Purpose To study the theoretical optical performance of 3 intraocular lens (IOL) designs in the presence of IOL decentration. Setting Optics Center, Bausch & Lomb, Rochester, New York, USA. Methods A ray-tracing program was used to evaluate the effect of IOL decentration on the optical performance of 3 silicone IOLs (LI61U, Bausch & Lomb; Tecnis Z9000, Advanced Medical Optics; and a new aberration-free IOL [SofPort AO, Bausch & Lomb]) in an experimental model eye. The study was done using pupil diameters of 3.0 mm, 4.0 mm, and 5.0 mm and IOL decentrations of 0 mm, 0.25 mm, 0.50 mm, 0.75 mm, and 1.00 mm. The modulation transfer functions were computed and plotted. A Monte Carlo simulation analysis with 1000 trials with IOL decentration randomly varying for each pupil size was performed. Results Decentration of LI61U and Tecnis Z9000 IOLs led to asymmetrical higher-order aberrations that adversely affected the optical performance of the model eye; performance was not affected with the aberration-free IOL because it lacks inherent spherical aberration. Optical performance with the aberration-free IOL was better than with the LI61U IOL as the former has less spherical aberration and did not introduce other aberrations when decentered. Performance with the aberration-free IOL was better than with the Tecnis Z9000 IOL for 3.0 mm, 4.0 mm, and 5.0 mm pupils when decentration exceeded 0.15 mm, 0.30 mm, and 0.38 mm, respectively. Performance with the LI61U IOL was better than with the Tecnis Z9000 IOL for 3.0 mm, 4.0 mm, and 5.0 mm pupils when decentration exceeded 0.3 mm, 0.5 mm, and 0.5 mm, respectively. Monte Carlo simulations showed the expected postoperative results of the LI61U IOL and aberration-free IOL would be repeatable and predictable, whereas the outcomes with the Tecnis Z9000 IOL would vary widely. Conclusions The optical performance of the model eye was not affected by decentration of an aspheric IOL designed to have no inherent spherical aberration. With decentration, the performance with the new IOL was better than with a conventional spherical IOL and an aspheric IOL designed to offset the spherical aberration of an average cornea.

Optical performance of 3 intraocular lens designs in the presence of decentration.

PURPOSE: To compare ocular and internal aberrations after femtosecond laser anterior capsulotomy and con- tinuous curvilinear capsulorrhexis in cataract surgery. METHODS: In this prospective study, anterior capsu- lotomy was performed during cataract surgery with an intraocular femtosecond (FS) laser (Alcon LenSx Inc) in 48 eyes. As a control group, continuous curvilinear cap- sulorrhexis (CCC) was performed in 51 eyes. Wavefront aberrometry, corneal topography, and objective visual quality were measured using the OPD-Scan (NIDEK Co Ltd). Vertical and horizontal tilt, coma, and visual quality metrics were evaluated separately to determine whether the source of aberrations was ocular or internal. Main outcome measures included postoperative residual re- fraction, uncorrected and corrected visual acuities, ocu- lar and internal aberrations, Strehl ratio, and modula- tion transfer function (MTF). RESULTS: No statistically signifi cant differences were noted between the FS and CCC groups, respectively, in postoperative sphere (!0.60"1.50 vs !0.50"1.40 diopters (D)), postoperative cylinder (1.30"1.01 vs 1.10"1.10 D), uncorrected distance visual acuity (0.86"0.15 vs 0.88"0.08), or corrected distance visual acuity (0.97"0.08 vs 0.97"0.06). The FS group had signifi cantly lower values of intraocular vertical tilt (!0.05"0.36 vs 0.27"0.57) and coma (!0.003"0.11 vs 0.1"0.15), and signifi cantly higher Strehl ratios (0.02"0.02 vs 0.01"0.01) and MTF values at all measured cycles per degree, compared to the CCC group. CONCLUSIONS: Capsulotomy performed with an in- traocular FS laser induced signifi cantly less internal aberrations measured by the NIDEK OPD-Scan aber- rometer compared to eyes that underwent CCC, which may result in better optical quality after the procedure. (J Refract Surg. 2011;27(10):711-716.) doi:10.3928/1081597X-20110913-01

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Internal aberrations and optical quality after femtosecond laser anterior capsulotomy in cataract surgery.

Efficacy and Safety of Femtosecond Laser-Assisted Cataract Surgery Compared with Manual Cataract Surgery: A Meta-Analysis of 14 567 Eyes.

Purpose To improve accuracy in intraocular lens (IOL) calculations and clarify the effect of various errors. Setting University eye hospitals, Mainz, Germany, and Vienna, Austria. Methods A numerical ray-tracing calculation has been developed for the pseudophakic eye. Individual rays are calculated and then undergo refractions on all surfaces of the IOL and cornea. The calculations do not use approximations; ie, the refractions are calculated exactly using Snell’s law. Rays can be calculated for any distance from the optical axis and for other parameter variations. The effects of aspheric surfaces can also be investigated. Instead of IOL powers, manufacturers’ IOL data (radii, refractive index, thickness) are used in the calculations for different IOL types. The resulting optical quality is visualized by using Landolt rings superimposed on the grid of retinal receptors. Results Intraocular lens design, corneal asphericity, and specific spherical aberration influence the visual quality of the pseudophakic eye significantly. The IOL refractive power is an ambiguous parameter that cannot characterize the visual outcome sufficiently accurately for an IOL implanted at a given position. The effects can be calculated only in numerical ray tracing, not in Gaussian optics. The accuracy of numerical ray tracing is independent of axial length. Therefore, very long or very short eyes gain the most from the higher accuracy of this approach. For average-size eyes, however, the results are the same as with SRK calculations. Conclusion Calculations in Gaussian optics should be replaced by state-of-the-art numerical methods, which can be run on any standard personal computer.

Ray tracing for intraocular lens calculation.

PURPOSE: To compare intraocular lens (IOL) power calculation and refractive outcome between patients who underwent laser refractive cataract surgery with a femtosecond laser and those with conventional cataract surgery. METHODS: In this prospective study, 77 eyes from 77 patients underwent laser refractive cataract surgery (laser group; Alcon LenSx femtosecond laser), and conventional cataract surgery with phacoemulsification was performed in 57 eyes from 57 patients (conventional group). Biometry was done with optical low coherence reflectometry (Lenstar LS900, Haag-Streit AG), and IOL calculation was performed with third-generation IOL formulas (SRK/T, Hoffer Q, and Holladay). The refractive outcome was analyzed using the mean absolute error (MAE; difference between predicted and achieved postoperative spherical equivalent refraction), and multivariable regression analysis was performed to compare the two groups. RESULTS: No significant differences were found between age, axial length, keratometry, and preoperative corrected visual acuity in the laser and conventional groups (P>.05; Mann-Whitney U test). At least 6 weeks after surgery, MAE was significantly lower in the laser group (0.38±0.28 diopters [D]) than in the conventional group (0.50±0.38 D) (P=.04). The difference was the greatest in short (axial length 26.0 mm, 0.33±0.24 vs 0.63±0.42) eyes. CONCLUSIONS: Laser refractive cataract surgery with a femtosecond laser resulted in a significantly better predictability of IOL power calculation than conventional phacoemulsification surgery. This difference is possibly due to a more precise capsulorrhexis, resulting in a more stable IOL position.

Comparison of IOL power calculation and refractive outcome after laser refractive cataract surgery with a femtosecond laser versus conventional phacoemulsification.

Purpose: To evaluate a computer program to modulate the visual impairment caused by intraocular lens (IOL) misalignment and visualize results obtained by numerical calculations. Setting: Department of Ophthalmology, Medical Faculty of Charles University, Prague, Czech Republic. Methods: The optic imagery of a Landolt circle was calculated using a ray-tracingcomputer program. Visual aberrations resulting from a decentered and/or tilted IOL were studied using this program and compared with theoretical calculations. Results: The IOL decentration and/or tilt shifted the postoperative refractive errorstoward myopia and astigmatism (oblique). The combination of IOL decentration and tilt produced a refractive error that depended on the relationship between the geometrical axes of the decentration and tilt. The refractive error can be enhanced or diminished depending on the relationship of these axes. Conclusions: These findings verify the results calculated by paraxial vergence equations. A ray-tracing program simulated the optic imagery for various kinds of IOL misalignment and IOL optic properties.

Computer modeling of visual impairment caused by intraocular lens misalignment

Background Intraocular lens (IOL) decentration and tilt may affect postoperative refractive errors through spherical aberration of the IOL. Methods Through a use of a ray-tracing program and by minimizing algorithm, we calculated theoretical refractive errors for various degrees of IOL decentration and tilt. We compared our results with those obtained by paraxial vergence calculations. Results IOL decentration and/or tilt shifted postoperative refractive errors toward myopia and astigmatism of oblique origin. For example, a 3-millimeter decentration of an IOL resulted in induction of approximately -2.00 diopters (D) sphere and +0.70 D cylinder. IOL tilt affected refractive errors to a lesser degree. The change in refractive error caused by a combination of IOL decentration and tilt depended on the relationship between the geometrical axes of decentration and tilt. In the case of the least favorable combination of 12 degrees of tilt and 3 mm of decentration, it can reach -7.00 D sphere and +4.00 D cylinder. Conclusions IOL decentration and/or tilt increase myopia and astigmatism. They are negligible for small decentrations, but could be sources of substantial postoperative refractive errors if the decentration or tile is large.

Changes in Refraction Induced by Change in Intraocular Lens Position

Objective: The use of specular microscopy to photograph in-vivo intraocular lens defects produced as a result of Nd-YAG laser posterior capsulotomy. Patients: Thirty randomly selected patients following Nd-YAG laser posterior capsulotomy. IOL: Lathe cut PMMA planoconvex. Results: IOL damage was found in 28 (93%) of IOLs. The most common damage was a crater with melted margins, craters with irregular margins and inclusions are also seen. Stellate defects in the intraocular lens were found in two cases. Conclusion: Specular microscopy performed during and after Nd-YAG laser capsulotomy demonstrates various kinds of intraocular lens defect.

Jiri Korynta

Papers

Computer modeling of visual impairment caused by intraocular lens misalignment

Changes in Refraction Induced by Change in Intraocular Lens Position

In-vivo Imaging of IOL Damage After Nd:YAG Laser Treatment