Show simple item record

dc.contributor.supervisorBuckhurst, Hetal
dc.contributor.authorOehring, Daniela
dc.contributor.otherFaculty of Health and Human Sciencesen_US
dc.descriptionTo evaluate current methods of assessing in vivo corneal biomechanics and to develop improved characterisation metrics a comprehensive evaluation of the physical attributes of the stress applied to the cornea is necessary, Figure 14 1. As such, the first part of the thesis evaluated the repeatability of the spatial and temporal distribution of the CST and ORA airflow and examined the influence of varying nozzle-to-cornea distance. Initial examination estimated the spatial distribution of an air puff (Experiment I: Two-dimensional shape characteristics of the CST air puff). Application of the CST air puff to samples of calcium alginate demonstrated that the device produced highly repeatable crater indentations (5.5%, n=100). The shape of the crater opening was found to show an elliptical conformation, which may have been influenced by the experimental set-up. Further analysis of the CST and ORA air puff characteristics was carried out with a gauge pressure sensor (Experiment II: Analysis and evaluation of the NCT air puff pressure flow). The maximal pressure was found centrally with the CST (96.4mmHg) after 15.013ms and lasted for 21.483ms; in comparison, the maximum pressure for the in ORA (91.7mmHg) occurred after 15.453ms and lasted for 23.061ms. The total spatial dimension of the CST and ORA air puffs were 45.2mm2 and 35mm2, respectively. The CST airflow pressure was highly repeatable, whereas the ORAs showed lower levels of repeatability. The nozzle-to-cornea distance showed a significant impact on the pressure magnitude. In Chapters III section 8 and 9 the study examined the components of the corneal deformation during application of the airflow pressure. The CST provides high-speed Scheimpflug imaging during the indentation procedure. Using these images, the anterior and posterior surface of the cornea were tracked over time and for the horizontal Scheimpflug-image plane. This procedure results in a so-called displacement matrix (LCST[c,t,d]) representing the complete response of the cornea to the air puff application. The matrix was composed of three components: the initial corneal shape, the globe retraction (so-called whole eye movement [WEM]) and the corneal deflection. The corneal deflection can be seen as the pure response of the cornea to an air puff, thus containing the information about the corneal strain during the air puff application (Experiment III: Globe movement during an air puff measurement). To isolate the corneal deflection component from the corneal deformation, the initial shape and the WEM need to be removed from the corneal deformation matrix. The initial corneal shape, could be removed by subtracting the y-position of the neutral plane at the initial frame from the corneal deformation. But the WEM required further investigation. The evaluation of the WEM via a high-speed camera system confirmed that the CST WEM represents the bulbus retraction and protraction caused by the air puff during NCT. The maximum globe retraction during NCT was found to be in the amplitude by 317µm after 21.8ms. The palpebral aperture size was shown to have no significant influence on the bulbus retraction. The accuracy of the amplitude and time were found to be 6.0%, and 1.4%, respectively. Significant nasal and temporal asymmetry was observed for the bulbus retraction, which was found to affect measures of IOP. The novel method determines the corneal deflection caused by an air puff which was corrected by the WEM. Knowing the distribution of the air puff (stress) and the isolated response of the cornea to an air puff (strain), novel corneal biomechanical properties were derived (Experiment VI: Novel ocular biomechanical parameter). By tracing the anterior and posterior corneal surface of the CST Scheimpflug images raw measures of dynamic corneal changes were extracted. Applying material engineering laws to these measures provided the means to derive novel kinematic and kinetic mechanical properties during dynamic load of the cornea. The within-subject repeatability for longitudinal and lateral strain were found to be in the order of 6.9% and 2.6%. Overall, the repeatability of the kinematic and kinetic parameters was found to be high. Since it is assumed, that the corneal biomechanical properties are dependent on IOP, the novel parameters needed to be evaluated for their variability and dependency across a wide range of IOPs (Experiment V: Evaluation of corneal biomechanical properties using ex vivo manometry). Using ex vivo porcine eyes and a manometry set-up, no significant differences were observed for variability of corneal biomechanics across manometry intraocular pressure (MIOP) levels between 13 to 50mmHg (ICC longitudinal strain 0.889, lateral strain 0.655). The IQR correlated positively with MIOP for the maximum strain, the time when the maximum strain occurred, hysteresis ( ) and EDyn ( ) (|r|>0.3 p<0.050). The EDyn and compressibility for longitudinal and lateral strain were significantly impacted by MIOP (pred. R2: 0.549, 0.404). Using kinematic and kinetic material properties as predictors, the MIOP was modelled by A1(length), EDyn ( ), A1(t) and ||(t0) (verification: adj. R2 of 0.789 [p=0.000]). Much of the literature on in vivo corneal biomechanics concerns the central cornea and little is known about the material properties of paracentral and peripheral regions. Therefore, to address this shortfall in the literature Chapter V section 11 describes a novel set-up to assess the peripheral cornea along the horizontal and vertical meridians with the CST and ORA (Experiment V: Experimental developments for assessing the regional variation of corneal biomechanics in vivo). The study examines the validity and reliability for off-central measures of corneal biomechanics. Paracentral and peripheral zones were considered as 40% and 80% distance from the centre, which equated to distances of 60% and 20% from the limbus respectively. Although the proposed method exhibited several limitations, nine clearly distinguishable corneal positions were assessed with the CST, and five during ORA-measurement. To provide a normative dataset for further studies, an exploratory one-visit, prospective, cross-sectional and partly randomised study was conducted, assessing healthy, human adults (n=113; 71% females; aged (24.5 ± 6.11) years) (Experiment VI: Ocular biometry). A novel method was developed to analyse the corneal thickness profile across the entire cornea; the data revealed four different profiles to characterise peripheral CT. The geometrical distortion of AS-OCT images using Casia SS-100 OCT was evaluated and it could be shown, that when using the recommended set-up, the geometrical distortion of the OCT images was negligible. A novel method to analyse the limbus architecture using AS-OCT imaging was proposed. The intra- and interobserver reliability of limbus and scleral thickness measurements using AS-OCT images was found to be high (ICC>0.8). The interdevice comparison between WAM-5500 and OPD-Scan III showed no significant difference. The study provides substantial and comprehensive knowledge of the ocular biometry in healthy human adults (study with the largest sample size in controlled conditions assessing the included parameters). Intraindividual differences in topography, corneal thickness, anterior chamber characteristics and scleral profile are discussed. The corresponding effect size of significant effects of demographics and intereye correlation on ocular biometry was found to be weak for gender and ethnicity but moderate to strong for age. The influence of both ocular biometry and demographical factors on measures of corneal biomechanics were examined (Experiment VII: Corneal biomechanics of healthy, human, adult eyes). The study found that the cornea could be described as a viscoelastic, damped system for longitudinal strain (stretch or elongation) and a highly oscillating system for lateral strain (compression). The cornea was found show homogenous tendencies in regards to its rigidity and EDyn whereas other material characteristics showed higher level of heterogeneity, largest variation was observed for compressibility. Demographic and biometric characteristics showed no significant effect on corneal biomechanics. Whereas the dynamic corneal response assessed by the CST and the ORA metrics were found to be highly dependent on the ocular architecture. Using ocular architecture and CST parameters concerning the dynamic corneal response as predictors, a linear regression model that explains the variability of ORA measures of CH and CRF to 99 and 98% is proposed.en_US

The thesis investigates methods of examining corneal biomechanics using non-contact tonometry and introduces novel techniques to investigate corneal material properties in vivo. A comprehensive systems analysis of the CorvisST (CST) and Ocular Response Analyser (ORA) was performed. Pressure sensors were used to characterisation the airflow produced by the CST and the ORA. Distinct differences were observed between the central airflow pressures between the two devices: the CST pressure was higher and of shorter duration. Scheimpflug high-speed imaging via the CST allowed components of the corneal deformation to be investigated and the development of a 3D deformation matrix (time, depth and spatial resolution) through tracing of the anterior and posterior corneal surface. Measures of whole eye movement (WEM) with CST were found to be robust. WEM demonstrated an asymmetric profile and a correction method was developed to address the corneal deformation matrix for this asymmetry. Novel methods for characterisation of intrinsic material characteristics of the cornea were developed using numerical and graphical analytical procedures. Application of these parameters was tested on enucleated porcine eyes across a wide range of manometry internal ocular pressure (MIOP). The dynamic E-Modulus was found to be most affected by MIOP change. To investigate the in vivo distribution and heterogeneity of the corneal biomechanics, a novel set-up allowed the mapping of corneal biomechanics across the cornea using the CST (central, paracentral, peripheral) and ORA (central, peripheral). Biometric and demographic grouping of subjects allowed detection of discriminating factors between individuals. The results suggest that the in vivo cornea of healthy human adults can be characterised as a viscoelastic, damped system for longitudinal strain and a highly oscillating system for lateral strain. The cornea is approximately homogenous for measures of rigidity and dynamic E-Modulus but other corneal material characteristics (longitudinal and lateral strain, hysteresis, damping and compressibility) demonstrated regional differences. The experimental design employed allowed for strict control of biometric and biomechanical intersubject variables, based on gold-standard techniques as well as newly-developed methods, thereby creating a normative database for future use.

dc.publisherUniversity of Plymouth
dc.rightsAttribution-NonCommercial-NoDerivs 3.0 United States*
dc.subjectBiomechanical propertiesen_US
dc.subjectAnterior ocular segmenten_US
dc.subjectCorneal hysteresisen_US
dc.subjectOcular Response Analyzeren_US
dc.subjectCorneal Young's modulusen_US
dc.subjectCorneal deflection behavioren_US
dc.subjectRegional variation of the corneal biomechanicsen_US
dc.subjectAirpuff tonometryen_US
dc.subjectNon contact tonometryen_US
dc.subjectAirstream NCTen_US
dc.subjectGlobe retractionen_US
dc.subjectWhole eye movementen_US
dc.titleOcular biomechanics of the anterior segmenten_US
dc.rights.embargoperiod12 monthsen_US

Files in this item


This item appears in the following Collection(s)

Show simple item record

Attribution-NonCommercial-NoDerivs 3.0 United States
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivs 3.0 United States

All items in PEARL are protected by copyright law.
Author manuscripts deposited to comply with open access mandates are made available in accordance with publisher policies. Please cite only the published version using the details provided on the item record or document. In the absence of an open licence (e.g. Creative Commons), permissions for further reuse of content should be sought from the publisher or author.
Theme by 
@mire NV