scholarly journals Differences in Simulated Refractive Outcomes of Photorefractive Keratectomy (PRK) and Laser In-Situ Keratomileusis (LASIK) for Myopia in Same-Eye Virtual Trials

2019 ◽  
Vol 17 (1) ◽  
pp. 287 ◽  
Author(s):  
Ibrahim Seven ◽  
Joshua S. Lloyd ◽  
William J. Dupps
Keyword(s):  
Finite Element Analysis ◽  
Laser In Situ Keratomileusis ◽  
Fe Model ◽  
Clinical Validation ◽  
Element Analysis ◽  
Forward Displacement ◽  
Refractive Outcomes ◽  
The Difference ◽  
Surgically Induced

The use of computational mechanics for assessing the structural and optical consequences of corneal refractive procedures is increasing. In practice, surgeons who elect to perform PRK rather than LASIK must often reduce the programmed refractive treatment magnitude to avoid overcorrection of myopia. Building on a recent clinical validation study of finite element analysis (FEA)-based predictions of LASIK outcomes, this study compares predicted responses in the validated LASIK cases to theoretical PRK treatments for the same refractive error. Simulations in 20 eyes demonstrated that PRK resulted in a mean overcorrection of 0.17 ± 0.10 D relative to LASIK and that the magnitude of overcorrection increased as a function of attempted correction. This difference in correction closely matched (within 0.06 ± 0.03 D) observed differences in PRK and LASIK from a historical nomogram incorporating thousands of cases. The surgically induced corneal strain was higher in LASIK than PRK and resulted in more forward displacement of the central stroma and, consequently, less relative flattening in LASIK. This FE model provides structural confirmation of a mechanism of action for the difference in refractive outcomes of these two keratorefractive techniques, and the results were in agreement with empirical clinical data.

Patient-Specific Finite Element Simulations of Standard Incisional Astigmatism Surgery and a Novel Patterned Collagen Crosslinking Approach to Astigmatism Treatment

2013 ◽  
Author(s):  
Ibrahim Seven ◽  
William J. Dupps
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Intraocular Pressure ◽  
Laser In Situ Keratomileusis ◽  
Patient Specific ◽  
Element Analysis ◽  
Collagen Crosslinking ◽  
Refractive Power ◽  
Refractive Outcomes

The cornea provides the majority of the eye’s refractive power and is the most important ocular determinant of retinal image quality. The refractive power of the cornea derives from its shape, and this shape is a function of the ocular biomaterial properties and loading forces such as the intraocular pressure (IOP). Finite element analysis has been explored as a tool for estimating the refractive outcomes of corneal interventions such as laser in-situ keratomileusis (LASIK) and photorefractive keratectomy (PRK)[1].

Comparison between the change in total corneal astigmatism and actual change in refractive astigmatism in transepithelial photorefractive keratectomy (tPRK), laser in situ keratomileusis (LASIK) and femtosecond laser assisted laser in situ keratomileusis (FsLASIK)

2021 ◽  
pp. 112067212110593
Author(s):  
Maja Bohac ◽  
Alma Biscevic ◽  
Violeta Shijakova ◽  
Ivan Gabric ◽  
Kresimir Gabric ◽  
...  
Keyword(s):  
Corneal Astigmatism ◽  
Laser In Situ Keratomileusis ◽  
Significant Difference ◽  
Actual Change ◽  
The Difference ◽  
Induced Changes ◽  
Surgically Induced ◽  
Refraction Data ◽  
Total Corneal Astigmatism

Purpose To compare changes in astigmatism by refraction and total corneal astigmatism after tPRK, LASIK and FsLASIK. Setting Specialty Eye Hospital Svjetlost, Zagreb, Croatia. Design Partially masked, semi-randomized, prospective, case-by-case, interventional, clinical study. Methods Patients with a stable refraction (-0.75DS to −8.00DS, astigmatism ≤1.00DC) underwent tPRK, LASIK or FsLASIK without complication. Astigmatism was measured at both corneal surfaces over the central 3.2 mm zone (approximately using Pentacam HRTM) preoperatively and 3 months postoperatively. Pentacam and refraction data were subjected to vector analysis to calculate the surgically induced changes in i) total corneal astigmatism (SIATCA) ii) any astigmatism by refraction (SIAR) and the vectorial difference (DV) between SIATCA and SIAR. Results Reporting key findings (p < .01), there was a significant difference between mean SIATCA and SIAR powers after tPRK (75eyes) but not after LASIK (100eyes) or FsLASIK (100eyes). Mean (±sd,95% CIs) values for DV powers were, tPRK −1.13DC(±0.71, −1.29 to −0.97), LASIK −0.39DC(±0.23,-0.44 to −0.34), FsLASIK −0.55DC(±0.38,-0.62 to −0.47). The differences were significant. For the tPRK and FsLASIK cases, linear regression revealed significant associations between I) SIATCA (x) &DV (z) powers (tPRK z = 1.586x-0.179, r  =  0.767, p < .01; FsLASIK z  =  0.442x-0.303, r  =  .484,p < .01), II) sines of SIATCA (x1) &DV (z1) axes (tPRK, z1 = 0.523 × 1 + 0.394, r = .650,p < .01; FsLASIK z1 = 0.460 × 1-0.308, r = .465,p < .01). Conclusions tPRK is more prone to unintended changes in astigmatism. The difference between SIATCA & SIAR after tPRK or FsLASIK is mediated by SIATCA. Photoablating deeper regions of the cornea reduces the gap between SIATCA & SIAR.

Finite Element Analysis Route to Achieve Accurate Resistivity Measurements in Diamond Anvil Cell

2013 ◽  
Vol 669 ◽  
pp. 279-282
Author(s):  
Xuan Guo Nan ◽  
Gang Peng ◽  
Bao Jia Wu
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Diamond Anvil Cell ◽  
Resistivity Measurement ◽  
Clear Understanding ◽  
Element Analysis ◽  
Steady Current ◽  
Diamond Anvil ◽  
The Difference

To have a clear understanding of the effect of electrode resistivity on the in-situ resistivity measurement under high pressure in a diamond anvil cell (DAC), we perform finite element analysis (FEA) to simulate the distribution of the steady current field in sample. The theoretical analysis reveals the origin of the effect. It is caused by the resistivity difference between electrodes and sample. And the more the difference of their resistivity is, the more obvious the effect is. All these will result in large resistivity error. However we find that reducing the resistivity difference between the electrode and sample can improve the results.

Thermal Modeling of a Railroad Tapered-Roller Bearing Using Finite Element Analysis

2012 ◽  
Vol 4 (3) ◽  
Author(s):  
Constantine M. Tarawneh ◽  
Arturo A. Fuentes ◽  
Javier A. Kypuros ◽  
Lariza A. Navarro ◽  
Andrei G. Vaipan ◽  
...  
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Surface Temperature ◽  
Elevated Temperatures ◽  
Roller Bearing ◽  
Fe Model ◽  
Element Analysis ◽  
Tapered Roller Bearing ◽  
Outer Race ◽  
Railroad Industry

In the railroad industry, distressed bearings in service are primarily identified using wayside hot-box detectors (HBDs). Current technology has expanded the role of these detectors to monitor bearings that appear to “warm trend” relative to the average temperatures of the remainder of bearings on the train. Several bearings set-out for trending and classified as nonverified, meaning no discernible damage, revealed that a common feature was discoloration of rollers within a cone (inner race) assembly. Subsequent laboratory experiments were performed to determine a minimum temperature and environment necessary to reproduce these discolorations and concluded that the discoloration is most likely due to roller temperatures greater than 232 °C (450 °F) for periods of at least 4 h. The latter finding sparked several discussions and speculations in the railroad industry as to whether it is possible to have rollers reaching such elevated temperatures without heating the bearing cup (outer race) to a temperature significant enough to trigger the HBDs. With this motivation, and based on previous experimental and analytical work, a thermal finite element analysis (FEA) of a railroad bearing pressed onto an axle was conducted using ALGOR 20.3™. The finite element (FE) model was used to simulate different heating scenarios with the purpose of obtaining the temperatures of internal components of the bearing assembly, as well as the heat generation rates and the bearing cup surface temperature. The results showed that, even though some rollers can reach unsafe operating temperatures, the bearing cup surface temperature does not exhibit levels that would trigger HBD alarms.

Composite Drive Shaft Coupling for Future Rotorcraft: A 3D Finite-Element Analysis

1988 ◽  
Author(s):  
R. N. Margasahayam ◽  
H. S. Faust
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Analytical Techniques ◽  
Fe Model ◽  
Design Development ◽  
Element Analysis ◽  
3D Finite Element ◽  
The Finite Element Analysis ◽  
Comparison Of The Results ◽  
3D Finite Element Method

Abstract A finite-element stress analysis of a one-piece, integrated, all-composite shaft and coupling is presented. In addition to a brief discussion of design-driving parameters, some limitations of the analytical techniques used for design development are described. The 3D finite-element method (FEM) was then used to evaluate critical stresses and strains experienced by the shaft coupling. A comparison of the results from the finite-element analysis and those from static bending, axial, and torsional tests conducted on these prototype shafts yielded excellent correlation. Some important considerations in the development of the FE model and the correlation of results with tests, especially in the design of composite materials, are addressed.

scholarly journals Use of 3D Scan of Weld Joint in Finite Element Analysis and Stochastic Analysis of Hot-Spot Stresses in Tubular Joint for Fatigue Life Estimation

2019 ◽  
Author(s):  
Mikkel L. Larsen ◽  
Vikas Arora ◽  
Marie Lützen ◽  
Ronnie R. Pedersen ◽  
Eric Putnam
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Welded Joints ◽  
Hot Spot ◽  
Strain Range ◽  
Fe Model ◽  
Design Codes ◽  
Element Analysis ◽  
Hot Spot Stress ◽  
Weld Toe

Abstract Several methods for modelling and finite element analysis of tubular welded joints are described in various design codes. These codes provide specific recommendations for modelling of the welded joints, using simple weld geometries. In this paper, experimental hot-spot strain range results from a full-scale automatically welded K-node test are compared to corresponding finite element models. As part of investigating the automatically welded K-joint, 3D scans of the weld surfaces have been made. These scans are included in the FE models to determine the accuracy of the FE models. The results are compared to an FE model with a simple weld geometry based on common offshore design codes and a model without any modelled weld. The results show that the FE model with 3D scanned welds is more accurate than the two simple FE models. As the weld toe location of the 3D scanned weld is difficult to locate precisely in the FE model and as misplacement of strain gauges are possible, stochastic finite element modelling is performed to analyse the resulting probabilistic hot-spot stresses. The results show large standard deviations, showing the necessity to evaluate the hot-spot stress method when using 3D scanned welds.

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