1. Discrepancy Between Scheimpflug and Placido-Based Topographies and Correlation with Astigmatism After Scheimpflug Topography-Guided LASIK Mohamed Abou Shousha, MD, 1,2 Osama S. Ibrahim, MD, PhD 2 1. Bascom Palmer Eye Institute, University of Miami, Miller School of Medicine, FL, USA 2. Faculty of Medicine, University of Alexandria, Alexandria, EGYPT The authors have no financial interest in the subject matter of this poster.

2. Purpose To compare  Keratometry (K) readings  K reading axes  Topographic astigmatism Scheimpflug- based topographer Then correlating presence of discrepancies to: postoperative astigmatism after Scheimpflug topography-guided LASIK Placido-based topographer obtained by: &

3. Methods Retrospective non-randomized study 40 eyes of 20 patients who underwent Scheimpflug topography-guided LASIK (Oculink) for the correction of primary ametropia.  K readings, their axes and topographic astigmatism obtained from the two topographers were compared  Presence of discrepancy was correlated to postoperative astigmatism after Scheimpflug topography-guided LASIK (Oculink).  Scheimpflug (ALLEGRO Oculyzer) and Placido-based (ALLEGRO Topolyzer) topographies were obtained.  Adjusting the head position to avoid head tilt was done in all measures.

4. Results: K readings In 50% of eyes (n=20), the two topographers showed more than 0.5 Diopter (D) difference in K readings, while in 5% of eyes (n=2) the difference was more than 1 D. In 95% of those eyes (n=19), the K readings obtained from the Scheimpflug topographer were steeper.

5. Avg K: 42.1 D Astigmatism: -3 D Axis: 23.8 Avg K: 42.1 D Astigmatism: -3 D Axis: 23.8 Avg K 43.3 D Astigmatism: -3.8 D Axis: 16.8 Avg K 43.3 D Astigmatism: -3.8 D Axis: 16.8 Difference in Avg K is 1.2 D, Steeper by Scheimpflug topographer Difference in K astigmatism is 0.8 D, more astigmatism by Scheimpflug topographer Difference in Avg K is 1.2 D, Steeper by Scheimpflug topographer Difference in K astigmatism is 0.8 D, more astigmatism by Scheimpflug topographer Placido-based topographer Scheimpflug- based topographer

6. Results: Topographic astigmatism In 15% of eyes (n=6), there was more than 0.5 D difference in topographic astigmatism between the two topographers. In 67% of those eyes (n=4) it was more in the Scheimpflug topographer.

7. Avg K: 44.7 D Astigmatism: -0.4 D Axis: 2.8 Avg K: 44.7 D Astigmatism: -0.4 D Axis: 2.8 Avg K: 44.9 D Astigmatism: -1.2 D Axis: 179.6 Avg K: 44.9 D Astigmatism: -1.2 D Axis: 179.6 Difference in K astigmatism is 0.8 D more astigmatism by Scheimpflug topographer Difference in K astigmatism is 0.8 D more astigmatism by Scheimpflug topographer Placido-based topographer Scheimpflug- based topographer

8. Results: K readings axes In 12.5% of eyes (n=5), there was more than 10 degrees difference In 7.5% of eyes (n=3) there was more than 20 degrees difference in K readings axes.

9. Avg K: 42.2 D Astigmatism: -2.8D Axis: 178 Avg K: 42.2 D Astigmatism: -2.8D Axis: 178 Avg K: 42.3 D Astigmatism: -3 D Axis: 168.5 Avg K: 42.3 D Astigmatism: -3 D Axis: 168.5 Placido-based topographer Scheimpflug- based topographer Difference in Axes is 10.5 Difference in Avg K is 0.1 D Difference in K astigmatism is 0.2 D Difference in Axes is 10.5 Difference in Avg K is 0.1 D Difference in K astigmatism is 0.2 D

10. Avg K: 42.1 D K astigmatism: -0.3 D Axis 164.4 Avg K: 42.1 D K astigmatism: -0.3 D Axis 164.4 Avg K: 42.9 D Astigmatism: -0.6 D Axis: 6.7 Avg K: 42.9 D Astigmatism: -0.6 D Axis: 6.7 Placido-based topographer Scheimpflug- based topographer Difference in Axes is 22.3 Difference in Avg K is 0.1 D Difference in K astigmatism is 0.3 D Difference in Axes is 22.3 Difference in Avg K is 0.1 D Difference in K astigmatism is 0.3 D

11. Results: Scheimpflug topography-guided LASIK (Oculink) Postoperative astigmatism of more than 1 D was present in 30% of eyes (n=12). 75% of those eyes (n=9) showed discrepancies between the two topographers with regard K readings, their axes and/or topographic astigmatism.

12. Conclusion: Discrepancies exist between Scheimpflug and Placido based topographers with regard K readings, their axes and topographic astigmatism. Cases that showed discrepancies had a higher incidence of postoperative astigmatism after Scheimpflug topography-guided LASIK.