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Editors Selection IGR 8-4

Anatomical Structures: Scleral shape and thickness

Comment by Crawford Downs on:

45638 Finite element modeling of the human sclera: Influence on optic nerve head biomechanics and connections with glaucoma, Norman RE; Flanagan JG; Sigal IA et al., Experimental Eye Research, 2011; 93: 4-12

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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. Norman et al. (509) have built finite element models of the human eye wherein an identical idealized ONH geometry was integrated into 11 eyespecific scleral shells whose shape and thickness were based on high-resolution MRI scans of 11 donor eyes. Norman analyzed the effects of scleral shell shape and scleral thickness on lamina cribrosa strain and deformation in these eyespecific 3D models to determine the range of values for individual eyes. He then used parametric axisymmetric models in a factorial analysis approach to determine how important eye shape (radius of curvature and axial length) and scleral thickness (as a function of distance from the ONH) were to lamina cribrosa deformation and strain.

It is not only the specific features of an individual eye, but also their particular combination that determine an ONH's biomechanical response

Their numerical results suggest that within eye models with identical ONHs wherein eye shape and scleral thickness are varied, the thickness of the immediate peripapillary sclera has by far the largest effect on laminar strain and deformation. Neither eye shape nor scleral thickness beyond the immediate peripapillary region had much effect on laminar biomechanics. This result is the most important, as it provides further evidence that peripapillary scleral thickness could serve as a biomarker ‐ in addition to scleral stiffness, and laminar geometry and stiffness ‐ for eyes in which elevated IOP is likely to cause particularly elevated ONH deformation and strain. As the authors acknowledge, these results should be viewed with some caution due to the simplifying assumptions employed, such as models' inability to consider regional laminar density or stiffness and the simplicity and uniformity of the identical laminar geometries. Also, this study did not compare models of different laminar or scleral stiffnesses, which previous studies suggest are as important as scleral thickness in determining laminar biomechanics. This body of work adds to the evidence that it is not only the specific features of an individual eye, but also their particular combination that determine an ONH's biomechanical response.

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