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Abstract
The Ocular Response Analyser (ORA; Reichert Ophthalmic Instruments, New York) is a clinical instrument that measures parameters called corneal hysteresis (CH) and corneal resistance factor (CRF), which are believed to be corneal biomechanical responses. These variables have experienced increasing interest in ophthalmic practice. However, there is little experimental work characterising the ocular determinants of CH and CRF. The aim of this thesis was to describe these variables clinically.
Three studies in young normals were conducted as a part of this dissertation. Firstly, normative cross-sectional data were collected, including the central corneal radius (autokeratometry), presumed corneal responses (ORA), intraocular pressure (ORA and Goldmann applanation tonometer (GAT)) and central corneal thickness (CCT; ultrasonic pachymetry). In a multivariable model, CRF was significantly related to GAT, and caused the association between CCT and GAT to become redundant. CH was related to CCT, but not GAT.
The diurnal variations of each parameter and their potential covariations were then studied. With the diurnal changes in GAT as the outcome variable, bivariate analyses showed a potential influence of the intradiem fluctuations in CCT and CRF, however the former predictor lost its significance to the latter in a multivariable model. The diurnal changes in CH were related to those of CCT, but not GAT.
Lastly, the influence of contact lens induced corneal swelling on the measured variables was investigated. The presence of corneal swelling caused an increase in GAT that could be predicted by the changes in CRF, and affected the relationships that CH and CRF had with CCT.
The positive association between CCT and GAT is commonly interpreted as increasing corneal rigidity causing tonometric estimation. Therefore, the current results indicate that CRF is also an estimate of corneal rigidity, given that CCT and CRF described the same underlying influence on GAT in the first two studies. Furthermore, CRF was able to predict the overestimation in GAT resulting from corneal changes associated with increased corneal hydration. The consistently observed associations between CH and CCT suggest that CH is also a measure of corneal behaviour, however the specific aspect/s quantified are unclear.