Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Guillermo L. Simón, MD.

Sarabel Simón, MD.

José Mª Simón, MD.

José Mª Simón, Jr., MD.

Cristina Simón, MD.

Introduction: Human Optics and the

Normal Cornea

The cornea is the highest diopter of human eye, accounting alone for about 43-44 diopters at corneal apex (about two thirds of the total dioptric power of the eye). It has an average radius of curvature of 7,8 mm. A healthy cornea is not absolutely transparent: it scatters almost 10 % of the incident light, primarily due to the scattering at the stroma.

The corneal geography can be divided into four geographical zones from apex to limbus, which can be easily differentiated in colour corneal video keratoscopy :

1- The central zone (4 central millimetres): it overlies the pupil and is responsible for the high definition vision. The central part is almost spherical and called apex.

2- The paracentral zone: where the cornea begins to flatten

3- The peripheral zone

4- The limbal zone

Refractive surgery refers to a surgical or laser procedure performed on the cornea, to alter its refractive power. The major refractive component of the cornea being its front surface, it is not difficult to understand that most refractive techniques have involved this frontal surface (PRK, radial keratotomies, …). Nevertheless, posterior surface of the

cornea also accounts, and that is the reason why a "posterior surface corneal topographer" like the Orbscan™ - Bausch & Lomb® was developed by Orbtek®, in the race for a more precise refractive surgery.

The cornea of an eagle is almost as transparent as glass: there is almost no scattering of incident light. That alone explains the resolution of an eagle eye being much better than ours. As we are never satisfied, we are now developing new tools and extremely promising laser surgical techniques that have proven to increase human being visual acuity by reducing corneal aberrations: we reduce diopters and also improve visual acuity. The new dream is "super-vision". Topographic and aberrometer-linked LASIK are on the way to achieve this goal of better- than-normal vision. Bausch & Lomb®’s Zywave™ combines topography and wavefront measurements to achieve customized computer controlled flying spot excimer laser ablation, which appears to be fundamental in treating irregular astigmatisms o retreating unsatisfied LASIK patients to regularize the corneal shape. Regularizing the corneal shape has the theoretical advantage of improving the quality of vision by means of reduction of halos, glare and any other optical aberrations. We are on the way to achieve an aberration-free visual system, though the influence of all other refractive surfaces (vitreous, lens,…) and interfaces still has to be ascertained.

In this chapter we will try to introduce the novice to this interesting new world of instruments recently developed due to the advent of refractive corneal surgery. We have tried to show different maps from different systems, trying to make an interesting basic atlas of corneal topography. There is no perfect system to assess true corneal surface shape, but we still have to rely on the instruments we have, waiting for new instruments and methods being developed for better accuracy. With that goal in mind BioShape AG® has developed the EyeShape™ system, based on a principle called fringe projection. Patterns of parallel lines are first imaged onto a reference and then onto the surface to be measured. Detection of the lines with a digital camera under a tilted angle yields distorted line patterns. The deviation of the detected lines from the original lines together with the tilt make it possible to calculate the absolute height at any point on the surface of the cornea (or not).

Instruments to Measure the Corneal Surface

The normal corneal surface is smooth: a healthy tear film neutralizes corneal irregularities. The cornea, acting as a convex "almost transparent" mirror, reflects part of the incident light. Different instruments have been developed to assess and measure this corneal reflex. These non contact instruments use a light target (lamp, mires, Placido disks,…) and a microscope or another optic system to measure corneal reflex of these light targets.

1- Keratometry

A keratometer quantitatively measures the radius of curvature of different corneal zones of 3 mm (diameter). The present day keratometer allows the operator to precisely measure the size of the reflected image, converting the image size to corneal radius using a mathematical relation r= 2 a Y/y where

r : anterior corneal radius

a: distance from mire to cornea (75 mm in keratometer)

Section II: Topography

Y.image size

y:mire size (64 mm in keratometer)

The keratometer can convert from corneal radius r (measured in meters) into refracting power RP (in Diopters) using the relationship:

RP = 337,5 / r

Modern -automated or notkeratometers also known as ophthalmometers directly convert from radius to diopters and inversely. They are mainly used to calculate the power of intraocular lenses through different formulas (Hoffer, SRK-T, SRK-II, Holladay, Enrique del Rio & S. Simón, …). Although the theory of measuring corneal reflex may appear to be simple, it is not, since eye movement, decentration or any tear film deficiency may difficult the measure creating errors. Modern video methods (topographers) can freeze the reflected cornea image, and perform the measurements once the image is captured on the video or computer screen, allowing greater precision. Note that most traditional keratometers perform measurements of the central 3 mm, while computerised topographers can cover almost the whole corneal surface.

2- Keratoscopy or Photokeratoscopy

It is a method to evaluate qualitatively the reflected light on the corneal surface. The projected light may be a simple flash lamp or a Placido disc target, which is a series of concentric rings (10 or 12 rings) or a tube (cone) with illuminated rings lining the inside surface. When we look at the keratoscope, an elliptical distortion of mires suggest astigmatism, and small, narrow and closely spaced mires suggest corneas that have high power (steep regions or short radius of curvature).

The use of keratoscopes is being abandoned in favour of computerized modern topographers which allow qualitative and quantitative measurements of the corneal surface, with higher definition and accuracy (more than 20 rings), and more sensitivity in the peripheral cornea.

•It is difficult to focus and align

•In most topographers, the patient is exposed to

high light

Large Placido disk systems work far away from the eye, while small Placido cones get much closer to the eye. While Placido disk systems easily create shadows caused by the nose and brow blocking the light of the rings, small cone systems fit under the brow and beside the nose, avoiding shadows, but can get in contact with large noses and make the patient blink and be afraid. With large Placido disk

3- Computerized Videokeratoscopy:

Modern Corneal Topographers

Corneal topography has gained wide acceptance as a clinical examination procedure with the advent of modern laser refractive surgery. It has many advantages over traditional keratometers or keratoscopes: they measure a large area of the cornea with a much higher number of points and produce permanent records that can be used for followup.

Basically, a projection corneal topographer consists of a Placido disk or cone (large or small) that illuminates the cornea by sending a mire of concentric rings, a video camera that captures the corneal reflex from the tear layer and a computer and software that perform the analysis of the data trough different computer algorithms. The computer evaluates the distance between a series of concentric rings of light and darkness in a variable number of points. The shorter the distance, the higher the corneal power, and inversely. Final results can be printed in colours or black-and-white.

The Placido disk (Figure 2) consists of a series of concentric dark and light rings in the configuration of a disk or a cone, of different sizes, depending on the number of rings and the manufacturer. Usually, it is better to have a large number of rings, since more corneal radius values can be measured. The mires of most systems exclude the very central cornea and the paralimbal area.

The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show a considerable amount of error when the required working distance between cornea and keratoscope is not maintained. The advantages of small cones (optimal illumination and the reduction of anatomically caused shadows) are in no proportion to the disad- vantage—poor depth of focus, resulting in poor

reproducibility. Which one should you choose, a small Placido cone or large Placido disk ? Not easy to answer: each family of topographers has advantages and disadvantages. Being no ideal instrument, topographer potential buyers will have to decide upon other important factors, like software ability to exactly reproduce real corneal height, number of rings, price, ….

There are two main groups of corneal topographers: those which use the principle of reflection (most), and those which use the principle of projection.

Let’s notice that the image captured by most topographers is produced by the thin tear layer covering the cornea that almost reproduces the shape or contour of the corneal surface. Most instruments perform indirect measurements of the corneal surface (reflection technique) and extrapolate to know the height of each point of the cornea. Reflection techniques amplify the corneal topographic distortions. Irregularities of the tear film layer like dry eye, mucinous film, or greasy film may distort the result of an otherwise normal topography, and a false diagnosis of corneal ectasia can be made, due to secretions over the cornea. In such cases, the topographic map turns out to be normal after a few blinks, or after proper cornea lubrication with an artificial tear substitute.

Euclid Systems Corporation® ET-800 uses a completely different method of topography called

Chapter 3: Fundamentals on Corneal Topography

Fourier profilometry using filtered blue light that induces fluorescence of a liquid that has been applied to the tear film before the examination. This projection technique visualizes the surface directly while a reflection technique amplifies the corneal topographic distortions.

Table 1:

Advantages and Disadvantages of Projection-Based

Systems Over Reflection-Based Ones.

Advantages:

Measurement of direct corneal height

Ability to measure: irregular corneal surfaces non-reflective surfaces

Higher resolution (theoretical)

Uniform accuracy across the whole cornea

Less operator dependent

Do not suffer from spherical bias

Disadvantages:

Not standard instruments (most are still prototypes): complex to use

need clinical experience validation

non standard presentation maps (more difficult to learn)

Longer examination time: longer image acquisition time longer image analysis

Fluorescein instillation needed (in some, like the

Euclid Systems Corporation® ET-800™)

Table 2:

Indications and Uses of Corneal Topographers:

The use of computerised Corneal Topography is indicated in the following conditions:

1- Preoperative and postoperative assessment of the refractive patient

2- Preoperative and postoperative assessment of penetrating keratoplasty

3- Irregular astigmatism

4- Corneal distrophies, bullous keratopathy

5- Keratoconus (diagnostic and follow-up)

6- Follow-up of corneal ulceration or abscess (Figure 3). 7- Post-traumatic corneal scarring

8- Contact lens fitting

9- Evaluation of tear film quality

10- Reference instrument for IOL-implants to see the corneal difference before and after surgery

11- To study unexplained low visual acuity after any surgical procedure (trabeculectomy, extracapsular lens

extraction, …).

12- Preoperative and postoperative assessment of

Intacs™ corneal rings (intrastromal corneal rings)

Figure 3: There are different methods of following the clinical course of a corneal ulceration or corneal abscess. While daily slit-lamp examination and daily photographs are invaluable, corneal topographic maps, being less "explorer dependant", can also be very useful in the follow-up.

Table 3:

Different Methods of Measuring Corneal Surface Used by Modern Corneal Topographers

Placido systems (small cone or large disk) are the most popular

Placido cone with arc-step mapping (Keratron™ from Optikon 2000®)

Placido disk with arc-step mapping (Zeiss Humphrey® Atlas™)

Polar Grid (ASTRAMAX™ 3-D stereo topographer, from Lasersight®, USA)

Slit-lamp topo-pachimetry (Orbscan™ - Bausch & Lomb®)

Fourier profilometry (Euclid Systems Corporation® ET-800™)

Fringe projection or Moiré interference fringes (EyeShape® from BioShape AG™)

Triangulation ellipsoid topometry (Technomed™ colour ellipsoid topometer)

Laser interferometry (experimental method, it records the interference pattern generated on the corneal surface by the interference of two lasers or coherent wave fronts)

(Tracey Technologies™.VFA)

Causes of Artefacts of the Corneal

Topography Map:

a-shadows on the cornea from large eyelashes or trichiasis (Figure 4).

b-ptosis or non-sufficient eye opening (Figure 5).

c-irregularities of the tear film layer (dry eye, mucinous film, greasy film)

d- too short working distance of the small Placido disk cone

e-incomplete or distorted image (corneal pathology) (Figure 6).

Understanding and Reading Corneal

Topography

The meaningful interpretation of topographic maps requires the examiner to have detailed knowledge and clinical experience on the patterns detailed in them. At first, one must understand how to read the colour scales. The untrained eye may find some confusion and sometimes misinterpretation in evaluating corneal maps. Modern topographers

(videokeratographers) use the Louisiana State University Color-Coded Map to display corneal superficial powers. The power values (measured in diopters) are preferred by clinicians over the radius values (measured in millimetres), although all topographers can map the corneas using both values.

Projection-based topography systems, adopted a similar colour scale to represent their height maps. High areas are depicted by warm colours, while low areas are depicted by cool colours.

Facing a corneal topography, care has to be taken to interpret coloured maps, since scales (and sometimes colour coding) can be modified in most topographers’ software. For patient examination manufacturer sets default values which are operator adjustable (diopter interval, radius interval). When operator adjusts the values to new parameters, colour scales are modified.

Rare are the topographers that directly measure the corneal elevation: most act by extrapolation from corneal curvature and power at each measured point. The Optikon 2000® Keratron™ is one of those systems that accurately maps aspheric surfaces by means of its own method of arc-step mapping.

The range of powers found in the normal cornea range from 39 D found at peripheral cornea, close to the limbus, to 48 D found at corneal apex.

The colours do not always represent an elevation map, they correspond to curvature values. Therefore, the cornea is most curved towards the center (green) and flattens out towards the periphery (blue). The nasal side becomes blue more quickly, indicating that the nasal cornea is flatter than the temporal. Some advanced instruments like the Optikon2000® Keratron™, are able to directly represent a coloured elevation map.

Apart from colour maps, most topographers also display values of simulated keratometry, that should be equivalent to those obtained by a keratometer. Simulated keratometry values are obtained form the radius values at the corneal position (3 central millimetres) where the reflection from the keratometer mires would take place.

Topographic Scales:

Two basic scales are commonly used: absolute and relative.

Absolute, Standardized or International Standard Scale: same scale for every map produced (Figure 7). Good for direct comparisons between different maps, for screening and for gross pathologies. It was designed to make only clinically relevant information obvious, by setting the interval between the contours of the power plot (i.e. in practice, the contours of colours) at 1,5 diopters (which means it has low resolution).

Section II: Topography

Relative, Normalized or Adaptative Colour Scale: different scale for each map (Figure 8). The computer determines maximum and minimum curvatures for the map and automatically distributes the range of colours. The computer contracts or expands its colour range according to the range of colours present in a given cornea. It is best suited for looking at variations for a particular cornea. It has the advantage of offering great topographic detail since incremental steps are smaller (around 0.8 diopters) giving high resolution, but suffers from some inconveniences: the meanings of colours are lost (explorer and clinician have to carefully check the meaning of the colours, according to the new scale), a normal cornea may look abnormal while abnormal corneas may appear closer to normal. With this scale, subtle features are made apparent, being good for detail.

Computer Displays: Presentation of

Topographic Information

When confronted to a topography display, either a printed report or on screen, one should study it in a structured way to avoid mistakes in interpretation, and get the most of it.

Proceed as follows:

•Check the name of the patient, date of exam and examined eye.

•Check the scale:

•type of measurement (height in microns, curvature in mm, power in diopters)

•step interval

•Study the map (type of map, form of abnormalities, …)

•Evaluate statistical information (cursor box, statistical indices when given …)

•Compare with topography of the other eye (always perform bilateral exams, when possible)

•Compare with the previous maps first

verifying they are in the same scale)

•Apply statistical analysis or other needed software application (contact lens fit, surgical modules, 3-D colour maps, neural networks, …)

•Explain the exam’s results to the patient

To present a corneal topography, each software application (i.e. each instrument) has a large number of computer displays. Most are produced form data of a single application, and are software dependent. Most instruments are able to show: a ring verification, a numerical display, a large number of corneal maps, a simulated keratometry, a meridional plot, and some can display a 3-D reconstruction of the corneal surface.

a) Ring verification (keratoscopic raw image) (Figure 1, in this chapter): displays a kerato-

Section II: Topography

scopic image of the Placido rings reflex on the examined cornea. It is a raw image, that allows qualitative evaluation of the image taken (irregularity of tear film layer, lids aperture, …), helping the examiner to either accept or reject the taken image. It is very useful when there is a question regarding the accuracy of the displayed data.

b) Numeric Display: of a number of corneal power values along several meridians shown in a radial display. Helpful to make the data amenable to statistical methods (Figures 9-10).