Comparison of Single-Scheimpflug and Dual-Scheimpflug Pachymetry Measurements and Effect of Decentration World Cornea Congress VI April 7-9, 2010 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.

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)

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

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.

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.

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 2006:177-182.

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 2006:177-182.

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.

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 2006:177-182. 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.

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.

Results Centered thinnest pachymetry are different (p<0.0001); Dual SPL: 554.3 ± 23.7µm OD, 555.4 ± 26.1µm OS Single SPL: 547.8 ± 24.6µm OD, 550.1 ± 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: -1.3 ± 2.0µm OD, -1.8 ± 1.8µm OS Single SPL: 7.1 ± 4.7µm OD, -13.1 ± 5.1µm OS

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.