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Editors Selection IGR 8-4

Ocular Biomechanics: CCT predictive for lamina cribrosa and peripapillary sclera?

Comment by Crawford Downs on:

25745 Central corneal thickness, lamina cribrosa and peripapillary scleral histomorphometry in non-glaucomatous chinese eyes, Ren R; Li B; Gao F et al., Graefe's Archive for Clinical and Experimental Ophthalmology, 2010; 248: 1579-1585

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Optic nerve head (ONH) biomechanics likely plays an important role in the development and progression of glaucoma, but it is not well understood. It has been hypothesized that central corneal thickness (CCT) ‐ a predictor for glaucomatous susceptibility ‐ may predict a similarly thin lamina cribrosa, and/or peripapillary sclera. Eyes with thinner ONH connective tissues may not withstand IOP as well as those with thicker tissues, and are thereby more susceptible to glaucomatous damage at lower IOPs. Ren et al. (563) have recently shown that there was no statistically significant association between CCT and either laminar or peripapillary scleral thickness in a histomorphometric study of 55 Chinese eyes. The work could have benefitted from the higher resolution, accuracy, and regional data afforded by newer 3D histomorphometry techniques, but the results are reasonably interpreted within the limits of the standard, single section, 2D approaches they employ.

Many interacting factors, including the structural stiffness of the sclera and lamina cribrosa, dominate the optic nerve head's biomechanical response to IOP

The authors conclude that these data "do not support an association between a thin cornea and increased structural glaucoma susceptibility." This conclusion is somewhat naïve, however, because recent work in ocular biomechanics indicates that many interacting factors dominate the ONH's biomechanical response to IOP. Modeling studies by Sigal and colleagues suggest that the most important determinants of ONH biomechanics are the structural stiffness of both the sclera and the lamina cribrosa, which is only partially determined by tissue thickness. The structural stiffness of a tissue is a combination of both its geometry (thickness, porosity, curvature, etc.) and its material properties (the stiffness of the constituent tissue itself). As a result, a patient may have a thin lamina and sclera that is structurally stiff because it's constituent extracellular matrix is particularly robust. Hence, the ONH's biomechanical response to IOP is determined by the particular combination of interacting factors in an individual eye, and not by a few independent features.

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