Effective corneal refractive diameter as a function of the object tangent angle in visual space PDF Print E-mail

J Cataract Refract Surg. 2005 Dec;31(12):2356-62. 

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.

This seems like what everyone is saying about LASIK. But who is Sandra Brown, and what happened in the case?


D. Malice The negligent acts and/or omissions of defendants as set out above constitute an entire want of care so as to indicate that the acts and/or omissions in question were the result of conscious indifference to the rights, welfare or safety of the Plaintiff, or that they constitute malice, as that term is defined by law, so as to give rise to an award of exemplary damages. Plaintiff would show that the acts and/or omissions of Defendants which, when viewed objectively from the standpoint of Defendants at the time of the occurrence, involved an extreme degree of risk, considering the probability and magnitude of the potential harm to others; and of which Defendants had actual, subjective awareness of the risk involved but, nevertheless proceeded with conscious indifference to the rights, welfare and safety of others. Plaintiff would show that the acts and/or omissions of the Defendants as set out above, constitute malice as that term is defined by law, so as to give rise to an award of exemplary damages against Defendants Alcon.  

E. Aggravated Assault  Defendants, by and through their employees, agents, vice-principals, knowingly engaged in conduct which resulted in Plaintiff's injuries; placed Plaintiff at risk for serious injuries; and/or were reasonably certain that their conduct would cause serious injury to Plaintiff. Accordingly, Defendants knowingly committed acts of aggravated assault which caused serious bodily injury to Plaintiff in violation of Penal Code Section 22.01.  

Systematic underablation in LASIK: ablation pattern identified by advanced topographical analysis  

J Cataract Refract Surg. 2003 Aug;29(8):1621-5.  

Brown SM, Campbell CE.  Department of Ophthalmology and Visual Sciences, Texas Tech University Health Sciences Center, Lubbock, Texas, USA. sandra.brown@ttuhsc.edu  

Topographical analysis based on the differential geometry of surfaces-curvature topography-was developed and applied to a patient after laser in situ keratomileusis. The patient had a minimal residual refractive error and normal best corrected visual acuity but had multiple visual aberrations, including ghosting and glare, unless the pupils were maximally constricted. The corneal loci responsible for the aberrations were difficult or impossible to identify on axial topographies but were readily identified with curvature topography. The patient's ablations appeared to be miniature versions of the intended ablation profiles, with small areas of emmetropic central cornea surrounded by annuli of rapidly increasing keratometric power; that is, systematic underablation. This may explain why some patients have visual aberrations with pupil diameters smaller than the programmed optical zones.  

Dr. Sandra Brown is a Lubbock, Texas ophthalmologist and author of this and many other published articles and medical studies on the importance of pupil size in refractive surgery.  

You can find more of her work at www.pubmed.gov. (In the search engine type "Brown SM".)