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

Zeiss Humphrey Systems® ATLAS™ Corneal Topography System Models 993 and Eclipse 995 are best sellers in the USA. They measure true elevation data (Figure 26, 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.

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 arte-

facts. 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.

At the time you read this chapter, Technomed® color ellipsoid topometer, might have been discontinued.

This Australian-made corneal topographer uses a 32 ring small Placido cone with over 15,000 measurement points, providing fairly detailed topography data over the central area of the human cornea. Coverage extends from a minimum ring diameter of 0.25mm to beyond 10mm. Images are captured automatically with a simple alignment system and stores the four best images. You simply position the instrument, guided by the intuitive 3D focusing target, and the software does the rest. Each video frame is analysed for centring, focus and movement. The best four frames are automatically captured and displayed in the image windows above - allowing the user to save the most suitable image for further analysis. The advanced analysis software corrects defocused, offcentred images and compensates for errors due to

misalignment. Multiple display options configurable to specific preference of the user are available: the E-300 is able to present a variety of different display options, with up to four images per screen. Examples are multiple images of the same type to identify trends, a difference display and a combination map which can present four different views (for instance, axial power, tangential power, elevation and video image) of one examination. It has a Contact Lens Fitting Software fully integrated with the rest of the software, supporting the fitting of multi-curve, toric, conic and aspheric RGP lenses, and simulated fluorescein display and a tear film clearance graph. The Contact Lens database provides standard lens designs and can be easily updated by the user.

The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. The 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 motorised 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 customised reports can be printed in less than a minute with any colour printer running under MS Windows ’95 ™ operating system.

A very interesting feature of this instrument is the Bull’s Eye Targeting™: the system automatically targets the apex position of a cone (keratoconus or other), providing a numerical index for that cone. An auto-alarm is activated so that any suspicious case of keratoconus (or excessive corneal elevation with an index higher than 10) is automatically detected and acoustically signalled as a peak detection warning window appears in the display after the image capture is complete. New users will appreciate this feature: a low index is not uncommon, and does not always mean that we face a pathologic cornea. High indices in a tangential map almost always mean that we face a keratoconus or another kind of corneal ectasia (Figure 28,29).

Peak detection can be triggered by any suspect peak, including mucous in the tear film, or localised areas of film break-up. In one such case, always have the patient close the eyes for a while and blink a few extra times before retaking the picture. In case of doubt, it is advisable to retake the picture again. The determination of the condition producing the corneal elevation needs to be confirmed by other clinical tests, like slit lamp examination or others.

The DICON ® CT-200™ software includes an optional refractive module that allows single analysis, trend analysis of multiple displays and a special package called VISX ® STAR S2™ Ablation Planner (Figure 30).

The VISX® STAR S2™ Ablation Planner is offered as an option and is intended to learn the control system for the Visx® laser. It offers a custom display of the CT 200 Elliptical Elevation Map, and access to the VISX® STAR S2™ control panel. It allows a simulated (not real) image of the before/after laser ablation for better comprehension of the procedure.

Developed by Dr. Robert B. Mandell is a simplified contact lens fitting software, with fluorescein simulation. You can design unique lenses for each cornea (personalised designs) and send the data directly to the manufacturer (via modem) or print the order sheet for faxing or mailing.

Figure 31: KERATRON™ Corneal Topographer (Optikon

2000® S.p.A, Italy -Europe).

The Keratron™ topographer is one of our preferred systems: it is a must if you are in refractive surgery. The Keratron Topographers offer automatic image capture. A patented corneal vertex detector system is housed inside a slight protrusion on either side of the cone. If you position the Keratron™ too close or too far, image capture just will not happen. Only when the system detects the vertex in the exact right position, image is automatically captured, thus obtaining more reproducible maps.

Introduced in 1994, the Keratron™ was the first hardware platform designed to get the most of an ARC STEP surface reconstruction, achieving accuracy and sensitivity, without smoothing of data or extrapolating to fill in topographic shadows. The Keratron's own method of arc-step mapping accurately maps aspheric surfaces. It uses a small Placido cone of rings.

It’s patented infra-red vertex detector sensor determines the exact position of the corneal vertex and begins constructing a web of "Arcs" between the intersections of 26 rings and 256 meridians, from the

vertex to the periphery. Defining corneal vertex position and starting measurements from it provide this topographer with high accuracy. Curvature and height are simultaneously derived from the length and shape of each arc. Mapping beginning at the corneal vertex, this instrument easily detects up central islands or minor defects. Each data point of the "web" is related to another one, thus eliminating inaccuracies of traditional Placido "concentric rings method" which take measurement of each point independently from one another, resulting in possible errors.

While most topographers first create an axial map and then convert the axial data into different maps, every Keratron’s map is calculated separately without conversions, thus decreasing probability of errors. Since the Keratron does not convert data, map error is minimal in all maps.

True corneal elevation (height) in microns as well as the traditional curvature maps are created. This system enables to map the image of a patient with bad fixation-through mathematics reconstruction. The system is fast and easy to use, working under MS Windows™ environment. The powerful software is the gem of the system: novice will find some difficulty but once you master it you will not want to get rid of this topographer.

You can design unique lenses for each cornea (personalised designs) and send the data directly to the manufacturer (via modem). A recently developed software by Jim Edwards, OD (patents pending) called WAVE Contact Lens Software uses a unique but logical approach to contact lens design by effectively creating a mirror image of the peripheral cornea in the lens design process. Contact lenses designed with Wave drape the cornea in a manner similar to a soft lens: they contour closely to the cornea. As the lens periphery matches the peripheral cornea, lens centration should be unsurpassed, even with reverse geometry lenses.

Optikon 2000® has made a small portable topographer called Scout Portable Topographer with the same features as the full size device: at the moment these lines are written it suffers from some

youth design defects that will be soon addressed by Opticon 2000®. It is available as slit-lamp model, hand-held model, table top model or surgical microscope model.

Topographers from Premier Laser Systems,

EyeSys Corneal Analysis System 2000 and EyeSys Vista Hand-held corneal topographer, have been the leading topographers in the USA for years but might have been discontinued at the moment you may read this chapter due to Premier Laser Systems’ bankruptcy. We have included them to honour the topographers we learned with, as most topographic

texts still refer to them. We hope that new partners in early future or potential buyers help to guarantee the survival of EyeSys topographers in this hard marketplace. At the moment these lines are written, service for EyeSys Corneal Analysis System 2000 and EyeSys Vista Hand-held corneal topographer is guaranteed by MarcoTechnologies™ from Jacksonville, Florida, for the US market.

Euclid Systems Corporation® ET-800 CTS is another interesting product in this round-up, since it uses a completely different method of topography called Fourier profilometry.

The technique uses the projection of 2 identical sine wave patterns onto the surface of the eye. The projection is done using filtered blue light that induces fluorescence of a liquid (fluorescein) that has been applied to the tear film before the examination. The resulting image is captured by a CCD camera. Two dimensional Fourier transform mathematics are used to calculate the phase shift of the projected wave pattern. The phase shift is directly related to the height information. This method analyses over 300,000 data points to achieve true elevation co-ordi- nates, with each point accurate to approximately the thickness of the tear film (about one micron). The problem is that thickness of the tear film varies with daytime, and is not the same for each patient.

The system uses no rings or Placido disk. It is quite fast (processing time: 4 seconds). The focusing mechanism is a live TV camera. It provides full

scleral and corneal coverage up to 22 x 17mm (useful to assess pterygium evolution). It is sold as the "only" topographer to measure true corneal elevation. Let’s observe again that most topographers measure corneal elevation by extrapolating from corneal reflex (thus interfered by tear film layer quality). It might well be the most precise method, each of the 300,000 data points being accurate to about 1 micron, but unfortunately it is not widespread enough to become a reference system. It still needs clinical validation, but offers an unprecedented cross sectional analysis capability that allows users to plot the heights of any meridian, or any horizontal or vertical slice of the cornea in real time.

This projection technique visualises the surface directly while a reflection technique amplifies the corneal topographic distortions. It measures with low light level for patient, offering full K analysis, "e" value analysis, cross sections, ellipsoidal difference map, full patient and radiological histories, and a easy to use four click exam wizard.