Comparison of Orbscan and Ultrasound Pachymetry Measurements

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Comparison of Orbscan and Ultrasound Pachymetry
Measurements
Faik Orucov, MD, Abraham Solomon, MD, Ziv Caspi,
David Landau, MD, Eyal Strassman, MD, Joseph
Frucht-Pery, MD
Department of Ophthalmology, Hadassah University
Hospital School of Medicine Hebrew University,
Jerusalem, Israel
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Purpose

• To compare central corneal thickness
  measurements (CCT) using Orbscan II (Bausch and
  Lomb, Rochester, NY) and Sonogage ultrasonic
  (US) pachymetry in normal eyes of large
  population.

Authors have no financial interests in any of the
mentioned products or companies
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Methods

• January 2002 and January 2006 (All LASIK
  patients)
• Hadassah University Hospital
• Missing records of any variables were excluded
  from the study.
• Thinnest point included for Orbscan II and for US
  pachymetry after at least 3 repeated
  measurements.
• The Student t test

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Rersults

• 6466 eyes were included in the study.
• Acoustic correction factor of 0.96
• Orbscan II measurements were significantly higher
  than US pachymetry measurements (553.8 41.2
  micron and 531.7 31.6 micron, respectively) (P lt
  0.0001).
• Corneal thickness measured by US pachymetry was
  approximately 22.1 19.7 microm (4) smaller than
  the thickness measured by Orbscan II with
  correction.

• A significant correlation was observed
  between ultrasound pachymetry and Orbscan
  slitscan pachymetry (Pearson correlation
  coefficient, r 0.887 P lt 0.001).

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Difference US Orbscan
P Positive 538.2 575.4
0.0001 Minimal 525.3 533.2
0.0001 Negative 514.1 477.9
0.0001

• The differences between orbscan and US pachymetry
  are divided to three as,
• High Positive Difference when orbscan measured
  thicker than 22 micron and
• High Negative Difference when orbscan measured
  thinner than 22 micron and
• Small Difference when measurements were 22
  micron.
• High positive differences appeared in thicker
  corneas
• High negative differences appeared in thin corneas

US Pachymetry
Orbscan pachymetry

• The cylinder was higher in negative differences
  than the positive differences (Mean -1.261.4
  diopter and -0.870.9 diopter Plt0.001) while
  simKs were same between groups ( P0.491 and
  P0.585 ).

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Conclusion

• Orbscan scanning-slit system obtained
  significantly different values for corneal
  thickness. (acoustic correction factor of 0.96)
• The differences between two devices are more in
  thick and thin corneas.
• The degree of variability within each group
  indicated that these two techniques are not
  clinically comparable.
• These differences are important for planning and
  measuring the effects of corneal refractive
  procedures.