Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009

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The residual bed thickness (RBT) of the cornea is the crucial factor contributing to the biomechanical stability of the cornea after LASIK. The flap as such does not contribute much after its repositioning to the stromal bed. This is easily seen by the fact that the flap can be easily lifted up even up to 1 year after treatment. The decreased RBT as well as the lamellar cut in the cornea both contribute to the decreased biomechanical stability of the cornea. A reduction in the RBT results in a long-term increase in the surface parallel stress on the cornea. The intraocular pressure (IOP) can cause further forward bowing and thinning of a structurally compromised cornea. Inadvertent excessive eye rubbing, prone position sleeping, and the normal wear and tear of the cornea may also play a role. The RBT should not be less than 250 μm to avoid subsequent iatrogenic keratectasias. Reoperations should be undertaken very carefully in corneae with RBT less than 300 μm. Increasing myopia after every operation is known as “dandelion keratectasia”.

The ablation diameter also plays a very important role in LASIK. Postoperative optical distortions are more common with diameters less than 5.5 mm. Use of larger ablation diameters implies a lesser RBT postoperatively. Considering the formula: Ablation depth [μm] = 1/3.

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optical zone diameter and the flap thickness are other parameters that have to be considered to avoid post-LASIK ectasia.

The flap thickness may not be uniform throughout its length. In studies by Seitz et al, it has been shown that the Moris Model One microkeratome and the Supratome cut deeper towards the hinge, whereas the Automated Corneal Shaper and the Hansatome create flaps that are thinner towards the hinge. Thus, accordingly, the area of corneal ectasia may not be in the center but paracentral, especially if it is also associated with decentered ablation. Flap thickness has also been found to vary considerably, even up to 40 µm, under similar conditions and this may also result in a lesser RBT than intended. It is known that corneal ectasias and keratoconus have posterior corneal elevation as the earliest manifestation. The precise course of progression of posterior corneal elevation to frank keratoconus is not known. Hence, it is necessary to study the posterior corneal surface preoperatively in all LASIK candidates.

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Figure 1.11C

Figure 1.11D

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Figure 1.11G

Figure 1.11H

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Scheimpflug imaging was patented by Theodor Scheimpflug in 1904 after he diskovered that when the planes within a camera intersect rather than be placed in parallel, the depth of focus is extended. In a typical camera, three imaginary surfaces exist: the film plane, lens plane and sharp image plane. These are parallel to each other such that the image of the object placed in the plane of sharp focus will pass through the lens plane perpendicular to the lens axis, and fall on to the film plane. The depth of focus is limited in such a camera. Figure 1.11B shows a Scheimpflug image of a flap tear. Thinning is seen secondary to loss of tissue where the flap was rotated away from the bed.

In a Scheimpflug camera, the three planes are not parallel but intersect in a line, called the “Scheimpflug line”. When the lens is tilted such that it intersects the film plane, the plane of sharp focus also passes through the Scheimpflug line, extending the depth of focus. Note that this results in mild image distortion, which is then corrected by the Pentacam system. A two-dimensional cross-sectional image results. When performing a scan, two cameras are used to capture the image. One centrally located camera detects pupil size and orientation, and controls fixation. The second rotates 180 degrees to