September 2016
Volume 57, Issue 12
Open Access
ARVO Annual Meeting Abstract  |   September 2016
Optic Nerve (ON) Sheath Traction on the Temporal Peripapillary Sclera in Adduction: Biomechanical Evidence for a Role in Normal Tension Glaucoma (NTG).
Author Affiliations & Notes
  • Andrew Shin
    Ophthalmology, Stein Eye Institute, UCLA, Los Angeles, California, United States
  • Joseph Park
    Ophthalmology, Stein Eye Institute, UCLA, Los Angeles, California, United States
  • Joseph L Demer
    Ophthalmology, Stein Eye Institute, UCLA, Los Angeles, California, United States
  • Footnotes
    Commercial Relationships   Andrew Shin, None; Joseph Park, None; Joseph Demer, None
  • Footnotes
    Support  NIH EY08313
Investigative Ophthalmology & Visual Science September 2016, Vol.57, 3563. doi:
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      Andrew Shin, Joseph Park, Joseph L Demer; Optic Nerve (ON) Sheath Traction on the Temporal Peripapillary Sclera in Adduction: Biomechanical Evidence for a Role in Normal Tension Glaucoma (NTG).. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3563.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : The historically central role of high intraocular pressure (IOP) in glaucomatous ON damage has been challenged by the observation that IOP is not elevated in many patients considered to have NTG. We propose that NTG may be caused by tractional deformation of the optic canal and lamina cribrosa (LC) by tautness of ON sheath in adduction recently recognized to occur in adduction (Demer, ARVO 2015).

Methods : Uniaxial tensile loading was used to characterize Young’s modulus in 4 fresh adult bovine tissues: ON, ON sheath, and peripapillary and peripheral sclera. Specimens were elongated at constant rate to failure during tension recording in a load cell under physiological temperature and humidity. Finite element analysis (FEA) using SolidWorks software was used to predict resulting stress/strain during adduction 0.1 radians beyond ON straightening. Physical dimensions of the FEA mesh were scaled to human histological and MRI data.

Results : Young’s modulus, a measure of stiffness of an elastic isotropic material, was averaged (±SD) for 5 specimens of each tissue: lowest for ON at 5.2±0.8 MPa and peripapillary sclera at 5.5±1.7 MPa, intermediate for peripheral sclera at 14±5 MPa, and highest for ON sheath at 45±13 MP. FEA indicated 405 kPa maximum stress and 17 % maximal strain induced in temporal side of the ON scleral canal. In the LC, maximum predicted stress was 80 kPa, greatest temporally. Predicted mechanical changes were ~30-fold larger than produced by simulated IOP increase to 50 mmHg.

Conclusions : ON sheath traction in adduction concentrates severe stress in the temporal peripapillary sclera in a distribution identical both to patterns of peripapillary atrophy and temporal ON head tilting typical of NTG, and ON deformations in adduction recently confirmed by OCT to occur in normal subjects and patients with NTG (Chang et al, ARVO'16). This IOP-independent stress could plausibly cause ON damage when accumulated over time, and thus cause NTG. Tractional ON damage may be particularly prevalent in patients with shorter ONs as in axial myopia. This study supports a new biomechanical concept of glaucoma as resulting at least in part from external traction on the ON sheath, rather than exclusively on pressure exerted within the eye.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

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