EyeWorld Asia-Pacific June 2012 Issue

June 2012 19 EWAP FEATURE the SIA to the pre-op Ks will give you the post-op Ks on the next case. “Toric IOLs should always be aligned with the steepest meridian of the cornea post-op,” he said. “You’re trying to predict where that is by adding the SIA to the pre-op astigmatism.” SIA for right eyes will differ from left eyes, “and that has to do with the incision architecture,” Dr. Hill said. “Thinking the number should be the same for all patients is a misconception.” Dr. Hill recently introduced a new tool that may help surgeons Tae-Im KIM, MD Associate Professor, Dept. of Ophthalmology, Yonsei University College of Medicine 50 Yonsei-ro, Sudaemun-gu, Seoul 120-752 Korea Tel. no. +82-2-2228-3570 Fax no. +82-2-312-0541 tikim@yuhs.ac F or the past decade, surgeons have tried several methods to correct preexisting astigmatism during cataract surgery, including making incisions in the cornea to alter the shape of the eye. Recently, the introduction of the advanced toric IOL has allowed for the prediction and correction of preexisting corneal astigmatism after cataract surgery. However, all of these anticipated surgical outcomes can be reached on the conditions of accurate assessment of preoperative corneal astigmatism and axis, and minimal induction of surgically induced astigmatism (SIA) during the procedure. Several methods have been introduced to evaluate corneal astigmatism before and after surgery. When comparing the amount of astigmatism, axis determination would be more heavily emphasized, because even a subtly disoriented axis of toric IOL implantation or making an incision may cause a reduction in the correcting effect or induce unwanted change. As the main article mentioned, the SIA varies not only according to location, size, and configuration of the incision itself, but also according to the rigidity of corneal tissue, blade material, and the surgical steps. We compared the amount of SIA between on-axis or temporal incisions. The results showed that incisions on the axis induced more SIA than horizontal corneal incisions and resulted in less refractive astigmatism and better uncorrected visual acuity at 6 months after surgery. The effect of suture on a cataract incision less than 3.0 mm is controversial. One report concluded that suture in a 2.75-mm incision induced more corneal astigmatism because of its tightness. However, an adequately tense suture may reduce unexpected corneal flattening induced by the incision. We also studied and compared the wound configurations formed between a diamond and metal blade using the RTVue with corneal module. Compared to the metal blade, the wound configuration from the diamond blade showed relatively low wound integrity in the posterior lip of the incision and caused persistent corneal edema. However, the blade material does not influence the severity of SIA. A patient undergoing cataract surgery has expectations that almost match one undergoing a refractive procedure. To perform cataract surgery without paying much attention to remaining refractive errors can no longer be justified. Therefore, personalized SIA-minimizing factors including location of the incision, wound structure, blade material, suture, and movement of the phaco or I&A tip during the surgical procedure should be promoted continuously. Editors’ note: Prof. Kim has no financial interests related to her comments. Views from Asia-Pacific calculate SIA, www.SIA-calculator. com, a “completely HIPAA- compliant” calculator with all data stored “in the cloud”. Conservatively, most surgeons need about 60 cases to determine their SIA, Dr. Hill said, although Dr. Koch and Li Wang, MD , assistant professor of ophthalmology, Cullen Eye Institute, BCM, believe surgeons who are consistent with incisional technique and placement may be able to use as few as 20 cases. They’ve published a study looking at post-op cataract incisions via optical coherence tomography to determine the effect on wound healing,1 and “a small Descemet’s detachment was present in 37% of eyes at 1 day postoperatively, decreased to 4% at 1 to 3 months, and was absent after 3 months,” Dr. Wang said. Dr. Kezirian noted SIA “is not stable until at least 3—and probably not until 6—months after surgery. “To really assess what has been induced by the procedure, you have to analyze longer-term data than most cataract surgeons collect,” he said. Dr. Brass said wounds may “remodel over some time”, and agreed that may affect SIA amounts. Dr. Hill, however, said wounds will start to stabilize between 4 and 6 weeks, depending on how much stretching was involved. Minimizing your SIA Operating on axis “will always diminish the astigmatism”, Dr. Brass said, but operating 90 degrees away will adversely affect the amount of corneal astigmatism. “If you flatten in one meridian, you will steepen in the other.” Dr. Brass typically works at 10 degrees for the left eye and 190 for the right. “The debate becomes operating on the step axis vs. staying where I am most comfortable,” he said. By using the toric calculator and choosing a lens with greater astigmatism correction, he is able to compensate for any induced astigmatism. Operating at 90 degrees with an average incision of about 2.5 mm will create an SIA “somewhere around 0.6-0.8,” Dr. Holladay said. “But a horizontal 2.5-mm incision induces 0.4-0.6 D in the horizontal meridian, or about 50% less SIA.” He added the Holladay IOL Consultant includes an “optimizer” showing the four incision locations where no residual astigmatism will be introduced. “Astigmatism goes to visual quality,” Dr. Kezirian said. “It behooves a surgeon to reduce the amount of SIA.” Surgeons who operate on a fixed axis need to perform a vector subtraction of pre-op and post-op corneal astigmatism and average them, “but they should do a histogram distribution” as well. If IOLs are placed on axis “the worst that happens is I over- or undercorrect, but I still get the full benefit of the IOL,” Dr. Kezirian said. “But moving the axis 15 degrees decreases the IOL efficacy by half. If I go 30 degrees, I’ve negated the benefit altogether. That reduces quality of vision because you have two toric surfaces interacting in an oblique axis and it can’t be corrected with glasses.” Dr. Hill “never operates in the same place—I move my incision so I’m always on the steep axis,” and said those who operate temporally will “always have variability” in their outcomes. “Surgeons who operate temporally are always going to make the horizontal meridian flatter than it was pre-op in the post-op. If that’s the case, they’re going to change the magnitude of astigmatism as well as the axis,” Dr. Holladay said, adding “there are only four possible places to put an incision so there’s no residual astigmatism.” “We have found that quite a bit of astigmatism on the posterior cornea can really influence outcomes,” Dr. Koch said. “We’re reformulating another toric IOL nomogram based on these data as well.” “The thing to remember about SIA is that it’s not the same for all eyes,” Dr. Hill said. “It’s easy to calculate a mean value, but difficult to anticipate an exact value for individual patients.” EWAP Reference 1. Wang L, Dixit L, Weikert MP, Jenkins RB, Koch DD. Incisional healing changes of clear corneal cataract incisions evaluated using Fourier-domain optical coherence tomography. Article in press, J Cataract Refract Surg . Editors’ note: Dr. Brass has financial interests with Alcon (Fort Worth, Texas, USA/Hünenberg, Switzerland). Dr. Hill has financial interests with Alcon and Haag-Streit (Koeniz, Switzerland). Dr. Holladay is the developer of the Holladay IOL Consultant programs. Dr. Koch has financial interests with Alcon and Abbott Medical Optics (Santa Ana, Calif., USA). Dr. Wang has no financial interests related to this article. Contact information Brass: +1-518-782-7827 Hill: +1-480-981-6130, hill@doctor-hill.com Holladay: holladay@docholladay.com Kezirian: +1-480-664-1800, guy1000@surgivision.net Koch: +1-713-798-6443, dkoch@bcm.edu Wang: +1-713-798-7946, liw@bcm.edu