الفهرس 
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Abstract
The demand on refractive surgeries is increasing every day because it offers marvelous results, but the fact that mostly it’s an elective surgery puts the safety of the procedure as the most important requirement.
Iatrogenic ectasia is one of the most important complications of the excimer laser procedures which are the commonest used procedures, knowing its properties and the factors leading to this sight threatening complication is very important.
Corneal Topography, Pachymetry and Pentacam are widely used before the procedures not only in screening whether the subject is fit for the procedure or not, but also as an important parameter in the excimer laser console.
But every now and then there is a border line case where there are different opinions about whether the procedure is safe or not, this grey zone needs to become clearer, many attempts were done but none of them proved to be able to stand alone and give a clear opinion, one of them is the corneal biomechanics measurement.
Corneal hysteresis is the difference between the inward and outward pressure values obtained during the dynamic bidirectional applanation process employed in the ocular response analyzer, as a result of viscous damping in the cornea.
The factors affecting corneal hysteresis can be classified into anatomical factors which include the central corneal thickness and physiological factors which include corneal viscoelasticity, intraocular pressure and age.
Up till now there are two devices which can measure the corneal biomechanics, the first is the ocular response analyzer which is an instrument used to assess the corneal hysteresis, Goldmann correlated intraocular pressure, corneal-compensated intraocular pressure and corneal resistance factor, it can also measure the central corneal thickness. It is based on dynamic bidirectional applanation. The other device is the Oculus Corvis which can measure the deformation amplitude, the applanation length, corneal velocity, intraocular pressure measurement and corneal thickness.
With the increased rate of refractive surgeries, Goldmann applanation tonometer has no longer been considered the ‘corner stone’ for measuring the intraocular pressure. There is a positive correlation between intraocular pressure and corneal hysteresis in glaucomatous subjects.
Corneal hysteresis is related to progressive glaucomatous visual field loss. The measurement of corneal hysteresis provides valuable information for monitoring patients with glaucoma, normal tension glaucoma and ocular hypertension. It is also useful in appreciation of the risk to develop glaucoma in glaucoma suspect by the corneal effort staging system. Corneal hysteresis recovers partially after treatment of glaucoma medically and surgically
Some clinical conditions such as Fuchs’ corneal dystrophy, keratoconus and high myopia may induce lowering of corneal biomechanical properties and hence corneal hysteresis. But diabetes mellitus induces increase in corneal hysteresis by increasing collagen cross linking.
Measurement of corneal hysteresis assists in predicting which patients stand the greatest risk for postoperative ectasia. It helps, in conjunction with corneal topography and pachymetry, in patient selection. It also helps in refractive surgery procedure selection, by specific knowledge of the biomechanical response of the cornea to different procedures.