Simulating the eye

Dr. Robert Park, a retinal surgeon at Tufts University School of Medicine, is simulating eye movement and the resultant stresses with Algor software in the hope of discovering more about retinal detachments.

According to Research to US-based Prevent Blindness, 25,000 cases of retinal detachment are treated each year. Seven thousand of those cases result in irreparable eye damage.

While doctors know what happens in the eye during the course of this disorder and have surgical treatments at their disposal, not enough is known about why retinal detachments occur in the first place so that they may develop a way to prevent them.

Dr. Robert Park, a retinal surgeon at the Ophthalmic Consultants of Boston and the Tufts University School of Medicine, is simulating eye movement and the resultant stresses with Algor’s Mechanical Event Simulation (MES) software in the hope of discovering more about retinal detachments.

Park’s research may help to explain why near-sighted eyes are more at risk for retinal detachment, provide better post-operative recovery instructions for patients undergoing retinal surgery or even lead to the discovery of techniques for the prevention of retinal detachments.

With a Bachelor of Science degree in Material Science and Engineering from the Massachusetts Institute of Technology and a medical degree from Albany Medical College, Park is in a unique position to understand retinal detachments from an engineering standpoint as well as a medical standpoint.

‘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,’ said Park.

‘As an engineer, my mind turned to trying to quantify the answers to those questions in terms of peak stresses. There is experimental research on retinal adhesivity and tensile strengths of eye tissues, but no one has ever modelled the whole eye to look at the stresses generated during movement.’

‘I began looking on the Internet for a FEA package in 2000,’ said Park. ‘I chose Algor’s FEA-based MES because it could predict stresses for a complete bio-mechanical model based on acceleration and deceleration as well as forces.’

What is a Retinal Detachment?

To understand what retinal detachment is, it is important to understand a little about inner eye anatomy. The eye is filled with a gel-like substance called vitreous humor, which consists of collagen fibres suspended in a matrix of water, glucosaminoglycans and proteoglycans. The vitreous fibres attach to the retina – the 100-230 micron thin layer of nerve tissue that is responsible for vision itself. The retina has 7 layers including a light-sensing photoreceptor layer, an intermediate cell layer and a layer of actual 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 very important layer of cells, called the pigment epithelium, separates the retina from the choroid.

With age, the vitreous humor contracts. The collagen fibres clump together and pockets of fluid develop between the clumped fibres until enough traction builds up on the vitreo-retinal interface to cause a separation from the retina. During vitreous separation, retinal tears can occur where there is a focal point of vitreo-retinal traction.

When the retina tears, fluid from the vitreous leaks underneath the retina. The fluid then overwhelms the pigment epithelium pump mechanism that keeps the retina in place and the retina detaches from the pigment epithelium, thus resulting in a detached retina. Without the retina in its proper anatomical position, the blood supply to the outer retina is lost and vision cannot be maintained.

Retinal detachments are more common in the middle aged and elderly. However, in addition to age-related causes, torn or detached retinas may be caused by traumatic accidents in a small percentage of cases. Treatments for retinal detachment include laser and freezing therapies in the early stages and surgery in more advanced cases.

Simulating eye movement

Park began by creating a very detailed model of the entire eye in Superdraw. Each part of the eye was created in a unique part so that the material properties of different types of tissues could be considered. Published bio-mechanical research was Park’s source for the linear material properties of the eye tissues.

Park then applied a rotation to the eye that represented a 30° saccadic eye movement, which is a rapid point-to-point shift in eye position that occurs when the focus of one’s attention shifts from one object to another. This very common type of movement accelerates the eye 125 RPM in about 17-19 milliseconds. The acceleration and deceleration curve that Park input was based on published impulse times of such movements from previous experimental measurements.

In a preliminary simulation, Park constrained the eye on the top and bottom with translational boundary conditions. This simulation produced high stresses around the constraints that Park believed were artificially large. Truncated portions of the muscles around the eye were then added to the model, replacing the constraints. These parts were fully constrained at their ends and mounted directly onto outer eye parts and resulted in better distribution of the applied loads.

The simulation captured 0.058 seconds at a capture rate of 1,500 steps per second. Park reviewed the magnitude and distribution of stresses. ‘The highest retinal stresses were in the super-temporal quadrant,’ said Park, referring to the top outside quadrant of the eye. ‘We know clinically that a vast majority of retinal detachments occur in this quadrant of the eye. I think it is too early in my research to call this a conclusive correlation, but it is a trend toward correlation.’ Correlating simulation results directly to experiments is not possible since stresses cannot be measured in an actual eye.

Looking to the future

Initial results have been promising, but Park still considers his research to be in its early stages. ‘The model I have now is quite basic,’ comments Park. In the near future, he plans to increase the complexity of the eye model, build smaller elements to better capture the eye’s behaviour and take advantage of MES’ surface contact capabilities to model the interfaces between different parts of the eye.

Once the model has been refined, Park will use that geometry to conduct a series of simulations. ‘I plan to simulate stresses generated by other types of eye movements as well as various types of body movements, such as walking and running,’ said Park. ‘Trauma induced by accidents in various head positions will also be included.’

Park’s long-term goal for his research is to discover how to prevent retinal detachment from occurring. However, along the way there may be some smaller but important advances. For example, Park’s research may have clinical applicability with regard to post-operative recommendations.

‘Retinal surgeons make recommendations to patients after retinal surgery to limit activity,’ said Park. ‘However, we don’t have real numbers about the kinds of activities that place the greatest stress on the retina. If the activities we limit after surgery are no greater than something uncontrollable like REM (rapid eye movement) sleep, does it make sense to make these recommendations at all?’ Park hopes to discover whether there are specific eye movements that place a greater stress on the retina than others and whether retinal surgeons can be more specific about what patients should avoid.

Another clinically relevant series of simulations will involve modelling different pathologic conditions. Myopia, or near-sightedness, is an obvious choice because people with myopia are disproportionately likely to experience retinal detachment. The myopic eye is slightly elongated and the dimensions of the structures such as the retina within the eye are different. In addition, the tension of tissues within the myopic eye is different than in normal eyes. ‘This research may eventually lead to an explanation of why myopic eyes are more at risk for retinal detachment than the general population,’ said Park.

Material nonlinearity represents another wide area for exploration. ‘As the model is refined, I want to experiment with human eye tissues to build a library of material properties at various temperatures and hydration states and work towards integrating material nonlinearity into the simulation,’ said Park. ‘I expect our experimental research into the material properties of eye tissues will uncover that eye tissues behave more like elastomers and polymers. At that point, we will incorporate the use of Algor’s nonlinear material models into our eye model.’

While much remains to be done, Park’s simulations are already yielding information that will serve as a starting point for a greater understanding of retinal detachment. Hopefully, his ongoing use of simulation software will advance our understanding about the stresses generated in the eye as a result of movement and will bring about some help for the thousands who suffer from retinal detachment each year.

This article appears courtesy of Algor Inc.

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