Comparison of Orbscan and Ultrasound Pachymetry Measurements

1
Comparison of Orbscan and Ultrasound Pachymetry
Measurements
Faik Orucov, MD, Abraham Solomon, MD,Ziv Caspi,
David Landau, MD, Eyal Strassman, MD, and Joseph
Frucht-Pery, MD
Department of Ophthalmology, Hadassah University
Hospital School of Medicine Hebrew University,
Jerusalem, Israel
RESULTS
BACKGROUND
An excimer laser surgery has become the most
common procedures performed for correction of
ametropias. The preoperative evaluation of
refractive laser surgeries include refraction,
topography and orbscan examination, Schirmers
test, pupil size, pachimetry, intraocular
pressure, slit lump and dilated fundoscopic
examination. Accurate assesment of corneal
thicness is an important part of the refractive
surgery workup.
6466 eyes were included in the study. Orbscan II
measurements were significantly higher than US
pachymetry measurements when applying the
manufacturer-recommended acoustic correction
factor of 0.96 (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) higher 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). Analysis of regression showed
a significant linear regression between the
values obtained with the devices (P lt 0.0001, F
test).
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.
Ultrasonic pachymetry
METHODS
Orbscan pacymetry
Preoperative central corneal thickness
measurements obtained by Sonogage ultrasonic
pachymetry and Orbscan II in patients who
underwent LASIK procedures between January 2002
and January 2006 were analyzed. Thinnest point
included for Orbscan II and for US pachymetry
after at least 3 repeated measurements.
Ultrasonic pachymetry and Orbscan pacymetry (
custom acoustic factor 0.96 )
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 ).
CONCLUSION

• Orbscan scanning-slit system obtained
  significantly different values for corneal
  thickness. 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.

Student t test was used to identify significant
differences between methods, and the correlation
between methods was determined using the Pearson
bivariate correlation.
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 (Mean
575.432.4 micron and 538.229.4 Plt 0.001,
orbscan vs US pachymetry ) while high negative
differences appeared in thin corneas ( Mean
477,9 44.1 micron and 514.1 45.4 micron Plt
0.001, orbscan vs US pachymetry ).