الفهرس 
Corneal anatomy and physiology.Only 14 pages are availabe for public view
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Abstract
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye’s total optical power. In humans, the refractive power of the cornea is approximately 43 dioptres.
Keratoconus (from Greek: kerato- horn, cornea; and konos cone), is a disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve. The cause of keratoconus is unknown. It usually appears in a patient during late teens or early twenties. The disease slowly progresses for 10 to 20 years as the cornea steepens and scars. Although both eyes may be affected, one eye is usually worse than the other. Vigorous eye rubbing can add to the disease process, therefore patients with keratoconus are advised to avoid rubbing their eyes.
Symptoms of keratoconus are frequent changing of glasses or contact lens prescriptions, blurring and distortion of vision, glare, light sensitivity and irritation.
Corneal topography is often used clinically for detecting and evaluating the severity of keratoconus. In clinical practice, the topographic diagnosis of keratoconus is often suggested by high central corneal power, large difference between the power of the corneal apex and of the periphery, and disparity between the 2 corneas of a given patient.
The principal approaches to the management of keratoconus have been limited, until recent years, to the use of spectacles, rigid gas permeable contact lens and, in more severe cases, to corneal surgery namely penetrating keratoplasty. Several alternative surgical techniques had been proposed such as deep anterior lamellar keratoplasty. In recent years, two procedures have been designed to manage keratoconus and other ectatic disorders: intrastromal corneal rings and corneal collagen cross-linking.
A recent surgical alternative to corneal transplant is the insertion of intrastromal corneal ring segments. A small incision is made in the periphery of the cornea and two thin arcs of polymethyl methacrylate are slid between the layers of the stroma on either side of the pupil before the incision is closed. The segments push out against the curvature of the cornea, flattening the peak of the cone and returning it to a more natural shape. The procedure offers the benefit of being reversible and even potentially exchangeable as it involves no removal of eye tissue.
The intrastromal rings available are Intacs and Ferrara Rings. Intacs are a patented technology and are placed outside the optical zone versus the smaller prismatic Ferrara rings that are placed just inside the 5 mm optical zone. Intacs were first approved for the treatment of keratoconus in July 2004. In common with penetrating keratoplasty, the requirement for some vision correction in the form of spectacles or hyDROPhilic contact lenses may remain subsequent to the operation. Potential complications of intrastromal rings include accidental penetration through to the anterior chamber when forming the channel, post-operative infection of the cornea, and migration or extrusion of the segments. Early studies on intrastromal corneal rings involved use of two segments to cause global flattening of the cornea. A later study reported that better results could be obtained for those cones located more to the periphery of the cornea by using a single Intacs segment. This leads to preferential flattening of the cone below, but also to steepening the over-flat upper part of the cornea
There is a new procedure that addresses primarily the pathophysiology of keratoconus and this is riboflavin UV-A rays induced cross-linking. Cross-linking of the cornea is a procedure that can increase the ties or chemical bonds between the fibers of the corneal collagen by means of a highly localized photo-polymerization using UV-A light and photosensitizer riboflavin DROPs. Riboflavin (Vitamin B2) has a dual function of acting as a photosensitizer for the production of oxygen free radicals, which induce physical cross linking of collagen, and it gives a ”shielding effect” by absorbing the UV-A irradiation (90%), thereby preventing damage to deeper ocular structures. UV-A light of 370 nm wavelength at 3 mW/cm 2 allows approximately 95% of the UV light to be absorbed into the cornea; thus there is no risk for damage to the lens and retina. Collagen cross-linking is the only treatment that deals with not only the refractive effects of the condition but the underlying pathophysiology.
If the treatment with CXL stops or slows the progression of keratoconus, while intracorneal rings reshape the cornea, a logical solution would be to combine the two treatment methods in order to synergize their effects.
Evidence suggests that the most effective sequence of procedures is placement of the intrastromal corneal segments (ICS) first and that the interval between the two steps (ICS and CXL) should be short, perhaps during the same session.