G.L. van der Heijde

The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox.

Scheimpflug slit images of the crystalline lens are distorted due to the refracting properties of the cornea and because they are obliquely viewed. We measured the aspheric curvature of the lens of 102 subjects ranging in age between 16 and 65 years and applied correction for these distortions. The procedure was validated by measuring an artificial eye and pseudophakic patients with intraocular lenses of known dimensions. Compared to previous studies using Scheimpflug photography, the decrease of the radius of the anterior lens surface with age was smaller, and the absolute value for the radius of the anterior and posterior lens surface was significantly smaller. A slight decrease of the posterior lens radius with age could be demonstrated. Generally, front and back surfaces were hyperbolic. Axial length was measured of 42 subjects enabling calculation of the equivalent refractive index of the lens, which showed a small, but highly significant decrease with age. These new findings explain the lens paradox and may serve as a basis for modelling the refractive properties of the lens.

Estimating the external force acting on the human eye lens during accommodation by finite element modelling.

Using new geometric information on the shape of the lens that has recently become available, a finite element model has been developed in order to estimate the forces that act on the lens during accommodation for a typical 29-year-old human eye. To investigate the influence of the anterior, posterior and central zonular fibres insertion regions, three models with different configurations were built. All three configurations appeared to be capable of inducing the required accommodative changes in the lens. Based on material properties from the literature, the estimated summed net force for each of the three models was approximately 0.08 N.

A new PRBA-based instrument to measure the shape of the cornea

We have developed a prototype-instrument providing a new way to measure the shape of the cornea. Our approach exploits properties of pseudo random binary arrays (PRBAs). Encoded in a coloured pattern (stimulus) that is mirrored to the eye PRBAs allow for unique characterization of positions both in the stimulus and the reflected image. This is used to come to an integral reconstruction of the cornea. It is demonstrated that the new technique contributes to a very robust measurement method.

Histopathologic ocular examination after lensectomy in a Macacus rhesus monkey, an unexpected event of secondary glaucoma.

In a Macacus rhesus monkey an unilateral lensectomy was performed on the 15th day of life to study the emmetropization proces. During a period of 20.5 months refractional state, axial length, corneal dioptric power and intraocular pressure were measured in the aphakic and fellow eye. Results showed that axial elongation of the aphakic eye was larger in comparison to the phakic fellow eye. Histopathologic examination of the aphakic eye revealed glaucomatous changes due to obstruction in the anterior chamber angle.

Physiological suppression demonstrated with the Aulhorn PDH

Physiological suppression was studied in 25 subjects using the Aulhorn phase-difference haploscope (PDH).To control fixation the eye movements were registered by means of an infra-red reflection technique.