LASIK Fundamentals, Surgical Techniques, and Complications - part 4 doc

ray of lenslets that consist of a matrix of small lenses (2,6). The light emerging from the eye is focused on a CCD camera by each lenslet to form a spot pattern. The spot pattern of an ideal subject with a perfect wavefront will be exactly the same pattern as the reference grid. The spot pattern of a subject with a distorted wavefront will create an irregular spot pattern. Displacements of lenslet images from their reference position are used to calculate the shape of the wavefront. Figures 9.5 and 9.6 show examples of spot patterns from a normal and a keratoconic subject with the Topcon Hartmann–Shack sensor. Although the spot pattern in the normal subject is regular, the spot pattern in the patient with keratoconus is markedly distorted. As the wavefront of each lenslet is perpendicular to the direction of the ray, i.e., displacement 142 Maeda Figure 9.5 Spot patterns in normal subject. Figure 9.6 Spot patterns in keratoconus. of their focusing spots, the wavefront of the measured subjects can be reconstructed from these spot patterns. Wavefront aberrations, the quantitative measure of wavefront distortions, are usually calculated using Zernike polynomials. The wavefront is expanded into sets of Zernike poly- nomials to extract the characteristic components of the wavefront. The Zernike polynomi- als are the combination of trigonometric functions and radial functions, and the terms of the Zernike polynomials, represented as Z (Fig. 9.7), are useful to show the wavefront aberra- tions because of their orthogonality (2). Examples of Zernike polynomials up to the fourth order are shown in Fig. 9.8. The zero order has one term that represents a constant. The first order represents tilt (two terms, one for the X axis and another for the Y axis). The second order includes three terms that represent defocus and astigmatism in the two directions. The third order has four terms that represent coma and trefoil aberrations. The polynomials can be expanded up to any arbitrary order unless there are enough numbers of measurements of points for calculations. Spectacles can correct only second-order aberrations, not the third and higher orders that represent irregular astigmatism. Using the Zernike coefficients of each term, monochromatic aberrations can be evaluated quantitatively (7). The wavefront can also be displayed as color-coded maps as shown in Figs. 9.5 and 9.6. The advancing part of a wavefront is shown by warmer colors and the trailing part of the wavefront is shown by cooler colors. 3. Adaptive Optics In general, optical systems, such as cameras, telescopes, fundus cameras, or spectacles, with lower aberrations have better optical properties. However, even if we minimize the aberrations in an optical system such as an astronomical telescope or a fundus camera, the aberrations induced by the atmosphere or the aberrations of normal human eyes are usually larger than the aberrations in the optic devices, and thus the final resolution of the images is reduced, i.e., the optical quality is limited by the higher-order aberrations of the subjects. Adaptive optics is the concept of intentionally designing the optics of the observatory system to compensate for the measured aberrations of the subject. As a result, the total aber- rations of the subject and observatory system are reduced, and one can observe with mini- mal aberrations. An example of an adaptive optical system is shown in Fig. 9.9. The wavefront sen- sor measures the aberrations of the subject, and this information is processed and sent to actuators. The shape of the deformable mirror is controlled by actuators to reshape the sur- face of the thin mirror to compensate for the aberration of the subject. Wavefront Technology and LASIK Applications 143 Figure 9.7 Equation of Zernike polynomials. 144 Maeda Figure 9.8 Graphic representations of Zernike polynomials. Babcock (8) arrived at the concept of adaptive optics in 1953, and this technology was used for military purposes for a long time. In 1991, much of the United States’ mili- tary work in adaptive optics was declassified, and the astronomical community applied this technology to their field (9). In 1997, Liang and Williams corrected the monochromatic aberrations of normal human eyes with adaptive optics and showed that this improved the contrast sensitivity of the eye, and the imaging of cone cells in the living human retina. With adaptive optics, images of the cone mosaic (short-, medium-, and long-wavelength- sensitive cones) in living human eyes were observed (10). If we use an excimer laser in place of the deformable mirror, we have the potential of correcting irregular astigmatism and obtaining supernormal vision by eliminating the in- herent optical aberrations of normal human eyes. Although this might not be classified as adaptive optics by the strictest definition, this concept has become one of the most dis- cussed topics in refractive surgery. C. WAVEFRONT-GUIDED ABLATION IN LASIK 1. Optical Quality in Current LASIK Procedures The question arises whether the current LASIK procedures are satisfactory. For correcting refractive changes, current LASIK procedures have reasonably good predictability for mild to moderate myopia and low hyperopia. As a result, most patients are satisfied with the out- come. This is the one of the major reasons why LASIK has been accepted by the public in such a short period. However, we have also noticed that the optical quality of the eye fol- lowing LASIK is not optimal, and LASIK patients sometimes complain about problems such as halo, glare, or difficulty with night driving. It is also very difficult to treat irregular astigmatism with conventional LASIK procedures. Thus we must realize that current tech- niques, while quite satisfactory, still have room to be improved. The effect of corneal shape on the optical quality of the eye can be calculated as corneal wavefront aberrations using corneal topography. It is possible to estimate the sur- gically induced higher-order aberrations of the eye from the measurement of corneal wave- front aberrations, because wavefront aberrations of the lens should be stable despite the re- fractive corneal surgeries. The measurement of corneal aberrations showed that higher-order aberrations of the cornea increased following PRK (11,12,13) and LASIK Wavefront Technology and LASIK Applications 145 Figure 9.9 Principles of adaptive optics. (12). This trend was more prominent for night vision (large pupil) than for day vision (small pupil). Also, there is a significant correlation between the surgically induced higher-order aberrations of the cornea and the attempted correction of the surgery (11,13). With the de- velopment of wavefront sensors, the increase of higher-order aberrations of the eye fol- lowing a conventional PRK procedure was confirmed (14). These results suggest that custom ablation methods that can correct irregular astig- matism or that can reduce surgically induced higher-order aberrations might reduce some of the problems of the current LASIK procedures. 2. Wavefront-Guided LASIK When wavefront-guided refractive surgery is performed, the following requirements should be satisfied for the wavefront sensing and the photo ablation. For the measurement of wavefront aberrations, wavefront sensors and softwares that can calculate the aberration are essential. Since 1999, many prototype wavefront sensor instruments have been intro- duced (Table 1). Figures 9.10, 9.11, and 9.12 show examples of wavefront sensors and their outputs. Although all of these instruments are prototypes, they should soon be com- mercially available. Based on the measured wavefront aberrations, not only spherical and cylindrical er- rors but also irregular astigmatism (higher-order wavefront aberrations) should be cor- rectable with the excimer laser. For that purpose, very fine processing of the corneal shape is required including asymmetrical or local ablations. Therefore laser instruments should be equipped with a flying spot scanning system or an equivalent mechanism, and an active eye tracking system is essential for precise ablation (Table 2). In addition, an algorithm that 146 Maeda Table 1 Examples of Wavefront Sensors Corneal Wavefront sensor Company Principle topography Excimer laser CustomCornea Zeiss Humphrey Hartmann–Shack Alcon/Summit/ wavefront Autonomous system WaveScan 20/10 Perfect Hartmann–Shack VISX Vision Zywave Technolas Hartmann–Shack Slit scanning Technolas (Orbscan IIz) Dresden Technomed Tscherning Wavelight, wavefront Schwind analyzer Ray-tracking Tracey Ray tracing refractometer Aberrometry ARK-10000 Nidek Optical path Videokeratoscope Nidek difference Wavefront Topcon Hartmann–Shack Videokeratoscope analyzer COAS Wavefront Hartmann–Shack Meditec Sciencies Wavefront Technology and LASIK Applications 147 can perform aberration correction (15) should be provided. The speed of light in the air is faster than that in the corneal stroma. Therefore the corneal stroma where the wavefront is delayed should be ablated in order to correct aberrations. The areas that are displayed with cooler colors in the wavefront map should be cut to reduce the wavefront aberrations in- cluding sphere, cylinder, and higher-order aberrations. On June 12, 1999, Seiler and his coworkers reported the first application of wave- front-guided LASIK. The early results in three eyes (16) were published by his group us- ing the Wavelight Allegretto excimer laser. The results of this report were promising, as all Figure 9.10 Alcon/Summit/Autonomous Custom cornea wavefront system. (Courtesy of Alcon/ Summit/Autonomous Inc.) 148 Maeda Figure 9.11 VISX WaveScan. (Courtesy of VISX.) three eyes gained up to two lines of visual acuity, and the wavefront deviations were re- duced by 27% on the average. In the United States, McDonald started the first wavefront- guided LASIK with the Autonomous system on October 1999. She has been performing a comparative study by doing conventional LASIK in one eye and the wavefront-guided LASIK in the other eye for myopia and hyperopia. Although wavefront-guided LASIK pro- duced similar uncorrected visual acuity compared to conventional LASIK, a reduction of higher-order aberrations by wavefront-guided LASIK was found in some cases. Also, VISX and other laser companies have started clinical trials that evaluate the wavefront- guided ablations. The safety and the efficacy of this procedure should be reported soon. Wavefront Technology and LASIK Applications 149 Figure 9.12 Topcon wavefront analyzer. (Courtesy of Topcon Inc.) Table 2 Examples of Excimer Lasers Eye tracking Laser Company Ablation (sampling rate) LadarVision 4000 Alcon/Summit/ Flying spot (0.8–0.9 mm) Laser radar tracker Autonomous (4000 Hz) Star S3 VISX Broad beam followed by Infrared tracker (60 Hz) scanning spot EC-5000CX Nidek Scanning slit followed Infrared tracker (60 Hz) by scanning spot MEL 70 G-Scan Asclepion-Meditec 1.5 mm flying spot Infrared tracker (50 Hz) Technolas 217Z B & L Dual-diameter flying Infrared tracker (120 Hz) spot (2 and 1 mm) Allegretto Wavelight 1 mm flying spot Infrared tracker (250 Hz) ESIRIS Schwind 1 mm flying spot Infrared tracker (300 Hz) LaserScan LSX Lasersight Flying spot (0.8–1 mm) Infrared tracker (60 Hz) D. SUMMARY It is reasonable for refractive surgeons to remove pathological irregular astigmatism or sur- gically induced aberrations that correlate with pupil diameter or attempt correction by re- fractive surgeries. On the other hand, we will need to know the clinical significance of su- pernormal vision, and also when we should correct for higher-order aberrations because aberrations of refractive surgery candidates do change with age (17,18). Wavefront-guided refractive surgery has just begun. Many aspects must be improved to obtain better results than the conventional techniques, as many newly developed surgi- cal procedures have problems for the first time. We need to know that conventional refrac- tive surgeries induce higher-order aberrations, and custom ablation appears to be the only solution. REFERENCES 1. J Liang, DR Williams, DT Miller. Supernormal vision and high-resolution retinal imaging through adaptive optics. J Opt Soc Am A 1997;14:2884–2892. 2. RK Tyson. Principles of Adaptive Optics. 2d ed. Boston: Academic Press, 1998. 3. MS Smirnov. Measurement of the wave aberration of the human eye. Biofizika 1961;6:687– 703. 4. B Howland, HC Howland. Subjective measurement of high-order aberrations of the eye. Sci- ence 1976;193:580–582. 5. J Liang, B Grimms, S Goelz, JF Bille. Objective measurement of wave aberrations of the hu- man eye with the use of a Hartmann–Shack wavefront sensor. J Opt Soc Am A 1994;11:1949– 1957. 6. LN Thibos, X Hong. Clinical applications of the Shack–Hartmann aberrometer. Optom Vis Sci 1999;76:817–825. 7. J Liang, DR Williams. Aberrations and retinal image quality of the normal human eye. J Opt Soc Am A 1997;14:2873–2883. 8. HW Babcock. The possibility of compensating astronomical seeing. Publ Astron Soc Pac 1953;65:229–236. 9. RQ Fugate, DL Fried, GA Ameer, BR Boeke, SL Browne, PH Roberts, RE Ruane, LM Wopat. Measurement of atmospheric wavefront distortion using scattered light from a laser guide star. Nature 1991;353:144–146. 10. A Roodra, DR Williams. The arrangement of the three cone classes in the living human eyes. Nature 1999;397:520–522. 11. CE Martinez, RA Applegate, SD Klyce, MB McDonald, JP Medina, HC Howland. Effect of papillary dilation on corneal optical aberrations after photorefractive keratectomy. Arch Oph- thalmol 1998;116:1053–1062. 12. T Oshika, SD Klyce, RA Applegate, HC Howland, MA El Danasoury. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 1999;127:1–7. 13. KM Oliver, RP Hemenger, MC Corbett, DPS O’Brart, S Verma, J Marshall, A Tomlinson. Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg 1997;13: 246–254. 14. T Seiler, M Kaemmerer, P Mierdel, HE Krinke. Ocular optical aberrations after photorefractive keratectomy for myopia and myopic astigmatism. Arch Ophthalmol 2000;118:17–21. 15. J Schwiegerling, RW Snyder. Custom photorefractive keratectomy ablations for the correction of spherical and cylindrical refractive error and higher-order aberration. J Opt Soc Am A 1998;15:2572–2579. 150 Maeda 16. M Mrochen, M Kaemmerer, T Seiler. Wavefront-guided laser in situ keratomileusis: Early re- sults in three eyes. J Refract Surg 2000,16:116–121. 17. T Oshika, SD Klyce, RA Applegate, HC Howland. Changes in corneal wavefront aberrations with aging. Invest Ophthalmol Vis Sci 1999;40:1351–1355. 18. A Guirao, C Gonzalez, M Redondo, E Geraghty, S Norrby, P Artal. Average optical perfor- mance of the human eye as a function of age in a normal population. Invest Ophthalmol Vis Sci 1999;40:203–213. Wavefront Technology and LASIK Applications 151 [...]... asymmetry (I-S value) between 1 .4 and 1.9 D as suspect and I-S Ͼ 1.9 D as suggestive of keratoconus Given these limits, we es- 160 Lu and Azar tablished three groups of central cornea power (K) ( 47 .2 D, 47 .2 48 .7 D, 48 .7D) and inferior–superior corneal power asymmetry (I-S value) (Ͻ1 .4 D, 1 .4 1.9 D, Ͼ1.9 D) and assigned 0, 1, and 2 points, respectively In other words, the higher the K and I-S value... Controls Ϫ 54. 97* (Ϫ86.67 to Ϫ23.27) Ϫ92.28* (Ϫ119.01 to Ϫ65.56) Ϫ109.15* (Ϫ132.97 to Ϫ85.33) Ϫ37.31* (Ϫ67.32 to Ϫ7.31) Ϫ 54. 18* (Ϫ81.63 to Ϫ26.73) Ϫ16.87 (Ϫ38.38 to 4. 63) P value Posterior best fit sphere (mm) (n = 180); mean difference (95% CI) P value Ͻ.001 Ͻ.001 Ͻ.001 008 Ͻ.001 182 Ϫ0 .42 * (Ϫ0.66 to Ϫ0.18) Ϫ0. 64* (Ϫ0. 84 to Ϫ0 .43 ) Ϫ0.63* (Ϫ0.81 to Ϫ0 .44 ) Ϫ0.22 (Ϫ0 .44 to 0.01) Ϫ0.20 (Ϫ0 .41 to 0.01)... of Radial Keratotomy study 10 years after the surgery Arch Ophthalmol 19 94; 112: 1298–1308 172 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Akpek et al MR Deitz, DR Sanders, MG Raanan, M DeLuca Long-term ( 5- to 12-year) follow-up of metalblade radial keratotomy procedures Arch Ophthalmol 19 94; 112:6 14 620 GM Kezirian, CM Gremillion Automated lamellar keratoplasty for the... topography, and modified Rabinowitz criteria Asymmetric anterior central corneal power between left and right eyes Յ1.9 D or Ͼ1.9 D is assigned 0 or 1 point, respectively If asymmetry Ͼ 1.9 D is present, the eye with the higher corneal power receives 1 point and the other eye receives 0 points K Ͻ 47 .2 D, 47 .2 48 .7 D, or 48 .7 D is assigned 0, 1, or 2 points, respectively I-S value Ͻ1 .4 D, 1 .4 1.9 D,... 181–208 GW Nyquist Rheology of the cornea: experimental techniques and results Exp Eye Res 1968;7:183–188 C Edmund Corneal elasticity and ocular rigidity in normal and keratoconic eyes Acta Ophthalmol 1988;66:1 34 140 TT Andreassen, AH Simonsen, H Oxlund Biomechanical properties of keratoconus and normal corneas Exp Eye Res 1980;31 :43 5 44 1 BH Schimmelpfenning, GO Waring Development of refractive keratotomy... for Myopia and Astigmatism St Louis, MO: Mosby–Year Book, 1992, pp 237–258 DV Leaming Practice styles and preferences of ASCRS members: 1993 survey J Cataract Refract Surg 19 94; 20 :45 9 46 7 MR Deitz, DR Sanders, MG Raanan Progressive hyperopia in radial keratotomy: long term follow-up of diamond knife and metal blade series Ophthalmology 1986;93:12 84 1289 GO Waring, M Lynn, PJ McDonnell, and the PERK... combines history, examination, topographical maps, and modified Rabinowitz criteria to establish stricter criteria for distinguishing normal controls, keratoconus suspects, and early and advanced keratoconus Smolek and Klyce (21) modified the Rabinowitz criteria and identified the anterior central corneal power (K) between 47 .2 and 48 .7 D as suspect and K Ͼ 48 .7 D as suggestive of keratoconus The modified... mechanical response of bovine, rabbit and human corneas J Biomech Eng 1992;1 14: 202–215 DD Koch The riddle of iatrogenic keratectasia J Cataract Refract Surg 1999;25 :45 3 45 4 Corneal Stability and Biomechanics after LASIK 37 173 R Turss, J Friend, M Reim, CH Dohlman Glucose concentration and hydration of the corneal stroma Ophthalmic Res 1971;2:253–260 38 W-M Yi, C-K Joo Corneal flap thickness in laser... watched to prevent corneal perforation during LASIK Figure 11.3 An Orbscan corneal tangential topography showing keratectasia after LASIK (Courtesy of Donald Sanders, M.D., Ph.D.) 168 169 atively (D) Left eye: topography showing marked inferonasal keratectasia 15 months postoperatively (From Ref 34. ) Figure 11 .4 (A) Pre -LASIK topography in the right eye (B) Pre -LASIK topography in the left eye (C) Right... suspect for keratoconus, and K Ͼ 48 .7 D was suggestive of keratoconus Likewise, inferior–superior central anterior corneal power asymmetry between 1 .4 and 1.9 D was considered suspect for keratoconus, and I-S value Ͼ 1.9 D was suggestive of keratoconus (21) Sensitivity and specificity of the modified Rabinowitz method were 96–100% and 85–89%, respectively (17,21) Figure 10.2 The Azar-Lu MEEI keratoconus . of central cornea power (K) ( 47 .2 D, 47 .2 48 .7 D, 48 .7D) and inferior–superior corneal power asymmetry (I-S value) (Ͻ1 .4 D, 1 .4 1.9 D, Ͼ1.9 D) and assigned 0, 1, and 2 points, respectively. In. suspects, and early and advanced keratoconus. Smolek and Klyce (21) modified the Rabinowitz criteria and identified the anterior central corneal power (K) between 47 .2 and 48 .7 D as suspect and K Ͼ 48 .7. corneal power receives 1 point and the other eye receives 0 points. K Ͻ 47 .2 D, 47 .2 48 .7 D, or 48 .7 D is assigned 0, 1, or 2 points, re- spectively. I-S value Ͻ1 .4 D, 1 .4 1.9 D, or Ͼ1.9 D is assigned