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4 S ince the introduction of femtosecond lasers for LASIK, refractive surgeons have been able to create round flaps, but as we learn more about the biomechanics of the cornea itself—including that it is elliptical—we've realized that a round flap may not be the most efficacious for the majority of our patients. Couple that knowledge with flap creation times that are as quick as 10 seconds with the iFS laser (Abbott Medical Optics, AMO, Santa Ana, Calif.), and refractive surgeons now have a bevy of options when creating LASIK flaps that was unimaginable even 5 years ago. When I first heard podium presentations about the potential advantages associated with oval/ elliptical flap creation, I was a bit skeptical. After all, we have been successfully using round flaps for the last 15 years. To evaluate just how oval the cornea is, I used digital video analysis to retrospectively measure 10 corneas. As shown in Figure 1, from limbus to limbus, the mean vertical diameter is 11.1±0.4 mm, while the mean horizontal diameter is 12.0±0.5 mm. This means that the cornea has a 7.9% horizontal ovalization. What this means in clinical practice is that oval flaps are a better anatomic match for the cornea as they fit symmetrically into the horizontally oval shape of the normal cornea. Perfectly round flaps occasionally rotate when they're repositioned, which can cause striae as the flap torques from its intended position in the bed. An oval flap, however, can only fit back into one place—it cannot be rotated, which ensures perfect alignment and all but eliminates the possibility of striae. We're also cutting slightly fewer peripheral corneal lamellae and fewer corneal nerves because there are fewer cuts being made vertically on the cornea; this may help retain more corneal structure strength and sensation than if we used round flaps. As Kerry Assil, M.D., said during one of his first presentations on oval flaps, 1 "Customized flap creation offers the potential for improved stromal bed exposure and biomechanical stability, as well as possibly reduced induction of opaque bubble layer, refractive aberration, dry eye, epithelial in- growth, striae, and inflammation." Why cylinder matters Mandel and colleagues reported that the majority of young patients in their practice had with-the-rule (WTR) astigmatism. 2 That intrigued me enough to determine how many of the patients at TLC Laser Eye Centers had WTR or oblique astig- matism who might be better served by a horizontal oval/elliptical flap. Since we know correcting the classic WTR astigmatism "bowtie" results in an ablation pattern that is hori- zontally oval, not round, it made sense that if an overwhelming num- ber of patients have WTR bowtie astigmatism, using an oval flap during the LASIK procedure could be beneficial. In our retrospective analysis of 218,913 eyes from pre-op evalua- tions recorded in our TLC database from 2000 to 2009, we defined WTR astigmatism as plus cylinder within 15 degrees of the 90-degree axis, against-the-rule (ATR) astigmatism as plus cylinder between 15 degrees of the 180-degree axis, and oblique astigmatism as all those eyes with an astigmatism oriented in the 60 degrees in between. Overall, we found that 42.4% had WTR astigmatism, 36.9% had oblique astigmatism, and 20.7% had ATR astigmatism. Extrapolating that data to preferred treatment patterns means about 80% of astigmatic eyes (WTR and oblique) can be best served by a horizontal oval/ elliptical treatment bed. I had the good fortune to review with Jack Holladay, M.D., some of the more detailed analyses on the amount and orientation of the astigmatism. When the orientation of the astigmatism was analyzed relative to the amount of spherical equivalent (SE) myopia, we found that ATR astigmatism was more common in very low myopes, but WTR astigmatism became more common after about 2.5 D of SE myopia. The percentage of eyes with WTR astigmatism slowly rose to rep- resent about 60% of eyes by 7.00 D of SE myopia. Conversely, ATR astig- matism decreased from 45% of very low myopes to about 10% of eyes by 7.00 D of myopia, while oblique astigmatism remained constant at about 30% of eyes (Figure 2). When the average amount of astigmatism was calculated for each astigmatic orientation for different levels of myopia, we found that all average amounts of astigmatism increased. The most dramatic increase was in the WTR astigma- tism group, which increased from an average of 0.4 D of WTR astigmatism in the 0-1 D SE group to a maximum average of 1.29 D in the –9.00-10.00 D SE myopia group (Figure 3). We also found that as the astigmatism level increased from 1.00 to 5.00 D, the percentage of eyes with WTR astigmatism increased from 37% to 66%, while oblique and ATR percentages decreased (Figure 4). So what do these patterns and trends mean for refractive surgeons? Patients with higher levels of myopia are also more likely to have higher levels of WTR and oblique astigmatism. At TLC, the average amount of myopia we're treating is around –4.00 D. Based on these calculations, the vast majority of our patients will greatly benefit from the horizontal oval/elliptical flaps, which corresponds to their horizon- tal oval ablation patterns. Personal technical pearls With the iFS laser, I routinely use a 100-micron flap depth, with inverted 110-degree side cut. I have found that the 5% "oversized" flap set with an 8.2 mm vertical default is the most effective to create the oval/elliptical flap with the iFS laser. EW Chicago 2012 4 Laser vision correction by Louis E. Probst, M.D. Matching corneal anatomy with customized flap architecture Louis E. Probst, M.D. " My experience has shown me that within the next few years, oval flaps will be the industry standard " An oval flap allows refractive surgeons to achieve a perfect flap alignment with better optical results AMO Saturday supplement_Chicago 2012-USE THIS ONE_Layout 1 4/20/12 4:25 PM Page 4

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