Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Figure 1.20 shows amount of elevation (color-coded) of the posterior corneal surface in microns (50 µm).

AGARWAL CRITERIA TO DIAGNOSE PRIMARY

POSTERIOR CORNEAL ELEVATION

1.Ratio of the Radii of anterior and posterior curvature of the cornea should be more than 1.2. In Figure 1.16 note the radii of the anterior curvature is 7.86 mm and the radii of the posterior curvature is 6.02 mm. The ratio is 1.3.

2.Posterior best fit sphere should be more than 52 D. In Figure 1.16 note the posterior best fit sphere is 56.1 D.

3.Difference between the thickest and thinnest corneal pachymetry value in the 7 mm zone should be more than 100 microns. The thickest pachymetry value as seen in Figure 1.16 is 651 microns and the thinnest value is 409 microns. The difference is 242 microns.

4.The thinnest point on the cornea should correspond with the highest point of elevation of the posterior corneal surface. The thinnest point as seen in Figure 1.16 bottom right picture is seen as a cross. This point or cursor corresponds to the same cross or cursor in Figure 1.16 top right picture which indicates the highest point of elevation on the posterior cornea.

5.Elevation of the posterior corneal surface should be more than 45 microns above the posterior best fit sphere. In Figure 1.16 you will notice it is 0.062 mm or 62 microns.

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CHAPTER 1: ORBSCAN

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Iatrogenic keratectasia may be seen in some patients following ablative refractive surgery (Figs 1.21 and 1.22). The anterior cornea is composed of alternating collagen fibrils and has a more complicated interwoven structure than the deeper stroma and it acts as the major stress-bearing layer. The flap used for Lasik is made in this layer and thus results in a weakening of that strongest layer of the cornea which contributes maximum to the biomechanical stability of the cornea.

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 mm to avoid subsequent iatrogenic keratectasias. Reoperations should be undertaken very carefully in corneae with RBT less than 300 mm. Increasing myopia after every operation is known as “dandelion keratectasia”.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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CHAPTER 1: ORBSCAN

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 [mm] = 1/3. (diameter [mm])2 x (intended correction diopters [D]), it becomes clear that to preserve a sufficient bed thickness, the range of myopic correction is limited and the upper limit of possible myopic correction may be around 12 D.

Detection of a mild keratectasia requires knowledge about the posterior curvature of the cornea. Posterior corneal surface topographic changes after Lasik are known. Increased negative keratometric diopters and oblate asphericity of the PCC, which correlate significantly with the intended correction are common after Lasik leading to mild keratectasia. This change in posterior power and the risk of keratectasia was more significant with a RBT of 250 µm or less. The difference in the refractive indices results in a 0.2 D difference at the back surface of the cornea becoming equivalent to a 2.0 D change in the front surface of the cornea. Increase in posterior power and asphericity also correlates with the difference between the intended and achieved correction 3 months after Lasik. This is because factors like drying of the stromal bed may result in an ablation depth more than that intended. Reinstein et al predict that the standard deviation of uncertainty in predicting the RBT

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

preoperatively is around 30 µm. [Invest Ophthalmol Vis Sci 40 (Suppl):S403, 1999]. Age, attempted correction, the 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 upto 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.

EFFECT OF POSTERIOR CORNEAL CHANGE ON

IOL CALCULATION

IOL power calculation in post-Lasik eyes is different because of the inaccuracy of keratometry, change in anterior and

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CHAPTER 1: ORBSCAN

posterior corneal curvatures, altered relation between the two and change in the standardized index of refraction of the cornea. Irregular astigmatism induced by the procedure, decentered ablations and central islands also add to the problem.

Routine keratometry is not accurate in these patients. Corneal refractive surgery changes the asphericity of the cornea and also produces a wide range of powers in the central 5 mm zone of the cornea. Lasik makes the cornea of a myope more oblate so that keratometry values may be taken from the more peripheral steeper area of the cornea, which results in calculation of a lower than required IOL power resulting in a hyperopic “surprise”. Hyperopic Lasik makes the cornea more prolate, thus resulting in a myopic “surprise” post cataract surgery.

Post-PRK or Lasik, the relation between the anterior and posterior corneal surface changes. The relative thickness of the various corneal layers, each having a different refractive index also changes and there is a change in the curvature of the posterior corneal surface. All these result in the standardized refractive index of 1.3375 no longer being accurate in these eyes.

At present there is no keratometry, which can accurately measure the anterior and posterior curvatures of the cornea. The Orbscan also makes mathematical assumptions of the posterior surface rather than direct measurements. This is

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

important in the Lasik patient because the procedure alters the relation between the anterior and posterior surfaces of the cornea as well as changes the curvature of the posterior cornea.

Thus direct measurements such as manual and automated keratometry and topography are inherently inaccurate in these patients. The corneal power is therefore calculated by the calculation method, the contact lens overrefraction method and by the CVK method. The flattest K reading obtained by any method is taken for IOL power calculation (the steepest K is taken for hyperopes who had undergone Lasik). One can still aim for -1.00 D of myopia rather than emmetropia to allow for any error, which is almost always in the hyperopic direction in case of pre Lasik myopes. Also, a third or fourth generation IOL calculating formula should be used for such patients.

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