Measurement of corneal tangent modulus using ultrasound indentation

Abstract

Biomechanical properties are potential information for the diagnosis of corneal pathologies. An ultrasound indentation probe consisting of a load cell and a miniature ultrasound transducer as indenter was developed to detect the force-indentation relationship of the cornea. The key idea was to utilize the ultrasound transducer to compress the cornea and to ultrasonically measure the corneal deformation with the eyeball overall displacement compensated. Twelve corneal silicone phantoms were fabricated with different stiffness for the validation of measurement with reference to an extension test. In addition, fifteen fresh porcine eyes were measured by the developed system in vitro. The tangent moduli of the corneal phantoms calculated using the ultrasound indentation data agreed well with the results from the tensile test of the corresponding phantom strips (R(2)=0.96). The mean tangent moduli of the porcine corneas measured by the proposed method were 0.089±0.026MPa at intraocular pressure (IOP) of 15mmHg and 0.220±0.053MPa at IOP of 30mmHg, respectively. The coefficient of variation (CV) and intraclass correlation coefficient (ICC) of tangent modulus were 14.4% and 0.765 at 15mmHg, and 8.6% and 0.870 at 30mmHg, respectively. The preliminary study showed that ultrasound indentation could be applied to the measurement of corneal tangent modulus with good repeatability and improved measurement accuracy compared to conventional surface displacement-based measurement method. The ultrasound indentation can be a potential tool for the corneal biomechanical properties measurement in vivo.

Glaucoma vs. Biomechanical Properties of Cornea

“…The corneal stiffness has been the focus of many researchers [4,5,8,9,13]. Stiffness can be determined experimentally for each individual patient during the diagnostics of glaucoma, for example by the indentation method [8,13]. This method allows to measure the stiffness repeatedly at different places and at different times.…”

Estimation of the Corneal Young’s ModulusIn VivoBased on a Fluid-Filled Spherical-Shell Model with Scheimpflug Imaging

“…The biomechanical properties of the cornea are associated with the development of corneal diseases such as keratoconus, ectasia after refractive surgery, and possible glaucoma progression [ 1 – 4 ]. Hence, there has been a recent surge of interest in assessing corneal biomechanical properties due to potential clinical implications [ 5 – 7 ]. Moreover, the biomechanical properties of the cornea have been proposed to directly affect intraocular pressure (IOP) measurements, especially for normal tension glaucoma [ 8 , 9 ], and are becoming recognized as necessary for the calibration of IOP in different moduli of tonometers [ 10 – 13 ].…”

High myopia induced by form deprivation is associated with altered corneal biomechanical properties in chicks

“…Applying the principle of an ultrasound-indentation system[ 28 ], we developed and validated (see below) a customized optical-coherence-tomography-indentation (OCT-indentation) system, and used it to measure in-situ corneal tangent modulus (TM) and corneal stiffness (CS). The OCT-indentation system consisted of a fiber-optic based time-domain OCT, an indenter of 1 mm diameter, a CCD camera, and data acquisition modules (see S1 Fig for the dimensions of the system).…”

“…In this study, we incorporated optical coherence tomography (OCT)[ 26 , 27 ] into our indentation system[ 28 ], and we used this novel OCT-indentation system to determine the impact of FD-induced high myopia on in-situ corneal biomechanical properties (CB), while IOP was maintained constant at one of three values in the physiological range. This system was designed with attention to the anatomical features of our target animal model, the chicken; namely: 1) the indenter probe was miniaturized (1 mm diameter) because of the steep corneal curvature in our small animals; 2) crystalline lens surface was used as a reference (see details in Methods ), to avoid confounding error due to eyeball movement during corneal indentation[ 29 ]; and 3) time-domain OCT (TD-OCT) was incorporated, to provide fast, high-resolution tracking of corneal and crystalline lens surfaces during indentation at different IOPs.…”

“…We chose TD-OCT as our first approach, because its reference arm allows an extended depth of detection, and because it costs less than frequency-domain OCT (FD-OCT). Data collected from this system were used to calculate corneal tangent modulus, by integrating with corneal geometrical parameters (corneal thickness and curvature) measured with other instruments[ 28 – 30 ]. As shown in this paper, our novel system is sensitive enough to measure small but significant changes in corneal biomechanical properties of highly myopic chicks.…”