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Editors Selection IGR 18-3

Basic Research: Scleral mechanics

Comment by Ian Sigal on:

24229 Peripapillary and posterior scleral mechanics--part II: experimental and inverse finite element characterization, Girard MJ; Downs JC; Bottlang M et al., Journal of Biomechanical Engineering, 2009; 131: 051012

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Girard et al. (IGR 11-3: 994) recently introduced a method to characterize scleral mechanical properties with two important advances: Firstly, they used a pressurization apparatus, which allowed them to measure the pressure-induced displacement of posterior monkey scleral shells loaded in a physiologically relevant regime. Secondly, their analysis accounted for the inhomogeneous (location-dependent), anisotropic (direction-dependent) and nonlinear (stretch-stiffening) behavior of the tissue.

They found that the sclera stiffened substantially with IOP, which is important, but was not surprising since it had been reported before. Their models also predicted that in the posterior sclera of monkey eyes the collagen fibers are oriented circumferentially forming a ring around the scleral canal, whereas further from the canal the fibers are irregularly arranged.

In the posterior sclera of monkey eyes the collagen fibers are oriented circumferentially forming a ring around the scleral canal

This is important because the existence of this ring had been hypothesized and this was the first evidence for it. Girard and colleagues proposed that such a ring may limit scleral canal expansion, although they warn that this may come associated with increased posterior lamina cribrosa deformation. Further evidence for this ring and its effects has recently been provided by Grytz et al.¹

Only two eyes were tested by Girard and colleagues, but the authors have since confirmed their findings in a larger study using sixteen eyes.² In this newer study they also found that the posterior sclera from old monkeys was significantly stiffer from that from young monkeys. Although their speculation that this may contribute to age-related susceptibility to glaucomatous vision loss is still to be demonstrated, it does illustrate the power of their techniques and how important it is to characterize scleral mechanics to understand the development and progression of glaucoma.

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