Mechanical analysis of an ocular globe, a required complexity during inflation test?

Abstract

Purpose : To describe the effect of varying the structure of biomechanical models on corneal deformation under progressing intraocular pressure (IOP).

Methods : A 3-D finite element model of an ocular globe was implemented to evaluate corneal deformation under various mechanical conditions during inflation testing. The eye model includes the cornea, limbus, sclera, iris, lens, muscles, anterior chamber and vitreous. The Ogden hyper-elastic model (µ=0.458 kPa, α=136.14) was suggested for the corneo-limbal structure, the Yeoh isotropic (C₁=0.81, C₂=56.05, C₃=2332.26 (MPa)) model for the sclera, the anterior chamber was modeled as a fluid cavity with progressing pressure [0-25 mmHg], while other eye components were incorporated as linear elastic material, each with different density and viscosity. This study investigates the interplay between several cases, including: Case 0 : restrained limbus; Case 1: unrestrained limbus with restrained sclera, Case 2 (Reference): unrestrained limbus and restrained sclera; Case 3: unrestrained limbus with restrained posterior sclera and free anterior sclera; Case 4: unrestrained limbus with restrained anterior sclera and free posterior pole; Case 5: effect of anterior chamber; Case 6: effect of vitreous; Case 7: effect of anterior chamber and vitreous; Case 8: effect of iris; Case 9: effect of Lens; Case 10: effect of muscle.

Results : The maximum apical displacement and maximum stress under IOP=15 mmHg was 0.22 mm, 0.013 MPa, the cornea underwent an increased stiffness above IOP>7 mmHg (Figure 1.a, Figure 2.a)). Cases 1,2 & 4 resulted in nearly similar displacements, with a slight increase when the limbus is unrestrained. Case 3 leads to reduced stiffness (25%), with increased displacement. Adding individual eye components (Cases 5-7), results were nearly similar to the reference. The presence of the iris slightly decreases the displacement, but leads to higher stress on corneal periphery. Meanwhile, the contribution of the muscles and lens together cannot be neglected as it reduces the corneal displacement with 50% (Figure 1.c-d, Figure 2.b)).

Conclusions : As the lens, iris and muscles each provide major contributions to the corneal deformation model, it is highly recommended to account for such internal eye contributions during ex vivo experiments.

This is a 2021 ARVO Annual Meeting abstract.

 

Figure 1: Comparison of the cases.

 

Figure2: a) Von Mises stress of the eye under IOP=15 mmHg, b) Ocular stress of Case 10.