporation of steps to reduce the quantum of postoperative astigmatism. Accurate determination of the preoperative astigmatism, knowledge of the SIA and an appropriate astigmatism reduction strategy play an important role. Preoperative astigmatism is traditionally measured using: 1. Manual keratometry, 2. Automated keratometry, 3. Corneal topography, 4. Optical Biometry. In an ideal situation the K-readings from all the devices will give similar readings, which, unfortunately, is hardly ever the case because devices don’t always measure the same areas of the cornea. The various methods to reduce postoperative astigmatism in cataract patients have been limbal relaxing incisions, paired opposite clear corneal incisions, toric IOLs and laser. However, toric IOLs have been increasing in popularity for various reasons.Satisfactoryresultswiththeselensesrequireproperworkupofpreoperative astigmatism, a good surgical technique and proper IOL handling/alignment. Toric IOLs are not used in patients with irregular astigmatism. Topography is a tool that may detect astigmatic irregularity which may go undetected in conventional keratometry. In case of major disagreement between the various devices it may be a good idea to perhaps not employ a toric IOL after proper explanations to the patient. In case of a minor disagreement, the axis of the astigmatism may be assessed using manual or automated keratometry and the actual magnitude may be taken from topography according to Dr Lane. Some surgeons would like to average the measurements after repeating them to ensure consistency in the values for each device. These devices don’t put due importance to the back surface of the cornea. Dr. Douglas Koch has shown in his recent study that the corneal back surface plays a significant role in terms of “fine-tuning the total astigmatism of the corneas”. The back surface is taken into account by tomographers, e.g. Orbscan (Bausch + Lomb), Pentacam (Oculus) and Galilei (Ziemer). These limitations may perhaps be answered by a group of devices— intraoperative wavefront aberrometers such as the ORA (Wavetec) or Clarity (Holos)—which allow surgeons to directly detect accurate refraction including astigmatism at the time of surgery after the cataract is removed. However, the surgeon should be ready with a wider inventory of toric IOLs in case the intraoperative measurements differ from the predicted preoperative values. The surgeon should also have a clear idea of his SIA and the various factors impacting it to optimize his astigmatic outcomes. Neglect of this component often leads to suboptimal results with toric IOLs. Intraoperative lens positioning along the proper axis is of paramount importance. The standard technique of marking with ink is considered to be inadequate. The emerging guidance systems play an increasingly important role for proper intraoperative lens placement.Guidance devices such as the SMI (Alcon, Fort Worth, Texas, USA/Hünenberg, Switzerland) take a photograph of the eye and match this intraoperatively so that landmarks are identified, ensuring proper lens positioning. The bottomline is no single device currently serves all functions with regard to corneal measurements and many believe that intraoperative aberrometry will become the standard of care in the future enabling practitioners to better achieve the target of emmetropia without astigmatism and possibly other higher aberrations such as spherical aberration and coma. Editors’ note: Dr. Chakrabarti is a consultant for Allergan (Irvine, Calif., USA) but has no financial interests related to his comments. How devices measure up Practitioners today are fortunate to have a variety of devices for measuring astigmatism at the ready. Here’s what’s available: Manual keratometry determines the quantity of astigmatism and the axis, according to Dr. Trattler. “It’s good for planning cataract surgery, but it doesn’t help us to figure out whether the cornea is regular or irregular,” he said. The IOLMaster considers corneal shape, using three measurements for astigmatism. “It’s a very rudimentary method, but it’s very accurate as far as helping us plan for the right intraocular lens power,” Dr. Trattler said. This will tell if astigmatism is present, how steep the cornea is, and help with surgical planning but will not identify irregular astigmatism. Corneal topography uses imaging technology to get a sense of the magnitude of the astigmatism and the shape of the cornea, Dr. Trattler explained. It can tell if the cornea is regular or irregular and if the patient has a condition such as keratoconus. The Pentacammeasures the shape of the cornea and the magnitude and regularity of the astigmatism. “You press it back and can determine the shape of the cornea and if there’s any regularity or irregularity,” Dr. Trattler said. The Galilei gives measurements for both anterior and posterior corneal curvatures. This can be helpful in considering what posterior astigmatism contributes, which has gained importance thanks to Dr. Koch’s new nomogram for implanting toric IOLs, which uses both measurements, Dr. Trattler explained. The Clarity and the ORA offer intraoperative wavefront measurements of astigmatism. These allow practitioners during surgery to measure the cornea through the power of the astigmatism. “It helps you to fine-tune your planning,” Dr. Trattler said. Devices such as the iDesign (Abbott Medical Optics, AMO, Santa Ana, Calif., USA), the iTrace (Tracey Technologies, Houston, Texas, USA), and the OPD (Marco, Jacksonville, Fla., USA) can analyze a combination of topography and wavefront measurements at the same time. These units can give corneal shape and also determine whether the astigmatism is symmetrical or asymmetrical. Optimizing outcomes How can practitioners best use devices to optimize results for astigmatic cataract patients? Dr. Lane stressed that it’s important to begin by distinguishing lenticular from corneal astigmatism. “Obviously the astigmatism associated with the lens will be absent following the removal of the cataract,” he said. “So you need to have an idea of what the post-operative corneal astigmatism will be.” He finds that’s best accomplished pre-op with the aid of different available tools. “Some of them are automated like the measurements that you would take with an IOLMaster or with the Lenstar,” he said. “Some of them have been around for many decades like manual keratometry, and some of them are looked at in terms of corneal topography or even OCT.” Intraoperatively, Dr. Lane sees systems such as the ORA and the Holos as serving an important function. “With the WaveTec aberrometry and in the future with Clarity aberrometry, that will help us to determine what the amount of astigmatism is after we’ve removed the cataract on the table, real-time,” he said. He also stressed the importance of using a guidance system for proper intraoperative lens placement. “We have guidance systems because with astigmatism not only is there an amount, there’s also a direction,” he said. “So you can choose the correct power implant, but if you put it in the wrong position you’ll be inaccurate in your correction of the astigmatism.” Guidance devices such as the SMI (Alcon, Fort Worth, Texas, USA/Hünenberg, Switzerland) take a photograph of the eye and match this continued on page 10