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

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Variability in Preoperative Measurement of
Astigmatism in Refractive PatientsInstrument
Variability or Patient Positioning?

• Peter Lee MD, Howard Gimbel MD, Maria Ferensowicz
  MA

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Financial Interest Disclosure

• The authors have no financial interest in any
  product, corporations, or companies referred to
  in this presentation

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Purpose

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

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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

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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 p0.000043 (highly
  significantly different), manifest refraction vs.
  OPD p0.389666 (not significantly different), and
  manifest refraction vs. Pentacam p0.000008
  (highly significantly different).

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Cylinder Power


highly significant
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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 p0.006629 (significantly different),
  manifest vs. OPD p0.000004 (highly significantly
  different), and manifest vs. Pentacam p0.613283
  (not significantly different).

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Cylinder Axis


significant highly significant
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Mean Absolute Difference in AxisManifest vs.
Topography
Dependent T-tests OPD vs Orbscan p0.487, OPD vs
Pentacam plt0.001, Orbscan vs Pentacam plt0.001
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Patient Positioning?
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Sample Patient

• How does a slight head tilt affect measurements
  of astigmatism by corneal topographies?

Before head tilt Before head tilt After head tilt After head tilt
Cylinder Power (D) Cylinder Axis (º) Cylinder Power (D) Cylinder Axis (º)
Orbscan 1.5 79 1.4 72
OPD 1.4 96 1.4 99
Pentacam 1.2 13 3.8 10
Patient positioning study in progress
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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.