Simulating the Mechanics of Lens Accommodation via a Manual Lens Stretcher

Abstract

The goal of this protocol is to mimic the biomechanics of physiological accommodation in a cost-efficient, practical manner. Accommodation is achieved through the contraction of the ciliary body and relaxation of zonule fibers, which results in the thickening of the lens necessary for near vision. Here, we present a novel, simple method in which accommodation is replicated by tensing the zonules connected to the lens capsule via a manual lens stretcher (MLS). This method monitors the radial stretching achieved by a lens when subjected to a consistent force and allows for a comparison of accommodating lenses, which can be stretched, to non-accommodating lenses, which cannot be stretched. Importantly, the stretcher couples to the zonules directly, and not to the sclera of the eye, thus only requiring the lens, zonules, and ciliary body rather than the entire globe sample. This difference can significantly decrease the cost of acquiring donor cadaver lenses by about 62% compared to acquiring an entire globe.

An Artificial Lens Capsule with a Lens Radial Stretching System Mimicking Dynamic Eye Focusing

“…During the ex vivo test of the biological lens (capsule) mimicking accommodation, the sclera together with ciliary body, zonules and lens was bonded to the end parts of different lens stretching system [ 2 , 12 , 23 , 24 , 25 , 26 ]. The surrounding tissues around the lens were cut radially to eliminate the circumferential contribution and separate the accommodation of lens [ 32 , 33 ].…”

“…During the stretching process, each arm moved simultaneously along the radial direction and applied stretch to the lens capsule to achieve the flatter surfaces of ALC, i.e., arriving at an unaccommodated state. Compared to previous similar applications [ 24 ], the bespoke LRSS offered a more precise motion by using a stepper motor (1.8° step angle, Model: 1705hs200a-nema 17, Ooznest, Brentwood, UK) together with a microcontroller and encoder (Model: S-lite, uStepper, Aalborg, Denmark). The motor was driven by a sub-step of 0.1° (3200 steps per revolution), resulting in a designed incremental linear step of 7 µm by the transmission.…”

“…There is a hyper-elastic stress–strain relationship of the capsule with mechanical testing, demonstrating variability of thickness and stiffness depending on age and spatial distribution [ 10 , 11 , 12 , 19 , 21 , 22 ]. To understand the lens accommodating process, ex vivo testing has been widely employed to replicate the contraction of ciliary muscle by applying and releasing stretch in a mechanical way on ex vivo lenses with zonules and ciliary body attached [ 2 , 12 , 23 , 24 , 25 , 26 ], but lacking the independent investigation on the capsule itself. The difference in the biomechanics of the lens with and without substance exists by the different force response of the filled and empty capsular bag [ 12 ], where the latter demonstrates the status after surgery.…”

The major influence of anterior and equatorial zonular fibres on the far-to-near accommodation revealed by a 3D pre-stressed model of the anterior eye

“…Functions of the zonular fibres have been explored by in vitro mechanical stretching to analyse their material properties [24,25], and by modelling studies to consider their macro or individual functions [26,27]. The supporting effect of the lens by the vitreous has also been considered by Ljubimova et al [28] who included the vitreous in their models and captured the influence of the vitreous body, namely the so-called "squeezing" effect.…”

Characterisation and Modelling of an Artificial Lens Capsule Mimicking Accommodation of Human Eyes

“…The accommodative function of the crystalline lens has been studied with ex vivo testing and numerical modelling [15]. The shape change of lenses during accommodation in vivo can be replicated by inducing radial stretch using a lens stretching device [16][17][18]. The deformation and reaction force of lenses by such devices have been used to investigate the aetiology of presbyopia by combining optical power and biometry measurements [19,20].…”