Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Chapter 38

Patient 3: Decentered ablation

This 36-year-old woman had LASIK in both eyes in 1998 and was referred to me because of a decentered ablation. The right eye was perfect, but she complained bitterly about permanent monocular diplopia and distorted halos in her left eye. A TopoLink LASIK was planned. The corneal topography taken prior to the TopoLink LASIK is shown in Figure 38-4, lower left, and Figure 38-5, lower right. A decentered myopic ablation is visible. The

ablation is decentered about 1.5 mm downwards and 1 mm temporally. We calculated a customized ablation based on the Orbscan II topographic map just described. The planned ablation pattern is shown in Figure 4, upper right. The scale is in m. The predicted outcome of corneal topography is shown in Figure 4, lower right (scale in diopters). I used the Hansatome to create a new flap with a thickness of 160 m and a diameter of 8.5mm (8.5mm suction ring). The surgery was uneventful. The ablation was

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In the post-keratoplasty group and in the post-trauma group, corrective cylinder was significantly reduced.The topographic success rate was defined as either the planned correction fully achieved or the attempted correction partially achieved (decrease of irregularity of more than 1 D on the differential map and / or increase of optical zone size by at least 1 mm). Success rate was highest (91%) in the decentered/small optical zones group, followed by the post-trauma group, which had a success rate of 83%. The lowest success rate was observed in the central island group, at 50%. Overall, 14 of the 29 eyes were reoperated (48%) due to regression of effect or undercorrection. The rate of reoperations was lowest in the decentered/small op-

tical zones group, at 36%, as compared to 50% in all other groups (Table 1).

These results demonstrate that TopoLink LASIK definitely works to significantly reduce irregularities in extremely irregular corneas. Results also showed that most eyes were undercorrected, which led us to adjust the algorithm. Finally, the problem of targeting the right spot on the cornea must be addressed. The results of group 4 (central islands) were poor, which suggests that we may not have hit the right target in these eyes with small and circumscribed irregularities. Ideally, the laser should be locked on a topographic map of the cornea prior to treatment, and that is what we are currently working on to improve results in these rare cases.

Table 1

Refraction, visual acuity, and corneal topography 12 months after TopoLink LASIK (UCVA: uncorrected visual acuity; SCVA: spectacle-corrected visual acuity; *: p = 0.01; **: p = 0.001)

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Results of TopoLink in Normal Eyes

In a prospective, non-comparative case series, we operated on 203 eyes of 203 patients between January 1999 and July 1999. Inclusion criteria were myopia of -1.00 to -12.00 D with or without astigmatism of up to -4.00 D. Patients were divided into two groups: Group 1 (low myopia) consisted of 114 patients with myopia of -1.00 to -6.00 D (mean, -3.83 +/-1.67 D), and astigmatism of 0 to -4.00 D (mean, -1.32 +/-1.06 D).

Group 2 (high myopia) consisted of 89 patients with myopia of -6.10 to -12.00 D (mean, -7.83 +/-1.38 D) and astigmatism of 0 to -3.50 D (mean, -1.06 +/-0.92 D).

No reoperations were performed in these series, and no complications occurred. Three months after surgery, 51 patients in the low myopia group and 40 patients in the high myopia group were available for follow-up. A total of 96.1% of patients in the low myopia group and 75% in the high myopia group were within +/-0.50 D of emmetropia. Uncorrected visual acuity was 20/20 or better in 82.4% of patients in the low myopia group and in 62.5% in the high myopia group; 20/25 or better in 98.0% in the low myopia group and in 70.0% of the high myopia group; and 20/40 or better in 100% of the low myopia group and 95.0% of the high myopia group. In low myopia, spectacle-corrected acuity at the higher levels improved as compared to preoperative values, and 13.7% (n=7) had a spectacle-corrected visual acuity of 20/12.5 or better. A total of 47.1% (n=24) saw 20/15 or better after TopoLink LASIK, as compared to the preoperative values of 5.9% (n=3) and 37.3% (n=19), respectively. Differences were statistically significant (p<0.01). However, when mean values (log scale) of spectacle-corrected visual acuity were compared, differences were not statistically significant (p=0.2).

The larger percentage of patients seeing 20/12.5 or 20/15 3 months postoperatively than preoperatively in the low myopia group may indicate an improvement of spectacle-corrected visual acuity due to the customized LASIK. In the high myopia group, no improvement was observed, even though a one-line improve-

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Figure 38-6: Beta-version of the Bausch & Lomb Surgical Aberrometer. The chinand headrest are visible on the right.

lenses, a so-called lenslet-array, and the resulting picture is captured by a CCD-camera (Figure 38-7). Ideally, each of the bright white spots focused by each of the small lenses should have the same intensity and pattern. This would equal a plane wavefront, which means a perfect optical system. As most eyes are not perfect optical systems, the white spots will have different intensities and / or patterns, indicating deviations of the wavefront from plano. The deviations from plano are calculated based on the image captured by the CCD-camera, and the actual wavefront deviation is depicted graphically in color-

coded maps. Figure 8 shows an example of astigmatism. The spherical error was not included. The 3D graph shows the typical “potato chip” pattern of astigmatism. The wavefront deviation is expressed in m above or below the ideal plane wavefront. The measured deviation, simply multiplied by a constant, can be used to perform the laser ablation. As such, wavefront-deviation guided ablations seem a very logical choice. As always, actually performing the procedure is not as easy as it looks, and my description of aberrometer technology is considerably simplified.

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Chapter 38

REFERENCES

1.Knorz MC. Broad-beam versus scanning-beam lasers for refractive surgery. Ophthalmic Practice 1997;15:142-145

2.Wiesinger-Jendritza B, Knorz MC, Hugger P, et al. Laser in situ keratomileusis assisted by corneal topography. J Cataract Refract Surg 1998;24:166-174

3.Applegate RA, Howland HC. Magnification and visual acuity in refractive surgery. Arch Ophthalmol 1993;111:1335-1342

4.Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography. J Cataract Refract Surg 1999;25:663-669

5.Pallikaris IG. Quality of vision in refractive surgery. J Refract Surg 1998;14:551-558

Michael C. Knorz, M.D.

Klinikum Mannheim

Theodor Kutzer Ufer 1-3,

Mannheim, Germany

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Introduction

The technological advances in refractive surgery techniques in the past decade have been overwhelming. For the first time ever there is a plausible chance of using corneal topography and eye aberration data to develop algorithms for optimized excimer laser ablations. The main objective is to obtain the best possible visual acuity. The first excimer (from the word excited dimer) lasers for refractive surgery started to operate at mid 1980’s and could only correct simple cases of myopia. With the evolution of these lasers we can talk today about point by point corneal ablations with “flying spot” lasers and correction of many other corneal abnormalities, such as irregular astigmatism.

The next obvious question that came to mind at the end of this decade is: if we have a precise method for measuring the front surface of the cornea and a laser that can “mold” it into whatever shape desired, what’s missing? Why are we not quite there yet? The answer is that there are still lots of aspects to be considered before refractive surgery gets close to perfection. One of these aspects comes from the current auto-refractors. It’s necessary to know how weak or strong is the optical system of the whole eye, including the lens. Corneal topography by it self doesn’t measure myopia and hyperopia, and in terms

Hartmann screens. Remember that the subject had to tell whether the light points were joined or not, because the examiner had no idea of what the light rays did after they entered the eye. Now imagine an opposite direction of propagation of the light rays. Imagine that we could shine a single light beam onto the fovea and, instead of asking the patient what he or she was seeing, in some way we could detect how the light rays came out of the eye. If we take a look at Figure 39-1 we might understand better the point that we want to make here.

We may notice that a point of light scattered at the retina of a normal eye generates at the exit

Figure 39-1: A dot of light reflecting at the fovea and leaving the eyes of three subjects with myopia, hyperopia and a normal eye (emetropic).

Figure 39-3: Waves in phase and out of phase

Figure 39-4. A plane wave front focuses light at a point that lies over the optical axis of the lens, but a distorted wave front focuses light at a displaced point. The amount of displacement determines the wave front distortion.

Figure 39-5. Schematic diagram of optical setup used by Liang to measure aberrations of the eye. A He-Ne laser beam (1) is focused at the back of the eye. In this first optical path the objective is to generate a small

spot of light at the retina, by adjusting position of lens (16). The accommodation system consists of a light bulb (5) that shines a picture (5), which is viewed by the eye. Lens (3) is shifted until the far point of the eye is found. The diffused light reflected at the retina return passing by all eye components (vitreous humor, crystalline, aqueous humor, cornea), goes through lens (16), reflects on the beam splitter (7) and continuous through lenses (8), (9) and (11), going through the stop (10). The stop eliminates reflections from the accommodation system, from the cornea and lens (16). Finally the wave-front hits at the HS sensor (12) and is focused at the CCD array (13). The CCD image is digitized in a “frame grabber” (14) and processed at an IBM PC, which displays the graphical information at the colored monitor (15).

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