1. Comparison of Single-Scheimpflug and Dual-Scheimpflug Pachymetry Measurements and Effect of Decentration World Cornea Congress VI April 7-9, Jennifer R. Lewis PhD, Ashraf M. Mahmoud, Robin F. Beran MD, Cynthia J. Roberts PhD
The authors of this poster have received research funding and travel expense reimbursement from Ziemer Group. J. Lewis is currently an employee of Ziemer USA.

2. Advantages of Rotating Scheimpflug Systems
No motion within a single slit image:
Biometry of Anterior Segment Structures along a meridian can be computed with confidence
Distinct from Anterior Segment OCT, for which each image (B scan) is composed of many scans (A scans)

3. Limitations of Rotating Scheimpflug Systems
Any scanning or rotating system is subject to eye motion during exam acquisition
Many parameters may be sensitive to errors of eye movement across scans (e.g. curvature)
Motion correction is required to generate appropriate corneal surfaces
Decentration in Rotating Scheimpflug Imaging 0
135
90
45
180
225
270
315

4. Scheimpflug geometry for corneal imaging
Apparent Corneal Thickness
apparent
thickness
cornea
slit lamp
Image Plane
Object plane
Lens Plane
Scheimpflug Line
45°
Scheimpflug geometry for corneal imaging
The posterior surface is viewed through the anterior surface, which produces an “apparent” corneal thickness due to refraction of the outgoing posterior ray at the anterior surface. This must then be corrected for “viewing angle” based on the known geometry.

5. Effect of Slit Light Decentration on Apparent Corneal Thickness
Decentered Left: Single SPL
Decentered Right: Single SPL
apparent
thickness
Apex
Apex
apparent
thickness X X
The slit light is not perpendicular to the surface.
Apparent slit image thickness depends on the direction of decentration.
The apparent slit image is thicker in the right view and thinner in the left view.

6. Decentration SPL: Right View The apparent thickness error of a 500 µm spherical cornea with a decentration of up to ±1 mm.
Apparent Deviation:
Decentration
Apparent thickness change
Roberts C and Züger BJ. “GALILEI™ Dual Scheimpflug Analyzer.” (Translated to Portuguese) In: Ambrosio R, Chalita MR, Chamon W, Schor P, Netto MV (eds). Wavefront, Anterior Segment Imaging and Corneal Topography: Latest concepts and technologies 2006:

7. Decentration SPL: Left View The apparent thickness error of a 500 µm spherical cornea with a decentration of up to ±1 mm.
Apparent Deviation:
Decentration
Apparent thickness change
Roberts C and Züger BJ. “GALILEI™ Dual Scheimpflug Analyzer.” (Translated to Portuguese) In: Ambrosio R, Chalita MR, Chamon W, Schor P, Netto MV (eds). Wavefront, Anterior Segment Imaging and Corneal Topography: Latest concepts and technologies 2006:

8. Effect of Slit Light Decentration on Apparent Corneal Thickness
Decentered Right: Dual SPL
Apex
apparent
corneal thickness X
The slit light is not perpendicular to the surface when decentered.
Apparent slit image thickness changes depending on direction to camera.
The apparent slit image is thicker in the right view and thinner in the left view.

9. Purpose
To test the clinical effect of a 1-mm decentration on thinnest pachymetry value and location comparing single- and dual-Scheimpflug imaging devices.
Decentration
Roberts C and Züger BJ. “GALILEI™ Dual Scheimpflug Analyzer.” (Translated to Portuguese) In: Ambrosio R, Chalita MR, Chamon W, Schor P, Netto MV (eds). Wavefront, Anterior Segment Imaging and Corneal Topography: Latest concepts and technologies 2006:
Above: Theoretical effect of decentration on a single-SPL (red or blue) and dual-SPL (green) in a model eye. The average from two cameras reduces the deviation by a factor of ten.

10. Materials and Methods N=21 Normal Subjects; Single Trained Operator
Two Devices
Single-Scheimpflug (Pentacam, Oculus)
Dual-Scheimpflug (Galilei, Ziemer Group)
Two Conditions: Aligned and Calibrated 1-mm horizontal decentratered (1mm to the operator’s right for both eyes)
Note: The Single SPL camera rotates position between right and left eyes. Therefore in this study, the relative position of the camera to the direction of decentration is opposite between right and left eyes for the Single SPL.
3 Repeated Measurements; first aligned, then decentered
Randomized Order of Devices and Right and Left Eye
Multiple analysis of variance (MANOVA) statistical analysis (SAS, p<0.05)
Compared the centered-condition mean thinnest pachymetry values between devices, differences in mean thinnest pachymetry between centered and decentered by device and by right and left eye, and location of the thinnest pachymetry.

11. Results Centered thinnest pachymetry are different (p<0.0001);
Dual SPL: ± 23.7µm OD, ± 26.1µm OS
Single SPL: ± 24.6µm OD, ± 27.5µm OS
Both devices’ thinnest pachymetry values are repeatable (p>0.05) and change when decentered (p<0.05)
The difference in thinnest pachymetry when decentered:
Dual SPL: ± 2.0µm OD, ± 1.8µm OS
Single SPL: ± 4.7µm OD, ± 5.1µm OS

12. Conclusions
The single Scheimpflug mean intra-subject difference is larger and changes sign, depending on the direction of misalignment relative to the camera.
The dual Scheimpflug is relatively insensitive to misalignment, with clinically insignificant differences between the aligned and misaligned conditions.
Both devices have statistically different mean aligned thinnest pachymetry values.
The Single SPL decentered thinnest pachymetry differences increase in magnitude and change sign, depending on the direction of decentration relative to the camera as theorized.
The Dual SPL is less sensitive to decentration by an order of 1/10th compared to the Single SPL.