Noninvasive Measurement of Scleral Stiffness and Tangent Modulus in Porcine Eyes

Abstract

PURPOSE. We investigated an indentation technique to measure the scleral stiffness and tangent modulus of porcine eyes. METHODS.The scleral load-displacement responses were measured with a universal testing machine as a function of IOP in 15 porcine eyes ex vivo using a 5-mm diameter cylindrical flatpunch indenter. The scleral radius of curvature and scleral thickness were measured using a DSLR camera (Alpha 900) and a camera-mounted stereomicroscope (M205C), respectively. The relationships between scleral stiffness, tangent modulus, and IOP were examined. RESULTS.The mean local scleral radius of curvature and scleral thickness were 7.86 6 0.49 and 1.03 6 0.14 mm, respectively. The average scleral stiffness and scleral tangent modulus of porcine eyes were 0.13 6 0.02 N/mm and 0.20 6 0.04 MPa at 15 mm Hg, respectively. The scleral stiffness and scleral tangent modulus were correlated positively with IOP (scleral stiffness, 0.989 < r < 0.999, P < 0.001; scleral tangent modulus, 0.989 < r < 0.999, P < 0.001).CONCLUSIONS. The scleral indentation technique can provide a noninvasive approach to measure scleral stiffness and tangent modulus.Keywords: sclera, glaucoma, myopia, tangent modulus, noninvasive measurement T he sclera constitutes over 70% of the outer envelope of the eyeball, has a complex multilayered structure, and has a central role in providing structural stability to the eye. The scleral biomechanical property is an important parameter characterizing the ocular structural integrity. The sclera has been shown to be more flexible and less load-bearing in myopes than in emmetropes. [1][2][3][4][5] It also has been shown that increased ocular rigidity (a measure describing the relationship between the change in IOP and the change in eyeball volume) is associated with the development of glaucoma, 6 and that scleral stiffness is correlated with increased prevalence of glaucoma and age. 7,8 While the importance of scleral properties is recognized, in vivo technique for measurement of the scleral properties is limited. Scleral properties, such as scleral stiffness and tangent modulus, are ascertained from load-displacement curves of the sclera. (Like other biological tissues, the sclera is a complex composite structure with many layers. In investigations of biomechanical properties, the detailed fine structures often are ignored, such that the properties measured are interpreted as the material property of the composite structure 9-11 under small deformation linear elastic regime.) The loads generally are imposed onto the eye using inflation methods, 9,12-17 and displacements are ascertained using speckle interferometry, 9,12,13,17 digital camera imaging, 15,16 or ultrasound speckle tracking.14 These inflation-based methods are destructive to the eye, and are unsuited for use in clinical study on the human eye in vivo.Instead of inflation, surface wave elastometry also can be used to measured elastic properties of the cornea. In surface wave elastometry (Refs. 18 and 19; Dupps W, et al. IOVS 2005;46:ARVO E-Abs...

A Mouse Model of Chronic Ocular Hypertension Induced by Circumlimbal Suture

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Repeatability of a novel corneal indentation device for corneal biomechanical measurement

“…One of the major advantages of our device is to provide standard parameters used in biomechanics, such as stiffness and tangent elastic modulus. The indentation device can also be applied to measure the scleral stiffness . This study evaluated the intrasession and intersession repeatability of the device in normal healthy corneas.…”

“…The indentation device can also be applied to measure the scleral stiffness. 23 This study evaluated the intrasession and intersession repeatability of the device in normal healthy corneas.…”

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“…Corneal indentation method is adapted for the development of measurement method for the corneal hysteresis behavior [4,5]. The corneal indentation tangent modulus (E| IOP ) at a fixed IOP are defined as […”

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