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M.K. Smolek, S.D. Klyce, E.J. Sarver, C. Tan; Corneal Ectasia Screening by Functional Optical Zone Area Analysis . Invest. Ophthalmol. Vis. Sci. 2006;47(13):568.
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Ectasias including keratoconus (KC), pellucid marginal degeneration (PMD) and ectasias of unknown origin are a controversial topic in refractive surgery. All LASIK and surface ablation keratoplasty candidates should be screened for ectasia–like or suspect topography in order to rule out latent or form–fruste disease. Clinicians and automated software algorithms have the greatest difficulty in distinguishing the suspect forms of KC and PMD because the transition from normal to abnormal topography is fuzzy and a precise cutoff is difficult to define. Functional Optical Zone (FOZ) area analysis may improve screening by establishing a more precise cutoff between normal and abnormal corneas.
TMS axial diopter topography maps (Tomey, Inc; n = 388) from the LSU Eye Center Clinic were catagorized by clinical experts into the following groups: normal (29), astigmatism (28), keratoconus suspect (19), keratoconus [mild (22), moderate (28), and severe (26)], pellucid marginal degeneration (28), myopic refractive surgery [LASIK (42), PRK (41), & RK (34)], contact lens–induced warpage (67), and penetrating keratoplasty (25). VolPro 6.95 (Sarver & Associates) extracted the Functional Optical Zone (FOZ) area in mm2 for zone diameters from 2 to 9 mm at 1 mm steps. The catagorical mean and standard deviation at each zone diameter was plotted and values were statistically compared with SigmaStat (SPSS, Inc).
Mean FOZ area was significantly larger for normal spherical corneas compared to KCS at all zone diameters (graph). However, cornea conditions such as clinically significant corneal astigmatism could not be distinguished from KCS by FOZ area alone.
FOZ area appears to be a robust indicator of normal corneas when compared to mild and suspect forms of ectasia, however, it cannot be used as a normality indicator if clinically significant amounts of astigmatism are present because astigmatism reduces the FOZ area. FOZ may be most useful when combined in an algorithm with additional corneal indices or aberration terms that are sensitive to corneal asymmetry and irregularity.
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