2
Corneal topography and its role in refractive surgery
Shehzad A Naroo and Alejandro Cervino
The cornea plays a fundamental role in both the structural integrity and the refractive state of the eye. Thus, both the determination and representation of its shape are important for refractive and surgical purposes, as well as in the diagnosis and evolution of several pathologies that express corneal shape alterations, such as keratoconus, marginal degeneration and other ectasias. The adult cornea is characterized by its specific distributions of curvature and thickness along the different meridians, distributions that are essential for the correct function of the cornea as the most important and powerful refractive element of the human eye.
History
Early interest in corneal topography dates back to Father Christopher Scheiner, who in 1619 compared corneal images to marbles. Using daylight he viewed the image formed when daylight shone through the cross-shaped glazing frame bars of his windows onto corneas and compared the images formed to those formed on marbles of a known size. Senff introduced the first concepts about human corneal topography in 1846, reporting that the anterior corneal surface flattens towards the limbus and compared the anterior surface of the cornea with a revolution ellipsoid. Henry Goode (1847) described the first keratoscope, which comprised a small luminous square held near to the eye. Helmholtz (1853) invented the ophthalmometer, and introduced the first doubling image system to avoid the problems caused by the continuous micro-move- ments of the eye that existed until then. This ophthalmometer was difficult to use,
but it served as a base for the development of the keratometers. In 1880 Antonio Placido introduced a flat disc with a series of concentric black and white rings, with the corneal reflections of the rings examined through a central aperture. It is illuminated from a light source above or beside the patient’s head. The Placido disc, as it became known, must be held normal to the line of sight or it will give a false impression of the toricity of the cornea. Gullstrand (1896) was the first to photograph the corneal image formed with the Placido disc.1,2
Classification of corneal topography
Since the early investigations of Javal and Helmholtz, a basic model of corneal topography has been established that uses the ellipse as a first-order approximation to the normal corneal profile. This classic model of the corneal contour corresponds to a surface with two zones, a central spherical zone of 4–5mm diameter and a peripheral zone that flattens towards the limbus. The central zone is responsible for the foveal image formation, and within this area of the cornea the changes in curvature are small, so often uniformity is assumed. However, it has been demonstrated by Bennett that this is not actually correct,3 but rather each point on the cornea is conical (as mentioned below). The anatomic centre of the apical zone rarely corresponds to the visual centre or the geometric centre, although most instruments assume this to be true. The position of the apex is independent of the geometric centre and is usually located 0.5mm on the temporal side with respect
to the geometric centre.4 Other classifications have also been developed, such as that of Rowsey and co-workers who considered the quantity and symmetry of peripheral flattening,5 and classified the corneas into essentially four types:
•Type A: paracentral zone is symmetrical (nasal–temporal difference less than 0.2mm), peripheral zone is symmetrical and the difference in flattening between the paracentral and peripheral zones is less than 0.2mm.
•Type B: paracentral zone is symmetrical, as is the peripheral zone, but the difference in flattening between both zones is more than 0.2mm.
•Type C: paracentral zone has a trace asymmetry (about 0.2mm), the peripheral zone is symmetrical and the difference in flattening between them is less than 0.2mm.
•Type D: paracentral zone has a nasal–temporal asymmetry and the peripheral zone is symmetrical, but the difference in flattening between the paracentral and peripheral zones is greater than 0.2mm.
The classification of the cornea into anatomic zones is considered inappropriate by several authors, because the cornea is a smooth surface, the curvature of
which is submitted constantly to subtle changes,3,6,7 which suggests that at any
individual point the cornea is conical and represented by Equation (2.1)
y2 = 2ro – px2 |
(2.1) |
where p is the shape factor of the cornea (see below) and ro is the central radius of curvature of the cornea.
However, in the central 3–4mm the changes are small, as mentioned above, and hence some level of uniformity is often assumed. The anterior peripheral cornea
10 ■ Refractive surgery: a guide to assessment and management
flattens with respect to the central curvature, a pattern mimicked by the posterior corneal curvature. The rate of flattening may be different along different meridians. The corneal asphericity is described in mathematical terms as being a prolate shape or a flattening ellipse. This shape of the cornea partially compensates the spherical aberration of the eye and improves the quality of the retinal image.
The technical requirements for a correct and reliable measurement of corneal topography were established by Bibby:8
•The units used to describe the corneal topography must not depend on the method of obtaining the values.
•The instrument must measure the total area of interest.
•All the information must be acquired simultaneously.
•The technique employed must be pre-
cise and reproducible.
Following these requirements, his work suggested mean values for the corneal shape of 0.85 ± 0.18 in 2100 eyes and, later, of 0.79 ± 0.15 in 32,000 eyes.
In the 20th century, the growth of the field of contact lenses, and later of refractive surgery, led to an increased interest in corneal topography. This, along with the parallel development of the computer technologies, resulted in great advances in corneal topographical analysis. Various workers have helped to develop better designs of photokeratoscopic systems and better graphic presentation and analysis of the data. Colour-coded topography maps were introduced by Klyce and later developed further by Maguire.9,10
Corneal shape
Evaluation of the corneal shape is of great importance in the monitoring and followup of corneal pathologies, contact lens fitting and refractive surgery, and in the evaluation of sequential temporal changes induced by contact lens wear, refractive surgery or orthokeratology. However, the description of the cornea may not be the same for a contact lens fit as for refractive surgery purposes, for example. Mandell described the cornea in three ways, according to the viewpoint required:11
•From a qualitative point of view, several corneal zones are considered: the central, paracentral, peripheral and limbal. Also, a division into optic and peripheral zones can be made for practical purposes, in which the central optic zone, with an almost constant curvature, is surrounded by a peripheral zone with a radius that progres-
Table 2.1 Corneal descriptors and their mathematical relations
Mathematical description |
Shape factor (p) |
Asphericity (Q) |
Corneal example |
Hyperbola shape |
p < 0 |
|
|
|
|
|
|
Parabola shape |
p = 0 |
|
|
|
|
|
|
Prolate shape |
0 < p < 1 |
Q < 0 |
Normal corneal |
(flattening ellipse) |
|
|
shape |
|
|
|
|
Circle |
p = 1 |
Q = 0 |
|
|
|
|
|
Oblate shape |
p > 1 |
Q > 0 |
Post-myopic laser |
(steepening ellipse) |
|
|
surgery or post- |
|
|
|
radial keratotomy |
sively increases. Near the apex the degree of change in the radius of curvature is very low, but it increases quickly towards the periphery. To establish the size of the central zone a 1D change criterion is usually accepted or, in other words, the area of the corneal surface at which the dioptric powers differ by less than 1D. In most cases this is a 4mm diameter central portion of the cornea.
•From a mathematical point of view, a simple mathematical expression is used to define the cornea as an ellipse or polynomial expression. In most normal corneas, the central zone is more curved than the peripheral zone, which means it has the form of a section of a prolate ellipse (with a positive shape factor and an asphericity in the range 0 to 1). There are studies that draw the conclusion that the different refractive groups have similar corneal
eccentricity values, but different values for the apical radius.12
•The third way to describe the corneal surface, as Mandell reports,11 is point
by point, which consists simply of a collection of values for the corneal radius of curvature or power found at different positions on the cornea. If all the adjacent numbers with the same value are connected in a contour map, the result obtained is transformed into an easily comprehensible pattern that gives a global image of the particular corneal shape.
A series of descriptors of the corneal shape has been defined to unify the different criteria of the range of normal corneal shapes. Also, some investigators have defined a number of corneal indexes to give a better understanding of corneal topography and its variations. The commonly used descriptors of corneal shape
are eccentricity (e), shape factor (p) and asphericity (Q). These indices are related by simple mathematical equations:
p = 1 – e2 |
(2.2) |
Q = –e2 |
(2.3) |
and |
|
Q = p – 1 |
(2.4) |
Table 2.1 shows corneal descriptors and their mathematical relations.
Some of the information collected by corneal topography is used to describe the corneal shape in easy-to-understand terms, which can both aid interpretation of the data and decipher the colour maps. A few examples of these are given below, although many corneal topography units have their own individual indices.
Corneal uniformity index or surface regularity index
The corneal uniformity index (CUI) or surface regularity index (SRI) represents the smoothness of the surface, a relation of the change in local corneal radius of curvature or corneal power over a determined area. It is evaluated from the frequency distribution of powers along the different meridians. This index is sometimes used to give a value to the visual acuity (VA), based on the assumption that the cornea is the only limiting factor in the patient’s VA; this is called the predicted corneal acuity (PCA).
Simulated keratometry readings
Simulated keratometry (SimK) readings are calculated using the steepest meridian from the central area along every meridian (SimK1), and the power and axis of the meridian orthogonal to the steepest (SimK2). These readings are useful substitutes of traditional keratometric measurements and have been reported as useful in the calculation of intraocular lens power.13 They are often taken as the steepest and flattest profiles, although if they
are calculated perpendicular to each other this may not be exactly correct.
Elevation is a relative measurement of corneal topography, and is described as the elevation difference with respect to a plane or to a flatter or steeper surface. Elevation may also be taken in relation to a reference sphere, which may be a floating sphere (i.e., related only to that cornea) or a fixed sphere (used to calculate the elevation of all corneas with that machine). Usually a floating sphere is used, and its radius of curvature is the mean radii for that cornea; all other data points are related to this reference sphere, which is termed the best-fit sphere (BFS).
Figure 2.1 shows a Holliday diagnostic summary (HDS) from the EyeSys 2000 machine.
Corneal topography measurement methods
Currently, numerous evaluation methods for corneal topography exist, some more precise than the others, but basically all are developments of the same fundamental theme. The idea is to make a three-dimen- sional reconstruction of the corneal surface, but difficulties arise when trying to represent a three-dimensional shape using a two-dimensional image. To do this some assumptions and simplifications are made:
Corneal topography and its role in refractive surgery ■ 11
•The working distance from the object point to the image is constant.
•The instrument axis is perpendicular to the corneal surface.
•The light from the object is reflected in the same meridian onto the plane on which the image is created (i.e., it is assumed that no circular inclination of the corneal surface occurs).
•The position of the image on the plane is unique for a determined surface.
•The image point is on a non-curved plane.
•The refractive index of the cornea is
the same for all individuals and remains constant in a particular patient.
Image capture in corneal topography can be divided into two basic types:
•Reflection techniques: the cornea works as a curved mirror and the reflected image is viewed directly or captured and analyzed. Examples of this technique are keratometry, keratoscopy and videokeratoscopy.
•Projection techniques: in this group of techniques, the cornea acts as a projection screen. An example of this is rasterstereography, which is used successfully in other areas of medicine
such as spinal curvature measurement.
Another technique is that of interferometry.
Using the cornea as a reflector system
Nearly all optometry practices have a keratometer. The main function of the keratometer is to measure the radius of curvature of the central portion of the front surface of the cornea.14 This result is usually obtained indirectly by measuring the angular size of the reflected image, formed by the cornea, of an object of known angular size; this is the first Purkinje image.6 In most instruments, this is an object with a linear size that is fixed or measurable at a predetermined distance from the image plane. Since it would be difficult to read off the reflected image height from the cornea, because of involuntary eye movements, the principle of doubling is used in keratometers. The image size is measured by lateral displacement of a doubled image (doubling may be achieved using a series of lenses, mirrors or, more commonly, prisms). In most keratometers, doubling takes place in one meridian only, along the line that joins the mires. Such an instrument must be rotated about its axis to align it with each of the principal meridians of the cornea in turn, and is therefore known as a two-position keratometer, such as a Javal–Schiötz type.
A one-position keratometer (such as the Bausch and Lomb type, see Figures 2.2 and 2.3) is an instrument in which variable doubling of mutually perpendicular pairs of mires is produced by two doubling devices in the corresponding meridians. The instrument is rotated about its axis to align the mires with both principal meridians of the cornea, and the images in each can then be brought into contact without further rotation.6
The primary use of the keratometer in contact lens practice is to measure the central radius of the cornea to determine the back optic zone radius of a contact lens
Figure 2.1 |
Figure 2.2 |
Holliday diagnostic summary (HDS) from the EyeSys 2000 machine. The box underneath the |
Bausch and Lomb style keratometer |
four maps shows some corneal parameters for this cornea measured over 3mm, except for the |
|
asphericity value (Q), which is measured over an area of 4.5mm. Note that the steepest and |
|
flattest refractive profiles (column 1) are not the same axes as the SimK axes (column 2) |
|
12 ■ Refractive surgery: a guide to assessment and management
a |
|
b |
|
|
|
|
|
|
Figure 2.3
Reflection of the mires from a Bausch and Lomb style keratometer. (a) The mires incorrectly aligned. (b) The mires correctly aligned when the keratometry readings are taken
that will produce the best fit. It is also used to check the radii of a corneal lens and to assess the fit of soft contact lenses. Changes in central corneal shape can also be detected with the keratometer, both quantitatively and qualitatively (by assessing the regularity of the mires), and in this capacity it is often used by clinicians to assist in the diagnosis of keratoconus.14
Keratometry has a number of inherent problems. The one-position keratometer described above assumes that the two principal meridians of the cornea are perpendicular. All keratometers measure corneal radius with pencils of light reflected by small areas, each situated not less than 1mm and up to about 1.7mm from the centre. The keratometer does not allow for decentration of the corneal apex or for corneal asphericity. The main source of error is focusing. If the mire images formed by the object are not focused accurately in the intended primary image plane, the radius measurement will be incorrect, since the object–image separation is then incorrect, and the unfocused mire images have a different separation from the sharply focused ones.15 These blurred images may not appear to be so if compensated for by the observer’s accommodation and his or her own uncorrected ametropia (especially astigmatism). Also, local distortion of the cornea in the region of the reflection area causes a corresponding distortion of the mire and renders focusing of the instrument uncertain.6,16
Adaptations of keratometers have been used to assess the peripheral corneal shape. New keratometers have been designed and modifications made to older designs. A modified Bausch and Lomb keratometer with the mire separation reduced from 64mm to 26mm and a series of offaxis fixation points was able to take measurements of the corneal periphery.11 Bennett describes a keratometer based on the Drysdale effect and used to measure the central and peripheral cornea.3
Videophotokeratoscopy
Modern corneal topography devices are an accumulation of techniques learned from the historical methods of keratoscopy and photokeratoscopy, described above. Highresolution video cameras record the reflection of the Placido disc mires from the patient’s cornea. Once the patient is aligned in front of the videophotokeratoscope, with the chin on the rest, the images are captured. The system is aligned when the tracking lights of the two superimposed laser beams reflected by the cornea are placed in the centre of the cross-hair target located in a box displayed on the monitor screen. The reflected image of the mires is recorded on a close-circuit video camera and analyzed by computer software to yield a representation of the corneal contour.
Examples of different machines
The machines mentioned here are just a few of the many types of topography units currently available. This is not intended to be an exhaustive list, but merely a representation of the variety of designs around. The two most widely used computer-assisted videophotokeratography systems are the EyeSys Corneal Analysis System (by EyeSys Technologies) and the Topographic Modelling System (TMS, by Tomey instruments), and these represent the two extremes of design. The EyeSys machines have a Placido disc and a longer working distance than their Tomey rivals, which have a Placido cone. A larger working distance makes the instrument less sensitive to small displacements of the eye, but has the disadvantage that the instrument is less compact. The cone systems use a shortened working distance, and the size of the cone means they are able to move closer to the eye, which allows a larger corneal coverage.
The TMS-1 uses a 25-ring Placido cone, with a total of 6400 data points, and utilizes a short working distance of approximately 70mm. The EyeSys-1 uses 16 rings, each
giving rise to 360 data points, a total of 5760 data points. The latest EyeSys topography unit, EyeSys-2000, uses 18 rings, but maintains a longer working distance. It still collects data from 360 points along each ring, to give a total of 6480 data points. The TMS-2, like its predecessor, uses a Placido cone, but increases the number of rings to 34, while maintaining 256 data points per ring over a maximum corneal diameter of 11.5mm. The latest offering from Tomey, the TMS-3, boasts an impressive automated image-capture system. It has 31 rings with 256 data points per ring, to give a total of 7936 data points. The automated image capture of the TMS-3 leads to a small sacrifice in corneal coverage, and offers up to a maximum diameter of 9.5mm.17,18
As corneal topography has become less of a research tool and more clinically widespread, the number of models available has increased. All use either the Placido cone system, as in the TMS units, or the Placido disc system, as in the EyeSys units. Most use a working distance that is between the two extremes of these two units. Haag-Streit and Oculus both offer the same unit, but packaged slightly differently. This machine has 22 rings on a Placido disc and offers 10,000 data points, which is currently the most of any topography unit; this device also has a very accurate collimating measurement for more accurate SimK readings. While a greater number of sampling points, in principle, allows the topography to be assessed in more detail, the validity of the device’s data depends on the algorithms used and, as yet, few comparative studies have been made on the performance of different units.
Topcon offer a novel system, the KR7000P, which is a combined autorefrac- tor–autokeratometer–topographer. As well as providing automated refraction and central corneal curvature readings along the two principal meridians, it gives the topography over a corneal diameter of 7mm, but it only offers 2600 data points in total. This unit can be used as a standalone machine with a built-in printer or can be linked to a desktop computer.
Once a patient is aligned on the topography machine and the cornea is in focus the actual image capture (automated or manual) is very quick, typically 33 milliseconds, as with the TMS-3 unit. Each data collection point measures the curvature at that reflected point on the cornea and all the data points are represented on a colour map display.
Presenting topography data
Two scales are commonly used to display topographic features; the absolute (also called standard scale) and the normalized
(also called the relative scale or autoscale). The absolute scale generates a colour-coded map with 1.0D increments on a pre-set scale, usually between 37D and 51D, and thus allows comparison of different corneas and different machines. The normalized scale uses smaller increments to span the range of dioptric powers of an individual cornea, and thus the same colour may not represent the same numerical value on different corneal pictures. The normalized scale is created by assigning the red range of colours to the steepest curvature of the cornea being examined, and the blue range of colours to the flattest curvature. The remaining colours are divided into equal step sizes and assigned their particular ranges (Figures 2.4 and 2.5).
The normalized scale is intended to render similar contours similar in appearance, irrespective of their absolute radius of curvature. Hence, the normalized scale, being more specific to an individual, is more sensitive in detecting subtle topographic changes in the anterior corneal surface. With both scales, steep areas are depicted by so-called ‘hot colours’ (i.e., reds and browns) and flatter areas are represented by ‘cold colours’ (i.e., blues and greens).
The keratometric data display gives details of the steepest and flattest corneal curvature in the central 3mm, 5mm and 7mm (the central data may be represented on the colour map). The profile map shows the corneal curvature in dioptric powers over the corneal surface by calculating the profile along the steepest and flattest meridians from the central 3mm zone.19 Most software algorithms assume that the corneal contour changes smoothly and hence ‘average’ the curvature over an area of a few square millimetres. Unfortunately, little information is available on this effect, which may be of importance in relation to ablation geometry in laser refractive surgery. Each local area of the cornea is generally a toric surface, rather than a sphericalone, and hence possesses both spherical and cylindrical power.
Sagittal and tangential data
In corneal topography, the light from the topographer mires is directed onto the cornea. Off-axis light, when reflected from a curved surface, gives rise to two focal points. One represents the radius of curvature normal to the reflected mires, known as the sagittal (or axial) reflection. The other focal point represents the radius of curvature that contains the reflected mires, the tangential image.
Sagittal and tangential data can be represented as different types of topography maps, and for the same cornea
Corneal topography and its role in refractive surgery ■ 13
|
|
|
Figure 2.4 |
|
Figure 2.5 |
Absolute (or standardized) scale of a |
|
Normalized (or relative) scale of the same |
corneal topography map. The scale is preset |
|
cornea as in Figure 2.4. In the map the |
by the machine manufacturer. (Note the |
|
dioptric scale has a much smaller range, |
large range of the dioptric scale) |
|
which enables differences in the radius of |
|
|
curvature to be detected more easily |
a |
Figure 2.6 |
|
|
Corneal topography maps |
|
|
of a patients’ eyes taken |
|
|
with the Orbscan |
|
|
topography unit. The map |
|
|
on the left in both (a) and |
|
|
(b) represents sagittal |
|
|
data and that on the right |
|
|
represents tangential |
|
|
||
|
data. Both (a) and (b) |
|
b |
||
show clearly how sagittal |
||
|
||
|
and tangential data can |
|
|
look very different for the |
|
|
same eye, but the main |
|
|
emphasis remains the |
|
|
same |
|
|
|
they may look different (Figure 2.6), although the main features of the map remain the same.
Using the cornea as projector system
The cornea was first used as a projector system to determine the corneal topography by Bonnett and Cochet (1962).20 It consists of the projection of a diffraction grid onto the corneal surface and the pattern produced by the grid is a function of the corneal topography. However, the cornea must first be made opaque to allow the grid pattern to be detected. Initially, talcum powder (in conjunction with topical anaesthesia) was used for this purpose, but more recently sodium fluorescein has been used. Accuracy of the measurements taken depends on the magnification used on the slit lamp and the way that the image is viewed or captured.7
Similar technology is adopted in the Orbscan topography unit (Orbtek Inc., Salt Lake City, Utah). The Orbscan takes 40 slit sections of the cornea during two scans, each scan lasting 0.7 seconds. Each slit section is similar to an optical section viewed through a slit lamp. Similar to Placido-based topography, the patient rests on a chin rest and the instrument is aligned using an XYZ manipulator base (see Figures 1.5 and 10.1). The image capture takes a total of 1.4 seconds and any eye movements render the image void.
The corneal curvature results are usually presented in the form of a contour map that shows height deviations from the best-fitting spheres, but a variety of other numerical descriptions can also be obtained. It has been shown that the measurement of anterior surface curvature, as assessed using calibrated standards, has a high accuracy and that the
14 ■ Refractive surgery: a guide to assessment and management
Figure 2.7
thickness measurement on human corneas has a high reproducibility.21,22
The default topography map that the Orbscan produces, the quad map, consists of four pictures (Figure 2.7). The quad map has maps of the anterior corneal height, the
Figure 2.8
Surgical options, ‘birds-eye’ view of a normal cornea (note the central steepening of the cornea)
The quad map consists of maps of the anterior corneal height, the posterior corneal height, the keratometric data and the pachymetry, but these maps can be altered to suit the user’s preferences (see text)
posterior corneal height, the keratometric data and the pachymetry, but these maps can be altered to suit the user’s preferences. Height maps indicate the relative height above or below a mean of the radii of curvature of the surface (anterior or posterior). The mean radius of curvature of the corneal surface, the BFS, is subtracted from all other radii of curvature points of the surface. Thus, the height maps do not indicate the curvature of the cornea at a particular point, but rather the relative height with regard to the BFS (similar to an Ordnance survey map, in which heights are shown with respect to sea level). The height maps use ‘hot’ colours to indicate areas that are higher than the BFS, and ‘cold’ colours to indicate areas that are lower than the BFS. The keratometric map shows the radius of curvature data of the cornea at any point. In the quad map, it is viewed as an overall value for the anterior and posterior corneal surfaces, but these surfaces can be viewed individually. The final map in the quad selection is a pachymetry map that shows the thickness of the entire area of cornea assessed.
Another option that the Orbscan allows is called ‘surgical options’. This view produces a three-dimensional schematic image of the examined eye. It can be adjusted to produce a view of the anterior or posterior cornea, or both simultaneously. The anterior lens can also be imaged using this option, although lens curvature data are not available directly. This type of three-dimen- sional schematic view is available on other types of topography systems too, but is usually calculated from radius of curvature data, whereas the Orbscan uses elevation data. Figure 2.8 shows a ‘birds-eye’ view of a normal cornea (note the central steepening of the cornea). Figure 2.9 shows a cornea that has undergone myopic photorefractive keratectomy (PRK), with the associated central corneal flattening. Figure 2.10 shows an eye after refractive keratectomy.
A new device recently available from the Birmingham Optical Group is the Oculus Pentacam system (Oculus, Giessen, Germany). This is discussed again in Chapter 10, as currently no published studies have used it. Essentially, it is a rotating Scheimplug camera and is able to image up to the fourth Purkinje image in a patient with a dilated pupil; otherwise, it is able to at least obtain data from three surfaces, like the Orbscan. The image creation and caption system is different to that of the Orbscan, so it remains to be seen how the two systems compare. The Pentacam is a table-mounted device and, similar to standard topography units, it uses an XYZ manipulator base with the patient lined up in front of the instrument with his or her chin upon a chin rest (Figure 2.11). The data are collected in around 2 seconds and approximately 25,000 data points are taken. The data can be represented as elevation data or radius of curvature data.
Figure 2.9
Surgical options, view of cornea that has undergone myopic PRK, with the associated central corneal flattening
Figure 2.10
Surgical options, view of an eye after radial keratotomy. (Courtesy of Orbtek)
Figure 2.11
The Oculus Pentacam system is a tablemounted device that uses an XYZ manipulator base with the patient lined up in front of the instrument and his or her chin upon a chin rest
Corneal topography in refractive surgery
Irregular and regular astigmatism can be observed using topography after cataract surgery and post-penetrating keratoplasty, which allows the surgeon to assess stability. For cases in which there is a lot of post-surgical astigmatism, corneal topography maps can be used to indicate potential areas of suture removal (Figure 2.12).
Corneal topography is a vital tool in refractive surgery. Pre-operative assessment checks for any contraindicated corneal conditions and dystrophies. Contact lens users who present for refractive surgery are advised to remove their lenses for a period of time before surgery to eliminate warpage induced by the contact lens. Warpage appears as an irregular topography picture with distortion that does not have a regu-
Corneal topography and its role in refractive surgery ■ 15
lar pattern. For a patient in whom warpage is observed, the topography is repeated on subsequent visits until no further changes are seen in the topography maps and only then is the patient considered suitable for surgery. Figure 2.13 shows a patient with corneal warpage in the right eye, but a relatively normal left eye.
Post-operative assessment is essential to check astigmatic results, stability and irregular healing. Different techniques of refractive surgery show characteristic postoperative changes. For example, myopic excimer laser surgery shows central flattening, whereas after hyperopic surgery a mid-peripheral flattening is seen. Post-ker- atotomy, steepening of the areas of surgical incision and an accompanying flattening of other areas of the cornea are seen.
Topography pictures taken at different visits allow the clinician to observe the
Figure 2.12
healing changes that occur to an eye over a period of time post-surgery (Figure 2.14). During aftercare appointments topography is often conducted to pick up abnormalities such as central islands, which can be identified clearly. Decentred zone ablations can also be identified with corneal topography. Areas of surface scarring, such as complications of corneal flaps, erosions and sutures, are also detectable.
Limitations of corneal topography
The quality of the anterior reflective surface of the cornea and inaccuracies in numerical assumptions can limit the usefulness of topography. Images are restricted nasally and superiorly because the recording mechanisms are eclipsed by the
A patient’s right eye after penetrating keratoplasty. The map labelled A was taken 1 week after surgery. Her corrected VA was 6/18, as there was some irregular astigmatism present. The surgeon removed one suture to try and make the astigmatism more regular. Picture B was taken a few minutes later, and gives corrected VA of 6/9 (Refraction: +6.00/–3.00 × 100). The main map shows the difference map obtained by subtracting map B from map A
Figure 2.13
Patient with corneal warpage in the right eye, but a relatively normal left eye
Figure 2.14
A cornea before and after photorefractive keratectomy. The initial refraction was –3.25/–0.25 × 175 and the 12 weeks post-surgical refraction was +1.00/–0.50 × 10. Picture A is the post-surgical map and picture B is the pre-surgical map. The larger picture is the difference between the two. The pre-surgical map has been subtracted from the postsurgical picture to demonstrate the area of cornea removed by ablation. It can be seen that a central area of approximately 5mm has been flattened (the actual laser setting for the ablation diameter was a 5.5mm optic zone and a total ablation zone of 6.5mm)
16 ■ Refractive surgery: a guide to assessment and management
nose, brow and upper eyelid. Superficial corneal scars and similar abnormalities confuse the topographies, especially if they are central. The patient’s ability to maintain fixation is vital.
Corneal topography and aberrometry
Currently, practically aberrometry is becoming a very popular technique in refractive surgery. It is used to create better ablation profiles and also to assess postoperative patients, especially those with complications. In fact, pre-operative wavefront aberrometry examination should help the surgeon decide whether a traditional refractive surgery procedure would solve the visual problems of the patient, or if a customized ablation is indicated. Some aber-
References
1Shah S and Naroo SA (1998). Corneal topography. Continued Medical Education.
J Ophthalmol. 2, 16–19.
2Corbett MC, Rosen ES and O’Brart D (1999). Corneal Topogrography.Principles and Applications, Ch 1, p. 3–11 (London: British Medical Journal Books).
3Bennett AG (1964). A new keratometer and its application to corneal topography.
Br J Physiol Opt. 21, 234–235.
4Edmund C (1987). Determination of the corneal thickness profile by optical pachometry. Acta Ophthalmol. 65, 147–152.
5Rowsey JJ (1984). Corneal topography. In
Contact Lenses: The CLAO Guide to Basic Science and Clinical Practice, Ed. Dabezies
OH Jr, Ch 4. (Orlando: Grune and
Stratton).
6Bennett AG and Rabbetts RB (1989). Measurements of ocular dimensions. In
Clinical Visual Optics, Second Edition, p. 457–483, Eds Bennett AG and Rabbetts RB (London: Butterworths).
7Dave T (1998). Current developments in measurements of corneal topography.
Contact Lens Anterior Eye 21, S13–S30.
8Bibby MM (1976). The Wesley–Jenssen System 2000 photokeratoscope. Contact Lens Forum 1, 37–45.
rometers are able to separate aberrations of the cornea from those of the whole eye, whereas other devices only give the wholeeye aberrations. The custom ablation techniques allow a method of correcting the whole eye’s aberrations on the corneal surface. In a similar way, traditional refractive surgery would correct a patient’s full ocular refraction on the cornea, even though some components of the prescription may be elsewhere, such as the crystalline lens.
Most corneal topography devices now have software that enable radius of curvature data to be interpreted to express the corneal high-order aberrations, often in terms of Zernike polynomials or Fourier analysis. Some newer topography devices now take aberrometry measurements in addition to corneal curvature data. The Nidek OPD device (Figure 2.15) has a Placido disc and, in addition to that, uses a skiascopy
9Klyce SD (1984). High resolution graphic presentation and analysis of keratoscopy. Invest Ophthalmol Vis Sci.
25, 1426–1435.
10 Maguire LJ, Singer Dem and Klyce SD
(1987). Graphic presentation of computer-analyzed keratoscope photographs. Arch Ophthalmol. 105, 223–230.
11 Mandell RB (1962). Methods to measure the peripheral corneal curvature. Part 3: Ophthalmometry. J Am Optom Assoc. 33, 889–892.
12 Sheridan M and Douthwaite WA (1989). Corneal asphericity and refractive error.
Ophthalmic Physiol Opt. 9, 235–238. 13 Ilango B, Shah S and Clark IH (2001). A
review of biometry techniques. Eye News 8, 24–28.
14 Sheridan M (1989). Keratometry and slit lamp biomicroscopy. In Contact Lenses, Third Edition, p. 243–259. Eds Phillips A and Stone J (London: Butterworths).
15 Applegate RA and Howland HC (1995). Non-invasive measurement of corneal topography. IEEE Eng Med Biol Mag. 14, 30–42.
16 Douthwaite WA and Evardson WT (2000). Corneal topography by keratometry. Br J Ophthalmol. 84, 842–847.
technique to measure ocular high-order aberrations. The combination of both techniques means that corneal aberrations can be separated from whole-eye aberrations.
Figure 2.15
The Nidek OPD
17 Douthwaite WA, Hough T, Edwards K and Notay H (1999). The EyeSys videokeratoscopic assessment of apical radius and p-value in the normal human cornea. Ophthalmic Physiol Opt. 19, 467–474.
18 Hilmantel G, Blunt RJ, Garrett BP, Howland HC and Applegate RA (1999). Accuracy of Tomey topographic modelling system in measuring surface elevations of asymmetric objects. Optom Vis Sci. 76, 108–114.
19 Roberts C (1998). A practical guide to the interpretation of corneal topography.
Contact Lens Spectrum 13, 25–33.
20 Bonnet R and Cochet P (1962). New method of topographical ophthalmometry
– its theoretical and clinical applications.
Am J Optom Arch Am Acad Optom. 39, 227–251.
21 Lattimore MR Jr, Kaupp S, Schallhorn SS and Lewis IV RB (1999). Orbscan pachymetry: Implications of a repeated measures and diurnal variation analysis.
Ophthalmology 106, 977–981.
22 Yaylali V, Kaufman SC and Thompson HW
(1997). Corneal thickness measurements with the Orbscan topography system and ultrasonic pachymetry. J Cataract Refract Surg. 23, 1345–1350.