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Variability in Preoperative Measurement of Astigmatism in Refractive Patients: Instrument Variability or Patient Positi

Variability in Preoperative Measurement of Astigmatism in Refractive Patients: Instrument Variability or Patient Positioning?. Peter Lee MD, Howard Gimbel MD, Maria Ferensowicz MA. Financial Interest Disclosure.

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Variability in Preoperative Measurement of Astigmatism in Refractive Patients: Instrument Variability or Patient Positi

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  1. Variability in Preoperative Measurement of Astigmatism in Refractive Patients:Instrument Variability or Patient Positioning? Peter Lee MD, Howard Gimbel MD, Maria Ferensowicz MA

  2. Financial Interest Disclosure • The authors have no financial interest in any product, corporations, or companies referred to in this presentation

  3. Purpose • To report the variability in preoperative measurements of astigmatism in refractive patients using three different corneal topographies and compared to manifest refraction.

  4. Methods • 1151 consecutive eyes underwent corneal topography by Orbscan, OPD and Pentacam and manifest refraction before refractive surgery (2005-2007) • All surgeries were done by one surgeon • Surgeon chose the measurement of astigmatic correction based on pre-operative data • Manifest refraction used most frequently

  5. Results – Cylinder Power • The pre-operative measurements of cylinder power (mean +/- SD) for the different modalities were: 0.88+/-0.89 (manifest refraction), 1.18+/-2.49 (Orbscan), 0.91+/-1.40 (OPD) and 1.06+/-1.38 (Pentacam). • Using dependent T-test for comparison: manifest refraction vs. Orbscan p=0.000043 (highly significantly different), manifest refraction vs. OPD p=0.389666 (not significantly different), and manifest refraction vs. Pentacam p=0.000008 (highly significantly different).

  6. Cylinder Power ** ** ** highly significant

  7. Results – Cylinder Axis • The pre-operative measurements of cylinder axis (mean +/- SD) for the different modalities were: 80.13+/-67.33 (manifest refraction), 86.74+/-75.63 (Orbscan), 90.61+/-70.07 (OPD), and 81.39+/-70.95 (Pentacam). • Using dependent T-test for comparison: manifest vs. Orbscan p=0.006629 (significantly different), manifest vs. OPD p=0.000004 (highly significantly different), and manifest vs. Pentacam p=0.613283 (not significantly different).

  8. Cylinder Axis ** * * significant ** highly significant

  9. Mean Absolute Difference in AxisManifest vs. Topography Dependent T-tests: OPD vs Orbscan p=0.487, OPD vs Pentacam p<0.001, Orbscan vs Pentacam p<0.001

  10. Patient Positioning?

  11. Sample Patient • How does a slight head tilt affect measurements of astigmatism by corneal topographies? * Patient positioning study in progress

  12. Conclusion • Inter-instrument variability and patient positioning and facial asymmetry add to the challenges of determining the true measurement of astigmatism in the refractive surgical population.

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