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Editors Selection IGR 18-2
Anatomical Structures: In vivo evaluation of ONH tissue biomechanics
Comment by Andrew Feola on:
70661 Verification of a virtual fields method to extract the mechanical properties of human optic nerve head tissues in vivo, Zhang L; Thakku SG; Beotra MR et al., Biomechanics and modeling in mechanobiology, 2017; 16: 871-887
The optic nerve head (ONH) experiences a complex biomechanical environment,1 likely relevant to the pathogenesis of glaucomatous optic neuropathy. To understand this environment it is critical to have an understanding of ONH tissue biomechanical properties, yet such measurements are challenging. Typically, these measurements are made on ex-vivo (post-mortem) tissue; in addition to possible post-mortem artifacts, there are other issues. For example, it is often difficult to obtain a full patient history (e.g., intraocular pressure [IOP] history, complete glaucoma status, or current treatments), and many ONH tissues are small and hard to access (e.g., the lamina cribrosa and neural tissues). The approach by Zhang et al. has the potential to overcome several of these issues by providing information about an individual's in vivo ONH material properties in a minimally invasive manner, which is to be commended.
This study used the virtual fields method (VFM) to estimate material properties, previously developed and used on a variety of materials.2 Here, Zhang et al. first verified the ability of VFM to estimate ONH material properties against a finite element model with known tissue properties. Then, they used VFM to estimate the stiffnesses of a single patient's prelaminar neural tissue and lamina cribrosa from OCT images obtained at two IOPs. This approach estimated the shear modulus (a measure of stiffness) of the prelaminar neural tissue and lamina cribrosa as 33.7 kPa and 63.5 kPa, respectively. These values are consistent with previously reported data, which is promising for future studies.
As the authors note, this is only a proof of principle study, thus there is still much work ahead to confirm this approach. In the future, it would be good to further validate this approach by comparing material properties of tissues directly determined from experimental tests to those predicted by VFM. It is also important to consider how VFM will perform during test/re-test experiments. While the robustness of the VFM method was examined relative to a finite element model, the variability in material properties, noise, motion, and other experimental conditions may be considerably larger in a clinical dataset. Specifically, as IOP can vary within and between patients, it is important to establish how variations in initial and final IOP impact the material properties estimated by VFM. Nonetheless, Zhang et al. demonstrate an exciting approach that could help us address questions that have yet to be answered. For example, do patients with elevated IOP but no glaucomatous damage have different ONH material properties compared to patients that eventually develop glaucoma? Or, how do material properties of ONH tissues change over time within individual patients? The potential for both longitudinal and cross-section in-vivo studies of ONH material properties may close gaps in our knowledge and help us better understand how the ONH's biomechanical environment influences risk of glaucomatous optic neuropathy.
References
- Burgoyne CF. A biomechanical paradigm for axonal insult within the optic nerve head in aging and glaucoma. Exp Eye Res 2011;93:120-132. doi:10.1016/j.exer.2010.09.005.
- Pierron F, Grediac M. The Virtual Fields Method: Extracting Constitutive Mechanical Parameters from Full-field Deformation Measurements. New York: Springer Science+Business Media 2012.
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