[Does the orientation of the IOL in the eye play a role? Imaging quality of foldable intraocular lenses measured utilizing point spread function (PSF-measurement) and scanning electron microscopy cross-sectional images]

Abstract

Background: Foldable intraocular lenses (IOLs) are established in ophthalmic surgery and increasingly more complex geometries are available. This study investigates different designs of the optics of the IOL as well as the imaging quality of these optics utilizing electron microscopic photographs and mathematical approximations.

Method: Included in this study were two different lens powers of common IOLs (+ 13 D and + 26 D). All studied IOLs were processed depending upon their water content: Aqueous IOLs with up to 25 % water content were fixed with 2.5 % glutaraldehyde in 0.1 M phosphate buffer (mPP), and then contrasted with 2 % OsO (4) and processed in an alcohol series of 30 - 100 % propanol. Subsequently, the IOLs were transferred into EPON (epoxy resin) and polymerization took place at 60 degrees C. After hardening, the samples were cut out with a leaves saw and divided into halves. Afterwards these were polished with glass and diamond knifes at the ultramicrotom (Ultracut, Rückert-Jung). After coal vaporization measurement took place in the scanning electron microscope (DSM 962, Zeiss). In the case of dry stored IOLs, the samples were sputtered with Au-Pd 20 nm and afterwards measurement was performed as described above. The measurement of the optical imaging quality was performed with correctly and inversely oriented IOLs according to the standard EN ISO 11979 -- 2 by means of a measuring system which works on the basis of the point spread function (PSF) and/or the modulation transfer function (MTF). From this the Strehl ratio was computed in each case.

Results: Utilizing a correlation of the electron microscopic investigation of the optics of different IOLs with their imaging quality (Strehl ratio), we were able to show that the optical design of the IOL has substantial influence on the imaging quality of an intraocular lens. In particular, with non-symmetrical designs of the optics (e. g., AR40 e; AMO) the orientation of an IOL in the eye is crucial; i. e., the Strehl ratio increased thereby from 0.68 to 0.87.

Conclusions: The design of asymmetrical optics is crucial and much attention should be paid to the fact that the imaging quality can substantially be affected. A correct orientation of the lens within the eye is very important with these asymmetrical optics.