Clinical validation of point-source corneal topography in keratoplasty.

Purpose: To validate the clinical performance of point-source corneal topography (PCT) in postpenetrating keratoplasty (PKP) eyes and to compare it with conventional Placido-based topography.

Methods: Corneal elevation maps of the anterior corneal surface were obtained from 20 post-PKP corneas using PCT (VU topographer, prototype; VU University Medical Center, Amsterdam, The Netherlands) and Placido-based topography (Keratron, Optikon 2000, Rome, Italy). Corneal surface parameters are calculated in terms of radius and asphericity.

Forward ray tracing for image projection prediction and surface reconstruction in the evaluation of corneal topography systems.

A forward ray tracing (FRT) model is presented to determine the exact image projection in a general corneal topography system. Consequently, the skew ray error in Placido-based topography is demonstrated. A quantitative analysis comparing FRT-based algorithms and Placido-based algorithms in reconstructing the front surface of the cornea shows that arc step algorithms are more sensitive to noise (imprecise).

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.

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.

Internal deformation of the human crystalline lens during accommodation.

Purpose: To describe the internal deformations in the crystalline lens that occur during accommodation.

Methods: A computer-based mechanical model of accommodation was created using the finite element method. The lens geometry of the model was based on in vivo measurements of human lenses in the accommodated state.

Constant volume of the human lens and decrease in surface area of the capsular bag during accommodation: an MRI and Scheimpflug study.

Purpose: A change in surface area of the capsular bag and a change in volume of the lens can indicate whether a change in the shape of the lens during accommodation is due to the compressibility or the elasticity of the lens material.

Methods: 3D magnetic resonance imaging (MRI) was used to image the complete shape of the lens in a group of five healthy subjects between 18 and 35 years of age. A parametric representation of the cross-sectional shape was fitted to the edges of the lens, which were determined with a Canny edge filter.

Precision and reliability of retinal thickness measurements in foveal and extrafoveal areas of healthy and diabetic eyes.

Purpose: To determine the precision and reliability of retinal thickness measurements with an optical coherence tomograph (Stratus OCT 3; Carl Zeiss Meditec, Dublin, CA) and a retinal thickness analyzer (RTA; Talia Technology Ltd., Neve-Ilan, Israel) in foveal, parafoveal, and perifoveal areas.

Methods: Three measurements of all areas were performed within 1 hour on the same day with each instrument in the eyes of healthy volunteers and diabetic patients.

Pseudo forward ray-tracing: a new method for surface validation in cornea topography.

Purpose: A pseudo forward ray-tracing (PFRT) algorithm is developed to evaluate surface reconstruction in corneal topography. The method can be applied to topographers where one-to-one correspondence between mire and image points can be established.

Methods: The PFRT algorithm was applied on a corneal topographer designed and constructed at the VU University Medical Center, Amsterdam, The Netherlands.

On the relationship between lens stiffness and accommodative amplitude.

The purpose of this study was to investigate the relationship between the stiffness of the material comprising the lens and the loss of accommodative amplitude with age. We used a validated mechanical model to determine the changes in the shape of the lens during accommodation and disaccommodation. The relative contribution of lens stiffness to loss of accommodative amplitude with age was determined by varying lens stiffness in the model.

The contribution of the posterior surface to the coma aberration of the human cornea.

Scheimpflug imaging was used to measure in six meridians the shape of the anterior and posterior cornea of the right eye of 114 subjects, ranging in age from 18 to 65 years. Subsequently, a three-dimensional model of the shape of the whole cornea was reconstructed, from which the coma aberration of the anterior and whole cornea could be calculated. This made it possible to investigate the compensatory role of the posterior surface to the coma aberration of the anterior corneal surface with age.

Stiffness gradient in the crystalline lens.

Background: While the overall stiffness of the lens has been measured in a number of studies, the knowledge about the stiffness distribution within the lens is still limited. The purpose of this study was to determine the stiffness gradient in the human crystalline lens. A secondary purpose was to determine whether the stiffness gradient depends on age.

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.

Spherical aberration of the anterior and posterior surfaces of the human cornea.

A ray-tracing procedure was applied to corrected Scheimpflug photography measurements to determine the spherical aberration of the anterior and posterior surfaces of the cornea. It was found that the total spherical aberration of the cornea increases slightly with age. The spherical aberration of the posterior corneal surface is negative at a young age and becomes positive at an older age.

Comment on "Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study".

In a recent paper, Koretz et al. [J. Opt.

Dynamic mechanical properties of human lenses.

The purpose of this study was to determine the shear compliance of human crystalline lenses as a function of age and frequency. Dynamic mechanical analysis was performed on 39 human lenses, ranging in age from 18 to 90 years, within the frequency range of 0.001-30 Hz.

Corneal surface reconstruction algorithm that uses Zernike polynomial representation.

We developed an algorithm that directly determines Zernike coefficients for the corneal anterior surface derived from the reflection image of a stimulus with pseudorandom encoding. This algorithm does not need to include calculation of corneal height maps. The numerical performance of the algorithm is good.

Radius and asphericity of the posterior corneal surface determined by corrected Scheimpflug photography.

Purpose: To obtain the shape of the posterior corneal surface in a healthy population, using Scheimpflug photography corrected for distortion due to the geometry of the Scheimpflug imaging system and the refraction of the anterior corneal surface.

Methods: The posterior corneas of 83 subjects, ranging in age from 16 to 62 years, were measured in the vertical meridian using corrected Scheimpflug photography. The aspherical shape of the anterior Corneal surface was also determined in conjunction with the correction of Scheimpflug images.