الفهرس 
CorneaOnly 14 pages are availabe for public view
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Abstract
Penetrating Keratoplasty (PK) is one of the most successful tissue transplants compared with transplantation of other tissues. Advances in microsurgical techniques have allowed the indications for keratoplasty to expand considerably. Although recently developed anterior and posterior lamellar procedures such as deep anterior lamellar keratoplasty, deep lamellar endothelial keratoplasty, and descemet’s stripping automated endothelial keratoplasty are evolving and gaining popularity, pkp still remains the primary surgical approach for extremely pathologic and deformed corneas.
Penetrating Keratoplasty (PK) graft central thickness is relevant measurement because of the direct relationship between corneal hydration and corneal thickness. Corneal thickness stabilizes by 3 months after PK. Central Corneal Thickness (CCT) is a predictor for outcome of PK. The greater the increase in thickness, the more likely the graft would fail. For this purpose, imaging of the anterior segment may play an important role in recognizing changes that may be otherwise unnoticeable during routine ophthalmological examination, and it may also give additional information to slit-lamp findings. Parallel to development of surgical technique in keratoplasty surgery, the accurate measurement of corneal topography, anterior chamber depth, and eye length has gained importance.
Pachymetry refers to the measurement of corneal thickness. It can used to assess corneal hydration and the corneal endothelial cells function in its dual role as a barrier to aqueous humor and as a metabolic pump. When functioning normally, the endothelial pump balances the leak rate to maintain the corneal stromal water content at 78% and the central corneal thickness (CCT) at " ~ "540 μm that is considered as the normal CCT.
The optical zone is the portion of the cornea that overlies the entrance pupil of the iris. The area of maximum curvature typically temporal to the center of the pupil is named corneal apex. The point located at the intersection of the patient’s line of fixation and the corneal surface is called the corneal vertex, which is represented by the corneal light reflex when the cornea is illuminated coaxially with fixation. The corneal vertex does not necessarily correspond to the corneal apex.
Until recently, ultrasound pachymetry was the most commonly used clinical method to measure CCT, and it is widely considered as the gold standard method for the measurement of corneal thickness. Although the required probe in contact with the cornea is associated with patients’ discomfort, the requirement for topical anesthesia, the risk of epithelial lesion (abrasion), and the risk of transmission of infections lead to preference of noncontact methods such as the optical coherence tomography, scanning slit pachymetry, specular microscopy, and corneal confocal microscopy.
Pentacam‐Scheimpflug uses the Scheimpflug principle in order to obtain images of the anterior segment. The Scheimpflug principle describes the optical properties involved in the photography of objects when their plane is not parallel to the film of the camera. It requires that the plane containing the slit beam and the image plane intersect at one point, with the corresponding angles equal.
Software is then used to construct a three‐dimensional image. A second camera captures eye movements and makes appropriate corrections. It calculates data for corneal topography (anterior and posterior corneal surface) and thickness, anterior chamber depth (ACD), lens opacification and lens thickness. It also provides data on corneal wave front of the anterior and posterior corneal surface using Zernike polynomials.
A Scheimpflug system images the anterior segment with a camera perpendicular to a slit beam, creating an optical section of the cornea and lens. The images taken during the examination are digitized in the main unit, and all image data is transferred to a computer interface bus. When the examination is finished, the computer calculates a 3-dimensional virtual model of the anterior segment, from which all additional information is derived. Approximately 25,000 data points are used to calculate topographic corneal thickness, corneal curvature, anterior chamber angle, volume, and height. Pachymetry and topography of the entire anterior and posterior surface of the cornea from limbus to limbus are calculated and displayed. Opacity in cornea and lens may be quantified. Rotating and dual-camera Scheimpflug imaging systems are in clinical use today for anterior segment imaging.