Assessing intraocular lens accommodation

Ben-nun and Alió¹ present a new concept for a potentially accommodating intraocular lens (IOL) that consists of a deformable silicone gel placed between 2 plates. The anterior plate has a central hole. When the posterior plate moves forward, the distance between the 2 plates is reduced, forcing the silicone through the central hole in the anterior plate to form a convex lens.

To evaluate whether the posterior plate moves in vivo, Ben-nun and Alió implanted a measuring gauge into monkey eyes. The gauge consisted of 2 plates separated by a spring with a circumferentially marked rod protruding from the posterior plate through the central opening in the anterior plate. Movement of the measuring rod in response to pilocarpine was determined by photographically recording the position of the marks on the rod in relation to the front surface of the anterior plate. The authors did not control for movement of the eye relative to the camera. Figure 1 shows the presence of angular rotation in the images of the measuring gauge, indicating lack of alignment of these serial images.

Figure 1: A reproduction of Figure 7 from the paper by Ben-nun and Alió before (A) and after (B) the administration of pilocarpine. Note that the entire measuring device in A is tilted down with respect to B. As a consequence, there is perspective distortion. More of the measuring rod is visible in A than in B. The measuring rod in A appears thinner than the measuring rod in B, and the spacing of the markings on the measuring rod in A are not equal to those in B. True movement of the measuring rod cannot be sorted from these artifactitious changes.

Ben-nun and Alió then implanted their accommodating IOL in monkey eyes. Ultrasound biomicroscopy (UBM) was used to measure the radius of curvature of the convexity of the silicone that extended through the central opening of the anterior plate under conditions of cycloplegia and cyclospasm. These UBM images show that in addition to the apparent change in radius of curvature of the convexity of the silicone, the cornea changed curvature, thickness, and relative location between images (Figure 2). Because it has been established that the cornea does not change shape or thickness during accommodation,^2–5 the imaging plane of the UBM relative to the eye changed between these images. Misalignment of the UBM probe of as little as 3 pixels between compared images of the eye can be responsible for significant artifactitious changes owing to perspective distortion.^6,7

Figure 2: A reproduction of Figure 8 from the paper by Ben-nun and Alió of UBM images of a primate eye after cycloplegia (left), baseline (middle), and cyclospasm (right). Note that as a result of perspective distortion, the cornea appears in different locations on each image and appears to have changed curvature and thickness. Because of these perspective distortions, true changes in the radius of curvature of the silicone convexity cannot be sorted from artifact.

Ben-nun and Alió did not control for spurious eye movements relative to their imaging devices. The resultant induced distortions in perspective confound their attempts to assess the presence of IOL accommodation. Without such controls, it is premature for the authors to conclude that their IOL is capable of in vivo accommodation.