EyeWorld India March 2017 Issue

March 2017 12 EWAP FEATURE How previous refractive surgery affects IOL power choice by Ellen Stodola EyeWorld Senior Staff Writer Surgeons weigh in on how prior surgery might impact calculations C hoosing the correct IOL power can be challenging, and it can be even more so in patients who have undergone previous refractive surgery. Douglas Koch, MD , professor and Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute at Baylor College of Medicine, Houston; Samuel Masket, MD , Advanced Vision Care, Los Angeles, and clinical professor, David Geffen School of Medicine, UCLA; and Daniel Chang, MD , Empire Eye and Laser Center, Bakersfield, California, commented on how previous surgery could cause difficulties for surgeons, how these difficulties can be overcome, and which formulas and IOLs can be helpful. AT A GLANCE • When doing calculations for a patient who has undergone previous refractive surgery, it’s important to remember that the relationship between the front and back surface of the cornea has changed. • Different steps should be taken depending on the procedure the patient has had. • When choosing an IOL, it’s important to account for induced spherical aberration. There may be a role for IOLs with extended depth of focus. Figure 1. A: Schematic conceptual relationship between the anterior and posterior corneal curvatures in the unoperated eye. B: Schematic conceptual corneal shape change after RK. C: Schematic conceptualized drawing of corneal shape after excimer photoab- lation for myopia. D: Schematic conceptualized drawing of corneal shape after hyperopic excimer photoablation. Source: Journal of Cataract & Refractive Surgery continued on page 14 Calculations after refractive surgery There are at least three issues, Dr. Koch said. “One is that the anterior corneal surface is more irregular, and, as a result, it’s more difficult to determine the true refractive power of the anterior corneal surface,” he said. “You’re not looking at uniformity of powers; you’re looking at variability.” Most formulas now try to use an average over the anterior corneal surface, but how you weigh various areas is tricky, Dr. Koch said. The second potential issue is that the posterior corneal power is unknown. “A basic assumption in IOL calculations is that you measure the front and you assume posterior power based on some fixed ratio compared to the front,” Dr. Koch said. However, he noted that such an assumption is erroneous, sometimes even in normal eyes, and in eyes that have undergone previous corneal refractive surgery, the relationship between front and back is no longer valid. Physicians have to make an estimate of posterior corneal power, while awaiting advances in technology that will allow them to comfortably obtain accurate measurements, Dr. Koch said. The third difficulty is that many lens calculation formulas use corneal power as one variable when calculating effective lens position. This can be hard to factor in if the patient has had previous refractive surgery. “You would need to use formulas that don’t take that into account or guess what the corneal power was prior to surgery,” he said. Dr. Chang agreed that there are several factors to consider. First is that not all refractive surgery is the same. Different treatment modalities, degrees of refractive correction, ablation and blend zone sizes, and even centration can affect the size and location of the active optical zone, he said. Dr. Chang said that of his patients coming in for cataract surgery, about 10% of them have had a previous refractive procedure. The physician must consider what surgery was done because predictions will vary depending on the refractive procedure performed, Dr. Masket said. If it was laser-based surgery, or photoablative, it can create a series of problems, he said. If it was an incisional or RK-type surgery, it creates a different set of problems. If the patient has undergone photoablative surgery, the ratio of the front and back surface of the cornea is changed. In the unoperated eye, the anterior and