Evaluation of free-form IOL topographies by clinically available topographers

Abstract

Purpose: 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.

Methods: 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.

Results: 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.

Conclusion: 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.