On the use of uniaxial tests on the sclera to understand the difference between emmetropic and highly myopic eyes

This study aims at assessing the differences between highly myopic eyes and emmetropic eyes in the biomechanical response to ex vivo uniaxial tests of the human sclera. Scleral strips were collected from two enucleated eyes with markedly different axial length (AXL) of 24 and 31 mm, respectively. Static stress measurements under uniaxial loading were carried out with a Zwicki-Line testing machine, specially designed for low load mechanical testing applications. Although all uniaxial stress-strain curves showed an exponential response typical of biological viscoelastic materials loaded within the physiological strain range, the qualitative behavior was ostensibly different between the two eyes. With respect to the 24 mm AXL eye, the 31 mm AXL eye mm showed a lower stiffness at physiological strains and a more evident stiffening at larger strains. It was concluded that the myopic eye is more compliant at low strains and stiffer at strains larger than the physiological ones. An interpretation of this behaviour is that the higher percentage of small diameter collagen in myopic sclera delays the process of fiber recruitment and shifts the tissue stiffening to higher strains.