J. Einighammer

Customized aspheric intraocular lenses calculated with real ray tracing.

PURPOSE: To calculate the exact geometry of custom intraocular lenses (IOLs) for pseudophakic eyes and theoretically predict the residual wavefront error by real ray tracing based on Snells law. SETTING: Centre for Ophthalmology, University Hospital, Tübingen, Germany. METHODS: Individual computer models were constructed based on measurements, including corneal topography and axial length. The geometry of custom spherical, aspheric, toric, and toric aspheric IOLs was calculated in an optimization process with real ray tracing to provide the minimum root mean square wavefront error. The geometric optical properties in terms of residual wavefront error was simulated and approximated by Zernike polynomials. RESULTS: Data from 45 pseudophakic eyes were used to construct the models. Defocus was almost completely corrected by the spherical IOL and astigmatism, by the toric IOL. The aspheric IOL strongly reduced spherical aberration but only slightly reduced total higher‐order aberrations (HOAs); both theoretical predictions corresponded to clinical investigations of wavefront measurements in pseudophakic eyes with a spherical or aspheric IOL. CONCLUSIONS: Real ray tracing calculated the exact geometry of custom IOLs to provide the minimum wavefront error, going beyond simple diopter information. Results show spherical aberration can be significantly reduced with aspheric IOLs. However, the limited possible reduction of total HOAs, even perfectly positioned custom aspheric IOLs, may be a reason for the unclear results in studies assessing the potential benefit to visual performance of currently used aspheric IOLs.

Calculating intraocular lens geometry by real ray tracing.

PURPOSE An implementation of real ray tracing based on Snells law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs). METHODS The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae. RESULTS Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11 +/- 0.43 diopters (D), cylinder -0.18 +/- 0.52 D, and axis 5.13 degrees +/- 30.19 degrees. Pearsons correlation coefficient was sphere r = 0.92, P < .01; cylinder r = 0.79, P < .01; and axis r = 0.91, P < .01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae. CONCLUSIONS It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history.

Biometric measurements inside the model eye using a two wavelengths Fourier domain low coherence interferometer

Abstract We present a setup to measure biometric data of the eye using Fourier domain interferometry. The measuring depth of a Fourier domain system is basically limited owing to the spectral resolution. Combining two spectral domain interferometers with different wavelength ranges creates two measurement sections and allows for a simultaneous biometric measurement in terms of corneal thickness, anterior chamber depth, and axial length. The necessary offset between both sections in the combined setup was calibrated with a known reference object. The setup was tested by measuring a self-constructed model eye. All biometric data of the model eye can be detected simultaneously. This system has a precision of 13 μm (standard deviation) and a trueness of 46 μm. The signal-to-noise ratio was 98 dB for the anterior part and 76 dB for the posterior part. In contrast to time domain interferometry, this setup does not need any mechanically moving parts. Owing to the short time frame of the biometric measurement, potential eye movements should have no influence on the result. In addition to the fast measurement, this setup provides the possibility to adjust the laser power of both sections independently. This could help in the case of dense cataract.