The Continuing Debate Over

ELEVATION vs. RADIUS OF CURVATURE DATA

BY MITCHELL M. LOFTIN, O.D.
JAN. 1997

In recent years, primary eyecare providers have been deluged by new diagnostic instruments based on microprocessors. These include video and digital imaging, confocal scanning laser ophthalmoscopy, optical coherence tomography, infrared laser polarimetry, color Doppler imaging, retinal flowmeter imaging, digital ICG angiography and computerized corneal topography. It is this last area of diagnostic technology that can immeasurably aid, but may also confuse clinicians.

Corneal topography is quickly becoming affordable to the average practitioner as the prices for processors and peripherals decrease. Thus, it's important to examine the means by which topography measures the cornea and how different devices compare clinically. Here, we compare and contrast Placido's disk based (PDB) systems with raster stereophotogrammetry (RSG) systems.

THE ABCs OF PDBs

PDB systems account for approximately 70 percent of the corneal topographers on the market today. Videokeratoscopy, a spin-off of the first keratoscopic disk invented by Placido in 1880, uses concentric rings projected onto the anterior corneal air/tear interface to create a virtual image that yields data points captured by a high resolution video camera. The topographer converts these data points into radii of curvature by measuring the position of each ring on the unknown elliptical curve of the cornea. All of the systems use proprietary polynomial algorithm software to produce two-dimensional and sometimes three-dimensional color maps. Processor speeds for handling these algorithms range from 486 66MHZ to 586 133MHZ systems. Some systems have modems for efficient data exchange between providers.

To understand how these systems work, take a brief look at the optics involved in creating the curvature maps. We can think of the cornea as a three-dimensional ellipsoidal structure that has X, Y and Z dimensions. The Z axis (sagittal depth) will be larger for corneas with shorter radii and smaller for corneas with longer radii.

Measurements taken between the edge of one dark ring and the next are referenced to and measured from the distance to the optical axis. The processor, through a series of complicated algorithms, calculates radii for different (X,Y) locations on the cornea. The result is an axial curvature or power map whose contour is based on a series of radii relative to the optical axis of the eye. At this point, we begin to see the first of the limitations of PDB systems. A system with all of its radii of curvature measurements referenced to the optical axis is, by definition, skewed towards rounding or 'sphericalizing' the overall topography. This explains the advent of tangential maps whose axes are centered at the point being instantaneously measured. These maps provide more information about topography outside the three millimeter optical zone.

AN OVERVIEW OF PDB TOPOGRAPHY SYSTEMS

Controversy continues to swirl about the effects of lateral decentration, defocus and optical aberrations which sometimes occur with PDB systems. Decentration is such an important issue that one company has incorporated three miniature cameras to improve alignment. Typically, alignment of any one patient can be roughly verified by viewing the videokeratographs to check for alignment of the vertex normal of the map with the center of the visible iris diameter.

Earlier studies stressed the significance of error in PDB systems from lateral decentration during alignment. Recent studies imply that error also results from defocus and using a spherically biased system to estimate the shape of an aspherical structure. As the optical engineers continue their debate, it becomes obvious to clinicians using these systems daily on atypical topographies that much is lacking in PDB systems.

One disadvantage is that it's difficult, if not impossible, to obtain full corneal coverage with steeply shaped corneas. The size of the concentric rings from the cornea increases with increasing radius of curvature (flatter base curve). The result is the opposite in steeply shaped corneas (i.e., keratoconus) leading to limitation in corneal coverage with PDB systems. With PDB systems, the average dimension of corneal coverage is approximately 9.93mm, using an average test sphere of 42.00DS. However, the average keratometric value for the general population is 43.25. For the average contact lens wearer, this is not a significant problem, but for keratoconus, pellucid marginal degeneration and keratoplasty patients, full corneal coverage of a steeply shaped topography is critical. These corneas tend to have extremely steeply shaped topographies greatly reducing the coverage to as little as five millimeters in some cases.

With PDB systems, the optics of reflectance greatly reduce the image size as the power of the convex reflecting structure increases. In addition, de-epithelialized surfaces, such as those seen with recently performed PRK and thermal keratectomy, are usually non-imageable. Finally, an early apparent lack of true data from the central cornea has been compensated for by advanced algorithms, additional cameras, additional paracentral rings and magnification systems. Each of the PDB topography systems attempts a different approach to 'patch' this area together.

We are left with the question: what is it that PDBs truly measure? The fact is that the radii of curvature measured by PDBs simply yield derivations of the cornea's somewhat ellipsoidal shape. For instance, consider the pellucid marginal, keratoconus and normal corneal shapes.

Keratoconus most often involves an inferotemporal or inferonasal steepening and elevation of the cornea as the bulging of ectasia progresses. Pellucid marginal degeneration also involves inferior steepening, but it is in conjunction with the formation of a ledge, usually inferiorly on the cornea. Caroline et al. characterize this as a classic double wing 'butterfly' very similar in appearance to an inferior keratoconus as viewed on a PDB system. Because the data is composed of best fit spheres and not true elevation, the true shape of the cornea is not known and the diagnosis may be missed. Tangential maps give more shape information, but not true elevation data. Thus, the system may not detect paracentral islands that can lead to apical bearing in contact lens fittings.

STEREO TRIANGULATION REDUCES MARGIN FOR ERROR

In contrast, raster stereophotogrammetry produces true topographical maps of the entire corneal surface using stereo triangulation. This technique, which was developed for the aerospace industry and is used by satellites, compares two or more views of the same object. These two vantage points allow a three-dimensional model of the object to be created. The RSG topographer uses a silver chrome grid on a glass substrate composed of horizontal and vertical lines spaced 0.2mm (200 microns) apart to project a real image onto the full diameter of the cornea. The intersection of horizontal and vertical lines produces a single image point whose X, Y and Z coordinates are obtained by instantaneous flash illumination. This image is then digitized and, using the known values of the working distance angle of offset and expected distortion, proprietary algorithms are used to produce true topographical elevation maps. To enhance deviations for analysis, an average theoretical sphere is mathematically created from the total information recorded, then subtracted to produce an elevation map. This information is not only capable of providing us with power maps indicating the curvature, but also the elevation data. The system gives full corneal coverage, even onto the sclera in some cases. It will also give pre-, post- and intraoperative measurements of de-epithelialized surfaces.

CASE #1: NO OBVIOUS SIGNS OF KERATOCONUS

A 35-year-old woman with gradual onset monocular triplopia of the left eye was initially diagnosed with irregular astigmatism. An ophthalmology colleague referred her to our contact lens clinic for an RGP lens. The patient had best corrected visual acuities of OD 20/20, OS 20/30 with diplopia. Pupils, ocular motility, intraocular pressures and posterior segment findings were normal. The slit lamp examination was also unremarkable. There were no signs of Vogt's striae or apical thinning. We noted an inferiorly curved Hudson-Stahli line roughly approximating a Fleischer's ring, as well as minimal superficial punctate staining inferiorly.

In this patient, both keratometry and retinoscopy were mildly distorted in the left eye with a slight scissoring effect. Keratometry readings were OD 47.12/48.12 @ 098 and OS 46.50/46.00 (distorted) @ 075. Videokeratoscopy of both eyes using a PDB topography system yielded the axial curvature maps seen in Figure 1. This map shows the immediate deficiency of axial maps to yield accurate data for a complex topographical surface. The map of the left cornea resembles the shape of a 'reverse keratoconus' map with an inferiorly decentered area of flattening. The center of this area lies approximately two millimeters below the optical axis and resembles a decentered ablation zone from PRK or an area of corneal warpage from previous wear of an improperly fitted RGP contact lens. The patient insisted that neither of these was the case and no other corneal findings suggested this.

FIG. 1: AXIAL CURVATURE MAPS OF CASE #1 AS DISPLAYED BY A PLACIDO'S DISK BASED CORNEAL TOPOGRAPHER.

Placing a large diameter RGP contact lens on the patient's eye yielded fluorescein patterns (Fig. 2) that revealed a large superior area of apical touch, with a smaller area of inferior touch.

FIG. 2: AFTER PLACING A LARGE DIAMETER RGP LENS, WE NOTED A LARGE SUPERIOR AREA OF APICAL TOUCH, WITH A SMALLER AREA OF INFERIOR TOUCH.

The right eye had an overall steepness higher than than the 47.20DS that is currently used by Maeda, Klyce et al. to diagnose keratoconus. Retesting the patient using raster stereophotogrammetry, we found the cone suggested by the fluorescein pattern (Fig. 3).

FIG. 3: USING RASTER STEREOPHOTOGRAMMETRY, WE FOUND THE CONE SUGGESTED BY THE FLUORESCEIN PATTERN IN FIGURE 2.

Although tangential maps of the PDB system also identified the areas of elevation (Fig. 4), they appeared disproportionate in size to those noted on the fluorescein pattern obtained with the diagnostic RGP lenses. In addition, the curvature maps do not tell us if the superior and inferior areas of steepening are due to depressions, as in pellucid marginal degeneration, or elevations, as in keratoconus. Because this patient is older than the typical age for keratoconus (teens to early 20s), we would have been highly suspicious of pellucid if not for the RSG elevation maps. The RSG system resolves this question with the diagnosis of atypical keratoconus or possible keratoglobus. We prescribed a Menicon gas permeable lens with decentered optical zone, and the patient achieved 20/20 visual acuity.

FIG. 4: TANGENTIAL MAPS PROVIDED BY THE PDB SYSTEM ALSO IDENTIFIED THE AREAS OF ELEVATION, BUT THEY APPEARED DISPROPORTIONATE IN SIZE TO THOSE NOTED ON THE FLUORESCEIN PATTERN.

CASE #2: RSG REVEALS PREVIOUSLY UNDETECTED DISCRETE ISLANDS

This 51-year-old woman, a former keratoconus patient who had undergone keratoplasty to her right eye 10 years ago, was referred to us for RGP fitting to correct irregular astigmatism. Her best corrected spectacle visual acuity was 20/25 OD with a refraction of OD -12.00
-6.50 x 024. Pupils, motility, pressure and posterior segments were normal. The patient had an intact 6.5mm clear corneal graft. As is typical of these patients, the graft resulted in a large degree of irregular corneal astigmatism (6.00D). Videokeratoscopy using a PDB system yielded the axial and tangential color maps in Figure 5.

FIG. 5: PDB SYSTEM YIELDS AXIAL AND TANGENTIAL COLOR MAPS OF POST KERATOPLASTY PATIENT (CASE #2).

Notice the maps show no discrete islands as is typically seen with this patient and is evident with the fluorescein pattern pictured in Figure 6. (Patient wearing fenestrated Softperm lens.) In fact, the PDB system utilized here predicts a uniform fluorescein pattern.

FIG. 6: FLUORESCEIN REVEALS DISCRETE ISLANDS POST KERATOPLASTY.

In contrast, the RSG system demonstrates the islands seen on the fluorescein pattern using its elevation data (Fig. 7). These islands are critical when attempting to fit these patients with large diameter, RGP or Softperm lenses. The areas of bearing can lead to significant staining and put the patient at higher risk for graft rejection. Not only is the PDB system unable to detect the two areas of elevation and thus predict the bearing pattern, it is also unable to image the full cornea diameter.

FIG. 7: USING ELEVATION DATA, RSG SYSTEM DEMONSTRATES ISLANDS SEEN ON FLUORESCEIN PATTERN ILLUSTRATED BY FIGURE 6.

CASE #3: SUBTLE DIFFERENCES AID DIAGNOSIS

This 32-year-old woman said she had noticed reduced visual acuity in both eyes for several months. We noted normal intra- and extraocular findings on pupils, motility, tonometry, biomicroscopy and ophthalmoscopy. Keratometry revealed slightly distorted mires of OD 42.50/46.00 @ 015, OS 42.00/45.50 @ 145. Videokeratoscopy suggested keratoconus in both eyes (Fig. 8).

FIG. 8: PDB SYSTEM SUGGESTED KERATOCONUS IN BOTH EYES.

Fluorescein revealed a normal, even, apical alignment pattern with an RGP lens on the right eye. However, significant apical bearing typical of keratoconus was present inferiorly in the left eye. The RSG elevation map (Fig. 9) shows a normal, astigmatic, with-the-rule pattern in the right eye (not keratoconus) and keratoconus inferiorly displaced in the left eye corresponding to the fluorescein patterns noted clinically.

FIG. 9: RSG ELEVATION MAP SHOWS NORMAL, WITH-THE-RULE ASTIGMATISM OD AND INFERIORLY DISPLACED KERATOCONUS OS.

CASE #4: WHERE IS THE CONE?

This 27-year-old man had undergone PKP in his left eye and was diagnosed with keratoconus of the right eye. He wore a fenestrated Softperm lens 6.50 base curve,
-10.00 power, 14.3 diameter OD and a bitoric RGP lenticulated design OS. Best corrected visual acuities were 20/30 OS and 20/20 OD. The graft was clear and stable in the left eye and the cone was not significant for apical staining or scarring. We observed Fleischer's ring and Vogt's striae in the right eye. We wondered then, "Where is the cone?"

A PDB system records the cone as an inverted 'butterfly' encroaching onto and just inferior to the corneal apex (Fig. 10). Despite attempts to wet the cornea, dry spots resulted in lost data. Also, the small overall diameter of the mires illustrates the less than limbal-to-limbal capabilities of PDB system on steeply shaped corneas. The butterfly shape of the axial map suggests pellucid marginal degeneration despite the long-standing history of keratoconus.

Now, compare this to the RSG color map (Fig. 11). As before, the axial map of the PDB system describes a moderate to large sized cone decentered inferior temporally in the right eye.

A second look at the elevation data shows a starkly different picture. First, the cone is located temporally and is much smaller than described by the PDB system. Second, the map itself has almost no dry spots due to use of fluorescein, and it extends to a full eight millimeters on both corneas. In addition, fluorescein patterns (not shown here) of rigid gas permeable diagnostic lenses again revealed that the cone was at the temporal 9 o'clock position of the cornea and not located inferior temporally as suggested by the PDB system.

TAKE ADVANTAGE OF ELEVATION DATA WITH RSG SYSTEMS

Numerous authors have noted that PDB systems have inherent shortcomings due to the nature of the measurement technique and the corresponding algorithms that manipulate the data to a graphical form. The significance to the contact lens practitioner is that apical clearance and apical alignment, although impossible to achieve in many pathological cases, are still valid overall goals. In distorted corneas, it's important to know the locations of areas of elevation so we can achieve adequate clearance. Harsh or abrasive bearing can lead to excessive apical scarring and staining in keratoconus and PKP patients. Because PDB systems provide only power data and not elevation information, they are at a disadvantage in giving accurate data for fitting severely distorted corneas. Typical contact lens programs found in PDB systems generally strive for apical clearance of 10 to 20 microns to provide adequate tear exchange. Because PDB systems cannot locate the areas of elevation, they are hard-pressed to meet these criteria.

In an even more critical situation, what confidence level can be placed in PDB systems in their current form to locate and accurately display central PRK islands and decentered PRK ablation zones? Don Johnson, M.D., who has performed over 1,700 PRK procedures in Westminster, B.C., says the best corrected visual acuity of patients with central islands will often be down as many as four lines. These patients also complain of ghosting, definite diplopia and a lot of glare and distortion problems. While other investigators argue the significance and permanence of central islands, it remains an important consideration in the postoperative healing process of PRK and PTK patients. Not only are RSG systems better able to detect these islands, they can also detect them earlier. RSG systems are the only systems we are aware of that are capable of interfacing with a Zeiss operating microscope to give color maps intraoperatively. They can also be configured to work with the existing equipment in a practitioner's office. All that's required is a compatible 486 to pentium 8MB RAM system and a high resolution slit lamp capable of being configured with a beam splitter to provide the needed image.

It seems intuitive that contact lens practitioners would desire a system which can easily be integrated into the prefitting workup. Since fluorescein staining is a necessary adjunct diagnostic procedure to detect dry eye syndrome, a measurement of corneal topography could be easily performed immediately prior to RGP lens insertion. Research at Bascom-Palmer in Florida reveals that RSG systems may be capable of providing additional guidelines for the diagnosis of dry eye patients through the use of RSG systems. RSG systems provide a superior means of providing elevation data, axial curvature, tangential curvature and location of PRK ablation zones relative to PDB systems. In addition, it seems conceivable that these systems could easily be configured to work with existing equipment found in most optometrists' offices at a reasonable price, thus increasing optometric access to modern topographical data.

As the limitations of PDB technology become more apparent, newer systems such as the ORBSCAN by Orbtek have also found their way into the eyecare market. This system uses 40 individually photographed 'slit scans' to provide elevation data of the anterior and posterior corneal surfaces, pachometry, anterior chamber depth as well as iris and crystalline lens topography. Recent studies using this instrument indicate its ability to give topographical data relevant to the posterior as well as anterior corneal surface. Lundergan reports of an early case of keratoconus in an eye with normal anterior surface topography, demonstrating early posterior elevation changes. Could this be typical of early keratoconus? With the current nationwide studies of keratoconus underway, it seems critical that this data be available to give us a clear picture of what is occurring in keratoconus. For those practitioners involved with postoperative corneal surgery and fitting keratoconus and PKP patients, this information could be most helpful.

Continued technological advancements require the eyecare profession to keep abreast of new devices and recognize the inherent limitations of presently used systems. Our clinic had the opportunity to work with the EyeSys Model 3 corneal topographer utilizing version 3.04 software. Our RSG system was the PAR (CTS) Corneal Analyzer utilizing version 1.2 software. Recent advances in the EyeSys system show significant improvements in the areas of weakness outlined here. Our clinical testing continues and we look forward to documenting these advances. CLS

This study was supported in part by a Research to Prevent Blindness unrestricted grant. The author has no financial interest in any of the companies mentioned in this article.

Dr. Loftin is a clinical assistant professor for the ophthalmology residency program at the University of Missouri-Columbia School of Medicine.

References are available upon written request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #20.