Zhongxia Zhu

Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen, Bavaria, Germany

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Publications (3)4.13 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: No commercially available device for measuring individual IOL surface topographies exists on the market. The purpose of this paper is to show the applicability of clinically available corneal topographers for measuring individual IOLs consisting of a spherical surface on one side and a freeform surface on the other side. Three measurement principles (Placido rings: Tomey TMS-2N, Scheimpflug: Oculus Pentacam, optical coherence tomography: Tomey CASIA) are applied in determining the IOLs' surface and compared against the design data used for producing the surfaces. Spherical and freeform IOLs are measured and analysed in both radius of curvature (ROC) and higher-order residual parameters by analysing the residuals. Repeatability and reproducibility measurements show a sub-μm precision for the TMS-2N system, while the Pentacam's values are located around 10μm and the CASIA system's values gather around 20μm. The TMS-2N system works best at detecting a sample's ROC and residual properties within the range of 8mm to 13.5mm mean ROC. In this range, the deviations from the theoretical ROC are about 45μm. The Pentacam doesn't have this limitation, but faces problems with exporting measurements of freeform surfaces. In some circumstances the program crashes and prevents the export. If being able to export the Pentacam measurements show an average deviation of 100μm from the theoretical ROC value. The CASIA system shows high amounts of noise which makes it not applicable in this field, having deviations of several 100μm from the theoretical ROC value. Residual comparison for the higher-order samples shows sub-μm precision for the TMS-2N, about 1μm precision for the Pentacam and several μm for the CASIA system. The values for the customized samples are slightly increased, several μm for the TMS-2N, up to 30μm for the Pentacam system and around 75μm for the CASIA system. The TMS-2N system is an appropriate device for measuring individual IOL surface topographies for ROCs between 8mm to 13.5mm. The Pentacam and CASIA induced relatively high level of variation and noise. Future application of the TMS-2N in this field will reveal its long-term statistics.
    Zeitschrift für Medizinische Physik 06/2012; 22(3):215-23. · 1.21 Impact Factor
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    ABSTRACT: To provide a mathematical calculation scheme for customized intraocular lens (IOL) design based on high resolution anterior segment optical coherence tomography (AS-OCT) of anterior eye segment and axial length data. We use the corneal and anterior segment data from the high resolution AS-OCT and the axial length data from the IOLMaster to create a pseudophakic eye model. An inverse calculation algorithm for the IOL back surface optimization is introduced. We employ free form surface representation (bi-cubic spline) for the corneal and IOL surface. The merit of this strategy is demonstrated by comparing with a standard spherical model and quadratic function. Four models are calculated: (1) quadratic cornea + quadratic IOL; (2) spline cornea + quadratic IOL; (3) spline cornea + spline IOL; and (4) spherical cornea + spherical IOL. The IOL optimization process for the pseudophakic eye is performed by numerical ray-tracing method within a 6-mm zone. The spot diagram on the fovea (forward ray-tracing) and wavefront at the spectacle plane (backward ray-tracing) are compared for different models respectively. The models with quadratic (1) or spline (3) surface representation showed superior image performance than the spherical model 4. The residual wavefront errors (peak to valley) of models 1, 2, and 3 are below one micron scale. Model 4 showed max wavefront error of about 15 µm peak to valley. However, the combination of quadratic best fit IOL with the free form cornea (model 2) showed one magnitude smaller wavefront error than the spherical representation of both surfaces (model 3). This results from higher order terms in cornea height profile. A four-surface eye model using a numerical ray-tracing method is proposed for customized IOL calculation. High resolution OCT data can be used as a sufficient base for a customized IOL characterization.
    Current eye research 06/2011; 36(6):579-89. · 1.51 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: The aim of this study is to represent the corneal anterior surface by utilizing radius and height data extracted from a TMS-2N topographic system with three different mathematical approaches and to simulate the visual performance. An iteratively re-weighted bi-cubic spline method is introduced for the local representation of the corneal surface. For comparison, two standard mathematical global representation approaches are used: the general quadratic function and the higher order Taylor polynomial approach. First, these methods were applied in simulations using three corneal models. Then, two real eye examples were investigated: one eye with regular astigmatism, and one eye which had undergone refractive surgery. A ray-tracing program was developed to evaluate the imaging performance of these examples with each surface representation strategy at the best focus plane. A 6 mm pupil size was chosen for the simulation. The fitting error (deviation) of the presented methods was compared. It was found that the accuracy of the topography representation was worst using the quadratic function and best with bicubic spline. The quadratic function cannot precisely describe the irregular corneal shape. In order to achieve a sub-micron fitting precision, the Taylor polynomial's order selection behaves adaptive to the corneal shape. The bi-cubic spline shows more stable performance. Considering the visual performance, the more precise the cornea representation is, the worse the visual performance is. The re-weighted bi-cubic spline method is a reasonable and stable method for representing the anterior corneal surface in measurements using a Placido-ring-pattern-based corneal topographer.
    Zeitschrift für Medizinische Physik 01/2010; 20(4):287-98. · 1.41 Impact Factor