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Civils widen their vision

Civil engineers take pride in their profession's immense diversity, but few would argue that it includes ophthalmology and the science of the human eye. The University of Dundee, however, is making such a claim. Mark Hansford investigates.

When, as recently as 1994, the Institution of Civil Engineers described civil engineering as 'managing the conception, innovation, promotion, design, construction, operation, maintenance and eventual removal of the amenities of modern civilisation', it probably thought it had things covered. But that was before civil engineers at the University of Dundee became leaders in ophthalmic research.

The university's department of civil engineering is using a combination of finite element analysis and lab tests to build predictive biomechanical models of the human cornea. From these models a 'virtual tissue' can be created, which will be used to improve understanding of the cornea's response to external agents, from contact lenses to car airbags.

'We are opening up a whole new area, ' explains the department's Tim Newson. 'This work should be seen as a first step towards the development of predictive tools that will enhance the research and clinical management of corneal injury, disease and surgery.'

The cornea plays a vital role in the optical performance of the eye. In combination with a surface tear layer it accounts for over two thirds of the eye' optical power. To achieve optimal image formation the cornea must maintain its shape and translucence, which can be degraded by disease, injury and the effects of surgery. Ability to predict the biomechanical response of the cornea to these kinds of trauma, and hence changes in optical performance, is of great clinical importance.

The diameter of the human cornea is approximately 12mm and varies in thickness from 0.67mm at the edge to 0.52mm at the centre, explains Newson.

'Thus the cornea can be thought of as a thin shell structure.'

The cornea is composed of a number of layers serving different functions but the biomechanical behaviour is dominated by the stroma, which represents about 90% of total corneal thickness. Around 78% of the stroma is water, but layered protein fibres accounting for 16% of the stroma give the cornea its strength, elasticity and form.

The stroma is divided into 300500 sheets of collagen - stromal lamaellae - lying parallel to the corneal surface. Each lamella is composed of long collagen fibrils embedded in a jelly-like substance. Fibrils lie parallel to each other through the length of the lamella and resist tensile forces within the cornea.

While these collagen fibrils have a Young's modulus in the order of 1.0GN/m 2along the fibril direction, the Young's modulus and the shear modulus of the jelly-like substance are only in the order of 10-5GN/m 2. As a result the stroma offers very little resistance to shear forces.

To get as full an understanding as possible of how this complex biostructure works, the six-man Dundee team has combined finite element modelling with pressure testing of real tissue specimens in the lab, using a laser system accurate to 1-2 Cyclic loading tests have shown considerable non-linearity, indicating a material that increases in stiffness with increasing pressure, explains Newson. 'Hence the material behaves in a similar manner to elastomers like rubber, but is even more extreme in its behaviour.'

Using this information, finite element models of the cornea have been developed using the Abaqus finite element package.

To simplify calculations the team used the relative uniformity of the cornea's surface and structure to assume a rotational symmetry. This reduced the modelling to a 'pie-slice' of the overall profile.

The team also had to find a way to simply and effectively model the connection of the translucent cornea to the larger diameter, opaque sclera at the limbus. 'The sclera has a stiffness of approximately 3-5 times that of the cornea, ' explains Newson, 'thus achieving fixity at the edge of the cornea is important.'

Models that use fully-fixed joints, pinned joints and inclined rollers have all been compared to those that include the full scleral structure.

The initial equilibrium state of the corneal structure includes an intra-ocular pressure (2 kN/m 2)applied to the lower surface of the model. This represents the hydrostatic pressure caused by the aqueous and vitreous humour inside the eye, which supports the whole eye structure.

At this stage, the finite element mesh has been simply modelled using quadrilateral finite elements, and the models have assumed simple elastic behaviour.

The next stage is to develop non-linear elasticity and more complex modelling that uses thin-shell elements and assumes the cornea to be a laminated composite material. Despite the initial crudeness, the models already offer a strong correlation to clinical studies, explains Newson.

'So far the study has investigated the effects of corneal thinning diseases, laser surgical procedures, and the behaviour of the cornea in response to contact lenses, ' he says.

'But further improvements to the model, such as non-linear elasticity, will provide a model that can actually be used in clinical practice.'

Email Dr Tim Newson at t. a. newson@dundee. ac. uk

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