![]() ![]() The cornea exhibits a non-linear viscoelastic response with varying amounts of hysteresis on different loading cycles.The cornea exhibits a non-linear stress versus strain response with progressive stiffening at high strains.Principles Confirmed with Ex Vivo TestingÄespite its weaknesses, ex-vivo destructive testing has successfully confirmed the following biomechanical principles: These error risks include altering fibril orientation (especially if corneal strips are used) and poor replication of in-vivo humidity conditions. While this type of investigation allows for extensive testing, including multiple loading patterns and measurement of various properties, the nature of testing ex vivo introduces the potential for error. Ex-vivo destructive testing techniques include strip extensiometry and pressure inflation of intact corneas. Biomechanical properties are measured by exposing corneal tissue to stress under controlled conditions with constant humidity. This type of evaluation involves testing either complete corneas or strips of cornea that have been removed from the eye. These techniques have been divided into ex vivo destructive tests and in vivo nondestructive tests. Several techniques have been developed for testing corneal tissue in an attempt to measure the mechanical properties of the cornea. The energy lost in this dissipation 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 refers to the ability of a substance to deform reversibly under stress. The cornea exhibits elastic and viscoelastic properties, which give it the quality of hysteresis. Freidenwald first described the viscoelastic properties of the cornea in 1937, followed, in subsequent decades, by Nyquist and Woo. The material properties of the cornea are responsible for its functionality. In addition, successful corneal treatments depend on interactions between biological and biomechanical factors and their impact on surrounding ocular tissues. An understanding of corneal biomechanics is important in describing disease states such as keratoconus and ectasia. While various methods have been devised to study the biomechanics of the cornea, only the Ocular Response Analyzer allows direct analysis of corneal biomechanical properties in the clinic. The physical composition of the cornea gives it viscoelastic properties, meaning it exhibits elements of both elasticity and viscosity. The biomechanics of the cornea affect its functional responses and greatly impact vision. 3.1.1 Principles Confirmed with Ex Vivo Testing. ![]()
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