Ahmed Elsheikh

Assessment of Corneal Biomechanical Properties and Their Variation with Age

Purpose: The aim of this study was to determine the stress-strain behavior of corneal tissue and how the behavior was affected by age. Methods: Human corneal specimens ranging in age between 50 and 95 years were tested under inflation conditions to determine their stress-strain behavior. The corneas were subjected to two load rates that represent dynamic and static loading conditions. The pressure-deformation results were analyzed using shell theory to derive the stress-strain behavior. Results: The corneas demonstrated clear nonlinear behavior with an initial low stiffness stage and a final high stiffness stage. The transition between the two stages coincided with intraocular pressures between 12 and 20 mmHg. There was a considerable increase in stiffness associated with both age and load rate. Equations were derived to describe the nonlinear stress-strain relationship of corneal tissue for any age between 50 and 95 years, and these equations are presented in a form suitable for use in numerical simulations. Conclusions: The cornea demonstrates considerable stiffening with age with the behavior closely fitting an exponential power function typical of collagenous tissue. The increase in stiffness could be related to the additional age-related nonenzymatic cross-linking affecting the stromal collagen fibrils.

Application of structural analysis to the mechanical behaviour of the cornea

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.

Comparative study of corneal strip extensometry and inflation tests

Strip extensometry tests are usually considered less reliable than trephinate inflation tests in studying corneal biomechanics. In spite of the evident simplicity of strip extensometry tests, several earlier studies preferred inflation tests in determining the constitutive relationship of the cornea and its other material properties, such as Youngs modulus and the hysteresis behaviour. In this research, the deficiencies of the strip tests are discussed and a mathematical procedure presented to take account of these deficiencies when obtaining the corneal material properties. The study also involves testing 10 pairs of porcine corneas using both strip extensometry and trephinate inflation techniques and the results are subjected to mathematical back analysis in order to determine the stress–strain behaviour. The behaviour obtained from the strip extensometry tests and using the new mathematical analysis procedure is shown to match closely the inflation test results.

Biomechanical properties of human and porcine corneas.

The suitability of porcine corneas as approximate models for human corneas in mechanical property characterisation studies is experimentally assessed. Thirty seven human donor corneas and thirty four ex-vivo porcine corneas were tested under inflation conditions to determine their short-term stress-strain behaviour and long-term creep behaviour up to 2.8 h (10,000 s). Vertical strips extracted from further 12 human corneas and 10 porcine corneas were subjected to stress-relaxation tests for up to 20 min at different stress levels. Human and porcine corneas were observed to have almost the same form of behaviour under short and long-term loading. They both exhibited non-linear stress-strain behaviour and reacted to sustained loading in a similar fashion. However, human corneas were significantly stiffer than porcine corneas. They also crept less under long-term loading and could sustain their stress state for longer compared to porcine corneas. These differences, in addition to others identified earlier in relation to corneal mechanical anisotropy, cast doubt on the suitability of porcine corneas as models for human corneas in mechanical studies.

Determination of the modulus of elasticity of the human cornea.

PURPOSE To determine the material behavior of the human cornea in the form of simple relationships between the modulus of elasticity and intraocular pressure (IOP) and to establish the effect of age on the material behavior. METHODS Human corneal specimens with age between 50 and 95 years were tested under inflation conditions to determine their behavior. The corneas were subjected to two extreme load rates to represent dynamic and static loading conditions. The pressure-deformation results were analyzed using shell theory to derive the relationship between the modulus of elasticity and IOP. RESULTS The corneas demonstrated a nonlinear hyperelastic behavior pattern with an initial low stiffness stage and a final high stiffness stage. Despite the nonlinearity of the pressure deformation results, the relationship between the modulus of elasticity and the applied pressure was almost linear. A considerable increase was noted in the values of the modulus of elasticity associated with both age and load rate. General equations were derived to calculate the values of the secant and tangent moduli of elasticity in terms of IOP for any age greater than 50 years, and these equations are presented in a simple form suitable for use in numerical simulations. CONCLUSIONS Adequate representation of corneal material behavior is essential for the accurate predictive modeling of corneal biomechanics. The material models developed in this work could be implemented in numerical simulations of refractive surgery procedures, corneal shape changes due to contact lens wear, and other applications.

Characterization of age-related variation in corneal biomechanical properties

An experimental study has been conducted to determine the stress–strain behaviour of human corneal tissue and how the behaviour varies with age. Fifty-seven well-preserved ex vivo donor corneas aged between 30 and 99 years were subjected to cycles of posterior pressure up to 60 mm Hg while monitoring their behaviour. The corneas were mechanically clamped along their ring of scleral tissue and kept in physiological conditions of temperature and hydration. The tissue demonstrated hyper-elastic pressure-deformation and stress–strain behaviour that closely matched an exponential trend. Clear stiffening (increased resistance to deformation) with age was observed in all loading cycles, and the rate of stiffness growth was nonlinear with bias towards older specimens. With a strong statistical association between stiffness and age (p < 0.05), it was possible to develop generic stress–strain equations that were suitable for all ages between 30 and 99 years. These equations, which closely matched the experimental results, depicted corneal stiffening with age in a form suitable for implementation in numerical simulations of ocular biomechanical behaviour.

Regional variation in the biomechanical properties of the human sclera

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 scleras real-life conditions in numerical simulations.

Assessment of the epithelium's contribution to corneal biomechanics

Determining the epitheliums contribution to corneal biomechanics is important for the predictive numerical simulation of corneal biomechanical behaviour in which the corneas 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.

Nanopatterning of Collagen Scaffolds Improve the Mechanical Properties of Tissue Engineered Vascular Grafts

Tissue engineered constructs with cells growing in an organized manner have been shown to have improved mechanical properties. This can be especially important when constructing tissues that need to perform under load, such as cardiac and vascular tissue. Enhancement of mechanical properties of tissue engineered vascular grafts via orientation of smooth muscle cells by the help of topographical cues have not been reported yet. In the present study, collagen scaffolds with 650, 500, and 332.5 nm wide nanochannels and ridges were designed and seeded with smooth muscle cells isolated from the human saphenous vein. Cell alignment on the construct was shown by SEM and fluorescence microscopy. The ultimate tensile strength (UTS) and Youngs modulus of the scaffolds were determined after 45 and 75 days. Alamar Blue assay was used to determine the number of viable cells on surfaces with different dimensioned patterns. Presence of nanopatterns increased the UTS from 0.55 +/- 0.11 to as much as 1.63 +/- 0.46 MPa, a value within the range of natural arteries and veins. Similarly, Youngs modulus values were found to be around 4 MPa, again in the range of natural vessels. The study thus showed that nanopatterns as small as 332.5 nm could align the smooth muscle cells and that alignment significantly improved mechanical properties, indicating that nanopatterned collagen scaffolds have the potential for use in the tissue engineering of small diameter blood vessels.

Mechanical anisotropy of porcine cornea and correlation with stromal microstructure

An experimental study was conducted to determine the variation of biomechanical properties with anatomical orientation in porcine corneas. Strip specimens extracted from fresh porcine corneas were subjected to cycles of uniaxial tension while monitoring their behaviour. The specimens were extracted from either the superior-inferior (vertical), temporal-nasal (horizontal) or diagonal direction. Comparisons of behaviour were limited to specimens taken from the same animal to avoid the natural variation of biomechanical properties in corneas from different animals. The specimens were subjected to three different strain rates to check if the behaviour comparisons were affected by the corneas viscoelasticity. Overall, vertical and horizontal specimens were found to have almost the same behaviour, and both were marginally stiffer than diagonal specimens (by 2-8%). This almost isotropic biomechanical behaviour was compatible with earlier indications of similar collagen fibril densities in the three main orientations of porcine corneas.