The Experts below are selected from a list of 5289 Experts worldwide ranked by ideXlab platform
Ahmed Elsheikh - One of the best experts on this subject based on the ideXlab platform.
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Review of ex-vivo characterisation of corneal biomechanics
'Elsevier BV', 2021Co-Authors: Junjie Wang, Xiaoyu Liu, Fangjun Bao, Bernardo T. Lopes, Lizhen Wang, Ashkan Eliasy, Ahmed Abass, Ahmed ElsheikhAbstract:The evaluation of the corneal Biomechanical Behaviour has important clinical applications. To name a few, the accuracy of the intraocular pressure measurement, the study of corneal ectatic diseases and the assessment and optimisation of corneal surgical procedures are all highly influenced by corneal biomechanics. Over the last 45 years different ex-vivo methods were developed to study corneal Biomechanical Behaviour. Different tissue maintenance, support, loading systems, as well as different monitoring strategies of corneal deformations were employed. In this review, the most important and commonly used methods are outlined, including strip extensiometry, inflation, compression, indentation and tissue separation testing. Their particularities, applications, pros and cons and main applications are discussed
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age related variations in the Biomechanical properties of human sclera
Journal of The Mechanical Behavior of Biomedical Materials, 2012Co-Authors: Brendan Geraghty, Ahmed Elsheikh, Paolo Rama, Stephen W Jones, Riaz AkhtarAbstract:Abstract This study examined age-related changes in Biomechanical Behaviour in the anterior, equatorial and posterior regions of the human sclera (white of the eye). Circumferential strip specimens were extracted from areas close to the limbus, equator and posterior pole in 45 donor scleras ranging in age between 51 and 84 years. The strips were subjected to cycles of uniaxial tension loading at a strain rate of 8% per minute while monitoring their load-deformation Behaviour. All specimens demonstrated nonlinear Behaviour with an initially low tangent modulus (a measure of material stiffness) increasing under higher stresses. The average ratios between the tangent modulus at a high stress of 1 MPa and that at a low stress of 0.05 MPa were 11.2±1.7, 12.0±1.7 and 12.4±1.5 for anterior, equatorial and posterior specimens, respectively. Stiffening was observed with age in all regions, but it was statistically significant only in the anterior region ( P
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regional variation in the Biomechanical properties of the human sclera
Experimental Eye Research, 2010Co-Authors: Ahmed Elsheikh, Brendan Geraghty, Daad Alhasso, Jonathan Knappett, Marino Campanelli, Paolo RamaAbstract:Abstract The study aimed to determine the variation in thickness and Biomechanical properties between the different regions of the human sclera. Thickness measurements were carried out along eight meridian lines extending from the posterior pole to limbus in 36 human donor scleras aged 52–96 years. Strip specimens were extracted from areas close to the limbus, equator and posterior pole, and tested under cycles of uniaxial tension. Two strain rates were considered to assess the viscoelasticity effects on the regional variation in material Behaviour. The results were used to derive the stress–strain Behaviour of each specimen and to calculate the tangent modulus at each stress level. The scleras had a variable thickness from maximum at the posterior pole to minimum close to the equator, and increasing again towards the limbus. All scleral specimens demonstrated nonlinear Behaviour with an initially low tangent modulus (a measure of stiffness) increasing gradually under higher stresses. With reference to specific stress levels, the Behaviour comparisons between regions showed a gradual growth in material tangent stiffness with progression from the posterior region towards the limbus. The viscoelasticity of the tissue, which was evident with significant increases in stress (157–203%) and tangent modulus (30.3–38.8%) with strain rate rise (from 8% to 200% per min), was associated with reductions in the regional variation in stiffness. The considerable variation in Biomechanical Behaviour found in this study should be useful in improving the accuracy of representing the sclera's real-life conditions in numerical simulations.
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assessment of the epithelium s contribution to corneal biomechanics
Experimental Eye Research, 2008Co-Authors: Ahmed Elsheikh, Daad Alhasso, Paolo RamaAbstract:Determining the epithelium's contribution to corneal biomechanics is important for the predictive numerical simulation of corneal Biomechanical Behaviour in which the cornea's five main layers are represented separately. Twenty-four corneal buttons were tested under posterior inflation conditions while monitoring their Behaviour using non-contact methods. The corneas were divided into two groups of 12; one with and one without the epithelium. Control of specimen hydration, temperature and pressure application rate, and limiting the programme to specimens within a small age range resulted in a narrow scatter of test results. On average, intact specimens were able to carry slightly more pressure at the same deformation, and experienced less average stress for the same strain, compared with specimens without the epithelium. These results indicated that the stiffness of the epithelium was considerably lower than that of the stroma, and might therefore be ignored in numerical simulation studies.
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application of structural analysis to the mechanical Behaviour of the cornea
Journal of the Royal Society Interface, 2004Co-Authors: Kevin Anderson, Ahmed Elsheikh, T A NewsonAbstract:Structural engineering analysis tools have been used to improve the understanding of the Biomechanical Behaviour of the cornea. The research is a multi-disciplinary collaboration between structural engineers, mathematical and numerical analysts, ophthalmologists and clinicians. Mathematical shell analysis and nonlinear finite-element modelling have been used in conjunction with laboratory experiments to study the Behaviour of the cornea under different loading states and to provide improved predictions of the mechanical response to disease and injury. The initial study involved laboratory tests and mathematical back analysis to determine the corneal material properties and topography. These data were then used to facilitate the construction of accurate finite-element models that are able to reliably trace the performance of cornea upon exposure to disease, injury or elevated intra-ocular pressure. The models are being adapted to study the response to keratoconus (a disease causing loss of corneal tissue) and to tonometry procedures, which are used to measure the intra-ocular pressure. This paper introduces these efforts as examples of the application of structural engineering analysis tools and shows their potential in the field of corneal biomechanics.
Antonio Heredia - One of the best experts on this subject based on the ideXlab platform.
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viscoelastic nature of isolated tomato solanum lycopersicum fruit cuticles a mathematical model
Physiologia Plantarum, 2010Co-Authors: Gloria Lopezcasado, Amador Salamanca, Antonio HerediaAbstract:Viscoelastic Behaviour of isolated tomato fruit cuticle (CM) is well known and extensively described. Temperature and hydration conditions modify the mechanical properties of CM. Mechanical data from previous transient-creep analysis developed in tomato fruit cuticle under different temperature and hydration conditions have been used to propose a rheological model that describes the viscoelastic nature of CM. As a composite material, the Biomechanical Behaviour of the plant cuticle will depend not only on the mechanical characteristics of the individual components by themselves but also on the sum of them. Based on this previous information, we proposed a two-element model to describe the experimental Behaviour: an elastic hookean element connected in parallel to a viscous element or Voigt element that will describe the mechanical Behaviour of the isolated CM and cutin under the studied conditions. The main parameters of the model, E 1 and E 2 will reflect the elastic and viscoelastic Behaviour of the cuticle. Relationship between these physical parameters and the change in CM properties were discussed in order to elucidate the rheological processes taking place in CM. This model describes both the influence of temperature and hydration and the Behaviour of the isolated cutin and the inferred contribution of the cuticle fraction of polysaccharides when the whole cuticle is tested.
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biomechanics of isolated tomato solanum lycopersicum l fruit cuticles the role of the cutin matrix and polysaccharides
Journal of Experimental Botany, 2007Co-Authors: Gloria Lopezcasado, Antonio J. Matas, Eva Domínguez, Jesus Cuartero, Antonio HerediaAbstract:The mechanical characteristics of the cuticular membrane (CM), a complex composite biopolymer basically composed of a cutin matrix, waxes, and hydrolysable polysaccharides, have been described previously. The Biomechanical Behaviour and quantitative contribution of cutin and polysaccharides have been investigated here using as experimental material mature green and red ripe tomato fruits. Treatment of isolated CM with anhydrous hydrogen fluoride in pyridine allowed the selective elimination of polysaccharides attached to or incrusted into the cutin matrix. Cutin samples showed a drastic decrease in elastic modulus and stiffness (up to 92%) compared with CM, which clearly indicates that polysaccharides incorporated into the cutin matrix are responsible for the elastic modulus, stiffness, and the linear elastic Behaviour of the whole cuticle. Reciprocally, the viscoelastic Behaviour of CM (low elastic modulus and high strain values) can be assigned to the cutin. These results applied both to mature green and red ripe CM. Cutin elastic modulus, independently of the degree of temperature and hydration, was always significantly higher for the ripe than for the green samples while strain was lower; the amount of phenolics in the cutin network are the main candidates to explain the increased rigidity from mature green to red ripe cutin. The polysaccharide families isolated from CM were pectin, hemicellulose, and cellulose, the main polymers associated with the plant cell wall. The three types of polysaccharides were present in similar amounts in CM from mature green and red ripe tomatoes. Physical techniques such as X-ray diffraction and Raman spectroscopy indicated that the polysaccharide fibres were mainly randomly oriented. A tomato fruit CM scenario at the supramolecular level that could explain the observed CM Biomechanical properties is presented and discussed.
Paolo Rama - One of the best experts on this subject based on the ideXlab platform.
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age related variations in the Biomechanical properties of human sclera
Journal of The Mechanical Behavior of Biomedical Materials, 2012Co-Authors: Brendan Geraghty, Ahmed Elsheikh, Paolo Rama, Stephen W Jones, Riaz AkhtarAbstract:Abstract This study examined age-related changes in Biomechanical Behaviour in the anterior, equatorial and posterior regions of the human sclera (white of the eye). Circumferential strip specimens were extracted from areas close to the limbus, equator and posterior pole in 45 donor scleras ranging in age between 51 and 84 years. The strips were subjected to cycles of uniaxial tension loading at a strain rate of 8% per minute while monitoring their load-deformation Behaviour. All specimens demonstrated nonlinear Behaviour with an initially low tangent modulus (a measure of material stiffness) increasing under higher stresses. The average ratios between the tangent modulus at a high stress of 1 MPa and that at a low stress of 0.05 MPa were 11.2±1.7, 12.0±1.7 and 12.4±1.5 for anterior, equatorial and posterior specimens, respectively. Stiffening was observed with age in all regions, but it was statistically significant only in the anterior region ( P
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regional variation in the Biomechanical properties of the human sclera
Experimental Eye Research, 2010Co-Authors: Ahmed Elsheikh, Brendan Geraghty, Daad Alhasso, Jonathan Knappett, Marino Campanelli, Paolo RamaAbstract:Abstract The study aimed to determine the variation in thickness and Biomechanical properties between the different regions of the human sclera. Thickness measurements were carried out along eight meridian lines extending from the posterior pole to limbus in 36 human donor scleras aged 52–96 years. Strip specimens were extracted from areas close to the limbus, equator and posterior pole, and tested under cycles of uniaxial tension. Two strain rates were considered to assess the viscoelasticity effects on the regional variation in material Behaviour. The results were used to derive the stress–strain Behaviour of each specimen and to calculate the tangent modulus at each stress level. The scleras had a variable thickness from maximum at the posterior pole to minimum close to the equator, and increasing again towards the limbus. All scleral specimens demonstrated nonlinear Behaviour with an initially low tangent modulus (a measure of stiffness) increasing gradually under higher stresses. With reference to specific stress levels, the Behaviour comparisons between regions showed a gradual growth in material tangent stiffness with progression from the posterior region towards the limbus. The viscoelasticity of the tissue, which was evident with significant increases in stress (157–203%) and tangent modulus (30.3–38.8%) with strain rate rise (from 8% to 200% per min), was associated with reductions in the regional variation in stiffness. The considerable variation in Biomechanical Behaviour found in this study should be useful in improving the accuracy of representing the sclera's real-life conditions in numerical simulations.
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assessment of the epithelium s contribution to corneal biomechanics
Experimental Eye Research, 2008Co-Authors: Ahmed Elsheikh, Daad Alhasso, Paolo RamaAbstract:Determining the epithelium's contribution to corneal biomechanics is important for the predictive numerical simulation of corneal Biomechanical Behaviour in which the cornea's five main layers are represented separately. Twenty-four corneal buttons were tested under posterior inflation conditions while monitoring their Behaviour using non-contact methods. The corneas were divided into two groups of 12; one with and one without the epithelium. Control of specimen hydration, temperature and pressure application rate, and limiting the programme to specimens within a small age range resulted in a narrow scatter of test results. On average, intact specimens were able to carry slightly more pressure at the same deformation, and experienced less average stress for the same strain, compared with specimens without the epithelium. These results indicated that the stiffness of the epithelium was considerably lower than that of the stroma, and might therefore be ignored in numerical simulation studies.
Gloria Lopezcasado - One of the best experts on this subject based on the ideXlab platform.
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viscoelastic nature of isolated tomato solanum lycopersicum fruit cuticles a mathematical model
Physiologia Plantarum, 2010Co-Authors: Gloria Lopezcasado, Amador Salamanca, Antonio HerediaAbstract:Viscoelastic Behaviour of isolated tomato fruit cuticle (CM) is well known and extensively described. Temperature and hydration conditions modify the mechanical properties of CM. Mechanical data from previous transient-creep analysis developed in tomato fruit cuticle under different temperature and hydration conditions have been used to propose a rheological model that describes the viscoelastic nature of CM. As a composite material, the Biomechanical Behaviour of the plant cuticle will depend not only on the mechanical characteristics of the individual components by themselves but also on the sum of them. Based on this previous information, we proposed a two-element model to describe the experimental Behaviour: an elastic hookean element connected in parallel to a viscous element or Voigt element that will describe the mechanical Behaviour of the isolated CM and cutin under the studied conditions. The main parameters of the model, E 1 and E 2 will reflect the elastic and viscoelastic Behaviour of the cuticle. Relationship between these physical parameters and the change in CM properties were discussed in order to elucidate the rheological processes taking place in CM. This model describes both the influence of temperature and hydration and the Behaviour of the isolated cutin and the inferred contribution of the cuticle fraction of polysaccharides when the whole cuticle is tested.
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biomechanics of isolated tomato solanum lycopersicum l fruit cuticles the role of the cutin matrix and polysaccharides
Journal of Experimental Botany, 2007Co-Authors: Gloria Lopezcasado, Antonio J. Matas, Eva Domínguez, Jesus Cuartero, Antonio HerediaAbstract:The mechanical characteristics of the cuticular membrane (CM), a complex composite biopolymer basically composed of a cutin matrix, waxes, and hydrolysable polysaccharides, have been described previously. The Biomechanical Behaviour and quantitative contribution of cutin and polysaccharides have been investigated here using as experimental material mature green and red ripe tomato fruits. Treatment of isolated CM with anhydrous hydrogen fluoride in pyridine allowed the selective elimination of polysaccharides attached to or incrusted into the cutin matrix. Cutin samples showed a drastic decrease in elastic modulus and stiffness (up to 92%) compared with CM, which clearly indicates that polysaccharides incorporated into the cutin matrix are responsible for the elastic modulus, stiffness, and the linear elastic Behaviour of the whole cuticle. Reciprocally, the viscoelastic Behaviour of CM (low elastic modulus and high strain values) can be assigned to the cutin. These results applied both to mature green and red ripe CM. Cutin elastic modulus, independently of the degree of temperature and hydration, was always significantly higher for the ripe than for the green samples while strain was lower; the amount of phenolics in the cutin network are the main candidates to explain the increased rigidity from mature green to red ripe cutin. The polysaccharide families isolated from CM were pectin, hemicellulose, and cellulose, the main polymers associated with the plant cell wall. The three types of polysaccharides were present in similar amounts in CM from mature green and red ripe tomatoes. Physical techniques such as X-ray diffraction and Raman spectroscopy indicated that the polysaccharide fibres were mainly randomly oriented. A tomato fruit CM scenario at the supramolecular level that could explain the observed CM Biomechanical properties is presented and discussed.
Ahmet Kurt - One of the best experts on this subject based on the ideXlab platform.
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comparison of Biomechanical Behaviour of maxilla following le fort i osteotomy with 2 versus 4 plate fixation using 3d fea part 3 inferior and anterior repositioning surgery
International Journal of Oral and Maxillofacial Surgery, 2009Co-Authors: Erkan Erkmen, Mustafa Sancar Atac, Ergun Yucel, Ahmet KurtAbstract:Abstract Having studied the effect of maxillary advancement and maxillary impaction in parts 1 and 2 of this research, the purpose of this study was to investigate the Biomechanical behavior of different fixation models in inferiorly and anteriorly repositioned maxilla following Le Fort I osteotomy. Two separate three-dimensional finite element models, simulating the inferiorly advanced maxilla at Le Fort I level, were used to compare 2- and 4-plate fixation. Model INF-2 resulted in 247 897 elements and 53 247 nodes and INF-4 consisted of 273 130 elements and 59 917 nodes. The stresses occurring in and around the bone and plate–screw complex were computed. The highest Von Mises stresses on the plates and maximum principal stresses on the bones were found in INF-2, especially under horizontal and oblique loads, when compared with INF-4. The present Biomechanical study shows that the traditionally used 4-plate fixation technique, following Le Fort I inferior and anterior repositioning surgery, without bone grafting, provides fewer stress fields on the maxillary bones and fixation materials.
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comparison of Biomechanical Behaviour of maxilla following le fort i osteotomy with 2 versus 4 plate fixation using 3d fea
International Journal of Oral and Maxillofacial Surgery, 2009Co-Authors: Mustafa Sancar Atac, Erkan Erkmen, Ergun Yucel, Ahmet KurtAbstract:Abstract Having studied the effect of maxillary advancement and maxillary impaction in parts 1 and 2 of this research, the purpose of this study was to investigate the Biomechanical behavior of different fixation models in inferiorly and anteriorly repositioned maxilla following Le Fort I osteotomy. Two separate three-dimensional finite element models, simulating the inferiorly advanced maxilla at Le Fort I level, were used to compare 2- and 4-plate fixation. Model INF-2 resulted in 247897 elements and 53247 nodes and INF-4 consisted of 273130 elements and 59917 nodes. The stresses occurring in and around the bone and plate–screw complex were computed. The highest Von Mises stresses on the plates and maximum principal stresses on the bones were found in INF-2, especially under horizontal and oblique loads, when compared with INF-4. The present Biomechanical study shows that the traditionally used 4-plate fixation technique, following Le Fort I inferior and anterior repositioning surgery, without bone grafting, provides fewer stress fields on the maxillary bones and fixation materials.
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comparison of Biomechanical Behaviour of maxilla following le fort i osteotomy with 2 versus 4 plate fixation using 3d fea part 1 advancement surgery
International Journal of Oral and Maxillofacial Surgery, 2008Co-Authors: Mustafa Sancar Atac, Erkan Erkmen, Ergun Yucel, Ahmet KurtAbstract:The study aimed to calculate the location and intensity of the maximum stress fields on the fixation plates and surrounding maxilla following Le Fort I osteotomies after advancement procedures using three-dimensional finite element analysis. The models were generated using skull CT scan data. Le Fort I osteotomy simulations were made and two separate impacted maxillary models were designed. The ADV-2 model has 2 plate fixations bilaterally at the piriform rims, the ADV-4 model has 4 plate fixations at the zygomatic buttresses and piriform rims. The stress fields on bone, plate and screws were computed for each model. Posterior occlusal loads were simulated on one side in the molar-premolar region, in all three directions, reflecting the chewing forces. The increased locations of highest Von Mises stresses on the plates and highest maximum principle stresses on the bones were determined in ADV-2 models especially under horizontal and oblique loads when compared with ADV-4 models. Evaluation of the highest Von Mises stress values and maximum principal stress revealed that oblique load in the ADV-2 model received the highest values. 4-plate fixation following Le Fort I advancement surgery exerts less stress on the maxillary bones and fixation materials than 2-plate fixation.