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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.18: Pre-enhancement axial curvature map, showing significant decentration of the ablation superior-temporally. (Courtesy of Dr. Bing Liu, General Air Force Hospital)

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CHAPTER 8: DECENTERED ABLATION

FIGURE 8.21: Postenhancement axial curvature map showing the decentration has been completely corrected. (Courtesy of Dr. Bing Liu, General Air Force Hospital)

corrected. Figure 8.22 shows the pre- (left) and postoperative (right) elevation maps. The difference map can be seen in Figure 8.23.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.22: Pre-enhancement and postenhancement axial elevation map. Note that when the vertex normal is not changed (as in this figure), the elevation map of the decentered ablation shows elevation where the cornea was actually ablated for a myopic treatment. This is because the area becomes flatter. Compare this map to the lower left corner of Figure 8.20, where the corneal-vertex normal is optimized (Target Z-axis). Using the optimized axis gives an elevated area where no adequate ablation was applied, consistent with the optimized enhancement ablation profile as indicated in Figure 8.20, upper right. (Courtesy of Dr. Bing Liu, General Air Force Hospital)

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CHAPTER 8: DECENTERED ABLATION

FIGURE 8.23: Pre-enhancement and postenhancement axial elevation difference map, showing pre-enhancement elevation (upper right), postenhancement elevation (lower right) and elevation difference (left), using a reference sphere of 7.85 mm. (Courtesy of Dr. Bing Liu, General Air Force Hospital)

CONCLUSION

When a patient with a history of refractive surgery presents with loss of best-corrected visual acuity and expresses

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

symptoms of glare, night vision difficulty, and monocular diplopia, decentered ablation needs to be ruled out. Understanding of elevation topography, and aberrometry is crucial for the diagnosis. Modern surgical options are presently evolving and gaining success in addressing this complication to improve visual quality in postrefractive eyes.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

INTRODUCTION

Presbyopia, is the final frontier for an ophthalmologist. In the 21st century the latest developments, which are taking place, are in the field of Presbyopia. In presbyopia, the nearest point that can be focused gradually recedes, leading to the need for optical prosthesis for close work such as reading and eventually even for focus in the middle distance. Presbyopic photorefractive keratectomy (PRK) has been tried. In this using the excimer laser, a mask consisting of a mobile diaphragm formed by two blunt blades was used to ablate a 10-17 micron deep semilunarshaped zone immediately below the papillary center, steepening the corneal curvature in that area.

Monofocal vision with LASIK has also been tried to solve the problem of presbyopia. The goal in such cases is to make the patient anisometropic. In this one eye is used for distance vision and the other for near vision. This is obviously not indicated in all subjects. The residual consequences are partial loss of stereopsis, asthenopia, headache, aneisokonia and decreased binocularity.

Guillermo Avalos1,2 started the idea of presbyopic LASIK. This is called the PARM technique. He held a live surgical conference in Mexico where he had invited the Agarwals to perform Phakonit and the no-anesthesia cataract surgery technique. There he discussed with them

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CHAPTER 9: PRESBYOPIC LASIK

the idea of Presbyopic Lasik and when they came back they started the technique.

PRINCIPLE

The objective is to allow the patient to focus on near objects while retaining his ability to focus on far objects, taking into account the refractive error of the eye when the treatment is performed. With this LASIK technique the corneal curvature is modified, creating a bilateral multifocal cornea in the treated optical zone. A combination of hyperopic and myopic LASIK is done aiming to make a multifocal cornea. We determine if the eye is presbyopic plano, presbyopic with spherical hyperopia or presbyopia with spherical myopia. These may also have astigmatism in which case the astigmatism is treated at the same time.

PROLATE AND OBLATE CORNEA

It is important for us to understand a prolate and oblate cornea before we progress further on the technique of Presbyopic LASIK. The shape of spheroid (a conoidal surface of revolution) is qualitatively prolate or oblate, depending on whether it is stretched or flattened in its axial dimension. In a prolate cornea the meridional curvature decreases from pole to equator and in an oblate cornea the

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