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.

Figure 1

Radius of curvature in mm plotted against the change in COP in dioptres (D) showing the optimal combinations of zonular angles that fit most closely to in-vivo data for the 16-year-old lens model using both sets of material properties^, when applied with a displacement set of 0.5 mm to the equatorial zonule, 0.6 mm to the anterior and posterior zonule.

Figure 2

The p-value contours showing relations between anterior and posterior zonular angles when the equatorial zonular angle was fixed to 2 degrees for both the anterior and posterior lens surface of models aged 16 and 35 years old two sets of using different material properties and capsular thickness.

Figure 3

The changes in radii of curvature (R) in mm and Central Optical Power (COP) in dioptres (D) for progressive stretching steps of the 16-year-old lens model using material properties of (a) Fisher and of (b) Wilde et al. and of 35-year-old lens model using material properties of (c) Fisher and of (d) Wilde et al..

Figure 4

The geometries and dimensions of the (a) 16-year-old lens and (c) 35-year-old lens as well as the anchorage regions of zonular fibres on lens capsule for the (b) 16-year-old lens and (d) 35-year-old lens.

Figure 5

Flow diagram indicating methodologic steps applied in the modelling procedure.

Figure 6

A representation of Finite Element lens models created showing a 16-year-old lens model with three parts of the zonule at angles of 20 degrees anterior, 4 degrees equatorial and 34 degrees posterior for six stages of deformation. The dashed contour indicates the non-deformed shape.