![]() In the anterior one third of the stroma, the lamellae are more narrowly interwoven than in the posterior two thirds. This layer is composed of 250-400 stacked lamellae. The corneal stroma makes up 90% of corneal thickness and is the main contributor to the cornea's strength and transparency. Understanding corneal macro- and microstructure allows for an understanding of its biomechanical properties. In the terminology of mechanical science, the cornea is a complex anisotropic composite with nonlinear elastic and viscoelastic properties. Biomechanical Properties of the Normal Cornea It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties. ‘Anisotropy' is the property of being directionally dependent, as opposed to ‘isotropy', which implies identical properties in all directions. ![]() The individual components remain separate and distinct within the finished structure. Most materials are linear for a specific range of stress values and beyond this range lose their linear elasticity and become nonlinear.Ĭomposite materials are materials made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. ![]() Nonlinear materials do not have a constant proportion of stress versus strain. For many materials, Young's modulus is essentially constant over a range of strains. It is defined as the ratio of the stress along an axis over the strain along that axis. Young's modulus, also known as the ‘tensile modulus' or ‘elastic modulus', is a quantitative measure of the stiffness of an elastic material. The energy lost in this process is called ‘hysteresis'. Viscoelastic materials exhibit characteristics of both viscosity and elasticity, resulting in energy dissipation when stress is applied. Viscous materials, on the other hand, flow when an external shear force is applied and do not regain their original shape when the force is removed. Elasticity is the ability of a substance to deform reversibly under stress. ![]() Several biomechanical terms help characterize the function of biologic materials. Corneal Biomechanical Properties Biomechanical Principles ![]()
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