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

pre and post-Lasik topographic patterns. One can detect from this any decentered ablations or any other complication of Lasik surgery.

Corneal topography is extremely important in cataract surgery. The smaller the size of the incision lesser the astigmatism and earlier stability of the astigmatism will occur. One can reduce the astigmatism or increase the astigmatism of a patient after cataract surgery. The simple rule to follow is that- wherever you make an incision that area will flatten and wherever you apply sutures that area will steepen. One can use the Orbscan to analyze the topogra-

phy before and after cataract surgery. For instance in an extracapsular cataract extraction one can check to see where the astigmatism is most and remove those sutures. In a phaco the astigmatism will be less and in phakonit (Fig 15) where the incision is sub 1.5 mm the astigmatism will be the least.

We can use the Orbscan to determine the anterior chamber depth and also analyze where one should place the incision when one is performing astigmatic keratotomy. The Orbscan can also help in a good fit of the contact lens with a fluorescein pattern.

SUMMARY

The Orbscan has changed the world of topography as it gives us an understanding of a slit scan three-dimensional picture. One can use this in understanding various conditions.

___________________________

Amar Agarwal, MD. M.S.; F.R.C.S.; F.R.C.Ophth

Nilesh Kanjani, MD. D.O; Dip N B; F E R C

Athiya Agarwal, M.D.; F.R.S.H. ; D.O.

Sunita Agarwal, MD. M.S.; F.S.V.H.; D.O.

Eye Research Centre &

Dr.Agarwal's Group of Eye Hospitals

Chennai (India), Bangalore (India), Trichy (India)

And Dubai (UAE)

19 Cathedral Road, Chennai600 086, India

15 Eagle Street, Langford Town, Bangalore-560

025, India

15- A Thillainagar, Trichy, India

Villa NO.2, Roundhouse, Al Wasl Rd, Jumeira,

PB 9168, Dubai

TEL- + 91 44 811 6233

FAX- + 91 44 811 5871

WEBSITEhttp://www.dragarwal.com E-MAIL- dragarwal@vsnl.com

General Considerations

Samuel Boyd, MD

of the corneal surface can significantly degrade the image.

Imaging techniques of the cornea are developing rapidly, mainly because of recent advances in refractive and cataract surgery. It is crucial to understand the significance of new imaging techniques and the relevant principles of corneal optics. The discussion of the most common clinical method of Placidobased corneal topography emphasizes important concepts of its clinical interpretation. Next, we will take care to review the principles and clinical significance related to this type of corneal topography.

Optical Properties of the Cornea

Several concepts are used to characterize optical properties of the cornea.

•Curvature: The curvature of the anterior and posterior surface of the cornea can be expressed as radii of curvature in millimeters or clinically more often in keratometric diopters.

•Shape: The shape of the anterior and posterior surface can be expressed in micrometers of elevation of the actual surface relative to a chosen reference surface (eg, sphere). These 2 concepts can characterize the overall shape and the macro-irregu- larities of the corneal surface (eg, corneal astigmatism).

•Local Surface: Smoothness of the surface is optically very important, and any micro-irregularities

•Power: Expressed as refraction in diopters, power is an optical property dependent on the shape of the surfaces and the refractive index of the surface of the cornea.

•Thickness and 3-D Structure: Changes in the 3-D structure (eg, after refractive surgery) can induce further changes of its shape because of biomechanical changes, such as altered elasticity of the remaining tissue.

The keratometric value is a concept inherited from keratometry and is calculated simply from radii of curvature, as follows: K = refractive index of 337.5/radius of curvature.

This concept is a simplification, ignoring the fact that the refracting surface is an air-tear interface, and it does not account for the oblique incidence of incoming light in the corneal periphery. The most important assumption is that the cornea is spherical.

The intact central corneal thickness of approximately 560 µm is considered enough to ensure long-term mechanical stability of the cornea. The peripheral thickness (aproximately 600 µm) is not well studied, but it is certainly clinically important in some refractive procedures as intracorneal rings, radial and astigmatic keratotomy and cataract surgery. With the advances in corneal imaging and widespread refractive surgeries, corneal behavior likely will be understood better.

Figure 1: Alcon Corneal Topography System - Placido Disk

Pattern Type.

Figure 2: Topographic Image Analysis Using Placido Disk System

The main advantages of this technology consists on its ability to make measurements from the central-anterior cornea and to display the data in a useful format with reasonable accuracy. This system provides detailed quantitative data about corneal contour. The accuracy of a particular system depends on the alternatives and the resolution it achieves at each of the stages between the generation of the mire pattern and the display of the information.(Art from Highlights of Ophthalmology collection of medical illustrations).

Corneal topography instruments used in clinical practice most often are based on Placido Reflective Image Analysis (Fig. 1). This method of imaging of the anterior corneal surface uses the analysis of reflected images of multiple concentric rings projected on the cornea (Fig. 2).

Corneal Topography – Principles

Multiple concentric rings of light are projected on the cornea. The reflected image is captured on a Charge-Coupled Device (CCD) camera. Computer software then analyzes the data and displays the results in a variety of color formats, pictures and scales.

Interpretation of Topographic Maps

Every map has a color scale that assigns particular color to a certain keratometric dioptric range. Never base an interpretation on color alone. The value in keratometric D is crucial in the clinical interpretation of the map and has to be looked at with the interpretation of every map.

Absolute maps have a preset color scale with the same dioptric steps, and dioptric minimum and maximum assigned to the same colors for particular instrument. These maps allow direct comparison of 2 different maps. Because the steps are in large increments (generally 0.5 D), their disadvantage is that they do not show subtle changes of curvature and can miss subtle local changes (eg, early keratoconus).

Normalized maps have different color scales assigned to each map based on instrument software that identifies the actual minimal and maximal keratometric dioptric value of a particular cornea. The dioptric range assigned to each color generally is smaller compared to the absolute map, and, consequently, maps show more detailed description of the surface. The disadvantage is that the colors of 2 different maps cannot be compared directly and have to be interpreted based on the keratometric values of their different color scales.

Surface Curvature Measurements in

Topography

Curvature/Power Map

Surface curvature measures how fast the surface bends at a certain point in a certain direction.

Axial curvature (usually known as sagittal curvature) measures the curvature at a certain point on the corneal surface in axial direction relative to the center. It requires the calculation of the center of the image, which cannot be measured directly.

Meridional or tangential curvature measures the curvature at a certain point on the corneal surface in meridional direction relative to the other points on the particular ring.

Meridional curvature maps are more sensitive measures of local curvature change. Axial curvature maps can be derived from meridional maps. Axial value at a certain point equals the average meridional curvature along the radius from the map center to the point of interest, thereby approximating the average refractive power.

Section II: Topography

Axial and meridional maps should be displayed theoretically in the units of radii of curvature (ie, mm) at each corneal surface point. For ophthalmologists who understand the clinically used units of D in ophthalmic practice better, the instruments display curvature in units of keratometric D and constitute so-called axial or meridional power maps.

Elevation Map

Elevation of a point on the corneal surface displays the height of the point on the corneal surface relative to a reference surface. The reference surface in most instruments was chosen to be a sphere. Best mathematical approximation of the actual corneal surface called best-fit sphere is calculated by instrument software for every elevation map separately. The same surface may appear different when mapped against different reference surfaces.

In laser refractive surgery, the refractive power is changed by removing tissue from the corneal surface, and elevation data appear more relevant for calculation of ablation depth and optical zones.

In practice the colors in the same region of elevation and axial curvature maps often are reversed. The vertical 90° axis is steeper in those incidences of with the rule corneal astigmatism (Fig. 3). Therefore, the superior area typically is depressed (blue on elevation map and steeper, red on axial map). The bow tie pattern on an axial map is just a different mathematical representation of an oval spherocylindrical surface. It is not seen on the elevation map that is created from the x, y, and z coordinates of the usual representation of data in a 3-D world.

Figure 3: Corneal topography and imaging. Symmetric "with the rule" astigmatism.(Art from Highlights of Ophthalmology collection of medical illustrations).

Figure 4: Color based, relative scales in topography.

Relative vs. Absolute Scales

When analyzing color topographic maps, it is very important to be aware of whether the scale is relative or absolute. The scale chosen is significant: a physiologically normal cornea can have its features exaggerated to look grossly abnormal using a relative scale and assigning color changes to 1/4D increments.

The example presented demonstrates the same corneal surface displayed using the relative and absolute scales with varying step ranges. Note the central steep area and the peripheral flattening demonstrating the aspheric shape of the cornea.

One precise and well-expressed example of the Placido-based Corneal Topography systems is the EyeMap EH-290 from Alcon (Fig. 5). It is an elec- tro-optical instrument capable of measuring the shape of the cornea in very fine detail. The EyeMap EH-290 is unique in that it offers unparalleled performance regarding clinical sensitivity, and intuitive operation.

Section II: Topography

algorithms, multiple views of the corneal surface, and an intuitive means to navigate and operate the system. The EyeMap EH-290 offers a very versatile software for multiple uses such as:

•Advanced Contact Lens Software

•Keratoconus Detection

•Corneal Statistical Information

•Advanced Communication Software

Figure 5: Alcon Corneal Topography System.

The EyeMap EH-290 analyzes the data collected using two separate algorithms (axial and instantaneous rate of change). This type of analysis provides the practitioner with very fine details regarding the condition of the corneal surface. This equipment also uses a 23 narrow ring modified Placido disk configuration that aids in the sensitivity of the system and provides total corneal coverage. The system, with its patented auto features, icons, and on-screen help text is easy for a technician to operate with minimal training.

The EyeMap EH-290 is based on over 30 years of R&D from two companies' experiences. Visioptic and Alcon pioneered in the use of multiple

Other Topography Systems

Orbscan Corneal Topography

The Orbscan corneal topography system uses mainly a scanning optical slit design that is fundamentally different from corneal topography that analyzes the reflected images from the anterior corneal surface with the Placido ring system. The high-resolution video camera captures 40 light slits projected through the cornea at a 45° angle, as seen during slit lamp examination. The instrument's software analyzes 240 data points per slit and calculates the corneal thickness and posterior surface of the entire cornea. This is a combination of reflective corneal topography and optical slit design.

The accuracy and repeatability of the instrument is limited to movement of the patient’s eye, ability of patients to keep the eye wide open, optically clear cornea, and the presence of corneal abnormalities. The other limitations of current optical slit technology are the inability to detect interfaces (eg, after LASIK flap) and the longer time of image acquisition and processing compared to standard Placido-based topography.

Most Important Clinical Applications

of Corneal Topography

Detection of corneal pathologic conditions most importantly keratoconus and other corneal degeneration: Corneal topography often is used clinically for detecting and evaluating the severity of

Chapter 4: Topography Systems

keratoconus (Fig. 6). In clinical practice, the topographic diagnosis of keratoconus often is suggested by high central corneal power, large difference between the power of the corneal apex and of the periphery, and disparity between the two corneas of a given patient.

Corneal topography has detected changes suggestive of early keratoconus in many patients without classic clinical manifestations of this disease, that often are classified as subclinical keratoconus. Corneal topography is not sufficient for the definitive diagnosis of keratoconus because other disorders, such as corneal warpage from contact lens wear, can simulate early keratoconus. No history of contact lens wear, corneal stromal thinning, and other clinical signs would confirm the diagnosis of keratoconus.

Several quantitative indices to diagnose keratoconus topographically have been proposed. For example, central steepening greater than 47.2 D, inferior-superior asymmetry index over 1.2 D, and high index of irregular astigmatism have been suggested for topographic diagnosis of keratoconus. Automatic diagnosis of corneal disorders based on corneal topography by the instruments has been improved recently with the use of mathematical algorithms called neural networks. These algorithms are programmed to improve by themselves with experience, and, after a period of training on several hundred corneal topographies, they are reported to be highly specific.

Screening before refractive surgery: The detection of keratoconus is of particular importance in patients who plan to undergo refractive surgery.

Evaluation of irregular astigmatism especially after penetrating keratoplasty:

Corneal topography is mostly valuable for detection of pre-operative and postoperative astigmatism, planning the removal of sutures, and postoperative fitting of contact lenses. Corneal topography is useful for evaluation of effects and stability of all refractive procedures. Its value for guiding refractive treatments currently is being evaluated.

Contact lens fitting: Corneal topography is especially valuable in fitting complex corneal surfaces (eg, after penetrating keratoplasty).

Cataract surgery after refractive surgery:

In phacoemulsification, an alternative method to obtain the right IOL power, when it is not posible to have the preoperative (refractive) values or to have the patient´s refraction, is by corneal topography. To obtain the central power of the cornea you can measure an average of the topographic values around the central (3 mm) zone in the topography or you may use the flattest value of the central cornea. The advantage is that you may read specifically the corneal power without being altered by refractive changes in the lens.

Although corneal topography is used every day more often in conjunction with wavefront maps, they are actually quite different. With corneal topography the surgeon is quite informed of the corneal shape before any surgical procedure. Even with the new equipment employing a colored wavefront graphic and a colored topography map together to generate the ablation pattern, the two images originated by the system are separate.

BIBLIOGRAPHY

1. Fedor, P. MD. Corneal Topography and Imaging. E-Medicine, 2002. Consulting Staff, Grosse

Pointe Ophthalmology, Henry Ford Eye Care Services.

2. Gills JP et al: Corneal topography: The state of the art. Slack Incorporated; 1995: 1-328.

Section II: Topography

3.Holladay JT: Corneal topography using the Holladay Diagnostic Summary. J Cataract Refract Surg 1997 Mar; 23(2): 209-21.

4.Kaufman HE, Barron BA, McDonald M, Kaufman SC: Companion Handbook to the Cornea. Butterworth Heinemann; 2000: 947-59.

5.Krachmer JH, Mannis JM, Holland EJ: Cornea. In: Mosby. 1997: 1-2253.

6.Liu Z, Huang AJ, Pflugfelder SC: Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system. Br J Ophthalmol 1999 Jul; 83(7): 774-8.

7.Merlin U. Fundamentals of Keratoscopy. Corneal topography. Slack Incorporated; 1996:74-76.

8.Rabbetts RB: Clinical Visual Optics. Butterworth Heinemann; 1998: 378-420.

9.Schuman JS: Ophthalmic imaging and diagnostics. Ophthalmology Clinics of North America 1998; 11: 1-490.

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Samuel Boyd, MD. Associate Editor

Highlights of Ophthalmology Director, Laser Section,

Associate Director, Retina and Vitreous, Clinica Boyd Ophthalmology Center Panama, Rep. of Panama.

E-mail: sboyd@thehighlights.com