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An alternative to reading glasses

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Hank Hogan

A s with death and taxes, presbyopia, the inability to focus close up, is one of life’s certainties. Nearly everyone suffers from it by age 50. Reading glasses, the traditional solution, work but aren’t perfect. Laser correction of the intraocular lens has been proposed as an alternative. However, the procedure involves the inside of the eye, which is hard to monitor, and irreversible ablation.

One culprit of presbyopia is thought to be increasing lens stiffness with age, which might be due to accumulating inelastic fibers within the lens. Laser ablation removes those fibers at precise locations where a beam is focused, but there has been no way to check progress during the process.

At the University of Michigan in Ann Arbor, biomedical engineering professor Matthew O’Donnell, graduate student Todd N. Erpelding and research scientist Kyle W. Hollman may have come up with a noninvasive way to measure elasticity that would let clinicians know whether more ablation is needed. The technique uses laser-generated microbubbles to map the elasticity of the lens.

The process begins with bubbles produced by laser-induced optical breakdown, which occurs when sufficient light is concentrated in a small volume. It creates a plasma that expands and forms a small, short-lived bubble. Before the bubbles diffuse, high-frequency sound waves are focused on them, driving them against the fibers. A microbubble is located by the equivalent of ultrasound imaging, and fiber elasticity can be determined from the bubble movement.

Researchers use laser-induced optical breakdown by focusing a pulsed laser to concentrate light intensity in a small volume. The resulting plasma expands and creates a small, short-lived bubble, in this case about 10 μm in diameter. The bubbles can be used to measure the elasticity of the lens at a given location. Images courtesy of Matthew O’Donnell, University of Michigan biomedical engineering department.

For their studies, the latest of which involved 41 pig lenses of various ages, the researchers used an Nd:glass laser from IntraLase Corp. of Irvine, Calif., operating at 1053 nm. The 800-fs, 13-μJ pulses produced microbubbles less than 100 μm in diameter. The size, controlled by pulse energy and other parameters, determined the maximum lifetime of the bubbles. O’Donnell noted that the ability to dial in a desired bubble lifetime is an asset for research and clinical applications.

The researchers focused the laser on a spot within the lens, forced the bubbles to move using a 1.5-MHz sound burst and then determined the location of each bubble using a 7.44-MHz pulse from the same ultrasonic transducer. They moved the focal point in millimeter steps and mapped the elasticity of the center (or nucleus) versus the edge (or cortex) of the pig lenses. They found that, as expected, there was increasing stiffness with age. Their results agreed with those obtained using much more invasive methods.

Microbubbles generated within a pig lens by laser-induced optical breakdown are monitored with ultrasound. Before bubble creation (left), the lens is clear. After the bubbles form (middle), they dissipate, with a lifetime determined by their size (right).

The microbubbles could potentially be created using the same laser employed for ablation. O’Donnell envisions a system in which manipulation and monitoring are combined. “That will be done clinically using just a laser scanning system similar to what will be done for corneal surgeries,” he said.

The time line for such a clinical device could be as long as five years or more, he noted. Although there is commercial interest in the technique, there is still the need to develop a product and to secure regulatory approval.

Jul 2006

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