Artificial lens helps to see clearly
Innovative design improves ability to focus on near objects
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
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
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
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.
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