EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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82 EWINTERNATIONAL September 2012 A new way to determine refractive operations efficacy by A.I. Myagkikh, Ph.D., Ost-Optik K Co., Ltd., Vladivostok, Russia I n the last 10 years, researchers and clinicians have made efforts to unify how the results of refractive operations are represented. The common goal has been to standardize output data to quickly evaluate the efficacy, safety, predictability, and stability of refractive surgery. One of the recommended graphical representations has been a histogram of Snellen's cumulative non-corrected visual acuity added with the cumulative corrected visual acuity before surgery.1 However, this kind of histogram is not completely successful for several reasons: 1.A cumulative representation does not consider what every operated eye contributes. 2.There is no descriptive presenta- tion about the difference in pre- and post-op visual acuity in connection with the operation performed. This is particularly obvious in the event of a pre-op low visual acuity. 3.A mathematical analysis of cumu- lative representations is difficult. In particular, it is impossible to calculate common parameters such as the average and disper- sion, both of which provide the most accurate characterization of the processes under consideration. Pre- and post-op visual acuity comparisons can provide informa- tion on the efficiency of refractive operations.2 Scientific continued from page 81 LASIK and show that recovery to normal levels takes on average 6 months. Figure 3 shows the average corneal sensation across these nine studies. In SMILE on the other hand, the anterior corneal anatomy is preserved and the anterior stromal nerve plexus is disrupted signifi- cantly less since there are no side- cuts created—no flap is created; this should result in fewer dry eye symp- toms and a faster recovery of post-op patient comfort. Early results seem to support this hypothesis. We have measured corneal sensation in 39 eyes after SMILE and the results compare favorably with the average data taken from similar published LASIK studies. Corneal sensation had recovered to the baseline level by 3 months after SMILE compared with 6-12 months after LASIK. Also, corneal sensation was only slightly depressed in the majority of eyes after SMILE at the day 1 post-op visit, whereas corneal sensation was found to be generally 0 in published LASIK studies reporting 1-day data. In summary, with the introduc- tion of the VisuMax femtosecond laser technology it has become clini- cally feasible to now create refractive lenticules of proper regularity with sufficient accuracy to meet and possibly exceed the accuracy of excimer laser tissue ablation for corneal refractive corrections. This enables Jose Ignacio Barraquer's orig- inal concept of keratomileusis to be effectuated through a minimally in- vasive pocket incision with maximal retention of anterior corneal inner- vational and structural integrity. It is the final frontier in the realization of the perfect refractive surgical technique for both patients and surgeons. EW References 1. Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6 month prospective study. Br J Ophthalmol. 2011;95:335-339. 2. Shah R, Shah S, Sengupta S. Results of small incision lenticule extraction: All-in- one femtosecond laser refractive surgery. J Cataract Refract Surg. 2011;37:127-137. 3. Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF. Depth-de- pendent cohesive tensile strength in human donor corneas: implications for refractive surgery. J Refract Surg. 2008;24:S85-89. 4. Knox Cartwright NE, Tyrer JR, Jaycock P, Marshall J. The effects of variation in depth and side cut angulation in sub-Bowman's keratomileusis and LASIK using a fem- tosecond laser: a biomechanical study. J Refract Surg. 2012 Jun;28(6):419-25. Editors' note: Dr. Reinstein practices at the London Vision Clinic, London, and is affiliated with the Department of Ophthalmology, Columbia University Medical College, New York, and the Centre Hospitalier National d'Ophtal- mologie, Paris. He has financial interests with Carl Zeiss Meditec and ArcScan Inc. (Morrison, Colo.). Contact information Reinstein: +44 207 224 1005, dzr@londonvisionclinic.com An ideal case would be a comparison under cycloplegia. How- ever, this type of screening can be time consuming. This is why in most refractive clinics, it is necessary to use the pre-op value of the cor- rected distance visual acuity (CDVA) and the post-op value of the uncor- rected distance visual acuity (UDVA). This comparison is performed as a calculation of UDVA to CDVA ratio for every operated eye and construc- tion of a graph of relevant distribu- tion (number of eyes versus value of calculated parameter) for all data sets. The ratio of the visual acuity purely expresses improvement achieved as a result of the operation, and construction of distributions considers the influence of every operation on the "big picture." Thus, by definition, Keff = UDVA post-op/CDVA pre-op, where Keff is the efficiency coefficient of the performed refractive operation for the given eye. The mathematical ratio may be calculated both in decimal represen- tation and directly as Snellen's fractions in accordance with the standard mathematical rules. Calcu- lations may also be made in logMAR scale using subtraction instead of division. Coincidence of a post-op result with a result planned in usual scales will be characterized by the following values: Keff =1, and in the logMAR scale, Keff=0. Figure 1 shows an example of data representation in the proposed Figure 1 Figure 2 Figure 3 format. For comparison, the same data are represented in a cumulative form in Figure 2. Examining the new approach In the theoretical ideal, the resulting distribution of efficiency coefficient should represent a delta function having a zero value according to all parameters and number of opera- tions in point 1.0 (Figure 3). In other words, all operated eyes should have a visual acuity that corresponds ex- actly to the corrected pre-op acuity to the fullest degree. However, it does not usually work this way for Figure 5 Source (all) : Alexander I. Myagkikh, Ph.D. many reasons—availability of errors in the determination of a visual acuity and calculations of operation, individual peculiarities of post-op eyesight recovery, and various me- Figure 4