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
BACKGROUND
RESULTS
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, Schirmer’s 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 < 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.
PURPOSE
800
700
A significant correlation was observed between ultrasound pachymetry and Orbscan slitscan
pachymetry (Pearson correlation coefficient, r = 0.887; P < 0.001). Analysis of regression showed a
significant linear regression between the values obtained with the devices (P < 0.0001, F test).
600
500
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.
400
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.
300
40
8. 4
63 .0
3
62 5 6
0. 8
618.0
59 6 0
5.
58 1 2
3.
57 6 4
0.
56 1 6
8. 8
545.6
53 2 0
3.
52 7 2
0.
51 2 4
8.
49 7 6
5. 8
48 .2
3
0
47 .8
0
46 3 2
8.
44 8 4
5. 0
43 .4
2
42 2 0
9.
37
Mean
METHODS
Ultrasonic
pachymetry
CCT
ORBSCCCT
Orbscan pacymetry
Ultrasonic
pachymetry and Orbscan pacymetry ( custom acoustic factor = 0.96 )
ACUSADD
The cylinder was higher in negative differences than the positive differences (Mean 1.26±1.4 diopter and -0.87±0.9 diopter; P<0.001) while simK’s were same between
groups ( P=0.491 and P=0.585 ).
CONCLUSION
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 575.4±32.4 micron and 538.2±29.4; P< 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; P< 0.001, orbscan vs US pachymetry ).
 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.