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Editors Selection IGR 22-1
Basic Science: Measuring scleral stiffness non-invasively
Comment by Crawford Downs on:
57212 Noninvasive measurement of scleral stiffness and tangent modulus in porcine eyes, Leung LK; Ko MW; Ye C et al., Investigative Ophthalmology and Visual Science, 2014; 55: 3721-3726
Scleral biomechanics likely plays an important role in the development and progression of glaucoma and myopia, but the contribution of scleral material properties (stiffness) to these diseases is not well understood. Experimental and numerical studies indicate that the peripapillary sclera is influential in determining ONH biomechanics because it defines the mechanical boundary condition for the contained ONH at the scleral canal, and scleral elongation is a central pathologic feature in myopia development. Leung and colleagues describe a method to measure scleral tangent modulus using a five-mm-diameter flat indenter in intact eyes. They tested this method in 15 porcine eyes ex vivo, and calculated scleral stiffness and tangent modulus as a function of IOP. Both scleral stiffness and tangent modulus were positively correlated with IOP, which matches previous results from inflation studies of both human and non-human primate scleral shells. The methodology used relies on several key assumptions for calculation of the reported parameters. First, the reaction of the intact eye to perturbation, in this case indention, involves stretching of the entire ocular coat including the cornea and ONH. Ocular rigidity is a well-known metric of this phenomenon. So, the change in IOP resulting from scleral indentation is confounded by the reactions of the cornea and ONH, which was not taken into account. Also, initial IOP was controlled with a needle into the anterior chamber during the indention experiments, and therefore some reflux into the needle would be expected with a one-mm-depth indention that occurred over three seconds, confounding the IOP response measurement. Finally, the calculations of scleral wall stress, and hence tangent modulus, were based upon Laplace's Law, which is only valid for perfectly spherical pressure vessels of uniform thickness and homogeneous material properties - none of these strictly apply to the eye. Hence, while indention may yet prove to be a plausible method to measure scleral biomechanics in vivo, additional validation is needed.
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