Discrepancy Between Scheimpflug and Placido-Based Topographies and Correlation with Astigmatism After Scheimpflug Topography-Guided LASIK

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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.
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Purpose

• To compare

• Keratometry (K) readings
• K reading axes
• Topographic astigmatism

obtained by
Scheimpflug- based topographer
Placido-based topographer

Then correlating presence of discrepancies to
postoperative astigmatism after Scheimpflug
topography-guided LASIK
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Methods

• Retrospective non-randomized study
• 40 eyes of 20 patients who underwent
• Scheimpflug topography-guided LASIK (Oculink)
• for the correction of primary ametropia.

• Scheimpflug (ALLEGRO Oculyzer) and Placido-based
  (ALLEGRO Topolyzer)
• topographies were obtained.
• Adjusting the head position to avoid head tilt
  was done in all measures.

• 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).

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Results K readings

• In 50 of eyes (n20), the two topographers
  showed more than 0.5 Diopter (D) difference in K
  readings, while in 5 of eyes (n2) the
  difference was more than 1 D.
• In 95 of those eyes (n19), the K readings
  obtained from the Scheimpflug topographer were
  steeper.

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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
Avg K 42.1 D Astigmatism -3 D Axis 23.8
Avg K 43.3 D Astigmatism -3.8 D Axis 16.8
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Results Topographic astigmatism

• In 15 of eyes (n6), there was more than 0.5 D
  difference in topographic astigmatism between the
  two topographers.
• In 67 of those eyes (n4) it was more in the
  Scheimpflug topographer.

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Difference in K astigmatism is 0.8 D more
astigmatism by Scheimpflug topographer
Placido-based topographer
Scheimpflug- based topographer
Avg K 44.7 D Astigmatism -0.4 D Axis 2.8
Avg K 44.9 D Astigmatism -1.2 D Axis 179.6
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Results K readings axes

• In 12.5 of eyes (n5), there was more than 10
  degrees difference
• In 7.5 of eyes (n3) there was more than 20
  degrees difference in K readings axes.

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Difference in Axes is 10.5 Difference in Avg K
is 0.1 D Difference in K astigmatism is 0.2 D
Scheimpflug- based topographer
Placido-based topographer
Avg K 42.2 D Astigmatism -2.8D Axis 178
Avg K 42.3 D Astigmatism -3 D Axis 168.5
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Difference in Axes is 22.3 Difference in Avg K
is 0.1 D Difference in K astigmatism is 0.3 D
Scheimpflug- based topographer
Placido-based topographer
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
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Results Scheimpflug topography-guided LASIK
(Oculink)

• Postoperative astigmatism of more than 1 D was
  present in 30 of eyes (n12).
• 75 of those eyes (n9) showed discrepancies
  between the two topographers with regard K
  readings, their axes and/or topographic
  astigmatism.

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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.