Map Interpretation Guide. Map Interpretation Guide. Software Version 5.2

Map Interpretation Guide Map Interpretation Guide Software Version 5.2

Map Interpretation Guide Software Version 5.2

contents Foreword... 4 Introduction... 5 Getting Started... 6 1 The Refractive Report... 6 1.1. Importance of color scale and step size in maps... 6 1.2. Indices displayed with the Refractive Report... 7 1.3. Evaluation of Refractive Reports... 10 2. The Keratoconus Report... 14 2.1. Importance of Instantaneous map and Keratoconus Indices... 14 2.2. Evaluation of Keratoconus Reports... 15 3. The Wavefront Report... 19 3.1. Zernike coefficients, higher order aberrations and shape of the cornea... 19 3.2. Understanding the relationship between HOAs, refraction, quality of vision, corneal shape and curvature... 20 3.3. Evaluation of Wavefront Reports... 22 4. The IOL Power Report... 27 4.1. About how GALILEI s Total Corneal Power is improving the IOL power calculation... 27 4.2. Evaluation of IOL Power Reports... 32 5. The Map x1... 34 5.1. Importance of the pachymetry map... 34 5.2. Other Map x1 options... 36 5.3. Evaluation of the Map x1 display... 37 6. The Map x4... 41 6.1. Importance of the Anterior BFTA Elevation map... 49 6.2. Evaluation of the Map x4 display... 53 7. Axial and Instantaneous Anterior-Posterior Topography... 55 7.1. Importance of the topography of both corneal surfaces... 56 7.2. Evaluation of the topography of both corneal surfaces... 57 8. BFS and BFTA Elevation maps... 62 8.1. Importance of the BFS Elevation map... 62 8.2. Importance of the BFTA Elevation map... 63 8.3. Evaluation of the Display showing BFS & BFTA Elevation maps of both surfaces... 64 9. The American Style Display... 68 9.1. Evaluation of the American Style Display... 70 10. The German Style Display... 72 10.1. Evaluation of the German Style Display... 72 11. Differential Maps... 75 Appendix... 79 3

Foreword After working in corneal topography for more than 10 years, I was the first user of a GALILEI system in Brazil in 2007. I am grateful to the Department of Ophthalmology, Federal University of São Paulo that accepted to receive my unit for research. If at that time I was passionate about the possibility to integrate Placido and Scheimpflug technologies, now I am delighted to see that the GALILEI is much more than a topographer. It is a multifunctional analyzer of the anterior segment of the eye with features and applications that serve more than only refractive surgery. More sophisticated and complete than other equipment, it gave us the opportunity to provide more accurate data in a simple to use device with easy to interpret exams. The CGA color scales and the Alternate Profile were developed to fulfill such a target. The CGA color scales and Alternate Profile use the strategy of establishing a fixed guide value that focuses on the yellow step of the best available color scale for each magnitude. This color scale guide is designed to ease the perception of borderline cases those cases we want to discover before and not after problems appear. This Map Interpretation Guide reflects my opinions on corneal tomography and how to use the GALILEI. I hope it is useful for all GALILEI users, especially those who are just beginning to use it. Carlos G. Arce, M.D. 4

Introduction The purpose of this guide is to facilitate the interpretation of GALILEI reports and displays and highlight how the choice of color scales can help guide interpretation. This guide provides example cases to help the user to understand relevant clinical patterns in maps included in the GALILEI reports and displays, focusing on three common cases: Example 1: Normal Spherical Cornea Example 2: Normal Astigmatic Cornea with Initial Posterior Surface Changes Example 3: Keratoconus (KC) with Irregular Astigmatism Maps of the Alternate Profile included in this guide correspond to my suggestions reported in the manual software (SW) version 5.2 update notes. The Alternate Profile was optimized to use the best available scale for each magnitude and incorporates the universal meaning of traffic light colors for all maps (Refer to the Appendix: Arce CG. SW version 5.2 update notes 2010 for more information). Maps of the Default Profile are those available in all GALILEI units in former SW versions. The Default Profile uses one default distribution of colors for all maps. Although the same data are used to produce maps of both profiles, the map patterns and colors may look different due to the scales used. Numerical values are shown only in the Alternate Profile and could be displayed on all maps. Note: These cases were acquired with SW version 5.0, which did not include the indices for KProb and Instantaneous Curvature that are included in later versions. Therefore these indices are reported as N/A in the reports. 5

Getting Started 1. The Refractive Report The basic Refractive Report contains the four standard maps that are displayed by other tomographers. These maps include Anterior Axial Curvature, Pachymetry, Anterior Elevation and Posterior Elevation (BFS) maps, shown in clockwise order. 1.1. Importance of color scale and step size in maps The Anterior Axial Curvature map is displayed in the Alternate Profile (Figure 1A) with the Default CGA 1 D color scale and is calculated with a keratometric refractive index of 1.3375. The Default Profile (Figure 1B) of the same map is presented with the Default Type I 1.5 D color scale. There are reasons why Anterior Axial maps with one diopter (D) scale are recommended and easier to understand. First, it is faster and simpler to count color steps one-by-one in 1 D increments. For example it is easier to quantify the difference between the flatter and steeper zones and to observe the asymmetry of dioptric profiles in different meridians or the increased number of diopters in multifocal corneas. Second, larger steps may mask typical patterns of asymmetry and astigmatism. Normal, predominantly spherical surfaces (with astigmatism less than 1 D) will not display the trade-mark bowtie pattern. In fact, other devices are set up with smaller steps to accentuate the bow-tie pattern so they are more easily observed. Smaller steps (< 1 D) are not necessary because slight astigmatism is not a problem and usually is a good sign of normality. In fact, very small differences in curvature add unnecessary noise on the maps. Furthermore, the intuitive universal meaning of colors is lost when extreme red or blue colored regions are achieved producing false positive patterns. Third, vertical or horizontal asymmetries larger than 1.4 D are usually considered risk factors for refractive surgery. Since the GALILEI Default CGA 1 D color scale has 6 green steps ranging from 41 D to 46 D, any asymmetry larger than 1 D will be evident, even if the surface has a curvature within the normal range. The appearance of a yellow region indicates that the surface curvature is on the borderline of being abnormally steep and any orange region indicates that the surface has already an abnormally steep curvature. The German CGA 20 µm scale used in the Pachymetry map of the Alternate Profile is much better, in my opinion, than the Default Type II 25 µm scale of the Default Profile or the ANSI Type III 10 µm used by other equipment. Although I would prefer to have only 5 or 6 6

green steps instead of 7 green steps, the thickness progression and distribution are more apparent when the yellow step is located in the middle of the color scale. The German style has 15 thicker steps of 20 µm each instead of only 11 steps as displayed with the Default style. The Anterior BFS Elevation (8-mm-diameter central zone) and Posterior BFS Elevation (7.8-mm-diameter central zone) maps have the ANSI CGA 5 µm scale in the Alternate Profile. Within a 5-mmdiameter central region of interest (ROI), the normal maximum peak of Anterior Elevation BFS was preliminarily established to be less than 10-12 µm and of the Posterior Elevation BFS to be less than 15-16 µm (Arce CG. Data not published; Blanco C, Nuñez X. Elevation and pachymetry values in normal corneas obtained by GALILEI. Boston, USA. ASCRS 2010). These values can be seen when the numbers are overlaid on the map or measured quickly using the mouse on any map location. To facilitate map interpretation, the Alternate profile provides 3 green steps in the maps to show the normal ± 5 µm values and 2 yellow steps to indicate such borderline values. The Default Profile uses the Default Type III 5 µm scale with the zero value in the second of 6 green steps, which may make it more difficult to detect any abnormality in the two elevation maps. 1.2. Indices displayed with the Refractive Report In addition to maps, relevant numerical indices corresponding to the maps are shown in the right column of the Refractive Report. The indices are presented in mm and diopters, when available. The average SimK values, K Steep, K Flat and steeper Astigmatism with angle are calculated and displayed at the top. Several indices from other maps (Anterior Instantaneous, Posterior Axial Curvature, and Total Corneal Power by ray tracing) and the anterior and posterior squared eccentricity (e 2 = -Q) are also displayed in the Refractive Report. The average normal anterior axial curvature has a 7.8 mm radius, which corresponds to an equivalent power of 43.27 D (range: 41 D to 47 D) using the 1.3375 keratometric refractive index (or 48.21 D using the 1.376 physiologic refractive index). The average normal posterior axial curvature has a 6.5 mm radius, which corresponds to an equivalent power of -6.15 D (range: -5.50 D to -6.8 D). The shape is reflected in the eccentricity (e) value. Normal corneal surfaces vary from a sphere (e 2 = 0) to a prolate (steeper at the center and flatter at the periphery) toric ellipsoid asphere (+1 > e 2 > 0). The anterior surface (mean value e 2 = +0.20 ± 0.16) is less prolate than the posterior surface (posterior e 2 > anterior e 2, mean value e 2 = +0.25 ± 0.16) in agreement with the normal corneal thickness distribution where the cornea is thinner at the center (Arce CG. Corneal shape 7

and HOAs may be used to distinguish corneas with keratoconus. Poster. Paris, France. ESCRS 2010). Although there are corneas with KC and e 2 < +1, and corneas with PMD that may achieve e 2 < 0, it seems there are no normal untouched corneas with e 2 < 0 or e 2 +1 (Arce CG, Trattler W, Dawson DG. Keratoconus and Keratoectasia. In Atlas of Corneal Pathology and Surgery. Boyd S. Editor-in-Chief. Jaypee Highlights Medical Publishers. Panama. 2010). The mean Total Corneal Power (TCP) by ray tracing, TCP steep, TCP flat and steeper Astigmatism with angle are also calculated automatically for a similar zone than the Sim K. The Axial Anterior Curvature in diopters calculated with a 1.3375 refractive index is also shown from the central (0 4 mm), mid-peripheral (4 7 mm) and peripheral (7 10 mm) regions. Other indices displayed are the central (0 4 mm), mid-peripheral (4 7 mm) and peripheral (7 10 mm) corneal thickness averages; the thinnest point thickness and location; the pupil size and location of pupil center, and the limbusto-limbus distance from Superior-Inferior and Temporal-Nasal. Missing indices may indicate that an earlier SW version was used to acquire the images or the result of poor quality data input. For example, when the pupil and/or limbus indices are not shown, the user can review the images in the Verify panel to verify the top view (TV) image focus and alignment to the fixation marks. The Refractive Report is the most commonly printed report used for preoperative refractive surgery screening around the world. The Refractive Report contains the traditional maps that have been standardized by previous topography devices. However, the Refractive Report may be insufficient for some cases because it compares anterior and posterior corneal surfaces using only BFS elevation maps. The posterior surface curvature map is not shown in this popular report and can be more instructive to aid early detection of pathology and screening for corneal disease. For this reason I suggest reviewing routine cases using two separate reports. A new customized report containing the curvature and elevation topography of both anterior and posterior corneal surfaces (refer to N 7 of this guide: Axial and Instantaneous Anterior-Posterior Topography). A second report using the Map x1 option and containing only the pachymetry map (refer to N 5 of this guide: Map x1). This is my preference in my Eye Clinic in Brazil and my suggestion for countries where pachymetry and topography exams are reported separately in order to fulfill independent codes used by health systems. We must remember that pachymetry is a different measurement and this exam deserves a separate consultation report from topography. 8

1.3. Evaluation of Refractive Reports Example 1: Normal Spherical Cornea The Alternate Profile (Figure 1A) of the Refractive Report shows a normal almost spherical anterior surface. There is an anterior axial curvature map without a bow-tie pattern (0.22 D of astigmatism and e 2 = 0.09) that tends to blue because the surface is uniformly flat (SimK Avg 41.09 D). The thinnest thickness value is 560 µm and there is asymmetric thickening of the peripheral cornea well observed by the change of color from green to blue (more nasal blue steps than temporal). There is an almost completely green anterior BFS map as should be expected when the surface uniformly fits the same radius of best fit sphere (±5 µm). The normal green horizontal bridge pattern on the posterior BFS map suggests that the posterior surface is toric, prolate and elliptical (the indices report 0.25 D of astigmatism and e 2 = 0.29). In the Default Profile (Figure 1B) there is an asymmetry of colors on the posterior BFS map with an orange zone at the temporal side of the central yellow (zero value) bridge. Despite a possible correlation with the asymmetric pachymetry map, this apparent asymmetry in elevation could be a mistaken hot zone since it is not observed in the respective map of the Alternate Profile. It also does not appear in the posterior curvature maps that will be shown later in this document (refer to N 7 of this guide: Axial and Instantaneous Anterior-Posterior Topography). Example 2: Normal Astigmatic Cornea With Initial Posterior Surface Changes This report shows a normal anterior prolate elliptical surface (e 2 = 0.31) with a typical steeper vertical (warmer) bow-tie representing a 1.5 D with-the-rule astigmatism. A normal horizontal green bridge pattern is found in the anterior BFS map of the Alternate Profile (Figure 2A). There is a false-positive orange zone in the anterior BFS map of the Default Profile (Figure 2B). The posterior surface is also elliptical but more prolate (e 2 = 0.72) than the anterior surface. The posterior BFS map also shows a normal horizontal bridge pattern with a yellow hot zone (border line values 15 µm) at the temporal side. Notice that this yellow hot zone is orange in the Default Profile in the posterior BFS map. There is a completely green pachymetry map indicating a normal thickness distribution and normal thinnest thickness value of 545 µm. There is a normal symmetric thickness progression (fewer than 10 steps of 20 µm per step). Border line yellow hot zones in the Alternate Profile are important to detect early topographic signs as they are indicators to search for risk factors in other maps. When these hot zones are temporal or nasal, 9

Figure 1A Figure 1B 10

Figure 2A Figure 2B 11

it usually announces an asymmetric bow-tie (irregular astigmatism) in the respective curvature maps. In this case such asymmetries will be described later (refer to N 2 of this guide: The Keratoconus Report and to N 7: Axial and Instantaneous Anterior-Posterior Topography). When they are located at the center of the bridge, they may correlate with steeper curvatures as with example N 3 that follows. Despite the differences in color distribution with the Default Profile, it is important to remember that each map represents the same surface and it is originated from exactly the same numeric values. Note: The astigmatism or cylinder value calculated from the axial map is relative to the center of the map. Misalignment of the device redcross to the center of the fixation marks (four white dots) can change the magnitude and angle of the axial curvature (related to the K steep and K flat values). The instantaneous curvature astigmatism values are less sensitive to decentration. Also the axis is less stable for lower magnitude cylinder values, e.g. when K steep = K flat. Example 3: KC with Irregular Astigmatism There are several clear indicators of KC and abnormal corneal deformation in maps and indices of this Refractive Report: a) Despite a SimK Avg of 46.86 D that is within normal range, there is an irregular with-the-rule astigmatism in both the anterior (3.28 D cylinder) and the posterior (-0.48 D cylinder) surfaces. b) The anterior curvature has a steep (red), very asymmetric vertical bow-tie appearance that resembles a doll-like pattern with a small head and a big colored skirt. c) The anterior surface has an inferior steepening of more than 52 D and a superior flattening less than 40 D ( 12 D of inferiorsuperior (I-S) dioptric difference). d) Both anterior and posterior surfaces (e 2 = 1.17 and e 2 = 1.23, respectively) are prolate with an asymmetric hyperbolic shape (e 2 > 1.0). e) The central corneal thickness (CCT) is 471 µm and the thinnest point is 467 µm (2 nd orange step). Both thicknesses are less than 500 µm, which is the borderline of the Alternate profile guide value that I use. f) The thinnest point is located 0.8 mm from the pupil centroid. Despite that this dislocation is less than 1 mm, its location induces a slight asymmetry of thickness progression and distribution reflected by 7 temporal steps and 11 nasal steps. For comparison, using the Alternate Profile, the normal progression profile is fewer than 10 steps (Arce CG, Use of optical pachymetry in diagnosis of keratoconus. Boston, ASCRS 2010). g) Since normal maximum values on BFS elevation maps should be below 16 µm, no more than 2 yellow steps are expected in the Alternate Profile (Figure 3A). Here however both anterior and posterior BFS maps already have yellow-orange horizontal bridge patterns. 12

Figure 3A Figure 3B 13

h) There is an abnormal central hot zone in both BFS elevation maps achieving almost 20 µm (third yellow step) in the anterior surface and 30 µm (already orange) in the posterior surface. i) When viewed in the Default Profile (Figure 3B), these same anterior and posterior surface hot zones are both displayed as orange-red hot zones. 2. The Keratoconus Report In the Keratoconus Report, the traditional Anterior Axial Curvature map is replaced by the Anterior Instantaneous (tangential) Curvature map. The other maps are the same shown in the Refractive Report and include the Pachymetry, Anterior Elevation and Posterior Elevation (BFS) maps, shown in clockwise order. 2.1. Importance of Instantaneous map and Keratoconus Indices The Anterior Instantaneous Curvature map concentrates on the localized curvature and every point correlates better with its real anatomical location on the corneal surface. As a consequence, smaller details are accentuated in the Instantaneous Curvature map. Such details are smoothed out in the Axial map of the same data. This Instantaneous map is calculated with the keratometric refractive index of 1.3375 and is displayed with the Default CGA 1.5 D scale in the Alternate Profile. In the Default Profile it is shown with the Default Type I 1.5 D scale. As we have seen, selecting different scales for the axial and instantaneous maps increases the chances of observing different curvature patterns. Sometimes one display appears normal whereas the other appears suspicious. The numerical indices of axial curvature and limbus measurements are replaced by the anterior surface Keratoconus Indices. Among these indices, the Inferior-Superior (I-S) index and the Standard Deviation of Corneal Power (SDP) are often increased in KC. The at risk I-S index based on GALILEI data has not been formally defined. Although usually an I-S index of more than 1.4 D is considered a risk factor for post-lasik ectasia based on data from other devices, an I-S index found in KC suspects of 1.20 D has also been reported (Li X, Yang H, Rabinowitz YS. Keratoconus: classification scheme based on video-keratography and clinical signs. J Cataract Refract Surg 2009; 35:1597 1603). On the other hand the Surface Regularity Index (SRI) approaches zero in spherical surfaces. The Keratoconus Prediction Index (KPI) is a compilation index that simulates the percent probability of KC based on the anterior surface analysis (Maeda N, Klyce SD, Smolek MK, Thompson 14

HW. Keratoconus Prediction Index. Invest Ophthalmol Vis Sci 1994; 35:2749 2757 and Mahmoud AM, Roberts C, Lembach R, etal. Simulation of machine-specific topographic indices for use across platforms. Optometry and Vision Sc 2006; 83:682 693). KPI and other indices are guide numbers reflecting the probability of keratoconus based on the anterior surface curvature features. Since this is a probability relying on topographic data from one dataset, it should not be the sole assessment tool in the diagnosis of KC and should be confirmed by clinical diagnosis. Based on my experience, GALILEI s KPI from 0 to 10% seems to correspond to normal or suspicious corneas; KPI from 10 to 20% to suspicious or borderline keratoconic corneas; KPI from 20 to 30 % to keratoconic or perhaps still suspicious corneas, and KPI more than 30% should be considered pellucid marginal degeneration (PMD) or KC (Arce CG. Data not published). The Keratoconus Probability (KProb) refers to a specificity and sensitivity validation of the reported KPI value based on the statistical analysis of a series of normal corneas and corneas with KC (Refer to the Roberts C. SW version 5.2 update notes for more information). 2.2. Evaluation of Keratoconus Reports Example 1: Normal Spherical Cornea The Keratoconus Report either in the Alternate (Figure 4A) or the Default (Figure 4B) Profile shows maps with similar patterns described for the Refractive Report. I-S index is 0.61 D indicating a small asymmetry within normal range. SDP is only 0.36 D and SRI is 0.41 D suggesting a regular and spherical anterior surface. ACP is 40.99 D confirming the tendency towards a flat surface. The KPI is 0%. Example 2: Normal Astigmatic Cornea With Initial Posterior Surface Changes In the Alternate Profile (Figure 5A) the Anterior Instantaneous Curvature map shows a D-shaped pattern reflecting asymmetry in the horizontal meridian. This D-shaped sign has been already reported as a risk factor for ectasia (Abad JC, Rubinfeld RS, Del Valle M, Belin MW, Kurstin JM. Vertical D: A novel topographic pattern in some keratoconus suspects. Ophthalmology 2007; 114:1020 1026). The I-S index is 1.29 D also indicating some inferior-superior asymmetry. The SDP index is 0.91 D, SRI is 0.93 D, ACP is 43.87 D and KPI is 0.9%, which are all within normal range. The aforementioned curvature asymmetry observed in the Alternative Profile display of the anterior surface is less pronounced in the Default Profile, as shown in Figure 5B. 15

Figure 4A Figure 4B 16

Figure 5A Figure 5B 17

Figure 6A Figure 6B 18

Example 3: KC with Irregular Astigmatism The Keratoconus Report shows similar patterns in maps found in the Refractive report in both the Alternate (Figure 6A) and the Default (Figure 6B) profile. However in the Anterior Instantaneous Curvature map (in the upper left panel) there is more than 54 D of inferior steepening and less than 36 D of superior flattening ( 18 D of dioptric difference). The I-S index is 10.18 D indicating significant inferiorsuperior asymmetry. The SDP index is 3.67 D, SRI is 1.81 D, ACP is 47.62 D and KPI is 100%, which are well within the typical abnormal range indicating KC. 3. The Wavefront Report 3.1. Zernike coefficients, higher order aberrations and shape of the cornea The Wavefront Report of GALILEI s SW 5.2 shows the total corneal higher order aberrations (HOAs) in microns (at left) and in diopters (at right). The Wavefront map in microns is similar to the Wavefront map in diopters, however the colors are inverted. The pie distributions of indices for both maps are exactly the same. Thus, I prefer to present the map of HOAs in microns (at left) and the pie distribution in diopters (at right). The GALILEI allows one to search the amount of corneal HOAs in any region of interest (ROI) from 3 to 6 mm. I normally initiate my evaluation with a 6-mm-diameter ROI and all Zernike coefficients active (when all indice symbol tabs are highlighted). It is important to mention that HOAs found in normal corneas have a different distribution and lower magnitude values than in KC, PMD or after corneal surgeries (Alió JL, Shabayek MH. Corneal higher order aberrations: a method to grade keratoconus. J Refract Surg 2006; 22:539 545). The first row of the Wavefront Report shows the Zernike coefficients for astigmatism and defocus. These second-order corneal aberrations are related to the corneal components of the patient s manifest refraction, thus they are often better understood in diopters instead of microns. The second row contains third-order corneal aberrations. Vertical (at left) and horizontal (at right) trefoil is very close to zero in normal corneas. In corneas with penetrating (PKP) or lamellar keratoplasty (Pesudovs K, Coster DJ. Penetrating keratoplasty for keratoconus: the nexus between corneal wavefront aberrations and visual performance. J Refract Surg 2006; 22:926 931), radial keratotomy (RK), paracentral scars, advanced PMD or corneas modified by edema, the trefoil terms are larger than ±0.20 µm due to paracentral and/or peripheral irregularities (Arce CG. Data 19

not published). The vertical (center left) and horizontal (center right) coma term may be used to evolve the progression of KC and PMD. Coma is one of the most important HOAs and a strong sign reflecting deformation of corneal surfaces (Gobbe M, Guillon M. Corneal wavefront aberration measurements to detect keratoconus patients. Cont Lens Anterior Eye 2005; 28:57 66). Asymmetric astigmatism, an asymmetric change of aspheric curvature of the anterior surface and/or asymmetry of thickness progression usually correlates with increased coma (Arce CG. Data not published). Third row has the quatrefoil and 4 th order astigmatism. Both terms tend to increase more than ±0.30 µm when the cornea has paracentral or peripheral deformation. As a rule of thumb, as the cornea is more deformed, the higher the 5 th to 8 th HOAs the system calculates. The most important fourth-order aberration is the spherical aberration (SA), located at the center of the third row. The normal SA value is in the range +0.15 µm to +0.30 µm and should be stable throughout life, when the cornea is not progressively deformed, surgically altered or diseased. Total root mean square (RMS) in diopters and microns is also shown at the bottom right of the numerical indices. Squared eccentricity (e 2 ) is equivalent to negative asphericity (e 2 = -Q). e 2 and Q are average values representing the shape of corneal surfaces. Prolate anterior surfaces have positive e 2 and oblate surfaces have negative e 2. When e 2 = 0, then the surface is spherical. An elliptical surface has e 2 from zero to < ±1. A parabolic surface has e 2 = ±1.0. A hyperbole has e 2 > ±1.0. The shape of anterior corneal surface correlates well with the amount of HOAs. More prolate anterior surfaces have less positive SA. When e 2 0.55, the SA is zero. 3.2. Understanding the relationship between HOAs, refraction, quality of vision, corneal shape and curvature Regardless of the cause, corneas with increased HOAs show a quantifiable decrease in visual performance that is pupil size dependent. The amount of coma has been corelated with the quality of vision in patients with asymmetric progression of KC or PMD. Trefoil, coma and quatrefoil have been correlated with the presence of glare and halos after corneal surgeries and IOL implantation. The amount of SA has a direct influence in the quality of vision and focusing of images by the eye. An SA of zero will produce a sharper image but shallow depth of focus. A larger SA (negative or positive) allows larger depth of focusing. Corneas after hyperopic or presbyopic surgery and with KC are more prolate and have negative SA, which partially explains their loss of sharp vision but their facility to focus near and far. 20

Corneas with KC become progressively steeper (positive Rabinowitz index when > 47.2 D), multifocal, and more prolate. Often steeper anterior surfaces with zones of > 50 D and clear topographic KC signs have eccentricity values higher than +0.80. And typically the posterior surface will precede the anterior surface to show a parabolic shape (e 2 = +1). The anterior surface initially has a symmetric (regular) with-the-rule (vertical, steeper ( warm or yellow/orange/red color) bowtie) astigmatism that increases (normal astigmatism < 3 D), showing larger difference of curvature values from the periphery to the center. Such multifocal corneas have more color steps on curvature maps. With the steepening of both surfaces, a nipple-, sagging- or globe-shaped cone appears depending on the size of the steeper central zone. The SA becomes less positive or is shifted to a negative value (SA 0 typically when the anterior e 2 > 0.60). In a practical view and for screening or diagnostic purposes, although there are corneas with KC with ellipsoid symmetric shape and e 2 < 1 in either anterior or posterior corneal surfaces, there are no normal untouched corneas with either anterior or posterior corneal surfaces with a parabolic symmetric shape and e 2 1 (Arce CG. Data not published). This observation supports e 2 = 1 to be an important cutoff value in KC diagnosis. Furthermore, when the anterior surface already has a hyperbolic shape (e 2 > +1), the cone peak is beginning to be dislocated temporal-inferiorly. Hyperopic or presbyopic surgery with excimer laser generates an anterior surface with e 2 1 and a posterior surface with e 2 < 1. After intracorneal femtosecond presbyopic surgery, we expect the e 2 of both surfaces to increase. It has been suggested that the best combination after hyperopic or presbyopic surgery is a SA of around -0.50 µm and a plano or a very low positive residual refraction. This is one of the reasons why these eyes may accept subjectively a variety of refractive corrections and remain with an unchanged visual acuity. On the other hand, the combination of a negative residual refraction with a negative corneal SA seems to be rejected by these patients. Therefore it is better not to recommend mono-vision or multifocal vision with negative refraction after corneal refractive surgery or intraocular lens (IOL) implantation in eyes with such prolate corneas. On the other hand, when a myopic LASIK or PRK ablation is required, the anterior surface becomes more oblate and flatter with more positive SA. The ideal combination in these eyes for best mono-vision, better near focus than far focus, or multifocal vision seems to be achieved by leaving a moderate negative (myopic) residual refraction. I understand that this concept explains partially why spherical IOLs (with SA between +0.18 to +0.25 µm) were so successful after being implanted in normal eyes (with SA between +0.20 to +0.30 µm), when a small myopic residual refraction was programmed. Similarly, it explains why monovision after incomplete myopic LASIK correction is better tolerated and accepted than mono-vision after hyperopic LASIK correction. 21

These concepts open the possibility to customize IOL implants according to the residual refraction that the surgeon and patient want to keep after surgery. After cataract extraction, the SA from the lens is removed. Therefore pseudophakic eyes will remain with an SA produced by the cornea and the IOL implanted. Spherical IOLs have positive SA, most of them between +0.18 to +0.25 µm. Currently available aspheric IOLs may have either negative SA (e.g. -0.20 µm for the Alcon Acrysoft IQ or -0.27 µm for the AMO Technis Multifocal IOL) or neutral SA (e.g. zero for the Bausch & Lomb SofPort, hydrophilic Rayner C-flex Aspheric or hydrophobic Mediphacos IOLs). Although one could compensate the normal positive corneal SA with the negative SA of certain IOLs, the policy to leave all eyes with zero or close to zero SA may not always be the best solution. There are very satisfied patients with larger positive or negative SA that would like to preserve their previous good multifocal quality of vision after IOL implantation. Their total SA should not be modified. Although we presently have one-size-fits-all IOLs, I foresee a time where corneal wavefront measured by the GALILEI would guide the manufacturing of better IOLs. 3.3. Evaluation of Wavefront Reports Example 1: Normal Spherical Cornea The e 2 of the anterior surface is 0.09, i.e. almost spherical. This value correlates well with the low 2 nd order astigmatism (0.19 @ 29.8, shown in the flatter axis) and 4 th order astigmatism (-0.01 µm), the slightly increased positive spherical aberration (+0.30 µm), and the low trefoil and quadrefoil in an ROI of 6 mm. Differences among total corneal wavefront-derived, SimK-derived (0.22 D @ 100, shown in the steeper axis), and ray-traced total corneal power-derived (0.12 D @ 141, shown in the steeper axis) astigmatisms reflect differences in the calculations of each value, differences in reference planes, and different sensitivities to measurement decentration. The size of the data zone or region of interest (ROI) can also change the astigmatism value. The wavefront region of interest can be selected to be between 3- to 6-mm-diameter central ROI. The region size is shown above the map. The SimK and the ray traced total corneal power astigmatism values are calculated from a 1- to 4-mm-diameter ring ROI. The horizontal coma is +0.42 µm suggesting some asymmetry in asphericity on this meridian. Although there is a relatively low total RMS (0.74 µm or 0.57 D), the pie chart confirms that defocus, coma and spherical aberrations are the most prominent aberrations in this cornea. The Wavefront Report is shown in the Alternate Profile, which I recommend for printing, in Figure 7A and in the Default Profile in Figure 7B. 22

Figure 7A Figure 7B 23

Figure 8A Figure 8B 24

Example 2: Normal Astigmatic Cornea with Initial Posterior Surface Changes (Figure 8) As expected, the e 2 of the anterior surface (0.31) is smaller than the e 2 of the posterior surface (0.72), thus the posterior surface is more prolate than the anterior surface. The 2 nd order astigmatism (1.15 D) is responsible for 93.5% of the total corneal aberrations. The coma term is less than ± 0.16 µm despite the slight D-like asymmetry found in the Instantaneous Anterior Curvature map. The defocus term is -0.18 D and other HOAs are less than ± 0.1 µm. SA is + 0.15 µm. Smaller SA would mean there is a tendency towards a steeper more multifocal prolate surface and therefore a sign of change in corneal shape. The total RMS is 1.19 D or 1.55 µm. The report, shown in the Alternate Profile, which I recommend for printing, is in Figure 8A and the Default Profile in Figure 8B. Example 3: KC with Irregular Astigmatism (Figure 9) The anterior (1.17) and posterior (1.23) e 2 reflect very prolate already hyperbolic corneal surfaces. Keratoconic corneas typically have an e 2 value close to +1.0 or higher. Defocus in this case is very low (+0.04 µm), vertical trefoil is higher than normal (+0.45 µm), and SA is already negative as expected (-0.15 µm). Other HOAs have low values. The 2 nd order astigmatism (1.56 D, 36.7%) and both vertical coma (-2.47 µm) and horizontal (-0.52 µm) coma (58.5%) together are responsible for most corneal HOAs. The vertical coma is probably the most important HOA to monitor KC. Vertical coma is also a numerical expression of corneal deformation. GALILEI may also show the total corneal coma either in µm or D. The amount of irregular astigmatism with a significant dioptric difference between the steeper and flatter curvatures and the Kranemann-Arce index (below) also reflect quantitatively the deformation of the corneal surfaces. The Kranemann-Arce index is the differential between the minimum negative and the maximum positive BFTA elevation values in microns within the 5-mm-diameter central zone. The normal Kranemann-Arce index was 10.72 ± 5.72 µm (maximum BFTA limit 25 30 µm) for the anterior surface and 22.49 ± 9.29 (limit 40 45 µm) for the posterior surface (Arce CG. Corneal shape and HOAs may be used to distinguish corneas with keratoconus. Poster. ESCRS; Paris, France. 2010). Qualitatively, the deformation indicative of corneas with KC is shown by (1) the irregular pattern of astigmatism found in 25

Figure 9A Figure 9B 26

the curvature maps, (2) the asymmetric pattern of aspheric change found on the BFTA elevation maps, and (3) the asymmetric pattern of thickness distribution found in the pachymetry maps (refer to N 5 of this guide: Map x1). The report, shown in the Alternate Profile, which I recommend for printing, is in Figure 9A and in the Default Profile in Figure 9B. 4. The IOL Power Report Both the Alternate (Figure 10A to 12A) and the Default Profile (Figure 10B to 12B) of the IOL Power Report show the Anterior Axial curvature map and the Total Corneal Power map by ray tracing from an 8-mm-diameter data zone, the Total Wavefront HOA map from a 6-mm-diameter data zone, and a top view of the Placido ring pattern reflected on the cornea. This report shows all indices including the SimK values required for standard IOL formulas. The average simulated keratometry (SimKavg) is actually the average of the flatter SimKf and the steeper SimKs. The astigmatism is the difference between SimKf and SimKs. The Anterior Instantaneous Curvature and the Total Corneal Power (ray traced) from a 1- to 4-mm-diameter ring zone are also shown. The Total Corneal Power (ray traced) is also calculated as the average of a central zone (0 to 4-mm-diameter), paracentral zone (4 to 7-mmdiameter), and peripheral zone (7 to 10-mm-diameter). Also shown are the RMS HOA, the SA, the central anterior chamber depth (ACD) or distance from the corneal endothelium to the anterior lens, the anterior chamber volume for an 8-mm-diameter central zone, the chamber angles in degrees at the superior, temporal, inferior and nasal direction, the nasal-temporal (horizontal) and superior-inferior (vertical) limbus diameter, the pupil diameter (approximately mesopic) and the Cartesian location of the pupil centroid relative to the center of the map. The average anterior segment length (ASL) or distance to the posterior lens requires measurement with a dilated pupil. 4.1. About how GALILEI s total corneal power is improving the IOL power calculation The most important value in this window is the 4-mm central average Total Corneal Power (TCP). This is the value that is used in the ASCRSsupported IOL calculator (iol.ascrs.org). The TCP is calculated by ray tracing the refractive power of the cornea using the anterior surface, the pachymetry and the posterior surface. Unlike the SimKavg, the TCP considers the pachymetry and the posterior surface curvature, as well as the actual index of refraction of each interface. The challenge in 27

Figure 10A Figure 10B 28

Figure 11A Figure 11B 29

Figure 12A Figure 12B 30

comparing the SimK to TCP is that both values calculations also have different reference planes and index of refractions. The SimK is based on the anterior surface curvature data however it uses an adjusted or keratometric index of refraction to estimate the pachymetry and posterior curvature, assuming an untouched, standard corneal shape and Gullstrand eye model. The TCP uses the actual pachymetry and posterior curvature, and standard index of refraction for the cornea and aqueous humor. The power depends on the method of calculation and the chosen reference plane, which is different for both values. The TCP values were calculated with the index of refraction of the cornea in version 5.2 and of the aqueous in version 5.2.1. Both values are referenced to the anterior corneal surface. The TCP map did not change appearance (and is still different from the SimK map) however the magnitude of TCP changed slightly, corresponding to the change in index of refraction. The ASCRS IOL calculator offers two formula depending on the software version used (pre-version 5.2.1 and version 5.2.1 and newer). The TCP is the average of every detected point in the selected ROI. This average was initially described for the total-mean (equivalent or Gaussian power) and total-optical (ray-traced following Snell law) Orbscan II corneal powers (Sónego-Krone S, López-Moreno G, Beaujon- Balbi O, Arce CG, et al. A direct method to measure the power of the central cornea after myopic laser in situ keratomileusis. Arch Ophthalmol 2004; 122:159 166), later coined as quantitative area topography (QAT). The SimKavg is an assumed or approximate total corneal power (Simulated K) calculated from the anterior surface curvature data. On the GALILEI, the SimK value was determined to be calculated from a 1- to 4-mm diameter ring-shaped ROI and is the mean of two values, the flat SimK and the steep SimK. Similar assumed or approximate total corneal powers are based on the anterior curvature (Axial and Instantaneous Curvatures) and the corneal power is then calculated using a keratometric refractive index of 1.3375. All corneal powers based only on anterior surface data require correction factors when used for altered corneas, including those that have undergone refractive surgery. The equivalent power of the cornea (thick lens formula) is the addition of the anterior and posterior curvatures including the ratio thickness/ refractive index of the cornea. It follows the thick lens formula of Gaussian optics. The size of the best central data zone for this total corneal power calculation has been established to be 2-mm diameter. These values can be used in IOL power calculations for untouched corneas, which have good congruency between both surfaces. Although GALILEI does not presently show the equivalent corneal power, it has been already used successfully for IOL power calculation after refractive surgery (Arce CG, Soriano ES, Weisenthal RW, Hamilton SM, et al. Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery. J Refract Surg 2009; 25:1061 1074). 31

Presently, there is more evidence that IOL power calculation using the ray traced Total Corneal Power from a central 4-mm-diameter data zone is more accurate for eyes that have undergone refractive surgery. This is the value that can be used to adjust the SimK values used in IOL formulas to calculate IOL power (Weikert MP, Shirayama M, Wang L, Koch DD. Role of posterior corneal power using the GALILEI dual Scheimpflug analyzer in standard IOL power calculation. San Francisco, 2008 ASCRS; Raju L, Wang L, Koch DD. Comparison of IOL power calculations using different average corneal power readings in post radial keratotomy patients. San Francisco 2009 ASCRS; Savini G, Hoffer KJ, Carbonelli M, Barboni P. Corneal power measurements by a dual- Scheimpflug analyzer for IOL power calculation in unoperated eyes. Boston 2010 ASCRS; Sandoval HP, Guenena MA, Solomon KD. Accuracy of a dual-scheimpflug analyzer to calculate post-lasik corneal power cataract surgery patients. Boston 2010 ASCRS). The use of SimKf and SimKs to calculate toric IOLs is a common practice around the world. However these values are derived from only the anterior surface and therefore they do not take into account the curvature of the posterior surface. Recently it was proposed to use the astigmatism and axis derived from the Total Corneal Power in order to avoid residual postoperative surprises (Barraquer R. Personal communication 2009). Although the SimK and ray-traced astigmatism values can be similar, there are corneas with different magnitudes and axis. The astigmatism value calculated from the entire cornea (including both anterior and posterior surfaces) should be more accurate in eyes that have undergone excimer laser refractive surgery and in eyes with incongruent corneal surfaces, i.e. crossed curvatures with against-therule astigmatism in the anterior surface and with-the-rule astigmatism in the posterior surface. 4.2. Evaluation of IOL Power Reports Example 1: Normal Spherical Cornea The SimKavg is 40.99 D, SimKf 40.88 D, SimKs 41.09 D, and the SimK astigmatism in the steeper axis is 0.22 @ 100. The Anterior Instantaneous Curvature value is not displayed since it was not available at the time of acquisition, using SW 5.0. The Anterior e 2 is 0.09. The Mean Total Corneal Power (Ray Traced) based on Software 5.0 is 40.45 D, which is almost 0.50 D smaller than the SimKavg. The ray traced flatter power is 40.39 D and the steeper 40.51 D with a smaller steep axis cylinder value of 0.12 D @ 141. This eye had an axial length of 24.60 mm measured by immersion ultrasound biometry. From the GALILEI data it is possible to calculate the central average distance from the epithelium to the anterior surface of the lens: 3.29 mm (anterior chamber depth from the corneal endothelium to the anterior lens surface) + 0.577 mm (central average pachymetry in a 4-mm-diameter ROI) = 3.867 mm. 32

The Central Average (0-4 mm) of Total Corneal Power (ray traced) based on Software 5.0 is 40.38 D. With an SA of +0.30 µm, a little still positive postoperative total SA after cataract surgery may be predicted if an aspheric monofocal IOL with negative SA (between -0.20 and 0.27 µm) is implanted in this eye. Using the SRK-T IOL formula, the power of an IOL with 118.7 of A constant for emmetropia will be +20.25 D with the SimKavg. The ray traced Total Corneal Power of SW 5.0 and SW 5.2 are not referenced to SimK and cannot be used directly in SimK formulae. Example 2: Normal Astigmatic With Initial Posterior Surface Changes The SimKavg is 43.63 D, SimKf 42.88 D, SimKs 44.37 D, and the SimK astigmatism in the steeper axis is 1.49 @ 85. The Anterior Instantaneous Curvature does not appear because exams were made with former SW 5.0. The Anterior e 2 is 0.31. The Mean Total Corneal Power (Ray Traced) from a 1-4-mm-diameter ROI is 43.34 D, a little smaller than the SimKavg. The ray traced flatter power is 42.63 D and the steeper 44.04 D with almost the same axis and cylinder of 1.41 D @ 87. This eye had 25.40 mm of axial length measured by immersion ultrasound biometry. From GALILEI data it is possible to calculate the central average distance from the epithelium to the anterior surface of the lens: 3.12 mm (anterior chamber depth from endothelium) + 0.554 mm (central average pachymetry in 4-mm-diameter ROI) = 3.674 mm. The Central Average (0-4 mm) of Total Corneal Power (ray traced) is 43.32 D. With an SA of +0.15 µm, an already negative postoperative total SA after cataract surgery may be predicted, if an aspheric monofocal IOL with negative SA is implanted in this eye. Using the SRK-T IOL formula, the power of an IOL with 118.7 of A constant for emmetropia will be +14.84 D with the SimKavg. Example 3: KC With Irregular Astigmatism The SimKavg is 46.86 D, SimKf 45.22 D, SimKs 48.50 D, and the SimK astigmatism in the steeper axis is 3.28 @ 95. The Anterior Instantaneous Curvature does not appear because exams were made with former SW 5.0. The Anterior e 2 is 1.17. The Mean Total Corneal Power (Ray Traced) from 1-4-mm-diameter ROI is 46.31 D, almost 0.50 D smaller than the SimKavg. The ray traced flatter power is 44.68 D and the steeper 47.94 D with almost the same axis and cylinder of 3.26 D @ 96. 33

This eye had 26.30 mm of axial length measured by immersion ultrasound biometry. From the GALILEI data it is possible to calculate the central average distance from the epithelium to the anterior surface of the lens: 2.30 mm (anterior chamber depth from endothelium) + 0.483 mm (central average pachymetry in a 4-mmdiameter ROI) = 2.783 mm. The Central Average (0-4 mm) of Total Corneal Power (ray traced) is 46.54 D. With an SA of -0.15 µm, more than -0.35 µm of postoperative total SA after cataract surgery may be predicted, if an aspheric monofocal IOL with negative SA is implanted in this eye. If a spherical IOL with positive SA (between +0.18 and +0.20 µm) is used the final total SA would be slightly positive or close to zero. In this eye a surgical correction of the astigmatism might be possible with a spherical toric IOL or an aspheric toric IOL with neutral SA (SA = 0). Using the SRK-T IOL formula, the power of an IOL with 118.7 of A constant for emmetropia will be +8.14 D with the SimKavg. 5. The MAP x1 The Alternate profile shows as a first option the same pachymetry map with the German CGA 20 µm scale, numerical values and overlays already described before (refer to Figure 1A from the Refractive Report). The Default Profile shows the Anterior Axial curvature map with the default Type I 1.5 D scale and without overlays as it was also described before (refer to Figure 2A from the Refractive Report). 5.1. Importance of the pachymetry map Although any available map may be shown, the Map x1 presentation is useful to report the pachymetry map separated from the topography study. The Pachymetry map of GALILEI gives much more information than ultrasound pachymetry. It shows the value and location of central corneal thickness (CCT) and the thinnest corneal pachymetry, the central, paracentral and peripheral average thickness. Pachymetry maps have a classical pattern with a uniform and symmetric distribution and progression, from a thinner central zone with the thinnest corneal point located at the center of the map within 1 mm from the pupil centroid. The pachymetry profile extends from the center with concentric rings and progressively thicker pachymetry towards the periphery. Since the normal difference between CCT and the thinnest corneal point thickness should always be less than 25 µm, both mark points have to be located within the same color step on a pachymetry map with the German CGA 20 µm scale. A borderline (Δ = 20 25 µm) or an already abnormal pachymetry difference would occur when the thinnest corneal point is located within the next warmer color 34