Low-energy dynamic indentation method for analysis of ophthalmic materials

Purpose. The purpose of the present work is to understand and study the mechanical behavior and critical parameters of ophthalmic polymers. The article introduces a novel low-energy indentation method that can be used to study and optimize the mechanical properties of ophthalmic materials. The technique has been developed in the frame of a larger study on the impact resistance of materials. Method. The low-energy dynamic indentation method is based on a lumped mass-spring model solved by a 4th-order Runge-Kutta numerical method. The model can be used to predict the material response to the indentation of a hemi-spherical tip and calculate the elasticity modulus of materials, dissipated energy during impact, residual deformation after impact, indentation depth and their conservative and nonconservative components. Results. As an example, two ophthalmic polymers were compared: CR-39 as the universal ophthalmic standard, and Superfin as Indo Lens U.S., standard. Results showed the model is in good agreement with experimental data and allowed to obtain elasticity moduli for both materials, which showed similar values. A larger conservative component of the displacement for Superfin was also obtained and a smaller calculated residual displacement, which is indicative of less deformed material after low energy impacts. Conclusions. The model can satisfactorily predict the behavior of materials under low energy indentation situations. In addition, it can be used to distinguish two apparently similar materials, such as CR-39 and Superfin, and classify them according to their response to these kind of indentations. The technique could be used as a very powerful tool to improve ophthalmic materialsPeer Reviewe

Low-energy dynamic indentation method for analysis of ophthalmic materials

Purpose. The purpose of the present work is to understand and study the mechanical behavior and critical parameters of ophthalmic polymers. The article introduces a novel low-energy indentation method that can be used to study and optimize the mechanical properties of ophthalmic materials. The technique has been developed in the frame of a larger study on the impact resistance of materials. Method. The low-energy dynamic indentation method is based on a lumped mass-spring model solved by a 4th-order Runge-Kutta numerical method. The model can be used to predict the material response to the indentation of a hemi-spherical tip and calculate the elasticity modulus of materials, dissipated energy during impact, residual deformation after impact, indentation depth and their conservative and nonconservative components. Results. As an example, two ophthalmic polymers were compared: CR-39 as the universal ophthalmic standard, and Superfin as Indo Lens U.S., standard. Results showed the model is in good agreement with experimental data and allowed to obtain elasticity moduli for both materials, which showed similar values. A larger conservative component of the displacement for Superfin was also obtained and a smaller calculated residual displacement, which is indicative of less deformed material after low energy impacts. Conclusions. The model can satisfactorily predict the behavior of materials under low energy indentation situations. In addition, it can be used to distinguish two apparently similar materials, such as CR-39 and Superfin, and classify them according to their response to these kind of indentations. The technique could be used as a very powerful tool to improve ophthalmic materialsPeer ReviewedPostprint (published version

Comportamiento a la fractura de materiales para lentes oftálmicas

La importancia que han adquirido polímeros orgánicos termoestables en el campo de la óptica oftálmica ha provocado una sustitución casi completa de los materiales inorgánicos tradicionales como el vidrio. La resina comercial CR-39 se ha convertido en el actual estándar mundial para la fabricación de lentes de bajo índice de refracción. En España, este material termoestable compite con el Superfin, otro estándar introducido por INDO S.A. en 1992. Para estudiar y comparar el comportamiento al impacto de estos dos materiales se han realizado una serie de ensayos de caída de dardo y se ha elaborado un modelo dinámico muelle-amortiguador de flexión de platos para simular su respuesta. Se ha validado el modelo y se ha podido determinar el módulo a flexión y la tensión de rotura. La comparación entre ambos materiales ha evidenciado que el Superfin muestra mejor comportamiento mecánico para la fabricación de lentes oftálmicas.Peer Reviewe

Comportamiento a la fractura de materiales para lentes oftálmicas

La importancia que han adquirido polímeros orgánicos termoestables en el campo de la óptica oftálmica ha provocado una sustitución casi completa de los materiales inorgánicos tradicionales como el vidrio. La resina comercial CR-39 se ha convertido en el actual estándar mundial para la fabricación de lentes de bajo índice de refracción. En España, este material termoestable compite con el Superfin, otro estándar introducido por INDO S.A. en 1992. Para estudiar y comparar el comportamiento al impacto de estos dos materiales se han realizado una serie de ensayos de caída de dardo y se ha elaborado un modelo dinámico muelle-amortiguador de flexión de platos para simular su respuesta. Se ha validado el modelo y se ha podido determinar el módulo a flexión y la tensión de rotura. La comparación entre ambos materiales ha evidenciado que el Superfin muestra mejor comportamiento mecánico para la fabricación de lentes oftálmicas.Peer ReviewedPostprint (published version

Models for prognostic variables in matched groups with censored data

SIGLEAvailable from British Library Document Supply Centre- DSC:D52207/84 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Comportamiento a la fractura de materiales para lentes oftálmicas

La importancia que han adquirido polímeros orgánicos termoestables en el campo de la óptica oftálmica ha provocado una sustitución casi completa de los materiales inorgánicos tradicionales como el vidrio. La resina comercial CR-39 se ha convertido en el actual estándar mundial para la fabricación de lentes de bajo índice de refracción. En España, este material termoestable compite con el Superfin, otro estándar introducido por INDO S.A. en 1992. Para estudiar y comparar el comportamiento al impacto de estos dos materiales se han realizado una serie de ensayos de caída de dardo y se ha elaborado un modelo dinámico muelle-amortiguador de flexión de platos para simular su respuesta. Se ha validado el modelo y se ha podido determinar el módulo a flexión y la tensión de rotura. La comparación entre ambos materiales ha evidenciado que el Superfin muestra mejor comportamiento mecánico para la fabricación de lentes oftálmicas.Peer Reviewe

Low Energy Dart Test for Mechanical Evaluation of Ophthalmic Materials

Martinez AB, Artus P, Dursteler JC, Arencon D. Low energy dart test for mechanical evaluation of ophthalmic materials. Optom Vis Sci 2009;86:979-87. Reproduced with permission. ©The American Academy of Optometry 2009.Purpose. Many impact tests fail to rigorously analyze the polymer behavior at impact, because they are performed in an energy range too different from real-life incidents, use specimens with other geometries than those of their final application, or they do not take in account polymer viscoelastic nature. A novel low energy impact method that overcomes current method limitations is presented for ophthalmic polymers and advances our understanding of the behavior of these materials under impact conditions. Method. Plate-shaped specimens of two known materials, CR-39 and Superfin, were tested in an energy range around their failure limit. A non-conservative model was proposed to predict the dynamic response of the specimens that did not fail. Both the deflection and indentation mechanisms were introduced in the model, which was solved using a fourth order Runge-Kutta numerical method. Damper coefficients that were introduced to model the energy dissipation and elastic modulus were obtained after the fitting process. Rupture stress and absorbed energy at failure were obtained from the specimens that failed. Results. Very good agreement between experimental and calculated data was observed. Under non-failure conditions, Superfin and CR-39 showed similar elastic modulus, although slightly larger energy dissipation was observed for CR-39. However, Superfin clearly outperformed CR-39 when measuring rupture stress and absorbed energy at failure with values 54% and 170% larger, respectively. Conclusions. Low energy impact methods are a very powerful tool to study and compare ophthalmic materials. The model satisfactorily predicted the behavior of materials in low energy impact conditions and can be used to obtain critical material characteristics. In this particular case, the method was used to quantify mechanical differences among CR-39 and Superfin. Of these two, the latter is the best performing material

Control of disability glare by means of electrochromic filtering glasses: A pilot study

Purpose: To evaluate the potential clinical usability of a new prototype of ophthalmic blue light filters developed by using electrochromic technology in pseudophakic patients complaining of glare. Methods: A prototype of electrochromic device was developed, with a specific frame that enclosed an electronic driver that allowed personalizing its function for each patient. A prospective, observational case series study was performed to test it. Five patients who had undergone cataract surgery with clear intraocular lenses and complaining of glare were included in the study. Main outcome measures were the results obtained in the satisfaction questionnaire that was delivered to patients. Then, visual acuity, contrast sensitivity, and reading ability data were evaluated with and without the prototype under different lighting conditions and different modes of the prototype after a complete month of use. Lens transmittance was also measured. Results: Visual acuity, contrast sensitivity and reading function did not change significantly with prototype use. The main activity for which the prototype was used was walking. Only one patient found that the dimming level was insufficient. No patients reported variable discomfort when passing tunnels, not sufficiently clear indoors, or put on and remove discomfort. The lenses slightly decreased their transmittance at the end of the study. Conclusion: Glasses based on electrochromic technology may be acceptable for outdoor/indoor use and for distance–near vision. Future studies with larger samples must be conducted to confirm the clinical usability of these glasses