In Vivo Noninvasive Measurement of Young’s Modulus of Elasticity in Human Eyes: A Feasibility Study

Abstract

Purpose Abnormal ocular biomechanical properties may be important for understanding the risk of glaucoma. However, there are no clinical methods for measuring standard material properties in patients. In this feasibility study we demonstrated proof-of-principle for a novel method, ultrasound surface wave elastography (USWE), to determine the in vivo Young’s modulus of elasticity of corneas in normal human eyes. Methods Twenty eyes of 10 healthy subjects (mean age 51.4 ± 7.2; ± SD, range 43–64 years) were studied. A spherical-tipped probe (3-mm diameter) was placed on closed eyelids and generated a gentle harmonic vibration at 100 Hz for 0.1 second. Wave speed propagation in the cornea was measured by USWE, and Young’s modulus was calculated from the wave speed. Associations between Young’s modulus and intraocular pressure (IOP), age, central corneal thickness, and axial length were explored by Pearson correlation. Statistical significance was determined by using generalized estimating equation models to account for possible correlation between fellow eyes. Results Mean IOP was 12.8 ± 2.7 mmHg. Mean wave speed in the cornea was 1.82 ± 0.10 m/s. Young’s modulus of elasticity was 696 ± 113 kPa and was correlated with IOP (r=0.57; p=0.004), but none of the other variables (p>0.1). Conclusions USWE is a novel non-invasive technique for measuring ocular biomechanical properties. Corneal Young’s modulus in normal eyes is associated with IOP, consistent with measurements in cadaver eyes. Further work is needed to determine elasticity in other ocular tissues, particularly the sclera, and if elasticity is altered in glaucoma patients.

A clinical method for estimating the modulus of elasticity of the human cornea in vivo

“…There are, however, techniques under development which may enable a value for corneal tissue elasticity to be obtained for the human cornea in vivo. These instruments include Brilluoin optical microscopy [20,21], corneal indentation [22], estimation of Young's modulus based on a fluid-filled spherical shell model with Scheimpflug imaging [23] and ultrasound surface wave elastography [24].…”

Brillouin microscopy

“…Some ex-vivo methods have been used to address this gap, but in-vivo imaging has been difficult. Ultrasound surface wave elastometry has been used for measurements in donor corneas [ 14 ], and recently in vivo [ 15 ]. OCT-based elastography has been attempted to measure variation in biomechanical strength between middle and posterior stroma [ 16 ].…”

Intraocular Pressure–dependent Corneal Elasticity Measurement Using High-frequency Ultrasound

“…The measured wave speeds of porcine corneas obtained using our method are comparable with those obtained using diagnostic ultrasound elastography in the literature. 26,27,36,40 IOP-dependent corneal wave speed was observed to be a linear function of IOP in the 7-29 mmHg IOP range. Increase in IOP beyond 29 mmHg could lead to nonlinear relation between wave speed and IOP.…”

“…[19][20][21][22] Elasticity imaging finds broad applications in medicine since disease processes or abnormalities alter tissues' elastic properties. 23,24 Quantitative elasticity imaging combines mechanical excitation of soft tissues (vibration, 22 shear waves generation, 25 and surface waves generation) 26,27 and synchronized motion tracking using coherent imaging systems (optical imaging, [28][29][30][31][32] ultrasound imaging, 19,20,22,25 and magnetic resonance imaging). 33,34 This results in images of tissue biomechanical properties, providing elastic contrasts that are potentially useful for clinicians in detecting abnormalities or monitoring progression of diseases.…”