Peer-Reviewed Literature: Evaluating Patients With the Orbscan II and Pentacam

Editor: Ming Wang, MD, PhD
Co-Editor: Tracy Swartz, OD, MS
Reviewer: Gregory J. McCormick, MD
Panel Members: Helen Boerman, OD, FAAO; Wei Jiang, MD; Lisa Martén, MD; Jason Noble, MD; Jay S. Pepose, MD, PhD; Renée Solomon, MD; Elizabeth Yeu, MD

Anterior segment surgeons are increasingly interested in screening laser vision correction candidates for keratoconus and determining the true corneal power in patients who have undergone laser refractive surgery for the purpose of IOL power calculations. The Orbscan II and Orbscan IIz (both from Bausch & Lomb, Rochester, NY) and the Pentacam Comprehensive Eye Scanner (Oculus, Inc., Lynwood, WA) allow for the direct measurement of both the anterior and posterior corneal surfaces, and these devices may be useful in the evaluation of the aforementioned patients. The following articles were reviewed:

1. Lim L, Wei R, Chan WK, et al. Evaluation of keratoconus in Asians: role of Orbscan II and Tomey TMS-2 corneal topography. Am J Ophthalmol. 2007;143:399-400.
2. Sonmez B, Doan MD, Hamilton R, et al. Identification of scanning slit-beam topographic parameters important in distinguishing normal from keratoconic corneal morphologic features. Am J Ophthalmol. 2007;143:401-408.
3. Rabinowitz YS. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. 1995;11:371-379.
4. Quisling S, Sjoberg S, Zimmerman B, et al. Comparison of Pentacam and Orbscan IIz on posterior curvature topography measurements in keratoconus eyes. Ophthalmology. 2006;113:1629-1632.
5. Luz A, Ursulio M, Castaneda D, Ambrosio R Jr. Corneal thickness progression from the thinnest point to the limbus: study based on a normal and a keratoconus population to create reference values [in Portuguese]. Arq Bras Oftalmol. 2006;69:579-583.
6. Ambrosio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract Refract Surg. 2006;32:1851-1859.
7. Buhren J, Kuhne C, Kohnen T. Defining subclinical keratoconus using corneal first-surface higher-order aberrations. Am J Ophthalmol. 2007;143:381-389.
8. Sonego-Krone S, Lopez-Moreno G, Beaujon-Balbi OV, et al. A direct method to measure the power of the central cornea after myopic laser in situ keratomileusis. Arch Ophthalmol. 2004;122:159-166.
9. Qazi M, Cua I, Roberts C, Pepose J. Determining corneal power using Orbscan II videokeratography for intraocular lens calculation after excimer laser surgery for myopia. J Cataract Refract Surg. 2007;33:21-30.
10. Borasio E, Stevens J, Smith GT. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula. J Cataract Refract Surg. 2006;32:2004-2014.

THE ORBSCAN II
The Orbscan II combines two technologies that allow surgeons to measure patients' corneal surfaces. A calibrated video and slit-scanning beam system measures the anterior segment's geometry, including anterior and posterior corneal surface elevation maps and regional pachymetry. A Placido-disc system calculates the curvature of the anterior corneal surface. The Orbscan II takes a total of 9,600 measurements of the anterior segment, which it uses to calculate slope and curvature in every direction. A tracking system compensates for involuntary ocular movements.

THE PENTACAM COMPREHENSIVE EYE SCANNER
The Pentacam is a tomographer that produces a three-dimensional image of the front and back surfaces of the cornea—a virtual picture of the anterior eye segment and the limbus-to-limbus measurements of corneal thickness. The system is based on the Scheimpflug camera, which rotates around a common axis while taking three-dimensional image slices of the anterior segment using the same central point. The common point allows the slices to be re-registered to compensate for saccadic movements.

SCREENING FOR KERATOCONUS
Screening for corneal abnormalities related to subclinical keratoconus is important to reducing patients' risk of corneal ectasia following LASIK. Lim et al¹ performed a study using the Orbscan II to evaluate 116 eyes with keratoconus or suspected keratoconus and 70 normal myopic eyes in an Asian population. Using the Orbscan II, the investigators found that the mean values of maximum posterior elevation, thinnest corneal pachymetry, 3-mm irregularity, and 5-mm irregularity were significantly different (P<.01) between the control group, the keratoconic group, and the group with suspected keratoconus. The mean maximum posterior elevation was 0.108 ±0.042 mm in the keratoconic group, 0.046 ±0.017 mm among the keratoconus suspects, and 0.026 ±0.008 mm in the control group. The thinnest pachymetric point was 458 ±59 µm in the keratoconic group, 504 ±40 µm in the keratoconus suspects, and 554 ±25 µm in the control group. The 3-mm irregularity was 6.00 ±2.60 D in the keratoconic group, 2.44 ±1.36 D in the keratoconus suspects, and 1.05 ±0.37 D in the control group. The 5-mm irregularity was 6.32 ±2.46 D in the keratoconic group, 2.46 ±1.19 D in the keratoconus suspects, and 1.38 ±0.39 D in the control group. Lim et al concluded that scanning, slit-based corneal topography with the Orbscan II provides data on the anterior corneal surface, posterior corneal surface, and pachymetry that are useful for the detection of keratoconus suspects.

Sonmez et al² studied parameters with the Orbscan IIz that are important in distinguishing normal from keratoconic eyes. The investigators found that the mean value for the irregularity index (3 mm) was 1.04 ±0.33 D in normal eyes (n = 207) and 4.20 ±2.15 D in keratoconic eyes (n = 47). The mean irregularity index at 5 mm was 1.33 ±0.36 D for normal eyes and 4.50 ±2.44 D in the keratoconic group. There was also a significant difference in the thinnest optical pachymetry, with a mean of 548 ±35 µm in normals and 472 ±57 µm in the keratoconic group. The researchers concluded that the device's metrics are useful in distinguishing normal corneas from those that may have keratoconus.

Sonmez et al also calculated the difference in inferior and superior corneal power to detect keratoconus. This difference in the tangential map was determined using corneal powers at five different locations above (30°, 60°, 90°, 120°, and 150°) and below (210°, 240°, 270°, 300°, and 330°) the horizontal meridian at 3-mm circles. There was a significant difference between control and keratoconic eyes with an average difference in inferior and superior corneal power of 0.41 ±0.64 D and 6.56 ±6.16 D, respectively. The investigators concluded that the difference in inferior and superior corneal power based on measurements with the Orbscan IIz is a useful metric to screen for evidence of keratoconus.

Additionally, Sonmez et al investigated the use of the Orbscan IIz to apply the skewed radial axis, a metric developed by Rabinowitz,³ in the evaluation of keratoconus. The skewed radial axis was calculated using the formula: skewed radial axis = 180° - (steep inferior axis - steep superior axis). For a 3-mm circle, the skewed radial axis was 34.6° ±39.1° for the control group and 64.5° ±41.8° for the keratoconic group, a significant difference (P<.001). The investigators concluded that—like the difference in inferior and superior corneal power, thinnest optical pachymetry, and irregularity indices—the skewed radial axis is a valuable metric in screening for keratoconus. They cautioned, however, that no single metric could be utilized to categorize an eye as having or not having keratoconus.

Quisling et al⁴ compared posterior-corneal-surface best-fit sphere, thinnest mean pachymetry, and posterior elevation measurements using the Orbscan IIz versus the Pentacam in eyes with keratoconus (N = 36). There was no significant difference between the two devices for the posterior-corneal-surface best-fit-sphere radius of curvature or thinnest optical pachymetry. The mean posterior elevation by best-fit sphere fixed to the apex of the posterior corneal surface was 34.86 µm (range, 3 to 120 µm) for the Pentacam and 48.50 µm (range, 11 to 118 µm) for the Orbscan IIz. Given that the thinnest pachymetry and posterior-surface best-fit sphere were equivalent between the devices, the researchers speculated that the mean difference in posterior elevation might be due to a difference in data analysis by the two machines. Based on their results, the investigators were not able to determine whether the Pentacam underestimated the posterior vault or the Orbscan IIz overestimated it. They concluded that surgeons and clinicians should become adept at interpreting data from the machine they use routinely.

The progression of pachymetric values from the thinnest point to the periphery in normal versus keratoconic eyes was evaluated by Luz et al⁵ using the Orbscan II and by Ambrosio et al⁶ using the Pentacam. Both groups of investigators found that pachymetric values increase faster in keratoconic corneas when measured from the thinnest point to the periphery. In the study by Ambrosio et al, corneal volume was derived based on regional pachymetric data, which also showed significant differences between normal and keratoconic eyes. Based on their results,the authors of the two studies concluded that pachymetry maps from the Orbscan II and Pentacam may be useful in distinguishing normal from keratoconic eyes.

Using data from the Orbscan II, Buhren et al⁷ calculated corneal higher-order aberrations in eyes with keratoconus, asymptomatic fellow eyes, and normal controls. The reserachers exported data from the Orbscan II and used software not provided with the device to generate an aberration profile. They found significantly more corneally induced coma in eyes with keratoconus than asymptomatic fellow eyes, which, in turn, had higher amounts of coma than normal controls (mean coma root mean square = 1.353 µm, 0.413 µm, and 0.168 µm, respectively). The use of discriminate analysis allowed differentiation between these groups with high degrees of sensitivity and specificity. These findings suggest that wavefront data may provide useful metrics for detecting keratoconus and that these metrics can be derived from measurements obtained with the Orbscan II.

DETERMINING CORNEAL POWER AFTER LASER REFRACTIVE SURGERY
Corneal power after laser refractive surgery is an increasingly important topic as more and more of these patients develop cataracts. Using conventional topographers and manual keratometers that measure only the anterior corneal curvature has proven to be less accurate in postphotoablative corneas than in virgin eyes. This difference is due in part to the altered relationship between the anterior and posterior corneal surfaces after laser refractive surgery. Sonego-Krone et al⁸ evaluated a direct method to measure corneal power after myopic LASIK using the scanning-slit technology of the Orbscan II by taking into account corneal thickness and anterior and posterior corneal curvature. The researchers investigated corneal power before and after myopic LASIK in 26 eyes and compared the Orbscan II and the clinical history method. They found the best agreement between the clinical history method and the Orbscan II corneal power determination to be with the 2-mm total optical power calculation (difference from refractive ∆ = 0.07 ±0.62 D) and the 4-mm total optical power calculation (difference from refractive ∆ = -0.08 ±0.53 D) These measurements can be readily obtained from the standard Orbscan II software. The investigators concluded that these methods of determining corneal power with the Orbscan II after LASIK have the potential to reduce the error induced by techniques that are based on the anterior corneal curvature alone.

Qazi et al⁹ assessed the accuracy of Orbscan II measurements for IOL power calculations in eyes with previous photorefractive surgery for myopia. Postoperative data collected after phacoemulsification were used to back-calculate corneal power using the Holladay 2 formula. The back-calculated corneal values were statistically compared at central Orbscan II curvature and power measurements of 3 to 6 mm. When these measurements were used prospectively the investigators concluded that 80.9% of eyes would achieve vision within ±0.50 D of their targeted refraction when a 4-mm optical power was used, 76.2% when a 5-mm total axial power was applied, and 42.1% when the historical method was employeed. The investigators concluded that the Orbscan II can be used to predict true corneal power more accurately than the history-based method and that the former may be particularly useful when pre-LASIK data are unavailable.

Borasio et al¹⁰ introduced a modified formula for Gaussian optics (known as the Borasio Edmondo Smith and Stevens formula) based on measurements of the anterior and posterior corneal surfaces with the Pentacam. The investigators evaluated 13 eyes with a history of laser refractive surgery that underwent phacoemulsification. They calculated corneal power using the Borasio Edmondo Smith and Stevens formula and predicted IOL power with either the SRK/T or Hoffer Q formula. They compared the actual postoperative refractive outcome to that predicted by the Borasio Edmondo Smith and Stevens formula, the clinical history method, and the Humphrey Atlas (Carl Zeiss Meditec, Inc., Dublin, CA) with the Holladay 2 formula. The researchers found the mean refractive error using the Borasio Edmondo Smith and Stevens formula to be +0.08 0.62 D, which was statistically significantly closer to the target than with the clinical history method or the Atlas topography system using the Holladay 2 formula. The investigators concluded that their method improved the accuracy of IOL power calculations for postphotoablative eyes.

THE BOTTOM LINE
The Orbscan II and the Pentacam are able to image and measure both the anterior and posterior corneal surfaces directly, which can be helpful for evaluating keratorefractive patients. By measuring both surfaces, features characteristic of keratoconus in the posterior corneal surface and regional pachymetry map can be identified, thus potentially improving the safety and efficacy of screening candidates for keratorefractive surgery. After laser vision correction, the Orbscan II and the Pentacam allow for the direct measurement of corneal power, and they appear to reduce errors that occur using devices that measure the anterior corneal surface alone. There may be differences between certain measurements obtained by the Orbscan II and the Pentacam, a situation suggesting that users should be familiar with the normative values for the specific device that they use. Evidence is mounting that having the capability of measuring both the anterior and posterior corneal surfaces provides useful information for cataract and refractive surgeons.

Reviewers
Dr. McCormick acknowledged no financial interest in the products or companies mentioned herein. Dr. McCormick may be reached at (802) 864-2010; drmccormick@vermontlaservision.com

Editor
Ming Wang, MD, PhD, Clinical Associate Professor of Ophthalmology at the University of Tennessee and Director of the Wang Vision Institute in Nashville, Tennessee. Dr. Wang acknowledged no financial interest in the products or companies mentioned herein. He may be reached at (615) 321-8881; drwang@wangvisioninstitute.com.

Co-Editor
Tracy Swartz, OD, Educational Director at the Wang Vision Institute in Nashville, Tennessee. Dr. Swartz acknowledged no financial interest in the products or companies mentioned herein. She may be reached at (615) 321-8881; drswartz@wangvisioninstitute.com.

Panel Members
Helen Boerman, OD, FAAO, is Assistant Clinical Operations Manager at the Wang Vision Institute in Nashville, Tennessee, and she is Staff Optometrist and Adjunct Facult, at the Indiana University School of Optometry in Bloomington. She acknowledged no financial interest in the products or companies mentioned herein. Dr. Boerman may be reached at (615) 321-8881; drboerman@wangvisioninstitute.com.
Wei Jiang, MD, is a general ophthalmologist practicing in Kaiser, California. She acknowledged no financial interest in the products or companies mentioned herein. Dr. Jiang may be reached at (925) 847-5065; wjiang70@yahoo.com.
Lisa Martén, MD, is a corneal fellow at the Wang Vision Institute in Nashville, Tennessee. She acknowledged no financial interest in the products or companies mentioned herein. Dr. Martén may be reached at (615) 321-8881; drmarten@wangvisioninstitute.com.
Jason Noble, MD, is a resident physician, Department of Ophthalmology and Vision Sciences at the University of Toronto. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Noble may be reached at (416) 844-5477; jason.noble@utoronto.ca.
Jay S. Pepose, MD, PhD, is Director of the Pepose Vision Institue and Professor of Clinical Ophthalmology at the Washington University School of Medicine, St. Louis. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Pepose may be reached at (636) 728-0111; jpepose@peposevision.com.
Renée Solomon, MD, is working at the Ophthalmic Consultants of Long Island in New York. She acknowledged no financial interest in the products or companies mentioned herein. Dr. Solomon may be reached at reneeoph@yahoo.com.
Elizabeth Yeu, MD, is a resident physician at the Rush University Medical Center in Chicago. She acknowledged no financial interest in the products or companies mentioned herein. Dr. Yeu may be reached at (312) 942-5315; elizabeth_yeu@rush.edu.