Biomechanics of the Sclera in Myopia: Extracellular and Cellular Factors

McBrien, Neville A.; Jobling, Andrew I; Gentle, Alex

doi:10.1097/opx.0b013e3181940669

Cited by 228 publications

(176 citation statements)

References 26 publications

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“…For example, scleral thinning and weakening have been reported in the early development of myopia in tree shrew eyes [6]. Increased scleral creep has also been reported in mammalian eyes developing myopia, particularly at the posterior pole of the eye [7,8]. The understanding of scleral mechanical properties and their alterations may provide insight into the potential association between glaucoma and myopia [9].…”

Section: Introductionmentioning

confidence: 99%

Ultrasonic Measurement of Scleral Cross-Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera

Tang

Liu

2012

Journal of Biomechanical Engineering

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Background. Scleral biomechanical properties may be important in the pathogenesis and progression of glaucoma. The goal of this study is to develop and validate an ultrasound method for measuring cross-sectional distributive strains in the sclera during elevations of intraocular pressure (IOP). Method of Approach. Porcine globes (n ¼ 5) were tested within 24 hs postmortem. The posterior scleral shells were dissected and mounted onto a custom-built pressurization chamber. A high-frequency (55-MHz) ultrasound system (Vevo660, VisualSonics Inc., Toronto) was employed to acquire the radio frequency data during scans of the posterior pole along both circumferential and meridian directions. The IOP was gradually increased from 5 to 45 mmHg. The displacement fields were obtained from correlation-based ultrasound speckle tracking. A least-square strain estimator was used to calculate the strains in both axial and lateral directions. Experimental validation was performed by comparing tissue displacements calculated from ultrasound speckle tracking with those induced by an actuator. Theoretical analysis and simulation experiments were performed to optimize the ultrasound speckle tracking method and evaluate the accuracy and signal-to-noise ratio (SNR) in strain estimation. Results. Porcine sclera exhibited significantly larger axial strains (e.g., À5.1 6 1.5% at 45 mmHg, meridian direction) than lateral strains (e.g., 2.2 6 0.7% at 45 mmHg, meridian direction) during IOP elevations (P's < 0.01). The strain magnitudes increased nonlinearly with pressure increase. The strain maps displayed heterogeneity through the thickness. The lateral strains were significantly smaller in the circumferential direction than the meridian direction at 45 mmHg (P < 0.05). Experimental validation showed that the ultrasound speckle tracking method was capable of tracking displacements at the accuracy of sub-micron to micron. Theoretical analysis predicted the dependence of the strain estimation SNR on the strain level, as well as signal processing parameters such as kernel size. Simulation results showed that ultrasound speckle tracking had a high accuracy for estimating strains of 1-5% and a high SNR for strains of 0.5-5%. Conclusions. A new experimental method based on ultrasound speckle tracking has been developed for obtaining cross-sectional strain maps of the posterior sclera. This method provides a useful tool to examine distributive strains through the thickness of the sclera during elevations of IOP.

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Section: Introductionmentioning

confidence: 99%

Ultrasonic Measurement of Scleral Cross-Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera

Tang

Liu

2012

Journal of Biomechanical Engineering

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“…The plastic strain-rate of the sclera, in response to stress, as it applies to myopia development, is discussed by Myers et al [1], Phillips et al [2], McBrien et al [3], Siegwart & Norton [4], Ku & Greene [5], Nash et al [6], Romano et al [7], and Downs et al [8], Uchio et al [16] using finite elements, calculate the stress fields caused by rapidly applied forces.…”

Section: Discussionmentioning

confidence: 99%

“…Several authors report the creep-rate stress response of collagen with applications to myopia development, including Myers et al [1], Phillips et al [2], McBrien et al [3], Siegwart & Norton [4], Ku & Greene [5], Nash et al [6], Romano et al [7], Genest et al [8], Glass [9], Downs et al [10]. Classical texts by Fung [11] and Ferry [12] provide basic equations and examples of the Maxwell and Kelvin-Voigt models.…”

Section: Introductionmentioning

confidence: 99%

Electrical analogs for Kelvin & Maxwell viscoelastic materials: Applications to cornea & sclera

Greene¹,

Medved²

2017

New Front Ophthalmol

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Purpose: The purpose of this report is to develop analog electrical circuits, using resistors, capacitors, and constant current sources, which automatically calculate the stress and strain-rate response of viscoelastic biomaterials in response to arbitrary loading history, and to compare these with experimental strain-rate results from sclera subjected to constant and square-wave pressure loads. Methods:Electro-mechanical models of reversible and irreversible viscoelasticity are analyzed, using series and parallel combinations of springs and dashpots from the Kelvin-Voigt, Maxwell, and Jeffrey's viscoelastic models. Experiments include strain-rate response of the sclera, applicable to the development of axial myopia. Results:The resulting strain ε(t) versus time t is shown to vary exponentially for Kelvin-Voigt, as a linear step-ramp for Maxwell, and as a curved step-ramp for Jeffrey's materials, consistent with experimental observations from cornea and sclera, corresponding to output voltage at the analog circuit capacitor V(t) in response to a step change in applied load σ(t) from 0 to σ o and a step change in applied current from 0 to Io at time t = 0. The cornea and sclera are rate-sensitive viscoelastic materials which stretch up to 12% in response to constant or repetitive loads. This is equivalent to an accumulated -9.00 diopters of axial myopia. Conclusions:The resulting analog equivalent circuits can be used as general purpose analog computers, to calculate system strain-rate ε(t), in response to arbitrary applied stress loading σ(t), including ramps, steps, sinusoids, and square waves, with variable intermittency factor. Results are applicable to collagen, cornea, sclera.

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“…Therefore, cross-linking treatment increases the ratio of elastic and viscous capacities of scleral tissue, and a robust elastic behaviour is a precondition for normal eye structure and function. Indeed, there is some evidence that the sclera of myopic eyes is less elastic and more viscous than that of normal eyes (Phillips et al 2000;McBrien & Gentle 2003;McBrien et al 2009). This indicates that cross-linking treatment would help to restore or maintain the elastic properties of the sclera.…”

Section: Cross-linking Effectsmentioning

confidence: 99%

“…Malignant myopia is associated with an excessive axial elongation of the eyeball probably caused by mechanical weakness, thinning and stretching of the sclera (Rada et al 2006;McBrien et al 2009;Sergienko & Shargorogska 2012). Therefore, various approaches were suggested to stiffen the biomechanically weakened scleral tissue by collagen cross-linking (Wollensak & Spoerl 2004;Wang et al 2012), and these approaches were critically discussed (Elsheikh & Phillips 2013).…”

Section: Introductionmentioning

confidence: 99%

Dose‐dependent collagen cross‐linking of rabbit scleral tissue by blue light and riboflavin treatment probed by dynamic shear rheology

Schuldt

Karl

Körber

et al. 2014

Acta Ophthalmologica

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ABSTRACT.Purpose: To determine the visco-elastic properties of isolated rabbit scleral tissue and dose-dependent biomechanical and morphological changes after collagen cross-linking by riboflavin/blue light treatment. Material: Scleral patches from 87 adult albino rabbit eyes were examined by dynamic shear rheology. Scleral patches were treated by riboflavin and different intensities of blue light (450 nm), and the impact on the visco-elastic properties was determined by various rheological test regimes. The relative elastic modulus was calculated from non-treated and corresponding treated scleral patches, and treatments with different blue light intensities were compared. Results: Shear rheology enables us to study the material properties of scleral tissue within physiological relevant parameters. Cross-linking treatment increased the viscous as well as the elastic modulus and changed the ratio of the elastic versus viscous proportion in scleral tissue. Constant riboflavin application combined with different blue light intensities from 12 mW/cm 2 up to 100 mW/cm 2 increased the relative elastic modulus of scleral tissue by factors up to 1.8. Further enhancement of the applied light intensity caused a decline of the relative elastic modulus. This might be due to destructive changes of the collagen bundle structure at larger light intensities, as observed by histological examination. Conclusion: Collagen cross-linking by riboflavin/blue light application increases the biomechanical stiffness of the sclera in a dose-dependent manner up to certain light intensities. Therefore, this treatment might be a suitable therapeutic approach to stabilize the biomechanical properties of scleral tissue in cases of pathological eye expansion.

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Biomechanics of the Sclera in Myopia: Extracellular and Cellular Factors

Cited by 228 publications

References 26 publications

Ultrasonic Measurement of Scleral Cross-Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera

Ultrasonic Measurement of Scleral Cross-Sectional Strains During Elevations of Intraocular Pressure: Method Validation and Initial Results in Posterior Porcine Sclera

Electrical analogs for Kelvin & Maxwell viscoelastic materials: Applications to cornea & sclera

Dose‐dependent collagen cross‐linking of rabbit scleral tissue by blue light and riboflavin treatment probed by dynamic shear rheology

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