Ophthalmology Research Today is a free monthly online journal that collates and summarizes the latest research about Ophthalmology, including details on eye surgery, myopia, cataracts. | ||||||||
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Effective corneal refractive diameter as a function of the object tangent angle in visual space.Brown SM, Freedman KA Cabarrus Eye Center, Concord, North Carolina 28025, USA. sbrownmd@carolina.rr.com PURPOSE: To determine whether the currently accepted method of selecting a minimum optical zone diameter for laser refractive surgery that is equal to or slightly greater than the dark-adapted pupil diameter provides a sufficient diameter of corneal surface to focus light arising from objects in the paracentral and peripheral visual field. SETTING: Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA. METHODS: An optical model of the anterior segment was developed to calculate the effective corneal refractive diameter (ECRD), which is the diameter of the area of cornea that refracts all incident light rays arising from an object through the physical pupil (PP). This model incorporates the patient variables of central anterior chamber depth (ACD), central corneal curvature (K(c)), and the diameter of the apparent entrance pupil (EP). The model was expanded to incorporate distant objects off the line of sight (LOS), described by their angular displacement from the fixation object in visual space (the object tangent angle delta(ob)). Results were calculated for the 360 meridian degree visual field (ie, for all objects in visual space perceptually displaced from the fixation object by angle delta(ob)). The effect of the prolate nature of the cornea was also investigated. RESULTS: The ECRD expanded rapidly as a function of PP and delta(ob) but was minimally influenced by K(c). Beyond a critical object tangent angle delta(c), light rays striking the corneal vertex were not refracted through the PP, and the ECRD became an annular surface centered on the corneal vertex. The delta(c) was not a function of K, but increased as the PP increased and decreased as the ACD increased. The prolate nature of the cornea had little influence on the ECRD, even for very peripheral light rays. CONCLUSIONS: The ECRD expands rapidly when considering distant objects only slightly displaced from the LOS. A patient treated with an optical zone equal to or slightly greater than the dark-adapted pupil diameter may experience vision quality loss for paracentral and midperipheral objects even under conditions of ambient indoor lighting. Published 13 February 2006 in J Cataract Refract Surg, 31(12): 2356-62.
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