Thesis (M.Sc.) - Cairo University - Faculty of Medicine - Department of Ophthalmology

Elasticity refers to how a material deforms in response to an external stress. An elastic material is one that regains its original form in a completely reversible displacement direction along the stress/strain pathway when the imposed stress is removed. Hysteresis refers to the energy lost during the stress/strain cycle. Viscous materials flow when an external shear stress is applied, but, unlike materials with elastic properties, they do not regain their original shape when the stress is removed. Corneal hysteresis (CH), which is the result of viscous damping in the corneal tissue, is an indicator of corneal biomechanical properties. Corneal biomechanical properties influence intraocular pressure measurement, undergo alterations in corneal pathology and following corneal refractive surgery. In this combined prospective and retrospective study, the premise was to determine if a correlation exists between the average keratometry reading detected by corneal topography and the corneal hysteresis measured by optical response analyzer in a group of patients undergoing evaluation for refractive surgery with low spherical errors of refraction and low or no astigmatism. The study included 150 eyes divided into 3 groups: Group A (mean K between 40 and 42 D): 50 eyes. Group B (mean K between 42 and 44 D): 50 eyes. Group C (mean K between 44 and 46 D): 50 eyes. We discovered that there was a clinically significant difference in corneal hysteresis between patients with flat and average keratometric readings and between average and steep keratometric readings There was also a statistically significant difference in corneal hysteresis between the flat and steep groups

Issued also as CD