Features

Increases in lower- and higher-order corneal aberrations mean that Laser in situ keratomileusis should be performed with caution in eyes with previous radial keratotomy, report Robert Montés-Micó and Teresa Ferrer-Blasco

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Until the early 1990s, radial keratotomy (RK) was the preferred refractive procedure for lower amounts of myopia. Many patients were treated with this technique.1 However, in many cases, the total amount of myopia was not corrected. Actually, post-RK patients come to our clinics to find a solution for their residual myopia.

Here, we report a case of a patient who was submitted to laser in situ keratomileusis (Lasik) to correct the residual amount of myopia after RK. Reported complications of Lasik in eyes with previous RK include intra-operative incision opening and wound dehiscence, epithelial defects, diffuse lamellar keratitis, decentred ablation, epithelial in growth and corneal haze.2,3

To the best of our knowledge, no previous report focusing on corneal aberrations caused by Lasik in eyes with previous RK has been published. The aim of this study was to assess the optical outcome of the Lasik procedure on a post-RK eye.

PATIENT

The patient was a 45-year-old male who elected conventional Lasik surgery over a 6mm optical zone to correct the following refractive error using Snellen letters and under photopic conditions (65cd/m2): L -2.75 -1.25 x 110, best spectacle visual acuity (BSVA) of 6/6. He had undergone RK nine years ago: L -4.75 (BSVA of 6/6). Figure 1 shows the location of the incisions on the cornea.

Preoperative evaluation included best-corrected visual acuity, manifest and cycloplegic refraction, slit lamp biomicroscopy, applanation tonometry, fundus examination, ultrasonic pachymetry and corneal topography.

SURGICAL PROCEDURE

Lasik surgery was performed using the VISX STAR S2 excimer laser (VISX Inc, Santa Clara, CA), with the following parameters: wavelength, 193nm radiant exposure (fluence), 160mJ/cm2 pulse repetition rate, 10Hz average ablation depth per pulse, 0.23µm on the cornea ablation zone diameter 6.0mm and transition zone 0.35mm. Aspiration air flow was used for debris removal.

Lamellar keratotomy was performed using the Carriazo-Barraquer microkeratome (Moria, Doylestown, PA) to create a flap of 8.5mm of diameter and 160 microns of depth. After ablation the flap was repositioned, the interface cleaned and perfect apposition was ensured by the preoperative corneal marks. The area was dried with a jet of air and Merocel sponges (Medtronic Xomed, Jacksonville, FL). Postoperatively tobramycin and dexamethasone eyedrops (Tobradex, Alcon-Cusi, Barcelona, Spain) three times daily for one week were used.

After Lasik the patient fulfilled the follow up protocol, with examination visits carried out at one day postoperatively to rule out any possible early complication, and then at one and two months. Data concerning postoperative Lasik included uncorrected visual acuity, manifest and cycloplegic refraction, slit lamp examination, applanation tonometry and corneal topography.

CORNEAL ABERRATIONS FROM CORNEAL TOPOGRAPHY

Videokeratographic data were obtained with computerised videokeratography Tomey TMS-2 (Tomey Inc, Salt Lake City, UT). Measurements were performed within the seven first seconds after blink to avoid the influence of the tear film break up on the corneal aberrations4,5 and they were repeated until a well focused and aligned image was obtained. Corneal elevation data were exported from the topographer onto floppy disks in ASCII files which contained information about corneal elevation, curvature, power and position of the pupil.

The slope data were fitted with Zernike polynomials up to the sixth order to determine aberration coefficients, from which the wavefront aberration function was reconstructed. The calculation of corneal wavefront aberration was performed using the CT-View 5.0 software (Sarver & Associates, Inc Merritt Island, Fl) for two pupil diameters: 4mm and 6mm. Zernike co-efficients were used to calculate the monochromatic corneal aberrations, represented by the terms, S3, S4, S5, and S6. S3 represents the third-order component of the wavefront aberration (coma-like aberration) S4 represents the fourth-order component of the wavefront aberration (spherical-like aberration). S5 and S6 are the fifth- and sixth-order components of the wavefront aberrations, respectively. Because of the linear independence of these terms, the total wavefront error was computed by summing all components, with overall image quality represented by (S3 + S4 + S5 + S6). The odd-order aberrations (S3 + S5) were added to examine the magnitude of coma-like aberrations, and the even-order aberrations (S4 + S6) were summated to evaluate the changes in spherical-like aberration. Corneal wavefront aberrations were calculated relative to the pupil centre instead of the corneal vertex (videokeratoscope axis) in order to evaluate the correlation between the corneal aberration and visual acuity. We also calculated the point spread function (PSF) which is defined as the light distribution of the image of a point source. The shape and width of the PSF depend upon the levels of aberrations and the shape of the pupil. The spot diagram was also calculated.

Both the PSF and the spot diagram inform us about how the optics through the first surface of the cornea affects the quality of the final image.

RESULTS

Two months after Lasik treatment the patient showed an uncorrected visual acuity of 6/7.5. Objective and subjective refraction was not possible due to the irregular pattern shown in retinoscopy and the poor quality of vision achieved with trial lenses. Figure 1 shows the pre- and postoperative topographic maps and the corneal spot diagrams before and after Lasik. Figure 2 shows the higher-order aberrations (3rd to 6th order) and PSFs before and after Lasik, computed for 4mm and 6mm pupil diameters.

Preoperative and postoperative RMS for low- and higher-order optical aberrations for both pupils are shown in Table 1. Low-order aberrations (defocus and astigmatism) increased after Lasik for both pupil diameters, and were considerably greater for a 6mm pupil (X2.38). Higher-order aberrations increased by a factor of about two after Lasik for both pupil diameters (X1.86 and X1.85, respectively). Spherical-like aberrations were dominant compared to coma-like aberrations for both pupil diameters before and after Lasik (about two times larger) however, Lasik induced larger changes in coma- than spherical-like aberrations (X1.41 and X1.91 compared to X1.19 and X1.81, for 4mm and 6mm pupils, respectively).

DISCUSSION

Lasik has proved to be an effective treatment for the correction of residual refractive errors after RK.2,3 Attia et al2 prospectively evaluated the results of Lasik in eyes with over-correction and under-correction after RK. Six months after Lasik, 91 per cent of eyes in the overcorrected group and 89 per cent in the undercorrected group were within ±1 dioptres of the intended correction, with a significant improvement in the uncorrected visual acuity in both groups. Francesconi et al3 evaluated Lasik used to treat RK induced hyperopia and found a reduction of mean spherical equivalent from +3.4 (±1.6D) preoperatively to -0.32 (±1.2D) postoperatively, and nearly 80 per cent of eyes between ±1D from emmetropia.

Wavefront technology analysis enables us to determine quantitatively the changes in the optics of the cornea induced by corneal refractive surgery. Despite the difficulty in obtaining post-Lasik refraction by conventional methods (retinoscopy and subjective), wavefront analysis is able to quantify residual refractive errors by means of low-order aberration evaluation (defocus and astigmatism). In this case, Lasik ablation of the cornea of a previous RK treatment increased the values of lower- and higher-order corneal aberrations. Up to now, there has been no report about the evaluation of corneal aberrations after Lasik ablation on previous RK eyes, thus, no comparison with previous results is possible.

From Figure 1 we can observe the corneal topography colour-coded maps before and after Lasik. Before Lasik the topographic map showed irregularities caused by the previous incisions of the RK and after Lasik. Despite the reduction in irregularities, the topography doesn't show the typical pattern of a post-myopic Lasik (central ablation). This indicates the difficulty in obtaining a regular surface by means of conventional Lasik ablation on previously irregular corneas. From this figure we can also observe the corneal spot diagram change after Lasik. Here, Lasik induces degradation of optical quality. The spot diagrams show that the image quality decreases in terms of the spread of the spots, directly related to the spread of the retinal image. Post-Lasik spot diagrams appear more widely spread than preoperative.

The maps of corneal higher-order aberrations before and after Lasik are shown in Figure 2 (for 4mm and 6mm pupil diameters). From the changes observed in the wavefront patterns of Figure 2 and the values shown in Table 1, we found an increase in corneal aberration post-Lasik, which was larger for a 6mm pupil diameter. These findings agree with previous studies which have shown the increase of corneal and optical (corneal + lenticular) aberrations after RK6,7 and Lasik8-12 procedures and with larger pupil diameters.

Considering the results in more detail, previous studies carried out by Applegate et al6,7 have shown that spherical-like aberration is dominant after RK in relation to coma-like aberrations. Our results agree with these findings, being around two times larger for spherical-like aberrations (Table 1). Several studies10-12 have reported that Lasik causes an increase in spherical and coma-like aberrations, with the main contribution being the increase in spherical-like aberration. Our results also agree with this conclusion, because spherical-like aberrations showed larger values in relation to coma-like aberration for both pupil diameters. However, we have to note that the increment post-Lasik in our patient is greater for coma-like compared to spherical-like aberration.

To explain this result, we may argue that in a previous RK eye, with radial incisions on the cornea, Lasik ablation doesn't result in the same effect on an area with an incision in relation to a incision-free area. This effect would be expected to produce asymmetry as a function of the incision-meridian of the cornea and consequently changes in coma-like aberration.

It is important to note that, although there is an increase in higher-order aberrations post-Lasik, we believe that the main problem in achieving good visual performance after Lasik treatment is the difficulty in removing low-order aberrations. The increase in low-order aberrations is considerably higher compared with higher-order aberrations (Table 1). It is our opinion that previous incisions through RK, combined with myopic Lasik ablation, increase the irregularities on the corneal surface making it difficult to predict post-Lasik refractive outcomes.

The spot diagram and PSF show that the image quality decreases after Lasik, and this could be explained by corneal irregularities. Consequently, some residual irregular refractive error may be common due to the difficulty for the ablation pattern to reduce previous refractive error. Corneal aberrations were calculated relative to the pupil centre to correlate the results with the visual acuity of the patient. This patient showed worse visual acuity after Lasik than previous surgery with his best-spectacle correction.

Following these results, we may conclude that conventional Lasik surgery in post-RK eyes should be performed with caution. The possible variability in optical and visual results should be addressed to the patient and the possible future consequences of re-treatments should be also considered. Perhaps customised wavefront-guided ablation, which factor into the ablation pattern the higher-order aberration terms, may solve this problem since more information about the optical properties of the cornea will be considered. However, this consideration should be addressed in future studies.

References

1 Waring GO, Lynn MJ, McDonell PJ. PERK Study Group. Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol, 1994112:1298-308.

2 Attia WH, Alió JL, Artola A, Munoz G, Shalaby AM. Laser in situ keratomileusis for under-correction and over-correction after radial keratotomy. J Cataract Refract Surg, 200127:267-72.

3 Francesconi CM, Nose RA, Nose W. Hyperopic laser-assisted in situ keratomileusis for radial keratotomy induced hyperopia. Ophthalmology, 2002109:602-5.

4 Montés-Micó R, Alió JL, Muñoz G, Charman WN. Temporal changes in optical quality of air-tear film interface at anterior cornea after blink. Invest Ophthalmol Vis Sci, 200445, 1752-1757.

5 Montés-Micó R, Alió JL, Muñoz G, Pérez-Santonja JJ, Charman WN. Postblink changes in total and corneal ocular aberrations. Ophthalmology, 2004111, 758-767.

6 Applegate RA, Hilmantel G, Howland HC. Corneal aberrations increase with the magnitude of radial keratotomy refractive correction. Optom Vis Sci, 199673:585-9.

7 Applegate RA, Howland HC, Sharp RP et al. Corneal aberrations and visual performance after radial keratotomy. J Refract Surg, 199814:397-407.

8 Oshika T, Klyce SD, Applegate RA, et al. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol, 1999127:1-7.

9 Hong X, Thibos LN. Longitudinal evaluation of optical aberrations following laser in situ keratomileusis. J Refract Surg, 200016:S647-50.

10 Moreno-Barriuso E, Merayo J, Marcos S, et al. Ocular aberrations before and after myopic corneal refractive surgery: Lasik-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci, 200142:1396-403.

11 Marcos S, Barbero S, Llorente L, Merayo-Lloves J. Optical response to Lasik surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci, 200142:3349-56.

12 Oshika T, Miyata K, Tokunaga T, et al. Higher order wavefront aberrations of cornea and magnitude of refractive correction in laser in situ keratomileusis. Ophthalmology, 2002109:1154-58.

◆ Robert Montés-Micó is Associate Professor of the Optics Department at the University of Valencia, Spain. Teresa Ferrer-Blasco is an optometrist practising in Valencia

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