Finite Element Modeling of the Human Sclera: Influence on ONH Biomechanics and Connections With Glaucoma

Abstract

Purpose: : Scleral thickness, especially near the optic nerve head (ONH) region, may be a factor of interest in the development of glaucomatous optic neuropathy. Here we investigate the influence on ONH biomechanics of inter-individual differences in scleral thickness.

Methods: : 11 enucleated human globes (7 normal and 4 with a reported history of glaucoma) were imaged using MRI at 80 µm isotropic resolution. The MRI scans were segmented to produce 3-D corneoscleral shells. An identical, idealized ONH geometry was inserted into each 3-D shell. Finite element software modelled the effect of pressurizing the eyes to an IOP of 50 mmHg and computed the resulting lamina cribrosa (LC) displacements and strains, of which first principal strain represents tissue stretching and third principal strain represents compression. An axisymmetric idealized model of the corneoscleral shell and ONH was constructed and parameterized to allow variations in shell geometrical features. A sensitivity analysis on this model using factorial analysis quantified the influence of shell geometry on ONH biomechanics over the geometric ranges measured from the individual eyes.

Results: : Significant variations were observed in the computed strains in the LC across the 11 3-D shells: mean first and third principal strains were 3.5% to 5.2% and -5.6% to -8.6%, respectively, while peak (95th percentile) first and third principal strains were 4.0% to 7.7% and -6.3% to -12.2%, respectively. These results imply large differences in individual ONH biomechanics. The sensitivity analysis found that scleral thickness adjacent to the ONH (within c. 2.5 mm) had the strongest influence on LC strain. Varying this parameter over the physiological range resulted in a change in peak strain in the LC and radial displacement of the scleral canal equivalent to a 15 mmHg change in IOP. Variations of scleral thickness further from the ONH and of globe radius had smaller effects on LC strains and scleral canal displacement.

Conclusions: : Physiologic inter-individual differences in scleral thickness can result in vastly different ONH biomechanics. In particular, physiological peripapillary scleral thickness variations can have a biomechanical effect equivalent to varying IOP by 15 mmHg.

Keywords: lamina cribrosa • intraocular pressure • sclera