R Rajendra Prasad 1 MD, RPEI 2 MBBS 1. R.P. Eye Institute,Vasant Kunj, New Delhi, India With the advances in keratorefractive surgical procedure, there have been tremendous growth in the population of refractive surgical patients namely LASIK in the last few years, so there have been rise in the incidence of LASIK complications too. One of the most dreaded, irreversible and visually threatening complications of keratorefractive surgical procedure is iatrogenic ectasia or post LASIK ectasia, none of us would want to come across in our refractive clinicalpractice. more predisposed to an anterior shift of cornea. But one of the greatest risk factor for the post LASIK ectasia is performing LASIK on a biomechanically weak cornea. LASIK If you look at the literature the incidence of post LASIK ectasia is estimated to be about 0.05% by most of the leading under reporting and lack of long term follow up after LASIK. It is likely that there are several thousand patients suffering from post LASIK ectasia. This is posing a greater challenge on the refractive surgeons to critically evaluate the refractive cases for corneal ectatic disorder and understand a little better about corneal biomechanics. If you look at the cornea (Figure 1) it s a complex viscoelastic structure having very least resistance to any applied force, but still maintains its contour and biomechanical architecture. Figure 1: Corneal structure collagen crosslinks and rigid anterior lamellae of the cornea, which provide its form and biomechanical strength. Corneal tissue is under constant intraocular pressure which pushes on the back surface of cornea to physically balance the atmospheric pressure from outside. A normal healthy cornea easily withstands forces applied on it and maintains its shape and contour. Any dissociation between the applied forces (intraocular pressure and atmospheric pressure) and biomechanical weakening of corneal tissue, the intraocular force would lead the posterior surface of the cornea, so the ectatic changes There are hundreds of reasons for the biomechanical weakening of cornea, but primary risk factors which may cause biomechanical weakening of cornea include, Inherent keratoconus, Low corneal hysteresis, Pellucid marginal degeneration, eyes with Forme frust keratoconus, thin cornea thickness < 250 microns which could be due creating thick Figure 2: Pressure and Cornea www. dosonline.org l 43
Figure 5: Falls asymmetric bowtie Figure 3: Forward bulging of cornea Figure 4: Ectasia evaluation devices which permanently disintegrate the corneal architecture and make it more susceptible to the applied forces leading to ectaticdisorder. Since large part of rigid anterior cornea is either removed in PRK or intersected in LASIK which permanently thin or weakens the cornea. It is highly important to identify preexisting ectatic changes, genetically predisposed ectaticdisorder keratoconus and high risk cases who are at risk of developing post Lasik ectatic changes. Performing a refractive procedure on high-risk cases may result into a blinding complication keretactectasia. We need to have devices, which could determine a real time eye image s depicting real shape and contour of the cornea the cornea. Ectasia evaluation underwent tremendous evolutionary quantitative and qualitative physical and optical properties of cornea. Initial devices were primarily based on curvature and refraction would only evaluate anterior surface of the cornea, but current devices are based on pachymetry and biometry to determine the real shape of altered corneas. Curvature of cornea can be measured with one of the very basic device keratometer, as radius of curvature of anterior corneal surface in mm., which is then converted into dioptric power by Snell s law Snell s la Keratometer behaves like a convex mirror so assumes cornea as an spherocylinder, assesses only ± 3 mm of mid peripheral irregular cornea which depart from the spherocylinder. It is does not give any information about the posterior surface of cornea. Placido disc based system is the basis of modern Placido topographer. In which a Placido target with a series of concentric illuminated rings projected on to the cornea to create bi- dimensional digital maps (x y axis) of anterior corneal shape. Also uses to detect corneal power. Placido topographer is unmatched in accuracy of curvature mapping and provides us greater corneal surface information, Axial power maps / Sagittal curvature keratoscopic axis for calculating the power at all the points along semi meridians on the corneal surface. Instantaneous power maps / Tangential curvature of a standard mathematical method for determining the local - A secondary focal point power theoretically valued analysis to provide realistic power of cornea in the periphery 44 l DOS Times - Vol. 18, No. 7 January, 2013
Figure 6: Figure 7: Schematic course of scan P ref =n/f coordinates of corneal surface. Construction of Elevation mapsis derived from curvature data by certain algorithm which is not exactly the elevation of corneal surface Elevation topography True topographic imaging implies shape and shape requires system There are two basic types of elevation topography 1. Scanning slit optical Scanning Slit based Elevation Topographer The principle of scanning slit-based topography is similar to a slit lamp corneal topography which uses slit scan optical surface of the cornea. An image of slit light intersectiing the cornea is used to obtain Two slits positioned at an angle of 45 degree to right and left of instrument axis. Twenty images are captured from each direction to obtain information from the cornea Scheimpflug based Elevation Topographer image of anterior eye segment and measures elevation data, max. 25.000 true elevation points with more measurement points in the center = pupil, assesses elevation and derivate topography by mathematic algorithms. So in both the situation we are getting relative elevation / curvature. Elevation derived from curvature in Placido based topography while curvature isderived from elevation in But the problem is when we need to differentiate curvature accuracy. Elevation derived from curvature maps can be true or not, depending to the original (curvature) map. (Figure 5), Difference between Elevation and curvature Figure 8: Scan Overview This is basically because Placido systems do not recognise the edges and elevation of a surface. Two objects with the different Elevations are depicted as same curvature. Whilecurvature is a reference based system changes in the reference point or viewing angle results in changes in curvature so a one curvature maps can have multiple elevation solutions. Integrated topography & Elevation. Curvature from Placido topographer while real placido ring technology in order to get accurate elevation and curvature data. First Integrated Topographer is Orbscan II e II Z which uses Placido topography for curvature data and slit scan optical pachymetry to derive elevation data to create tri - dimensional from 3000 points to assess anterior and posterior surfaces of cornea, corneal thickness and anterior chamber depth. Second integrated topographer is Galilei which uses Placido derived topography to assess x and y axis from anterior surface of cornea, assesses slope and curvature with high accuracy. posterior corneal surface in a larger area. Scheimpflug Camera www. dosonline.org l 45
Figure 9(a): 4 Map Composite Display Figure 9(b): Steep Normal Cornea to make better military maps. To get a higher depth of focus, plane and subject plane are parallel to each other, giving limited depth of focus. intersection. Which says: To get a higher depth of focus the three planes, the picture plane, the objective plane and the intersection so that images along the optical axis of the eye the anterior eye segment. The camera starts rotation, and captures images in 180 over the whole eye segment, so at the end we get photos over 360 digree. While taking 50 images, the angle between the single images is 180 /50 = 3.6. During one scan the system takes maximum 50 photos with 500 true elevation points per picture, so at the end we get 25.000 true elevation points. meshed in the center, the pupil- the most important area, because of the rotation. compensates inside the movements of the patients eye. The system calculates out of all the single pictures during one scan a 3D-model of the anterior eye segment. Scan Overview Picture, current Camera-Slit position and the actual Image current camera-slit position through the eye and the left/ right orientation of the patient s eye. same time, the current sectional plane will be shown in the orientation display. Scheimpflug Image and Densitometry the densitometry of the lens is shown in the diagram. surface, the yellow one the lens surface. The software will set these lines automatically and they are the basis for further calculation of the 3D-model. 3D-Model of the anterior eye segment are shown in a rotatable model for demonstration to the patient. Keratometer values Anterior Chamber Analysis calculated by the system; Coloured Maps front Elevation map can have three selectable reference bodies: Matches perfect to astigmatic Cornea Matches optimal to the true shape of the cornea Interpretation Would start with the 4 map composite display (Figure 9a) which assess data from Elevation front and back, sagittal curvature front and pachymetry map. cornea (both anterior and posterior surfaces). The sagittal curvature appears normal as would be expected from the normal symmetric anterior eleva- tion and the pachymetry map reveals a normal thickness with a normal pachymetry distribution. 46 l DOS Times - Vol. 18, No. 7 January, 2013
Figure 10: Form Fruste Keratoconus Figure 12: Standard Best Fit sphere BFS Figure 11: Classical Keratoconus This map (Figure 9b) demonstrates a normal with-the-rule astigmatic cornea both the anterior and posterior elevations demonstrate a similar pattern as does the anterior sagittal curvature. The curvature maps reveals a steep cornea (K1 = 47.6, K2 = 50.2) but the elevation maps do not reveal any suspicious areas. The pachymetry map is well centered with a thinnest reading of 546 microns. This is a normal astigmatic cornea with steep curvature, but otherwise normal The anterior elevation map (Figure 10) shows a minor island that is still within the normal range. The posterior elevation, If the surgeon had only relied on anterior curvature and central corneal thickness reading this patient would have central corneal thickness of 520 microns). This demonstrates the importance of having accurate posterior elevation data in addition to anterior surface. 4 map composite display (Figure 11) of classic Keratoconus. Both anterior and posterior elevations show a prominent island of positive deviation (maximal for ante- rior and of the pachymetry map (thinnest reading 480 microns). The Tangential curvature map also shows inferior steepening, but again does not accurately locate the cone. Figure 13: Difference between BFS and corneal contour Belin Ambrosio ectatic disorder of cornea based on anterior corneal curvature data derived from corneal topography, pachymetric graphs and indices and the elevation maps. For refractive surgery screening he suggested using a standard Anterior Elevation map if the difference between BFS and But the problem with BFS is that any abnormal portion of the ectasia, the cone or apical protrusion will have the effect of steepening the BFS. the apex of the cone and the BFS and reduce the sensitivity of the test. Then Belin designed a new screening display called as valid data from within the 9.0 mm central cornea, excluding That more closely approximates the individual s normal cornea and then to compare the actual corneal shape to this new reference shape. www. dosonline.org l 47
Figure 14: Figure 15: Difference Elevation map Figure 16: Figure 17: Enhanced Ectasia Display have increased sensitivity and abnormal area of eleveation is excluded from the enhanced BFScalculation. Difference Elevation Maps Shows a relative change in elevation from the standard elevation map to the exclusion map. Maps contain only 3 colors, each one corresponding to the amount of elevation change that occurs when moving between the baseline elevation map and the exclusion map. The green on the difference map represents a change in elevation of less than 6 microns on the front surface and 8 microns on the back surface of the cornea and are typically within the range seen in normal eyes. The yellow areas represent a change between 6 and 12 microns for the front surface and 8 to 20 microns for the back The red represents areas where the elevation difference between the 2 maps is 12 microns anteri- orly or 20 microns posteriorly and are the magnitude typically seen in eyes with known Keratoconus. As you can see from (Figure 17), while the front surface does not show much change from the baseline to the exclusion elevation map (the map is all green), the posterior surface has substantial change (central area of red). This case would be an example of ectasia susceptibility. This case illustrates the importance of not just relying on central corneal thickness and anterior curvature. In this case, both the thinnest corneal reading (OS) was below 500, the pachymetric progression graphs were borderline OD and abnormal OS and the enhanced elevation maps show changes (RED) on the posterior surface OU while the anterior surface remains normal. more evident in the lower one (PTI) in both eyes. Topography map below (Figure 18) reveals form fruste keratoconus and the patient was rejected from refractive surgery. Pentacam anterior segment analysis of the same patient (Figure 18) reveals normal pachymetry (normal distribution The anterior & posterior elevation reveals a slightly decentered apex leading to a False Positive inferior steepening on a curvature map. This case illustrates the limitation on curvature analysis 48 l DOS Times - Vol. 18, No. 7 January, 2013
Figure 18: Form Fruste Keratoconus? Figure 21: Thick abnormal cornea Figure 19: Elevation map Figure 20: Interlocking, red on red effect reference based measurement and in this case, inaccurately axis or orientation and does not have the false positive rate commonly seen with curvature maps. Principle of Interlocking So we have seen that some of the parameter leads us off the track when diagnosing ectasia? Diagnosis of ectasia based on only one parameter may lead to a wrong diagnosis. Some corneas may have parameters that differ slightly from normal values and still be perfectly physiological and pathology-free. For this reason, to make a correct diagnosis we must refer to different principles and interlocking relationship of all of the parameters that are needed to measure the cornea: Like curvature, thickness, anterior and posterior elevation distribution pattern of each of these elements. The key corneal parameters whose position we must study: A Maximum curvature, maximum anterior and posterior elevation and the point with minimal pachymetry B Coincidence of the highest points of the anterior and The highest curvature point (usually shown in red and which we shall call RED ) and the thinnest point (RED) coincide: highly suspect cornea, alert! The highest curvature point (RED) coincides with the highest anterior and posterior points (also shown with warm colors and which we shall also refer to as RED ): highly suspect cornea, alert! The highest curvature point (RED), the thinnest point (RED) and the highest anterior and posterior points of the corneal surface (RED) all coincide: based on these elements ectasia can be diagnosed! We have referred to this interfacing of parameters as the Many a time we come across a situation where thickness of the cornea becomes the basis of ectasia diagnosis. high-risk case for post LASIK ectasia. case for refractive surgery. t s not the thinnest or thickest part of the cornea, which is factor in the evaluation of corneal ectatic disorder. Corneal thickness values at the thinnest point (TP) are determined and the averages of thickness values of the points within twenty-two imaginary circles centered on the TP with increased diameters at 0.4mm- steps are calculated to create the CTSP. Percentage of increase of thickness (PIT) is calculated for each position from the CTSP using a simple formula: (CT @ x - TP) / TP, Where x represents the averages of thickness values www. dosonline.org l 49
Figure 22: Thin normal cornea Figure 23: Corneal thickness progression graph of the points on imaginary circles centered on the TP with increased diameters from 0.4 to 8.8mm. The CTSP and PIT graphs provide very relevant clinical data to differentiate normal thin cornea, and ectatic cornea, CTSP and PIT are highly different in Keratoconus than normals. We also found that Keratoconus have thinner corneas with less volume and faster and more abrupt increase in these parameters from the thinnest point (TP) towards the periphery than normal corneas. Corneal thickness progression graph In the clinical case the x-axis represents the increase in thickness from the center (left) of the corneal periphery examination (red line) is pathological because it is not parallel to the line of reference, indicating a loss of tissue toward the periphery with respect to a normal pachymetry gradient for the thickness of this cornea (broken line). used in conjunction with the classic ones provided by corneal topography to test the hypothesis that the CTSP and PIT increase the sensitivity for the detection of very early forms of Keratoconus, Figure 24, case 12 shows distinctly abnormal pachymetry distribution with marked inferior-temporal displacement and a thinnest reading of 440 microns. This example shows the importance of looking at the pachymetry distribution, which may be the single abnormal References Figure 24: corneal thickness progression graph 1 2 Hodge C, Lawless M, Sutton G, Post LASIK Keratectasia. J Cataract 3 forward shift after laser in situ keratomileusis. J Cataract Refract Surg. combined Placido Scanning Slit system. J Cataract Refract Surg.2012 5 Ambrosio R, Klyce SD Corneal topographic and pachymetric screening 50 l DOS Times - Vol. 18, No. 7 January, 2013