In-vivo high-speed biomechanical imaging of the cornea using Corvis ST and digital image correlation

In-vivo corneal biomechanical characterization has gained significant clinical relevance in ophthalmology, especially in the early diagnosis of eye disorders and diseases (e.g. keratoconus). In clinical medicine, the air-puff-based tonometers such as Ocular Response Analyzer (ORA) and Corvis ST have been used in the in-vivo biomechanical testing. In the test, the high-speed dynamic deformation of the cornea under air-puff excitation is analyzed to identify the abnormities in the morphological and biomechanical properties of the cornea. While most existing measurements reflect the overall corneal biomechanical properties, in-vivo high-speed strain and strain rate fields at the tissue level have not been assessed. In this study, 20 subjects were classified into two different groups: the normal (NORM, N = 10) group and the keratoconus (KC, N = 10) group. Image sequences of the horizontal cross-section of the human cornea under air puff were captured by the Corvis ST tonometer. The macroscale mechanical response of the cornea was determined through image analysis. The high-speed evolution of full-field corneal displacement, strain, velocity, and strain rate was reconstructed using the incremental digital image correlation (DIC) approach. Differences in the parameters between the NORM and KC groups were sta-tistically analyzed and compared. Statistical results indicated that compared with the NORM group, the KC corneas absorbed more energy (KC: 8.98 +/- 2.76 mN mm; NORM: 4.79 +/- 0.62 mN mm; p-value <0.001) with smaller tangent stiffness (KC: 22.49 +/- 2.62 mN/mm; NORM: 24.52 +/- 3.20 mN/mm; p-value = 0.15) and larger maximum deflection (KC: 0.99 +/- 0.07 mN/mm; NORM: 0.92 +/- 0.06 mN/mm; p-value <0.05) on the macro scale. Further, we also observed that The maximum displacement (KC: 1.17 +/- 0.06 mm; NORM: 1.06 +/- 0.07 mm; p-value <0.005), velocity (KC: 236 +/- 29 mm/s; NORM: 203 +/- 17 mm/s; p-value <0.01), shear strain (KC: 24.43 +/- 2.59%; NORM: 20.26 +/- 1.54%; p-value <0.001), and shear strain rate (KC: 69.74 +/- 11.99 s(-1); NORM: 54.84 +/- 3.03 s(-1); p-value <0.005) in the KC group significantly increased at the tissue level. This is the first time that the incremental DIC method was applied to the in-vivo high-speed corneal deformation measurement in combination with the Corvis ST tonometer. Through the image registration using incremental DIC analysis, spatiotemporal dynamic strain/strain rate maps of the cornea can be estimated at the tissue level. This is constructive for the clinical recognition and diagnosis of keratoconus at a more underlying level.