Refractive change induced by the LASIK flap in a biomechanical finite element model.

PURPOSE: To study the effect of varying four parameters on the refractive change induced by the LASIK flap. METHODS: Using a variety of patient-specific data such as topography, pachymetry, and axial length, a finite element model is built. The model is used in a non-linear finite element analysis to determine the response and change in optical power of the cornea as a function of a material property of the cornea (corneal elasticity), flap diameter and thickness, and intraocular pressure. RESULTS: The central flattening or hyperopic shift occurred atop the flap in all four of the simulated eyes tested with the creation of the LASIK flap. Of the four parameters tested, modulus of elasticity (Young's modulus) had the most profound effect on the results of hyperopic shift, varying from <0.5 diopters (D) in the least elastic (stiffest) cornea to >2.0 D of hyperopic shift in the most elastic cornea. The depth of the lenticular cut was the second-most significant parameter tested varying from 0.24 D at 100 microns to 1.25 D at 275 microns of depth. Varying intraocular pressure demonstrated less difference, and varying corneal flap diameter demonstrated the least difference in induced refractive change on the model. The hyperopic shift was noted to be greater in hyperopic than in myopic eyes (simulated) tested. CONCLUSIONS: Three-dimensional finite element analysis modeling of actual patient data could lead to a better understanding of the biomechanical response of corneal tissue to the lenticular flap creation and potentially for ablation patterns produced by the excimer laser. Understanding these biomechanical responses may lead to greater predictability and improvement of visual outcomes.

The FUBI system for solar rating nonprescription eyewear.

BACKGROUND: In an effort to develop a comprehensive method of rating nonprescription eyewear for its ability to protect the eye against solar damage, the FUBI (Fashion, Ultraviolet, Blue, and Infrared) System is presented. The system presents a numeric value, from Oto 100, for each of the three known harmful portions of the solar spectrum: ultraviolet UV), blue/violet (B), and infrared (IR). A fourth value was determined for the fashion (F) of the eyewear as it relates to protection of the eye against reflected or scattered radiation that is not transmitted through the eyewear. METHODS: The numeric value of the system for UV, B, and IR was derived by taking the average transmittance of radiation through each tested lens and weighting it by multiplying that value by a relative toxicity factor (RTF) for each waveband of solar radiation tested. The RTF was derived by multiplying the approximate level of radiation reaching a specified anatomic part of the eye at sea level for each wavelength tested (Elambda) by the inverse of the value of its action spectrum (sensitivity, Slambda) on that part of the eye. This weighted average transmitted percentage of radiation was then deducted from 100 to derive the FUBI value for the UV, B, and IR range. The numeric value for F was derived by measuring the scattered or reflected light from five known sources of luminance at a fixed distance around opacified lenses on each tested frame. RESULTS: The FUBI values for six known commercial products of nonprescription eyewear are presented for comparison. CONCLUSIONS: The FUBI system presents a comprehensive, scientifically valid means of rating nonprescription eyewear for solar protective value. The system will provide consumers of nonprescription eyewear with more useful, comparative information about the protective qualities of eyewear against solar radiation.