Ophthalmology

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To the Editor:

I commend Munoz et al on their well-designed and well-conducted study on the clinical effect of a combination of multifocal intraocular lenses (MIOL) including the newest rotationally asymmetric MIOL, which appeared in the March 2012 issue of the Journal of Refractive Surgery.¹ They found that contrast sensitivity was comparable to that achievable with a monofocal implant and, remarkably, 92.5% of patients were spectacle-independent for all distances, with a mean postoperative spherical equivalent within 0.50 diopters (D) of emmetropia in 89% of cases. This is notably superior to the benchmark standards set only a few years ago for refractive outcomes after cataract surgery, suggesting that at least 55% of patients should be within 0.50 D of emmetropia.²

The authors should also be praised for providing details regarding the A-constant value and the formula used for IOL power selection.

Regarding the mildly myopic mean postoperative spherical equivalent obtained with both lenses, I question whether the authors chose the IOL power with the closest value to emmetropia and whether they believed that final mild myopia would be preferable. Also, given that they used the SRK-T formula in all cases, and no axial length range values were provided, would the authors recommend the employed formula even for eyes shorter than 22.0 mm, which is not recommended by the Royal College of Ophthalmologists guidelines in the United Kingdom.³

Understanding that the authors randomized the eyes receiving the first of the two MIOLs, perhaps collecting information regarding ocular dominance would have allowed a better interpretation of the obtained results. It seems that having had a rotationally asymmetric IOL in one eye helped overcome some of the drawbacks of the diffractive MIOLs, such as moderate halos and insufficient intermediate visual acuity. The sample size was relatively small (40 patients), and the randomization process may have resulted in dominant eyes being chosen to receive the rotationally asymmetric MIOL, which had been previously found to allow superior intermediate visual acuity and induce less photic phenomena.⁴

Gianluca Carifi, MD

London, United Kingdom

The author has no financial or proprietary interest in the materials presented herein.

We appreciate the comments of Dr Carifi regarding our article describing the clinical effect of the combination of refractive and diffractive multifocal intraocular lenses (MIOLs), including the new rotationally asymmetric Lentis Mplus MIOL (Oculentis GmbH, Berlin, Germany).¹

We agree with Dr Carifi that the SRK-T formula should not be used in short eyes. The power of the MIOL was selected using the SRK-T formula in 74 (92.5%) eyes with axial length longer than 22.0 mm; however, in 6 (7.5%) eyes with axial length shorter than 22.0 mm, the Hoffer-Q formula was used.^2,3 The Holladay II formula is another valid option for short eyes.⁴

We chose the MIOL power with the closest value to emmetropia. It is not necessary to aim for mild final myopia to improve intermediate vision when the proposed combination of MIOLs is used. The Acri.Lisa MIOL (Acri.Tec GmbH, Hemmingsdorf, Germany) has poor tolerance to defocus, which is exactly contrary in the case of the Lentis Mplus MIOL.⁵ This can be easily understood by observing the defocus curve of both MIOLs (Fig). The defocus curve of the Lentis Mplus MIOL shows a plateau shape between +1.00 and −1.00 diopters (D) of defocus (distance vision), with visual acuity around 0.8 decimal (20/25) or better, whereas the Acri.Lisa shows a sharp shape and poor tolerance to defocus with visual acuity around 0.5 decimal (20/40) at +1.00 and −1.00 D of defocus. This means that emmetropia is a requisite to achieve adequate uncorrected far vision when using a diffractive MIOL, whereas mild defocus can be easily tolerated with the Lentis Mplus model. In our opinion, this represents a clear advantage of the Lentis Mplus MIOL design, as it may reduce the need for secondary interventions to improve uncorrected vision.

We also agree with Dr Carifi that ocular dominance is an important issue. Patients without ocular dominance before cataract progression and with similar axial length in both eyes were included, as we wanted to compare the monocular performance of each MIOL in a contralateral-eye basis in addition to the primary aim of the study, which was the binocular performance of the combined system. However, the presence of ocular dominance does not contraindicate the combination of MIOLs with different designs or additions as proved by previous studies.⁶ It is noteworthy that the majority (82.5%) of patients in our study did not prefer the vision of one eye over the other when specifically asked about this item. Future studies should further address the issue of ocular dominance when combining refractive and diffractive MIOLs.

Gonzalo Muñoz, MD, PhD, FEBO

César Albarrán-Diego, MSc

Valencia, Spain

The author has no financial or proprietary interest in the materials presented herein.