Indirectly assessing changes in corneal properties with OCT speckle after crosslinking in porcine eyes.

The aim of this study was to assess the effect of the standard crosslinking (CXL) procedure on corneal properties and subsequent changes in collagen bonds formation using optical coherence tomography (OCT) corneal speckle statistics and vibrational spectroscopy. Porcine eyes with intact corneal epithelium were randomly selected to one of the four study groups: (1) untreated eyes moistened with phosphate-buffered saline (PBS); (2) eyes after the epithelial debridement and riboflavin application; (3) eyes after CXL procedure according to the Dresden protocol; and (4) eyes after corneal epithelial debridement, regularly moistened with PBS. Before and after this selection, each eyeball was subjected to the constant intraocular pressure of 20 mmHg. Then, ocular biometry was performed and the central cornea was imaged using spectral-domain OCT. Following this, a nonparametric approach to speckle modeling (the Contrast Ratio (CR)) was utilized within the region of interest for each B-scan covering the central corneal stroma. To verify whether the CXL performed ex-vivo results in formation of new bonds in the cornea, Fourier Transform Infrared Attenuated Total Reflectance (ATR-FTIR) spectra of dried corneas, dissected from examined eyeballs, were collected and analyzed. Corneal epithelium removal alone or with the riboflavin application leads to a statistically significant decrease in the CR median value (the Wilcoxon signed-rank test, p < 0.05). However, the most pronounced change in CR median value, which decreases with the increased number of scatterers, was shown after the complete CXL procedure including riboflavin soaking and UVA irradiation (the Wilcoxon signed-rank test, p = 0.004). Analysis of ATR-FTIR spectra revealed influence of UVA irradiation on collagen matrix. The study has shown the increased dehydration accompanied by almost no alteration of collagen native triple-helical structure. Significant changes have been observed for bands related to collagen crosslinks. Specifically, the predominant changes occurred in the sugar region from 1150 to 975 cm-1, as well as in the absorbance of carbonyl groups. Furthermore, the ratio of two Amide I components at approximately 1660 cm-1 and 1675 cm-1 decreased after UVA irradiation. Together, these results provided the evidence for the creation of new corneal crosslinks. In conclusion, this study clearly indicates that the UVA exposure causes the substantial difference in optical scattering occurring in corneal stroma as a result of the induced biochemical changes at the molecular level in this tissue assessed with ATR-FTIR. The proposed speckle-based methodology brings a new insight into the development of OCT technology useful in an indirect assessment of some collagen changes.

Tear Film Surface Quality in Modern Daily Disposable Contact Lens Wear.

OBJECTIVES: As reported previously, tear film surface quality (TFSQ) should be considered in contact lens (CL) fitting. This study followed noninvasive keratograph tear film break-up time (NIKBUT) in CL wearers for 12 months to validate its clinical utility in predicting CL performance. METHODS: Fifty-five subjects (M/F=17/38) aged 26±4 years were prescribed silicone hydrogel or hydrogel CLs. The study included baseline measurements without CLs; 2 visits for CL fitting and control; follow-up after 3, 6, and 12 months of CL wear; and postwear visit without CLs. Ocular Surface Disease Index (OSDI), 8-Item Contact Lens Dry Eye Questionnaire (CLDEQ-8), first and mean NIKBUT (F/M-NIKBUT), fluorescein tear film break-up time (FBUT), and ocular surface staining were evaluated. RESULTS: Post hoc analysis of each pair of visits showed differences between baseline and all CL visits for F-NIKBUT, M-NIKBUT, FBUT, and corneal staining. No difference was reported in symptoms. In addition, differences between baseline and postwear visits were noted in OSDI, M-NIKBUT, FBUT, and corneal staining, with three of the latter parameters showing a downward trend. CONCLUSIONS: No changes in TFSQ and symptoms were reported over 12 months. Introducing NIKBUT as part of routine CL fitting is advised to improve CL fit and predict success.

A 12-month Prospective Study of Tear Osmolarity in Contact Lens Wearers Refitted with Daily Disposable Soft Contact Lenses.

SIGNIFICANCE: Tear osmolarity has been reported as one of the best diagnostic tests for dry eye disease and its management. Contact lens wear is thought to influence tear homeostasis. Understanding how regular wear of contact lenses and their type affect tear osmolarity is important. PURPOSE: The purpose of this study was to assess changes in tear osmolarity and ocular symptoms over a period of 12 months following refitting contact lens wearers with daily disposable contact lenses. METHODS: Fifty subjects aged (mean ± SD) 26 ± 4 years were refitted with either silicone-hydrogel (delefilcon A, SiHy, n = 34) or hydrogel (omafilcon A, Hy, n = 16) daily disposable soft contact lenses. Study included seven visits: baseline measurements without contact lenses; two visits for contact lens fit and selection; follow-up measurements after 3, 6, and 12 months of contact lens wear; and a post-study visit without contact lenses. Lens type selection was based on a set of objective measurements. Subjects were instructed to follow strict wearing rules. Impedance-based osmometer was used to collect samples from the lower tear meniscuses. The Ocular Surface Disease Index (OSDI) and the eight-item Contact Lens Dry Eye Questionnaire were used to assess subject-reported ocular symptoms and contact lens discomfort, respectively. RESULTS: Downward trend of tear osmolarity was noted over the time course of study. Statistically significant differences were noted in tear osmolarity (Friedman test, χ = 21.91 [P < .001], and χ = 13.89 [P = .003]) for the right and left eyes, respectively. A statistically significant decrease in tear osmolarity was apparent only for initially symptomatic subjects (for OSDI ≥13: χ = 26.25, P < .001; for OSDI <13: χ = 5.65, P = .13). Statistically significant differences were not noted in OSDI and the eight-item Contact Lens Dry Eye Questionnaire between the baseline and 12-month visit (χ = 5.92 [P = .12] and R = 0.015 [P = .87], respectively). There was a difference between baseline and post-study visits (P = .002) in OSDI, showing lower score at the latter visit. CONCLUSIONS: Refitting with daily disposable contact lenses was demonstrated to lower tear osmolarity.

Assessing Corneal Speckle in Optical Coherence Tomography: A New Look at Glaucomatous Eyes.

SIGNIFICANCE: Optical coherence tomography speckle is used here in an unconventional way as the indirect source of information on tissue microstructure. The study reveals that the corneal speckle of glaucoma suspects has a similar relationship between the parameters of scattering exhibited in glaucoma patients. PURPOSE: The purpose of this study was to evaluate the potential of optical coherence tomography imaging of the cornea in early diagnosis of glaucoma. METHODS: Corneas of 64 subjects of three groups, including 18 primary open-angle glaucoma patients, 24 glaucoma suspects with normal levels of IOP and uncompromised visual field, and 22 age-matched controls, were imaged with spectral optical coherence tomography. A central region of interest in each optical coherence tomography image visualizing the stroma was automatically extracted, and the intensity data within that region were fit with the generalized gamma distribution. Its parameters describe the scatterer cross section and scatterer density and indirectly assess corneal microstructure. In addition, subjects underwent measurements of IOP, visual field, Heidelberg Retinal Tomography, retinal nerve fiber layer thickness, and biometry. Group means of all parameters were tested for equality. Within each group of subjects, correlation was evaluated between the statistical parameters of the corneal speckle. RESULTS: Glaucoma suspects had statistically significantly different IOP, visual field parameters, mean retinal fiber layer thickness, and central corneal thickness from those of glaucoma patients but not from those of the control group. The parameters of the corneal speckle were not significantly different between the groups, but they showed a markedly higher and statistically significant coefficient of determination for glaucoma patients and suspects than that for the control group (0.927 and 0.707 vs. 0.336). CONCLUSIONS: The results indicate that glaucoma suspects have similar relationship between the corneal scatterer cross section and scatterer density to that exhibited in the glaucoma patients but markedly different from that of healthy controls.

Supporting Dry Eye Diagnosis with a New Method for Noninvasive Tear Film Quality Assessment.

SIGNIFICANCE: Noninvasive high-speed videokeratoscopy equipped with specific software has shown potential for assessing the homeostasis of tear film, providing clinicians with a fast and consistent tool for supporting dry eye diagnosis and management. PURPOSE: The purpose of this study was to evaluate the efficacy of a recently proposed method for characterizing tear film dynamics using noninvasive high-speed videokeratoscopy in assessing the loss of homeostasis of tear film. METHODS: Thirty subjects from a retrospective study, of which 11 were classified as dry eye and 19 as healthy, were included. High-speed videokeratoscopy measurements were performed using E300 videokeratoscope (Medmont Pty., Ltd., Melbourne, Australia). Raw data were analyzed using a recently proposed method to estimate the dynamics of the tear film based on a fractal dimension approach. This method provides three time-varying indicators related to the regularity of the reflected rings: tear film surface quality indicator, breaks feature indicator, and distortions feature indicator. From each indicator, five parameters were extracted and analyzed, including noninvasive breakup time, mean value of the indicator in the stability phase, mean value of the indicator in the whole interblink interval, mean value of the indicator in the leveling phase, and the general trend of the time series. Receiver operating characteristic curves were used to determine the sensitivity and specificity of each parameter in dry eye detection. RESULTS: The best discrimination performance between dry eye and healthy subjects was achieved with the breaks feature indicator noninvasive breakup time parameter, with an area under the curve of 0.85. For a cutoff value of 10 seconds, the sensitivity was 100% and the specificity was 84%. CONCLUSIONS: The analyzed method improves the assessment of tear film homeostasis in comparison with previous high-speed videokeratoscopy methods showing higher potential in assisting dry eye diagnosis.

Assessment of Tear Film Using Videokeratoscopy Based on Fractal Dimension.

SIGNIFICANCE: The proposed automated approach for estimating the quality of the tear film closes the gap between the manual and automated assessment, translating the high-speed videokeratoscopy technology from scientific laboratories to a clinical practice. PURPOSE: To develop and test a new method for characterizing Tear Film Surface Quality with high-speed videokeratoscopy utilizing a fractal dimension approach. METHODS: The regularity of the reflected pattern in high-speed videokeratoscopy (E300; Medmont) depends on tear film stability. Thus, determining tear film stability can be addressed by estimating the fractal dimension of the reflected pattern. The method is tested on 39 normal subjects. The results of the fractal dimension approach are compared with those obtained using previously proposed automated method, based on a gray-level co-occurrence matrix approach, and with subjective results obtained by two operators that were assessing the video recordings in ideal conditions. RESULTS: Fractal dimension method was less affected by eye movements and changes in the videokeratoscopic image background than gray-level co-occurrence matrix method. Median difference of the noninvasive break-up time between manual and automated methods was 0.03 s (IQR = 4.47 s) and 0.0 s (IQR = 2.22 s) for gray-level co-occurrence matrix and fractal dimension approaches, respectively. Correlation coefficient with manual noninvasive break-up time was r = 0.86 (P < 0.001) for gray-level co-occurrence matrix approach, and r = 0.82 (P < 0.001) for fractal dimension approach. Significant statistical difference was found between noninvasive break-up measurements of manual and gray-level co-occurrence matrix method (P = 0.008). CONCLUSIONS: The proposed method has the potential to characterize tear film dynamics in more detail compared to previous methods based on high-speed videokeratoscopy. It showed good correlation with manual assessment of tear film.

Scleral asymmetry as a potential predictor for scleral lens compression.

PURPOSE: To identify the position and magnitude of lens compression due to short-term miniscleral contact lens wear, as well as evaluating the usefulness of scleral asymmetry as a predictor for scleral lens decentered compression. METHODS: Fourteen healthy subjects (mean ± S.D.: 29.2 ± 6.0 years) wore a highly gas-permeable spherical haptic miniscleral contact lens during a 5-h period. Corneo-scleral height Fourier profilometry was captured using an Eye Surface Profiler (www.eaglet-eye.com) before and immediately after lens removal. Scleral asymmetry, lens compression location and magnitude were processed using custom-made algorithms, both globally and for scleral quadrants. RESULTS: Miniscleral contact lenses do not set uniformly on the ocular surface, with the largest decentration seen along the horizontal meridian. The greatest flexural stress exerted by the lens on the ocular surface occurs at the point coinciding with the inner diameter landing point of the lens and not with its overall diameter. Scleral asymmetry was significantly correlated with compression location (R = 0.71, p = 0.002) and compression magnitude (R = 0.81, p < 0.001), showing its potential as compression predictor. CONCLUSION: Larger amounts of scleral asymmetry will lead to more decentration of spherical haptic scleral lenses. Objective and accurate methods, like the one presented here, could help the practitioner prevent cases of scleral blanching or discomfort due to an excessive compression by the lens.

Interaction of lysozyme with a tear film lipid layer model: A molecular dynamics simulation study.

The tear film is a thin multilayered structure covering the cornea. Its outermost layer is a lipid film underneath of which resides on an aqueous layer. This tear film lipid layer (TFLL) is itself a complex structure, formed by both polar and nonpolar lipids. It was recently suggested that due to tear film dynamics, TFLL contains inhomogeneities in the form of polar lipid aggregates. The aqueous phase of tear film contains lachrymal-origin proteins, whereby lysozyme is the most abundant. These proteins can alter TFLL properties, mainly by reducing its surface tension. However, a detailed nature of protein-lipid interactions in tear film is not known. We investigate the interactions of lysozyme with TFLL in molecular details by employing coarse-grained molecular dynamics simulations. We demonstrate that lysozyme, due to lateral restructuring of TFLL, is able to penetrate the tear lipid film embedded in inverse micellar aggregates.

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.

Scleral changes with accommodation.

PURPOSE: To assess whether the sclera changes its shape during accommodation, quantify those changes and ascertain whether they depend on age and refractive power. METHODS: Twenty-two subjects, aged between 19 and 84 years old were recruited for the study. Young subjects were grouped according to their refractive power as emmetropes (seven subjects) and myopes (seven subjects). Data were obtained with a height corneo-scleral topographer (Eye Surface Profiler) with coverage area far beyond the limbus. Lenses of different power were used to stimulate accommodative demand of 0.0 D, 2.5 D and 4.0 D. Elevation differences between measurements acquired while at the intermediate or close target and infinity where calculated for each participant for the horizontal meridian for a 16 mm chord. RESULTS: We found that sclera undergoes significant changes in its shape with accommodation in young subjects able to accommodate but not for those with limited accommodation. For a 4.0 D stimulus at the nasal side the scleral change amounted on average to 390 ± 330 µm (mean ± S.D.) for the young group. This phenomenon was more pronounced in myopes (for a 4.0 D target; nasal part: 560 ± 350 µm (mean ± S.D.)) than emmetropes (for a 4.0 D target; nasal part: 220 ± 120 µm (mean ± S.D.)). Differences were also found between different meridians. CONCLUSIONS: Scleral shape undergoes changes with accommodation and this phenomenon is more pronounced in myopes than emmetropes.

The effect of aberrations on objectively assessed image quality and depth of focus.

The effects of aberrations on image quality and the objectively assessed depth of focus (DoF) were studied. Aberrometry data from 80 young subjects with a range of refractive errors was used for computing the visual Strehl ratio based on the optical transfer function (VSOTF), and then, through-focus simulations were performed in order to calculate the objective DoF (using two different relative thresholds of 50% and 80%; and two different pupil diameters) and the image quality (the peak VSOTF). Both lower order astigmatism and higher order aberration (HOA) terms up to the fifth radial order were considered. The results revealed that, of the HOAs, the comatic terms (third and fifth order) explained most of the variations of the DoF and the image quality in this population of subjects. Furthermore, computer simulations demonstrated that the removal of these terms also had a significant impact on both DoF and the peak VSOTF. Knowledge about the relationship between aberrations, DoF, image quality, and their interactions is essential in optical designs aiming to produce large values of DoF while maintaining an acceptable level of image quality. Comatic aberration terms appear to contribute strongly towards the configuration of both of these visually important parameters.

Objective Measures of Pre-lens Tear Film Dynamics versus Visual Responses.

PURPOSE: To ascertain whether there is a correlation between objective measures of pre-lens tear film surface quality (TFSQ) dynamics and visual quality responses. METHODS: Thirty one subjects wore a pair of silicone hydrogel lenses for 1 hour before measurements. Acquisition of TFSQ dynamics was conducted for each subject in suppressed blinking conditions. A Placido disk videokeratoscope was fitted with an internal target and a video streaming output. Subjects were asked to focus on the target and indicate the just noticeable decrease in visual quality. The recorded videos and the subjective responses were saved for further analysis, for which a new videokeratoscopy image processing technique, based on calculating a texture feature (homogeneity), was developed. The estimated times of lens dewetting were then associated with the subjective responses. RESULTS: The dewetting point of the lens was identified in 82% of recordings. The group median time of subjective response was 3.5 s post-dewetting point, which corresponded to about 4.3% decrease in TFSQ. In the remaining measurements, either no substantial change in TFSQ or changes related to post-blink slow movement of the lipid layer were observed. The former was not associated with any noticeable change in subjective visual quality whereas the latter usually created visually identifiable temporal disturbance resulting in a subjective response. A small but statistically significant correlation (r = 0.32, p < 0.001) was found between objectively estimated point of dewetting and the subjective response, corresponding to just noticeable change in visual quality. CONCLUSIONS: The new technique proved to be sufficiently sensitive to identify the distinct two-phase process of pre-lens TFSQ dynamics. Changes in objectively determined pre-lens TFSQ can also be identified subjectively, but there exists, in general, a certain lag between the two responses. The estimated times of lens dewetting could be utilized for in vivo assessment of contact lens wetting parameters.

Principles of operation, accuracy and precision of an Eye Surface Profiler.

PURPOSE: To introduce a newly developed instrument for measuring the topography of the anterior eye, provide principles of its operation and to assess its accuracy and precision. METHODS: The Eye Surface Profiler is a new technology based on Fourier transform profilometry for measuring the anterior eye surface encompassing the corneo-scleral area. Details of technical principles of operation are provided for the particular case of sequential double fringe projection. Technical limits of accuracy have been assessed for several key parameters such as the carrier frequency, image quantisation level, sensor size, carrier frequency inaccuracy, and level and type of noise. Further, results from both artificial test surfaces as well as real eyes are used to assess precision and accuracy of the device (here benchmarked against one of popular Placido disk videokeratoscopes). RESULTS: Technically, the Eye Surface Profiler accuracy can reach levels below 1 µm for a range of considered key parameters. For the unit tested and using calibrated artificial surfaces, the accuracy of measurement (in terms of RMS error) was below 10 µm for a central measurement area of 8 mm diameter and below 40 µm for an extended measurement area of 16 mm. In some cases, the error reached levels of up to 200 µm at the very periphery of the measured surface (up to 20 mm). The SimK estimates of the test surfaces from the Eye Surface Profiler were in close agreement with those from a Placido disk videokeratoscope with differences no greater than ±0.1 D. For real eyes, the benchmarked accuracy was within ±0.5D for both the spherical and cylindrical SimK components. CONCLUSIONS: The Eye Surface Profiler can successfully measure the topography of the entire anterior eye including the cornea, limbus and sclera. It has a great potential to become an optometry clinical tool that could substitute the currently used videokeratoscopes and provide a high quality corneo-scleral topography.

Simplifying numerical ray tracing for two-dimensional non circularly symmetric models of the human eye.

Ray tracing is a powerful technique to understand the light behavior through an intricate optical system such as that of a human eye. The prediction of visual acuity can be achieved through characteristics of an optical system such as the geometrical point spread function. In general, its precision depends on the number of discrete rays and the accurate surface representation of each eye's components. Recently, a method that simplifies calculation of the geometrical point spread function has been proposed for circularly symmetric systems [Appl. Opt.53, 4784 (2014)]. An extension of this method to 2D noncircularly symmetric systems is proposed. In this method, a two-dimensional ray tracing procedure for an arbitrary number of surfaces and arbitrary surface shapes has been developed where surfaces, rays, and refractive indices are all represented in functional forms being approximated by Chebyshev polynomials. The Liou and Brennan anatomically accurate eye model has been adapted and used for evaluating the method. Further, real measurements of the anterior corneal surface of normal, astigmatic, and keratoconic eyes were substituted for the first surface in the model. The results have shown that performing ray tracing, utilizing the two-dimensional Chebyshev function approximation, is possible for noncircularly symmetric models, and that such calculation can be performed with a newly created Chebfun toolbox.

Computational aspects of the through-focus characteristics of the human eye.

Calculating through-focus characteristics of the human eye from a single objective measurement of wavefront aberration can be accomplished through a range of methods that are inherently computationally cumbersome. A simple yet accurate and computationally efficient method is developed, which combines the philosophy of the extended Nijboer-Zernike approach with the radial-basis-function-based approximation of the complex pupil function. The main advantage of the proposed technique is that the increase of the computational cost for a vector-valued defocus parameter is practically negligible in comparison to the corresponding scalar-valued defocus parameter.

Clinical utility of spectral analysis of intraocular pressure pulse wave.

BACKGROUND: To evaluate the clinical utility of spectral analysis of intraocular pressure pulse wave in healthy eyes of a control group (CG), patients having glaucomatous optic disc appearance or ocular hypertension, and patients with primary open angle glaucoma or primary angle closure glaucoma. METHODS: This is a prospective study that enrolled 296 patients from a single glaucoma clinic. Age matched CG consisted of 62 individuals. Subjects underwent comprehensive clinical diagnostic procedures including intraocular pressure (IOP) measurement with dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT). DCT time series were analyzed with custom written software that included signal preprocessing, filtering and spectral analysis. An amplitude and energy content analysis, which takes into account non-stationarity of signals but also provides methodology that is independent of IOP and ocular pulse amplitude (OPA) levels, was applied. Spectral content up to the 6th harmonic of the pressure pulse wave was considered. Statistical analyses included descriptive statistics, normality test, and a multicomparison of medians for independent groups using Kruskal-Wallis test. RESULTS: GAT IOP showed statistical significance (Kruskal-Willis test p < 0.05) for three out of 10 considered multiple comparisons, DCT IOP and OPA showed statistically significant results in five and seven cases, respectively. Changes in heart rate and central corneal thickness between the groups were statistically significant in two cases. None of the above parameters showed statistically significant differences between CG and the suspects with glaucomatous optic disc appearance (GODA). On the other hand, spectral analysis showed statistically significant differences for that case. CONCLUSIONS: Spectral analysis of the DCT signals was the only method showing statistically significant differences between healthy eyes and those of GODA suspects.

Age-related changes in corneal pulsation: ocular dicrotism.

PURPOSE: To evaluate age-related changes in the shape of corneal indentation pulse (CIP) in relation to simultaneously registered blood pulsation (BPL) and electrocardiogram (ECG) signals in healthy subjects. METHODS: Seventy-seven subjects were recruited for the study, including 36 young subjects (aged 23 to 32 years) and 41 older subjects (aged 44 to 72 years) with no reported ocular and cardiovascular abnormalities and free of any systemic diseases. Corneal indentation pulse was measured with a noninvasive ultrasonic distance sensor. Simultaneously, BPL and ECG were acquired with a pulse oximeter placed on the earlobe and Eindhoven triangle setup, respectively. Correlation analysis was applied to identify the interactions between the parameters of the averaged signals, estimated with dedicated signal processing algorithms, for all subject measurements and separately for both considered groups. RESULTS: Twenty-nine out of 41 older subjects showed evidence of a double peak-shaped CIP waveform that could be interpreted as ocular pulse dicrotism. None of the young subjects exhibited this phenomenon. For the young group, significant correlations (p < 0.05) were found between the CIP crest time of a single peak-shaped CIP waveform and the other shape parameters of the CIP, BPL, and ECG signals. Similarly, for the older group of subjects, significant corresponding correlations were only found between the preliminary crest time of a double peak-shaped CIP waveform. CONCLUSIONS: Using noninvasive corneal indentation pulse measurement revealed, for the first time, an ocular pulse dicrotism. This might be a natural sign of aging or an early indication of hemodynamic aspects of cardiovascular diseases.

Tear film dynamics on soft contact lenses.

PURPOSE: To analyze and model the dynamics of tear film surface quality (TFSQ) in a group of subjects with healthy eyes, before and during contact lens (CL) wear, and in a group of subjects with dry eyes (DEs). METHODS: Four sets of TFSQ measurements with lateral shearing interferometry were retrospectively analyzed on two groups of subjects. The first group included 13 CL wearers for which TFSQ measurements corresponding to baseline, Best CL, and Worst CL were selected. The second group included 13 DE subjects. The acquired TFSQ time series were fit with a power-exponential model. Tear film surface quality stability time, best TFSQ, and integrated poststability time characterizing the tear film deterioration process were derived. RESULTS: The proposed power-exponential model was better suited (higher correlation values) for the TFSQ dynamics on CL rather than for those of baseline and DE measurements. The average baseline TFSQ Stability Time was significantly longer (p < 0.001) than those in the DE group and with both the best and worse CL. The average Best TFSQ achieved at baseline was statistically significantly better (p = 0.03) than that for the DE group. The average best TFSQ was significantly better (p < 0.01) for the Best CL than for that of the Worst CL. Deterioration of TFSQ on both best and Worst CL was substantially faster than that achieved for the DE group. CONCLUSIONS: The observed distinct change in the sign of the TFSQ velocity on contact lenses suggests a two-phase dynamics in which the postblink stability phase is followed by a phase of dewetting. Lens material properties influence the first phase but play little role after the dewetting process occurs.

Zernike vs. Bessel circular functions in visual optics.

PURPOSE: We propose the Bessel Circular Functions as alternatives of the Zernike Circle Polynomials to represent relevant circular ophthalmic surfaces. METHODS: We assess the fitting capabilities of the orthogonal Bessel Circular Functions by comparing them to Zernike Circle Polynomials for approximating a variety of computationally generated surfaces which can represent ophthalmic surfaces. RESULTS: The Bessel Circular Functions showed better modelling capabilities for surfaces with abrupt variations such as the anterior eye surface at the limbus region, and influence functions. From our studies we find that the Bessel Circular Functions can be more suitable for studying particular features of post surgical corneal surfaces. CONCLUSIONS: We show that given their boundary conditions and free oscillating properties, the Bessel Circular Functions are an alternative for representing specific wavefronts and can be better than the Zernike Circle Polynomials for some important cases of corneal surfaces, influence functions and the complete anterior corneal surface.

Measuring intraocular pressure with OCT: the first approach.

The variability of corneal OCT speckle statistics is indirectly related to changes in corneal microstructure, which may be induced by intraocular pressure (IOP). A new approach is considered, which attempts to estimate IOP based on corneal speckle statistics in OCT images. An area (A) under trajectories of contrast ratio with respect to stromal depth was calculated. The proposed method was evaluated on OCT images from the ex-vivo study on porcine eyeballs and in-vivo study on human corneas. A statistically significant multivariate linear regression model was obtained from the ex-vivo study: IOP = 0.70 · A - 6.11, in which IOP was precisely controlled in the anterior chamber. The ex-vivo study showed good correlation between A and IOP (R = 0.628, at the least) whereas the in-vivo study showed poor correlation between A and clinical air-puff tonometry based estimates of IOP (R = 0.351, at the most), indicating substantial differences between the two studies. The results of the ex-vivo study show the potential for OCT speckle statistics to be utilized for measuring IOP using static corneal imaging that does not require corneal deformation. Nevertheless, further work is needed to validate this approach in living human corneas.