Abstract
Purpose :
Ocular accommodation decreases with age as a result of a multitude of factors. While increasing lens stiffness plays a role, also other factors are considered. One of these factors is the change in orientation of the zonular fibers. In this study, we investigate the sensitivity of the accommodative amplitude to the orientation of the zonular fibers.
Methods :
A finite element mechanical model of the accommodative apparatus was made in COMSOL, consisting of the crystalline lens and zonular fibers. The shape of the crystalline lens and stiffness of the cortex and nucleus represented the 45-year-old lens, according to Burd (Burd, Judge & Cross, 2002). The model included zonular fibers as annular sheets surrounding the lens, and extended to an outer diameter of 6.33 millimeters. This outer diameter represents the diameter of the ciliary body in the accommodated state. The orientation of the anterior and posterior zonular fibers was varied between 0 - 10 degrees and 0 – 19 degrees, respectively. In each case the outer end of the zonular fibers were stretch by a prescribed displacement of 0.4 millimeters in four steps. (Dis)Accommodation was determined as the difference between the initial state and the stretched state of the lens.
Results :
Sixty-six (66) different combinations of orientations of anterior and posterior angels were modeled and evaluated. The accommodative amplitude ranges from 5.0 to 6.2 diopters, indicating that a difference in zonular fiber orientation can have an effect on the accommodative amplitude as large as 1.2 diopters.
The change in accommodative amplitude (AA) as a function of fiber orientation was expressed as d(AA)/d(θ), and indicates the sensitivity of the accommodative amplitude to the orientation of the zonular fibers. d(AA)/d(θ) ranged from 0.10 to 0.53 Diopter/degree. Specifically, for small anterior angles (~ 2 degrees) and larger posterior angles (14-16 degrees), AA showed the greatest sensitivity to the orientation of zonular fibers.
Conclusions :
The orientation of zonular fibers has a significant influence on the accommodative amplitude. In specific angle ranges, the accommodative amplitude can change by one diopter with only a few degrees of variation of the orientation of zonular fibers. Therefore, changing the orientation of zonular fibers with age could contribute to the development of presbyopia.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.