Contribution of posterior corneal astigmatism to total corneal astigmatism.

PURPOSE: To determine the contribution of posterior corneal astigmatism to total corneal astigmatism and the error in estimating total corneal astigmatism from anterior corneal measurements only using a dual-Scheimpflug analyzer. SETTING: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. DESIGN: Case series. METHODS: Total corneal astigmatism was calculated using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism, and the changes with age were analyzed. Vector analysis was used to assess the error produced by estimating total corneal astigmatism from anterior corneal measurements only. RESULTS: The study analyzed 715 corneas of 435 consecutive patients. The mean magnitude of posterior corneal astigmatism was -0.30 diopter (D). The steep corneal meridian was aligned vertically (60 to 120 degrees) in 51.9% of eyes for the anterior surface and in 86.6% for the posterior surface. With increasing age, the steep anterior corneal meridian tended to change from vertical to horizontal, while the steep posterior corneal meridian did not change. The magnitudes of anterior and posterior corneal astigmatism were correlated when the steeper anterior meridian was aligned vertically but not when it was aligned horizontally. Anterior corneal measurements underestimated total corneal astigmatism by 0.22 @ 180 and exceeded 0.50 D in 5% of eyes. CONCLUSIONS: Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism. Selecting toric intraocular lenses based on anterior corneal measurements could lead to overcorrection in eyes that have with-the-rule astigmatism and undercorrection in eyes that have against-the-rule astigmatism. FINANCIAL DISCLOSURE: The authors received research support from Ziemer Group. In addition, Dr. Koch has a financial interest with Alcon Laboratories, Inc., Abbott Medical Optics, Inc., Calhoun Vision, Inc., NuLens, and Optimedica Corp.

Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm.

PURPOSE: To evaluate the accuracy of refractive prediction of 4 intraocular lens (IOL) power calculation formulas in eyes with axial length (AL) greater than 25.0 mm and to propose a method of optimizing AL to improve the accuracy. SETTING: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA, and Department of Ophthalmology, Goethe University, Frankfurt am Main, Germany. DESIGN: Case series. METHODS: Refractive prediction errors with the Holladay 1, Haigis, SRK/T, and Hoffer Q formulas were evaluated in consecutive cases. Eyes were randomized to a group used to develop the method of optimizing AL by back-calculation or a group used for validation. Further validation was performed in 2 additional data sets. RESULTS: The optimized AL values were highly correlated with the IOLMaster AL (R(2) from 0.960 to 0.976). In the validating group, the method of optimizing AL significantly reduced the mean numerical errors for IOLs greater than 5.00 diopters (D) from +0.27 to +0.68 D to -0.10 to -0.02 D and for IOLs of 5.00 D or less from +1.13 to +1.87 D to -0.21 to +0.01 D, respectively (all P<.05). In 2 additional validation data sets, this method significantly reduced the percentage of eyes that would be left hyperopic. CONCLUSIONS: The proposed method of optimizing AL significantly reduced the percentage of long eyes with a hyperopic outcome. Updated optimizing AL formulas by combining all eyes from the 2 study centers are proposed. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Noninvasive assessment of tear stability with the tear stability analysis system in tear dysfunction patients.

PURPOSE: To evaluate tear film stability in patients with tear dysfunction and an asymptomatic control group by using the novel, noninvasive Tear Stability Analysis System (TSAS). METHODS: In this prospective case-control study, 45 patients with dysfunctional tear syndrome (DTS) were stratified into three groups (1, 2, and 3/4) based on clinical severity, with higher scores indicating more severe symptoms; 25 asymptomatic control subjects were evaluated. TSAS measurements were performed with the RT-7000 Auto Refractor-Keratometer (Tomey Corporation, Nagoya, Japan). Images of ring mires projected onto the cornea every second for 6 seconds were captured and analyzed. Focal changes in brightness were calculated as numerical ring breakup (RBU) values, and the elapsed time when the cumulative values (RBU sum) exceeded a threshold was defined as the ring breakup time (RBUT). RESULTS: RBUTs in the DTS groups were all significantly lower than those in the control subjects, with the lowest values found in DTS 3/4. RBUT was significantly shorter in DTS 3/4 than in DTS 1 (P<0.001). The change in RBU sum over a 6-second period in the DTS groups combined or between the individual groups was statistically significant (P<0.001), as was the difference between the 1- and 6-second values. For distinguishing between asymptomatic controls and DTS, the sensitivity and specificity of a 5.0-second RBUT cutoff were 82.0% and 60.0%, respectively. CONCLUSIONS: The TSAS may be a useful, noninvasive instrument for evaluating tear stability and for classifying DTS severity.

Comparison of accuracy of intraocular lens calculations using automated keratometry, a Placido-based corneal topographer, and a combined Placido-based and dual Scheimpflug corneal topographer.

PURPOSE: To compare the accuracy of intraocular lens (IOL) power calculations with automated keratometry, a Placido-based corneal topographer, and a combined Placido-based and dual Scheimpflug corneal topographer. METHODS: Retrospectively, 75 eyes of 62 patients who had phacoemulsification with implantation of the SN60WF IOL were analyzed. Corneal powers were measured using 5 techniques: (1) automated keratometry (AutoK, IOL Master), (2) simulated keratometry from the Placido-based corneal topographer (SimKP, Atlas), (3) simulated keratometry from the combined Placido-based and dual Scheimpflug corneal topographer (SimKP+DS, Galilei), (4) total corneal power of the steep and flat meridians over a central 1.0-4.0 mm zone using ray tracing (TCPMeridian, Galilei), and (5) TCP over the central 4.0-mm zone by ray tracing (TCPCentral, Galilei). For each IOL implanted, prediction errors were determined by comparing the predicted refractions calculated with the Holladay 1 formula to the actual refraction that was obtained 3-4 weeks postoperatively. The surgeon factor was optimized for each method of corneal power measurement. The accuracy of IOL power calculation using these corneal powers was determined by calculating the mean absolute prediction error (MAE) and the percentage of eyes with prediction errors within 0.5, 1.0, and 1.5 diopters (D). RESULT: The MAEs were 0.37, 0.39, 0.39, 0.41, and 0.42 D for the AutoK, SimKP, SimKP+DS, TCPMeridian, and TCPCentral methods, respectively. There were no significant differences among groups. Over 93% of eyes had MAEs within 1.0 D for all methods of corneal power measurement. CONCLUSIONS: The accuracy of IOL power calculation was comparable with AutoK, the Placido-based corneal topographer, and the combined Placido-based and dual Scheimpflug corneal topographer.

Repeatability of corneal power and wavefront aberration measurements with a dual-Scheimpflug Placido corneal topographer.

PURPOSE: To evaluate the repeatability of the Galilei dual-Scheimpflug analyzer in measuring corneal curvature, wavefront aberrations, pachymetry, and anterior chamber depth (ACD). SETTING: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. METHODS: Three consecutive measurements were performed in 1 eye of each subject. The following were evaluated: (1) mean total corneal power at the central, paracentral, and peripheral zones (0.0 to 4.0 mm, 4.0 to 7.0 mm, and 7.0 to 8.0 mm, respectively) and posterior corneal power (K(avg)); (2) corneal higher-order wavefront aberrations (6.0 mm pupil); (3) mean pachymetry at the central, paracentral, and peripheral zones; and (4) ACD. Repeatability was assessed by calculating the within-subject standard deviation (SD), coefficient of variation (COV), and intraclass correlation coefficient (ICC). RESULTS: The study enrolled 20 subjects. The SD was 0.09 diopter (D), 0.05 D, and 0.19 D for central, paracentral, and peripheral total corneal power, respectively, and 0.03 D for posterior K(avg). The COV ranged from 0.10% to 0.35%, and the ICC was 0.996 or more (P<.001). For 3rd-order coma and trefoil, the SD was 0.08 mum and 0.09 mum, respectively. For 4th-order spherical aberration, astigmatism, and tetrafoil, the SDs were lower (0.02 mum, 0.04 mum, and 0.09 mum, respectively). The SD was 1.68 mum, 1.98 mum, and 2.82 mum for central, paracentral, and peripheral pachymetry, respectively, and 0.04 mm for ACD. CONCLUSION: Dual-Scheimpflug measurements of corneal power, pachymetry, ACD, and corneal aberrations for Zernike terms in the middle of the Zernike tree showed excellent repeatability.

Comparison of corneal powers obtained from 4 different devices.

PURPOSE: To assess the repeatability and comparability of anterior corneal power values obtained from the Galilei Dual Scheimpflug Analyzer (Ziemer, Port, Switzerland), Humphrey Atlas corneal topographer (Carl Zeiss, Jena, Germany), IOLMaster (Carl Zeiss), and a manual keratometer (Bausch & Lomb Inc, Rochester, New York, USA). DESIGN: Prospective, comparative study. METHODS: Prospectively, 20 subjects were enrolled. Three sets of corneal power measurements were obtained by a single observer using the Galilei, Atlas topographer, IOLMaster, and manual keratometer. Repeatability of the 3 measurements from each device was evaluated by means of coefficient of variation, standard deviation (SD), and intraclass correlation coefficient. An analysis of variance was used to compare the differences in corneal powers among devices. The Bland and Altman method also was performed to assess agreement in measurements between devices. Vector analysis was used to compare the astigmatism values obtained from different devices. RESULTS: For each device, the coefficient of variation of repeated measurements was lower than 0.22%. The SD of 3 repeated measurements ranged from 0.042 to 0.096 diopters (D). The intraclass correlation coefficients were higher than 0.99 in all devices. Mean central corneal powers were 43.80 D, 43.88 D, 43.92 D, and 43.76 D for the Galilei, Atlas, IOLMaster, and manual keratometer, respectively. SDs of the differences between devices ranged from 0.07 D for Galilei and IOLMaster to 0.14 D for Galilei and Atlas. For astigmatism, the mean astigmatism values for the Galilei, Atlas, IOLMaster, and manual keratometer were 0.54 D at 84 degrees, 0.51 D at 88 degrees, 0.62 D at 88 degrees, and 0.52 D at 87 degrees, respectively. CONCLUSIONS: The corneal power measurements from these 4 devices were highly reproducible, comparable, and correlated.