A diffractive trifocal IOL (FineVision, PhysIOL) provides a peak modulation transfer function for intermediate vision.
"By combining the two diffractive profiles, and manipulating the height of the steps that controls the distribution of energy between the different orders, the profile of a diffractive trifocal IOL (FineVision, PhysIOL, Liège, Belgium) can be achieved," said Dr Damien Gatinel, PhD.
The trifocal IOL provides three focal points - facilitating intermediate vision - in contrast to bifocal IOLs that provide near and distance vision.
Very near foci capture about 4% of the light and may not supply the retina with sufficient light for quality images, he explained.
The design of these lenses is also based on harmonic progression that is used to combine two profiles for designing the trifocal IOL.
"Looking at these characteristics explains why patients might report halos and why patients need bright illumination in order to read," said Dr Gatinel, of the Foundation Rothschild, Paris, France.
An experimental comparison of four multifocal IOLs on the optical bench indicated that apodized lenses transfers light energy to facilitate near and distance vision with increasing pupil size, whereas non-apodized IOLs do not.
Dr Gatinel and colleagues compared four diffractive IOLs to evaluate the optic quality of the IOLs:
All four IOLs have the same power (22 D). The FineVision and the AcrySof IOLs are apodized.
The investigators used a certain instrument (PMTF, Lambda-X, Nivelles, Belgium) to evaluate the lenses. The aperture over which the IOL optic is analysed can be varied without moving the lenses. Analysis of diffractive IOLs is performed using through-focus modulation transfer function (MTF) plots for the resolution of 50 cycles per mm.
In addition, images of the 1951 USAF target through the artificial eye were taken at the different positions of a CCD camera, corresponding to the position where the MTF was maximal, Dr Gatinel explained.
With a 3 mm pupil - using the through-focus MTF - all four of the IOLs favoured distance vision over near vision.
With a 3.75 mm pupil, the height of the MTF peak for distance vision increased at the expense of the near vision. With dilated pupils (4.5 mm), apodized lenses had the most dramatic increases in the peak height for distance vision. At these diameters, the trifocal FineVision IOL had a peak for intermediate vision, whereas the other IOLs did not.
In a discussion of the 1951 USAF target images, Dr Gatinel explained that for distance vision with increasing pupil size (4.5 mm) the image contrast was higher in apodized models because of the less defocused light of the near foci - compared with what was observed with a small pupil (3 mm).
The near vision images were similar among the four IOLs with a small pupil. With a larger pupil, the image was more constant with the Tecnis IOL and the Acri.Lisa IOL - which are not apodized lenses - compared with the other two IOLs.
For intermediate vision with the 3- and 4.5-mm pupils only the FineVision IOL provided an identifiable image.
Summary of findings
This experiment showed that apodization allows a transfer of energy from near to distance with an increase in the size of the aperture, according to Dr Gatinel.
Also, the images with the non-apodized IOLs retained a relatively constant energy ratio for the different foci.
The contrast modulation is more affected than image resolution.
"The FineVision IOL seems to provide three effective focal points compared with the other bifocal IOLs," he concluded.