PRESBYOPIA

When viewed from an engineer's perspective, the increasing stiffness of the crystalline lens with age can account for most of the loss of accommodation that is present in presbyopic eyes, said harvey Burd MA, DPhil, MiCE, CEng, Oxford University, Oxford, UK. 'The picture that is emerging from our preliminary data is that the change in stiffness of the lens together with the reduced movement of the ciliary body contribute about 80 per cent of the loss of accommodation in presbyopic eyes. The remaining 20 per cent of accommodation loss is related to changes in the shape and bulk of the lens,' he told the XXiX Congress of the ESCRS.

In his analysis, Dr Burd used a technique called the finite element method to separate out the contribution of each of the factors involved in the loss of accommodation with age. Engineers use the same approach to determine if an airplane will be able to fly, before it makes its maiden flight, he noted. 'The finite element method is one method of modelling engineering systems theoretically before they're actually constructed. so we thought, why not apply the system to understanding the human eye? if an accurate model can be devised, it becomes possible to look at the effect of different parameters and quantify their role in presbyopia,' he added.

Finite element eye model Dr Burd noted that the finite element method involves a very detailed mathematical modelling of the design and materials of the subject under investigation, based on the laws of physics. in the case of an airplane, the finite element model would include such parameters as the materials used in constructing the plane, the geometry of the wing, and the power of its engines. 'The idea is that the computer program simulating the performance aggregates the behaviour of all of these elements in order to give it a prediction of the behaviour as a whole. so it's a very neat way of assessing performance. Of course, if the performance is insufficient in the simulation model, it is very easy to adjust it at a computer design stage, compared to when you're actually in the air finding a problem with the wing,' Dr Burd said.

In the case of an investigation of accommodation and presbyopia in the human eye, the main parameters in the finite element model would be the mechanical, geometric and optical properties of the components of the eye that are involved in accommodation. These parameters reflect the various molecular changes that occur with age. The schematic eye model Dr Burd and his associates used was based on the published literature regarding the shape and anatomical configuration of structures such as the crystalline lens, the lens capsule, the zonule and the ciliary body, as well as the changes that occur in these structures with age. They also incorporated into their model the findings obtained through the physical testing of the stiffness of the crystalline lens at different ages. Their technique was to use a specially designed rig to spin the lenses and to define the measured amount of deformation that occurred in terms of a Young's modulus of elasticity. They based the amount of stretch they imposed on the lens on the data obtained from MRi studies. The MRi studies also show a slight decline in ciliary movement with age, which they also incorporated into the finite element model.

'What we see is that in the young lens the nucleus is very, very soft, about a 10th of the stiffness of the cortex. They both stiffen with age. Between 29 to 45 years of age the stiffness of the nucleus increases by a factor of about 20 and the cortex by a factor of about two. At about 45 years, the stiffness of the cortex and nucleus are similar and then the stiffness of the nucleus rockets off exponentially into older age. This is a key part of understanding the presbyopia story,' he said. Using the stiffness measurements and the literature-based eye model, Dr Burd and his associates created finite element meshes of a 29-year-old lens and a 45-yearold lens. They then created an animation of the deformations that would occur in the different lenses and in related structures in response to an accommodative stimulus. The animation showed the change in thickness and curvature was very strong in the 29-year old lens. however, the 45-yearold lens, though still deforming, has smaller thickness variations, a reduced change of curvature and a reduced ciliary body movement.

Dr Burd noted that the 29-year-old eye accommodates, on average, by about 8 D. Between 29 and 45 years of age there is loss of about 5 D of accommodation. in the finite element model, the increased stiffness of the lens alone that occurs between those ages would cause a loss of 3 D of accommodation. The reduced movement of the ciliary body, which may itself be a result of the increased lens stiffness, accounts for another dioptre of accommodation loss, he said. 'The lens contains crystalline proteins in a compact globular form. With age they denature which means they open up. The denatured proteins form cross-links, which increases the stiffness of the lens substance. As a result, the lens which was very flexible in the young eye becomes increasingly less able to accommodate as one gets older', he added.