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Joseph Park, JoAnn Giaconi, Kouros Nouri-Mahdavi, Simon K. Law, Laura Bonelli, Anne L Coleman, Joseph Caprioli, Joseph L Demer; Finite Element Analysis (FEA) of Anatomical Factors Exaggerating Optic Nerve (ON) Strain During Adduction Tethering in Primary Open Angle Glaucoma (POAG) Without Elevated Intraocular Pressure (IOP).. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6172.
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© ARVO (1962-2015); The Authors (2016-present)
ON tethering in adduction has been proposed to cause or contribute to optic neuropathy in POAG. However, since ON tethering in adduction is nearly universal, pathogenicity is plausible only if other factors exaggerate effects of ON tethering in POAG. We therefore explored possible anatomical pathological factors by FEA that reflects individual anatomical variations in patients.
Axial length (AL) and globe to apex distance (GA) were measured in high resolution axial magnetic resonance images obtained with surface coils in 22 matched normal controls and 23 POAG patients with IOP always < 21 mmHg. From these, one typical control and 3 patients were selected for representative FEA. For the normal control, AL was 24.0mm and GA 41.0mm. The short orbit POAG case had AL 23.3mm and GA 33.2mm. The long eye case had AL 28.1mm and GA 36.3mm. The combined case had both long AL 27.4mm & short orbit 35.1mm. Normal tissue properties were obtained by tensile loading of tissues from 28 fresh human orbits. The FEA simulated 6° adduction after ON straightening and had identical boundary conditions, normal material properties, and dimensions except AL and GA.
Mean AL in POAG was 25.4±1.6 mm, significantly longer than controls at 24.6±1.3mm (P<0.007). Mean GA for POAG at 37.8±3.5mm was significantly less than controls at 39.4±3.3mm (P<0.032). In all example cases, FEA showed strain concentration in the temporal ON head, retrolaminar axonal tissue, and temporal peripapillary sclera. Mean temporal ON strain was 2.6% (max. 5.3%) for the control, 4.4% (max. 9.6%) for short orbit, 4.6% (max. 11.0%) the long AL, and 6.3% (max. 12.9%) for long AL and short orbit. Corresponding mean peripapillary strains were 2.9% (max.3.5%), 2.8% max.3.2%), 4.1% (max. 4.9%), and 4.1% (max.5.0%), respectively.
Assuming normal tissue biomechanical properties, anatomical combinations of large globe and short orbit are predicted to more than double ON and peripapillary strain during adduction tethering. Globes are significantly larger and orbits shorter on average in patients with POAG than in normal subjects, supporting the proposed mechanism of repetitive strain injury from adduction in POAG.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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