There"s constantly new research into the applications and accuracy of advanced technology that eventually ends up in our offices. One such device that is becoming a fixture in many optometric offices is the corneal topographer. Recent research studies looked at the accuracy of corneal topography vs. other methods of quantifying corneal curvature, contour and thickness. This report is the first of occasional columns I"ll write on diagnostic technologies.

Repeatable Measures
The Orbscan topographer from Bausch & Lomb Surgical and the Atlas unit from Zeiss Humphrey Systems have been the subjects of several recent research studies, particularly those focusing on refractive surgery outcomes.

Orbscan is a multi-surface corneal topographer that gives you a series of printouts called "Quad Maps." These maps show anterior corneal surface elevation (top left), posterior corneal elevation (top right), surface topography (bottom left) and corneal pachymetry (bottom right). These four views are essential when evaluating refractive surgery candidates.

One abstract from the Associa-tion for Research in Vision and Ophthalmology (ARVO) conference suggests that the instrument is repeatable and predictable within a session and over time. These researchers report that measurements on 31 different eyes during four separate sessions--including one day, one week and four weeks postoperatively--showed no statistical difference in any of the 23 parameters they measured.₁

The top two elevation maps in the Orbscan printout are particularly valuable in evaluating hyperopes for refractive surgery. Many surgeons have had mixed results in hyperopic LASIK ablation. One reason for this may be the location of the corneal apex. Hyperopic excimer ablation occurs in the peripheral cornea and essentially creates a central apex. If a patient has a decentered apex but the surgery creates one on center, the patient experiences a "double apex" effect. This often results in induced astigmatism and little hyperopic correction. The Orbscan elevation maps can help determine whether a patient"s corneal apex is or is not centered prior to surgery.

Early Keratoconus
These maps can also help diagnose subclinical, early or forme fruste keratoconus preoperatively. One Tokyo-based study shows that the Orbscan identified keratoconus and subclinical keratoconus with 97% sensitivity and 100% specificity.₂

This study also suggests that the Orbscan allows clinicians to detect keratoconus earlier than they would otherwise. This is because the device takes readings of the posterior corneal surface and corneal thickness. Keratoconus manifests earlier posteriorly than it does in the anterior surface. Having the ability to detect subclinical keratoconus may help prevent post-LASIK irregularity and complications related to ectasia.

Another study of LASIK patients in Mexico City used elevation maps from the Zeiss Humphrey Atlas topographer. These results indicate that the Atlas is useful for calculating the ablation postoperatively.₃ Subjects were myopic and myopic/astigmatic patients, and researchers measured their elevation topography before and one month after LASIK.

They measured microns of ablation by comparing pre- and post-op elevation topographies as well as the spheres of reference topography reports. The researchers then compared this to the ablation in the excimer laser surgery report. The mean difference between the results of the Zeiss Humphrey analysis and the laser ablation log was only 10.85 microns. These researchers also found two new ablation patterns in LASIK patients: "U" and circular patterns.

Pachymetry Values
Pachymetry is an essential tool when we"re evaluating patients for refractive surgery. One study compared the corneal thickness measurements Orbscan obtains with ultrasound pachymetry. In 74 eyes the mean of the differences was only 0.95 microns for the Orbscan I vs. ultrasound pachymetry, and -13.63 microns for the Orbscan II.₄

These results indicate the Orbscan device achieves a high degree of agreement and correlation with pachymetry.

In another study of Orbscan"s ability to measure corneal thickness, researchers in Paris measured corneal thickness and curvature on 60 eyes of patients diagnosed with Marfan"s syndrome, a disease associated with flattening corneas. They found, for the first time, a strong association with corneal thinning and stromal disruption in Marfan"s patients.₅

Another Parisian study shows that the Orbscan"s capabilities for measuring the anterior chamber depth are highly reproducible and correlate strongly with measurements that A-scan ultrasonography can obtain.₆ This could provide a great benefit when obtaining measurements for phakic and cataract intraocular lens implants.

Corneal Warpage
Optometrists Kenneth Lebow and Robert M. Grohe observed patterns of superior corneal flattening associated with inferior corneal steepening in both keratoconus and contact lens warpage. Using the Zeiss Humphrey Atlas topography unit to map a retrospective group of 100 eyes diagnosed with either keratoconus or contact lens warpage, they could then look at specific topographic measurements to make a strong differentiation--with 98% sensitivity and 94% specificity.₇

Their study shows that keratoconic eyes had steeper flat K readings, greater variation in axial curvature, greater corneal toricity, and larger shape factors and corneal irregularity measurements (CIMs). This information, only found in topography, uniquely differentiates contact lens-induced warpage from true keratoconus.

Corneal topography has proven itself as a predictable and reproducible tool in quantifying many measurements of a patient"s cornea. It has become an essential tool for evaluating patients prior to refractive surgery. And, clinicians are finding more unique capabilities for this device, including diagnosing such conditions as early keratoconus and Marfan"s syndrome, and using topography as an important tool for cataract and other surgical procedures.

Dr. Karpecki has no financial interests in any of the products mentioned.

1. Herderick EE, Roberts C, et al. Repeatability of the Orbscan II in a normal population. ARVO 2001 abstract no. 4819-B838.
2. Bessho K, Maeda N, et al. Automated keratoconus detection using data from anterior and posterior corneal surfaces. ARVO 2001 abstract no. 4800-B819.
3. Erazo J, Boca C, Velasco R, et al. Elevation topography in LASIK: Reported vs. obtained ablation. ARVO 2001 abstract no. 3253-B395.
4. Beltran F, Silva C, et al. Validity of corneal thickness measurements by two elevation topography systems. ARVO 2001 abstract no. 4805-B824.
5. Sultan G, Piscella PJ, et al. Orbscan analysis of the cornea in Marfan"s disease. ARVO 2001 abstract no. 4829-B848.
6. Allouch C, Touzeau O, et al. Ocular biometric measurements with a slit-lamp method (Orbscan). ARVO 2001 abstract no. 4801-B821.
7. Lebow KA, Grohe RM. Differentiating contact lens-induced warpage from true keratoconus using corneal topography. CLAO J 1999 Apr; 25(2)114-22.