The loss of accommodative amplitude is one of the changes that individuals must face as they mature. After the treatment of age-related tear-film abnormalities, correcting distance vision is the most important step toward rehabilitating accommodation in otherwise healthy presbyopic patients. Excimer laser technology has enhanced our ability to eliminate the common ametropic refractive states, and custom ablation applications, through advances in wavefront detection, soon will practically guarantee aberration-free visual results. Treatment of the presbyopic eye that leaves no spherical or chromatic aberrations, does not diminish contrast sensitivity, and restores accommodation will certainly be an enjoyable advancement for doctors and patients alike.

THEORIES OF PRESBYOPIA
Several possible explanations for the development of presbyopia have been postulated. Ronald Schachar, MD, of Dallas showed that the equatorial zonular fibers originate in the anterior aspect of the ciliary muscle. Presbyopia, he maintains, may be due to the continued growth of the lens and the consequential slackening of zonular fibers, which would prevent the ciliary muscle from increasing equatorial zonular tension and the central lens curvature during accommodation.

This theory of presbyopia differs from the classic description of the accommodation mechanism described by Hermann von Helmholtz, MD, of Berlin, Germany. Further, Dr. Schachar's theory was not supported by a monkey-model study that showed the lens equator to move away from the sclera and decrease lens diameter, as predicted by Dr. von Helmholtz and contrary to Dr. Schachar's prediction.1

According to the disaccommodation theory, continued lenticular growth throughout life and age-related changes in zonular insertions lead to an inability of the zonular fibers to hold the aging lens in the relatively flattened unaccommodative state.2 No evidence has yet supported this idea, however. Other researchers have proposed that a hardening of the lens could lead to presbyopia, because the lens capsule would be unable to exert sufficient force on the sclerotic lens to cause accommodation.

Some investigators have implicated age-related changes in capsular elasticity, combined with a thickening of the lens capsule, in the development of presbyopia. Contradictory results of various studies and an uncertainty regarding the capsule's role in accommodation make the nature of capsular involvement in presbyopia unclear, however.3-5

Other researchers have proposed that changes in the ciliary muscle may account for presbyopia, but a study in a rhesus monkey model found that structural and neuromuscular changes in the muscle's accommodative apparatus did not parallel the progressive decline in accommodative amplitude. The changes also seemed insufficient to explain presbyopia.6 Nevertheless, the extent of accommodative movement of the monkey's ciliary muscle does decrease with age.7,8 Similarly, a histological study of the human ciliary muscle identified several aspects that change with age: (1) The total area of the ciliary muscle decreases; (2) the muscle's length decreases almost by one-half in adults aged 30 to 85 years; (3) the areas of the longitudinal and reticular portions of the muscle decrease; (4) the area of the circular portion increases; (5) the connective tissue in the muscle's longitudinal portion increases; and (6) the distance from the muscle's inner apex to the scleral spur decreases.9

MANAGING PRESBYOPIA WITH IOL IMPLANTATION

Background and Methodology
Despite several trials to correct presbyopia with scleral implants, scleral incisions, and presbyopia laser ablation, many consider the implantation of an IOL that could treat presbyopia successfully to be the most physiologic solution. Among the different methods posited as a model for presbyopia management, the best future option would likely be an IOL that could be implanted through a small incision during cataract surgery, especially considering that cataract surgery is practiced worldwide.

We selected two models of IOLs currently being implanted in an attempt to determine which would best treat presbyopia. We included 27 patients (54 eyes) in a study designed to compare the results of implanting the Array multifocal lens (AMO, Irvine, CA) versus the AT-45 CrystaLens (C&C Vision, Aliso Viejo, CA). All subjects underwent bilateral cataract extraction with phacoemulsification. Sixteen patients received the multifocal IOL, and 11 patients received the accommodative IOL. During a mean follow-up period of 1 year, none of the subjects underwent another procedure to improve the visual outcome.

Objectives
The study's objectives were to examine two alternatives for managing presbyopia and to evaluate the optical performance of the accommodative lens. Our primary aim was to follow patients implanted with both lens models and evaluate their distance and near visual acuity. We analyzed subjects' UCVA and BCVA at distance and near, their manifest refraction for distance, the amount of plus lens power needed at near, and their near visual acuity while using the distance correction.

Results
At the end of the follow-up period, both subject groups had improved UCVA at distance (due to cataract extraction) and at near. As regards distance vision, most of the study's subjects had UCVA of greater than 20/25. The improvement of near visual acuity was more pronounced in the patients implanted with multifocal IOLs. We found no significant difference in BCVA at near between the groups. All patients had a mean BCNVA of 20/25 or more with the addition of a mean plus lens of + 1.1 ± 0.4 D.

One aim of presbyopic surgery is to enable patients to read without glasses. Interestingly, we observed that patients who were implanted with multifocal IOLs had better near vision while using their distance correction than those implanted with accommodative IOLs.

One important drawback of implanting the multifocal IOL, however, was a postoperative decrease in contrast sensitivity. In our study, although all of the patients lost a degree of contrast sensitivity following surgery, this decrease did not persist for more than 6 months and did not affect subjects' visual function. This temporary disadvantage did not occur in patients implanted with the accommodative IOL.

Conclusions
In addition to restoring distance vision in cataract patients, implanting a multifocal or accommodative IOL may constitute a solution for the problem of presbyopia. The hinged plate-haptic AT-45 IOL is designed to allow the lens optic to move forward significantly in the eye in order to provide accommodation upon contraction of the ciliary muscle. Its contraction is considered essential to lens function and controls the amount of accommodation recovered. The Array lens is a zonal progressive IOL with five concentric zones on its anterior surface. It has a 6.0-mm foldable optic and PMMA haptics, which are 13 mm in diameter. We find that the possibility of implanting this lens through a clear corneal incision via the same technique used for implanting a monofocal foldable IOL makes using the Array more practical than using the AT-45 lens.

Our 6-month study determined that both lenses were able to restore subjects' visual acuity, although a few months passed before the multifocal group regained normal contrast-sensitivity values. We found that implanting multifocal IOLs restored better near vision to presbyopic patients by simulating accommodation—more than that restored by the accommodative lenses. The advantages of performing astigmatically neutral incisions is an additional benefit of multifocal versus accommodative lenses; the multifocal group experienced less induced astigmatism because of the smaller size of the incision (2.8 mm vs 3.5 mm with the Array and AT-45 IOLs, respectively).

Appropriate patient selection, accurate biometry, and recent surgical techniques are the keys to successfully treating presbyopia with IOLs. In the near future, new modalities of accommodative IOLs that can be implanted or injected through microincisions may treat presbyopia more successfully than current models.

Jorge L. Alió, MD, PhD, is Head of the Department of Refractive Surgery, Instituto Oftalmológico de Alicante, and Professor and Chairman of the Ophthalmology Department, Miguel Hernández University Medical School, Alicante, Spain. Dr. Alió holds no financial interest in the products mentioned herein. He may be reached at +34 96 515 00 25; jlalio@oftalio.com.
Ahmed Galal, MD, MSc, is a clinical fellow at the Department of Refractive Surgery, Instituto Oftalmológico de Alicante, and Miguel Hernández University Medical School in Alicante, Spain. Dr. Galal holds no financial interest in the products mentioned herein. He may also be reached at +34 96 515 00 25; jlalio@oftalio.com.
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