In-vivo corneal pulsation in relation to in-vivo intraocular pressure and corneal biomechanics assessed in-vitro. An animal pilot study.

The aim was to ascertain whether the characteristics of the corneal pulse (CP) measured in-vivo in a rabbit eye change after short-term artificial increase of intraocular pressure (IOP) and whether they correlate with corneal biomechanics assessed in-vitro. Eight New Zealand white rabbits were included in this study and were anesthetized. In-vivo experiments included simultaneous measurements of the CP signal, registered with a non-contact method, IOP, intra-arterial blood pressure, and blood pulse (BPL), at the baseline and short-term elevated IOP. Afterwards, thickness of post-mortem corneas was determined and then uniaxial tensile tests were conducted leading to estimates of their Young's modulus (E). At the baseline IOP, backward stepwise regression analyses were performed in which successively the ocular biomechanical, biometric and cardiovascular predictors were separately taken into account. Results of the analysis revealed that the 3rd CP harmonic can be statistically significantly predicted by E and central corneal thickness (Models: R2 = 0.662, p < 0.005 and R2 = 0.832, p < 0.001 for the signal amplitude and power, respectively). The 1st CP harmonic can be statistically significantly predicted by the amplitude and power of the 1st BPL harmonic (Models: R2 = 0.534, p = 0.015 and R2 = 0.509, p < 0.018, respectively). For elevated IOP, non-parametric analysis indicated significant differences for the power of the 1st CP harmonic (Kruskal-Wallis test; p = 0.031) and for the mean, systolic and diastolic blood pressures (p = 0.025, p = 0.019, p = 0.033, respectively). In conclusion, for the first time, the association between parameters of the CP signal in-vivo and corneal biomechanics in-vitro was confirmed. In particular, spectral analysis revealed that higher amplitude and power of the 3rd CP harmonic indicates higher corneal stiffness, while the 1st CP harmonic correlates positively with the corresponding harmonic of the BPL signal.

Age-Related Changes in Corneal Deformation Dynamics Utilizing Scheimpflug Imaging.

PURPOSE: To study age-related changes in corneal deformation response to air-puff applanation tonometry. METHODS: Fifty healthy subjects were recruited for a prospective study and divided into two equal age groups (≤ 28 and ≥ 50 years old). Up to three measurements by a corneal deformation analyser based on the Scheimpflug principle were performed on the left eye of each subject. Raw Scheimpflug images were used to extract changes in anterior and posterior corneal profiles, which were further modelled by an orthogonal series of Chebyshev polynomial functions. Time series of the polynomial coefficients of even order exhibited a dynamic behavior in which three distinct stages were recognized. A bilinear function was used to model the first and the third stage of corneal dynamics. Slope parameters of the bilinear fit were then tested between the two age groups using Wilcoxon rank sum test and two-way non-parametric ANOVA (Friedman) test. RESULTS: Statistically significant changes (Wilcoxon test, P<0.05) between the age groups were observed in the phase of the second applanation dynamics for the posterior corneal profile. In a two-way comparison, in which the corneal profile was used as a dependent variable, statistically significant changes (ANOVA/Friedman test, P = 0.017) between the groups were also observed for that phase. CONCLUSION: Corneal biomechanics depend on age. The changes in corneal deformation dynamics, which correspond to mostly free return of the cornea to its original shape after the air pulse, indicate that the age related differences in corneal biomechanics are subtle but observable with high speed imaging.

Assessing subject-related variations of the Ocular Response Analyzer parameter calculation.

BACKGROUND: The aim was to study the relationships between the output parameters of the Ocular Response Analyzer (ORA) and those calculated from the raw ORA data and to ascertain the subject-related variations of ORA parameter calculation procedures. METHOD: Six subjects were recruited for a prospective study. Up to 32 measurements by ORA were performed in series on the dominant eye of each subject. A relationship was examined between Goldmann-correlated intraocular pressure values (IOPg) obtained from the standard ORA output and IOPg' calculated from raw ORA data with a custom-written procedure. The same analysis was carried out for the parameters of corneal hysteresis (CH and CH'). Data and statistical analysis included Epanechnikov kernel smoothing, orthogonal linear regression, hypothesis testing and bootstrap techniques. RESULTS: The group average (mean ± standard deviation) IOPg and CH values were 11.6 ± 1.8 mmHg and 10.7 ± 1.7 mmHg, respectively. A strong correlation was found between IOPg and IOPg' and also between CH and CH' parameters. There was a significant (Behrens-Fisher test, p < 0.001) difference between subjects for both IOPg and CH calculations, in terms of the regression slope parameter. CONCLUSIONS: Subject-related variations of ORA parameter calculation were demonstrated. This could indicate that currently employed estimators of IOP parameters include unreported algorithmic procedures that may lead to biased results.

Corneal deformation dynamics in normal and glaucoma patients utilizing scheimpflug imaging.

The purpose of this study was to explore the corneal deformation dynamics recorded in raw Scheimpflug images of CorVis ST tonometer to evaluate age- and glaucoma-related changes in corneal biomechanics. This was a prospective study in which 18 subjects were recruited and divided into three equal groups (healthy young, healthy older and older glaucoma group). Up to three measurements by CorVis ST tonometer were acquired on the left eye of each subject. Raw Scheimpflug images of CorVis ST were used to extract changes in anterior and posterior corneal profiles, which were further modeled by a Chebyshev polynomial expansion of optimally determined order. Corneal deformation dynamics were studied via time-varying series of Chebyshev polynomial coefficients, in which three phases of corneal deformation were determined. The first and the last phase of corneal dynamics were fitted with bilinear functions. Further, the slope of each linear trend in the model was tested between the groups. Statistically significant differences were observed in the final return phase of corneal dynamics between the older group of subjects and those of glaucoma subjects indicating possible biomechanical differences in their tissues that could only be observed with minimum applied pressure.