Inflation experiments and inverse finite element modelling of posterior human sclera.

The complexity of inverse finite element modelling methods used in ocular biomechanics research has significantly increased in recent years in order to produce material parameters that capture microscale tissue behaviour. This study presents a more accessible method for researchers to optimise sclera material parameters for use in finite element studies where macroscale sclera displacements are required. Five human donor sclerae aged between 36 and 72 years were subjected to cycles of internal pressure up to 61 mmHg using a custom-built inflation rig. Displacements were measured using a laser beam and two cameras through a digital image correlation algorithm. Specimen-specific finite element models incorporating regional thickness variation and sclera surface topography were divided into six circumferential regions. An inverse finite element procedure was used to optimise Ogden material parameters for each region. The maximum root mean squared (RMS) error between the numerical and experimental displacements within individual specimens was 17.5 µm. The optimised material parameters indicate a gradual reduction in material stiffness (as measured by the tangent modulus) from the equator to the posterior region at low-stress levels up to 0.005 MPa. The variation in stiffness between adjacent regions became gradually less apparent and statistically insignificant at higher stresses. The study demonstrated how inflation testing combined with inverse modelling could be used to effectively characterise regional material properties capable of reproducing global sclera displacements. The material properties were found to vary between specimens, and it is expected that age could be a contributing factor behind this variation.

Outcome of Cytomegalovirus Colitis in Inflammatory Bowel Disease with Different Regimes of Ganciclovir.

BACKGROUND Cytomegalovirus (CMV) infection is common in individuals with inflammatory bowel disease (IBD) and is responsible for relapse, increased severity, and poor outcome if left untreated. Ganciclovir is the mainstay of treatment but data regarding its use, mode of administration, and duration of treatment is poorly described. We reviewed the practice of treating CMV colitis with different regimes of ganciclovir at a district NHS hospital to compare the clinical outcome. METHODS 35 patients with IBD and concurrent diagnosis of CMV infection were evaluated. The parameters studied were clinical outcome in term of clinical response, length of hospital stay, readmission, or colectomy with three different regimes of ganciclovir, in addition to treatment for IBD. RESULTS 35 patients with IBD (ulcerative colitis = 23, Crohn's disease = 5, Indeterminate colitis = 7) and positive diagnosis of CMV infection were studied. Clinical outcome with two weeks of intravenous (IV) ganciclovir regime was superior than one week of IV ganciclovir and two weeks of oral Valganciclovir in term of clinical response on day 15 (95.8% vs 74%, 24.3%, respectively p = 0.45) and colectomy rate within 3 months (6.25% vs 27.3%, vs 25%, respectively). CONCLUSION CMV colitis is associated with poor outcome in patient with IBD if left untreated. 2 weeks IV ganciclovir was associated with a better outcome than 1 week of IV treatment or oral treatment.

Stress free configuration of the human eye.

Numerical simulations of eye globes often rely on topographies that have been measured in vivo using devices such as the Pentacam or OCT. The topographies, which represent the form of the already stressed eye under the existing intraocular pressure, introduce approximations in the analysis. The accuracy of the simulations could be improved if either the stress state of the eye under the effect of intraocular pressure is determined, or the stress-free form of the eye estimated prior to conducting the analysis. This study reviews earlier attempts to address this problem and assesses the performance of an iterative technique proposed by Pandolfi and Holzapfel [1], which is both simple to implement and promises high accuracy in estimating the eye's stress-free form. A parametric study has been conducted and demonstrated reliance of the error level on the level of flexibility of the eye model, especially in the cornea region. However, in all cases considered 3-4 analysis iterations were sufficient to produce a stress-free form with average errors in node location <10(-6)mm and a maximal error <10(-4)mm. This error level, which is similar to what has been achieved with other methods and orders of magnitude lower than the accuracy of current clinical topography systems, justifies the use of the technique as a pre-processing step in ocular numerical simulations.

Strain-rate sensitivity of porcine and ovine corneas.

Knowledge of strain-rate sensitivity of corneal tissue is important for improving the understanding of the tissue's response to mechanical actions and the accurate numerical simulation of corneal biomechanical behaviour under the effects of disease and surgery. In the study, fresh and well-preserved porcine and ovine corneal buttons were subjected to uniaxial tension loads with seven different strain rates ranging between 0.8 and 420% per minute. All specimens exhibited increased stiffness (as measured by the tangent modulus) with higher strain rates. However, clear differences in their behaviour were observed. While ovine corneas showed gradual, consistent and mostly statistically significant increases in stiffness with all elevations in strain rate, porcine corneas' response was significant over only a limited range of low strain rates. The effect of strain rate on the material's stress-strain behaviour was considered in the formation of three sets of constitutive models including: (i) a model based on a simple exponential stress-strain relationship, (ii) the Ogden model that considers the tissue's hyperelasticity but not anisotropy, and (iii) a third model by Holzapfel, Gasser and Ogden that considers both hyperelasticity and anisotropy. The three models are introduced to enable consideration of the strain rate effects in simulations employing finite element programs with varying capabilities or in modelling applications in corneal biomechanics which may or may not require consideration of mechanical anisotropy.

Biometry of the cornea in myopic chinese patients.

PURPOSE: To develop a close-fit mathematical model of corneal anterior and posterior topography and investigate the variation of corneal topography with biometric parameters in a group of young adult myopic Chinese patients. METHODS: Corneal topography data were acquired from 112 eyes of 61 myopic patients (mean age: 24.10±3.66 years). Videokeratoscopic images were recorded for anterior and posterior corneal surfaces using an Orbscan IIz topography system (Bausch & Lomb), and elevation data were fit to ellipsoid, hyperboloid, and paraboloid equations. Axial length, anterior chamber depth, refractive error, corneal curvature, central corneal thickness, and white-to-white distance were also measured and statistically analyzed. RESULTS: The ellipsoid equation provided the best fit for the anterior and posterior corneal surfaces in all patients. Corneal asymmetry slightly improved the accuracy of the mathematical model and reduced errors of fit with the clinical data. Strong evidence of right and left corneal symmetry was obtained in the form of similar corneal curvature, shape factor, and thickness values. The refractive error showed strong correlation with axial length (r=-0.586, P<.001) but not with corneal curvature or anterior chamber depth, indicating that myopia may be related to scleral stretching and globe elongation rather than changes in corneal curvature or anterior chamber depth. CONCLUSIONS: Axial length appears to be the main morphological parameter related to myopia. Myopic corneas took the form of a rotationally asymmetric prolate ellipsoid that gradually flattened towards the periphery. The study also provided evidence of symmetry between an individual's right and left corneas.

The influence of lamellar orientation on corneal material behavior: biomechanical and structural changes in an avian corneal disorder.

PURPOSE: Retinopathy, globe enlarged (RGE) is an inherited genetic disease of chickens with a corneal phenotype characterized by loss of tissue curvature and changes in peripheral collagen fibril alignment. This study aimed to characterize the material behavior of normal and RGE chicken corneas under inflation and compare this with new spatial- and depth-resolved microstructural information to investigate how stromal fibril architecture determines corneal behavior under intraocular pressure (IOP). METHODS: Six RGE chicken corneas and six age-matched normal controls were tested using trephinate inflation and their stress-strain behavior determined as a function of posterior pressure. Second harmonic generation mulitphoton microscopy was used to compare the in-plane appearance and degree of through-plane interlacing of collagen lamellae between normal and mutant corneas. RESULTS: RGE corneas displayed a 30-130% increase in material stiffness [E(tangent)(RGE) = 0.94 ± 0.18 MPa to 3.09 ± 0.66 MPa; E(tangent)(normals) = 0.72 ± 0.13 MPa to 1.34 ± 0.35 MPa] (P ≤ 0.05). The normal in-plane disposition of anterior collagen in the peripheral cornea was altered in RGE but through-plane lamellar interlacing was unaffected. CONCLUSIONS: This article demonstrates changes in corneal material behavior in RGE that are qualitatively consistent with microstructural collagen alterations identified both herein and previously. This study indicates that, in general, changes in stromal fibril orientation may significantly affect corneal material behavior and thereby its response to IOP.