*Benjamin. J. Coldrick1, J. G. Swadener1, Leon. N. Davies2
1Biomedical Engineering Research Group, Aston University, Birmingham, B4 7ET
2Ophthalmic Research Group, Aston University, Birmingham, B4 7ET
Key Words: structure, biomechanics, finite element analysis
Finite element analysis is a useful tool in understanding how the accommodation system of the eye works. Further to simpler FEA models that have been used hitherto, this paper describes a sensitivity study which aims to understand which parameters of the crystalline lens are key to developing an accurate model of the accommodation system. A number of lens models were created, allowing the mechanical properties, internal structure and outer geometry to be varied. These models were then spun about their axes, and the deformations determined. The results showed the mechanical properties are the critical parameters, with the internal structure secondary. Further research is needed to fully understand how the internal structure and properties interact to affect lens deformation.
A fundamental attribute of human vision is the ability to change focus, so that objects both near and far can be seen. In the young eye, this ability is controlled by the accommodation system, where humans are able to change focus due to the crystalline lens changing shape (Figure 1). For distance vision, the lens assumes the relaxed position, where it is in the lowest power form. This is achieved by relaxation of the ciliary muscle, which, in turn, pulls the zonular fibres that surround the equator of the lens, flattening the lens . For the lens to form its most powerful shape, for near vision, the ciliary muscle contracts, causing the zonular fibres to relax. This causes the lens to change shape as the capsule, which surrounds the lens, applies an inwards pressure on the lens . Although accommodative ability is present in the young human eye, the amplitude of the response declines with age. Consequently, by approximately 50-55 years of age, the lens is no longer able to be formed into is most powerful form, resulting in the loss of ability to focus on near objects; this condition is known as presbyopia.
Relaxed ciliary muscle
Lens in least powerful shape
Contracted ciliary muscle
Lens in most powerful shape
Figure 1: Schematic of eye showing accommodation stages, the left hand side represents the lens in its low power form
Given the ubiquity of presbyopia, it is important to understand why it develops so that potential preventative and therapeutic techniques can be developed. Although functional near vision can be achieved with simple methods such as spectacles or contact lenses, or more advanced methods such as intraocular lenses, these are not ideal. The ideal method of treating presbyopia would be to restore the dynamic change in power that a young lens can achieve, therefore restoring the full range of near and far vision .
Finite element analysis has been used in a number of previous studies to analyse the accommodation system [3-6]. While some approaches differ, many similarities occur either in the model geometry or the model mechanical properties. The current study will explore these models to lay the groundwork for a more detailed FEA model of the accommodation system.