Corneal shape, curvature, and thickness profile can be generated from a variety of technologies, such as Placido disk–based topography and elevation-based devices (including scanning-slit systems and Scheimpflug imaging). Each technology conveys different information about corneal curvature, anatomy, and biomechanical function. Also, computerized topographic and tomographic systems may display other data: pupil size and location, indices estimating regular and irregular astigmatism, estimates of the probability of having keratoconus, simulated keratometry, and corneal asphericity. Other topography systems may integrate wavefront aberrometry data with topographic data. Although this additional information can be useful in preoperative surgical evaluations, no automated screening system can supplant clinical experience in evaluating corneal imaging.
Corneal topography provides highly detailed information about corneal curvature. Topography is evaluated using keratoscopic images, which are captured from Placido disk patterns that are reflected from the tear film overlying the corneal surface and then converted to computerized color scales (Fig 1-7). Because the image is generated from the anterior surface of the tear film, irregularities in tear composition or volume can have a major impact on the quality and results of a Placido disk–based system. Because of this effect, reviewing the Placido image (image of the mires) prior to interpreting the maps and subsequent numerical data is extremely important. In addition, Placido disk–based systems are referenced from the line that the instrument makes to the corneal surface (termed the vertex normal). This line may not necessarily be the patient’s line of sight or the visual axis, which may lead to confusion in interpreting topographic maps. For a more extensive discussion of other uses of computerized corneal topography, refer to BCSC Section 3, Clinical Optics, and Section 8, External Disease and Cornea. Generally, data from the reflection of the mires generated by the topographic instruments are presented not only numerically but—more important for clinical evaluation—also as an image, with corneal curvature typically represented utilizing axial and tangential methods.
Figure 1-7 Placido imaging of the cornea. A, The ring reflections of the Placido imaging device can be seen on this patient’s cornea. This image is then captured and analyzed.
(Courtesy of M. Bowes Hamill, MD.), B, The printout of the captured Placido image is seen in the lower right hand corner of this image with the different calculated color maps displayed in the other corners. (Courtesy of M. Bowes Hamill, MD.)
Axial power and curvature
Axial power representation derives from the supposition that the cornea is a sphere and that the angle of incidence of the instrument is normal to the cornea. Axial power is based on the concept of “axial distance” (Fig 1-8). As can be seen from the illustration, axial power underestimates steeper curvatures and overestimates flatter curvatures. This representation also is extremely dependent on the reference axis employed—optical or visual. Maps generated from the same cornea but using different reference axes look very different from one another. Axial power representations actually average the corneal powers and thereby provide a “smoother” representation of corneal curvature than does the tangential, or “instantaneous,” method. Recall that the curvature and power of the central 1–2 mm of the cornea are generally not well imaged by Placido disk techniques but can be closely approximated by the axial power and curvature indices (formerly called sagittal curvature); however, the central measurements are extrapolated and thus are potentially inaccurate. These indices also fail to describe the true shape and power of the peripheral cornea. Topographic maps displaying axial power and curvature provide an intuitive sense of the physiologic flattening of the cornea but do not represent the true refractive power or the true curvature of peripheral regions of the cornea (Fig 1-9).
Instantaneous power and curvature
A second method of describing the corneal curvature on Placido disk–based topography is the instantaneous radius of curvature (also called meridional or tangential power). The instantaneous radius of curvature is determined by taking a perpendicular path through the point in question from a plane that intersects the point and the visual axis, while allowing the radius to be the length necessary to correspond to a sphere with the same curvature at that point. The curvature, which is expressed in diopters, is estimated by the difference between the corneal index of refraction and 1.000, divided by this tangentially determined radius. A tangential map typically shows better sensitivity to peripheral changes with less “smoothing” of the curvature than an axial map shows (Fig 1-9). In these maps, diopters are relative units of curvature and are not the equivalent of diopters of corneal power. The potential benefit of this method’s increased sensitivity is balanced by its tendency to document excessive detail (“noise”), which may not be clinically relevant.
Figure 1-8 Schematic representation of the difference between axial distance (axial curvature) and radius of curvature for 2 points on a curved surface. Points C1 and C2 represent the centers of curvature of their respective surface points. Points A1 and A2 represent the endpoints of the axial distances for the given axis. As can be seen, local, steeper areas of curvature are underestimated, whereas flatter areas are overestimated.
(Adapted from Roberts C. Corneal topography: a review of terms and concepts. J Cataract Refract Surg. 1996;22(5):624–629, Fig 3.)
Figure 1-9 Examples of curvature maps. A, Axial (sagittal); B, instantaneous (tangential).
(Courtesy of J. Bradley Randleman, MD.)
For routine refractive screening, most surgeons have the topographic output in the axial (sagittal) curvature mode rather than the instantaneous (tangential) mode.
Corneal topography and astigmatism
A normal topographic image of a cornea without astigmatism demonstrates a relatively uniform color pattern centrally with a natural flattening in the periphery (Fig 1-10). Regular astigmatism is uniform steepening along a single corneal meridian that can be fully corrected with a cylindrical lens. Topographic imaging of regular astigmatism demonstrates a symmetric “bow-tie” pattern along a single meridian with a straight axis on both sides of center (see Fig 1-10B). The bow-tie pattern on topographic maps is an artifact of Placido-based imaging; that is, because the Placido image cannot detect curvature at the central measurement point, the corneal meridional steepening seems to disappear centrally and become enhanced as the imaging moves farther from center.
Irregular astigmatism is nonuniform corneal steepening from a variety of causes that cannot be corrected by cylindrical lenses. Irregular astigmatism decreases best-corrected visual acuity (BCVA; also called corrected distance visual acuity, CDVA) and may reduce contrast sensitivity and increase visual aberrations, depending on the magnitude of irregularity. Rigid gas-permeable and hard contact lenses can correct visual acuity reductions resulting from corneal irregular astigmatism by bridging the irregular corneal surface and the contact lens with the tear film. For more information on irregular astigmatism, see BCSC Section 3, Clinical Optics.
Figure 1-10 Normal corneal topographic patterns. A, Round; B, symmetric bow tie.
(Courtesy of J. Bradley Randleman, MD.)
Corneal topography is very helpful in evaluating eyes with irregular astigmatism. Topographic changes include nonorthogonal steep and flat meridians (ie, not 90° apart), Figure 1-11. Asymmetry between the superior and inferior or nasal and temporal halves of the cornea may also be revealed by corneal topography, although these patterns are not necessarily indicative of corneal pathology. In contrast, wavefront analysis can demonstrate higher-order aberrations (such as coma, trefoil, quadrafoil, or secondary astigmatism). The ability to differentiate regular from irregular astigmatism has clinical significance in keratorefractive surgery. Traditional excimer laser ablation can treat spherocylindrical errors but does not effectively treat irregular astigmatism. Topography-guided ablation may be useful in treating irregular astigmatism not caused by early corneal ectatic disorders.
Figure 1-11 A curvature map showing nonorthogonal axes, which may indicate pathology that would contraindicate refractive surgery.
(Courtesy of Gregg J. Berdy, MD.)
Limitations of corneal topography
In addition to the limitations of the specific algorithms and the variations in terminology among manufacturers, the accuracy of corneal topography may be affected by other potential problems:
misalignment (misaligned corneal topography may give a false impression of corneal apex decentration suggestive of keratoconus)
instability (test-to-test variation)
insensitivity to focus errors
limited area of coverage (central and limbal)
decreased accuracy of corneal power simulation measurements (SIM K) after refractive surgical procedures
decreased accuracy of posterior surface elevation values in the presence of corneal opacities or, often, after refractive surgery (with scanning-slit technology)
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.