Rob G.L. van der Heijde

Equivalent refractive index of the human lens upon accommodative response

Purpose. To experimentally verify the suggestion of Gullstrand (1909), i.e., that the equivalent refractive index of the human lens increases with accommodation. Methods. The left eye of five subjects was focused on different accommodation stimuli, while the right eye was imaged with Scheimpflug photography in order to obtain the shape of the lens and cornea during accommodation. The procedure was then repeated, but instead of using the Scheimpflug camera, the accommodative response of the right eye was measured objectively with an aberrometer. The axial length was measured with a Zeiss IOL-master. Combining the results of these measurements made it possible to correct the digital Scheimpflug images for corneal and lenticular refraction, and to simultaneously calculate the equivalent refractive index of the lens for all different accommodative stimuli. Furthermore, a two-compartment model of the lens was constructed, with a nucleus and a cortex. Results. In all five subjects there was no significant change in the equivalent refractive index of the lens as a function of accommodation. The mean equivalent refractive index was 1.435 ± 0.008. Furthermore, the accommodative response appeared to be lower than the accommodative stimulus (i.e., accommodative lag). It appeared to be possible to model the optical power of the lens, based on the geometry of cortex and nucleus. Based on a refractive index of 1.406 for the nucleus, the mean refractive index of the cortex was 1.381. Conclusions. Gullstrand suggested that there would be an increase in the equivalent refractive index with accommodation; the intra-capsulary mechanism of accommodation. However, we found that the equivalent refractive index of the lens does not change with accommodation when the accommodative lag is taken into account. Furthermore, it appeared to be possible to simulate the accommodative process of a subject with a two-compartment model with constant refractive indices.

Topographer reconstruction of the nonrotation-symmetric anterior corneal surface features.

Purpose. The purpose of this study is to demonstrate the performance of a topographer (the VU topographer, prototype development, VU University Medical Center, Amsterdam, The Netherlands) that uses a color-coded stimulus pattern to reconstruct both the rotation–symmetric and nonrotation–symmetric shape features of the anterior corneal surface. Methods. Spherical surfaces, toric surfaces, the Rand surface (surface with peripheral corrugations), and sample eyes were measured. A ring topographer (Keratron, Optikon 2000, Rome, Italy) and the Haag-Streit ophthalmometer (Haag Streit, Bern, Switzerland) were used for comparison. Results. All three instruments produced similar values for the radii of curvature of spherical surfaces with a tolerance of 0.02 mm. The Keratron gave underestimated values for the astigmatic power of toric surfaces (>0.25 D for toric surfaces with astigmatism >9 D). Because it eliminates skew ray error, only the VU topographer was able to reconstruct the correct shape of the Rand surface in contrast with ring topographers. The effect of skew ray error was also observed in the surface reconstruction of a radially keratotomized (RK) eye. There was height difference of 2.75 ± 1.25 &mgr;m between the output of the VU topographer and the output of the Keratron. Conclusion. The VU topographer is just as accurate in reconstructing the rotation–symmetric features of the anterior corneal surface as the ring topographers but is superior in recovering the nonrotation–symmetric shape features.

Performance in specular reflection and slit-imaging corneal topography.

Purpose. Assessment of the relative performance in measuring corneal shape and corneal aberrations for two specular reflection topographers: Keratron Placido Ring Topographer, VU Topographer, and two slit-lamp imaging instruments: Orbscan II and Topcon SL-45 Scheimpflug. Methods. Corneal height maps of the anterior corneal surface were obtained from a group of 34 subjects with all four instruments; posterior corneal surface height maps were only obtained with the two slit-lamp imaging instruments. Corneal surface shapes are calculated in terms of radius of curvature and asphericity fitting an aspheric model. Wave aberrations for the anterior corneal surface and the total cornea are determined up to and including sixth order Zernike convention by means of ray tracing. Results. Clinical relevant differences were observed for radius of curvature of the anterior corneal surface, where the slit-imaging instruments measure higher values (mean difference = 0.05 mm, p < 0.05) and anterior corneal astigmatism for which the Orbscan II measures higher values than the VU Topographer [mean difference = 0.174 &mgr;m (0.134 Equivalent Diopters), p < 0.01]. Small significant differences were observed for asphericity and spherical aberration of the anterior corneal surface; however, these are not clinically relevant. Clinically relevant differences were also observed for posterior radius (difference = 0.135 mm p < 0.001), total corneal astigmatism (difference = 0.207 &mgr;m (0.159 Equivalent Diopters), p = 0.001), and central corneal thickness (CCT) (difference = −18.6 &mgr;m, p < 0.001). The differences found for total corneal coma and trefoil were not clinical relevant. Furthermore, the precision of the specular reflection topographers is superior to that of the slit-lamp instruments by at least a factor of two. Conclusions. For traditional spectacle and contact lens applications, the corneal topographers are interchangeable except for measuring anterior radius of curvature. However, for more modern techniques as customized corneal refractive surgery, the subtle differences (e.g., total corneal astigmatism and CCT) between the instruments are clinically relevant.

Development of a ciliary muscle-driven accommodating intraocular lens

PURPOSE: To develop a ciliary muscle–driven accommodating intraocular lens (IOL) that has a large and predictable range of variable power as a step toward spectacle independence. SETTING: Department of Physics and Medical Technology, VU University Medical Center, Amsterdam, The Netherlands. METHODS: A concept IOL that has a rotating focus mechanism and a mechanical frame that can operate within the range of ciliary muscle contraction of a typical 60‐year‐old human eye was designed. Prototypes were made to test the IOLs mechanical performance in an enucleated pigs eye using a laboratory lens‐stretching device that mimics the action of the human ciliary muscle. Changes in focal length during stretching were measured by laser‐based ray tracing and a videocamera system. To rotate the 2 lenses in the IOL with variable optical power, a frame that allows the displacement and force of the ciliary muscle to be transferred by the capsular bag was designed. RESULTS: Ray tracing showed that the modulation transfer function (MTF) of the IOL in different accommodative states did not deviate to a great extent from the MTF of a monofocal IOL. During stretching experiments, the prototype IOL achieved 8.0 diopters of accommodation. CONCLUSIONS: Evaluation of an accommodating IOL that meets the requirements for a spectacle‐independent solution to presbyopia showed that the mechanical and optical designs must be further optimized to improve optical quality and functionality.