Accuracy of Total Corneal Astigmatism Measurements With a Scheimpflug Imager and a Color Light-Emitting Diode Corneal Topographer
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Scheimpflug principle
Astigmatism
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Two new methods of corneal analysis with Scheimpflug images will be introduced in this paper. The first study was to document the light scattering of the cornea itself using a newly modified Scheimpflug camera with higher magnification. Details of the slit images of the cornea could be evaluated more precisely than those of images taken with an existing camera. The second study was to evaluate the corneal epithelial barrier function by measuring the intensity of the light scattering originating from instilled fluorescein excited through filters. Fluorescein accumulation in the damaged epithelium and diffusion with time into the stromal tissue could be observed in two-dimensional images and the changes could also be documented by measuring the intensity and extent of light scattering. These newly developed methods will be useful for studies of the cornea.
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Objective To analyze the spherical aberration changes of the eye and anterior surface of cornea for myopia after laser in situ keratomileusis (LASIK) with OPD-SCAN.Methods Preoperative and postoperative aberration examinations were performed on 73 eyes of 40 myopia patients,which were continuously selected and assigned to three different groups according to the diopter:low diopter,media diopter and high diopter.The spherical aberration of the eye and anterior surface of cornea in 6mm pupil were computed.Comparison of the spherical aberration among preoperation,one week after operation and three months after operation were adopted with one-way ANOVA.Post Hoc Tests were used to compare every two groups of them.Linear correlation was used to analyze the relationship between the diopter before the surgery and the spherical aberration after the surgery.t-test was performed to analyze the differences of the spherical aberration of the eye and anterior surface of cornea between three months after LASIK and those of preoperation.Results Spherical aberration of eye and anterior surface of cornea showed signifcant differences among preoperation,one week and three months postoperatively in each group(P0.01).The spherical aberration RMS of the eye and anterior surface of cornea were directly correlated with the diopter before the surgery (P0.01).The differences of spherical aberration of eye and anterior surface of cornea between 3 months postoperation and preoperation in each group were statistically different.Conclusion LASIK can induce the increasing of spherical aberration of the eye and anterior surface of cornea.And it has positive correlation with preoperative diopter.The spherical aberration of anterior surface of cornea increases more than that of the eye.
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We report 100 cases of postkeratoplasty astigmatism, ranging from 4 to 20.5 diopters, treated with the following procedures: relaxing incisions, relaxing incisions with augmentation sutures, and repeated relaxing incisions with augmentation sutures according to the degree of astigmatism. Patients were divided into three study groups (group 1 for preoperative defects ranging from 4 to 10 diopters, group 2, from 10.5 to 15 diopters, and group 3 for defects exceeding 15 diopters). Mean preoperative astigmatism was 7.15 +/- 1.67, 12.56 +/- 1.37, and 17.50 +/- 1.51 diopters in each group, respectively. The net decrease in astigmatism after surgery was 5.53 +/- 1.44, 9.68 +/- 1.80, and 14.87 +/- 1.69 diopters in each group, respectively. Postoperatively, spectacle-corrected visual acuity improved by two Snellen lines in the group 1, five in the group 2, and nine in the group 3. The mean time to achieve stable results after surgery was 2.2 +/- 0.3, 2.7 +/- 0.4, and 4.2 +/- 0.7 months in each group, respectively (two patients in group 2 and all patients in group 3 underwent a two-step procedure). The relaxing incisions procedure, eventually modified by either adding compression sutures or repeating the incisions after 8-10 weeks, offers the possibility of a wide range of corrections for postkeratoplasty astigmatism with acceptable postoperative periods for visual rehabilitation.
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PURPOSE: To evaluate inter-device agreement between Placido topography (iTrace; Tracey Technologies, Houston, TX) and Scheimpflug tomography (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) for measuring corneal power and cylinder and axis of astigmatism. METHODS: Observational case series of 54 eyes from 54 subjects with no ocular disease. Main outcome measures were corneal power, cylinder power, and axis of astigmatism and their agreement was assessed by Bland–Altman analysis. RESULTS: For corneal power and corneal cylinder, 95% limits of agreement (LoA) were considered good (−0.38 to 0.45 diopters [D] and −0.49 to 0.27 D, respectively). In contrast, the 95% LoA for corneal astigmatism axis exceeded the clinically relevant margins (−14.8 to 13.5): 28 eyes (52%) had a greater than 5° difference, 10 eyes (19%) had a greater than 10° difference, and 4 eyes (7%) had a greater than 20° difference between instruments. This absolute difference was significantly correlated with average corneal cylinder (Spearman’s r = −0.379, P = .005) but not with average corneal power. In eyes with corneal astigmatism 2 D or greater, the 95% LoA for axis were −8.7° to 6.7°, whereas in those with corneal astigmatism less than 1 D, the 95% LoA for axis were −19.1° to 16.6°. CONCLUSIONS: Placido topography and Scheimpflug tomography show good agreement for corneal power and cylinder, but not for corneal astigmatism axis. These instruments could be used interchangeably only in eyes with corneal astigmatism of 2 D or greater. [ J Refract Surg. 2014;30(1):49–53.]
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Photorefraction of a sample of 93 infants of ages 1 day to 12 months showed that 63 percent of the subjects had astigmatism of 0.75 diopter or greater, and 12 percent greater than 2 diopters. Seventy percent of these astigmatisms were in the horizontal-vertical meridians. By comparison, only 8 percent of a sample of 26 adults tested by the same method showed astigmatism (all 0.75 to 1 diopter). The high incidence of infant astigmatism has implications for critical periods in human visual development and for infant acuity.
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Eleven patients with high astigmatism after penetrating keratoplasty underwent a new form of relaxing keratotomy, the trapezoidal relaxing keratotomy, originally described by Ruiz. The average preoperative astigmatism was 7.89 diopters (range 4.25 to 12.50 diopters) with an average reduction in cylinder of 5.00 diopters (range 0-9.5 diopters). No complications were experienced.
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Purpose To demonstrate a Scheimpflug-based imaging procedure for investigating the depth- and time-dependent strain response of the human cornea to inflation testing of whole eye globes. Methods Six specimens, three of which with intact corneal epithelium, were mounted in a customized apparatus within a humidity and temperature-monitored wet chamber. Each specimen was subjected to two mechanical tests in order to measure corneal strain resulting from application of cyclic (cyclic regimen) and constant (creep regimen) stress by changing the intra-ocular pressure (IOP) within physiological ranges (18–42 mmHg). Corneal shape changes were analyzed as a function of IOP and both corneal stress-strain curves and creep curves were generated. Results The procedure was highly accurate and repeatable. Upon cyclic stress application, a biomechanical corneal elasticity gradient was found in the front-back direction. The average Young's modulus of the anterior cornea ranged between 2.28±0.87 MPa and 3.30±0.90 MPa in specimens with and without intact epithelium (P = 0.05) respectively. The Young's modulus of the posterior cornea was on average 0.21±0.09 MPa and 0.17±0.06 MPa (P>0.05) respectively. The time-dependent strain response of the cornea to creep testing was quantified by fitting data to a modified Zener model for extracting both the relaxation time and compliance function. Conclusion Cyclic and creep mechanical tests are valuable for investigating the strain response of the intact human cornea within physiological IOP ranges, providing meaningful results that can be translated to clinic. The presence of epithelium influences the results of anterior corneal shape changes when monitoring deformation via Scheimpflug imaging in inflation experiments of whole eye globes.
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