Abstract
Purpose: :
To compare the results of intraocular lens (IOL) calculations for cataract surgery using a formula based in eye ray tracing and the SRK-T formula. Both results are compared to see which spherical equivalent (SE) approaches the most to the desired refraction (in this case emmetropia).Using this formula based in eye ray tracing we calculate IOL by direct measurments, instead of linear regressions like the SRK-T formula, having another option for IOL calculation.
Methods: :
We made a retrospective, single surgeon post surgical analysis on 53 patients (176 eyes, all of them between 21.5 mm. and 28 mm. of axial length), they were analyzed using different parameters: mean anterior corneal radius, asphericity, mean posterior corneal radius, corneal thickness (all of them obtained by Scheimpflug image analysis); axial length, anterior chamber depth, lens thickness (obtained by A-scan immersion technique); and other data as actual SE, post operatory desired SE (obtained subjectively and objectively).After that we calculated an IOL by SRK-T formula and we calculated the IOL with the formula based in eye ray tracing. It consists in an objective measurement method that determines intraocular spacing by means of corrected Scheimpflug images, giving relationships for key anterior segment parameters incorporating aspheric surfaces. These parameters are analyzed, using an established formula and based on the principle of a meridional ray impinging on a surface that follows a straight line using the radial height at the origin, angle in radians between the ray, optical axis and the distance, with a technique developed for use with a common spreadsheet computer program.Then it was compared the post surgical SE using SRK-T recommended IOL, and the theoretical post surgical SE using the IOL formula based on eye ray tracing, to see which one approaches the most to the desired SE.
Results: :
The IOL calculations, on eyes between 21.5 mm. and 28 mm. mean 23.76 mm. (SD +/- 1.53 mm.), based on SRK-T showed a mean SE 0.05 (SD +/- 1.48) meanwhile the ones based on the eye ray tracing showed a mean SE 0.12 (SD +/- 0.92), then we compared both results to see which one approaches the most to emmetropia and there was no statistically significant difference (P 0.7600) between the group calculated by SRK-T and the group calculated by the eye ray tracing based formula, for a desired SE.
Conclusions: :
The formula based on eye ray tracing showed to be as reliable as the conventionally used SRK-T for IOL calculation, in eyes whose axial length was between 21.5 mm. and 28 mm. adding a new tool when it comes to IOL calculation for cataract surgery patients.
Keywords: intraocular lens