16 Intraocular Lens Tilt
Intraocular lens (IOL) position in the postoperative period is commonly seen by slit-lamp examination. Ultrasound biomicroscopy (UBM) can be used to quantify the IOL position changes in the late postoperative period. 1 Clinical Purkinje image assessment has long been the method used in evaluating IOL position. 2 More recently, optical coherence tomography (OCT) has been used for this purpose. 3 OCT has its own advantages of being noncontact and less time consuming. High-speed anterior-segment (AS) OCT (Visante, Carl Zeiss Meditec, Dublin, CA) of 1310-nm wavelength has been used for IOL imaging.
16.1 Lens Position Module in OCT
Anterior-segment OCT (Visante, Carl Zeiss Meditec) has the option of including the type of scans that we require for evaluation in the specific mode as one module. As seen in Fig. 16.1, the lens position module includes the AS single mode raw mode and high-resolution raw mode, and normal AS scan mode can also be used. The limitation of normal AS scan mode is that whenever the vertex is not centered, the image capture does not happen. Hence, raw image mode is used.
16.2 Image Acquisition
The pupil should be dilated to at least 6 mm before examination. The headrest and the chin rest of the OCT are adjusted to guarantee a perpendicular position of the patient’s head for each examination. Cross-sectional imaging of the IOL is carried out using the Visante AS OCT (Carl Zeiss Meditec). The eye is focused on the target in the viewing eyepiece of the patient. AS single-scan mode and high-resolution scans are taken in all four axes (0–180 degrees, 45–225 degrees, 315–135 degrees, and 270–90 degrees). In AS single scan, the examiner should capture the image when the reflection of the IOL is visible (Fig. 16.1) and ensure that the corneal vertex is centered and the limbus is seen clearly. In high-resolution cornea mode, the reflection of the IOL with respect to the iris is seen. Iris pupillary margins are seen as high reflective edges, and the IOL convex surfaces are seen as a curved line of reflection (Fig. 16.2).
16.3 Method to Calculate Intraocular Lens Tilt
The images then are analyzed using the caliper tools in the software of the AS OCT for iris vault (distance in millimeters between the iris margin and the anterior surface of the IOL at the pupillary plane [D1, D2]) (Fig. 16.3). Using MatLab software version 7.1 (Mathworks, Natick, MA), AS single-scan images are analyzed. A straight line (L) passing through the limbus on either side of the image is marked as the reference line. A second line (1) passing through the horizontal axis of the IOL (Fig. 16.4) is marked. The horizontal axis of the IOL is determined by the following method. The image from OCT is converted to binary for subsequent extraction of edge coordinates. The selected points on the anterior and posterior arc edges of IOL are obtained. The mathematical representation to fit the anterior and posterior arc of IOL is derived from the equation of the circles passing through the given points (Fig. 16.4). The intersection points of the two circles are joined to form the horizontal axis of the IOL. This is executed in all four quadrants (180–0 degrees, 225–45 degrees, 315–135 degrees, and 270–90 degrees). The slopes are calculated for both the straight lines (L,1). When the reference line along the limbus and the IOL optic are parallel, the optic is not considered to be tilted. The angle (θ) in degrees between the two lines (L and 1) is determined. The slope ratio is calculated by dividing the slope of IOL by the slope of the limbus.
16.3.1 Tilt in Normal Intraocular Lens
We reported the tilt of a normal IOL in the capsular bag to be 1.52 ± 0.9 degrees. 3 However, we noted that the tilt was not significant enough to affect the vision of the patients. The mean ocular residual astigmatism was 0.24 ± 0.47 diopters. No significant correlation was found between ocular residual astigmatism and postoperative best-corrected visual acuity. Of the four axes, the 180- to 0-degree axis showed the IOL contour completely in all 100% of the examined eyes (Fig. 16.5). The mean distance D1 was 0.80 ± 0.6 mm, and the mean distance D2 was 0.83 ± 0.57 mm. On comparison with different types of IOLs, no significant difference in position was noted. There were three groups of IOLs: a single-piece acrylic IOL (Appasamy Associates, Chennai, India); a single-piece, foldable acrylic IOL (Akreos; Bausch & Lomb, Rochester, NY); and a single-piece PMMA (polymethylmethacrylate) IOL (Appasamy Associates). No significant difference was found among the IOL groups with respect to the slope ratio (P = 0.431, Kruskal-Wallis test), postoperative vision (P = 0 .935), and angle (θ; P = 0.333).