Strain heterogeneity measurement of mouse sclera by stereo digital image correlation under inflation testing

Scleral biomechanics has emerged as an important topic in eye research, particularly for understanding the pathophysiology of diseases such as myopia and glaucoma. For the biomechanical characterization of scleral tissue, the three-dimensional digital image correlation (3D-DIC) method has been widely used as an accurate optical tool for full-field deformation measurements during ex vivo inflation testing in different species like cow, pig, human, rat, and tree shrew, with a large range of eye size roughly between 38 mm and 7 mm. Mouse has been used as the common model in glaucoma and more recently in myopia research, but until now, due to the small eye size (~ 4 mm in diameter), no stereo-DIC has been proposed for whole-eye inflation testing. We present the first setup of inflation testing of intact mouse eyes for measuring the strain field of the sclera by the 3D-DIC method during quasi-static pressure loading, along with an analysis of the variables required to design a stereo-DIC system. In addition, we investigated the biomechanical characterization of mouse sclera, and the evolution of the sclera strain distribution during a loading-unloading cycle. We discovered that the scleral strain distribution during quasi-static linear changes of intraocular pressure (IOP) follows a Lognormal distribution whose standard deviation represents a metric of the biomechanical heterogeneity of the strain response.