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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

meridional curvature continually increases. The optical surfaces of the normal human eye both cornea and lens is prolate. This shape has an optical advantage in that spherical aberration can be avoided. Following LASIK the prolateness of the anterior cornea reduces but is insufficient to eliminate its spherical aberration. Thus one should remember the normal cornea is prolate. When myopic LASIK is done the cornea becomes oblate. When hyperopic LASIK is done the cornea becomes prolate.

Every patient treated with an excimer laser is left with an oblate or prolate shaped cornea depending upon the myopia or hyperopia of the patient. The approach to improve visual quality after LASIK is to apply geometric optics and use the patient’s refraction, precise preoperative corneal height data and optimal postoperative anterior corneal shape in order to have a customized prolate shape treatment.

TECHNIQUE

First of all a superficial corneal flap is created with the microkeratome. The corneal flap performed with the microkeratome must be between 8.5 to 9.5 mm in order to have an available corneal surface for treatment of at least 8 mm. In this way, the laser beam does not touch the hinge of the flap. In India the Bausch and Lomb Lasik machine is

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CHAPTER 9: PRESBYOPIC LASIK

FIGURE 9.1: Hyperopic lasik done on the cornea.

Myopic prolate cornea produced

used and in Mexico the Apollo machine is used. Once the flap has been created a hyperopic ablation in an optical zone of 5 mm is done (Fig. 9.1). The treated cornea now has a steepness section. The cornea is thus myopic, prolate. This allows the eye to focus in a range that includes near vision but excludes far vision.

With this myopic-shaped cornea, one now selects a smaller area of the central cornea that is concentric with

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 9.2: Myopic lasik done. Myopic ablation of 4 mm optical zone performed to create a central oblate cornea

the previous worked area. The size of the area is a 4 mm optical zone. A myopic LASIK is now done with the 4 mm optical zone (Fig. 9.2).

The resulting cornea now has a central area (oblate) that is configured for the eye to focus on far objects and a ring shaped area that allows the eye to focus on near objects (Fig. 9.3). The flap is now cleaned and replaced back in position.

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CHAPTER 9: PRESBYOPIC LASIK

FIGURE 9.3: Schematic diagram of a presbyopic cornea in which hyperopic and myopic lasik has been done. The patient can thus focus for near and distance

KERATOMETRY AND PACHYMETRY

Pachymetry is not important for this procedure. The preoperative keratometry reading is extremely important. The postoperative keratometer reading should not exceed 48 D. The keratometer reading should be taken from topography and not from a manual keratometer machine. For each hypermetropic dioptre corrected, the corneal curvature increases in 0.89 keratometric dioptres as an

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

average. It is recommended to treat patients with keratometry in the range between 41 to 43 D to obtain postoperative curves under 48 D. If the cornea is more than 48 D, it produces undesired optical alterations like glare, halos, decreased visual acuity and decreased contrast sensitivity. The preoperative and postoperative keratometer readings should be nearly the same for the patient to be comfortable.

If astigmatism is present, it is recommended to use as a limit 2.5 D. One should also remember there is an induced astigmatism of 0.5 to 0.75 D created by the corneal shape after the surgery and this can decrease one or two lines of uncorrected visual acuity.

Plano Example

Now let us look at treating presbyopic patients who are basically plano for distance.

Example 1

Let us take a patient who is plano for distance and is 20/ 20. For near on addition of + 2 D the patient is J1. The preoperative keratometer let us say is 41 D.

There are three steps in the presbyopic LASIK treatment.

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CHAPTER 9: PRESBYOPIC LASIK

Step 1 For distance—No treatment is required as the patient is plano 20/20

Step 2 For near—Hyperopic LASIK is done of + 2 D. A 5 mm optical zone is taken. We have already mentioned that each dioptre of hyperopia corrected changes the corneal curvature by 0.89 D, which is approximately 1 D. So the keratometer changes from 41 to 43 D (approximately)

Step 3 Myopic LASIK of –1 D with a 4 mm optical zone. So keratometer now becomes 42 D.

Regression occurs for hyperopia treatment to about 1 D, so we have done myopic ablation of –1 and not –2 D. The preoperative keratometer reading was 41 D and postoperative keratometer reading is 42 D, which is nearly the same.

Hyperopic Examples

Now let us look at presbyopic LASIK being performed in a hyperopic eye.

Example 2

Let us take a patient who is hyperopic for distance and is 20/20 with + 1 D. For near on addition of + 3 D the patient is J1. The preoperative keratometer let us say is 42 D.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

There are three steps in the presbyopic LASIK treatment.

Step 1 For distanceHyperopic LASIK is done of +1 D with a 5 mm optical zone. So keratometer changes from 42 D to 43 D.

Step 2 For nearHyperopic LASIK is done of +3 D. A 5 mm optical zone is taken. We have already mentioned that each dioptre of hyperopia corrected changes the corneal curvature by 0.89 D, which is approximately 1 D. So the keratometer changes from 43 to 46 D (approximately)

Step 3 Myopic LASIK of minus 2 D with a 4 mm optical zone. So keratometer now becomes 44 D.

Regression occurs for hyperopia treatment to about 1 D, so we have done myopic ablation of –2 and not –3 D. The preoperative keratometer reading was 42 D but after making the patient plano it is 43 D. The postoperative keratometer reading is 44 D, which is nearly the same.

Though we have to correct totally 4 D for hypermetropia we take it in two steps. One should not do it in one step as that much hyperopia corrected in one step makes the central cornea too steep to perform the myopic ablation.

Example 3

Let us take a patient who is hyperopic for distance and is 20/20 with + 3 D. For near on addition of + 3 D the patient is J1. The preoperative keratometer let us say is 44 D.

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CHAPTER 9: PRESBYOPIC LASIK

The preoperative keratometer reading is 44 D and we have to correct 3 D for distance and 3 D for near. So if we do presbyopic Lasik we will make the keratometer reading 50 D. So, one should not treat such patients with presbyopia LASIK.

Myopic Example

Now let us look at myopic patients.

Example 4

Let us take a patient who is myopic for distance and is 20/ 20 with –2 D. For near on addition of + 2 D the patient is J1. This means the patient is plano for near. The pre-operative keratometer let us say is 43 D.

There are three steps in the presbyopic LASIK treatment.

Step 1 For distancePatient is myopic so no treatment is required.

Step 2 For nearHyperopic LASIK is done of +2 D. A 5 mm optical zone is taken. We have already mentioned that each dioptre of hyperopia corrected changes the corneal curvature by 0.89 D, which is approximately 1 D. So the keratometer changes from 43 to 45 D (approximately)

Step 3 Myopic LASIK of –3 D with a 4 mm optical zone. So keratometer now becomes 42 D.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Regression occurs for hyperopia treatment to about 1 D, so we have done myopic ablation of –3 and not –4 D. The preoperative keratometer reading was 43 D but patient was myopic by 2 D, so actually the keratometer reading should be 41 D. The postoperative keratometer reading is 42 D, which is nearly the same.

We did myopic ablation of 3 D, as patient is myopic of 2 D and presbyopic of 2 D. Regression factor taken is 1 D.

Figures 9.4 and 9.5 show the pre and postoperative corneal topography in a patient in whim presbyopic Lasik

FIGURE 9.4: Preoperative topography of a patient before presbyopic lasik

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CHAPTER 9: PRESBYOPIC LASIK

FIGURE 9.5: Postoperative topography of a patient after presbyopic lasik

has been done. This patient was 20/20 and J1 postoperatively without glasses.

Figure 9.6 shows another case in which presbyopic lasik was done. The picture on the left is the preop picture and the one on the right is the postoperative picture.

Figure 9.7 is the same patients topographic photos showing pre and post surgery in a 3 D pattern. This idea of

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