Sensitivity of mechanical strain in human peripapillary region to adduction tethering evaluated by hyperelastic characterization and finite element analysis (FEA)

Abstract

Purpose : Traction on the optic nerve (ON) and its sheath in adduction may constitute a mechanism of glaucomatous optic neuropathy. We tested human tissues to define hyperelastic & anisotropic properties, and employed these data in FEA to predict combinations of tissue properties potentially associated with traction-induced optic neuropathy in adduction.

Methods : 22 pairs of fresh, post-mortem human eyes were loaded uniaxially in orthogonal directions at constant rate up to 10% strain under physiological temperature & humidity. Five regions were tested: anterior, equatorial & posterior sclera, ON sheath, and circumpapillary sclera demarcating the scleral canal. The Ogden hyperelastic model (N=1, μ₁ and α₁) was fit to each specimen. We implemented FEA to investigate ON traction in 6° adduction beyond ON straightening. FEA was repeated varying combinations of tissue parameters throughout ranges of observed variations in eyes tested.

Results : Since orthogonal loadings suggested isotropic behavior of all tissues except for circumpapillary sclera that was stiffer circumferentially than radially, anisotropic modeling was employed only there. Ogden parameters (Median, 5^th-95^th percentile) of each tissue are listed on the illustration. FEA showed that maximum lamina cribrosa deformation in adduction occurs on temporal side. Sensitivity analysis showed the most extreme tractional deformation (19% max. strain, 1.4MPa max. stress) at upper bounds of ON sheath & posterior sclera “stiffness,” combined with lower bound circumpapillary sclera “stiffness.” The least deformation (12% strain, 0.7MPa stress) was for opposite combinations of these “stiffnesses.”

Conclusions : We have now characterized, in the physiologic range, the hyperelastic properties of most human ocular tissues loaded by ON traction in adduction. While tractional deformation due to adduction tethering is predicted to vary widely due to individual variations in tissue parameters, the most severe stress and strain at the temporal peripapillary region are expected when the ON sheath and posterior sclera are relatively “stiff,” but the circumpapillary ring is not. Since current measurements were made in specimens without history of glaucoma, pathological tissues might have biomechanical properties outside ranges reported here.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.