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Optical performance of monofocal versus multifocal intraocular lenses

2008; Lippincott Williams & Wilkins; Volume: 34; Issue: 11 Linguagem: Inglês

10.1016/j.jcrs.2008.06.043

1873-4502

Damien Gatinel,

Intraocular Surgery and Lenses

In their recent paper, Ortiz et al.1 studied the optical performance of monofocal and multifocal intraocular lenses (IOLs) in the human eye. Their main conclusion was that of the IOLs studied, the hybrid refractive–diffractive IOL was the least affected by pupil diameter in terms of intraocular aberrations. This IOL type also showed a smaller increase in optical aberrations during pupil dilation. I question the validity of the authors' comparison of monofocal and multifocal IOLs because they failed to verify 2 conditions. 1. The authors used a reconstructed wavefront to compare the optical quality of the pseudophakic eyes analyzed in the study. Therefore, these metrics relied on the underlying assumption that the wavefront was accurately reconstructed by the Hartmann-Shack system or at least that the fidelity of the wavefront reconstruction was equal for all tested IOLs. Classic data analysis from a Hartmann-Shack wavefront sensor does not consider the quality of individual spots formed by the lenslet array. Only the displacement of spots is needed for computing the local slope of the wavefront over each lenslet aperture. The underlying assumption is that the wavefront is locally flat over the face of the lenslet, which is not true for diffractive IOLs. Diffractive IOLs separate the incident light into 2 converging waves: distance wavefront and near wavefront. The local phase of the wavefront collected by the Hartmann-Shack lenslet array shows rapid phase shifts, and the wavefront is significantly distorted over the area of the lenslet on a very fine spatial scale. These microfluctuations scatter light, resulting in blurred or double spots. Light centration may be difficult or arbitrary to localize, even for an aberrometer with a high spatial resolution such as the Complete Ophthalmic Analysis System (Wavefront Science, Inc.). These limitations, which have been reported,2–4 cause the actual diffracted wavefront to be undersampled and inaccurately sampled. Therefore, the final reconstructed wavefront does not capture every characteristic of the actual wavefront. Finally, the Fourier-calculated metrics may largely overestimate the optical quality of diffractive IOLs. As the nondiffractive part of the spherical AcrySof ReSTOR IOL (Alcon Laboratories) has the same geometric (ie, surface curvature) and material characteristics as the monofocal AcrySof MA60, no difference in the distance spherical aberration wavefront should be measured. Hence, the ReSTOR's reduced positive spherical aberration, as measured in this study, may have been caused by an artifact that resulted from inadequate centroid detection. It seems hazardous for the authors to derive any clinically relevant conclusion regarding the value of spherical aberration for bifocal diffractive IOLs at various pupil diameters. The retinal image formed by the IOL would be impaired by some defocused light diverging from the near foci and forming concentric halos around the center of the retinal point spread function (PSF). Additionally, 20% of the light is diffracted in higher diffraction orders and the effect of this is not measured by the Hartmann-Shack wavefront-sensing instruments. 2. The objective optical quality provided by a diffractive IOL depends not only on the phase of the wavefront, but also on the light intensity (ie, energy) at the focal plane. The incident light received by a multifocal IOL is divided between the different focal lengths. Therefore, only a fraction of the incident light is directed toward the distance foci. Even in the hypothetical situation of a refractive or diffractive multifocal IOL achieving a nonaberrated image at the distance focal plane, the light transmission efficiency would be inferior to that of a nonaberrated monofocal IOL. This is important because human vision does not work well in low light. In the case of a hybrid refractive–diffractive IOL, the amount of light diffracted toward the distance foci would increase with the pupil diameter; however, it would remain less than that refracted by a monofocal IOL for a large pupil diameter. Unfortunately, this point, although crucial in the frame of objective in vivo optical quality evaluation, was omitted by Ortiz et al.1 and other authors also.5,6 Multifocal pseudophakic IOLs represent a growing and competitive market, and it is urgent to develop accurate and objective standards for measuring and reporting the optical quality of eyes implanted with diffractive optics. Although I believe that some of the conclusions drawn by Ortiz et al. are not valid, the general scope of the paper is extremely valuable. Currently, we believe there are no commercially available methods to accurately reconstruct the ocular wavefront after diffractive IOL implantation because of the rapid phase variations caused by bifocal diffractive optics. It must also be emphasized that if the modulation transfer function (MTF) is a powerful technique for expressing the effects of optical systems in Fourier series from the phase wavefront reconstruction, it may not work well (or need numerous terms) when the details are similar to sharp, discontinuous phase variations, such as those caused by diffractive optics. Double-pass aberrometry is based on the actual (not calculated) PSF measurement. It may enable clinicians to capture the direct effect of the light impinging on the patient's retina and may be more adapted to provide metrics such as the MTF or Strehl ratio for distance correction.