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Artificial lens helps to see clearly

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Innovative design improves ability to focus on near objects

Gary Boas

As the population ages, researchers are looking for new ways to restore vision lost to cataracts or to other medical conditions. One technique involves replacing the lens of the affected eye with an artificial intraocular lens.

Millions of intraocular lenses are implanted during cataract surgery worldwide every year. Although these procedures successfully restore vision, the eye’s ability to focus on near objects — known as accommodation — suffers significantly.

A recently developed artificial intraocular lens could help improve the eye’s ability to focus on near objects following implantation during cataract surgery or other medical purposes, an ability that often suffers significantly with such implants. The lens achieves this by using shifting optical elements that take advantage of the space perpendicular to the optical axis. In contrast, most artificial lenses move along the optical axis.

Investigators have described a number of designs for accommodative intraocular lenses; however, some require large surgical incisions, increasing the risk of optical distortions and postoperational complications. Other designs fail to offer high enough contrast, lead to reduced peripheral vision or are highly sensitive to misalignments.

In the Aug. 21 issue of Optics Express, Michiel C. Rombach of AkkoLens International BV in Breda, and Aleksey N. Simonov and Gleb Vdovin of Flexible Optical BV in Delft, both in the Netherlands, reported a two-element accommodative intraocular lens.

Based on a varifocal cubic Alvarez lens, the researchers’ design consists of a planoconvex anterior element and a posterior optical element. This arrangement offers sufficient contrast over the entire range of accommodation of 4 diopters and, thus, could contribute to improved accommodative vision in patients with intraocular lens implants.

The two-element lens consists of a planoconvex anterior element and a posterior optical element.

In eyes with a natural crystalline lens, accommodation is driven by the contraction and relaxation of the muscles, fibers and capsular bag that hold the lens in place. When the eye focuses on a distant point, the ciliary muscle is relaxed, allowing for a flattened lens and relatively low focal power. During accommodation, however, the anterior surface of the lens shifts forward and the posterior surface moves slightly backward, creating a strong curvature and high focal power.

Young eyes may exhibit accommodation ranges of higher than 10 diopters but lenses begin to harden at about 35 years of age, leading to a gradual decrease in accommodative power. Later, at roughly 60 years of age, some eyes begin to develop cataracts, which affect accommodation still further.

Artificial intraocular lenses can help to address this loss of accommodative power. However, almost all currently available accommodative intraocular lens, and most of those in development, move along the optical axis of the eye (much like the lengthening of a camera lens when zooming in at closer distances).

With these designs, the relatively small amount of space along the optical axis limits the extent of accommodation. The investigators’ lens overcomes this limitation by using shifting optical elements to exploit the space perpendicular to the optical axis — a direction of movement well suited to the human eye. Thus, it enables improvements in accommodation of 3 to 4 diopters.

Lens testing and fabrication

To demonstrate the potential of the lens, the researchers manufactured and characterized a prototype with a clear aperture of ~5.7 mm and an outer diameter of ~11.7 mm. They measured the imaging quality of the lens in air, not in a simulated in vivo environment of the anterior chamber of the eye. To determine the range of accommodation and the aberrations produced by the lens (such as astigmatism and coma) for a lateral shift, they measured the wavefront of a collimated laser beam after it passed through the lens.

Researchers tested the lens by measuring the wavefront of a collimated laser beam. The experiments revealed that the measured wavefront showed little deviation from the calculated spherical wavefront, while the focal power increased considerably.

The beam of a HeNe laser operating at 543.5 nm, made by Melles Griot of Carlsbad, Calif., first passed through a 5.4-mm diaphragm and illuminated the anterior optical element; both the anterior and the posterior elements were mounted on translation stages to enable precise positioning in the X and Z directions.

The diverging beam was collimated by a 50-mm-focal-length objective and its wavefront measured by a 127-subaperture Shack-Hartmann wavefront sensor with hexagonal geometry made by OKO Technologies BV in Delft.

The experiments showed that the deviation of the measured wavefront from the calculated spherical wavefront — caused by astigmatism and coma — was no more than ~0.7 waves for a lateral shift of 0.75 mm. At the same time, the focal power of the intraocular lens changed by ~40 diopters in air, or ~4 diopters in the eye.

Further study is needed to assess how biological responses such as opacification, shrinkage or hardening of the capsular bag due to surgical trauma might affect the efficacy of such an accommodative intraocular lens with shifting optical elements.

In fact, the researchers are in the later phases of animal trials that will help to optimize the lens for cataract and presbyopic patients. They expect to obtain permission in the near future for trials in humans. Clinical studies will take another two years after that, Rombach said.

Production of the intraocular lens has been almost as challenging as design and optimization. Although the materials chosen are used in existing intraocular lenses and proven to work, fabrication of the devices is slightly more complicated.

Typically, intraocular lenses are polished as one of the final steps of the production process. But this is difficult with the nonspherical, free-form optics of the currently described lens. For this reason, the researchers turned to recently developed, commercially available lathing machines with accuracies of about 10 nm. Without these, Rombach explained, production of the high-quality free-form optics would not have been possible.

They will continue to use this technology as they streamline production for larger batch capabilities.

“We will stay with the lathing technology for larger batches because this gives us nearly [lens by lens] production flexibility [as well as] options to make custom lenses for [the] correction of higher-order aberrations such as astigmatism. The lathing has shown to be competitive to other production technologies,” Rombach said.

Oct 2006
BiophotonicsResearch & TechnologySensors & Detectors

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