Applications of corneal topography and tomography: a review

Keratoconus

Keratoconus is a progressive eye disorder primarily affecting individuals in adolescence and early adulthood. The ectatic changes in the cornea cause thinning and cone-like steepening leading to irregular astigmatism and reduced vision. Keratoconus is a complex disorder with a multifaceted aetiology and pathogenesis, including genetic, environmental, biomechanical and cellular factors. Environmental factors, such as eye rubbing, UV light exposure and contact lens wearing, are associated with disease progression. On the cellular level, a complex interplay of hormonal changes, alterations in enzymatic activity that modify extracellular membrane stiffness, and changes in biochemical and biomechanical signalling pathways disrupt collagen cross-linking within the stroma, contributing to structural integrity loss and distortion of normal corneal anatomy. Clinically, keratoconus is diagnosed through clinical examination and corneal imaging. Advanced imaging platforms have improved the detection of keratoconus, facilitating early diagnosis and monitoring of disease progression. Treatment strategies for keratoconus are tailored to disease severity and progression. In early stages, vision correction with glasses or soft contact lenses may suffice. As the condition advances, rigid gas-permeable contact lenses or scleral lenses are prescribed. Corneal cross-linking has emerged as a pivotal treatment aimed at halting the progression of corneal ectasia. In patients with keratoconus with scarring or contact lens intolerance, surgical interventions are performed.

The KERATO Biomechanics Study 1: A Comparative Evaluation Using Brillouin Microscopy and Dynamic Scheimpflug Imaging

PURPOSE

To assess the corneal biomechanical properties in normal individuals and patients with keratoconus using the Brillouin optical scanning system (Intelon Optics) (BOSS) and compare them with ultra-high-speed Scheimpflug imaging (Corvis ST; Oculus Optikgeräte GmbH).

METHODS

Sixty eyes from 60 patients (30 normal and 30 keratoconus) were included in this prospective, single-center, comparative, non-interventional study. Corneal biomechanics were evaluated using the Corvis ST and the BOSS. With the BOSS, each corneal image was acquired three times, measuring 10 locations within an 8-mm diameter. Parameters extracted included mean, maximum, and minimum Brillouin shift. These 10 points were also grouped into superior, central, and inferior regions. BOSS repeatability was assessed using the coefficient of repeatability and coefficient of variation. Furthermore, normal individuals and patients with keratoconus were compared using the Corvis ST and BOSS.

RESULTS

The BOSS exhibited good repeatability, with coefficient of repeatability ranging from 0.098 to 0.138 GHz for single points in normal individuals and 0.096 to 0.149 GHz for patients with keratoconus. Statistical analysis revealed significant differences between normal individuals and patients with keratoconus, indicating softer corneas in keratoconus, observed with both the Corvis ST and BOSS. Specifically, the BOSS showed significant differences in mean, inferior, and superior mean, maximum, and minimum Brillouin frequency shift (all P < .05), whereas the Corvis ST displayed highly significant differences in stiffness parameter at first applanation, stress strain index, deformation amplitude ratio, and inverse integrated radius (all P < .001).

CONCLUSIONS

Corneal biomechanical measurements proved highly repeatable and effectively demonstrated significant differences between normal individuals and patients with keratoconus using both the BOSS and the Corvis ST. [J Refract Surg. 2024;40(8):e569-e578.].

A method for extracting corneal reflection images from multiple eye images

The incident light reflected from the cornea is rich in information about the human surroundings, and these reflected rays are imaged by the camera, which can be used for research on human consciousness and gaze analysis, and produce certain help in the fields of psychology, human computer interaction and disease diagnosis. However, limited by the low corneal reflection ability, when a high-definition camera captures corneal reflecting rays, a large amount of color and texture interference from the iris can seriously contaminate the corneal reflection images, resulting in low usability and ubiquity of corneal reflection images. In this paper, we propose a corneal reflection image extraction method with multiple eye images as input. We align the iris regions of multiple eye images with the help of iris localization method, and by comparing multiple iris regions, we obtain the complementary iris regions, so that the iris interference in the corneal reflection region can be stripped completely. A large number of experiments have demonstrated that our work can effectively mitigate iris interference and effectively improve the quality of corneal reflection images.

Algorithm and software for field distortion correction in a commercial SD-OCT for corneal curvature assessment

Accurate assessment of corneal curvatures using frequency domain optical coherence tomography (OCT) with galvanometer scanners remains challenging due to the well-known scan field distortion. This paper presents an algorithm and software for correcting the distortion using only two simple measurements in which a readily available standard sphere is positioned in different depths in front of the OCT scanner. This offers a highly accessible and easily reproducible method for the field distortion correction (FDC). The correction was validated by measuring different spherical phantoms and conducting corneal curvature measurements of ex vivo porcine corneas using a commercial spectral-domain OCT system and a clinically approved swept-source OCT as a reference instrument. Thus, the error in radius measurements of spherical phantoms was reduced by >90% and astigmatism by >80% using FDC. In explanted porcine eyes, the error in astigmatism measurements with the Telesto was reduced by 75% for power and 70% for angle. The best fitting sphere radius was determined up to a deviation of 0.4% from the Anterion. This paper describes a correction algorithm for OCT immanent distortion that is applicable to any scanning OCT setup and enables precise corneal curvature measurements. The MATLAB software for the FDC is publicly available on GitHub.

Current concepts in the management of cataract with keratoconus

This review analyzed all pertinent articles on keratoconus (KCN) and cataract surgery. It covers preoperative planning, intraoperative considerations, and postoperative management, with the aim of providing a simplified overview of treating such patients. Preoperatively, the use of corneal cross-linking, intrastromal corneal ring segments, and topo-guided corneal treatments can help stabilize the cornea and improve the accuracy of biometric measurements. It is important to consider the advantages and disadvantages of traditional techniques such as penetrating keratoplasty and deep anterior lamellar keratoplasty, as well as newer stromal augmentation techniques, to choose the most appropriate surgical approach. Obtaining reliable measurements can be difficult, especially in the advanced stages of the disease. The choice between toric and monofocal intraocular lenses (IOLs) should be carefully evaluated. Monofocal IOLs are a better choice in patients with advanced disease, and toric lenses can be used in mild and stable KCN. Intraoperatively, the use of a rigid gas permeable (RGP) lens can overcome the challenge of image distortion and loss of visual perspective. Postoperatively, patients may need updated RGP or scleral lenses to correct the corneal irregular astigmatism. A thorough preoperative planning is crucial for good surgical outcomes, and patients need to be informed regarding potential postoperative surprises. In conclusion, managing cataracts in KCN patients presents a range of challenges, and a comprehensive approach is essential to achieve favorable surgical outcomes.

Corneal Tomography in a Commercially Available Optical Coherence Tomography Device and Comparison With a Rotating Scheimpflug Camera

OBJECTIVE

To compare the outcomes of corneal tomography obtained with an anterior-to-posterior segment optical coherence tomography device (Revo, Optopol Technologies Sp. z o.o.) and a rotating Scheimpflug camera (Pentacam AXL, Oculus Optikgeräte, Wetzlar, Germany).

METHODS

In healthy subjects, agreement in stereometric parameters commonly used in cataract and refractive surgery was assessed. Comparison between the devices was performed using correlation coefficients, the Bland-Altman method, and a paired t test.

RESULTS

Results of right eyes of 78 patients (47 women) aged 25.24±5.56 years were analyzed. The flat and steep anterior keratometry values were significantly higher for Revo than Pentacam (43.65±1.58 D vs. 42.99±1.47 D; P = 0.000, and 44.53±1.57 D vs. 43.82±1.49 D; P =0.000, respectively) and showed excellent correlation ( r =0.978 and 0.974, respectively). The results for maximal keratometry manifested a higher difference (45.89±1.69 D vs. 44.27±1.51 D for Revo and Pentacam, P =0.000) but were also strongly correlated ( r =0.871). Revo showed significantly lower corneal thickness values than Pentacam, both for apex pachymetry (535.54±32.45 vs. 550.74±31.55 μm; P =0.000) and for thinnest pachymetry (522.58±32.16 vs. 547.25±31.95 μm; P =0.000). The correlation coefficients for anterior and posterior corneal elevation at the thinnest point showed weak positive correlation ( r =0.179 and 0.185), while the correlation for corneal asphericity was moderate ( r =0.317).

CONCLUSIONS

There was a significant difference between keratometric values obtained with Revo and the Pentacam AXL, although the measurements showed excellent correlation. Similar results were found for corneal thickness measurements, but not for corneal surface elevation and corneal asphericity.

Medmont Meridia™: Much More Than a Corneal Topographer

In recent years, advanced diagnostic instruments have become widely available in optometric practices, offering various functions that optimize clinical information gathering. This article focuses on the Medmont Meridia™, a state-of-the-art multipurpose diagnostic device with corneal topography and cutting-edge features. Corneal topography is pivotal in the early diagnosis of corneal disorders, determining baseline ocular surface assessment, helping in contact lens fitting, and monitoring ocular health over time. The Medmont Meridia boasts Placido-disc-based imaging with extensive corneal coverage. Furthermore, the Meridia accurately measures the horizontal visible iris diameter, pupil diameter, and palpebral fissure width, which assists in making contact lens parameter decisions. Additionally, it offers sagittal height data for scleral lens design and first lens selection, streamlining the fitting process. Beyond its topography capabilities, the Meridia excels as a comprehensive dry eye assessment tool. With features like tear meniscus height, tear film surface quality, and meibography capabilities, it aids in diagnosing dry eye and monitoring its progression. The device also provides customizable dry eye reports with integrated grading scales and questionnaires, making dry eye management, patient education, and compliance more accessible. In conclusion, the Medmont Meridia consolidates a multitude of examination tools in a single instrument, enhancing practice efficiency and elevating patient care and communication. Its versatility and accuracy make it an invaluable asset in optometric practices worldwide.

Artificial intelligence for detecting keratoconus

BACKGROUND

Keratoconus remains difficult to diagnose, especially in the early stages. It is a progressive disorder of the cornea that starts at a young age. Diagnosis is based on clinical examination and corneal imaging; though in the early stages, when there are no clinical signs, diagnosis depends on the interpretation of corneal imaging (e.g. topography and tomography) by trained cornea specialists. Using artificial intelligence (AI) to analyse the corneal images and detect cases of keratoconus could help prevent visual acuity loss and even corneal transplantation. However, a missed diagnosis in people seeking refractive surgery could lead to weakening of the cornea and keratoconus-like ectasia. There is a need for a reliable overview of the accuracy of AI for detecting keratoconus and the applicability of this automated method to the clinical setting.

OBJECTIVES

To assess the diagnostic accuracy of artificial intelligence (AI) algorithms for detecting keratoconus in people presenting with refractive errors, especially those whose vision can no longer be fully corrected with glasses, those seeking corneal refractive surgery, and those suspected of having keratoconus. AI could help ophthalmologists, optometrists, and other eye care professionals to make decisions on referral to cornea specialists. Secondary objectives To assess the following potential causes of heterogeneity in diagnostic performance across studies. • Different AI algorithms (e.g. neural networks, decision trees, support vector machines) • Index test methodology (preprocessing techniques, core AI method, and postprocessing techniques) • Sources of input to train algorithms (topography and tomography images from Placido disc system, Scheimpflug system, slit-scanning system, or optical coherence tomography (OCT); number of training and testing cases/images; label/endpoint variable used for training) • Study setting • Study design • Ethnicity, or geographic area as its proxy • Different index test positivity criteria provided by the topography or tomography device • Reference standard, topography or tomography, one or two cornea specialists • Definition of keratoconus • Mean age of participants • Recruitment of participants • Severity of keratoconus (clinically manifest or subclinical) SEARCH METHODS: We searched CENTRAL (which contains the Cochrane Eyes and Vision Trials Register), Ovid MEDLINE, Ovid Embase, OpenGrey, the ISRCTN registry, ClinicalTrials.gov, and the World Health Organization International Clinical Trials Registry Platform (WHO ICTRP). There were no date or language restrictions in the electronic searches for trials. We last searched the electronic databases on 29 November 2022.

SELECTION CRITERIA

We included cross-sectional and diagnostic case-control studies that investigated AI for the diagnosis of keratoconus using topography, tomography, or both. We included studies that diagnosed manifest keratoconus, subclinical keratoconus, or both. The reference standard was the interpretation of topography or tomography images by at least two cornea specialists.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted the study data and assessed the quality of studies using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. When an article contained multiple AI algorithms, we selected the algorithm with the highest Youden's index. We assessed the certainty of evidence using the GRADE approach.

MAIN RESULTS

We included 63 studies, published between 1994 and 2022, that developed and investigated the accuracy of AI for the diagnosis of keratoconus. There were three different units of analysis in the studies: eyes, participants, and images. Forty-four studies analysed 23,771 eyes, four studies analysed 3843 participants, and 15 studies analysed 38,832 images. Fifty-four articles evaluated the detection of manifest keratoconus, defined as a cornea that showed any clinical sign of keratoconus. The accuracy of AI seems almost perfect, with a summary sensitivity of 98.6% (95% confidence interval (CI) 97.6% to 99.1%) and a summary specificity of 98.3% (95% CI 97.4% to 98.9%). However, accuracy varied across studies and the certainty of the evidence was low. Twenty-eight articles evaluated the detection of subclinical keratoconus, although the definition of subclinical varied. We grouped subclinical keratoconus, forme fruste, and very asymmetrical eyes together. The tests showed good accuracy, with a summary sensitivity of 90.0% (95% CI 84.5% to 93.8%) and a summary specificity of 95.5% (95% CI 91.9% to 97.5%). However, the certainty of the evidence was very low for sensitivity and low for specificity. In both groups, we graded most studies at high risk of bias, with high applicability concerns, in the domain of patient selection, since most were case-control studies. Moreover, we graded the certainty of evidence as low to very low due to selection bias, inconsistency, and imprecision. We could not explain the heterogeneity between the studies. The sensitivity analyses based on study design, AI algorithm, imaging technique (topography versus tomography), and data source (parameters versus images) showed no differences in the results.

AUTHORS' CONCLUSIONS

AI appears to be a promising triage tool in ophthalmologic practice for diagnosing keratoconus. Test accuracy was very high for manifest keratoconus and slightly lower for subclinical keratoconus, indicating a higher chance of missing a diagnosis in people without clinical signs. This could lead to progression of keratoconus or an erroneous indication for refractive surgery, which would worsen the disease. We are unable to draw clear and reliable conclusions due to the high risk of bias, the unexplained heterogeneity of the results, and high applicability concerns, all of which reduced our confidence in the evidence. Greater standardization in future research would increase the quality of studies and improve comparability between studies.

Upper Eyelid Blepharoplasty: Surgical Techniques and Results-Systematic Review and Meta-analysis

BACKGROUND

Upper eyelid blepharoplasty is a surgical procedure that aims to correct the typical changes that occur with aging to the periorbital area. The outcomes of this surgery are aesthetic, as well as functional. Many studies have described an impact on the cornea, intraocular pressure, dry eye syndrome, and visual quality. The aim of this systematic review is to compare the different surgical techniques and their outcomes.

METHODS

The authors performed a literature review through online databases PubMed, Web of Science, Clinicaltrials.gov, and CENTRAL libraries. Information was collected about the surgery techniques and the functional and aesthetic outcomes as well as complications of the interventions. Six types of upper blepharoplasty surgery were studied. Data were analyzed using Cochrane RevMan.

RESULTS

Twenty studies were included in our systematic review and nine in our meta-analysis. We presented results about intraocular pressure, central corneal thickness, flattest keratometry, steepest keratometry, corneal astigmatism, visual acuity, Schirmer test 1 and 2, tear film break-up time and the ocular surface disease index questionnaire, according to type of surgery. Our meta-analysis showed no significant results.

CONCLUSIONS

No significant results were found; however, many studies reported an impact of upper blepharoplasty surgery in the outcomes studied. Only a small number of complications were reported, and patients were satisfied with the aesthetic outcomes.

LEVEL OF EVIDENCE III

This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors https://www.springer.com/00266 .

Keratoconus Diagnosis and Treatment: Recent Advances and Future Directions

Keratoconus is a disorder characterized by progressive corneal thinning and steepening that may result in significant visual impairment secondary to high astigmatism, corneal scarring, or even corneal perforation. Early detection and screening of keratoconus are essential for effective management and treatment. Several screening methods, such as corneal topography and tomography, corneal biomechanics, and genetic testing, are being developed to detect keratoconus at an early stage. Once detected, prevention of progression is the mainstay of keratoconus management. Corneal collagen cross-linking is a minimally invasive treatment option that can slow or halt the progression of keratoconus. Additionally, recent studies have investigated the potential use of copper sulfate eye drops (IVMED-80) and extracellular vesicles to prevent the progression of keratoconus as non-invasive treatment options. For visual rehabilitation, currently available treatments include scleral lenses, intracorneal ring segments, corneal allogenic intrastromal ring segments, and deep anterior lamellar keratoplasty. The safety and efficacy of these emerging treatment options for keratoconus are currently being investigated.

Bibliometric and visualized analysis of myopic corneal refractive surgery research: from 1979 to 2022

Background

Myopic corneal refractive surgery is one of the most prevalent ophthalmic procedures for correcting ametropia. This study aimed to perform a bibliometric analysis of research in the field of corneal refractive surgery over the past 40 years in order to describe the current international status and to identify most influential factors, while highlighting research hotspots.

Methods

A bibliometric analysis based on the Web of Science Core Collection (WoSCC) was used to analyze the publication trends in research related to myopic corneal refractive surgery. VOSviewer v.1.6.10 was used to construct the knowledge map in order to visualize the publications, distribution of countries, international collaborations, author productivity, source journals, cited references, keywords, and research hotspots in this field.

Results

A total of 4,680 publications on myopic corneal refractive surgery published between 1979 and 2022 were retrieved. The United States has published the most papers, with Emory University contributing to the most citations. The Journal of Cataract and Refractive Surgery published the greatest number of articles, and the top 10 cited references mainly focused on outcomes and wound healing in refractive surgery. Previous research emphasized "radial keratotomy (RK)" and excimer laser-associated operation methods. The keywords containing femtosecond (FS) laser associated with "small incision lenticule extraction (SMILE)" and its "safety" had higher burst strength, indicating a shift of operation methods and coinciding with the global trends in refractive surgery. The document citation network was clustered into five groups: (1) outcomes of refractive surgery: (2) preoperative examinations for refractive surgery were as follows: (3) complications of myopic corneal refractive surgery; (4) corneal wound healing and cytobiology research related to photorefractive laser keratotomy; and (5) biomechanics of myopic corneal refractive surgery.

Conclusion

The bibliometric analysis in this study may provide scholars with valuable to information and help them better understand the global trends in myopic corneal refractive surgery research frontiers. Two stages of rapid development occurred around 1991 and 2013, shortly after the innovation of PRK and SMILE surgical techniques. The most cited articles mainly focused on corneal wound healing, clinical outcomes, ocular aberration, corneal ectasia, and corneal topography, representing the safety of the new techniques.

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.

Changes of the effective optical zone after small-incision lenticule extraction and a correlation analysis

The objective of the study is to observe the changes in the effective optical zone (EOZ) after small incision lenticule extraction (SMILE) and explore possible correlations with some influencing factors. In total, 133 eyes after SMILE were divided into the mild to moderate myopia group (- 1.75 D to - 5.75 D, 70 eyes) and the high myopia group (- 6.00 D to - 9.50 D, 63 eyes). The postoperative EOZ was calculated by utilizing the corneal tangential curvature map. Changes in EOZ (△-OZ) were monitored and compared between the two groups. Pearson correlation analysis was conducted to determine the correlation between △-OZ and corneal high-order wavefront aberrations. Multicollinearity analysis and ridge regression analysis were performed to assess the correlation between △-OZ and some corneal parameters. After SMILE, the horizontal EOZ (H-EOZ), vertical EOZ (V-EOZ), and average EOZ (A-EOZ) were significantly smaller than the programmed optical zone (POZ) in both groups (p < 0.05). The difference between V-EOZ and POZ (△V-OZ) and the difference between A-EOZ and POZ (△A-OZ) showed more significant changes in the high myopia group than in the mild to moderate myopia group, and △V-OZ was significantly larger than the difference between H-EOZ and POZ (△H-OZ) in the high myopia group. In both groups, the total high-order aberration (T-HOA) and spherical aberration (SA) both increased after SMILE, and they had a similar significant negative correlation with A-EOZ. Moreover, there was a significant negative correlation between △-OZ and Km (X1), Q-value (X2), spherical equivalent (SE, X3), ablating depth (AD, X4) and △e (X6), and a significant positive correlation between △-OZ and △Q (X5). △H-OZ was expressed as Y1, △V-OZ as Y2, and △A-OZ as Y3. The multiple linear regression equations were as follows: Y1 = 3.683 - 0.065X1, Y2 = 1.549 - 0.469X2 - 0.059X3, Y3 = 4.015 - 0.07X1 - 0.03X3, Y1 = 1.337 - 0.005X4 + 0.413X5, Y2 = 1.265 + 0.469X5, and Y3 = 0.852 - 0.002X4 - 0.398X6. The correlation degree with △A-OZ was ranked as Km > △Q > Q-value > AD > e-value > △e > SE > △Km, as represented by the ridge regression analysis. The EOZ was irregularly reduced after SMILE, which should be taken into consideration in the design of POZ, especially for high myopia. Consideration of the refractive diopter and corneal topography is advised for the design of POZ, the latter of which has greater reference significance.

Acoustic Micro-Tapping Optical Coherence Elastography to Quantify Corneal Collagen Cross-Linking: An Ex Vivo Human Study

Purpose

To evaluate changes in the anisotropic elastic properties of ex vivo human cornea treated with ultraviolet cross-linking (CXL) using noncontact acoustic micro-tapping optical coherence elastography (AμT-OCE).

Design

Acoustic micro-tapping OCE was performed on normal and CXL human donor cornea in an ex vivo laboratory study.

Subjects

Normal human donor cornea (n = 22) divided into 4 subgroups. All samples were stored in optisol.

Methods

Elastic properties (in-plane Young's, E, and out-of-plane, G, shear modulus) of normal and ultraviolet CXL-treated human corneas were quantified using noncontact AμT-OCE. A nearly incompressible transverse isotropic model was used to reconstruct moduli from AμT-OCE data. Independently, cornea elastic moduli were also measured with destructive mechanical tests (tensile extensometry and shear rheometry).

Main Outcome Measures

Corneal elastic moduli (in-plane Young's modulus, E, in-plane, μ, and out-of-plane, G, shear moduli) can be evaluated in both normal and CXL treated tissues, as well as monitored during the CXL procedure using noncontact AμT-OCE.

Results

Cross-linking induced a significant increase in both in-plane and out-of-plane elastic moduli in human cornea. The statistical mean in the paired study (presurgery and postsurgery, n = 7) of the in-plane Young's modulus, E = 3 μ , increased from 19 MPa to 43 MPa, while the out-of-plane shear modulus, G, increased from 188 kPa to 673 kPa. Mechanical tests in a separate subgroup support CXL-induced cornea moduli changes and generally agree with noncontact AμT-OCE measurements.

Conclusions

The human cornea is a highly anisotropic material where in-plane mechanical properties are very different from those out-of-plane. Noncontact AμT-OCE can measure changes in the anisotropic elastic properties in human cornea as a result of ultraviolet CXL.

Patient selection for corneal topographic evaluation of keratoconus: A screening approach using artificial intelligence

Background

Corneal topography is a clinically validated examination method for keratoconus. However, there is no clear guideline regarding patient selection for corneal topography. We developed and validated a novel artificial intelligence (AI) model to identify patients who would benefit from corneal topography based on basic ophthalmologic examinations, including a survey of visual impairment, best-corrected visual acuity (BCVA) measurement, intraocular pressure (IOP) measurement, and autokeratometry.

Methods

A total of five AI models (three individual models with fully connected neural network including the XGBoost, and the TabNet models, and two ensemble models with hard and soft voting methods) were trained and validated. We used three datasets collected from the records of 2,613 patients' basic ophthalmologic examinations from two institutions to train and validate the AI models. We trained the AI models using a dataset from a third medical institution to determine whether corneal topography was needed to detect keratoconus. Finally, prospective intra-validation dataset (internal test dataset) and extra-validation dataset from a different medical institution (external test dataset) were used to assess the performance of the AI models.

Results

The ensemble model with soft voting method outperformed all other AI models in sensitivity when predicting which patients needed corneal topography (90.5% in internal test dataset and 96.4% in external test dataset). In the error analysis, most of the predicting error occurred within the range of the subclinical keratoconus and the suspicious D-score in the Belin-Ambrósio enhanced ectasia display. In the feature importance analysis, out of 18 features, IOP was the highest ranked feature when comparing the average value of the relative attributions of three individual AI models, followed by the difference in the value of mean corneal power.

Conclusion

An AI model using the results of basic ophthalmologic examination has the potential to recommend corneal topography for keratoconus. In this AI algorithm, IOP and the difference between the two eyes, which may be undervalued clinical information, were important factors in the success of the AI model, and may be worth further reviewing in research and clinical practice for keratoconus screening.

Detection of Corneal Ectasia Using OCT Maps of Pachymetry and Posterior Surface Mean Curvature

PURPOSE

To quantify the abnormal corneal thinning and posterior surface steepening that is observed in keratoconus with an Ectasia Index.

METHODS

Optical coherence tomography (OCT) was used to image the corneas of normal individuals and patients with varying stages of keratoconus (manifest, subclinical, and forme fruste). Maps of corneal pachymetry and posterior surface mean curvature were generated, and an Ectasia Index was calculated by multiplying Gaussian fits obtained from the two types of maps. Repeated five-fold cross-validation was used to evaluate the ability of the Ectasia Index to differentiate between normal and keratoconic eyes. The classification performance of the Ectasia Index was compared to minimum pachymetry and maximum posterior mean curvature.

RESULTS

Thirty-two eyes from 16 normal individuals, 89 eyes from 63 patients with manifest keratoconus, 16 eyes from 15 patients with subclinical keratoconus, and 26 eyes from 26 patients with forme fruste keratoconus were included in the study. During cross-validation, 100% of the eyes with manifest (89 of 89) and subclinical (16 of 16) keratoconus were correctly classified by the Ectasia Index. The average classification accuracy for the forme fruste keratoconus group was 63 ± 21% (16.4 of 26). The specificity for the normal group was 91 ± 10% (29.1 of 32). The Ectasia Index had a higher sensitivity for keratoconus detection and similar specificity in comparison to minimum pachymetry and maximum posterior mean curvature.

CONCLUSIONS

The Ectasia Index could be a valuable additional metric for clinicians to consider when screening for keratoconus. [J Refract Surg. 2022;38(8):502-510.].

Fourier analysis on irregular corneal astigmatism using optical coherence tomography in various severity stages of keratoconus

PURPOSE

To investigate the diagnostic capability of Fourier indices in detecting clinical or subclinical keratoconus (KC).

DESIGN

Prospective cross-sectional study METHODS: : The study included 126 eyes with clinical KC (50 KC without any corneal scar, 50 KC with anterior corneal scar, and 26 KC with posterior scar having a history of acute corneal hydrops), 50 with topographic KC (without clinical signs), 50 with pre-topographic KC (normal topography without clinical signs), and 50 controls. Corneal tomographic data were obtained using anterior segment optical coherence tomography (OCT). Fourier analysis decomposed dioptric data from both anterior and posterior corneal surface into spherical, regular astigmatism, asymmetry, and higher-order irregularity components. The discriminating ability of the Fourier indices of pre-topographic KC, topographic KC, and clinical KC from controls were assessed after quantitative Fourier analysis of irregular corneal astigmatism.

RESULTS

Posterior asymmetry and higher-order irregularity components were significantly greater in pre-topographic KC eyes than those in controls (p<0.001 for both), with the highest area under the receiver operating characteristic curve (AUROC) of 0.778 and 0.709, respectively. The same was true for anterior asymmetry, posterior asymmetry, and posterior higher-order irregularity components in topographic KC (AUROC of 0.945, 0.941, and 0.893, respectively), whereas it was >0.948 for all Fourier components in clinical KC.

CONCLUSIONS

Fourier analysis using OCT can evaluate anterior and posterior corneal irregular astigmatism of various KC stages, from very mild to advanced, including severe cases with corneal scar. Irregular astigmatism indices from the posterior corneal surface showed the highest AUROC values in discriminating early KC stages.

Performances of Corneal Topography and Tomography in the Diagnosis of Subclinical and Clinical Keratoconus

Aim

The purpose of the study was to assess the efficacy of topographical and tomographical indices given by the Pentacam (pachymetric, tomopetric, and aberometric) in clinical and subclinical keratoconus (KCN) diagnosis.

Material and Methods

In this observational analytic retrospective study, patients with abnormal findings in topography and tomography maps but with no signs on clinical examination (subclinical KCN group, sKCN), patients with clinical keratoconus (KCN group), and healthy subjects (Control group) were evaluated.

Results

The KCN group proved significantly different (p < 0.001) values of the investigated parameters than the Control group. Eleven out of 28 investigated parameters proved significantly different in the sKCN group compared to controls (p < 0.001). Two topographic measurements, namely I-S (cut-off = 1.435, a large value indicates the presence of KCN) and CCT (cut-off = 537, a small value indicates the presence of KCN), showed AUCs equal to 1 [0.999 to 1]. Six other Pentacam measurements, including Back maximum keratometry (Back Kmax) proved to be excellent parameters for case-finding and screening. In distinguishing sKCN from normal eyes, Pentacam index of vertical asymmetry (IVA), inferior-superior difference (I-S) value, thinnest point (TP), Belin Ambrosio Enhanced Ectasia Display (BAD_D) and root mean square total (RMS total) performed best.

Conclusions

In distinguishing sKCN from normal eyes, Back Kmax, IVA, I-S, and RMS total values demonstrated higher accuracy and utility. Six indices, namely ISV, IVA, KISA, PRC, RMS-HOA, and Back Kmax demonstrate excellent utility in case-finding and screening for clinical KCN.

Comparison of Javal-Schiøtz keratometer, Orbscan IIz and Pentacam topographers in evaluating anterior corneal topography

CLINICAL RELEVANCE

Inter-instrument variation in anterior corneal shape (ACS) measurement has a consequence for ocular clinical practice.

BACKGROUND

To consider inter-instrument variability in keratometry measurements across the ACS and to explore instrument protocols for determining ACS keratometric analogues (KAs).

METHODS

  Mean keratometry/KAs of the right eye were recorded using Javal-Schiøtz keratometer (J-S), Orbscan IIz and Pentacam from 124 subjects (78 females; mean ± SD age: 24.71 ± 6.61 years). Mean radii of curvature were obtained for 1-mm wide annular zones extending up to 6 mm (horizontally) and 4 mm (vertically) from the apex for Orbscan and Pentacam. Zonal mean radius of curvature was calculated by averaging keratometry values for all measured points within the zone.

RESULTS

KA (mean ± SD): Horizontal: Orbscan (7.80 ± 0.31 mm) and J-S (7.82 ± 0.29 mm) were not significantly different (p = 0.072). Pentacam (7.86 ± 0.29 mm) was significantly flatter than J-S (p < 0.001) and Orbscan (p < 0.001). Vertical: Orbscan (7.64 ± 0.31 mm) was significantly steeper than J-S (7.67 ± 0.29 mm, p < 0.005) and Pentacam (7.70 ± 0.29 mm, p < 0.001). Pentacam was significantly flatter than J-S (p < 0.001) and significant flatter than Orbscan across the entire profile (1-4 mm zones horizontal and vertical, p < 0.001).                        LoAs (CI): J-S/Orbscan: ±0.75 mm (0.05-0.18); J-S/Pentacam: ±0.72 mm (0.01-0.12); Pentacam/Orbscan: ±0.16 mm (0.04-0.08). There was a +0.03 mm positive bias for Orbscan compared to J-S, +0.06 mm positive bias for Orbscan compared to Pentacam and -0.03 mm negative bias for Pentacam compared to J-S.

CONCLUSIONS

Algorithms used by Orbscan and Pentacam to solve the peripheral paraxial ray problem produce significantly different KAs. Instrument-specific KAs cannot be used inter-changeably between instruments. Differences in KA between instruments are not significant for ocular surgery, but may influence rigid contact lens fitting. Pentacam measures flatter than Orbscan and J-S.

Repeatability and reproducibility of the galilei G6 and its agreement with the pentacam® AXL in optical biometry and corneal tomography

OBJECTIVES

To assess the repeatability and reproducibility of corneal measurements performed by the Galilei G6 and determine its agreement with the Pentacam® AXL.

METHODS

105 eyes underwent examination by both devices, measuring nine parameters. Paired t-tests, Deming Regression, and Bland-Altman plots were used to determine agreement. Analysis of Variance was used to determine repeatability and reproducibility.

RESULTS

Measurements showed no clinically significant differences between the two devices. On average, the Galilei G6 measured axial length longer than the Pentacam® AXL by 0.05 mm ± 0.03 mm (p < 0.001), just 0.2% of the mean value and therefore clinically insignificant. It measured central corneal thickness and anterior chamber depth 3.77 μm ± 7.71 μm longer (p < 0.001) and 0.04 mm ± 0.07 μm shorter (p < 0.001), respectively. They are also clinically insignificant, constituting just 0.7% and 1% of the mean values. The results showed evidence of repeatability and reproducibility. Only measurements of corneal cylinder showed some clinically significant variance.

CONCLUSIONS

The Galilei G6 and Pentacam® AXL measurements show evidence of repeatability, reproducibility, and agreement for examined parameters. Certain caution needs to be applied in cases with moderate or severe corneal cylinder, due to discrepancies in repeatability and reproducibility of corneal cylinder measurements with the Galilei G6.

Trends in Corneal Topography and Tomography Imaging for Keratoconus Management

Purpose

To describe trends in the use of corneal topography and tomography for the management of keratoconus patients at a single academic institution and to identify how these trends may be associated with different procedural interventions.

Patients and Methods

In this retrospective cohort study, keratoconus patients treated from 2012 to 2019 were identified. The electronic health record was reviewed for the presence of corneal topography or tomography imaging completed within seven days of the first visit and the highest level of intervention at the first and most recent visit. Patients were grouped as pediatric (<18 years) or adult (≥18 years). Chi-square tests and linear regressions were used to evaluate trends and to determine which factors were predictive for receiving corneal collagen cross-linking (CXL) versus other surgical interventions (intrastromal corneal ring segments, lamellar keratoplasty, or penetrating keratoplasty) by the most recent visit.

Results

A total of 873 keratoconus patients met inclusion criteria. The use of corneal topography at the first visit remained relatively consistent from 2012 to 2019, while corneal tomography usage at the first visit increased from 3.9% in 2015, when corneal tomography was introduced, to 52.8% in 2019. Each year was associated with an 11.2% ± 1.9% increase in the use of corneal tomography at the first visit in pediatric patients and 6.7% ± 0.5% in adult patients. Use of corneal tomography at the first visit was a significant predictor for receiving CXL procedures (P < 0.001) and a negative predictor for requiring other surgical interventions (P = 0.032) when controlling for the year of the first visit.

Conclusion

Obtaining corneal tomography at the first visit has become the standard of care in keratoconus, especially for pediatric patients. Moreover, the increase in corneal tomography imaging has coincided with an increase in CXL procedures and decrease in other surgical interventions.

Automated vs. human evaluation of corneal staining

BACKGROUND AND PURPOSE

Corneal fluorescein staining is one of the most important diagnostic tests in dry eye disease (DED). Nevertheless, the result of this examination is depending on the grader. So far, there is no method for an automated quantification of corneal staining commercially available. Aim of this study was to develop a software-assisted grading algorithm and to compare it with a group of human graders with variable clinical experience in patients with DED.

METHODS

Fifty images of eyes stained with 2 µl of 2% fluorescein presenting different severity of superficial punctate keratopathy in patients with DED were taken under standardized conditions. An algorithm for detecting and counting superficial punctate keratitis was developed using ImageJ with a training dataset of 20 randomly picked images. Then, the test dataset of 30 images was analyzed (1) by the ImageJ algorithm and (2) by 22 graders, all ophthalmologists with different levels of experience. All graders evaluated the images using the Oxford grading scheme for corneal staining at baseline and after 6-8 weeks. Intrarater agreement was also evaluated by adding a mirrored version of all original images into the set of images during the 2nd grading.

RESULTS

The count of particles detected by the algorithm correlated significantly (n = 30; p < 0.01) with the estimated true Oxford grade (Sr = 0,91). Overall human graders showed only moderate intrarater agreement (K = 0,426), while software-assisted grading was always the same (K = 1,0). Little difference was found between specialists and non-specialists in terms of intrarater agreement (K = 0,436 specialists; K = 0,417 non-specialists). The highest interrater agreement was seen with 75,6% in the most experienced grader, a cornea specialist with 29 years of experience, and the lowest was seen in a resident with 25,6% who had only 2 years of experience.

CONCLUSION

The variance in human grading of corneal staining - if only small - is likely to have only little impact on clinical management and thus seems to be acceptable. While human graders give results sufficient for clinical application, software-assisted grading of corneal staining ensures higher consistency and thus is preferrable for re-evaluating patients, e.g., in clinical trials.

Comparison of Anterior Corneal Aberrometry, Keratometry and Pupil Size with Scheimpflug Tomography and Ray Tracing Aberrometer

This study aimed to assess the anterior corneal wavefront aberrations, keratometry, astigmatism vectors and pupil size between Pentacam HR® (Oculus Optikgeraete GmbH, Wetzlar, Germany) and iTrace® (Tracey Technologies Corp., Houston, TX, USA). In this observational study, 100 eyes (50 healthy volunteers) were scanned in mesopic light condition with a Pentacam HR® and iTrace®. Anterior corneal aberrations (spherical aberration (Z40), vertical coma (Z3 − 1), horizontal coma (Z3 + 1)), keratometry in the flattest (K1) and steepest meridian (K2), mean astigmatism, astigmatic vectors (J0 and J45), and pupil size were measured. We found a significant difference in Z40 (Pentacam®: +0.30 ± 0.11 µm and iTrace®: −0.03 µm ± 0.05 µm; p < 0.01) with no correlation between the devices (r = −0.12, p = 0.22). The devices were in complete agreement for Z3 − 1 (p = 0.78) and Z3 + 1 (p = 0.39), with significant correlation between the machines (r = −0.38, p < 0.01 and r = −0.6, p < 0.01). There was no difference in K1, K2 and mean astigmatism. J0 was negative with both devices (against-the-rule astigmatism), but there was no correlation. J45 was negative with the Pentacam HR® (more myopic oblique astigmatism) but significantly correlated between the devices. Pupil size was smaller with Pentacam HR® (p < 0.01). In summary, these devices cannot be used interchangeably. Corneal Z40 was significantly different with more negative Z40 with iTrace® compared to Pentacam HR®. iTrace® operates with lower illumination, giving larger pupil size than Pentacam HR®, which uses intense blue light during measurement. No correlation was found for J0. Pentacam HR® had a trend to record more negative J45 (myopic oblique astigmatism).

Comparing Changes in Corneal Astigmatism Using Scheimpflug Camera after Epiblepharon Correction Surgery

Purpose: To evaluate the changes in corneal astigmatism before and after epiblepharon correction surgery with a Scheimpflug camera.Methods: From April, 2019, to June, 2020, 48 eyes of 24 patients underwent epiblepharon correction surgery. The patients were divided into two subgroups by age and their best corrected visual acuity (BCVA) and corneal anterior and posterior astigmatism were compared before and after correction surgery.Results: There was no difference in BCVA after the correction surgery. To assess the changes in the keratometric value and astigmatism after epiblepharon surgery, a Scheimpflug camera was used. The anterior flat and mean keratometry changed from 42.10 ± 1.46 to 42.43 ± 1.10 D (p = 0.035) and from 42.97 ± 1.26 to 43.21 ± 1.15 D (p = 0.012), respectively, while the posterior corneal astigmatism changed from 0.44 ± 0.31 to 0.35 ± 0.17 D (p = 0.047). There were no significant changes in steep keratometry (p = 0.191) or anterior corneal astigmatism (p = 0.126).Conclusions: There were significant changes after epiblepharon correction surgery in anterior keratometry reading and posterior corneal astigmatism.

Effect of Implantable Collamer Lens on Anterior Segment Measurement and Intraocular Lens Power Calculation Based on IOLMaster 700 and Sirius

Purpose

To investigate the possible effect of an implantable collamer lens (ICL) on ocular biometrics and intraocular lens (IOL) power calculation.

Methods

Ocular measurements were taken preoperatively and at the two-month follow-up using IOLMaster 700 and Sirius in 85 eyes (43 patients) who had previously undergone ICL surgery. IOL power was calculated using either IOLMaster 700 (Barrett Universal II formula) or Sirius (ray-tracing). All data were compared using the paired t-test.

Results

The difference between preoperative and postoperative anterior chamber depth (ACD), lens thickness (LT), and keratometry on the steep axis (K2) measured by IOLMaster 700 was statistically significant (p < 0.001). In 11 of 85 eyes, IOLMaster misjudged the anterior surface of the ICL as that of the lens, leading to an error in ACD and LT. There were no significant differences between preoperative and postoperative axial length (AL) (p = 0.223), white to white (WTW) (p = 0.100), keratometry on flat axis (K1) (p = 0.117), or central corneal thickness (CCT) (p = 0.648), measured using IOLMaster. The difference in IOL power calculated using the Barrett II formula was significant (p = 0.013). Regression analysis showed that AL and K had the greatest influence on IOL calculation (p < 0.001), and ACD and LT had less influence (p = 0.002, p = 0.218, respectively). K1 and K2 were modified to exclude the influence of K2, and modified IOLs showed no difference between pre and postoperation (p = 0.372). Preoperative and postoperative ACD measured using Sirius were significantly different (p < 0.001); however, the IOL power calculated using ray-tracing technology showed no significant differences (p > 0.05).

Conclusions

The ocular biometric apparatus may misjudge the anterior surface of the lens, resulting in measurement errors of ACD and LT, which has little effect on the calculation of IOL power when using IOLMaster 700 (Barrett Universal II formula) and Sirius (ray-tracing).

Time-varying regularity of changes in biomechanical properties of the corneas after removal of anterior corneal tissue

Background

The corneal biomechanical properties with the prolongation of time after corneal refractive surgery are important for providing a mechanical basis for the occurrence of clinical phenomena such as iatrogenic keratectasia and refractive regression. The aim of this study was to explore the changes of corneal elastic modulus, and stress relaxation properties from the 6-month follow-up observations of rabbits after a removal of anterior corneal tissue in simulation to corneal refractive surgery.

Methods

The anterior corneal tissue, 6 mm in diameter and 30–50% of the original corneal thickness, the left eye of the rabbit was removed, and the right eye was kept as the control. The rabbits were normally raised and nursed for 6 months, during which corneal morphology data, and both of corneal hysteresis (CH) and corneal resistance factor (CRF) were gathered. Uniaxial tensile tests of corneal strips were performed at months 1, 3, and 6 from 7 animals, and corneal collagen fibrils were observed at months 1, 3, and 6 from 1 rabbit, respectively.

Results

Compared with the control group, there were statistical differences in the curvature radius at week 2 and month 3, and both CH and CRF at months 1, 2, and 6 in experiment group; there were statistical differences in elastic modulus at 1, 3, and month 6, and stress relaxation degree at month 3 in experiment group. The differences in corneal elastic modulus, stress relaxation degree and the total number of collagen fibrils between experiment and control groups varied gradually with time, and showed significant changes at the 3rd month after the treatment.

Conclusions

Corneas after a removal of anterior corneal tissue undergo dynamic changes in corneal morphology and biomechanical properties. The first 3 months after treatment could be a critical period. The variation of corneal biomechanical properties is worth considering in predicting corneal deformation after a removal of anterior corneal tissue.

Paradox of complex diversity: Challenges in the diagnosis and management of bacterial keratitis

Bacterial keratitis continues to be one of the leading causes of corneal blindness in the developed as well as the developing world, despite swift progress since the dawn of the "anti-biotic era". Although, we are expeditiously developing our understanding about the different causative organisms and associated pathology leading to keratitis, extensive gaps in knowledge continue to dampen the efforts for early and accurate diagnosis, and management in these patients, resulting in poor clinical outcomes. The ability of the causative bacteria to subdue the therapeutic challenge stems from their large genome encoding complex regulatory networks, variety of unique virulence factors, and rapid secretion of tissue damaging proteases and toxins. In this review article, we have provided an overview of the established classical diagnostic techniques and therapeutics for keratitis caused by various bacteria. We have extensively reported our recent in-roads through novel tools for accurate diagnosis of mono- and poly-bacterial corneal infections. Furthermore, we outlined the recent progress by our group and others in understanding the sub-cellular genomic changes that lead to antibiotic resistance in these organisms. Finally, we discussed in detail, the novel therapies and drug delivery systems in development for the efficacious management of bacterial keratitis.

A comparison of central corneal thickness measured using noncontact methods and ultrasonic pachymetry

Aim

The aim of this study is to compare central corneal thickness (CCT) measurements in healthy eyes using ultrasonic pachymetry (UP) with values obtained using anterior segment spectral domain optical coherence tomography (SD-OCT), the Scheimpflug camera system, and noncontact specular microscopy.

Material and Methods

One hundred and twenty-five eyes of 117 healthy individuals (56 women, 61 men) were included. All patients underwent consecutive CCT measurement with UP, the Scheimpflug camera, anterior segment SD-OCT, and specular microscopy. The four devices could be read. Measurements from the four devices were classified in pairs and compared using the paired t-test. Relations between the devices were analyzed using the Pearson test. An alpha error level of P < 0.05 was regarded as statistically significant.

Results

The mean age of the healthy individuals included in the study was 34.75 ± 14.55 years (20-76). Mean CCT values were 541.21 ± 29.24 μm with UP, 539.26 ± 29.27 μm with the Scheimpflug camera system, 532.21 ± 29.53 μm with SD-OCT, and 534.45 ± 29.20 μm with specular microscopy. The differences in measurements at the paired t-test were statistically significant (P < 0.001 for all). However, measurements using the four devices exhibited significant mutual correlation (P < 0.001 for all).

Conclusions

CCT can be measured using different devices. Noncontact methods have now begun replacing UP, regarded as the gold standard. Although CCT values obtained from different devices were correlated, the difference between these measurements was statistically significant. Therefore, the same device should be used for measurement in the diagnosis and follow-up of diseases such as corneal ectasia and glaucoma where corneal thickness is important.

Agreement of Anterior Segment Parameters Between Schiempflug Topography and Swept-Source Optic Coherence Based Optic Biometry in Keratoconus and Healthy Subjects

PURPOSE

The aim of this study is to compare anterior segment parameters, including corneal thickness (CCT), keratometry and anterior chamber depth (ACD), and white to white corneal diameter (WTW), obtained by Pentacam Schiempflug imaging and intraocular lens (IOL) Master 700 swept-source optic coherence tomography biometry in keratoconus patients and healthy subjects.

METHODS

This prospective cross-sectional instrument agreement analysis includes 88 eyes of 50 keratoconus patients and 87 eyes of 50 healthy subjects. Biometry was performed using IOL Master 700, and topography was performed using Pentacam. The keratometry values (Kf, Ks, Km, and Kmax), ACD, WTW, CCT, axial length (AL), anterior chamber angle (ACA), and lens thickness (LT) were evaluated. Levels of agreement between devices were evaluated by Bland-Altman plots with 95% limits of agreement.

RESULTS

Intraocular lens Master 700 showed higher WTW, ACD, pupil diameter, and CCT values than Pentacam in both the keratoconus and control groups. However, there were no statistically significant differences in flat keratometry (Kf) and steep keratometry (Ks) values between the groups.

CONCLUSION

Pentacam and IOL Master 700 may be used interchangeably in normal eyes and keratoconus eyes for the measurement of keratometry values and axis; however, these two devices should not be considered interchangeable for WTW, ACD, pupil diameter, and CCT measurements in both keratoconus patients and healthy subjects.

Application of a scheimpflug-based biomechanical analyser and tomography in the early detection of subclinical keratoconus in chinese patients

Background

In vivo corneal biomechanics evaluation has been used to help screen early keratoconus in recent years. This study is to evaluate the value of a Scheimpflug-based biomechanical analyser combined with tomography in detecting subclinical keratoconus by distinguishing normal eyes from frank keratoconus (KC) and forme frusta keratoconus (FFKC) eyes in Chinese patients.

Methods

Study design: diagnostic test. This study included 31 bilateral frank keratoconus patients, 27 unilateral clinically manifesting keratoconus patients with very asymmetric eyes, and 79 control subjects with normal corneas. Corneal morphological and biomechanical parameters were measured using a Pentacam HR and a Corvis ST (OCULUS, Wetzlar, Germany). The diagnostic ability of computed parameters reflecting corneal biomechanical and morphological traits [including the Belin-Ambrósio deviation index (BAD_D), the Corvis biomechanical index (CBI) and the tomographic and biomechanical index (TBI)] was determined using receiver operating characteristic (ROC) curve analysis and compared by the DeLong test. Additionally, the area under the curve (AUC), the best cut-off values, and the Youden index for each parameter were reported. A novel corneal stiffness parameter, the stress-strain index (SSI), was also compared between KC, FFKC and normal eyes.

Results

Every morphological and biomechanical index analysed in this study was significantly different among KC, FFKC and normal eyes (P = 0.000). The TBI was most valuable in detecting subclinical keratoconus (FFKC eyes), with an AUC of 0.928 (P = 0.000), and both forms of corneal ectasia (FFKC and frank KC eyes), with an AUC of 0.966 (P = 0.000). The sensitivity and specificity of the TBI was 97.5 and 77.8 % in detecting FFKC and 97.5 and 89.7 % in detecting any KC, respectively, with a cut-off value of 0.375. The morphological index BAD_D and the biomechanical index CBI were also very useful in distinguishing eyes with any KC from normal eyes, with AUCs of 0.965 and 0.934, respectively. The SSI was significantly different between KC, FFKC and normal eyes (P = 0.000), indicating an independent decrease in corneal stiffness in KC eyes.

Conclusions

The combination of a Scheimpflug-based biomechanical analyser and tomography could increase the accuracy in detecting subclinical keratoconus in Chinese patients. The TBI was the most valuable index for detecting subclinical keratoconus, with a high sensitivity and specificity. Evaluation of corneal biomechanical properties in refractive surgery candidates could be helpful for recognizing potential keratoconic eyes and increasing surgical safety.

Quality-of-Life Improvement After Scleral Lens Fitting in Patients With Keratoconus

OBJECTIVES

To assess the quality-of-life improvement after scleral lens fitting in patients with keratoconus.

METHODS

We conducted a prospective observational study of all consecutive patients with keratoconus starting scleral lens fitting in 2019, from January 2 to December 27, at the Dijon University Hospital, France. Quality of life was assessed before and after wearing scleral lenses for 3 months through the National Eye Institute Visual Function Questionnaire 25 (NEI-VFQ 25) questionnaire. We investigated factors potentially influencing quality of life: final visual acuity, keratoconus stage, and maximum keratometry reading.

RESULTS

In total, 41 eyes of 24 patients were successfully fitted with scleral lenses, 92.9% were 18 mm in diameter and 7.1% were 17 mm. The visual acuity improved by -0.4±0.3 logarithm of the minimum angle of resolution (P<0.001) after scleral lens fitting. Visual gain increased with keratometry and keratoconus severity (P<0.001; P<0.01). The NEI-VFQ 25 score increased by 19.5±19.1 points on average (P<0.001). No association was found between the factors under study and improvement in quality of life.

CONCLUSION

Our study confirms the functional improvement in patients with keratoconus fitted with scleral lenses, especially in patients with more advanced disease. Moreover, it suggests a major role of these lenses in improving quality of life, independently of the severity of the disease and the visual gain.

Current Developments in Corneal Topography and Tomography

Introduction: Accurate assessment of the corneal shape is important in cataract and refractive surgery, both in screening of candidates as well as for analyzing postoperative outcomes. Although corneal topography and tomography are widely used, it is common that these technologies are confused. The aim of this study was to present the current developments of these technologies and particularly distinguish between corneal topography and tomography. Methods: The PubMed, Web of Science and Embase databases were the main resources used to investigate the medical literature. The following keywords were used in various combinations: cornea, corneal, topography, tomography, Scheimpflug, Pentacam, optical coherence tomography. Results: Topography is the study of the shape of the corneal surface, while tomography allows a three-dimensional section of the cornea to be presented. Corneal topographers can be divided into large- and small-cone Placido-based devices, as well as devices with color-LEDs. For corneal tomography, scanning slit or Scheimpflug imaging and optical coherence tomography may be employed. In several devices, corneal topography and tomography have been successfully combined with tear-film analysis, aberrometry, optical biometry and anterior/posterior segment optical coherence tomography. Conclusion: There is a wide variety of imaging techniques to obtain corneal power maps. As different technologies are used, it is imperative that doctors involved in corneal surgery understand the science and clinical application of devices for corneal evaluation in depth.

Comparison of two novel swept-source optical coherence tomography devices to a partial coherence interferometry-based biometer

To evaluate the repeatability and agreement of corneal and biometry measurements obtained with two swept-source optical coherence tomography (SSOCT) and a partial coherence interferometry-based device. This is a cross-sectional study. Forty-eight eyes of 48 patients had three consecutive measurements for ANTERION (Heidelberg Engineering, Germany), CASIAII (Tomey, Japan) and IOLMaster500 (Carl Zeiss Meditec, USA) on the same visit. Mean keratometry (Km), central corneal thickness (CCT), anterior chamber depth (ACD) and axial length (AL) were recorded. Corneal astigmatic measurements were converted into vector components—J0 and J45. Intra-device repeatability and agreements of measurements amongst the devices were evaluated using repeatability coefficients (RCs) and Bland–Altman plots, respectively. All devices demonstrated comparable repeatability for Km (p ≥ 0.138). ANTERION had the lowest RC for J0 amongst the devices (p ≤ 0.039). Systematic difference was found for the Km and J0 obtained with IOLMaster500 compared to either SSOCTs (p ≤ 0.010). The ACD and AL measured by IOLMaster500 showed a higher RC compared with either SSOCTs (p < 0.002). Systematic difference was found in CCT and ACD between the two SSOCTs (p < 0.001), and in AL between ANTERION and IOLMaster500 (p < 0.001), with a mean difference of 1.6 µm, 0.022 mm and 0.021 mm, respectively. Both SSOCTs demonstrated smaller test–retest variability for measuring ACD and AL compared with IOLMaster500. There were significant disagreement in keratometry and AL measurements between the SSOCTs and PCI-based device; their measurements should not be considered as interchangeable.

Agreement between Scheimpflug Camera and the Swept-source Optical Coherence Tomography Measurements in Keratometry and Higher-order Aberrations

PURPOSE

To evaluate the compatibility of corneal curvature and astigmatism, and higher-order aberrations (HOAs) measured by the Scheimpflug camera Pentacam HR and the swept-source optical coherence tomography ANTERION.

METHODS

This prospective study included normal subjects with no ophthalmic history. Steep keratometry (K), flat K, astigmatism and its axis of the anterior and posterior surfaces, total corneal power, and HOAs using the two instruments were compared. To compare the mean values of the measurements, a paired t-test was used. Bland-Altman analysis was applied to assess the agreement between the two devices.

RESULTS

Fifty-three eyes of 53 subjects were evaluated. There were statistically significant differences for steep K, astigmatism, and vector J0, J45 in the anterior surface and total corneal power between the two devices (p &lt; 0.05). There were also significant differences in the most of the keratometric values of the posterior corneal surface (p &lt; 0.05) except J0 (p = 0.410). Both devices showed strong positive correlations in steep K, flat K, astigmatism (r &gt; 0.81, p &lt; 0.001) with wide ranges of a 95% limit of agreement. Vectoral components were significantly correlated (r &gt; 0.78, p &lt; 0.001) with narrow 95% limit of agreement, except J45 of the posterior surface (r = 0.39, p = 0.004). In the corneal HOAs, there were statistically significant differences in the vertical coma, horizontal trefoil, spherical aberration, and root mean square of each fifth- and sixth-order Zernike coefficient (p = 0.043, p = 0.041, p &lt; 0.001, p &lt; 0.001, and p &lt; 0.001, respectively). Other HOAs showed moderate to strong positive correlations (r &gt; 0.37, p &lt; 0.05). Most HOAs, except for the horizontal trefoil, showed clinically acceptable agreements. The total root mean square of HOAs was not significantly different between the two devices (p = 0.122).

CONCLUSIONS

Most of the keratometric values cannot be used interchangeably. However, the vectoral component of astigmatism showed clinically good agreement. Several HOAs have statistically significant differences; however, almost all HOAs showed acceptable agreements, except for the horizontal trefoil.

CLEAR - Evidence-based contact lens practice

Evidence-based contact lens -->practice involves finding, appraising and applying research findings as the basis for patient management decisions. These decisions should be informed by the strength of the research study designs that address the question, as well as by the experience of the practitioner and the preferences and environment of the patient. This reports reviews and summarises the published research evidence that is available to inform soft and rigid contact lens history and symptoms taking, anterior eye health examination (including the optimised use of ophthalmic dyes, grading scales, imaging techniques and lid eversion), considerations for contact lens selection (including the ocular surface measurements required to select the most appropriate lens parameter, lens modality and material selection), evaluation of lens fit, prescribing (teaching self-application and removal, adaptation, care regimen and cleaning instructions, as well as -->minimising risks of lens wear through encouraging compliance) and an aftercare routine.

Relationship between corneal biomechanical parameters and corneal sublayer thickness measured by Corvis ST and UHR-OCT in keratoconus and normal eyes

BACKGROUND

To explore the relationship between corneal biomechanical parameters and corneal sublayer thickness using corneal visualization Scheimpflug technology (Corvis ST) and ultrahigh-resolution optical coherence tomography (UHR-OCT) in clinical and suspected keratoconus and normal eyes.

METHODS

Cross-sectional prospective study. A total of 94 eyes of 70 participants were recruited. Twenty five eyes of 19 keratoconus patients, 52 eyes of 34 patients showing high risk of developing keratoconus according to the Belin/Ambrosio Enhanced Ectasia Display, and each eye of 17 normal subjects were enrolled. All participants underwent Corvis ST, Pentacam, and UHR-OCT examinations at the same time. Stiffness parameter A1 (SP-A1), deformation amplitude ratio (DA ratio), and other biomechanical parameters were recorded using Corvis ST. The vertical and horizontal thickness profiles of central 3 mm corneal epithelium, Bowman's layer, and stroma as measured by the perpendicular distance between the neighboring interfaces were generated using UHR-OCT. The flat keratometry and steep keratometry were obtained using Pentacam. Analysis of correlation was applied to explore the association between variables.

RESULTS

Most of the biomechanical parameters and corneal sublayer thickness profiles showed statistical differences among three groups. A statistically significant linear relationship was noted between SP-A1 and DA ratio in all three groups. SP-A1 was found to be positively correlated with epithelial and Bowman's layer thickness in the keratoconus (KC) group, and with stromal thickness in all three groups. In the normal and suspected keratoconus (SKC) groups, only stromal thickness was included in the stepwise linear regression to predict SP-A1, whereas in the KC group, steep keratometry and Bowman's layer thickness were included.

CONCLUSIONS

Significant and different correlations were noted between corneal stiffness and corneal sublayer thickness in different groups, indicating that corneal sublayers may play different roles in maintaining corneal biomechanical stability between keratoconus and normal eyes.

Comparison of the effects of femtosecond laser energy on corneal endothelium at two different dissection levels in femtosecond laser-assisted deep anterior lamellar keratoplasty for keratoconus

PURPOSE

The objective of this study is to compare the endothelial safety of femtosecond laser (FSL) energy at two different dissection depths in FSL-assisted deep anterior lamellar keratoplasty (FSDALK) for keratoconus.

METHODS

This prospective double-armed interventional study included 25 eyes from 21 patients with advanced keratoconus who underwent FSDALK (big bubble technique) at a trephination depth of 110 µm in group I (11 eyes) and 80 µm in group II (14 eyes)-all of which were anterior to the Descemet's membrane (DM). Visual acuity measurement, anterior and posterior segment examination, corneal tomography, and specular microscopy were performed preoperatively and at 3, 6, and 12 months, postoperatively. Endothelial cell density, coefficient of variation, percentage of cell hexagonality, and pachymetry were used to evaluate endothelial safety.

RESULTS

No statistically significant difference in any of the endothelial safety parameters was found between the two groups. The mean 12-month postoperative endothelial cell loss rate was 17.46% and 12.91% in group I and II, respectively (P = 0.345). Most of the endothelial cell loss occurred during the first 3 months after surgery. Group II showed statistically greater improvement in the mean keratometry values at all follow-up visits.

CONCLUSION

The endothelial safety profiles of lamellar FSL cuts at 110 µm and 80 µm anterior to the DM are comparable. Cuts as deep as 80 µm anterior to the DM can be safely applied without causing significant injury to the endothelium. Further studies are needed to compare the endothelial safety profiles of different FSL platforms available in the market. Trial registration PACTR201901615323963. Registered 24 November 2018-retrospectively registered.

Phakic intraocular lens: Getting the right size

Phakic intraocular lenses (IOL) are a boon for patients who want spectacle independence but are unable to get refractive correction through laser platforms due to high refractive error or certain corneal contraindications. Phakic IOL's (PIOL) have their own set of complications and challenges, the most important being getting the sizing right. This paper attempts to solve the problem of accurate sizing of PIOL's. Parameters needed for calculating the ideal size of PIOL's have been studied in a step by step manner using all possible tools depending upon the availability and preference of the surgeon. The pros and cons of using a particular tool for measurements have been highlighted along with illustrative case examples to help surgeons who are starting PIOL implantation surgery.

Repeatability and reliability of measurements obtained by the combined Scheimpflug and Placido-disk tomography in different stages of keratoconus

OBJECTIVE

The objective of this study is to evaluate the repeatability and reliability of corneal parameters in different stages of keratoconus patients using a combined Scheimpflug-Placido disc analysis system.

MATERIALS AND METHODS

In this prospective study, three consecutive measurements were performed by the same observer using Scheimpflug-Placido disc anterior segment analysis device in keratoconus patients. Flattest and steepest simulated keratometry and corneal volume, corneal aberrations, thinnest corneal thickness, symmetry index, keratoconus vertex and Baiocchi-Calossi-Versaci index were recorded. Keratoconic eyes were divided into four stages using the Amsler-Krumeich classification. Repeatability was evaluated using within the subject standard deviation, repeatability index (Ri) and coefficient of variation; reliability was evaluated by intraclass correlation coefficient (ICC). Pearson correlation coefficients were used to assess the correlation between the parameters evaluated.

RESULTS

Two hundred sixty-one eyes of 261 keratoconus patients were included in the study. The repeatability for all corneal curvature parameters decreased as the keratoconus severity increased, and there is a positive correlation between keratometry of the apex and corneal curvature parameters (p < 0.05) except mean simulated keratometry. The corneal aberrations were repeatable in all keratoconus subgroups (Ri < 0.34 μm). There are also positive correlations between keratometry of apex and corneal aberrations (p < 0.05) except total high-order root mean square and spherical aberration. The reliability was excellent (ICC > 0.90) for all indices except keratoconus vertex back.

CONCLUSION

The parameters used in the diagnosis and follow-up of keratoconus in the Sirius corneal tomography system may differ more than expected at stages 3 and 4 of the disease. Awareness of this situation may be helpful in planning follow-up and treatment.

Differences between Scheimpflug and optical coherence tomography in determining safety distances in eyes with an iris-fixating phakic intraocular lens

Purpose

To investigate the agreement and reliability of anterior segment optical coherence tomography (AS-OCT) and Scheimpflug imaging in measuring the distance from the anterior edge of an iris-fixated phakic intraocular lens (IF-pIOL) to the corneal endothelium.

Methods

Anterior segment configuration was assessed in a total of 62 eyes of which 25 hyperopic and 37 myopic eyes, all corrected with an IF-pIOL. Measurements were performed by two independent observers using AS-OCT (Visante, Model 1000, Carl Zeiss Meditec Inc.) and Scheimpflug imaging (Pentacam HR, Oculus Optikgerate). The distance from the anterior edge of the pIOL to the endothelium was measured in five different positions using both modalities with their corresponding pIOL software. The measurements as well as the inter- and intra-observer reliability of the two imaging modalities were then compared.

Results

Distance measurements for all positions performed by AS-OCT were found to be significantly larger than those performed by Scheimpflug imaging, with mean differences ranging from 0.11 to 0.22 mm. Both instruments exhibited good inter- and intra-observer reliability.

Conclusion

Anterior pIOL edge to endothelium distance measurements by AS-OCT and Scheimpflug imaging have good intra- and inter-observer reliability. However, as AS-OCT provides larger measurements, these two modalities cannot be used interchangeably. Correction of this difference might be essential for proper decision-making during pre-operative screening for pIOL implantation and post-operative safety monitoring.

Electronic supplementary material

The online version of this article (10.1007/s00417-020-04874-7) contains supplementary material, which is available to authorized users.

Biometric Measurement of Anterior Segment: A Review

Biometric measurement of the anterior segment is of great importance for the ophthalmology, human eye modeling, contact lens fitting, intraocular lens design, etc. This paper serves as a comprehensive review on the historical development and basic principles of the technologies for measuring the geometric profiles of the anterior segment. Both the advantages and drawbacks of the current technologies are illustrated. For in vivo measurement of the anterior segment, there are two main challenges that need to be addressed to achieve high speed, fine resolution, and large range imaging. One is the motion artefacts caused by the inevitable and random human eye movement. The other is the serious multiple scattering effects in intraocular turbid media. The future research perspectives are also outlined in this paper.

Repeatability of whole-cornea measurements using a new swept-source optical coherence tomographer

PURPOSE

The aim of this study was to evaluate the repeatability of several corneal parameters provided by a high-resolution swept-source optical coherence tomographer (SS-OCT).

METHODS

One eye from each of 74 patients was measured five times consequently using the ANTERION SS-OCT. The following corneal parameters were analyzed: average, steep and flat keratometry (K), astigmatism for anterior, posterior and total at 3-mm, average K and astigmatism at 6-mm, anterior and posterior eccentricity, higher-order aberrations (HOA) and spherical aberration (Z40), and anterior and posterior best-fit sphere at 8-mm. The intrasubject standard deviation (S_w), coefficient of variation (CoV), coefficient of repeatability (CoR) and intraclass correlation coefficient (ICC) were calculated for each parameter to assess the repeatability.

RESULTS

We have not found statistically significant differences between repeated measurements (p > 0.05). Repeatability was good considering the different metrics used. S_w values were <0.09, varying from 0.035 (posterior average K at 6-mm) to 0.0878 (anterior flat K at 3-mm). CoV values were also low and similar among the different parameters (from 0.08% to 0.21%), except for anterior, posterior and total astigmatism (from 2.25% to 8.46%). Both anterior and posterior eccentricity, and corneal aberrations (HOA and Z40) CoV values were also high. The CoR values were low for all parameters showing those related to the posterior cornea the lowest values (about 0.01). ICC values were >0.98.

CONCLUSIONS

The ANTERION SS-OCT showed good repeatability when reconstructed different parameters for the whole cornea. This device produces measurements with high repeatability that could be useful for clinical research.

Effects of shell thickness of hollow poly(lactic acid) nanoparticles on sustained drug delivery for pharmacological treatment of glaucoma

Structural designing of carriers with extended drug release profiles is critically important for achieving long-acting drug delivery systems toward efficient managements of chronic diseases. Here, we present a strategy to exploit the effects of the shell thickness of hollow poly(lactic acid) nanoparticles (HPLA NPs) in sustained glaucoma therapy. Formulations based on pilocarpine-loaded HPLA NPs with tailorable shell thickness ranging from 10 to 100 nm were shown to be highly compatible with human lens epithelial cells in vitro and with rabbit eyes in vivo. Specifically, shell thickness regulated the release of pilocarpine, with thick shells (~70 to 100 nm) providing sustained drug release performance but limited drug-loading efficiency, whereas ultrathin shells (~10 nm) induced the opposite effects. Remarkably, moderately thick shells (~40 nm) showed the most effective release profile of pilocarpine (above the therapeutic levels of ~10 µg/mL for over 56 days). In a rabbit model of glaucoma, single intracameral administration of an HPLA NP-based formulation with shell thickness of ~40 nm sustainably alleviated ocular hypertension for over 56 days, consequently protecting the structural integrity of the corneal endothelium, preserving the electrophysiological functions of the retina, and attenuating retinal and optic nerve degeneration in progressively glaucomatous eyes. The findings therefore implied a promising use of shell thickness effects in the development of long-acting drug delivery systems for pharmacological treatment of chronic ocular diseases. STATEMENT OF SIGNIFICANCE: Owing to their large surface areas and modifiable structures, nanoparticles have been considered as a promising platform for drug delivery; however, achieving drug nanocarrier systems with reduced burst release and sustained therapeutic efficacy remains challenges. This work presents the first report on rational design of hollow poly(lactic acid) nanocarriers for tailoring the structure-property-function relationships toward effective treatment of glaucoma. The shell thickness of the hollow nanocarriers is demonstrated to have influential impacts on pilocarpine encapsulation efficiency and release profile, indicating that the most sustained delivery performance (maintaining the release of pilocarpine above therapeutic level over 56 days) can be obtained for the polymeric nanoparticles with moderate shell thickness of ~40 nm.

Corneal Epithelial Thickness Mapping After Photorefractive Keratectomy for Myopia

PURPOSE

To evaluate the longitudinal changes in epithelial thickness after photorefractive keratectomy (PRK) and correlate these with refractive changes.

METHODS

This prospective study included 52 eyes of 52 candidates for myopic PRK. Along with standard ophthalmic examinations, corneal epithelial thickness mapping by anterior segment optical coherence tomography was performed. Epithelial thickness maps of 9-mm diameter were divided into 25 sectors, including a central 2-mm zone and eight octants within para-central (2 to 5 mm), midperipheral (5 to 7 mm), and peripheral (7 to 9 mm) annular zones. All PRK treatments were performed using the Technolas Teneo 317 model 2 excimer laser (Bausch & Lomb, Rochester, NY) and an aspheric profile with a 6-mm diameter optical zone. Follow-up was at 1, 3, and 6 months.

RESULTS

Repeated thickness measures before and after PRK at different follow-up times showed a significant difference in thickness separately in various zones (P < .001). A significant decrease in thickness was seen 1 month after PRK in all zones. Afterward, epithelial thickening continued in all zones and reached the preoperative thickness in the midperipheral and peripheral zones 6 months later, whereas the thickness in the central 5-mm zone was significantly thicker than before surgery. There was also a significant correlation between changes in spherical equivalent and epithelial thickness from before to 6 months postoperatively in the paracentral and peripheral zones.

CONCLUSIONS

There was a marked decrease in the epithelial thickening pattern at 1 month after PRK, with gradual thickening at 3 and 6 months. Changes in epithelial thickness and spherical equivalent were significant only for the para-central peripheral zone. [J Refract Surg. 2019;35(10):632-641.].