Interaction of Anatomical Factors Increases Posterior Ocular Strain During Adduction Tethering of the Optic Nerve (ON) in Primary Open Angle Glaucoma (POAG) Without Statistically Elevated Intraocular Pressure (IOP)

Abstract

Purpose : Prior investigations of POAG without statistically elevated IOP have emphasized single factors in possible pathogenesis. Using finite element (FE) simulations, we investigated possible interaction of anatomical factors to explore variations in resulting mechanical strain on the ON head during adduction tethering as potential causes of optic neuropathy in POAG.

Methods : Target-controlled, axial magnetic resonance imaging with surface coils was employed to measure anatomical factors of 10 normal controls and 10 POAG patients with untreated IOP not exceeding 21mmHg. Mean globe axial length AL, globe diameter at the equator (GDE), and coordinate of the orbital apex (COA) relative to globe center were investigated as anatomical factors that affect adduction tethering. Then, computational FE models of each subject’s anatomy were designed using Solidworks 2019. Other dimensions, boundary conditions, & average normal tissue material properties were identical for all models. Simulations of 6° incremental adduction from 26° were performed for each subject FE model.

Results : Individually, each anatomical factor in patients with POAG was statistically indistinguishable from controls, except AL. Mean AL (±SD) was larger at 26.5±1.4mm for POAG than 24.4±1.1mm for controls (P=0.001) and GDE in POAG was 26.2±1.1mm, larger than the control at 25.3±0.9mm (P=0.054). Mean COA, which reflects ON path, was similar in the 2 groups (P>0.5). In both groups, FE simulation showed strain on the temporal side of the ON & peripapillary sclera during adduction tethering, but significantly greater in POAG than in controls. Peripapillary sclera strain was 4.7±0.9% for POAG & 3.9±0.9% for controls (P=0.016); lamina cribrosa strain was 4.9±0.8% for POAG & 4.2±0.6% controls (P=0.034); & retrolaminar ON strain was 6.9±0.9% for POAG & 5.8±0.9% for controls (P=0.012).

Conclusions : Subject-specific FE simulations based only on observed combinations of anatomical variations patients with POAG suggest abnormally great strain on the temporal ON & peripapillary sclera during adduction tethering. While single anatomical factors alone do not distinguish POAG patients, consideration of variations in tissue material properties in addition to anatomical combinations might further distinguish the pathology of POAG.

This is a 2020 ARVO Annual Meeting abstract.