Comparison of Placido, Scheimpflug and Combined Dual Scheimpflug-Placido Technologies in Evaluating Anterior and Posterior CLMI, SimK's as well as Kmax, in Keratoconic and Postrefractive Surgery Ectasia
Authors: Mauger Thomas F , Mahmoud Ashraf M , Roberts Cynthia J , Chheda Lena V , Kuennen Rebecca A , Hendershot Andrew J , Lembach Richard G 
Published: April 2012

ABSTRACT

Purpose
To calculate and compare cone location and magnitude index (CLMI), Kmax and other corneal measures derived from three different technologies, Placido, Scheimpflug, and a combination dual Scheimpflug-Placido device, from the same group of eyes with keratoconus and postrefractive surgery corneal ectasia.

Methods
Keratoconus (n = 26) eyes of (n = 19) subjects and postrefractive surgery ectasia (n = 5) eyes of (n = 5) subjects were selected to have measurements performed using the Keratron Scout, Pentacam HR and Galilei Dual Scheimpflug Analyzer. Device-generated SimK's and device-specific CLMI and Kmax indices as well as map data, were exported from each device. Index values for multiple exams were averaged. The map data were processed using The Ohio State University Corneal Topography Tool (OSUCTT) to calculate CLMI parameters, Kmax and SimK values using consistent algorithms on all three devices. Maps were averaged before calculation for multiple examinations. Repeated measures analysis of variance and post- hoc analysis were used to identify differences between devices.

Results
The anterior axial CLMI calculated from the Keratron data was significantly higher than CLMI for the Galilei (p = 0.0443) or Pentacam (p < 0.0004) with keratoconus, 12.23 compared with 11.20 and 11.00 diopters, respectively. Kmax was also significantly higher in the Keratron than the Galilei (p = 0.0063) or the Pentacam (p < 0.0002). Galilei and Pentacam were not significantly different from each other in either CLMI (p = 0.6287) or Kmax (p = 0.2115). The anterior CLMI values for the postrefractive surgery ectasia eyes were not significantly different between devices. Posterior CLMI values were calculated from the Galilei and Pentacam data and were −2.60 and −2.46 diopters (p = 0.1173) for keratoconus and −2.66 and −3.04 diopters (p = 0.2242) for postrefractive surgery ectasia.

Conclusion
The small cone Placido measured dioptric values that were greater than the pure Scheimpflug system, but the difference was only about 1 diopter, which is not relevant clinically in evaluating and managing ectasia. The combined dual Scheimpflug-Placido system produced measured dioptric values between the other two technologies. The anterior CLMI calculations accurately predicted keratoconus with all three devices. The posterior CLMI in ectasia may be a potentially valuable calculation in demonstrating asymmetric steepening.

How to cite this article
Mauger TF, Mahmoud AM, Roberts CJ, Chheda LV, Kuennen RA, Hendershot AJ, Lembach RG. Comparison of Placido, Scheimpflug and Combined Dual Scheimpflug-Placido Technologies in Evaluating Anterior and Posterior CLMI, SimK's as well as Kmax, in Keratoconic and Postrefractive Surgery Ectasia. Int J Keratoco Ectatic Corneal Dis 2012;1(1):44-52.
• C Roberts is a Consultant for Oculus Optikgerate GmbH and Ziemer Ophthalmic Systems AG, and has an interest in the GALILEI.
• A Mahmoud has an interest in the GALILEI.
• T Mauger, L Chheda, R Kuennen, A Hendershot, and R Lembach have no financial interests.