Mechanics of Individual-Specific Models of the Corneo-Scleral Shell in Glaucoma

Abstract

 

Purpose:

 

Previous work suggests that IOP-induced forces acting on the optic nerve head (ONH) depend strongly on scleral thickness and globe diameter, among other factors (IOVS, 46:4189). Here we study the biomechanics of whole human globes, with emphasis on force transmission to the ONH.

 

Methods:

 

Individual-specific finite element computer models of the corneo-scleral shell and lamina cribrosa (LC) were reconstructed from µMRI scans of 3 enucleated, ostensibly normal, human post mortem eyes, as described elsewhere. Simplified axisymmetric models with homogeneous thickness based on the mean thickness and curvature of each scleral shell were also created. Commercial software (Ansys Inc, USA) was used to predict ocular response to an IOP increase from 5 to 50mmHg in all models. Tissue properties were as previously described (IOVS, 46:4189). Median and peak (95^th percentile) magnitudes of IOP-induced equivalent stress and principal strains were used to quantify the biomechanical response.

 

Results:

 

In individual-specific models, global mean stresses were 10.5, 8.2 and 10.4 times the IOP (xIOP). There were substantial variations in stress and strain from one region to another, with peak stresses of (xIOP): 18.7, 14.1 and 17.4. Longitudinally, stresses and strains were maximal at the equator, and up to 2x those at the poles. Stresses also varied circumferentially and were as much as 40% higher on one side than the other at the equator. In the peripapillary region the stresses were relatively axisymmetric in some eyes (5% variation) and asymmetric in others (30% variation). Stresses and strains were more homogeneous in the simplified models, e.g. in the simplified model of eye 1 global mean and peak stress were (xIOP) 10.8 and 11.4.

 

Conclusions:

 

Physiologically accurate models of human globes confirm that scleral geometry has a substantial influence on ocular biomechanics. The magnitude and axial symmetry of the forces exerted on the ONH by the sclera vary between individuals, and could potentially influence ONH sensitivity to IOP.  

 

Keywords: computational modeling • sclera • intraocular pressure