الفهرس 
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Abstract
Biomechanics is concerned with the medical application of mechanical concepts. Eye biomechanic helps us to understand its normal function and predict changes in response to forces such as changes in IOp, and surgical intervention.
The cornea reacts to stress as a visco-elastic material. The visco-elastic response consists of an immediate deformation followed by a rather slow deformation. The immediate elastic response seems to reflect the immediate elastic properties of the collagen fibers, and the steady state elastic response reflects the properties of the corneal matrix.
Since awareness of corneal biomechanics; studies have renewed to measure such a property. A particularly interesting technique has emerged during the last few years; Ocular Response Analyzer (ORA), that enables a dynamic measure for IOp including two applanation events, respectively. Theoretically, these two pressures should be the same, but this is not the case, which represents the resistance of the corneal tissue to applanation due to its visco-elastic properties. The difference between the inward and outward pressures is termed hysteresis and is measured in millimeters of mercury. It is a measure of: corneal damping capacity, visco-elasticity and energy absorption capability of cornea. This device also provides a pressure measurement that is significantly less affected by the cornea than other methods of tonometry. Hence, CH is a viscoelastic property that reflects the difference in behavior under loading and unloading. It is a func¬tion of corneal viscous damping properties, i.e. energy absorption capabilities.
These new parameters may be clinically useful in a number of different areas including: Identification of corneal diseases such as keratoconus and Fuchs’ dystrophy, glaucoma diagnosis and management, screening potential keratorefractive surgery candidates, and accurate IOp measurement.
As a pre-operative assessment, changes in CH due to many factors, diseases and previous ocular surgeries, were recorded. Some increasing, some decreasing, and others still doubtful, till now, as the following:
• Aging process lowers CH.
• In diabetic patients, the studies done, till now, were controversial.
• Chronic smoking increases CH.
• In twins, the monozygotic correlations were higher than dizygotic for CH.
• In corneal diseases, as keratoconus and fuchs, CH decreases significantly.
• CH 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.
• In soft contact lens wearers, CH was not changed nor associated with corneal swelling induced by the soft contact lens.
As a post-operative assessment, CH changes were recorded after many ocular surgeries, some increasing, some decreasing, and others still doubtful, till now, as the following:
• In the field of 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 trabeculectomy and IOp lowering therapy the CH increased and significantly correlated with the decrease in IOp.
• After insertion of corneal intra stromal rings, there were no significant changes in the CH.
• After collagen cross linking with UV-A-riboflavin, there were no significant differences in CH.
Assessment of corneal biomechanics, as CH and CRF, using the ORA, and documentation of the measured values, are important before any ocular surgical procedure, valuable in many directions, as detecting the occult diseases were proved to affect the corneal biomechanics in their early stages. Also they will be of extreme importance in decision making for the appropriate surgical manneuver, or shifting from a planned manneuver to another for the safe of the patient’s eye, and they will help us to compare the pre-operative findings with the post-operative results to catch the exact changes in corneal tissues by the surgery.
Finally, we must note that all studies, done till now, are tailed ” for further, long term studying and investigations” .