K 635
Optimized calculation of intraocular lenses
H. Lahdo, J. Wahl, P.-R. Preußner
Background: Intraocular lenses are mostly estimated by approximation formulae which are influenced by measurements, estimations, manufacturers specifications and more or less accurate calculation algorithms without showing their contribution to the result. Efforts to find the best mean for a patient collective dont necessarily lead to the best lens also for highly myopic or hyperopic eyes. Our aim is to get a measure of reliability also for these eyes. Therefore, we have to clearly separate: 1.) measurements including error estimates, 2.) accurate optic calculation, 3.) prognosis of the most probable post-operative anterior chamber depth.
Method: We use a ray-tracing program for which the error in the result is always below 0.01dpt, whatever the size of the eye is. The following manufacturers specifications are employed for the lens: anterior and posterior curvature radius, central thickness and refractive index. So far 25 lens types (approximately 1000 lenses) from the companies Acri.Tec, Allergan, Alcon, Morcher and Pharmacia-Upjohn are included. The measurement error of the final results is calculated by an error propagation algorithm based on the individual measurement errors (laser interferometry, ultra-sound, corneal topography). A least-square-fit is used to predict the most probable post-operative anterior chamber depth which is proposed to the physician.
Results: After measuring pre- and post-operative values for various lenses we have determined fit-parameters for the most probable post-operative anterior chamber depth. These data are permanently updated in our department. Several errors in the manufacturers specifications have been identified by internal consistence checks and subsequently corrected together with the manufacturers.
Conclusion: Indeed the described proceedings dont improve the accuracy of intraocular lens calculation for the average eye, but the more it does for eyes in the marginal range of the distribution.
Universitäts-Augenklinik, Langenbeckstr. 1, D-55101 Mainz