1. Peter Lee MD, Howard Gimbel MD, Maria Ferensowicz MA
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 ( )
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= (highly significantly different), manifest refraction vs. OPD p= (not significantly different), and manifest refraction vs. Pentacam p= (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: / (manifest refraction), / (Orbscan), / (OPD), and / (Pentacam).
Using dependent T-test for comparison: manifest vs. Orbscan p= (significantly different), manifest vs. OPD p= (highly significantly different), and manifest vs. Pentacam p= (not significantly different).

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

9. Mean Absolute Difference in Axis Manifest 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?
Before head tilt
After head tilt
Cylinder
Power (D)
Axis (º)
Orbscan
1.5 79
1.4 72
OPD 96 99
Pentacam
1.2 13
3.8 10
* 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.