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Editors Selection IGR 18-1

Anatomical structures: What do resilient nerve heads look like ?

Comment by Crawford Downs on:

69210 What is a typical optic nerve head?, Voorhees AP; Grimm JL; Bilonick RA et al., Experimental Eye Research, 2016; 149: 40-47

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Optic nerve head (ONH) biomechanics has been hypothesized to play an important role in the development and progression of glaucoma, but it is not well understood. The dearth of available data is due to the technical challenges involved in the measurement of ONH tissue mechanical properties (stiffness) and the complexity of the ONH and scleral geometry. Further complicating the study of ONH biomechanics is the biologic variability in the load-bearing structure, which includes geometry (scleral thickness, neural canal shape and size, laminar pore size and beam thickness, etc.), and tissue stiffness, which may change with age, pathology, extracellular matrix (ECM) composition, and connective tissue remodeling.

Voorhees, Sigal and coworkers have constructed a series of idealized parametric finite element models of the human eye, within which various geometric and tissue stiffness parameters can be varied to determine which combinations of statistically typical and/or atypical stiffnesses and geometries of the eye, lamina cribrosa, and sclera lead to typical or atypical biomechanical responses of the ONH. Similar to the findings of their previously published work, results show that the input factors interact in unpredictable ways, and atypical ONHs do not necessarily yield atypical biomechanical responses, and typical ONHs can often exhibit atypical biomechanical responses. Further, their results suggest that the laminar and scleral stiffness most influence the ONH's biomechanical response to IOP, which supports their previous work.

As the authors acknowledge, these results should be viewed with some caution due to the simplifying assumptions necessary to construct the models. The most important limiting assumptions are the model's inability to consider either regional laminar density or stiffness, or regional differences in laminar and scleral geometry, as well as their limitation to perfectly circular scleral canals. All these remaining factors likely work with the considered parameters in complex ways to contribute to an individual ONH's susceptibility to IOP-related glaucomatous damage, and more work is needed to elucidate these mechanisms.

Results suggest that the laminar and scleral stiffness most influence the ONH's biomechanical response to IOP

This paper is important in that it reminds us that simple biomarkers that aim to predict ONH biomechanical behaviors with only one or two measures may yield an incomplete picture of true ONH biomechanics. Fortunately, recent advances in OCT and other imaging technologies are improving, which may lead to a more comprehensive assessment of ONH biomechanical behavior in vivo.

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