By bouncing sound waves off tiny bubbles generated inside eye tissue, a biomedical engineer at The University of Texas at Austin is developing a new tool that could improve the outcome of laser eye surgery and address common eye conditions.
Stanislav Emelianov and colleagues in the Department of Biomedical Engineering will use the microbubbles generated by a pulsed laser to determine the stiffness and other biomechanical properties of eye components, such as the outer eye layer called the cornea. These bubbles will be created at pinpoint locations within eye tissue, using a femtosecond pulsed laser that initially modifies the cornea during LASIK surgery to improve vision.
The researchers will also use the five-year, $1.6 million in funding from the National Institutes of Health to study whether pulsed laser microbubbles can effectively provide a new laser treatment for presbyopia. This difficulty in focusing on close objects develops in everyone during middle age.
In addition, these microbubbles could provide the first tool for evaluating the biomechanical properties of vitreous humor – the fluid in the main eye chamber. Because changes in this fluid often occur in advance of diseases that affect the retina or other internal eye structures, assessing the properties of vitreous humor using microbubbles could reveal how these diseases develop.
“Many eye surgeons routinely use lasers to treat different conditions,” said Emelianov, “so it would be easy for them to adapt to using a laser to probe the properties of eye structures, which would lead to more informed decisions before performing surgeries.”
Americans undergo about 1.5 million LASIK surgeries each year, with thousands developing complications or less-than-perfect vision. The surgery involves using a laser or sharp blade to turn the outer portion of the cornea into a circular flap of tissue with an untouched “hinge,” and vaporizing some cornea tissue underneath that flap with an excimer laser treatment.
When the flap is readjusted over the treated area, the change in overall shape of the cornea improves vision. However, cornea tissue can restructure itself after LASIK surgery, detracting from the desired outcome.
Emelianov compared a LASIK-treated cornea to a table where the tissue removed from within the cornea represents one of the supportive legs.
“If you remove one table leg, the remaining legs will bend to absorb more weight or stress,” said the assistant professor of biomedical engineering. “The same stress redistribution occurs inside the cornea after LASIK surgery. Sometimes, residual stresses are too much, and new tissue will grow within the cornea, or the existing tissue will readjust, affecting the final cornea shape.”
Using microbubbles to probe the stiffness of cornea tissue before LASIK would allow eye surgeons to better understand how much tissue to safely remove to achieve optimal results.
To further advance the pulsed laser technology, Emelianov will draw on the expertise of ophthalmologist H. Grady Rylander and optical scientist Thomas Milner (the William J. Murray Jr. Fellow in Engineering No. 3, and the Charles M. Rowe Fellow in Engineering, respectively). With Milner, for instance, Emelianov is developing new ultrasound transducers and optical probes that will detect the vibrations of microbubbles created at certain points in the cornea. How easily a bubble vibrates after it is generated within cornea tissue indicates how stiff, or elastic, the tissue is.
A different elasticity test will be performed when eye tissue that is softer than the cornea needs to be probed. This will involve “poking” the side of a microbubble with a wave of sound generated by an ultrasound device. For instance, the rounded, flexible lens of the eye that sits behind the cornea could be probed this way. The ability to displace a bubble within the crystalline lens material would reveal how stiff the material is.
The same microbubbles that probe the properties of the lens could be converted into a form of laser treatment for conditions such as presbyopia. People may lose the ability to see close objects because the lens in the eye that changes shape to help focus light becomes less flexible with age. Alternatively, the muscles surrounding the lens, which are responsible for changing its shape, can weaken over time.
Regardless of the cause, treating the outer rim of the lens with a series of pulsed-laser microbubbles could make the lens more flexible and easier for lens muscles to reshape.
For both tests, Emelianov, and theoretical acoustics experts Yurii Ilinskii and Evgenia Zabolotskaya from the Applied Research Laboratories, will develop algorithms and computer models to predict microbubble behavior. Emelianov and collaborators will initially test their predictions in a jello-like material that has similar properties to eye tissue, in animal eyes obtained from slaughter houses and in human eyes donated for research purposes.
“In many types of eye surgery, it would be helpful to measure the biomechanical properties of the tissue being worked with,” said Emelianov. “Microbubbles produced by a femtosecond pulsed laser could provide a single tool for making those measurements.”
University of Texas at Austin. August 2007.