Abstract
Purpose :
Prior models of extraocular mechanics have artificially constrained globe translation either by rigidly fixation eye center, or suspending it with unphysiological virtual springs. In order to clarify the role connective tissues on globe rotation & translation, we developed an FEM of incremental adduction induced by active contractility of extraocular muscles (EOMs), including the suspensory tissues & optic nerve (ON).
Methods :
The model implemented hemisymmetric geometries of bilaminar EOMs, ON, orbital wall, & orbital fat were obtained from MRI of 5 healthy adults, and measured constitutive tissue properties. Active & passive strain energies of EOMs were defined using ABAQUS (Dassault Systemes) software. The eye was rotated by EOM deformations caused by twitch activation. Ocular center was not fixed, but the globe was instead suspended by Tenon’s fascia and an anatomically realistic pulley system, including orbital layer insertions. Starting from 26° adduction, the medial rectus was commanded to incrementally contract, and the lateral rectus to relax. We alternatively modeled absence of orbital fat, versus fat with Young’s modulus ranging up to 7 KPa.
Results :
During incremental adduction from 26 to 32°, EOM, ON & connective tissue tensions were physiologically plausible, and stress & strain were concentrated at the optic disc. Maximum principal strain was 6% in peripapillary sclera, lamina cribrosa, & ON sheath, while minimum principal strain was -6% in peripapillary sclera, lamina cribrosa, & disc. Von mises stress averaged 30MPa in lamina cribrosa. Without orbital fat, the globe translated 0.1mm nasally but 1.2mm posteriorly, which is non-physiological. With 7KPa stiffness orbital fat, translation was 0.17mm nasally but only 0.33mm posteriorly, which is physiologically realistic.
Conclusions :
This physiologically plausible method of simulating EOM activation along with an anatomically realistic suspension system can provide realistic input to model biomechanical behavior of active and passive tissues in the orbit to clarify biomechanical consequences of ON traction during adduction.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.