الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Corneal hysteresis is an important indication of the biomechanical properties of the cornea. It is an indicator of viscous damping in the cornea during inward and outward applanation pressure events that is the ability of the tissue to absorb and dissipate energy, a property that is determined by the visco-elastic properties of the corneoscleral shell.
CH can be measured in vivo using ORA, In which a patented dynamic bi-directional applanation process enables measurement of corneal biomechanical properties using an applied force-displacement relationship.
An air jet similar to that used in traditional air-puff tonometers generates force / pressure on the cornea. It also provides a pressure measurement that is significantly less affected by the cornea than other methods of tonometry.
Some factors also play a role in CH value, CH in normal eyes was higher than that in patients with ocular disease such as keratoconus and Fuchs’ corneal dystrophy Also CH values related to the severity of the disease as it is decreasing with the severity of the disease. Aging process lowers CH. It has positive but moderate correlation to the CCT; the higher the CCT the higher the CH. In glaucoma patients, CH is significantly lower than average with a much wider range when compared to normal subjects.
With the ever increasing popularity of refractive surgery, the knowledge of the factors that determine the biomechanical properties of the eye and their importance in the management of disease process has gained importance. Better understanding of corneal biomechanics might allow for improved predictability of refractive surgery outcomes.
As a post-operative assessment, CH changes were recorded after many refractive surgeries, some increasing, some decreasing, and others still doubtful, till now, as the following:
• In the field of Laser refractive surgery it was proved that LASIK causes a reduction in CH.
• After cataract surgery with clear corneal incision, using phaco-emulsification and IOL implantation the CH decreased on the first day, then returned to normal preoperative values a short time after that.
• After insertion of corneal intra stromal ring segments, there were no significant changes in the CH.
• After Keratoplasty the CH proved to be decreased.
After collagen cross linking with UV-A-riboflavin, there were no significant differences in CH.