A primary indicator of glaucoma is intraocular pressure (IOP). Precision measurement of the IOP depends on accurate measurement of corneal radius, thickness and biomechanical properties. While in vivo methods for measurement of corneal thickness and radius are available, methods to measure the corneal biomechanical properties have yet been developed. To address this gap, a non-invasive indentation-based method to measure corneal biomechanical elastic tangent modulus in vivo was developed in this study. The feasibility and accuracy of the approach was first investigated on a universal testing machine (UTM) with a customized indenter, and tested using pressurized silicone rubber cornea. Analysis and comparison with elastic properties from three-point bending test showed excellent...[ Read more ]

A primary indicator of glaucoma is intraocular pressure (IOP). Precision measurement of the IOP depends on accurate measurement of corneal radius, thickness and biomechanical properties. While in vivo methods for measurement of corneal thickness and radius are available, methods to measure the corneal biomechanical properties have yet been developed. To address this gap, a non-invasive indentation-based method to measure corneal biomechanical elastic tangent modulus in vivo was developed in this study. The feasibility and accuracy of the approach was first investigated on a universal testing machine (UTM) with a customized indenter, and tested using pressurized silicone rubber cornea. Analysis and comparison with elastic properties from three-point bending test showed excellent agreement with an average measurement difference under 0.10%. Afterward, the method was tested on porcine eyes ex vivo and rabbit eyes in vivo respectively. Biological porcine and rabbit corneas are viscoelastic in nature while the liquid in the eye chamber supporting the cornea is also viscoelastic. Indentation tests showed that the systematic viscoelastic contributions becomes negligible only at high indentation rate, and the rate-independent corneal tangent modulus can be reliably measured at indentation rates above 20 mm/min for pressurized porcine eyes and for rabbit eyes in vivo

On this basis, a handheld corneal indentation device (CID) suited for use in clinical setting was developed. Comparisons with UTM showed that the average measurement difference is smaller than 1% in silicone rubber cornea and in porcine eye ex vivo. Further testing showed that the intraclass correlation coefficient (ICC) for the CID measurement is 0.993 and 0.890 in porcine eyes ex vivo and human eye in vivo respectively. With corneal properties in vivo available from CID, the effect of corneal properties in individual patient can be accounted for in IOP measurements to improve screening and diagnosis of glaucoma in patients.