LASIK Fundamentals, Surgical Techniques, and Complications - part 5 ppsx

vary from unit to unit, and poor quality blades should uniformly be rejected and replaced. Blades should never be reused unless expressly designed for such use by the manufacturer. It has been shown that dull blades produce thinner flaps and increase the risk of irregular flaps (5,6). Many centers allow for the same blade to be used on both eyes of the same pa- tient, but not more than this. Even in this usage it has been demonstrated that the second flap cut is somewhat thinner than the first (6). C. GUIDELINES FOR LASER SETTINGS It is a popular misconception that lasers are computer-controlled devices that are incapable of producing a less than perfect outcome. In reality these devices, while capable of great accuracy, must be carefully maintained and calibrated to achieve proper performance. When setting up the excimer laser the following parameters merit special attention: laser room environment, fluence testing, and beam homogeneity. The laser room environment is an important variable that must be controlled. The room should be kept cool for optimal laser performance. This varies from laser to laser, but a temperature of 18 to 24 degrees Celsius should be maintained. The ambient humidity should be kept low (40 to 50% relative humidity) and as steady as possible. Humidity can significantly affect the ablation rate of corneal tissue, with overcorrections more likely in very dry conditions and undercorrections more likely in more humid conditions (13). The room should be kept free from particulate debris, which can adversely affect ablation reg- ularity if it is deposited on the optical system of the laser. This can be achieved with an ac- tive filtration system using a HEPA (high efficiency particulate air) filter. These systems are capable of removing particles as small as 0.1 micron, including bacteria. The system should be in operation continuously, as shutdown periods will allow particulate matter to coat the optics of the laser system. Laser fluence is a measure of the energy density and is described as the amount of en- ergy applied per unit area with each pulse. This is measured in millijoules per centimeter squared (mJ/cm 2 ). The minimum fluence necessary for proper photoablation of the cornea is approximately 50 to 60 mJ/cm 2 . The fluence of the laser should be checked before every ablation, which is usually automatically performed in most lasers. If the fluence is low, this is an indication that the gas concentrations should be raised to prevent undercorrections, which may occur due to inadequate tissue ablation. Gas levels that are too high can cause higher fluence and overcorrections. Beam homogeneity is a measure of the consistency of the distribution of energy ap- plied over the ablation area. Homogeneity is an important parameter in broad beam lasers, and if it is poor, it can lead to an irregular ablation and potential loss of best spectacle cor- rected visual acuity. It is of much less significance in small scanning spot/slit lasers, as any inhomogeneity present would be spread out evenly over the ablation area. Manual verifi- cation of beam homogeneity is essential in broad beam delivery systems. This is usually ac- complished by test ablations into appropriate substrate materials as provided by the indi- vidual laser manufacturer. Visual evidence of poor homogeneity is reason to cancel the procedure until the problem can be remedied. 1. Adjustments to Standard Laser Nomograms It should come as no surprise to any experienced surgeon that the laser is very much like any other surgical instrument in that an adjustment must be included in the treatment plan 194 Lahners and Hardten to allow for different surgical variables. While the primary determinant of the tissue abla- tion pattern and final visual outcome is the treatment design that is included in the laser software, this does not lessen the importance of developing personal nomograms. Today’s surgeon has the advantage of using ablation software that has evolved through several gen- erations and provides for advanced features such as wider optical zones, multiple treatment zones, transition zones, and software to reduce surface irregularities such as central islands. However, each laser is used in different settings, and surgeons have individual variations in technique and operating environment. To allow for these differences, individual nomo- grams should be developed to improve the accuracy and predictability of treatments. In- creasing the standardization of the technique will improve the standard deviation of the achieved effect, and adjusting the nomogram by the average achieved effect will move the mean towards the desired result, which is typically emmetropia. There are many factors that can contribute to the final visual and refractive outcome after LASIK. These include patient age, gender, preoperative keratometry, preoperative pachymetry, degree of planned correction, laser type, software version, room temperature, room humidity, facility altitude, accuracy in laser calibration, depth of keratectomy, degree of corneal hydration used by the surgeon, time that the flap is raised and the total procedure duration, whether or not forced air (or vacuum) is used across the stromal bed during abla- tion, postoperative inflammation (e.g., diffuse lamellar keratitis), and postoperative medi- cations. It is the job of the surgeon to control as many of these factors as possible (e.g., tem- perature, humidity, technique, time of surgery, postoperative medications), as this is the first step in achieving precise outcomes. For the factors that are not controllable, it is im- portant to examine how they affect refractive outcomes using statistical methods so that the accuracy of the ablation can be improved, decreasing the rate of over- and undercorrec- tions. Differences in technique can make a large difference in the final ablation. Even small variables should be sought out and eliminated if possible. For example, lifting the flap with nontoothed forceps instead of an irrigating cannula removes the risk of accidental intro- duction of fluid into the interface. Even a small amount of fluid can hydrate the stroma, re- sulting in decreased tissue ablation and undercorrection. The dryer the stromal tissue is, the more tissue is ablated per pulse of the laser (13). Increasing the amount of time that the flap is raised increases the evaporation and decreases the hydration of the bed. This can result in significantly increased ablation and thus overcorrection. The time to perform the entire technique from start to finish, and especially the time while the flap is lifted, should be con- sistent from case to case. If the surgeon chooses to wipe the bed, then the same technique should be performed in every case, including the dampness/dryness of the sponge and the number of wipes during the ablation. When choosing an initial nomogram one should choose one from a surgeon who ideally has experience on the same laser, at the same fa- cility, and who uses a similar technique. This will give the beginning surgeon a starting point. Patient age is a very important consideration when designing nomograms. The same laser will typically have more effect in an older patient. Older patients also have less ac- commodative amplitude, which will make them less tolerant of hyperopia. We are less ag- gressive when treating myopia in patients over 40 for these reasons. Some surgeons will in- clude an adjustment for mild residual myopia in older patients where the nondominant eye treatment is reduced by 5 to 10%. Younger patients show a greater tendency towards re- gression and can tolerate small overcorrections owing to their increased accommodative Microkeratomes and Laser Settings 195 amplitudes. For these reasons it is appropriate to be somewhat more aggressive in younger patients. The development of a personal nomogram begins with the collection of data includ- ing the patient’s age, refractive error, gender, pachymetry, keratometry, and postoperative manifest refractions, including data with the longest follow-up time possible; typically 6 months to 1 year is ideal. While in most cases the patient’s refractive error will stabilize at 3 months, higher refractive errors may take longer, sometimes 6 to 12 months. It is impor- tant to collect and include this data, if it is available. The data is then analyzed using mul- tiple regression analyses, and the resulting information is used to create a personal nomo- gram. There are also commercial programs on the market that will assist in nomogram development (Table 1). Other factors should also be considered. As was mentioned above, it may be desir- able to attempt slight hyperopia to achieve fewer myopic undercorrections in younger pa- tients, and mild myopia on overage in older patients to reduce undesirable hyperopia. It must be remembered that treatments for overcorrection of myopia can be technically more difficult than the initial surgery, especially if a small flap was originally used, and the re- treatment requires the cutting of a new flap. Because of the relative difficulty and poorer predictability in hyperopic treatments, some surgeons structure their nomograms so that the rate of enhancement for an overcorrection is less than the rate of enhancement for an un- dercorrection. The nomograms should also be altered to allow for differences in corneal response following PRK, LASIK, radial keratotomy, or penetrating keratoplasty. Concerning re- treatments, several trends can be observed. Patients who have shown large amounts of re- gression may be more likely to underrespond to retreatments as well. For this reason, some surgeons are more aggressive when treating an initial undercorrection. We tend to use the same nomogram for retreatment of initial undercorrections, unless it is clear why the pa- tient originally underresponded, attempting to avoid problems with overtreatment. When treating overcorrections after previous LASIK, most surgeons reduce the correction, be- cause these eyes have a tendency to overrespond to the second treatment. The frequency of updating nomograms is a matter of personal choice. Some surgeons use a more rapid cycle when recalculating their nomograms and change nomograms every few months. This can allow for a more rapid adjustment to a new technique, new instru- ments, or other factors. Other surgeons prefer to make fewer adjustments on their nomo- grams, opting to refine their nomogram only when larger patient numbers and more follow- up data indicate that a change is necessary. Beginning refractive surgeons should begin data collection and analysis in a timely manner but should avoid the tendency to start changing the nomogram until an adequate amount of data has been collected at a stable time point postoperatively. 196 Lahners and Hardten Table 1 Commercially Available Outcomes and Data Analysis Software Software name Designed by Contact information The Refractive Surgery Guy M. Kezirian, MD and 480-348-9299; Consultant Jack T. Holladay, MD www.RefractiveConsultant.com LASIK/PRK Outcomes Perry S. Binder, MD 858-756-4462; email: Analysis LASIKSupport@aol.com As an example we have included our nomograms from two surgeons (David R. Hardten, MD and Richard L. Lindstrom, MD) using the Visx Star S3 Smoothscan in Min- neapolis, MN (Tables 2–4). The altitude of our facility is approximately 1000 feet above sea level, our temperature between 68 and 74 degrees F, and our humidity 20 to 40%. To use the nomogram we convert to spherical equivalent and then use the percentage adjust- ment indicated to change the sphere of the patient’s refractive error. We enter cylinder di- rectly as we have not found that this parameter requires independent adjustment. The dif- ferences between the nomograms illustrate effects of variation in technique between two surgeons using the exact same laser and operating setting. Based on our experience we have found that the factors that influence our results most are age and preoperative refractive error. This does not mean that other factors do not contribute to the accuracy of the treatment. Using multiple regression analyses the surgeon can determine the contribution of different variables and calculate adjustment factors for each. While setting up nomograms can be a laborious task for the busy clinician, it is an im- portant step that should not be overlooked, as it can contribute greatly to more accurate sur- gical outcomes and more satisfied patients. Microkeratomes and Laser Settings 197 Table 2 Myopic LASIK. (David R. Hardten, M.D.; VISX Star S3 Smoothscan, Minneapolis, MN) 0–0.9 D 1–1.9 D 2–3.9 D 4–5.9 D 6–7.9 D 8–11.9 D Age (yrs) (%) (%) (%) (%) (%) (%) 20–29 58 28 11 4 Ϫ10 30–39 45 22 8 1 Ϫ2 Ϫ5 40–49 32 14 3 Ϫ1 Ϫ4 Ϫ6 50–59 19 7 0 Ϫ2 Ϫ5 Ϫ7 60ϩ 91 0Ϫ3 Ϫ6 Ϫ8 Achieved effect ϭ 1.11 PRK setting Ϫ 0.012 Age ϩ 0.93, based on 1 month results with 5% additional regres- sion to 1 year. Use the above adjustment to spherical equivalent (patient’s spherical equivalent is the horizontal axis and age is the vertical); enter full cylinder without correction. Table 3 Myopic LASIK. (Richard L. Lindstrom, MD; VISX Star S3 Smoothscan, Minneapolis, MN) 0–0.9 D 1–1.9 D 2–3.9 D 4–5.9 D 6–7.9 D 8–11.9 D Age (yrs) (%) (%) (%) (%) (%) (%) 20–29 68 32 10 0 Ϫ5 Ϫ9 30–39 45 22 3 Ϫ4 Ϫ7 Ϫ10 40–49 32 10 Ϫ3 Ϫ8 Ϫ10 Ϫ11 50–59 19 Ϫ3 Ϫ9 Ϫ12 Ϫ13 Ϫ14 60–69 Ϫ14 Ϫ15 Ϫ16 Ϫ16 Ϫ17 Ϫ17 Achieved effect ϭ 1.26 PRK setting Ϫ 0.023 Age ϩ 1.53, based on 1 month results with 5% additional regres- sion to 1 year. Use the above adjustment to spherical equivalent (patient’s spherical equivalent is the horizontal axis and age is the vertical); enter full cylinder without correction. D. CONCLUSION LASIK continues to evolve into a very safe and effective technique. The proper selection of microkeratome settings and the development of personal laser nomograms are important elements of a successful LASIK procedure. It is only by diligent attention to details and the continuous analysis of variables that we can continue to advance the state of the art, while providing the greatest accuracy and best possible vision for our patients. REFERENCES 1. C Argento, MJ Cosentino, G Valenzuela. Influence of keratometry on the flap size. ASCRS Symposium on Cataract, IOL, and Refractive Surgery, San Diego, 1998, p 119. 2. YI Choi, SJ Park, BJ Song. Corneal flap dimensions in laser in situ keratomileusis using the In- novatome automatic microkeratome. Korean J Ophthalmol 2000;14(1):7–11. 3. HS Dua, JV Forrester. The corneoscleral limbus in human corneal epithelial wound healing. Am J Ophthalmol 1990;110(6):646–656. 4. PS Binder, M Moore, RW Lambert, DM Seagrist. Comparison of two microkeratome systems. J Refract Surg 1997;13(2):142–153. 5. E Donnenfeld, R Wertheimer, A Wallerstein, H Perry, L Landrio, E Rahn. Predictors of corneal flap thickness in LASIK surgery. ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, 1998, p 63. 6. FR Villareal, PR Valdes, EB Garza. Reproducibility of corneal flap thickness with Hansatome microkeratome: comparison between first and fellow eye using the 180-micron head. ASCRS Symposium on Cataract, IOL and Refractive Surgery, Boston, 2000, p 14. 7. WM Yi, CK Joo. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999;25(8):1087–1092. 8. A Behrens, B Seitz, A Langenbucher, MM Kus, C Rummelt, M Kuchle. Evaluation of corneal flap dimensions and cut quality using the Automated Corneal Shaper microkeratome. J Refract Surg 2000;16(1):83–89. 9. R Suarez, R Yee. Are manual microkeratomes reliable? ASCRS Symposium on Cataract, IOL and Refractive Surgery, Boston, 2000, p 33. 10. T Seiler, K Koufala, G Richter. Iatrogenic keratectasia after laser in situ keratomileusis. J Re- fract Surg 1998;14:312–317. 11. SP Amoils, MB Deist, P Gous, PM Amoils. Iatrogenic keratectasia after laser in situ ker- atomileusis for less than Ϫ4.0 to Ϫ7.0 diopters of myopia. J Cataract Refract Surg 2000;26(7): 967–977. 12. CR Munnerlyn, SJ Koons, J Marshall. Photorefractive keratectomy: a technique for laser re- fractive surgery. J Refract Surg 1988;14:46–52. 198 Lahners and Hardten Table 4 Hyperopic LASIK. (David R. Hardten, MD; VISX Star S3 Smoothscan, Minneapolis, MN) 0–1.9 D 2–3.9 D 4–6 D After RK After LASIK Age (yrs) (%) (%) (%) (%) (%) 20–29 0 10 20 Ϫ4 Ϫ14 30–39 10 18 25 Ϫ2 Ϫ10 40–49 17 23 28 Ϫ1 Ϫ1 50–59 30 30 30 0 4 60 35 32 31 2 8 Columns to far right are for consecutive hyperopia. Use the above adjustment to spherical equivalent (patient’s spherical equivalent is the horizontal axis and age is the vertical); enter full cylinder without correction. 199 14 Centration of LASIK Procedures MARSHA C. CHEUNG Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, U.S.A. CHUN CHEN CHEN and DIMITRI T. AZAR Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston, Massachusetts, U.S.A. A. EFFECT OF IMPROPER CENTRATION Many corneal procedures such as LASIK and photorefractive keratectomy demand proper centration on the cornea. The optical zone is the part of the cornea that refracts light rays to form the image on the fovea. Many corneal surgical procedures can cause scarring in the pe- ripheral cornea, leaving behind a central optical zone. If this optical zone is too small or im- properly centered, visual function can be adversely affected by glare or irregular astigma- tism. Glare and blurred images are especially noted at night when the pupil is dilated, thereby demanding the largest scar-free optical zone. Numerous other complications such as monoc- ular diplopia, unpredictable visual acuity outcomes, and poor contrast sensitivity can also be attributed to improper centration of the optical zone (2). Since the majority of refractive surgeries such as LASIK (1) operate on eyes that can be corrected to 20/20 visual acuity and (2) are elective procedures, all these complications affecting visual outcome are significant. By careful attention to proper centration of corneal procedures, many of the optical problems following the refractive surgical procedures including LASIK may be decreased. The question then arises as to what method should be used to determine the optical zone and how this zone should be centered. Many axes of the eye can be described such as the optical axis, visual axis, pupillary axis, line of sight, line of fixation, etc. Over the years, confusion and conflicting definitions over these various axes have been sources of much controversy surrounding the question of what is the proper centration technique. In this chapter, we review the relevant definitions, examine the evolution of the current centration methods, and describe the most current clinical approach to centering corneal refractive surgery. B. INITIAL CENTRATION TECHNIQUES: USING THE OPTICAL AXIS AND THE VISUAL AXIS Every optical system has an optical axis defined by the line passing through the center of curvature of each component of the system (3). However, the human eye does not repre- sent a perfectly aligned optical system, so the eye cannot be assigned an optical axis. As shown in Fig. 14.1, the incoming ray of light hits a primary nodal point and then continues toward the fovea from a second nodal point with the identical angle to the optical axis (4). The Gullstrand model of the eye deals with the nonideal nature of the human eye by con- sidering it a centered system with a pair of nodal points. The visual axis is an interrupted line that connects the point of fixation with the fovea, passing through multiple nodal points (3). Figure 14.2 shows how the eye would be if it were a perfectly centered optical system. 200 Cheung et al. A B Figure 14.1 (A) The simplified schematic eye: in this simplified model of the eye as an optical system, the visual axis connects the object to the fovea via two nodal points. This model relates the object and image sizes and distances but does not take into account the real path of light as it passes through the human eye. (From Ref. 1.) (B) Pencils of parallel light rays in an emmetropic eye will be bent by the cornea and lens to focus on the retina. (From Ref. 59.) The visual axis may be useful in terms of allowing optical calculations for the eye such as the refraction and magnification of objects and their images. However, given that these models are based on a false assumption of the human eye being a centered system, these models are insufficient in making optical calculations for patients with especially decen- tered systems. Centration of LASIK Procedures 201 Figure 14.2 If the human eye were a perfectly centered system, then all optical elements includ- ing the corneal intercept of the visual axis, the corneal light reflex, the corneal center of curvature, and the foveal image would all be perfectly aligned. (From Ref. 7.) The initial proposed centering techniques in the 1980s were based on this erroneous assumption that the eye is centered according to this visual axis. Despite the theoretical use- fulness of the early models, they cannot be used for the human eye. There are numerous el- ements that may contribute to a patient’s eye being a decentered system—for example, an eccentric pupil or a large angle between the visual and optical axes. While the visual axis may be useful for theoretical circumstances and mathematical calculations relating objects and image sizes and distances, it is not of much value when evaluating the path of actual rays as they pass through the human eye. C. CURRENT CENTRATION TECHNIQUES: CENTERING BASED ON THE ENTRANCE PUPIL In the mid-1980s, Walsh and Guyton and then Uozato and Guyton transformed the initial thinking on centering techniques (5,6). They emphasized that the former method of using the visual axis of the eye was poorly defined and inaccurate; they reported that these meth- ods should not be used for centering corneal surgical procedures. Instead, these authors brought to light the notion of centering based on the entrance pupil. D. THE ENTRANCE PUPIL As shown in Fig. 14.3, the pupil can be thought of as it exists in three different planes: the entrance pupil, the real pupil, and the exit pupil. When we look at the human eye, we see a virtual image of the pupil and the iris that is based on the refractive properties of the cornea and aqueous. The entrance pupil is that virtual image of the pupil. Clinically, measurements have been taken showing that the entrance pupil is approximately 0.30 mm anterior to and 14% larger than the human pupil (8). It is located about 5 mm behind the front surface of the cornea (6). As the patient fixates on an object, a collection of light rays will fall onto the eye surface, but only those that specifically are within the boundaries of the entrance pupil will go into the eye. Similarly, the exit pupil of the eye is the image of the real pupil formed by refraction through the crystalline lens. 202 Cheung et al. Figure 14.3 The pupil can be thought of as it exists in three different planes: the entrance pupil, the real pupil, and the exit pupil. The entrance pupil is a virtual image of the pupil based on the re- fractive properties of the cornea and aqueous, located approximately 0.30 mm anterior to the real pupil and approximately 14% larger. The exit pupil is the virtual image of the real pupil formed by the refraction through the crystalline lens. (From Ref. 1.) E. THE OPTICAL ZONE There is a critical portion of the cornea used to see the fixation point—this portion of the cornea overlies the entrance pupil and is termed the optical zone as described by Maloney which is shown in Fig. 14.3 (9). The optical zone is centered on the “line of sight” which matches the chief ray in geometrical optical terminology, but only in a perfectly aligned op- tical system (10). This line of sight or chief ray joins the object to the center of the entrance pupil to the foveal image as shown in Fig. 14.4. Since the entrance pupil is circular, the bun- dle of rays passing through it is circular as well. These rays strike the corneal surface in a circular fashion—the optical zone—which is centered on the intersection of the line of sight with the cornea. These definitions are of critical importance to the corneal surgeon. Uozato and Guy- ton first emphasized that the intersection of this line of sight, or the chief ray, is located at the desired center of the optical zone for corneal surgical procedures such as LASIK or PRK (6). They also pointed out how irregular and unpredictable refraction and glare may result from any corneal scarring or irregularities overlying the entrance pupil. On the other hand, irregularities or scarring that are peripheral to the optical zone of the cornea affect only the light rays that do not reach the fovea; in other words, scarring peripheral to the corneal surface over the entrance pupil will not affect the foveal image. However, light from peripheral locations in the patient’s visual field does pass through the eccentric por- tions of the cornea to reach the entrance pupil; thus peripheral corneal irregularities or scar- ring can affect the patient’s peripheral image quality (6). Figure 14.5 displays how the light rays pass through the optical zone to reach the fovea. It also shows the path of peripheral rays to reach the parafovea. A corneal scar within the optical zone can scatter the light, leading to a blurred foveal image as shown in Fig. 14.6. Figure 14.7 shows how in LASIK, even a properly centered ablation zone can have periph- eral rays outside of the optical zone leading to unwanted visual side effects such as glare. F. THE PUPILLARY AXIS AND THE ANGLE LAMBDA The pupillary axis has been described as the line perpendicular to the cornea that passes through the center of the entrance pupil and is shown relative to the line of sight in Fig. 14.8 (1,6). It also passes through the center of curvature of the corneal surface. Therefore clini- cally it can be located as the surgeon centers the corneal light reflex in the center of the pa- tient’s pupil; in doing so, it is important that the surgeon take great caution to sight monoc- ularly from directly behind the light source. While the pupillary axis and the line of sight Centration of LASIK Procedures 203 Figure 14.4 The line of sight is defined as the line connecting the fixation point to the center of the entrance pupil. In geometrical optic terms, this is equivalent to the chief ray. (From Ref. 58.) [...]... SLACK, 1996, pp 108–109 Centration of LASIK Procedures 48 49 50 51 52 53 54 55 56 57 58 59 227 HD Hoskins, M Kass Becker-Shaffer’s Diagnosis and Therapy of the Glaucomas 7th ed St Louis, MO: CV Mosby, 1989, p 71 PS Hersh, BH Schwartz-Goldstein Corneal topography of phase III excimer laser photorefractive keratectomy Ophthalmology 19 95; 102:963–978 MG Mulhern, A Foley-Nolan, M O’Keefe, PI Condon Topographical... optical zone 0 0 .5 1 2 3 4 0 16 31 61 86 100 Source: From Maloney, 1990 222 Cheung et al Table 2 Mean Decentration Shown in Recent Studies Total mean decentration Study Procedure Cavanaugh et al (13) PRK 0.40 mm Lin (24) Amano et al (26) Schwartz-Goldstein and Hersh (29) PRK PRK PRK 0.34 mm 0 .51 mm 0.46 mm Terrell et al (42) PRK Pallikaris and Siganos (55 ) Mulhern et al (50 ) PRK LASIK PRK LASIK PRK 0.41... refractive outcomes in LASIK Centration of LASIK Procedures 2 25 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 EJ Ellis, DG Hunter Centering refractive corneal surgical procedures In: DT Azar, ed Refractive Surgery Stamford, CT: Appleton & Lange, 1997, pp 1 25 134 PS Binder Optical problems following refractive surgery Ophthalmology 1986;93:739–7 45 WB Lancaster Terminology... displacement/shift and drift from the study by Azar and Yeh (A) Low displacement (r ϭ 0.10) and low drift index (0.23) (B) Low displacement (r ϭ 0.20 mm) and high drift index (1.30) (C) High displacement (r ϭ 0.67 mm) and low drift index (0.00) (D) High displacement (0. 95 mm) and high drift index (3.27) The postoperative visual acuities for these patients were (A) 20/ 15, (B) 20/30, (C) 20/20, and (D) 20/40... of LASIK complications occur intraoperatively Preoperative complications are related to inadequate preparation for surgery (Table 1) Postoperative complications are almost always related to events that occurred during surgery Many complications unique to LASIK are microkeratome related With improvement in microkeratome technology, the incidence of LASIK complications has been substantially reduced and. .. centration and best corrected visual acuity ( 15, 17,34) Klyce and Smolek evaluated decentration from the center of the pupil and found that the amount of decentration did not correlate with best corrected visual acuity ( 15) Lin et al did studies using corneal topography to calculate the SRI, a measure of irregular astigmatism, and found that it did not correlate with decentration (17) On the other hand, a handful... applied clinical optics In: Albert and Jacobiec, eds Principles and Practice of Ophthalmology Philadelphia: W.B Saunders, 2000, p 53 37 15 Surgical Caveats for Managing Difficult Intraoperative Situations SAMIR G FARAH Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, U.S.A DIMITRI T AZAR Massachusetts Eye and Ear Infirmary, Schepens Eye Research Institute, and Harvard Medical School, Boston,... in 53 .2% overall, and in 44% and 63 .5% , respectively Experienced examiners were not significantly more accurate, and images of the 1.0 D scales were significantly more correct than Centration of LASIK Procedures 217 those of 0 .5 D scales They stated, therefore, that subjective analysis of postoperative corneal topography by itself is not sufficient to predict potential patient complaints after PRK, and. .. study by Stulting et al (2), the incidence of intraoperative complications decreased from 2.1% during the first 3 months to 0.7% during the last 9 months of the study, proving that the complication rates can be reduced as the surgical team gains experience Most intra- and postoperative complications are common to myopic and hyperopic LASIK Several complications may be prevented if slit lamp examination... laser photorefractive keratectomy Ophthalmology 19 95; 102:42–47 SA Klein, RB Mandell Axial and instantaneous power conversion in corneal topography Invest Ophthalmol Vis Sci 19 95; 36:2 155 –2 159 J Wang, DA Rice, SD Klyce A new reconstruction algorithm for improvement of corneal topographical analysis Refract Corneal Surg 1989 ;5: 379–387 DT Azar, PC Yeh Corneal topographic evaluation of decentration in photorefractive . dis- turbance (50 ). According to these results, decentration less that 0 .5 are optimal, those be- tween 0 .5 and 1.0 mm are acceptable, and those greater than 1.0 mm are considered severe and. MN) 0–0.9 D 1–1.9 D 2–3.9 D 4 5. 9 D 6–7.9 D 8–11.9 D Age (yrs) (%) (%) (%) (%) (%) (%) 20–29 58 28 11 4 Ϫ10 30–39 45 22 8 1 Ϫ2 5 40–49 32 14 3 Ϫ1 Ϫ4 Ϫ6 50 59 19 7 0 Ϫ2 5 Ϫ7 60ϩ 91 0Ϫ3 Ϫ6 Ϫ8 Achieved. www.RefractiveConsultant.com LASIK/ PRK Outcomes Perry S. Binder, MD 85 8-7 5 6-4 462; email: Analysis LASIKSupport@aol.com As an example we have included our nomograms from two surgeons (David R. Hardten, MD and Richard