Regional variation in the biomechanical properties of the human sclera

The study aimed to determine the variation in thickness and biomechanical properties between the different regions of the human sclera. Thickness measurements were carried out along eight meridian lines extending from the posterior pole to limbus in 36 human donor scleras aged 52-96 years. Strip specimens were extracted from areas close to the limbus, equator and posterior pole, and tested under cycles of uniaxial tension. Two strain rates were considered to assess the viscoelasticity effects on the regional variation in material behaviour. The results were used to derive the stress-strain behaviour of each specimen and to calculate the tangent modulus at each stress level. The scleras had a variable thickness from maximum at the posterior pole to minimum close to the equator, and increasing again towards the limbus. All scleral specimens demonstrated nonlinear behaviour with an initially low tangent modulus (a measure of stiffness) increasing gradually under higher stresses. With reference to specific stress levels, the behaviour comparisons between regions showed a gradual growth in material tangent stiffness with progression from the posterior region towards the limbus. The viscoelasticity of the tissue, which was evident with significant increases in stress (157-203%) and tangent modulus (30.3-38.8%) with strain rate rise (from 8% to 200% per min), was associated with reductions in the regional variation in stiffness. The considerable variation in biomechanical behaviour found in this study should be useful in improving the accuracy of representing the sclera's real-life conditions in numerical simulations.