"Eye Engineer," Feature Article, October 2003

eye engineer

A surgeon takes a biomechanical approach to the retina.

This article was prepared by staff writers in collaboration with outside contributors.

Perhaps 30,000 people in the United States suffer retinal detachments every year from just one of the leading causes. According to a 1995 report by the American Academy of Ophthalmology, one in 10,000 people has a retina detach because of a tear or break, a condition called rhegmatogenous retinal detachment.

Doctors know what happens in the eye during the course of the disorder and treatment is about 90 percent successful, according to Research to Prevent Blindness Inc., an eye-health organization. More knowledge of how retinal detachments occur in the first place could lead to prevention.

That's why Robert Park, an assistant professor of ophthalmology at the University of Arizona in Tucson, is using finite-element analysis to study stresses on the eye. Park hopes his research, which began when he worked at the Tufts University School of Medicine, could lead to prevention or to better healing.

Park combines a bachelor's degree in material science and engineering from the Massachusetts Institute of Technology with a medical degree from Albany Medical College. "As a retinal surgeon, I became curious about which areas of the eye are most susceptible to damage and what happens when the eye is in motion," Park said. "As an engineer, my mind turned to trying to quantify the answers to those questions in terms of peak stresses."

The eye contains a gel-like vitreous humor—collagen fibers suspended in a matrix of water and proteins. The fibers attach to the retina—the thin layer of nerve tissue that is responsible for vision. The retina is 100 to 230 micrometers thick and has seven layers, including a light-sensing photoreceptor layer, an intermediate cell layer, and a layer of nerve cells that attach directly to the brain. A layer of blood vessels, called the choroid, separates the retina from the thick white outer layer called the sclera. A layer of cells called the pigment epithelium separates the retina from the choroid.

A finite-element model shows the change in the stresses on the human eye in fractions of a second during a common rapid movement.

Trauma or complications of age can tear a retina. Fluid leaking through the tear can separate the retina from the pigment epithelium. Then blood supply is lost, and vision suffers. Treatments include laser and freezing therapies in the early stages and surgery in more advanced cases.

Using Mechanical Event Simulation software from Algor Inc. of Pittsburgh, Park modeled a human eye in a moderate 30-degree saccadic movement, a rapid point-to-point rotation that occurs with a shift in the focus of attention. The model accelerated the eye to 125 rpm in about 18 milliseconds, rotated it for 20 ms, and decelerated in 18 ms. The acceleration and deceleration curve Park used was based on published experimental data.

First, Park constrained the eye on the top and bottom with translational boundary conditions. But, stresses seemed artificially large. He added portions of the muscles around the eye, replacing the constraints. Fully constrained at their ends and mounted directly onto outer eye parts, they resulted in better distribution of the applied loads.

The simulation captured 0.058 second in 1,500 steps. Park said the model showed the highest retinal stresses in the top outside quadrant of the eye, where a majority of retinal detachments occur. He added that it is too early in his research to call it a conclusive correlation.

"The model that I have now is quite basic," he said. He plans to increase its complexity and build smaller elements to better capture the eye's behavior.

Park plans to study the effects of body movements, such as walking and running, as well as trauma induced by
accidents.

His long-term goal is to prevent retinal detachment. His research may also some day influence post-operative recommendations on patient activity.