Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Figure 1-7: Concept of Spherical vs. Aspherical Ablation Profile as Obtained by the "Flying Spot" Type Laser

This cross section of the anterior globe compares a spherical ablation profile (S) to an aspherical profile (A). A spherical treatment results in a corneal surface (1) which has the same radius (R1) throughout its curvature. The common center of the spherical curvature is shown at (C1). By comparison an aspherical ablation profile, made possible by the "flying spot" type excimer laser, is defined as one which has varying curvatures across the treatment zone. In the aspherical example (A), the central curvature

(2) has longer radius (R2) than the mid-peripheral curvature (3), which has shorter radius (R3). The centers of curvature for the two areas of the cornea are different (C2 and C3). The curvature change between these two areas is gradual. Thus, the central cornea has a "flatter" curvature than the mid-peripheral cornea in this case. The dotted line represents the pre-op corneal curvature. (After

Boyd´s "Atlas of Refractive Surgery").

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that they believe the future of lasers is in scanning technology. The Japanese company Nidek and the U.S. company LaserSight also manufacture scanning lasers. The Nidek laser involves a slit that can be moved across the surface like the rectangular beam of a slit lamp . The Meditec is similar. The Visx laser has recently been modified ("Smooth Scan") to achieve a scanning effect. Although it is a broad beam laser, the smooth scan modification allows the broad beam to be broken up into individual beams that scan

the surface. It is predicted that the smoother ablation that results will improve results of surgery. McDonnell explicitly adds, however, that improved surface smoothness has yet to be proven in a prospective, randomized trial to translate into improved visual acuities.

The Nidek and Autonomous lasers are now commercially available, with recent approval by the U.S. Food and Drug Administration (FDA). Other scanning lasers, such as the Technolas and LaserSight are now approved in USA by FDA.

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Eye Tracking Systems

Another advantage of the scanning lasers is that they can be used in combination with eye tracking technology and computer controlled mirrors (Fig. 1-8) to move the spot automatically in microseconds to compensate for eye movements. At least theoretically, such a laser is not dependent on absolute fixation and can thereby improve the quality of the surface.

As described by McDonnell, landmarks are identified at the beginning of the procedure. Without eye-tracking systems, if the patient looks slightly away from the fixation target while a broad beam laser is being used, the surgeon must quickly release the foot pedal and stop the ablation. With eye tracking technology, however, the laser immediately registers the movement of the eye and moves the spot accordingly without interrupting the surgery. Technologically, some of these eye tracking devices are quite impres-

Figure 1-9: How Corneal Tissues are Affected with Different Refractive Techniques

In this figure, you will clearly observe the differences in corneal tissue invasion comparing incisional keratotomy to laser in situ keratectomy (LASIK). (A) This cross section view of the cornea shows the tissue penetration of the diamond knife in RK with a deep incision of 500 microns reaching down close to Descemet's. The space shown between the arrows demonstrates the thin, untouched and intact corneal area. The corneal tissue strength is significantly weak and unstable because of the radial cuts. (B) This represents the corneal depth reached in LASIK by the excimer laser in a patient with -8.00 (myopic) diopters. The higher the myopia, the larger the ablation but limited by Jose Barraquer's thickness law. In this case the ablation depth reaches 240 microns (160 microns: corneal flap + 80 microns: stromal laser ablation). The rest of the corneal stroma is untouched (between arrows). (After Boyd´s "Atlas of Refractive Surgery").

molecules at high speed (Fig. 1-2). Tissue ablation in LASIK reaches an average of 250 microns (Fig.1-9 B) from the original surface of the cornea. On the other hand, during incisional keratotomy (radial keratotomy for myopia and astigmatic keratotomy for astigmatism) the depth of incision into the corneal stroma reaches down to 500 microns, close to Descemet's membrane and almost 90% of the corneal thickness (Fig. 1-9-A).

This major difference between the two techniques reveals how the stroma is significantly weakened in incisional keratotomy thereby affecting the strength and stability of the globe. LASIK involves no heat damage, no permanent scarring, not even a thermal effect.

In the long run, RK patients carry two swords of Damocles over their heads. One is the threat of a blow to the eye severe enough to cause a rupture. The RK patient is always more susceptible to rupture because the corneal scars will never be as strong as the original cornea. The second threat is that these scars apparently stretch or relax with time, which may give the patient more effect than the original result. An undercorrected patient moves toward a better result, but a properly corrected or overcorrected patient moves into hyperopia, and can become quite farsighted.

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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 videokeratoscopy :

1- The central zone (4 central millimeters): 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 “su- per-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 or 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 dioptric 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. Corneal maps of rare cases and complications can be found in the different chapters of this book. Please refer to them for better knowledge. 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

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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)

Y. image size

y: mire size (64 mm in keratometer)

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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 2-2) 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 anatomi-

Figure 2-2: The Placido cone consists of a series of concentric dark and light rings in the configuration of 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: notice that while describing the technical characteristics of videokeratographers some manufactures count both clear and dark rings, while others only count light ones. The mires of most systems exclude the very central cornea (where the video camera or CCD is located) and the paralimbal area. Picture shows a large cone of the Haag-Streit® Keratograph CTK 922™ with 22 rings (dark and light rings). (Published with permission from Haag-Streit® AG International).

cally caused shadows) are in no proportion to the disadvantage—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.

Euclid Systems Corporation® ET-800 uses a completely different method of topography called Fourier profilometry using filtered blue light that induces fluorescence of a liquid that has been applied to the tear film before the examination. This

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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 dystrophies, bullous keratopathy

5- Keratoconus (diagnostic and follow-up)

6- Follow-up of corneal ulceration or abscess

(Figure 2-2).

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 2-3: 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™)

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)

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Causes of Artefacts of the Corneal

Topography Map:

a- shadows on the cornea from large eyelashes or trichiasis (Figure 2-4).

b- ptosis or non-sufficient eye opening

(Figure 2-5)

Figure 2-6: An advanced corneal herpetic keratopathy produces an irregular completely distorted corneal map in which no regular pattern can be identified. Notice that the low-vision patient is unable fixate the fixation light.

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 millimeters), 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.

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