Inflation experiments and inverse finite element modelling of posterior human sclera

Geraghty, Brendan; Abass, Ahmed; Eliasy, Ashkan; Jones, Stephen; Rama, Paolo; Kassem, Wael; Akhtar, Riaz; Elsheikh, Ahmed

doi:10.1016/j.jbiomech.2019.109438

Cited by 14 publications

(8 citation statements)

References 30 publications

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“…following earlier experimental studies [57,59,60], Fig 13. The purpose of splitting the sclera into three regions was to characterise regional mechanical properties of scleral tissue using circumferential regions of isotropic elements to replicate macroscale sclera displacements. Scleral materials were characterised as μ s1 = 0.441, α s1 = 124.5, μ s2 = 0.349, α s2 = 138.5, μ s3 = 0.308 and α s3 = 162.2.…”

Section: Peripheral-zone Designsupporting

confidence: 61%

Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

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Purpose This study investigates how both the peripheral zone design and corneal shape affect the behaviour of soft contact lenses on-eye. Methods In this study, soft contact lenses of varying nominal cylindrical powers and peripheral zone designs—a single-prism gravity-based stabilised lens (G1P), two-prism blink-based stabilised lens (B2P) and four-prism blink-based stabilised lens (B4P)—were generated as finite element models. The on-eye simulation results were analysed to identify the impact of each peripheral zone design (Each with different volume ratios) on the effective power change (EPC) when worn by a subject. Topographies of three eyes of varying average simulated anterior corneal curvature (flat, average & steep) were used in this study. Results The volume of the lens’s peripheral zone as a ratio of the total lens volume (Vp) recorded very weak correlations with the effective power change (EPC) among the three investigated designs when they were fitted to the flat eye (R = -0.19, -0.15 & -0.22 respectively), moderate correlations with the average eye (R = 0.42, 0.43 & 0.43 respectively) and strong correlations with the steep eye (R = 0.91, 0.9 & 0.9 respectively). No significant differences were noticed among the three investigated designs and none of the cylindrical lenses designed with axis 90° recorded EPC values outside the acceptance criteria range (ACR) of ±0.25 D. No significant differences in EPC were recorded among the three designs G1P, B2P and B4P (p>0.6) when they were designed with three axes at 90°, 45° and 0°. Moving the toric lens axis away from 90° dragged the EPC to the negative side and most of the investigated lenses with axes at 45° and 0° recorded EPCs outside the ±0.25D range. Conclusions In all cases, the shape of the cornea had a more dominant effect on EPC when compared to the peripheral zone design. Corneal shape influences the soft toric contact lens’s on-eye power change more than the lens design.

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Section: Peripheral-zone Designsupporting

confidence: 61%

Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

et al. 2020

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“…In order to keep the focus on the geometrical effect only, single 1 st order (N = 1) Ogden hyperelastic material model was used for the whole eye globe where the material parameters were set to average values of μ 1 = 0.07 MPa and α 1 = 110.836 based on our previous knowledge. [62][63][64] The Ogden constitutive strain energy equation can be expressed as [65] U ¼…”

Section: Plos Onementioning

confidence: 99%

Limbus misrepresentation in parametric eye models

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To assess the axial, radial and tangential limbus position misrepresentation when parametric models are used to represent the cornea and the sclera. Methods This retrospective study included 135 subjects aged 22 to 65 years (36.5 mean ±9.8 STD), 71 females and 64 males. Topography measurements were taken using an Eye Surface Profiler topographer and processed by a custom-built MATLAB code. Eye surfaces were freed from edge-effect artefacts and fitted to spherical, conic and biconic models. Results When comparing the radial position of the limbus, average errors of-0.83±0.19mm,-0.76 ±0.20mm and-0.69±0.20mm were observed within the right eye population for the spherical, conic and biconic models fitted up to 5mm. For the same fitting radius, the average fitting errors were-0.86±0.23mm,-0.78±0.23mm and-0.73±0.23mm for the spherical, conic and biconic models respectively within the left eye population. For the whole cornea fit, the average errors were-0.27±0.12mm and-0.28±0.13mm for the spherical models,-0.02 ±0.29mm and-0.05±0.27mm for the conic models, and-0.22±0.16mm and 0.24±0.17mm for the biconic models in the right and left eye populations respectively. Conclusions Through the use of spherical, conic and biconic parametric modelling methods, the eye's limbus is being mislocated. Additionally, it is evident that the magnitude of fitting error associated with the sclera may be propagating through the other components of the eye. This suggests that a corneal nonparametric model may be necessary to improve the representation of the limbus.

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“…The phenomenon of viscoelastic characteristics including strain rate dependency, hysteresis, creep and stress relaxation has been observed consistently in soft biological tissues such as the sclera (Elsheikh et al, 2010;Geraghty et al, 2020), cornea (Elsheikh, Kassem & Jones, 2011;Kazaili, Geraghty & Akhtar, 2019) and tendon (Robinson et al, 2004;Zuskov et al, 2020). Similar to the other biological tissues, ligaments inherit viscoelastic characteristics meaning they exhibiting both elastic and viscous behaviour, hence they are history-and time-dependent (Bonner et al, 2015;Fung, 1993;Ristaniemi et al, 2018).…”

Section: Introductionmentioning

confidence: 95%

Viscoelastic characteristics of the canine cranial cruciate ligament complex at slow strain rates

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Ligaments including the cruciate ligaments support and transfer loads between bones applied to the knee joint organ. The functions of these ligaments can get compromised due to changes to their viscoelastic material properties. Currently there are discrepancies in the literature on the viscoelastic characteristics of knee ligaments which are thought to be due to tissue variability and different testing protocols. The aim of this study was to characterise the viscoelastic properties of healthy cranial cruciate ligaments (CCLs), from the canine knee (stifle) joint, with a focus on the toe region of the stress-strain properties where any alterations in the extracellular matrix which would affect viscoelastic properties would be seen. Six paired CCLs, from skeletally mature and disease-free Staffordshire bull terrier stifle joints were retrieved as a femur-CCL-tibia complex and mechanically tested under uniaxial cyclic loading up to 10 N at three strain rates, namely 0.1%, 1% and 10%/min, to assess the viscoelastic property of strain rate dependency. The effect of strain history was also investigated by subjecting contralateral CCLs to an ascending (0.1%, 1% and 10%/min) or descending (10%, 1% and 0.1%/min) strain rate protocol. The differences between strain rates were not statistically significant. However, hysteresis and recovery of ligament lengths showed some dependency on strain rate. Only hysteresis was affected by the test protocol and lower strain rates resulted in higher hysteresis and lower recovery. These findings could be explained by the slow process of uncrimping of collagen fibres and the contribution of proteoglycans in the ligament extracellular matrix to intra-fibrillar gliding, which results in more tissue elongations and higher energy dissipation. This study further expands our understanding of canine CCL behaviour, providing data for material models of femur-CCL-tibia complexes, and demonstrating the challenges for engineering complex biomaterials such as knee joint ligaments.

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Inflation experiments and inverse finite element modelling of posterior human sclera

Cited by 14 publications

References 30 publications

Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

Limbus misrepresentation in parametric eye models

Viscoelastic characteristics of the canine cranial cruciate ligament complex at slow strain rates

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