Current pseudoaccommodating IOLs may provide good distance and near vision after cataract¹ and clear lens surgery.² Differences in visual outcomes achieved with these versus other IOL options come from its optical principle (ie, shape of its surfaces) and the role of pupil size.³

Traditionally, refractive and diffractive principles have been used to create multifocality from near to distance. Montés-Micó et al⁴ showed that the light distribution between distance and near foci plays an important role in the final retinal image. Out-of-focus images created by multifocality are responsible for some reduction in contrast sensitivity.

To overcome these limitations, Jacobi et al⁵ developed a new concept termed asymmetrical bilateral multifocal IOL implantation, whereby two binocularly implanted bifocal diffractive IOLs with different light distribution between the foci work together to enhance visual performance of the IOL. A distant-dominant multifocal IOL (light distribution, 70% for far and 30% for near focus) (Acri.Twin 737D; Acri.Tec GmbH, Berlin, Germany) is implanted in one eye, thus rendering it dominant for distance vision. Alternately, a near-dominant multifocal IOL (light distribution, 30% for far and 70% for near focus) (Acri.Twin 733D; Acri.Tec GmbH) is implanted into the fellow eye, rendering it dominant for near vision. Both Acri.Twin models (Figure 1) are bifocal diffractive IOLs with different light distributions for distance and near focus.

ASYMMETRIC IMPLANTATION
We recently evaluated 343 consecutive patients who underwent bilateral implantation of the distance-weighted 737D and near-weighted 733D.⁶ Monocular and binocular distance BCVA and near BCVA and distance contrast sensitivity under photopic and mesopic conditions were determined. Binocularly, the Acri.Twin system allowed good distance and near vision (0.031 ±0.059 and 0.005 ±0.024 logMAR, respectively). Contrast sensitivity with the Acri.Twin system was within normal limits under photopic and mesopic conditions. Considering our results, and based on the asymmetrical bilateral light distribution, the Acri.Twin IOL system provides pseudoaccommodation and excellent results in cataract and clear lens patients.

When performing binocular implantation of these IOLs, keep in mind the different light distributions between the distance- and the near-weighted IOL models. Two drawbacks of asymmetrical bilateral implantation should be considered. First, locating the dominant eye in cataract patients is problematic, because a stronger cataract in the dominant eye could lead to a shift in dominance. Second, the possible binocular intolerance to different light distribution between eyes could lead to an explant of the near-weighted IOL. We have performed 15 unilateral explantations of the near-weighted 733D IOL and exchanged it for the distance-weighted 737D IOL. This created a symmetrical implantation. No ocular dominance is required for bilateral implantation of the distance-dominant IOL, and the light distribution between eyes is symmetric, avoiding binocular intolerance.

SYMMETRIC IMPLANTATION
We have also evaluated 50 patients who underwent bilateral implantation of the distance-dominant diffractive bifocal 447D IOL (Acri.Tec GmbH) (Figure 2).⁷ Again, monocular and binocular distance BCVA and near BCVA as well as distance contrast sensitivity under photopic and mesopic conditions were determined. Six months postoperatively, binocular means of distance BCVA and near BCVA were 0.02 ±0.04 logMAR and 0.04 ±0.03 logMAR, respectively. Contrast sensitivity was within normal limits under both conditions. Therefore, bilateral implantation of the distance-dominant bifocal 447D IOL is an effective alternative for asymmetrical bilateral bifocal IOL implantation, solving the ocular dominance choice and binocular tolerance problem.

ACRI.LISA IOL IMPLANTATION
A new optical concept, refractive/diffractive optics, has been applied to IOL designs. The Acri.LISA 366D IOL (Acri.Tec GmbH) (Figure 3) uses this hybrid concept to reduce the side effects associated with conventional refractive and diffractive multifocal IOLs. This IOL is based on symmetrical implantation with a 65/35 ratio of light percentage between distance and near foci. It was developed to improve previous outcomes in presbyopia correction.

We bilaterally implanted the Acri.LISA in 81 patients.⁸ After 3 months, binocular means of distance and near BCVA were 0.048 ±0.111 logMAR and 0.012 ±0.0084 logMAR, respectively. Contrast sensitivity was within normal limits under photopic and mesopic conditions. Binocular contrast sensitivity was better than monocular contrast sensitivity at all spatial frequencies under photopic and mesopic illumination levels. Therefore, we found the Acri.LISA 366D IOL to provide a satisfactory full range of vision. This IOL shows a high level of uncorrected and corrected distance and near vision as well as improved contrast sensitivity under photopic and mesopic conditions. Future studies should include longer follow-up and comparison with other IOLs based on the same diffractive/refractive concept.

CONCLUSION
When implanting these IOLs, consider the optical design of each IOL and the patient characteristics. For a symmetrical implant, the Acri.LISA IOL should be used in patients with cataracts, high ammetropia, and presbyopia. Our outcomes showed that patients achieved good distance, near, and intermediate visual acuity.

The 447D IOL is also indicated for the same patient population. Similar outcomes for distance and intermediate vision are achieved, but patients may have to use near vision spectacles in some situations. The 737D IOL should be considered for use in high hyperopic eyes or as a piggyback implant onto a monofocal IOL.⁹ The asymmetric implant should be considered using the 447D and 443D IOLs (Figure 2) in patients with high myopia (and anisommetropia) and dominance.

José F. Alfonso, MD, PhD, is Head of the Refractive Surgery Department at the Instituto Oftalmológico Fernández-Vega, in Oviedo, Spain. Dr. Alfonso states that he has no financial interest in the products or companies mentioned. He may be reached at j.alfonso@fernandez-vega.com.

1. Alfonso JF, Fernández-Vega L, Baamonde B, Montés-Micó R. Prospective visual evaluation of apodized diffractive intraocular lenses. J Cataract Refract Surg. (In press).
2. Fernández-Vega L, Alfonso JF, Rodríguez PP, Montés-Micó R. Clear lens extraction with multifocal apodized diffractive intraocular lens implantation. Ophthalmology. (In press).
3. Alfonso JF, Fernández-Vega L, Baamonde B, Montés-Micó R. Correlation of pupil size with visual acuity and contrast sensitivity after implantation of an apodized diffractive intraocular lens. J Cataract Refract Surg. 2007;33:430-438.
4. Montés-Micó R, EspaÒa E, Bueno I, et al. Visual performance with multifocal intraocular lenses: mesopic contrast sensitivity under distance and near conditions. Ophthalmology. 2004;111:85-96.
5. Jacobi FK, Kammann J, Jacobi WK, et al. Bilateral implantation of asymmetrical diffractive multifocal intraocular lenses. Arch Ophthalmol. 1999;117:17-23.
6. Alfonso FJ, Fernández-Vega L, SeÒaris A, Montés-Micó R. Quality of vision with the Acri.Twin asymmetric diffractive bifocal intraocular lens system. J Cataract Refract Surg. 2007;33:197-202.
7. Fernández-Vega L, Alfonso JA, Baamonde B, Montés-Micó R. Symmetric bilateral implantation of a distance dominance diffractive bifocal intraocular lens. Unpublished data.
8. Alfonso JF, Fernández-Vega L, SeÒaris A, Montés-Micó. Prospective study of the bifocal Acri.LISA intraocular lens. Unpublished data.
9. Alfonso FJ, Fernández-Vega L, Baamonde MB. Secondary diffractive bifocal piggyback intraocular lens implantation. J Cataract Refract Surg. 2006;32:1938-1943.