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Editors Selection IGR 21-2

Clinical Examination Methods: other: Measuring Elasticity in Vivo

Comment by Ross Ethier on:

74266 In Vivo Noninvasive Measurement of Young's Modulus of Elasticity in Human Eyes: A Feasibility Study, Sit AJ; Lin SC; Kazemi A et al., Journal of Glaucoma, 2017; 26: 967-973

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Sit and coworkers describe the application of ultrasound surface wave elastography to measure the stiffness (Young's modulus, or E) of the cornea in ostensibly normal volunteers. The motivation for this proof-of-concept study is a body of evidence that the stiffness of ocular tissues, particularly the peripapillary sclera and lamina cribrosa, modulate the effects of intraocular pressure on the sensitive cells of the optic nerve head and can thus influence glaucoma risk in individual patients. Although many techniques exist for measuring tissue stiffness in a laboratory setting, clinical measurements are more challenging. The technique used here is clever: a small mechanical probe was placed on the closed eyelid and vibrated to cause tiny displacement waves to propagate laterally in the cornea, which were then imaged using a standard ultrasound system. Finally, the wave speed was obtained by image analysis and a theoretical model was used to infer corneal E from wave speeds. E was correlated with IOP measured by GAT, as expected, but not with age, central corneal thickness, and axial length, although this lack of correlation was possibly due to the small cohort (20 eyes in 10 subjects). Advantages of the technique include its relatively simplicity, non-invasive nature, and rapidity. Importantly, E is a direct measure of inherent tissue properties, which is attractive compared to systems such as the Ocular Response Analyzer that produce biomechanical outcome measures that are harder to interpret.

E is a direct measure of inherent tissue properties, which is attractive compared to systems such as the Ocular Response Analyzer that produce biomechanical outcome measures that are harder to interpret

Drawbacks include the assumptions required in the theoretical model used to infer E: these include simplifications of the tissue geometry and neglect of several complex tissue properties (anisotropy and viscoelasticity). The authors wish to now extend the approach to measure stiffness of posterior ocular tissues - this will be more challenging, possibly due to imaging issues and also because the above-mentioned assumptions of the theoretical model become less correct near the optic nerve head. If further validated, this technique would provide a muchneeded tool for interrogating posterior ocular tissue biomechanical properties.

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