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

Figure a8: Marginal pellucid degeneration

Stromal corneal disease include a variety of inflammatory and non-inflammatory disorders, like Terrien’s marginal degeneration, Mooren’s ulceration, pellucid marginal degeneration an others. Picture displays true elevation map (left) and axial map (right) of a pellucid marginal degeneration, a narrow band of corneal thinning located 1-2 mm from the inferior limbus. Observe the flattening of the central cornea (true elevation and axial maps) along the vertical axis. Extensive peripheral guttering leads to irregular against-the-rule astigmatism, such as this arching inferior bow tie visible in the axial map. These topographic findings are characteristic: they help establishing diagnosis even in patients without slit-lamp typical findings.

Figure a9: Contact lens overuse (warpage)

Different types of contact lenses have different impact on corneal surface and different indications. We can classify them into three main groups: soft, rigid gas permeable (RGP) and hard (PMMA). The last are no longer considered suitable for making contacts, and are only prescribed in special cases. Rigid gas permeable contact lenses a relatively popular: they offer good visual performance, they can be polished, they tolerate most known cleaning solutions, and custom designs are possible. The bad side also exists, since they require individualised fitting (by means of k readings, topographic maps, …), they are not easily tolerated at first, and induce with relative ease changes of the shape of the cornea: the process of changes is termed warpage. It is thought due to mechani-

cal pressure on the cornea, although other factors like oxygen deficiency have not been excluded. Many topographic patterns may result, like the one in the picture, depending upon the fit (size, curvature, …) and position of the lens. In this c ase, observe the inferior steepening in the axial map causing meridian asymmetry as a result of superior riding contact lens. The true elevation map shows corneal surface irregularity (orange).

Cessation of the lens wear and good ocular lubrication result in return to corneal former shape.

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FUNDAMENTALS ON CORNEAL TOPOGRAPHY

Figure a10: Curved arcuate keratotomies (astigmatic keratotomies)

Most refractive efforts have concentrated on altering the shape of the cornea, which is the main diopter of the eye. Topography is valuable in the preoperative assessment and planning of the surgery. Picture displays both true elevation (left) and axial maps (right) of an astigmatic patient who underwent astigmatic keratotomies. Two paired circumpherential relaxing incisions centered on the steep axis result in focal steepening (orange-red in true elevation map) and central flattening in that meridian (blue). The final result is 0.13D of astigmatism in the 3mm central cornea.

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Figure a12: Photorefractive keratectomy PRK

To correct myopia, the excimer laser removes more tissue from the centre than the periphery of the treatment zone (ablation zone). The ablation profile is different for every model of laser. The map on the left represents the spherical approach (axial curvature) of a patient who suffered myopic photorefractive keratectomy. Only the “true elevation” map (on the right) shows the transition area, where dioptric powers are very high (ring in red).

Figure a13: Subtraction map in a PRK

The most effective way of displaying the changes in a cornea that undergoes a refractive procedure are difference maps. The change induced by surgery is obtained by subtracting the preoperative map (upper small axial map) from the postoperative map (lower small axial map). The image on the right shows the result (in terms of dioptric variation, axial curvature) of a myopic PRK. In red, the ablation zone. In orange, the transition zone, which is easily delineated in the postoperative axial map which shows that the central cornea has been flattened (lower small axial map on the left).

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Figure a14: True curvature analysis in pre/post PRK

The comparison between preoperative and postoperative true curvature analysis of the same PRK patient shows no variations of the peripheral cornea after surgery.

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FUNDAMENTALS ON CORNEAL TOPOGRAPHY

Figure a18: Myopic and hyperopic keratomileusis (LASIK)

Shown are two “true curvature” maps of both myopic (left) and hyperopic (right) keratomileusis.

To correct myopia, excimer laser removes a central disc of corneal stroma, resulting in central flattening (blue) and the presence of a relative peripheral steepening ring (red). Corneal topographic changes similar to those seen after photorefractive keratectomy (PRK) occur after LASIK for myopia.

To correct hyperopia, the excimer laser does just the opposite: it removes an annulus or ring of tissue from the mid-periphery (blue) to steepen the central cornea (red).

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Figure a19: Intrastromal segmented graft for the correction of high myopias

Picture shows a “true curvature” map of a left eye cornea that received an intrastromal segmented graft for the correction of high myopia. The map is similar to that of a myopic LASIK, but less regular.

Figure a20: Fluorescein simulation in RGP contact lens

Contact lens fitting applications are used to help choosing the best lens for every case, by simulating the fluorescein film pattern and contact lens position of rigid contact lenses (RGP and PMMA). The simulated fluorescein feature is intended to reduce fitting time by viewing the effect of changing lens parameters on a personalised basis, depending on the patient’s corneal exam. Let’s notice that the true “in vivo” result of any computerised fluorescein test may vary due to differences caused by lid action on the lens (aperture and weight).

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Figure 2-23: Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995

TOPOGRAPHERS CURRENTLY AVAILABLE

Zeiss Humphrey Systems® ATLAS™ Corneal

Topography System Models 993 and Eclipse 995 (Figure 2-23, with permission)

Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995 are best sellers in the USA. They measure true elevation data (Figure 2-24, with permission) through an advanced arc-step algorithm (similar to Optikon 2000® Keratron™), by means of 20-22 ring conical Placido disk. The Atlas Eclipse 995 offers ultra-low illumination and increased peripheral coverage (limbus to limbus). They also offer automatic pupil measurement. Software displays are viewed in a 10,4 “ TFT 640x480 pixel resolution in 18 bit colour; they include: photokeratoscope view, axial map, tangential map, numeric view, and profile view. Very interesting optional software packages are available at a price: MasterFit™ contact lens module, corneal elevation map, corneal irregularity map, refractive power map, keratoconus detection map, VisioPro™ ablation planing software and Healing Trend/ STARS™ display.

Figure 2-24: Elevation Map

Technomed® Color Ellipsoid Topometer

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 disadvantage, poor depth of focus, resulting in poor reproducibility.

The Color Ellipsoid Topometer compensates defocusing errors with software and hardware, by means of a triangulation measurement., enhancing precision and theoretically avoiding measuring artefacts. It is the only Placido (30 ring) system with colour coded rings (three coloured rings). By means of a laser, it measures 10800 points, providing real height values and has ray tracing software. A new module enables topography-driven laser ablation. This unit is specially useful in diagnosing postoperative problems in a refractive practice, specially in those cases with a loss of vision that cannot be explained. The Color Ellipsoid Topometer can predict the quality of vision based on the shape of the cornea and pupil.

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Chapter 2

Figure 2-25: DICON ® CT200

DICON ® CT200 (Figure 2-25, with permission)

The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. The

Figure 2-26: Dicon’s CT-200™ can explore the whole cornea (apex, and limbus to limbus) thanks to an offset fixation. Patient fixates different green lights: shown is a quadruple view of right eye corneal maps display a nasal fixation, including 3-D reconstruction with a 45º tilt (left and down). Optional software (Multiview™) provides total cornea coverage using the mentioned multiple fixation targets. Limbal measurements aren’t always reliable, being subject to many artefacts.

DICON® CT200 is a cheap easy to use instrument with autofocus and autoalignment that eliminate joystick and explorer subjectivity, thus improving repeatability. The big Placido disk cone in managed from the computer by means of the mouse. Final alignment (up and down) and focusing (forwards and backwards) are automatically performed by the motorized instrument head.

It can explore the whole cornea (apex and limbus to limbus) thanks to an offset fixation. The patient can fixate different green lights, to allow complete cornea coverage. Offset-fixation mapping allows for more precise mapping of the central 3mm of the cornea. More true data points from the apex and true limbus-to-limbus measurements over a large corneal area provide for better coverage without extrapolation.

Nevertheless, we miss a different chin rest to allow faster exams by eliminating the need for patient’s head re-centration from one eye to the other.

The system generates maps in seconds and detailed customized reports can be printed in less than a minute with any colour printer running under MS Windows ’95 ™ operating system.

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Contents

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Subjects Index

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