FEB 2014

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/274531

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Page 41 of 114

incision that is temporal in order for you to account for all of the changes that happen from cataract surgery," he said. How to determine SIA SIA can be determined through both preop and postop measurements, p hysicians say. The cornea is typically stable enough for SIA calculations by three to six months, although it can show stability as early as four to six weeks. Dr. Wang and colleagues deter- mine SIA through vector analysis with preop and stable postop Ks. "In terms of the sample size, if t he surgeon is consistent with inci- sional technique and placement, 20 cases may be used to calculate the SIA. If the standard deviation of the SIA is large, more cases are needed," she said. Dr. Hill developed an online tool for surgeons to determine their own SIA, available at www.SIA-calculator.com. The calculator is free and requires only a login and password. Each surgeon can enter his or her cases, creating a private database of information. "The calculator will determine a value for SIA that can be sorted by incision type, location, and size, as well as patient age and right eye vs. left eye. It is optimal to enter about 60 cases to have a true representa- tion," Dr. Hill said. With 15,000 cases entered into the calculator, it is estimated that for many surgeons, a 2.4 mm clear corneal temporal incision will have an estimated 0.40 D of SIA ±0.05 D. "This number can be used until an actual value is determined," Dr. Hill said. He said that as physicians enter their SIA information into the calcu- lator, they should see a variation in the amount of SIA between cases, a number that physicians interviewed agreed is important to observe. "This is because the amount of SIA is multifactorial," Dr. Hill said. "Even if the keratometry values do not seem to change all that much, a change in the orientation of the steep meridian is another represen- tation of SIA." Dr. Hill explained the reason for this change: "Astigmatism is a vector, having both magnitude (the power difference between principal meridians) and direction (the orien- tation of the steep meridian). The value given by the SIA calculator represents a mean value for a certain area, such as temporal, superior temporal, and superior." Dr. Holladay said that the methods above only account for corneal changes. To get the total SIA, s urgeons must use the preop Ks and postop refraction vector to account for all changes. An exact toric calculator is also vital in these cases, Dr. Holladay said, even when the SIA is accurate. Toric calculators that estimate the necessary toricity of an IOL for a given amount of corneal astigma- t ism and SIA use a constant of 1.46; 1.46 D of toricity is assumed to cor- rect 1.0 D of corneal astigmatism. This value is only true for a 22 D IOL, a 44 D mean K for an IOL 5.0 mm posterior to the corneal vertex. For any other values, the ratio changes. For a 10 D IOL to correct 1 D of corneal astigmatism, you need 1.75 D, and for a 34 D IOL you only need 1.2 D. Two calculators exist that can take these exact measure- ments, he said—his own Holladay IOL Consultant (Bellaire, Texas) and Abbott Medical Optic's (Santa Ana, Calif.) toric calculator. Alcon (Fort Worth, Texas) has this in the Verion System and is also working on a next-generation online calculator that can calculate an exact measure- ment, he said. "A doctor who wants to get good results needs to be using an exact toric IOL calculator and cannot use one that has the approximation method because that gives you the wrong answer except for the nomi- nal values," Dr. Holladay said. EW Editors' note: Drs. Hill and Kezirian have no financial interests related to this article. Dr. Holladay has financial interests with the Holladay IOL Consultant, Abbott Medical Optics, and WaveTec Vision (Aliso Viejo, Calif.). Dr. Wang has financial inter- ests with Ziemer (Port, Switzerland). Contact information Hill: hill@doctor-hill.com Holladay: holladay@docholladay.com Kezirian: guy1000@surgivision.net Wang: liw@bcm.edu T. 800.461.1200 | www.innovativexcimer.com Improved Clinical Outcomes of CXL and PRK with Amoils Epithelial Scrubber Epithelial Removal Has Never Been Easier Corneal Xlinking, PRK & Advanced Surface Ablation E^YV_b]U`YdXU\Ye]bU]_fQ\Y^ only 5 - 7 seconds 1f_YTQ\S_X_\TQ]QWUd_ surrounding tissue =Y^Y]YjUd_dQ\`b_SUTebUdY]U >_^UUTV_bceRcUaeU^dcSbQ`Y^W Visit us at the ASCRS Booth 2032 Epithelial Removal Has Never Been Easier Visit us at the ASCRS Booth 2032 Epithelial Removal Has Never Been Easier Visit us at the ASCRS Booth 2032 Epithelial Removal Has Never Been Easier Visit us at the ASCRS Booth 2032 Visit us at the ASCRS Booth 2032 Cor PRK & Advanced Sur Visit us at the ASCRS Booth 2032 neal Cor Xlinking, PRK & Advanced face Ablation Sur Visit us at the ASCRS Booth 2032 Xlinking, PRK & Advanced face Ablation oved Clinical Outcomes of CXL and Impr PRK with Amoils Epithelial Scrubber oved Clinical Outcomes of CXL and PRK with Amoils Epithelial Scrubber oved Clinical Outcomes of CXL and PRK with Amoils Epithelial Scrubber _ V Y ^ E only 5 - 7 seconds Y _ f 1f ounding tissue surr ] Y ^ Y = U b ] e Y \ U X d Y ` U ] b _ only 5 - 7 seconds Q ] Q T \ _ X _ S \ Q T ounding tissue T U S _ b ` \ Q d _ d U j Y ] ^ Y \ Q f _ ] U _ d U W U ] Y d U b e T ^ _ > e a U c R e c b _ V T U U ^ W ^ Y ` Q b S c d ^ U e . 800.461.1200 | www.innovativexcimer T . 800.461.1200 | www.innovativexcimer .com . 800.461.1200 | www.innovativexcimer February 2014 38-45 Cataract_EW February 2014-DL2_Layout 1 1/30/14 10:18 AM Page 39

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