1. Authors: Leonidas Traipe Magdalena Rios Edison Villagra Allister Gibbons Marcelo Hidalgo Claudia Goya Authors have no financial interest

2.  They represent the true shape of the cornea – independent of the position of the corneal surface to the visual axis.  They provide more precise results than curvature maps Curvature maps assume that the visual axis and reference axis pass through the apex of the cornea

3.  To describe the variation in elevation topography that happens when the body of reference is changed in a Scheimpflug based system using Allegro Oculyzer (Wavelight Inc.).  To determine specific differences between: Best fit Sphere Ellipsoid Toric Ellipse

4.  Sample: We retrospectively analyzed 90 consecutive keratoconus patients (149 eyes), diagnosed by clinical and topographical criteria. 72.2% Male // 27+-9 years old  The sample was divided in 4 different stages described by Krumeich (1998) and later modified by Alio (2006). Eyes: Stage I=55 // Stage II=44 // Stage III=25 // Stage IV=25 KRUMEICH J, DANIEL J, KNÜLLE A. 1998. Live-epikeratophakia for keratoconus. J Cataract Refract Surg. 24:456-463. ALIO J, SHABAYEK M. 2006. Corneal Higher Order Aberrations: A method to grade keratoconus. J Refract Surg. 22:539-545.

5. Analysis Plot – Points of elevation (in micrometers) over/under the body of reference in the central 3 mm. Valid for the anterior and posterior surface (Belin M, 2005). We used constant distances between each of the 9 points analyzed in each patient. We did not modify the axis of reference of the topographic maps. 8,00 3,57 0,00 6,00 -1,02 -10,00 11,00 2,05 -5,00 9,00 2,55 -3,00 9,00 -0,91 -8,00 10,00 0,36 -7,00 8,00 1,76 - 4,00 6,00 0,02 -6,00 1 2 3 4 12,00 4,40 -1,00 5 6 7 8 9 Red = Minimum value of elevation. Blue = Maximal value of elevation. White = Mean value of elevation. This procedure was applied to all eyes included in this sample. For each body of reference and for both corneal surfaces.

6. Elevation Point Analyzed Body of reference with statisticall diferences in comparison with the other bodies. P- value Anterior SurfacePoint 1 (Central) BFS Toric Ellipse<0.01 Ellipsoid<0.01 Posterior SurfacePoint 1 (Central) BFS Toric Ellipse<0.03 Ellipsoid<0.05 Absolutely no difference in central elevation point comparing Ellipsoid and Toric Ellipse.

7.  The only other study point with significant differences was point 6 (the most inferior): Only in Keratoconus Stage 2 Only between BFS and Toric Ellipse (p=0.046)  All other study points were non statistically significant comparing all bodies of reference to one another (ANOVA).

8. To graph the difference between using different bodies of reference in all patients studied (Stage I). Elevation in micrometers

9.  The central elevation (Point 1) has statistically significant variation in: Both Corneal Surfaces Between all different bodies of reference In the 4 stages of keratoconus severity  Also significantly different: All elevations comparing BFS and Toric Ellipse Most elevations comparing BFS and Ellipsoid  Except in elevation study point 6

10. STAGE 3STAGE 4 STAGE 1 STAGE 2 um Study Points 1-9

11.  As expected BFS revealed higher elevation differences.  This could probably show earlier ectatic disease increasing the tests sensitivity but decreasing test specificity.  We found no differences in using Ellipsoid or Toric Ellipse as bodies of reference.

12.  Posterior elevation subtraction values are more important In early stage KC they tend to be more abnormal than anterior elevation changes. This difference diminishes as the KC progresses.  Central elevation is where most differences are found between bodies of reference.