Introduction and basics
Computerized corneal topography analysis is the measurement of the curvature of the corneal surface. This tool is based on the principles of keratometry and photokeratoscopy developed in 1880 by Placido. He placed a planar target with concentric alternating black-and-white rings in front of a patient’s eye and then observed the shape of the rings in the virtual image of that target created from the reflection of the patient’s anterior corneal surface. If the cornea is spherical, the rings appear circular and concentric. Deviations of the corneal shape appear as either distortions in shape or eccentricity of the rings.
Photokeratoscopy provides the user with only qualitative information about the curvature of the cornea, changes that accompany surgery, and progressive corneal abnormalities. The keratometer yields quantitative data, but only at four points. These points are located at approximately the 3-mm optical zone along two perpendicular meridians. One pair of points is aligned along the steepest axis of the corneal surface, with the second pair 90 degrees away. The keratometry has fundamental limitations in that it is able only to measure points along the annulus of the 3-mm optical zone.
With the capability of modern computers and software technology to qualify the data obtained from reflected Placido disc images, it has become feasible and practical to precisely analyze the radius of curvature (mm) and corresponding refractive power (diopter) on the corneal surface from inside the 1-mm optical zone to outside the 9- to 11-mm optical zone. This information is then translated into a complete color-coded map. The map is interpreted much like other topographic maps. These topographic maps provide the ability to monitor corneal curvature changes from the apex to the periphery.
There are a variety of corneal topographer systems available. Some use a back-lit conical dish as its Placido target; other systems use a cylindric light cone as the Placido target. With either a conical dish or a cylindric light cone, a Placido ring image is produced on the cornea.
The Orbscan IIz, Galilei, and Oculus Pentacam ( Fig. 41.1 ) are the latest in the state-of-the art technologies for mapping the surfaces of the cornea and for anterior segment analysis. The Orbscan takes multiple cross-sectional scans of the cornea with an advanced Placido disc system and is able to analyze elevation and curvature measurements on both the anterior and posterior surfaces of the cornea, white-to-white measurement, anterior chamber depth, angle kappa, and corneal pachymetry values. Galilei analyzer merges two technologies, the rotating Scheimpflug and Placido technology, into one measurement, leading to accurate values of the posterior and anterior surfaces. The dual Scheimpflug approach offers accurate pachymetry readings and needs only to rotate 180 degrees. Pentacam is a rotating Scheimpflug camera that generates Scheimpflug images in three dimensions, with the dot matrix fine-meshed in the center as a result of the rotation. It takes a maximum of 2 seconds to generate a complete image of the anterior segment. Any eye movement is detected by a second camera and corrected in the process. The pachymetry and topography of the entire anterior and posterior surfaces of the cornea from limbus to limbus are calculated and depicted in Fig. 41.2 . The analysis of the anterior eye segment includes calculation of the chamber angle, chamber volume, and height. Images of the iris and anterior and posterior surfaces of the lens are also generated. The densitometry of the lens is automatically qualified. This chapter’s focus is on the cornea.
Most corneal topography systems available today can generate various map displays. When performing computerized corneal topography for prerefractive surgery screening, diagnosis of a corneal pathology, or contact lens fitting, the most commonly used maps are discussed in the following.
Axial map or sagittal map
This is the most widely used and simplest of all topographic displays. It shows the curvature of the anterior surface of the cornea as a topographic map in diopteric values and measures it in an axial direction relative to the center.
Every map has a color scale. Cool colors, such as blue and green represent flatter areas of the cornea, whereas the warmer colors of orange and red represent steeper areas of the cornea. The analysis should include the keratometric values and should not be interpreted based on the colors alone.
Corneal irregularity measurement (CIM) and shape factor measurements are statistical indices that some topography units, such as Humphrey Atlas, provide on their axial map printout. The increase or decrease of these two over time indicates a change in the progress or healing of a condition. CIM values less than 0.5 indicate a normal-shaped cornea, and 1.0 or higher indicates corneal surface irregularities. Shape factor 0 to 0.3 is normal. Shape factor more than 1.0 indicates high irregularities ( Fig. 41.3 ).
Elevation map
This is the difference in height between the measurements of the cornea and a reference shape called best fit. This value can be negative if the measurement is less than the reference and positive if it is greater than the reference. The reference shape could be a best-fit sphere, best fit ellipsoid, or a toric reference shape ( Fig. 41.4 ).
Corneal thickness map
This describes corneal thickness measurements distributed across the cornea.
Other types of topography displays include tangential map, true net power, refractive map, keratometry map, multivue map ( Fig. 41.5 ), differential map, photokeratoscopic view, profile view, and so on.
Clinical uses
Of all currently available technology, the corneal topography is the best to provide specific and detailed information about the curvature of the cornea. It offers an exact evaluation of the profile of the cornea and a better interpretation and control of some of the pathologic conditions that can occur and affect the cornea.
Variations that occur in corneal topography can be the result of the changes of the corneal stroma and epithelium. Tissue loss and scars cause a flattening of the area and increase the curvature of the cornea around the lesion. With thinning processes, such as keratoconus and pellucid marginal degeneration, thin tissues actually protrude and therefore the curvature of the cornea becomes greater.
The most commonly performed laser eye surgeries, such as photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK) for the correction of myopia, hyperopia and astigmatism, and corneal crosslinking (CXL) for the treatment of keratoconus, can greatly benefit from corneal topography. Corneal topography can identify irregular astigmatism and potential problems, as well as early forms of keratoconus and pellucid marginal degeneration. Corneal topography precisely measures the alterations produced in corneal shape and follows any regression or remodeling that may occur over time.
Corneal topography is useful for the correct evaluation of high corneal astigmatism. These cases often pose significant challenges when performing retinoscopy, automatic refraction, and standard keratometry.
Corneal topography is very helpful in assessing the quality of the surface of the cornea, the stability and the effect of the surgery after CXL, radial keratectomy, and the selective removal of sutures after penetrating or lamellar keratoplasty. Also it assists in choosing the site of incision in cataract surgery to minimize postoperative astigmatism.
Corneal topography is very beneficial in determining the power and axis of corneal astigmatism in implanting toric intraocular lenses in cataract surgery.
In addition, corneal topography is used in fitting contact lenses, especially gas-permeable lenses. It is also useful in monitoring and evaluating contact lens effect on the cornea in long-term contact lens wearers.
Corneal topography analysis in refractive surgery
The development and evaluation of keratorefractive surgery have benefited from the parallel advances made in the field of corneal topography analysis.
The major advantage of laser refractive surgery is the precision with which the excimer laser ablates corneal tissue. Consistent, accurate centration of the procedure is one component of the technique that is critical to its success.
From the topographic map of a cornea, it is possible to determine the amount of spherocylindric aberration before surgery and objective measurement of surgical results, as well as the precise location of the ablation zone in laser refractive surgery.
Preoperative analysis
The normal cornea is aspheric (see Fig. 41.3 ), being steepest centrally with progressive flattening toward the periphery.
A commonly encountered corneal topographic finding is that of regular and symmetric astigmatism with a bowtie shape. A with-the-rule regular corneal cylinder is vertically aligned ( Fig. 41.6 , lower left), whereas against-the-rule corneal astigmatism takes the form of a horizontally aligned bowtie pattern ( Fig. 41.7 ).
