In vivo identification of the retinal layer containing photopigments in OCT images through correlation with two-photon psychophysics.

Two-photon vision enables near-infrared light perception in humans. We investigate the possibility to utilize this phenomenon as an indicator of the location of the outer segments of photoreceptor cells in the OCT images. Since two-photon vision is independent on OCT imaging, it could provide external to OCT reference relative to which positions of retinal layers visible in OCT imaging could be measured. We show coincidence between OCT imaging of outer retinal layers and two-photon light perception. The experiment utilizes an intrinsic nonlinear process in the retina, two-photon absorption of light by visual photopigments, which triggers perception of near-infrared light. By shifting the focus of the imaging/stimulus beam, we link the peak efficiency of two-photon vision with the visibility of outer segments of photoreceptor cells, which can be seen as in vivo identification of a retinal layer containing visual photopigments in OCT images. Determination of the in-focus retinal layer is achieved by analysis of en face OCT image contrast. We discuss experimental methods and experimental factors that may influence two-photon light perception and the accuracy of the results. The limits of resolution are discussed in analysis of the one-photon and two-photon point spread functions.

Description of a new method to calculate the equator of the crystalline lens using AS-OCT images: Accuracy in non-dilated measurements.

OBJECTIVE: To establish a methodology for objectively estimating the Lens Equatorial Plane (LEP) from clinical images, comparing LEP with dilated versus non-dilated pupils. METHODS: A cohort of 91 eyes from 60 patients undergoing preoperative assessments for cataract surgery was evaluated. Anterior Segment Optical Coherence Tomography (AS-OCT) images were analysed under conditions of pharmacologically induced pupil dilation versus a non-dilated pupil. Geometrical parameters, including LEP, intersection diameter (ID), lens thickness (LT), anterior and posterior lens thickness were automatically calculated by applying standard image processing techniques to clinical AS-OCT images. RESULTS: Significant differences in lens parameters, including LEP, were observed between dilated and non-dilated conditions (all p < 0.001). A strong linear correlation was found across all geometrical variables under both conditions (r[LEP] = 0.64, r[ID] = 0.78, r[LT] = 0.99, all p < 0.001); enabling reliable correction of these differences. CONCLUSION: The study introduces an objective methodology for LEP calculation, emphasising the need to consider the eye's physiological state during preoperative measurements. Incorporating LEP into future intraocular lens (IOL) power calculation formulas and replacing the habitual effective lens position may potentially improve the accuracy of IOL power estimation and thus postoperative visual outcomes.

Anterior scleral thickness in Marfan syndrome: A quantitative analysis.

PURPOSE: To investigate the anterior scleral thickness (AST) in patients with Marfan syndrome (MFS). METHODS: A prospective, cross-sectional study was conducted at the Department of Ophthalmology, Ghent University Hospital, Ghent, including patients with a genetically confirmed clinical diagnosis of MFS and age-, gender- and axial length-matched controls. Subjects with known corneal, conjunctival or scleral pathology and a history of ocular surgery, including pars plana vitrectomy, recent contact lens use or high-grade astigmatism were excluded. Subjects underwent non-cycloplegic autorefraction, Scheimpflug-based corneal tomography, axial length measurement and spectral-domain optical coherence tomography (OCT). AST was manually measured at 1 mm (AST1), 2 mm (AST2) and 3 mm (AST3) from the scleral spur, temporally and nasally. RESULTS: A total of 56 subjects (28 subjects in the MFS group and 28 matched subjects in the control group) were included in this study. In patients with MFS, AST was significantly reduced compared to matched controls, both overall and at every analysed measuring point in the nasal and temporal areas (p < 0.001). Central corneal thickness (CCT) and mean keratometry (Kmean) values were significantly lower in patients with MFS (p < 0.05). A positive correlation was found between nasal AST and CCT in patients with MFS. No correlation was found between AST and Kmean or between AST and axial length. In patients with MFS with ectopia lentis, compared to those without, temporal AST3 was significantly lower (p < 0.05). AST was significantly lower in patients with MFS harbouring a variant predicted to cause haploinsufficiency compared to those with a variant expected to lead to a dominant negative effect for both nasal and temporal measurements. CONCLUSION: Based on anterior segment OCT measurements, AST of patients with MFS is significantly lower compared to matched controls.

Comparison of Scheimpflug Corneal Tomography and Anterior Segment Optical Coherence Tomography Measurements in Corneal Cystinosis: A Case Series.

PURPOSE: To report the clinical course and compare the utility of Scheimpflug tomography (ST) and anterior segment optical coherence tomography (AS-OCT) for central corneal thickness (CCT) and corneal densitometry (CD) assessment in patients with corneal crystals owing to nephropathic cystinosis. METHODS: A retrospective chart analysis of three patients with nephropathic cystinosis and the presence of corneal cystine crystals in both eyes was performed. All patients underwent clinical examination and anterior segment photography, ST, and AS-OCT scans. Corneal densitometry was exported from built-in proprietary software for ST and from custom-made validated software for AS-OCT. Anterior segment optical coherence tomography images were rescaled to grayscale units from 0 (maximum transparency) to 100 (minimum transparency) to match built-in ST densitometry readings. Furthermore, the mean pixel intensity, representative of CD, was calculated from the pixels corresponding to the segmented cornea. RESULTS: All three patients had pathognomonic cystine crystals deposits in the cornea and were treated with cysteamine medications that resulted in clinical improvement. The CCT measured using ST exhibited a range from 560 to 958 µm. Conversely, when assessed with AS-OCT, the CCT varied within the range of 548 to 610 µm. Both examinations could be performed, but in the more severe cases, AS-OCT showed far greater utility to estimate CD. In four of six eyes examined, ST showed disproportionate CCT values, compared with the AS-OCT, whereas reliable CD measurements were only available in AS-OCT. CONCLUSION: The AS-OCT could be considered a baseline ocular measurement in cystinosis and in the evaluation of disease progression and treatment efficacy.

Machine learning-based prediction of tear osmolarity for contact lens practice.

PURPOSE: This study addressed the utilisation of machine learning techniques to estimate tear osmolarity, a clinically significant yet challenging parameter to measure accurately. Elevated tear osmolarity has been observed in contact lens wearers and is associated with contact lens-induced dry eye, a common cause of discomfort leading to discontinuation of lens wear. METHODS: The study explored machine learning, regression and classification techniques to predict tear osmolarity using routine clinical parameters. The data set consisted of 175 participants, primarily healthy subjects eligible for soft contact lens wear. Various clinical assessments were performed, including symptom assessment with the Ocular Surface Disease Index and 5-Item Dry Eye Questionnaire (DEQ-5), tear meniscus height (TMH), tear osmolarity, non-invasive keratometric tear film break-up time (NIKBUT), ocular redness, corneal and conjunctival fluorescein staining and Meibomian glands loss. RESULTS: The results revealed that simple linear regression was insufficient for accurate osmolarity prediction. Instead, more advanced regression models achieved a moderate level of predictive power, explaining approximately 32% of the osmolarity variability. Notably, key predictors for osmolarity included NIKBUT, TMH, ocular redness, Meibomian gland coverage and the DEQ-5 questionnaire. In classification tasks, distinguishing between low (<299 mOsmol/L), medium (300-307 mOsmol/L) and high osmolarity (>308 mOsmol/L) levels yielded an accuracy of approximately 80%. Key parameters for classification were similar to those in regression models, emphasising the importance of NIKBUT, TMH, ocular redness, Meibomian glands coverage and the DEQ-5 questionnaire. CONCLUSIONS: This study highlights the potential benefits of integrating machine learning into contact lens research and practice. It suggests the clinical utility of assessing Meibomian glands and NIKBUT in contact lens fitting and follow-up visits. Machine learning models can optimise contact lens prescriptions and aid in early detection of conditions like dry eye, ultimately enhancing ocular health and the contact lens wearing experience.

OCT based corneal densitometry: the confounding effect of epithelial speckle.

Corneal densitometry is a clinically validated method for objectively assessing the transparency of stroma. The technique is currently dominated by Scheimpflug technology. Still, optical coherence tomography (OCT), in which examination of the statistical properties of corneal speckle is undertaken, has also been considered to assess corneal densitometry. In-vivo, the stroma is observed via the epithelium. However, the effect of this external layer on stromal densitometry has not been considered as yet. This study aims to quantify the influence of epithelium integrity on corneal OCT densitometry. OCT images from eleven freshly enucleated porcine eyes before and after epithelial debridement were used. OCT densitometry was investigated at different stromal depths using four metrics of speckle statistics. Results indicate that there exist statistically significant differences in speckle statistics for a given stromal depth depending on the presence or absence of the epithelium. The estimation error in speckle statistics can reach over 20% depending on the stromal depth. The anterior stroma densitometry values are the ones most affected by epithelial integrity. In conclusion, if OCT densitometry stromal parameters are to be considered in absolute terms, it is essential to consider the confounding effect of the epithelial layer in the analysis.

Ocular Surface Homeostasis After Scleral Lens Usage.

OBJECTIVES: The aim of this prospective study is to examine the effects of 5 hours of well-fitted, mini-scleral contact lens (mini-SL) wear on the tear film cytokine expression in healthy eyes. METHODS: Twenty-three healthy participants were included in the study. One eye of each participant was selected at random, and a mini-SL measuring 16.5 mm in diameter was fitted by an experienced contact lens specialist. The contact lens remained in place for 5 hours. Precorneal tear fluid was collected using capillary tubes at three different time points: baseline before SL insertion (T0), after 5 hours of SL wear (T1), and 3 hours after SL removal (T2). The concentration of 40 inflammatory cytokines at the three different time points was determined using multiplex bead assay. RESULTS: Mini-scleral lens wear did not result in significant changes in the cytokine-to-protein ratio after 5 hours of wear on a healthy eye. CONCLUSIONS: Although a well-fitted mini-SL reduces the rate at which the precorneal tear film is refreshed, 5 hours of lens wear did not appear to significantly affect the tears cytokine-to-protein ratio, suggesting that scleral lenses have minimal impact on corneal cytokine expression.

Relationship between corneal tissue and shape in short-term soft contact lens wear.

PURPOSE: To investigate which morphometric and ocular surface tissue parameters are affected by short-term soft contact lens (CL) wear and to assess whether they carry related or independent information. METHODS: Twenty-two healthy participants wore silicone hydrogel (SiHy; MyDay, CooperVision) soft CLs for 8 h in their left eye. Corneal tomography and corneoscleral topography were captured before and immediately after CL wear. Central corneal thickness (CCT), corneoscleral parameters (limbus position and corneoscleral junction [CSJ] angle) and corneal tissue parameters (corneal transparency and homogeneity) were evaluated. RESULTS: Corneoscleral parameters (limbus position and CSJ angle) were independent of corneal tissue parameters (transparency and homogeneity) at baseline and after CL wear. CCT was independent of all the other parameters examined at baseline, but baseline values of corneal tissue parameters were moderately correlated with CCT change (transparency: r = -0.51; p = 0.007), homogeneity: r = -0.46; p = 0.02). CONCLUSIONS: A complete characterisation of ocular surface changes following CL wear should consider corneoscleral topography and corneal densitometry simultaneously, since they carry complementary information.

The influence of soft contact lens material on the corneoscleral profile.

PURPOSE: To objectively quantify changes in corneoscleral profile, as evaluated by the limbus position and corneoscleral junction (CSJ) angle, as a consequence of wearing different soft contact lens (CL) materials. METHODS: Twenty-two healthy participants wore silicone hydrogel (SiHy, MyDay, CooperVision) and hydrogel (Hy, Biomedics 1 day extra, CooperVision) soft CLs for 8 h per lens in their left eye. In each session, corneoscleral topography was captured before and immediately after CL removal with an Eye Surface Profiler. Previously validated automatic and objective algorithms for limbal position and CSJ angle calculation were applied to 360 semi-meridians to investigate the effect of short-term CL wear on corneoscleral topography, globally and by sectors, depending on the soft CL material worn. RESULTS: Short-term soft CL wear significantly impacted limbal position (SiHy: 120 ± 97 µm, Hy: 128 ± 85 µm) and CSJ angle (SiHy: 0.57 ± 0.36°, Hy: 0.55 ± 0.40°); all p < 0.05. A statistically significant difference was found between the sectors with regard to limbus position and CSJ angle before CL wear that remained following lens wear (all pairwise comparisons, p < 0.001). Although individual differences were observed, there was no evidence that one material caused more substantial corneoscleral alterations. CONCLUSION: Corneoscleral profile parameters were altered significantly following 8 h of soft CL wear. The observed changes in limbus position and CSJ angle support the importance of participant-material biocompatibility.

Typical localised element-specific finite element anterior eye model.

Purpose: The study presents an averaged anterior eye geometry model combined with a localised material model that is straightforward, appropriate and amenable for implementation in finite element (FE) modelling. Methods: Both right and left eye profile data of 118 subjects (63 females and 55 males) aged 22-67 years (38.5 ± 7.6) were used to build an averaged geometry model. Parametric representation of the averaged geometry model was achieved through two polynomials dividing the eye into three smoothly connected volumes. This study utilised the collagen microstructure x-ray data of 6 ex-vivo healthy human eyes, 3 right eyes and 3 left eyes in pairs from 3 donors, 1 male and 2 females aged between 60 and 80 years, to build a localised element-specific material model for the eye. Results: Fitting the cornea and the posterior sclera sections to a 5th-order Zernike polynomial resulted in 21 coefficients. The averaged anterior eye geometry model recorded a limbus tangent angle of 37° at a radius of 6.6 mm from the corneal apex. In terms of material models, the difference between the stresses generated in the inflation simulation up to 15 mmHg in the ring-segmented material model and localised element-specific material model were significantly different (p < 0.001) with the ring-segmented material model recording average Von-Mises stress 0.0168 ± 0.0046 MPa and the localised element-specific material model recording average Von-Mises stress 0.0144 ± 0.0025 MPa. Conclusions: The study illustrates an averaged geometry model of the anterior human eye that is easy to generate through two parametric equations. This model is combined with a localised material model that can be used either parametrically through a Zernike fitted polynomial or non-parametrically as a function of the azimuth angle and the elevation angle of the eye globe. Both averaged geometry and localised material models were built in a way that makes them easy to implement in FE analysis without additional computation cost compared to the limbal discontinuity so-called idealised eye geometry model or ring-segmented material model.

Corneoscleral junction angle in healthy eyes assessed objectively.

PURPOSE: To introduce a fully objective method to measure corneoscleral junction (CSJ) angle and evaluate the CSJ angle in healthy eyes. METHODS: Corneoscleral topography (Eye Surface Profiler, ESP) was acquired from the right eye of 105 healthy Caucasian subjects, ranging from 18 to 59 years old. From the raw anterior eye height data, the topographic limbus was automatically demarcated in 360 semi-meridians. Further, in limbal location, the CSJ angle was automatically calculated from corneoscleral height data using neighbouring auxiliary points for angle calculation. Additionally, CSJ angle was statistically analysed regionally. RESULTS: The mean CSJ angle was 177.5 ± 1.1°. There is a mean difference of 7.7 ± 3.7° between the steepest (smallest) and flattest (largest) CSJ angle within the same eye. There exist statistically significant differences between temporal (178.2 ± 1.4°) and nasal (176.4 ± 1.1°) regions (paired t-test, p < 0.001), and between superior (178.1 ± 1.1°) and inferior (177.9 ± 1.1°) regions (p = 0.038). CSJ angle is correlated with limbus position (r = 0.43, p < 0.001). CONCLUSION: CSJ angle is rotationally asymmetric. CSJ varies regionally, being the smallest (steepest) in the nasal region. Significant rough changes in CSJ angle were observed for some healthy individuals.

Corneal tissue changes following short-term soft contact lens wear of different materials.

PURPOSE: To study the effect of different soft contact lens (CL) materials during short-term wear on corneal tissue. METHODS: Twenty-two healthy participants wore both silicone hydrogel (MyDay, CooperVision) and hydrogel soft CLs (Biomedics 1 day extra, CooperVision) for 8 h per lens. In each session, Scheimpflug images were captured before and immediately after CL removal. Images were analysed using the densitometry distribution analysis, a technique from which two parameters, α (corneal transparency) and ß (corneal homogeneity), were estimated. In addition, the central corneal thickness changes after CL wear and the influence of the CL material on corneal transparency were evaluated. RESULTS: The ß parameter (homogeneity) increased by 5% after wearing both CL materials (paired t-test, p < 0.001). However, the α parameter (transparency) only increased in half of the participants. No material was found to be more determinant in causing the corneal densitometry changes. Statistically significant but not clinically relevant changes in corneal thickness were observed. CONCLUSIONS: Biomarkers of corneal tissue integrity (α and ß) were affected by short-term soft contact lens wear. The observed changes in corneal transparency and homogeneity were not clinically relevant but support the importance of participant-material biocompatibility.

Speckle Contrast as Retinal Tissue Integrity Biomarker in Patients with Type 1 Diabetes Mellitus with No Retinopathy.

PURPOSE: To study the retinal and choroidal layers in type 1 diabetes mellitus (DM1) without diabetic retinopathy (DR), using speckle contrast of optical coherence tomography (OCT) images as a tissue biomarker in comparison with healthy subjects. METHODS: OCT Spectralis images of 148 eyes, 84 from DM1 patients without DR signs, and 64 belonging to the control group, were collected. The speckle contrast and thickness of the inner retinal layer (IRL), the outer retinal layer (ORL), and the choroidal layer in the nasal parafoveal area (N3), were prospectively analyzed. RESULTS: A statistically significant difference (p = 0.001) in the IRL thickness between groups was observed, being thicker in the DM1 group. There were no differences in the ORL and choroidal thicknesses between groups. A statistically significant difference (p = 0.02) in the IRL speckle contrast was obtained, being lower in the DM1 group. The maximum speckle contrast was reached in the ORL for both groups, although in the DM1 group, it occurs closer to the choroid, at 64 ± 8 µm (p = 0.008). CONCLUSIONS: Statistically significant differences were found in speckle contrast and thickness between the control and the DM1 group, suggesting an IRL alteration of DM1 patients, supporting the retinal neurodegeneration before DR signs are observed.

Suspect glaucoma detection from corneal densitometry supported by machine learning.

PURPOSE: To discriminate suspect glaucomatous from control eyes using corneal densitometry based on the statistical modeling of the pixel intensity distribution of Scheimpflug images. METHODS: Twenty-four participants (10 suspect glaucomatous and 14 control eyes) were included in this retrospective study. Corneal biomechanics was assessed with the commercial Scheimpflug camera Corvis ST (Oculus). Sets of 140 images acquired per measurement were exported for further analysis. After corneal segmentation, pixel intensities corresponding to different corneal depths were statistically modeled for each image, from which corneal densitometry in the form of parameters α (brightness) and ß (homogeneity) was derived. After data pre-processing, parameters α and ß were input to six supervised machine learning algorithms that were trained, tested, and compared. RESULTS: There exists a statistically significant difference in α and ß parameters between suspect glaucomatous and control eyes (both, P < 0.05/N, Bonferroni). From the implemented supervised machine learning algorithms, the K-nearest neighbors (K-NN) algorithm reached 83.93% accuracy to discriminate between groups only using corneal densitometry parameters (α and ß). CONCLUSION: Densitometry of the anterior cornea including epithelium on its own has the potential to serve as a clinical tool for early glaucoma risk assessment.

Influence of eye tilt on corneal densitometry.

PURPOSE: To investigate whether Pentacam densitometry readings are affected by corneal tilt. METHODS: In a prospective study, the right eyes of 86 healthy participants aged 42.8 ± 20.0 years (range 18-79 years) were imaged using Scheimpflug tomography. Elevation maps were exported to calculate corneal tilt using custom-made software, and densitometry readings were acquired directly from the corneal densitometry analysis add-on to the standard software Oculus Pentacam HR. Simple mediation analysis was applied to study age as a confounding factor in the correlation between corneal tilt and corneal densitometry. RESULTS: Corneal tilt and corneal densitometry are not independent from one another because age is significantly correlated with both corneal tilt (r = 0.50, p < 0.001) and corneal densitometry (r = 0.91, p < 0.001). Only 3.8% of the correlation between tilt and densitometry operates directly, while the remaining 96.2% depends on age. CONCLUSIONS: Corneal tilt plays a role in corneal densitometry readings, even though the interaction is strongly influenced by age. Age is a well-known factor in densitometry readings that should be taken into consideration when interpreting Scheimpflug densitometry.

Influence of Author's Gender on the Peer-Review Process in Vision Science.

PURPOSE: To investigate the gender gap in first/last authors in vision science and whether gender affects manuscript review times. DESIGN: Observational retrospective database study. METHODS: First/last author's gender and country were assigned to 30 438 PubMed records (data derived from Q1-Q2 Ophthalmology journals for 2016-2020). Using mixed models, the influence of First Author Female (FAF) and Last Author Female (LAF) were evaluated on the manuscripts' review timeline. This analysis was performed globally and in predefined subgroups (English names, Asian names, specific topics). Additionally, the gender GAP was explored by country, journal, and research topics. RESULTS: The percentages of FAF/LAF were unevenly distributed by country; in the top 30 ophthalmology journals, FAF accounted for 40.0%±6.7% of the publications whereas LAF accounted for 27.1%±4.9%. Overall, FAF/LAF papers underwent significantly longer times to be reviewed (up to +10 days) and accepted (+5 days). These differences persisted when only English names-easily recognizable worldwide-were considered, but not for Asian names. Delays >1 month to get published were found for FAF in 3 of 4 topics analyzed (eg, amblyopia). CONCLUSIONS: Significant differences were found in both review and acceptance times for FAF or LAF papers. The causes for this are likely multifactorial and could be explained by a combination of gender bias and by women's concerns with being held to higher standards, something that has been previously documented, thereby perhaps delaying the rebuttal to reviewers. Increased awareness of this source of potential bias may assist in the implementation of preventive and corrective measures.

Outcomes of Human Leukocyte Antigen-Matched Allogeneic Cultivated Limbal Epithelial Transplantation in Aniridia-Associated Keratopathy-A Single-Center Retrospective Analysis.

PURPOSE: To assess the efficacy and safety of human leukocyte antigen-matched allogeneic cultivated limbal epithelial stem cell grafts in the treatment of aniridia-associated keratopathy (AAK). METHODS: Six eyes of 6 patients with severe AAK received an allogeneic stem cell graft between January 2010 and March 2017. Anatomical and functional results were assessed at 6 months, 1 year, 2 years, and the final follow-up visit available. Safety analysis was performed by considering all perioperative and postoperative adverse events and additional surgeries required during the follow-up period. RESULTS: The mean follow-up was 53.6 months (range 24-104 months). In most patients (80%), there was an early improvement of the keratopathy postoperatively, which slowly regressed during longer follow-up. At the final follow-up, 4 of the eyes were graded as failure and 1 eye was graded as partial success. Grading the sixth eye was not possible because of an adverse event. None of the patients maintained a total anatomical success in the long-term. Only 1 patient maintained a modest improvement in best-corrected visual acuity from hand motion to counting fingers. Four serious adverse events were recorded in 2 patients. CONCLUSIONS: Severe AAK remains a challenging condition to manage. Transplantation of allogenic ex vivo cultivated limbal stem cells may provide a temporary improvement in ocular surface stability, but anatomical and functional results are poor in the long-term. The eyes are prone to adverse events, and any surgical treatment should take this into consideration.

Limitations of Reconstructing Pentacam Rabbit Corneal Tomography by Zernike Polynomials.

The study aims to investigate the likelihood of Zernike polynomial being used for reconstructing rabbit corneal surfaces as scanned by the Pentacam segment tomographer, and hence evaluate the accuracy of corneal power maps calculated from such Zernike fitted surfaces. The study utilised a data set of both eyes of 21 rabbits using a reverse engineering approach for deductive reasoning. Pentacam raw elevation data were fitted to Zernike polynomials of orders 2 to 20. The surface fitting process to Zernike polynomials was carried out using randomly selected 80% of the corneal surface data points, and the root means squared fitting error (RMS) was determined for the other 20% of the surface data following the Pareto principle. The process was carried out for both the anterior and posterior surfaces of the corneal surfaces that were measured via Pentacam scans. Raw elevation data and the fitted corneal surfaces were then used to determine corneal axial and tangential curvature maps. For reconstructed surfaces calculated using the Zernike fitted surfaces, the mean and standard deviation of the error incurred by the fitting were calculated. For power maps computed using the raw elevation data, different levels of discrete cosine transform (DCT) smoothing were employed to infer the smoothing level utilised by the Pentacam device. The RMS error was not significantly improved for Zernike polynomial orders above 12 and 10 when fitting the anterior and posterior surfaces of the cornea, respectively. This was noted by the statistically non-significant increase in accuracy when the order was increased beyond these values. The corneal curvature calculations suggest that a smoothing process is employed in the corneal curvature maps outputted by the Pentacam device; however, the exact smoothing method is unknown. Additionally, the results suggest that fitting corneal surfaces to high-order Zernike polynomials will incur a clinical error in the calculation of axial and tangential corneal curvature of at least 0.16 ± 01 D and 0.36 ± 0.02 D, respectively. Rabbit corneal anterior and posterior surfaces scanned via the Pentacam were optimally fitted to orders 12 and 10 Zernike polynomials. This is essential to get stable values of high-order aberrations that are not affected by Zernike polynomial fittings, such as comas for Intracorneal Ring Segments (ICRS) adjustments or spherical aberration for pre-cataract operations. Smoothing was necessary to replicate the corneal curvature maps outputted by the Pentacam tomographer, and fitting corneal surfaces to Zernike polynomials introduces errors in the calculation of both the axial and tangential corneal curvatures.

Assessing and compensating for the confounding factors in Scheimpflug-based corneal densitometry.

Scheimpflug-based corneal densitometry is a clinically verified method for assessing corneal transparency. Nevertheless, the estimates of corneal densitometry appear to be correlated with age and eye biometry parameters, such as the anterior chamber depth or the pupil size, and that ensues a convoluted conditional estimation problem, where it is difficult to interpret the results. This study aims at devising a methodology for compensating for such confounding factors by using, as a research platform, a commercially available Scheimpflug camera that allows exporting images in a dynamic fashion, allowing averaging the results from multiple acquisitions. Two approaches are considered, one based on appropriately normalizing the line densitometry signal and one based on image histogram equalization. Then, three parameters for describing corneal densitometry are derived including the mean value of backscatter and the scale and shape parameters of the Weibull distribution estimated in regions of interest encompassing parts of corneal stroma. The results show that, unlike the non-normalized measures, the proposed approaches lead to parameters that are not correlated with age nor the eye biometry.

Effect of Corneal Tilt on the Determination of Asphericity.

Purpose: To quantify the effect of levelling the corneal surface around the optical axis on the calculated values of corneal asphericity when conic and biconic models are used to fit the anterior corneal surface. Methods: This cross-sectional study starts with a mathematical simulation proving the concept of the effect that the eye's tilt has on the corneal asphericity calculation. Spherical, conic and biconic models are considered and compared. Further, corneal asphericity is analysed in the eyes of 177 healthy participants aged 35.4 ± 15.2. The optical axis was determined using an optimization procedure via the Levenberg-Marquardt nonlinear least-squares algorithm, before fitting the corneal surface to spherical, conic and biconic models. The influence of pupil size (aperture radii of 1.5, 3.0, 4.0 and 5.0 mm) on corneal radius and asphericity was also analysed. Results: In computer simulations, eye tilt caused an increase in the apical radii of the surface with the increase of the tilt angle in both positive and negative directions and aperture radii in all models. Fitting the cornea to spherical models did not show a significant difference between the raw-measured corneal surfaces and the levelled surfaces for right and left eyes. When the conic models were fitted to the cornea, changes in the radii of the cornea among the raw-measured corneal surfaces' data and levelled data were not significant; however, significant differences were recorded in the asphericity of the anterior surfaces at radii of aperture 1.5 mm (p < 0.01). With the biconic model, the posterior surfaces recorded significant asphericity differences at aperture radii of 1.5 mm, 3 mm, 4 mm and 5 mm (p = 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively) in the nasal temporal direction of right eyes and left eyes (p < 0.01, p < 0.01, p < 0.01 & p < 0.01, respectively). In the superior-inferior direction, significant changes were only noticed at aperture radii of 1.5 mm for both right and left eyes (p = 0.05, p < 0.01). Conclusions: Estimation of human corneal asphericity from topography or tomography data using conic and biconic models of corneas are affected by eyes' natural tilt. In contrast, the apical radii of the cornea are less affected. Using corneal asphericity in certain applications such as fitting contact lenses, corneal implant design, planning for refractive surgery and mathematical modelling when a geometrical centre of the eye is needed should be implemented with caution.