Ahmed Abass

Transverse depth-dependent changes in corneal collagen lamellar orientation and distribution

It is thought that corneal surface topography may be stabilized by the angular orientation of out-of plane lamellae that insert into the anterior limiting membrane. In this study, micro-focus X-ray scattering data were used to obtain quantitative information about lamellar inclination (with respect to the corneal surface) and the X-ray scatter intensity throughout the depth of the cornea from the centre to the temporal limbus. The average collagen inclination remained predominantly parallel to the tissue surface at all depths. However, in the central cornea, the spread of inclination angles was greatest in the anterior-most stroma (reflecting the increased lamellar interweaving in this region), and decreased with tissue depth; in the peripheral cornea inclination angles showed less variation throughout the tissue thickness. Inclination angles in the deeper stroma were generally higher in the peripheral cornea, suggesting the presence of more interweaving in the posterior stroma away from the central cornea. An increase in collagen X-ray scatter was identified in a region extending from the sclera anteriorly until about 2 mm from the corneal centre. This could arise from the presence of larger diameter fibrils, probably of scleral origin, which are known to exist in this region. Incorporation of this quantitative information into finite-element models will further improve the accuracy with which they can predict the biomechanical response of the cornea to pathology and refractive procedures.

A wide-angle X-ray fibre diffraction method for quantifying collagen orientation across large tissue areas: application to the human eyeball coat

A quantitative map of collagen fibril orientation across the human eyeball coat, including both the cornea and the sclera, has been obtained using a combination of synchrotron wide-angle X-ray scattering (WAXS) and three-dimensional point mapping. A macromolecular crystallography beamline, in a custom-modified fibre diffraction setup, was used to record the 1.6 nm intermolecular equatorial reflection from fibrillar collagen at 0.5 mm spatial resolution across a flat-mounted human eyeball coat. Fibril orientation, derived as an average measure of the tissue thickness, was quantified by extraction of the azimuthal distribution of WAXS scatter intensity. Vector plots of preferential fibre orientation were remapped onto an idealized eyeball surface using a custom-built numerical algorithm, to obtain a three-dimensional representation of the collagen fibril architecture.

Changes in Scleral Collagen Organization in Murine Chronic Experimental Glaucoma

PURPOSE The organization of scleral collagen helps to determine the eyes biomechanical response to intraocular pressure (IOP), and may therefore be important in glaucoma. This study provides a quantitative assessment of changes in scleral collagen fibril organization in bead-induced murine experimental glaucoma. METHODS Wide-angle X-ray scattering was used to study the effect of bead-induced glaucoma on posterior scleral collagen organization in one eye of 12 CD1 mice, with untreated fellow eyes serving as controls. Three collagen parameters were measured: the local preferred fibril directions, the degree of collagen anisotropy, and the total fibrillar collagen content. RESULTS The mouse sclera featured a largely circumferential orientation of fibrillar collagen with respect to the optic nerve head canal. Localized alteration to fibril orientations was evident in the inferior peripapillary sclera of bead-treated eyes. Collagen anisotropy was significantly (P<0.05) reduced in bead-treated eyes in the superior peripapillary (Treated: 43±8%; CONTROL 49±6%) and midposterior (Treated: 39±4%; CONTROL 43±4%) sclera, and in the peripapillary region overall (Treated: 43±6%; CONTROL 47±3%). No significant differences in total collagen content were found between groups. CONCLUSIONS Spatial changes in collagen fibril anisotropy occur in the posterior sclera of mice with bead-induced chronic IOP elevation and axonal damage. These results support the idea that dynamic changes in scleral form and structure play a role in the development of experimental glaucoma in mice, and potentially in human glaucoma.

Automotive Driveline Modelling, Inverse-Simulation and Compensation

A nonlinear automotive driveline model and its inverse simulation are developed. The model contains four inertias comprising engine, clutch, transmission, coupling, and wheels. In this paper both nonlinear effects arising from the clutch and backlash are considered. The backlash in the proposed model is sandwiched between the compliance of the clutch and the compliance of the drive shafts. A basic simulation approach is outlined for obtaining the Horowitz linear time-invariant equivalent (LTIE) of the automotive driveline model by a nonparametric identification using a Windowed Fourier Transformation (WFT) method. The inverse simulation model is used as a compensator and is shown to achieve calibrated uncertainty reductions over the crossover frequency range. The reduction in the equivalent linear uncertainty of the nonlinearity available from the inverse compensation is demonstrated by the WFT analysis.

Automotive driveline control by a nonlinear nonparametric QFT method

A new nonlinear Quantitative Feedback Theory (QFT) methodology is presented and applied to the control of the automotive driveline with nonlinear clutch and backlash. A driveline model is presented which incorporates the nonlinear clutch characteristics and the coupling backlash realistically sandwiched by compliant inertias. The model gives responses that are representative of typical driveline systems, with the backlash accentuating the magnitude of oscillatory shuffle response. A Nonparametric (NP) identification method is given for directly developing the QFT templates of the Linear Time-Invariant Equivalent (LTIE) plant model set. The identification uses a Discrete Fourier Transform (DFT) to convert the system data to the frequency domain (FD). To mitigate against windowing errors the frequency response (FR) data is locally smoothed by a sliding frequency weighting function. The LTIE set is obtained as set of NP identified models obtained directly from typical vehicle experimental input-output test data. Non-minimum phase (NMP) QFT bounds are determined from the nominal NP plant by determining the Hubert transform of the NMP plant. The controller design methodology is applied to the model and it is shown that predefined robust tracking and disturbance response performance specifications can be systematically obtained, without subsequent heuristic parameter tuning.

SAXS4COLL: an integrated software tool for analysing fibrous collagen-based tissues

SAXS4COLL is an interactive computer program for reduction and analysis of small-angle X-ray scattering data from fibrous collagen tissues, combining data reduction, bespoke background subtraction, semi-automated peak detection and calibration.

Assessment of the Ocular Response Analyzer as an Instrument for Measurement of Intraocular Pressure and Corneal Biomechanics

Abstract Purpose: The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP. Materials and Methods: A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r2 between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA’s measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results. Results: The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = −2) to (k1 = −2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets. Conclusions: The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea’s stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.

Non-Orthogonal Corneal Astigmatism among Normal and Keratoconic Brazilian and Chinese populations

ABSTRACT Purpose: To investigate the prevalence of non-orthogonal astigmatism among normal and keratoconic Brazilian and Chinese populations. Methods: Topography data were obtained using the Pentacam High Resolution (HR) system ® from 458 Brazilian (aged 35.6 ± 15.8 years) and 505 Chinese (aged 31.6 ± 10.8 years) eyes with no history of keratoconus or refractive surgery, and 314 Brazilian (aged 24.2 ± 5.7 years) and 74 Chinese (aged 22.0 ± 5.5 years) keratoconic eyes. Orthogonal values of optical flat and steep powers were determined by finding the angular positions of two perpendicular meridians that gave the maximum difference in power. Additionally, the angular positions of the meridians with the minimum and maximum optical powers were located while being unrestricted by the usual orthogonality assumption. Eyes were determined to have non-orthogonal astigmatism if the angle between the two meridians with maximum and minimum optical power deviated by more than 5° from 90°. Results: Evidence of non-orthogonal astigmatism was found in 39% of the Brazilian keratoconic eyes, 26% of the Chinese keratoconic eyes, 29% of the Brazilian normal eyes and 20% of the Chinese normal eyes. Conclusions: The large percentage of participants with non-orthogonal astigmatism in both normal and keratoconic eyes illustrates the need for the common orthogonality assumption to be reviewed when correcting for astigmatism. The prevalence of non-orthogonality should be considered by expanding the prescription system to consider the two power meridians and their independent positions.

Positions of Ocular Geometrical and Visual Axes in Brazilian, Chinese and Italian Populations

ABSTRACT Purpose: To identify the relative positions of geometrical and visual axes of the eye and present a method to locate the visual center when the geometrical axis is taken as a reference. Methods: Topography elevation data was collected using a Pentacam HR ® topographer from 2040 normal eyes of 1020 healthy participants in Brazil, China and Italy. A three-dimensional, rotation algorithm, a first-order Zernike polynomial fit and a nonlinear least-squares error function was used within an optimization function to locate the geometrical axis and the visual center of each eye. Results: The right eyes of participants were significantly more tilted than left eyes throughout the topography scanning process (p < 0.001). The visual centers were always located in the nasal-superior quadrant, although the visual centers of fellow eyes were not symmetrically located. Mean distances between the visual center and the geometrical center in right eyes were 0.8 ± 0.29 mm, 0.56 ± 0.18 mm and 0.91 ± 0.34 mm among Brazilian, Chinese and Italian participants, respectively, and located at angular positions of 38.7 ± 24.5°, 23.0 ± 29.8° and 23.1 ± 28.1° from the nasal side. However, in left eyes, mean distances were 0.76 ± 0.33 mm, 0.45 ± 0.12 mm and 0.75 ± 0.33 mm at polar angles from the nasal side of 59.3 ± 29.0°, 50.6 ± 44.5° and 61.8 ± 34.1°, respectively. Conclusions: Fellow eyes do not perform similarly during the fixation process, with right eyes tending to tilt more than left eyes, and the visual centers of the fellow eyes positioned differently relative to the geometrical centers.

Driveline Launch Control by a Test-Based Nonparametric QFT Method

Abstract A novel Nonparametric (NP) Nonlinear Quantitative Feedback Theory (QFT) robust control design method is experimentally applied to the driveline launch, tracking and shuffle control problem. In the experimental setup the engine Air Bleed Valve (ABV) duty cycle (DTY) is used as an excitation input signal and the vehicle wheel speed is the controlled output. The input signal is formed as PRBS with different amplitudes and different Perturbation Periods (PPs). The NP-QFT design technique uses the discrete Hilbert transform to determine the necessary phase shift to produce the stable minimum phase (SMP) nominal plant required by the QFT approach for the non minimum phase (NMP) driveline plant. The tracking performance for the driveline with the designed controller is tested on a chassis dynamometer by several step speed demands and was found to fall within the specified response boundaries. The controlled system disturbance rejection is tested by applying a step torque to the chassis dynamometer rollers against the wheel rotation direction. The resulting speed disturbance is rejected and remains within the required limits. In contrast to previously proposed techniques the new approach does not require any parametric model and relies entirely on the non-parametric frequency response characterisation of the vehicle driveline dynamics which can be obtained quickly from experimental input output data. The NP-QFT technique is entirely systematic and does not rely on trial and error (cut and try) procedures and should significantly speed the industrial development of driveline control systems.