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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

basis treatment with excimer laser.26 For these patients, and for those suffering an irregular astigmatism after trauma or refractive surgery, a custom-tailored, topo- graphy-based ablation, which has been adapted to the corneal irregularity, would be the best approach to improve not only their refractive problem but also to improve their quality of vision.

This treatment was the first step in customized ablation depending mainly on the corneal topography as well as the refraction for calculating the treatment. It aimed at obtaining the best corrected visual acuity that can be attained by wearing hard contact lenses. Its requirements were an excimer laser with spot scanning technology, in which a small laser spot delivers a multitude of single shots fired in diverse positions to fashion the desired ablation profile. The laser spot is programmable, thus any profile could be obtained. A videokeratography system that provides an elevation map at high resolution is needed, and specific software is used to create a customized ablation program for the spot scanner laser.

Methods

The aim of this study was to fashion a regular corneal surface in 41 eyes of 41 patients with irregular astigmatism induced by LASIK: 27 eyes (51,9%) had irregular

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astigmatism with a defined pattern; 14 eyes (48,1%) had irregular astigmatism without a defined pattern.

All cases were treated with a Plano Scan Technolas 217 C-LASIK Scanning-spot Excimer laser (Bausch & Lomb, Chiron Technolas GmbH, Doranch, Germany) assisted by a C-SCAN Color-Ellipsoid-Topometer (Technomed GmbH, Germany). We performed several corneal topographies from same eye; the software of the automated corneal topographer selected the four exactly equal. These corneal maps, the refractive error, the pachymetry value and desired k-readings calculated for each patient were sent to Technolas by modem. The information was analyzed and a special software program for each patient was created, including it in the Technolas 217 C-LASIK excimer laser by system modem.

The basis for the topography-assisted procedure was the preoperative topography.12,27 This data was transferred into true height data and the treatment for correcting the refractive values in sphere and astigmatism, taking into account the corneal irregularities, was calculated. After that, a postoperative topography was simulated. With this technique, real customized treatment should become a reality, not only treating the refractive error but also improving the patient’s visual acuity.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Results

After 3 months of the surgery: The mean preoperative UCVA improved from 20/80 ±0.25 (range 20/400 - 20/60) to 20/40±0.54 (range 20/100 - 20/32); mean preoperative BCVA improved from 20/60±0.20 (range 20/200 - 20/32) to 20/32±0.15 (range 20/60 - 20/25). This proved to be statistically significant (p<0.001).

Even though emmetropia was our goal, it was considered more important to achieve a regular corneal surface. The spherical equivalent of the individual refraction was taken into account in determining the corneal k-value. Preoperatively, mean sphere was -0.26 ±4.50 D (range –5.75 to +3.70 D) and mean cylinder was – 1.71±3.08 D (range –6.00 to +2.56 D). Three months after surgery, the mean sphere was 0.70±1.25 D (range –1.75 to +1.50 D) and the mean cylinder was -0.89±1.00 D (range –1.92 to +1.00).

Corneal topography improved significantly in those cases that presented an irregular astigmatism with a defined pattern. The mean Corneal Surface Quality improved from 45% (range 35% - 60%) to 76.6% (range 60.06% - 96.43%). The corneal surface is left smooth and the RayTracing improved in the peak distortion, coinciding with the improvement of the visual acuity (Figs 7.5A and B). In 60.29% patients the visual aberrations disappeared.

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FIGURE 7.5A

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.5B

FIGURES 7.5A AND B: Topolink: Preoperative (A) and postoperative

(B) corneal topography with raytracing

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At 3 months of follow up, the safety of the procedure was 74.31%, the efficacy (Fig. 7.6) in%UCVA 20/40 was 63.68% and the predictability for the spherical equivalent within the ± 1D zone was 68.23%.

Discussion

Using the corneal topographic map as a guide, excimer laser ablation can be used to create a more regular surface with improved visual acuity. In a program consisting of a combination of phototherapeutic and photorefractive ablation patterns, the amount of tissue to be removed is calculated on the basis of the diameter and steepness of the irregular areas of the corneal surface. At present, customized ablation based on topography can improve spectacle-corrected visual acuity.

Limitations for this technique exist. With this procedure some irregular astigmatisms cannot be corrected. Some patients could not be selected as candidates for Topolink because any of the following criteria were present:

1.Different between steep and flat meridians more than 10D at the 6.0 mm treatment area.

2.Corneal pachymetry was not thick enough (< 400 mm).

3.Diameter of the corneal topography more than 5.0 mm.

4.Corneal topography showing an irregular astigmatism with undefined pattern (irregularly irregular).

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.6: Safety of the Topolink procedure at 3 months

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This preliminary study showed that topographicassisted LASIK (Topolink) could be a useful tool to treat irregular astigmatism. This technique was, as aforementioned, the early stage of developing customized ablation. The surgeon depends only on the Placido topographic images, their precision and their reproducibility. To the moment, this cannot provide us with the actual customization and we are still left with some patients waiting for a solution.

The Future

A new view of customization could be achieved with more reliable instruments (elevation topography, aberrometer, etc.). As aforementioned, wavefront analysis (aberroemtery, Fig. 7.7) can measure the refractive state of the entire internal ocular light path 8. Using this technology, it has been shown that using only the refractive error of the eye to treat the ammetropia can greatly increase optical aberrations within the eye.28 Increases in wavefront aberrations are evident after both PRK and LASIK,29 and increased spherical aberration has been shown to occur in cases of increased corneal astigmatism.30 This increase in spherical aberration and coma will interfere with visual function, particularly in low-light conditions where the pupil size increases, increasing the effect of aberrations

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 7.7: Aberrometry, Clinical example

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within the eye, a condition that is diminished in daylight where the pupil constricts.31

We are now conducting the second phase of a study incorporating the data of the wavefront analysis using the ZyWave aberrometer (Bausch and Lomb, CHIRON Technolas GmbH, Doranch, Germany) together with the elevation topography of the Orbscan II (Orbtek, Bausch and Lomb Surgical, Orbscan II corneal topography, Salt Lake City, Utah, USA) to correct ametropia.

To the moment the system is under trial, and is only applicable to regular virgin corneas. With the proper development of the technique, we think that it would provide us with the real customized ablation necessary not only for our desperate irregular astigmatism patients but also for obtaining a super vision for ametropes who are to be treated for the first time.

Other Procedures

Automated Anterior Lamellar Keratoplasty

This technique was originally designed to treat superficial stromal disorders, but it has also been used for the treatment of difficult cases of irregular astigmatism, with very poor results.32 The surgeon performs phototherapeutic keratectomy or a microkeratome lamellar resection to 250-400 μm

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