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Li Wang - One of the best experts on this subject based on the ideXlab platform.
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astigmatism induced by intraocular lens tilt evaluated via ray tracing
Journal of Cataract and Refractive Surgery, 2018Co-Authors: Mitchell P Weikert, Abhinav Golla, Li WangAbstract:Purpose To evaluate astigmatism induced by aspheric and toric intraocular lens (IOL) tilt using a ray-tracing model. Setting Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. Design Experimental study. Methods Ray-tracing eye models with aspheric IOLs (16.0 diopters [D], 22.0 D, and 28.0 D) and toric IOLs (16.0 D, 22.0 D, and 28.0 D each with toricities of 1.50 D, 3.75 D, and 6.00 D) were used. The IOLs were tilted from 1 to 10 degrees horizontally around a 90-degree vertical meridian. Toric IOLs were aligned at 90 degrees and 180 degrees to correct with-the-rule (WTR) and against-the-rule (ATR) corneal astigmatism, respectively. Astigmatism at the corneal plane induced by IOL tilt was calculated. Results Induced astigmatism increased with increasing IOL tilt and power. Horizontal tilt around a vertical meridian induced ATR astigmatism. For 5 degrees of tilt, induced astigmatism was 0.08 D, 0.11 D, and 0.14 D for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Ten degrees of IOL tilt produced 0.33 D, 0.44 D, and 0.56 D of induced astigmatism for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Tilting toric IOLs aligned at 90 degrees around a vertical meridian increased the magnitude of induced ATR astigmatism. Tilting toric IOLs aligned at 180 degrees decreased the magnitude of induced WTR astigmatism. Conclusions Tilting aspheric IOLs horizontally around a vertical meridian induced ATR astigmatism. Tilting toric IOLs aligned at 90 degrees increased ATR astigmatism, resulting in overcorrection. Tilting toric IOLs aligned at 180 degrees decreased WTR astigmatism, producing undercorrection.
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contribution of posterior corneal astigmatism to total corneal astigmatism
Journal of Cataract and Refractive Surgery, 2012Co-Authors: Douglas D Koch, Mitchell P Weikert, Mariko Shirayama, Richard Jenkins, Li WangAbstract: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.
Mitchell P Weikert - One of the best experts on this subject based on the ideXlab platform.
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astigmatism induced by intraocular lens tilt evaluated via ray tracing
Journal of Cataract and Refractive Surgery, 2018Co-Authors: Mitchell P Weikert, Abhinav Golla, Li WangAbstract:Purpose To evaluate astigmatism induced by aspheric and toric intraocular lens (IOL) tilt using a ray-tracing model. Setting Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. Design Experimental study. Methods Ray-tracing eye models with aspheric IOLs (16.0 diopters [D], 22.0 D, and 28.0 D) and toric IOLs (16.0 D, 22.0 D, and 28.0 D each with toricities of 1.50 D, 3.75 D, and 6.00 D) were used. The IOLs were tilted from 1 to 10 degrees horizontally around a 90-degree vertical meridian. Toric IOLs were aligned at 90 degrees and 180 degrees to correct with-the-rule (WTR) and against-the-rule (ATR) corneal astigmatism, respectively. Astigmatism at the corneal plane induced by IOL tilt was calculated. Results Induced astigmatism increased with increasing IOL tilt and power. Horizontal tilt around a vertical meridian induced ATR astigmatism. For 5 degrees of tilt, induced astigmatism was 0.08 D, 0.11 D, and 0.14 D for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Ten degrees of IOL tilt produced 0.33 D, 0.44 D, and 0.56 D of induced astigmatism for 16.0 D, 22.0 D, and 28.0 D aspheric IOLs, respectively. Tilting toric IOLs aligned at 90 degrees around a vertical meridian increased the magnitude of induced ATR astigmatism. Tilting toric IOLs aligned at 180 degrees decreased the magnitude of induced WTR astigmatism. Conclusions Tilting aspheric IOLs horizontally around a vertical meridian induced ATR astigmatism. Tilting toric IOLs aligned at 90 degrees increased ATR astigmatism, resulting in overcorrection. Tilting toric IOLs aligned at 180 degrees decreased WTR astigmatism, producing undercorrection.
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contribution of posterior corneal astigmatism to total corneal astigmatism
Journal of Cataract and Refractive Surgery, 2012Co-Authors: Douglas D Koch, Mitchell P Weikert, Mariko Shirayama, Richard Jenkins, Li WangAbstract: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.
Douglas D Koch - One of the best experts on this subject based on the ideXlab platform.
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contribution of posterior corneal astigmatism to total corneal astigmatism
Journal of Cataract and Refractive Surgery, 2012Co-Authors: Douglas D Koch, Mitchell P Weikert, Mariko Shirayama, Richard Jenkins, Li WangAbstract: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.
Kristian Naeser - One of the best experts on this subject based on the ideXlab platform.
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Influence of Posterior Corneal Astigmatism on Total Corneal Astigmatism in Eyes With Keratoconus
Cornea, 2016Co-Authors: Giacomo Savini, Kristian Naeser, Domenico Schiano-lomoriello, A. MularoniAbstract:To measure posterior corneal astigmatism (PCA) and investigate its influence on total corneal astigmatism (TCA) in eyes with keratoconus. Keratometric astigmatism (KA), PCA, and TCA were investigated by means of a dual Scheimpflug analyzer in patients with keratoconus. Vector analysis was carried out with the Naeser polar value method. We enrolled 119 eyes. PCA magnitude averaged 0.77 ± 0.43 diopters (D) and exceeded 0.50, 1.00, and 2.00 D in 73.9%, 21.8%, and 16.8% of eyes, respectively. PCA averaged 0.95 ± 0.48, 0.55 ± 0.28, and 0.70 ± 0.35 D in eyes with with-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism. The steepest posterior meridian was oriented vertically (between 61 and 119 degrees) in 55.5% of eyes, thus generating ATR astigmatism. The difference between the location of the steepest meridian of KA and that of TCA was >10 degrees in 8.4% of eyes. On average, KA overestimated TCA in eyes with WTR astigmatism by 0.16 D and underestimated TCA in eyes with ATR astigmatism by 0.22 D. The PCA power oriented along the steeper anterior corneal meridian averaged −0.83 ± 0.40, −0.40 ± 0.37, and −0.53 ± 0.43 D for WTR, ATR, and obliquely astigmatic eyes, respectively. Linear regression disclosed a statistically significant correlation (P < 0.0001, r2 = 0.16) between the Meridional powers of TCA and PCA. In eyes with keratoconus, PCA displays large, variable values and is correlated to TCA. The influence of PCA on TCA cannot be disregarded when planning astigmatism correction by toric intraocular lenses.
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conversion of keratometer readings to polar values
Journal of Cataract and Refractive Surgery, 1990Co-Authors: Kristian NaeserAbstract:Abstract Corneal astigmatism is a complex entity that has direction and magnitude. This study reports a new method to describe corneal astigmatism within the with- and against -the-rule concept. Each net astigmatism of the maximal power M in the meridian α may be divided into two dioptric components: a with-the-rule astigmatism projected on the 90-degree meridian and an against-the-rule component projected on the 180-degree meridian. The former figure has the dioptric value M × sin2α, the latter M × cos2α. The polar value is defined as the difference between these magnitudes: M × (sin2#x00D7; - cos2α). The polar value calculates the balance between the with- and against-the-rule components for any given net astigmatism. The entire model allows an exact description of surgically- induced with- or against-the-rule astigmatism following cataract extraction. The advantage of the model is that a corneal astigmatism may be expressed by a single figure. The system enables each surgeon to evaluate the contribution of the preoperative astigmatism, incision type, suture technique, and postoperative treatment on the final astigmatism. This in turn allows the surgeon to estimate a number of different surgical techniques empirically. By disposing of and choosing between several known techniques the surgeon may be able to minimize final astigmatism even in cases of significant preoperative astigmatism.
Jackson D Coleman - One of the best experts on this subject based on the ideXlab platform.
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very high frequency ultrasound biometry of the anterior and posterior chamber diameter
Journal of Refractive Surgery, 2004Co-Authors: M J Rondeau, Gyorgy Barcsay, Ronald H Silverman, Dan Z Reinstein, Rupa Krishnamurthy, A Chabi, Ted T Du, Jackson D ColemanAbstract:PURPOSE: To measure the largest diameter of the anterior chamber (AC) and posterior chamber (PC) dimension and its orientation and determine the relationship with the principal keratometric meridians. METHODS: Twenty-eight eyes of 14 subjects were scanned with high frequency (50 MHz) ultrasound in sequential Meridional scan planes at 30° increments. Observer identified angle and ciliary sulcus recess boundaries in each patient scan set were fit with an elliptical model to obtain the ellipse semi-major axis corresponding to the largest diameter and its Meridional orientation. Anterior and posterior chamber diameters from raw data and model fit were compared using linear statistics. Circular statistics were used to compare the orientation of the largest diameter for raw ultrasound measurements, model estimations of largest diameter, and autorefractor determined keratometric axes. RESULTS: The mean model diameters were anterior chamber OD 12.07 mm (0.32 SD); anterior chamber OS 12.06 mm (0.36 SD); posterior chamber OD 12.35 mm (0.42 SD); posterior chamber OS 12.33 mm (0.43 SD). The general trend for orientation of the meridian of largest diameter was in the horizontal meridian. In over 35% of eyes the difference between AC or PC meridian and the flat keratometric axis was greater than 20 degrees. CONCLUSIONS: Accurate and reproducible anterior segment biometry depends on visualization of structures and minimization of eye and head movement error. The range and standard deviation of the diameter and orientation measures suggests anatomic variation is sufficient to require biometry for proper sizing and placement of intraocular devices that use angle or sulcus fixation. [J Refract Surg 2004;20:454-464]