الفهرس 
Anatomy Of The CorneaOnly 14 pages are availabe for public view
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Abstract
Keratoconus and pellucid marginal degeneration (PMD) are progressive corneal degenerations characterized by asymmetric bilateral corneal thinning in most cases. The etiology of KC includes degeneration of epithelial cells followed by breaks in the Bowman layer, releasing degradative enzymes. These enzymes cause an increase in collagenase activity, thus breaking down stromal collagen and resulting in stromal thinning.
Pellucid marginal degeneration etiology is not well established, but collagen abnormalities are also present.
Both conditions result in corneal distortion and, therefore, reduced vision. Because of the irregular astigmatism, these patients cannot be adequately corrected with spectacles or soft contact lenses.
Historically, management of Keratoconus and PMD was limited to gas-permeable contact lenses followed by penetrating keratoplasty at the advanced stages. However, penetrating keratoplasty is associated with significant risks, including intraocular surgery risk, endothelial cell loss during the procedure, long period of rehabilitation, prolonged use of steroids and glaucoma risk, anisometropia, irregular astigmatism, and graft rejection.
To address the corneal wall weakening of these ecstatic conditions, procedures have been developed to treat Keratoconus and PMD by reinforcing the strength of the cornea. These procedures include epikeratoplasty and lamellar keratoplasty.
Intracorneal ring segments (Intacs; Intacs Addition Technology, Inc, Des Plaines) represent a new treatment of Keratoconus, PMD and iatrogenic postoperative corneal ectasias by both tissue-additive and tissue tenting effects. The expansion of the segment reshapes the cornea to stabilize the disease, enable comfortable correction, and achieve better vision with glasses or contact lenses. The placement and dimensions of the Intacs implants help to reshape the cornea normalizing the cornea’s architecture caused by keratoconus.
Intacs ring implantation in eyes with keratoconus was first performed in 1997. The Food and Drug Administration (FDA) granted limited approval with a Humanitarian Device Exemption for the placement of Intacs in patients with keratoconus in July 2004. The FDA approved the Intacs procedure for patients older than 20 years old who have experienced progressive deterioration in their vision, have clear central corneas with a minimal thickness of 450 μm at the proposed incision site, and for whom corneal transplantation is the only remaining option to improve visual function.
Early studies, specifically those performed before Humanitarian Device Exemption approval, used the manual channel makers. This technique has a higher risk of perforation or need for removal of segments because of poor positioning. To avoid these issues, the lamellar channel can be created using a femtosecond laser.
The principle of the IntraLase femtosecond laser is to accomplish surgery with little collateral damage. This ultrashort-pulsed laser allows for non-thermal laser-tissue interaction, called photodisruption, through very small pulse energies.
Tissue resection is achieved by precise placement of microphotodisruptions scanned at high repetition rates controlled by the computer.