The multifocal intraocular lens (MIOL) offers the pseudophakic patient the possibility of far, intermediate, and near vision without the use of spectacles,1–7 achieving emmetropia, the functional goal of cataract surgery. However, even a small amount of astigmatism can lead to substantial impairment of multifocality.7–9 To understand how astigmatism and its correction can interfere with the multifocal effect of an MIOL, we attempted to reproduce the behavior of 3 MIOLs in the presence of 1.0 diopter (D) of induced astigmatism.
Materials and Methods
The laser optical bench used in this study has been described.10 The laser device measures the shape and light intensity of focal spots. Three MIOLs were studied: 1 Pharmacia 811E, a diffractive IOL; 1 AMO Array MPC25NB, a far-dominant, 5-zone refractive IOL with a spherical convex surface; 1 Domilens Progress 1, a near-dominant progressive refractive IOL. One monofocal IOL, the Pharmacia 722A, was used as a control.
Astigmatism was induced by interposing a +1.0 D cylinder lens between the IOL and the television camera. Astigmatism was corrected by adding a −1.0 D cylinder lens in front of the IOL on the same axis. The number of focal spots, their intensity, and the effect of induced astigmatism and its correction on the shape and brightness of the spots were assessed for each lens.
The monofocal IOL (control) produced a focal spot of high brightness (Figure 1, A). Inducing astigmatism modified the spot into 2 focal lines divided by the metric distance of 1.0 D in the optical bench (Figure 1, B). Correcting the astigmatism restored 1 focal spot with the same shape as the original one but with a decrease in light intensity of approximately 20% (Figure 1, C).
The Pharmacia 811E diffractive bifocal IOL produced 2 focal spots corresponding to the far and near foci; 44.8% of the light was concentrated at the far focus and 37.0% at the near focus (Figure 2, A); 18.2% was scattered by the secondary orders of diffraction. Inducing astigmatism created 2 pairs of focal lines, corresponding to the far and near foci (Figure 2, B). Correcting the astigmatism restored the original 2 focal spots, with a decrease in light intensity of 20% (Figure 2, C).
The AMO Array produced 6 main focal spots. Approximately 50% of the light energy was concentrated at the far foci of 20.0 and 20.5 D (19.7% and 29.6%, respectively), and 20.9% at the near focus of 23.5 D (Figure 3, A). Inducing astigmatism created several pairs of focal lines, with a significant dispersion of light and interference among the various focal lines (Figure 3, B). Correcting the astigmatism restored the original series of focal spots, with a decrease in light intensity of 20% (Figure 3, C).
Owing to the progressive aspheric surface, the Domilens Progress 1, a near-dominant refractive MIOL, did not produce well-defined focal spots, creating instead a progressive change in light intensity from the far to the near foci (Figure 4, A). It was therefore impossible to identify the various pairs of anterior and posterior focal lines surrounding Sturm's conoid (Figure 4, B). Correcting the astigmatism led to a decrease in light intensity of about 20% of the progressive shift from the far to the near focal spots (Figure 4, C).
The study was designed to assess light distribution through MIOLs by looking at focal spot light intensity. The effect of postoperative astigmatism and its correction was reproduced using a laser optical bench.
Some studies have proposed inducing slight simple myopic astigmatism to ensure a higher depth of field in patients implanted with a monofocal IOL.11 However, with MIOLs, astigmatism is not only useless but may even compromise the correct functioning of these lenses.
Multifocal IOLs have demonstrated a notable division of light energy among secondary foci; this characteristic is more evident in lenses with aspheric optics, such as the Domilens Progress 1 MIOL. It is possible that cortical amplification of image intensity exists and is partly able to compensate for the lower brightness of images focused on the retinal plane.12 Nevertheless, in a previous study,13 we demonstrated that the spatial resolution threshold in near vision—obtained with high-pass resolution perimetry—was similar in 2 groups of pseudophakic patients with conventional and progressive multifocal IOLs. Other studies14–21 have highlighted the decreased contrast sensitivity in patients with MIOLs compared with patients with monofocal IOLs and worse near vision in patients with progressive MIOLs than in patients with other MIOLs.22 These results can be explained by the excessive division of light energy produced by MIOLs, especially those with aspherical optics. This suggests that there is a threshold below which the cortical amplification of image intensity cannot occur.
Our study demonstrated that the presence of astigmatism after cataract surgery can be a critical factor in the functioning of MIOLs. This is due to the drop in light energy through the expanded conoids of Sturm, related to the multiple foci of the MIOLs, and the possible interference between the posterior focal line of the nearest focus and the anterior focal line of the next focus. The optical correction of the astigmatic ametropia is able to restore focal spots characterized by lower brightness and thus produce a reduced quality of vision. The reduction in light intensity is due to light reflection on the surfaces of the lenses interposed between the IOL and the television camera to induce astigmatism and correct it.
In conclusion, it appears that astigmatically neutral surgery or surgical correction of pre-existing astigmatism is essential with MIOL implantation to minimize the decrease in contrast sensitivity.
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