EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/274531
E W CATARACT 38 February 2014 by Erin L. Boyle EyeWorld Senior Staff Writer Surgically induced astigmatism caused by multiple factors SIA can be a key value in toric IOL selection and is often determined by preop and postop measurements S urgically induced astigma- tism (SIA) serves as a reference point for many surgeons in determining the astigmatism that is created by incisions in cataract sur- g ery and can be useful in toric IOL selection. "Surgically induced astigmatism is the change in both the power and, to a lesser degree, the orientation of the principal meridians following a corneal incision," said Warren E. Hill, MD, East Valley Ophthalmol- ogy, Mesa, Ariz. Traditionally, surgeons use preoperative and postoperative K readings to determine their SIA vector, said Jack T. Holladay, MD, clinical professor of ophthalmology, Baylor College of Medicine, Houston. But that way could change, he said. "Even though that's the way we normally do it, the fact is, that's not the best way," he said. Dr. Holladay is working on incorporating the postop refraction measurement. He has found that the postop refractive astigmatism is more than just changes in the cornea on the front and the back surface. Other factors are also involved, he said. "For example, if you took some- one who had a spherical cornea and you made a temporal incision, what we're finding is that person ends up with a little induced against-the-rule on his refraction even though the K readings show that he got a little with-the-rule. What that tells us is there's more going on than just the change in the front surface of the cornea," Dr. Holladay said. Using the average SIA is not always a reliable method of account- ing for astigmatism induced by surgery in all cases, according to Guy M. Kezirian, MD, president, SurgiVision Consultants, Scottsdale, Ariz. "The assumption that many surgeons have is that their average surgically induced astigmatism is going to predict the SIA that they'll have on the next case. This is seldom true," said Dr. Kezirian. When he ran an analysis of 4,000 eyes with good preoperative and postoperative keratometry read- ings, he found that the magnitude of the standard deviation of the SIA was usually greater than the mean. This translates to the variation from case to case being larger than the average amount, he said. "So for surgeons to use an aver- age of their SIA as an assumption of what's going to happen in the next case ignores all of the variation that they have with each eye," he said. He advocated careful placement of incisions as a way to reduce in- duced astigmatism: "It's far better to accept that SIA is not consistent and to use either on-axis incisions or or- thogonal-axis incisions to limit the variability in induced astigmatism than it is to use a calculator to deter- mine how to alter the orientation of the IOL," Dr. Kezirian said. Dr. Hill noted that, "Looking at more than 15,000 cases submitted for analysis on www.SIA-calculator.com, there is typically a wide variation in the amount of SIA from one case to the next for the same surgeon using the same incision type, location, and size." Factors involved The amount of SIA that can occur is based on numerous factors, Dr. Hill said, including: • Incision location, size, and architecture • The corneal radius, thickness, and rigidity • The stretching of the incision as the folded IOL passes through it "The key determinants of SIA are the location, size, and architec- ture of the cataract incision and the suture use (temporary effect only). Another important but poorly understood factor is the individual biologic response of the patient's cornea," said Li Wang, MD, PhD, associate professor, Baylor College of Medicine. She said wound healing could also have an impact on SIA, with wound remodeling different for each patient. The biomechanical changes in the cornea are not the only determi- nants of SIA, Dr. Holladay said, although that could account for about half of it. "The factors that determine the refractive change that we get from cataract surgery are, number one, the changes in the front surface from the cataract incision. That by far is the biggest component," he said. The second factor is back surface changes in the cornea, he said. Douglas Koch, MD, colleague to Dr. Wang at Baylor College of Medicine, has shown that surgeons overesti- mate with-the-rule astigmatism and underestimate against-the-rule by just measuring the front surface by about a quarter diopter, Dr. Holladay said. The normal decentration and tilt associated with IOLs can add ap- proximately another quarter diopter of against-the-rule astigmatism, he said. "The total then is about half a diopter of against-the-rule astigma- tism change from a normal cataract D o you know what your surgically induced astigmatism is? I would g uess that the majority of surgeons do not. In this column, we discuss toric IOLs. When considering astigmatism correction, whether it is with toric IOLs or femtosecond laser or manual incisions, the surgically induced astigmatism (SIA) should be taken into consideration. Most cataract surgeons understand that SIA is important but are unsure what factors determine it or the m ethod to calculate their own SIA. We asked several experts to explain SIA and describe how to determine the value. Bonnie An Henderson, MD, cataract editor Cataract editor's corner of the world The SIA Calculator, which can be found at www.SIA-calculator.com, can help surgeons determine their surgically induced astigmatism in cataract surgery. Source: Warren E. Hill, MD, FACS 38-45 Cataract_EW February 2014-DL2_Layout 1 1/30/14 10:18 AM Page 38