The importance of parameter choice in modelling dynamics of the eye lens

Abstract

The lens provides refractive power to the eye and is capable of altering ocular focus in response to visual demand. This capacity diminishes with age. Current biomedical technologies, which seek to design an implant lens capable of replicating the function of the biological lens, are unable as yet to provide such an implant with the requisite optical quality or ability to change the focussing power of the eye. This is because the mechanism of altering focus, termed accommodation, is not fully understood and seemingly conflicting theories require experimental support which is difficult to obtain from the living eye. This investigation presents finite element models of the eye lens based on data from human lenses aged 16 and 35 years that consider the influence of various modelling parameters, including material properties, a wide range of angles of force application and capsular thickness. Results from axisymmetric models show that the anterior and posterior zonules may have a greater impact on shape change than the equatorial zonule and that choice of capsular thickness values can influence the results from modelled simulations.

The effects of mechanical strain on mouse eye lens capsule and cellular microstructure

“…Gross lens shape change caused by strain results microstructural level changes in all the regions of the lens that we examined. This provides critical information that will be useful in expanding current biomechanical models of lens shape changes (Reilly and Ravi, 2010;Reilly, 2014;Wang et al, 2017). Furthermore, the identification of a relationship between microstructure and tissue biomechanics supports the idea that age-dependent microstructural alterations, such as an increase in stiffness of cells or matrix (Starodubtseva, 2011;Lacolley et al, 2017), could have biomechanical consequences at the tissue level.…”

A Modeling Approach for Investigating Opto-Mechanical Relationships in the Human Eye Lens

“…Young's modulus E or shear modulus G. This log-log linear equation [25] is described as: log( ) = log( ) + (3) where a and b are material dependent coefficients that were determined for porcine lenses: a=0.093 and b=9.29 [25] which have similar elastic shear moduli to young human lenses [29], [reviewed in 30]. Taking the eye lens as nearly incompressible [31], [32] Young's and shear modulus can be linearly related: E=3G [10], [30]. The calculated Young's moduli E at the central plateau and at the lens poles are 0.82 kPa and 0.04 kPa respectively.…”

“…Yet a number of modelling approaches simplify these forces as emanating from a single point [5], [6], [7], [8], [9]. Recently it was shown that separating directions of zonular force across the three sections makes a substantial difference to the shape change and renders the modelled simulation closer to the changes in shape seen in the biological lens [10]. This is fundamental for understanding the mechanical behaviour of the different zonular sections and for providing insights needed to understand the accommodative process and its loss with age.…”

“…Such insights may resolve the conflicts between major accommodative theories [11], [12], [13], [14]. Recent modelling [10] and experimental studies on monkey lenses [15] suggest that the equatorial zonular section is less effective when compared to the anterior and posterior zonular sections in altering central curvatures and optical power of the lens during accommodation. This has been previously postulated [12], [16].…”

“…Finite Element lens models were created based on human lenses from five different ages, covering the age range from the second to the sixth decade of life. A parametric analysis of 990 different combinations of zonular angles and hence directions of force on the lens, was conducted for each model using an exhaustive search scheme developed in a previous study [10]. The resultant changes in lens thickness and stress field distributions were analysed to investigate the optomechanical relationship and how this may alter with age.…”