Corneal biomechanical metrics in eyes with refraction of -19.00 to +9.00 D in healthy Brazilian patients

Corneal Biomechanical Parameters after 60-Year-Old

Purpose

To determine the distribution of corneal biomechanical parameters in an elderly population.

Methods

This cross-sectional study was conducted in subjects above 60 years living in Tehran. The participants were selected using multi-stage cluster sampling. Corneal biomechanical parameters were measured in a randomly selected subsample of this population using the Reichert Ocular Response Analyzer (Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA).

Results

Of 470 subjects, the data of 420 participants aged over 60 years were analyzed (mean age: 69.3 ± 6.5 years and range: 61-88 years), 363 (86.4%) of whom were male. The mean and standard deviation of corneal hysteresis (CH) and corneal resistance factor (CRF) were 8.37 ± 1.55 mmHg (95% confidence interval [CI]: 8.02-8.72) and 9.06 ± 1.70 mmHg (95% CI: 8.69-9.44), respectively. The mean CH was 8.27 ± 1.54 mmHg in men and 9.25 ± 1.28 mmHg in women, and the mean CRF was 9.00 ± 1.71 mmHg in men and 9.63 ± 1.37 mmHg in women. According to the results of multiple linear logistic regression analysis, CH had a significant association with younger age (β = -0.05, P = 0.032), female sex (β = 1.83, P < 0.001), reduced maximum keratometry (β = -0.22, P = 0.06), and increased anterior chamber volume (β = 0.01, P = 0.007). CRF had a significant correlation with a younger age (β = -0.06, P = 0.02), female sex (β = 1.01, P = 0.05), central corneal thickness (β = 0.02, P < 0.001), and reduced maximum keratometry (β = -0.39, P = 0.010).

Conclusion

The mean CH and CRF values were low in this sample of the Iranian population aged over 60 years indicating the weaker elasticity of the corneal connective tissue.

In Vivo Corneal Biomechanical Properties in a Selected Chinese Population, Measured Using the Corneal Visualization Scheimpflug Technology

Purpose: To evaluate the repeatability and reproducibility of recalculated dynamic corneal response (DCR) parameters and the biomechanical-compensated intraocular pressure (bIOP) derived from the Corneal Visualization Scheimpflug Technology (Corvis ST), as well as to study the variations of DCR parameters and their relationship with demographic, and ocular characteristics.

Methods: A total of 544 healthy subjects were recruited in this study and a series of ophthalmological examinations were performed on their right eyes. Three repeated measurements were obtained at 3-min intervals for 291 of the participants to ensure repeatability. A sum of 100 participants was examined twice within 2-h intervals using two different Corvis ST in the reproducibility study. The repeatability and reproducibility of 37 parameters, including 36 DCR parameters and bIOP, were assessed by the coefficient of repeatability (CR), coefficient of variation (CV), intraclass correlation coefficient (ICC), and within-subject standard deviation (sw). Pearson’s correlation coefficients and stepwise multivariate linear regression models were performed to investigate whether the DCR parameters were related to demographic and ocular characteristics.

Results: Of all the 37 parameters, 34 showed excellent (ICC ≥0.90) or good (ICC ≥0.75) repeatability while 27 of the 37 parameters showed excellent (ICC ≥0.90) or good (ICC ≥0.75) reproducibility. In particular, a CV of less than 20% was found for all DCR parameters and bIOP. A fraction of 14 out of 36 DCR parameters was selected for correlation analysis, based on measurement reliability and clinical relevance in referring to previous literature. Age was negatively associated with the Highest concavity delta arc length (HCdArcL) and peak distance (PD) but it positively correlated with the Whole Eye Movement Max Length (WEMML). Intraocular pressure (IOP) and central corneal thickness (CCT) were negatively associated with the deformation amplitude ratio (DARM) [1 mm], A1 Velocity (A1V), and PD, while positively related to the stiffness parameter at applanation 1 (SP-A1). The bIOP was negatively associated with A1V but positively associated with A2 Velocity (A2V). The anterior chamber volume (ACV) was negatively associated with the pachy slope (PS), WEMML, and SP-A1.

Conclusion. The Corvis ST showed good precision for the repeatability and reproducibility of 36 DCR parameters and bIOP parameters in healthy eyes. The IOP, CCT, bIOP, Km, and ACV significantly influenced the DCR parameters of the eyes.

Association of Corneal Biomechanics Properties with Myopia in a Child and a Parent Cohort: Hong Kong Children Eye Study

Associations between corneal biomechanics, axial elongation and myopia are important but previous results are conflicting. Our population-based study aimed to investigate factors associated with corneal biomechanics, and their relationships with myopia in children and adults. Data from 3643 children and 1994 parents showed that children had smaller deformation amplitudes (DA) than parents (p < 0.001). A larger DA was significantly associated with elongated axial length (AL; children: ß = 0.011; adults: ß = 0.0013), higher corneal curvature (children: ß = 0.0086; adults: ß = 0.0096), older age (children: ß = 0.010; adults: ß = 0.0013), and lower intraocular pressure (IOP; children: ß = −0.029; adults: ß = −0.031) in both cohorts. The coefficient of age for DA in children was larger than in adults (p < 0.001), indicating that the DA change with age in children is faster than in adults. DA was significantly associated with spherical equivalent (p < 0.001) resulting from its correlation with AL and corneal curvature. In conclusion, the cornea is more deformable in adults than in children, whereas corneal deformation amplitude increases faster with age in children than that in adults, along with AL elongation. Longer AL, steeper corneal curvature, older age and smaller IOP correspond to a more deformable cornea. The association between corneal deformation amplitude and refraction was mediated via AL and corneal curvature.

Corneal densitometry in patients with keratoconus undergoing intrastromal Ferrara ring implantation

OBJECTIVE

Evaluation of central corneal densitometry changes following Ferrara corneal ring segment implantation in patients with keratoconus, especially the correlation between corneal densitometry and keratometry.

METHODS

Retrospective, non-comparative, interventional study based on the review of medical records of patients diagnosed with keratoconus who underwent Ferrara corneal ring segment implantation. Pre and post-operative corneal densitometry measurements obtained with Pentacam HR (Oculus, Wetzlar, Germany) were analyzed. The follow-up time was 3 months, and data comparison was made, using specific statistical analysis, with the data of 3 months postoperatively.

RESULTS

The study sample consisted of 43 eyes of 36 patients. The mean corrected visual acuity improved from 0.82 LogMAR preoperatively (SD ± 0.33) to 0.19 LogMAR (SD ± 0.13) postoperatively. The mean spherical equivalent varied from -4.63 (SD ± 3.94) preoperatively to -2.16 (SD ± 2.63) postoperatively. Asphericity varied from -0.69 (SD ± 0.32) preoperatively to -0.27 (SD ± 0.31) postoperatively. The mean maximum K was 54.01D (SD ± 3.38) preoperatively and 51.50D (SD ± 2.90) postoperatively. The mean anterior densitometric value was 18.26 (SD ± 2.03) preoperatively and 17.66 (SD ± 1.84) postoperatively.

CONCLUSION

Corneal densitometry is an interesting technology that should be studied in keratoconus patients. Our results suggest that the corneal densitometry in the cornea's anterior layer reduces after ICRS implantation and correlates with corneal keratometry. Further studies should be performed to increase the knowledge in this field.

Corneal Biomechanical Assessment with Ultra-High-Speed Scheimpflug Imaging During Non-Contact Tonometry: A Prospective Review

BACKGROUND

In recent years, increasing interest has arisen in the application of data from corneal biomechanics in many areas of ophthalmology, particularly to assist in the detection of early corneal ectasia or ectasia susceptibility, to predict corneal response to surgical or therapeutic interventions and in glaucoma management. Technology has evolved and, recently, the Scheimpflug principle was associated with a non-contact air-puff tonometer, allowing a thorough analysis of corneal biomechanics and a biomechanically corrected intraocular pressure assessment, opening up new perspectives both in ophthalmology and in other medical areas. Data from corneal biomechanics assessment are being integrated in artificial intelligence models in order to increase its value in clinical practice.

OBJECTIVE

To review the state of the art in the field of corneal biomechanics assessment with special emphasis to the technology based on ultra-high-speed Scheimpflug imaging during non-contact tonometry.

SUMMARY

A meticulous literature review was performed until the present day. We used 136 published manuscripts as our references. Both information from healthy individuals and descriptions of possible associations with systemic diseases are described. Additionally, it exposed information regarding several fields of ocular pathology, from cornea and ocular surface through areas of refractive surgery and glaucoma until vascular and structural diseases of the chorioretinal unit.

Biomechanical Properties in Different Types of Thin Corneas in Menoufia Population

Background

To evaluate and compare corneal hysteresis (CH) and corneal resistance factor (CRF) in normal thin (NT) healthy corneas with central corneal thickness (CCT) of 470–500 μm with matched thickness in keratoconus suspect (KCS) and keratoconus (KC) eyes.

Methods

A total of 103 eyes in three groups were included prospectively: NT, KCS, and KC groups based on clinical examination and Pentacam findings. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured using the ocular response analyzer (ORA). CCT, CH, and CRF were compared between the three groups and statistically analyzed by variance tests.

Results

The three groups consisted of 44 NT, 26 KCS, and 33 KC. The mean CH measured was 8.689 ± 1.775, 9.051 ± 1.1190, and 8.129 ± 0.8539 mmHg in NT, KCS, and KC eyes, respectively. The mean CRF was 8.441 ± 1.663, 8.337 ± 1.114, and 7.2422 ± 1.3110 mmHg in NT, KCS, and KC eyes, respectively. Within the range of central corneal thickness (470–500 μm), only mean CRF was statistically significantly different between the NT and KC (P < 0.05); there was no statistically significant difference between NT and KCS, nor was the mean CH between each group (P > 0.05).

Conclusions

CRF only can be helpful in differentiating KC from NT eyes; KCS could not be predicted with either corneal biomechanical metrics. There was no benefit from CH in differentiating between the three study groups.

IMI - Clinical Myopia Control Trials and Instrumentation Report

The evidence-basis based on existing myopia control trials along with the supporting academic literature were reviewed; this informed recommendations on the outcomes suggested from clinical trials aimed at slowing myopia progression to show the effectiveness of treatments and the impact on patients. These outcomes were classified as primary (refractive error and/or axial length), secondary (patient reported outcomes and treatment compliance), and exploratory (peripheral refraction, accommodative changes, ocular alignment, pupil size, outdoor activity/lighting levels, anterior and posterior segment imaging, and tissue biomechanics). The currently available instrumentation, which the literature has shown to best achieve the primary and secondary outcomes, was reviewed and critiqued. Issues relating to study design and patient selection were also identified. These findings and consensus from the International Myopia Institute members led to final recommendations to inform future instrumentation development and to guide clinical trial protocols.

Four-year changes in corneal biomechanical properties in children

BACKGROUND

To determine four-year changes of corneal biomechanical parameters in Iranian children aged seven to eleven years and their correlation with optical components.

METHODS

In this four-year prospective cohort study, 468 children aged seven to eleven years who were initially evaluated in 2012 were re-evaluated in 2016-2017. Multi-stage stratified cluster sampling was applied. Cycloplegic refraction, biometry using LENSTAR/BioGraph, and corneal biomechanical assessment using Ocular Response Analyzer (ORA) were undertaken for each participant. The corneal biomechanical parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), areas under the peaks 1 and 2 (p1 and p2 areas) and irregularity indices (A and B indices).

RESULTS

All biomechanical parameters except A index decreased in phase 2. The mean changes of CH and CRF were 0.68 ± 0.16 mmHg (for both parameters) during four years. The mean difference in CH and CRF was 0.23 ± 0.23 and 0.24 ± 0.23 mmHg in females and 1.03 ± 0.23 and 0.96 ± 0.23 mmHg in males, respectively. Different age groups showed varying amounts of decrease in all parameters except for A index. The age group 'ten years' experienced the smallest decrease in CH (0.02 ± 0.48 mmHg) and CRF (0.20 ± 0.47 mmHg) and the age group 'eleven years' showed the greatest decrease in CH (1.41 ± 0.35 mmHg) and CRF (0.99 ± 0.34 mmHg). According to linear regression analysis, CH and CRF had a significant direct relationship with corneal power and an inverse relationship with axial length (p < 0.001).

CONCLUSION

Age and sex are influencing factors on the ORA parameters. Older age is associated with reduced biomechanical parameters and reductions are more significant in males than females. Axial elongation and corneal flattening decrease CH and CRF.

Anterior Segment Biometry and Their Correlation with Corneal Biomechanics in Caucasian Children

PURPOSE

To assess the relationship between the corneal biomechanical parameters and the anterior segment parameters in Caucasian children.

METHODS

This study included 293 eyes from 293 healthy children aged between 6 and 17 years. Corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated with the Ocular Response Analyzer, axial length (AL) with IOLMaster and the anterior segment with Pentacam. Anterior segment parameters obtained were the following: central corneal thickness (CCT), corneal volume (CV), anterior chamber depth (ACD), anterior chamber volume (ACV) and mean anterior and posterior keratometry. Two multiple linear regression models were constructed to assess the association between CH and CRF with anterior segment parameters. A value of p < 0.05 was taken as the criterion for statistical significance in all analyses.

RESULTS

The mean CH and CRF were 12.12 ± 1.71 and 12.30 ± 1.89 mmHg, respectively. Multiple linear regression revealed that CH and CRF were associated negatively with AL in both models, and positively with CCT and CV in the first and second model, respectively. Meanwhile ACD, ACV or mean keratometry did not correlated with CH and CRF. Moreover, when CCT was in the model, it explained more variability for both CH (22.1%) and CRF (30.9%) than when CV was included (16.2% for CH and 16.5% for CRF).

CONCLUSIONS

CH and CRF were correlated positively with CCT and CV, and negatively with AL in healthy Caucasian children. Moreover, corneal parameters were the most contributory variables to CH and CRF changes.

Biomechanical Properties of the Cornea Using a Dynamic Scheimpflug Analyzer in Healthy Eyes

PURPOSE

To investigate biomechanical properties of the cornea using a dynamic Scheimpflug analyzer according to age.

MATERIALS AND METHODS

In this prospective, cross-sectional, observational study, participants underwent ophthalmic investigations including corneal biomechanical properties, keratometric values, intraocular pressure (IOP), and manifest refraction spherical equivalent (MRSE). We determined the relationship of biomechanical parameters and ocular/systemic variables (participant's age, MRSE, IOP, and mean keratometric values) by piecewise regression analysis, association of biomechanical parameters with variables by Spearman's correlation and stepwise multiple regression analyses, and reference intervals (RI) by the bootstrap method.

RESULTS

This study included 217 eyes of 118 participants (20-81 years of age). Piecewise regression analysis between Corvis-central corneal thickness (CCT) and participant's age revealed that the optimal cut-off value of age was 45 years. No clear breakpoints were detected between the corneal biomechanical parameters and MRSE, IOP, and mean keratometric values. Corneal velocity, deformation amplitude, radius, maximal concave power, Corvis-CCT, and Corvis-IOP exhibited correlations with IOP, regardless of age (all ages, 20-44 years, and over 44 years). With smaller deformation amplitude and corneal velocity as well as increased Corvis-IOP and Corvis-CCT, IOP became significantly increased. We provided the results of determination of confidence interval from RI data using bootstrap method in three separate age groups (all ages, 20-44 years, and over 44 years).

CONCLUSION

We demonstrated multiple corneal biomechanical parameters according to age, and reported that the corneal biomechanical parameters are influenced by IOP.

Corneal Biomechanical Properties of Keratoconic Eyes Following Penetrating Keratoplasty

Objectives

To investigate the corneal biomechanical properties of keratoconic eyes following penetrating keratoplasty (PKP).

Materials and Methods

Thirty-five patients (70 eyes) were enrolled to this prospective study. Operated and contralateral keratoconic eyes were defined as Group 1 and 2, respectively. All patients underwent ophthalmological examination and measurements of corneal biomechanical properties by Ocular Response Analyzer (ORA), intraocular pressure (IOP) by Goldmann applanation tonometry, and central corneal thickness (CCT) by Pentacam. Shapiro-Wilk W test was performed to test normality of the data. The statistical significance was evaluated with the paired t-test and Wilcoxon signed ranks test. Pearson correlation and Spearman rho tests were used for correlation analysis.

Results

The average age and male/female ratio were 31.34±11.65 (15-60) years and 21/14, respectively. The mean values of the data obtained from Group 1 and 2 respectively were: corneal hysteresis (CH): 9.35±1.66, 8.18±1.84 mmHg (p=0.013), corneal resistance factor (CRF): 9.48±1.96, 7.14±2.05 mmHg (p<0.001), IOPcc: 16.90±4.32, 14.26±3.69 mmHg (p=0.004), IOPg: 15.45±4.61, 10.91±3.97 mmHg (p<0.001), IOPapl: 14.26±3.11, 13.09±2.54 mmHg (p=0.046), and central corneal thickness (CCT): 545.64±60.82, 442.60±68.14 μM (p<0.001). The positive correlation between CH and CRF was moderate (r=0.444) in Group 1 and strong (r=0.770) in Group 2. There was a moderate negative correlation between CH and IOPcc in both groups (r=-0.426, r=-0.423), but CH was not correlated with IOPg or IOPapl in either group. There were weak to strong positive correlations between CRF and all IOP values in both groups. There was no correlation between CRF and CCT in Group 1 (r=0.075) and a very weak correlation in Group 2 (r=0.237). Only IOPcc and IOPg were strongly correlated in both groups.

Conclusion

Better understanding of corneal biomechanical properties is essential for elucidating the pathophysiology and diagnosis of several corneal pathologies such as keratoconus. The biomechanical properties of keratoconic eyes seem to be closer to normal values after PKP.

Age-related change in corneal biomechanical parameters in a healthy Caucasian population

PURPOSE

To determine the effect of aging on corneal biomechanical parameters measured via ocular response analyzer in a homogenous healthy Caucasian population.

METHODS

A total of 2039 Caucasian adults were consecutively recruited and divided into seven groups according to decades of age. The difference in mean corneal hysteresis (CH), mean corneal resistance factor (CRF), mean Goldmann-correlated intraocular pressure (IOPg), and mean corneal-compensated IOP (IOPcc) between decades of age were investigated. The strength of the correlations between corneal biomechanical parameters, and between each biomechanical parameter and age were evaluated. The effect of age on each corneal biomechanical parameter was analyzed.

RESULTS

Mean age of the participants (1173 female and 866 male) was 43.30 ± 14.64 years. Mean CH, CRF, IOPcc, and IOPg were 11.49 ± 1.89 mmHg, 11.40 ± 2.30 mmHg, 15.01 ± 3.11 mmHg, and 15.72 ± 3.80 mmHg, respectively. There were significant differences in mean CH, CRF, IOPcc, and IOPg between groups (p < 0.001 for all parameters). There was a significant negative correlation between age and CH (r = -0.067 and p = 0.003), and a significant negative correlation between age and CRF (r = -0.053 and p = 0.017). There was a significant positive correlation between age and IOPg (r = 0.25 and p < 0.001), and between age and IOPcc (r = 0.20 and p < 0.001). Linear regression analysis showed that for every 1-year increase CH decreased 0.011 mmHg, CRF decreased 0.004 mmHg, IOPcc increased 0.053 mmHg, and IOPg increased 0.047 mmHg.

CONCLUSIONS

Aging can cause significant changes in corneal biomechanical parameters. Corneal biomechanical parameters were correlated with each other, and each was correlated with aging.

Corneal Biomechanics in Ectatic Diseases: Refractive Surgery Implications

BACKGROUND

Ectasia development occurs due to a chronic corneal biomechanical decompensation or weakness, resulting in stromal thinning and corneal protrusion. This leads to corneal steepening, increase in astigmatism, and irregularity. In corneal refractive surgery, the detection of mild forms of ectasia pre-operatively is essential to avoid post-operative progressive ectasia, which also depends on the impact of the procedure on the cornea.

METHOD

The advent of 3D tomography is proven as a significant advancement to further characterize corneal shape beyond front surface topography, which is still relevant. While screening tests for ectasia had been limited to corneal shape (geometry) assessment, clinical biomechanical assessment has been possible since the introduction of the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, USA) in 2005 and the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) in 2010. Direct clinical biomechanical evaluation is recognized as paramount, especially in detection of mild ectatic cases and characterization of the susceptibility for ectasia progression for any cornea.

CONCLUSIONS

The purpose of this review is to describe the current state of clinical evaluation of corneal biomechanics, focusing on the most recent advances of commercially available instruments and also on future developments, such as Brillouin microscopy.

TFOS DEWS II Sex, Gender, and Hormones Report

One of the most compelling features of dry eye disease (DED) is that it occurs more frequently in women than men. In fact, the female sex is a significant risk factor for the development of DED. This sex-related difference in DED prevalence is attributed in large part to the effects of sex steroids (e.g. androgens, estrogens), hypothalamic-pituitary hormones, glucocorticoids, insulin, insulin-like growth factor 1 and thyroid hormones, as well as to the sex chromosome complement, sex-specific autosomal factors and epigenetics (e.g. microRNAs). In addition to sex, gender also appears to be a risk factor for DED. "Gender" and "sex" are words that are often used interchangeably, but they have distinct meanings. "Gender" refers to a person's self-representation as a man or woman, whereas "sex" distinguishes males and females based on their biological characteristics. Both gender and sex affect DED risk, presentation of the disease, immune responses, pain, care-seeking behaviors, service utilization, and myriad other facets of eye health. Overall, sex, gender and hormones play a major role in the regulation of ocular surface and adnexal tissues, and in the difference in DED prevalence between women and men. The purpose of this Subcommittee report is to review and critique the nature of this role, as well as to recommend areas for future research to advance our understanding of the interrelationships between sex, gender, hormones and DED.

Comparison of Corneal Biomechanical Properties between Indian and Chinese Adults

PURPOSE

To investigate the difference in corneal hysteresis (CH) and corneal resistance factor (CRF) between Indian and Chinese populations.

DESIGN

Population-based cross-sectional study.

PARTICIPANTS

Three hundred eighty-two Singaporean Indian persons and 764 Singaporean Chinese 50 years of age or older were included from the Singapore Indian Eye Study and Singapore Chinese Eye Study, respectively.

METHODS

Participants underwent standardized systemic and ocular examinations and interviewer-administered questionnaires for risk factor assessment. The CH and CRF were measured with the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, NY). Information on genetic ancestry was derived using principal component analysis. Linear regression models were used to investigate the association of CH and CRF with potential risk factors.

MAIN OUTCOME MEASURES

Corneal hysteresis and CRF.

RESULTS

After excluding participants with a history of intraocular surgery, a diagnosis of glaucoma suspect or glaucoma, refractive surgery, or presence of corneal abnormalities, CH and CRF readings were available for 382 Indian persons. For each Indian participant, 2 Chinese participants were selected and matched for age and gender (n = 764). There were no differences in the clinical measurements of CH (10.6±1.6 mmHg; P = 0.670) or CRF (10.3±1.7 mmHg; P = 0.103) between the ethnic groups. However, after adjusting for covariates, Indian persons had, on average, 0.18-mmHg higher CH levels than in Chinese (95% confidence interval [CI], 0.02-0.38; P = 0.031). Consistently, CH level was correlated significantly with genetic ancestry in the Southeast Asian population. Corneal resistance factor level was not associated independently with self-reported ethnicity (95% CI, -0.10 to 0.29; P = 0.335).

CONCLUSIONS

Chinese have lower CH than Indian persons, and this disparity may reflect biomechanical differences of the cornea.

Corneal biomechanical data and biometric parameters measured with Scheimpflug-based devices on normal corneas

AIM

To analyze the correlations between ocular biomechanical and biometric data of the eye, measured by Scheimpflug-based devices on healthy subjects.

METHODS

Three consecutive measurements were carried out using the corneal visualization Scheimpflug technology (CorVis ST) device on healthy eyes and the 10 device-specific parameters were recorded. Pentacam HR-derived parameters (corneal curvature radii on the anterior and posterior surfaces; apical pachymetry; corneal volume; corneal aberration data; depth, volume and angle of the anterior chamber) and axial length (AL) from IOLMaster were correlated with the 10 specific CorVis ST parameters.

RESULTS

Measurements were conducted in 43 eyes of 43 volunteers (age 61.24±15.72y). The 10 specific CorVis ST data showed significant relationships with corneal curvature radii both on the anterior and posterior surface, pachymetric data, root mean square (RMS) data of lower-order aberrations, and posterior RMS of higher-order aberrations and spherical aberration of the posterior cornea. Anterior chamber depth showed a significant relationship, but there were no significant correlations between corneal volume, anterior chamber volume, mean chamber angle or AL and the 10 specific CorVis ST parameters.

CONCLUSIONS

CorVis ST-generated parameters are influenced by corneal curvature radii, some corneal RMS data, but corneal volume, anterior chamber volume, chamber angle and AL have no correlation with the biomechanical parameters. The parameters measured by CorVis ST seem to refer mostly to corneal properties of the eye.

The Relationship between Corvis ST Tonometry and Ocular Response Analyzer Measurements in Eyes with Glaucoma

It is important to compare the results of Corneal Visualization Scheimpflug Technology instrument (CST) measurements and Reichert Ocular Response Analyzer (ORA) parameters. The purpose of the study was to investigate the association between CST measurements and ORA parameters in ninety-five patients with primary open-angle glaucoma. Measurements of CST, ORA, axial length (AL), average corneal curvature (CC), central corneal thickness (CCT) and intraocular pressure (IOP) with Goldmann applanation tonometry (GAT) were carried out. The association between CST and ORA parameters was assessed using linear regression analysis, with model selection based on the second order bias corrected Akaike Information Criterion index. Measurements from ORA (corneal hysteresis [CH] and corneal response factor [CRF]) had high intraclass correlation coefficients (ICC) and low coefficients of variation, but some CST parameters showed much lower reproducibility, namely: A1 length, A2 length, highest concavity time and peak distance. Of 12 CST parameters tested, 8 were significantly correlated with CH and 10 were significantly correlated with CRF, however, the magnitude of the correlation coefficients were weak to moderate at best. The optimal model to explain CH using CST measurements was given by: CH = -76.3 + 4.6*A1 time + 1.9*A2 time + 3.1 * highest concavity deformation amplitude + 0.016*CCT (R² = 0.67, p <0.001). Similarly, the optimal model for CRF was given by: CRF = -53.5 + 4.2*A1 time + 1.9*A1 length + 20.8*A1 deformation amplitude + 0.8*A2 time + 0.017*CCT (R² = 0.73, p <0.001). ORA parameters show higher reproducibility than CST measurements. Although many CST parameters are significantly related to ORA parameters, the strengths of these relationships are weak to moderate.

Corneal Biomechanics Determination in Healthy Myopic Subjects

Purpose. To determine the corneal biomechanical properties by using the Ocular Response Analyzer™ and to investigate potential factors associated with the corneal biomechanics in healthy myopic subjects. Methods. 135 eyes from 135 healthy myopic subjects were included in this cross-sectional observational study. Cornea hysteresis (CH), corneal resistance factor (CRF), cornea-compensated intraocular pressure (IOPcc), and Goldmann-correlated intraocular pressure (IOPg) were determined with the Reichert Ocular Response Analyzer (ORA). Univariate and multivariate regression analyses were performed to investigate factors associated with corneal biomechanics. Results. The mean CH and CRF were 9.82 ± 1.34 mmHg and 9.64 ± 1.57 mmHg, respectively. In univariate regression analysis, CH was significantly correlated with axial length, refraction, central corneal thickness (CCT), and IOPg (r = −0.27, 0.23, 0.45, and 0.21, resp.; all with p ≤ 0.015), but not with corneal curvature or age; CRF was significantly correlated with CCT and IOPg (r = 0.52 and 0.70, resp.; all with p < 0.001), but not with axial length/refraction, corneal curvature, or age. In multivariate regression analysis, axial length, IOPcc, and CCT were found to be independently associated with CH, while CCT and IOPg were associated with CRF. Conclusions. Both CH and CRF were positively correlated with CCT. Lower CH but not CRF was associated with increasing degree of myopia. Evaluation of corneal biomechanical properties should take CCT and myopic status into consideration.

Corneal biomechanics in normal Saudi individuals

Purpose

The aim of this study was to determine and relate corneal biomechanical metrics with demographic, tomographic and refractive data in healthy Saudi people.

Design

Prospective, cross-sectional, observational study.

Participants

The study included 215 normal Saudi adult individuals.

Methods

Corneal hysteresis (CH) and corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) and corneal-compensated intraocular pressure (IOPcc) were measured by ocular response analyzer. The central keratometry (CK), central corneal thickness (CCT), and anterior chamber depth (ACD) were measured using Pentacam system. The spherical equivalent of refraction (SER) was obtained by an Auto-refractometer.

Results

The mean CH was 11.16 ± 2.11, CRF was 11.07 ± 2.31 and IOP was 15.12 ± 3.5. Mean CK, ACD, CH and CRF were distinct among gender with the significant P values of 0.05, 0.006, 0.020, and 0.047 respectively. CRF was negatively correlated with ACD (r = −0.146, P = 0.032). A positive correlation was found between CRF and SER (r = 0.176, P = 0.010), CCT (r = 0.447, p = 0.000) and CH (r = 0.878 and p = 0.000). CH was negatively correlated with IOPcc (−0.433, p = 0.000). A positive correlation was found between CH and ACD (r = −0.14, p = 0.044), SER (r = 0.617, p = 0.014), CCT (r = 0.412, p = 0.000) and IOPg (r = 0.183, p = 0.007).

Conclusion

This study demonstrated a distinct difference among gender values of corneal hysteresis and corneal resistance factor being higher in female Saudi subjects. CH and CRF values were higher in Saudi subjects than values in other populations. This may suggest the presence of ethnic differences in ocular parameters and support the importance of establishing population norms for corneal biomechanical parameters.

In vivo human corneal deformation analysis with a Scheimpflug camera, a critical review

Corneal morphological analysis has greatly improved in recent years, providing physicians with new and reliable parameters to study. Moreover, today corneal functional too is a routine analysis, thanks to biomechanical evaluation allowed by an ocular response analyzer (Reichert Ophthalmic Instrument, Depew, NY, USA). Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany), that relies on the ultrahigh speed Scheimpflug camera, is a new device providing corneal deformation parameters measured ny scanning the cornea response to an air puff; it is an instrument able to measure intraocular pressure too. This device could open up a whole new prospective in screening, detecting and managing corneal diseases, intraocular pressure measurement and in evaluating surgical procedures involving the cornea. This paper provides a comprehensive explanation of Corvis ST measurement principles and parameters and a literature review of scientific studies.

Discriminant Value of Custom Ocular Response Analyzer Waveform Derivatives in Forme Fruste Keratoconus

PURPOSE

To evaluate the performance of corneal hysteresis (CH), corneal resistance factor (CRF), 37 Ocular Response Analyzer (ORA) waveform parameters, and 15 investigator-derived ORA variables in differentiating forme fruste keratoconus (KC) from normal corneas.

DESIGN

Case-control study.

METHODS

Seventy-eight eyes of 78 unaffected patients and 21 topographically normal eyes of 21 forme fruste KC patients with topographically manifest KC in the contralateral eye were matched for age, the thinnest point of the cornea, central corneal thickness, and maximum keratometry. Fifteen candidate variables were derived from exported ORA signals to characterize putative indicators of biomechanical behavior, and 37 waveform parameters were tested. Differences between groups were assessed by the Mann-Whitney test. The area under the receiver operating characteristic curve (AUROC) was used to compare the diagnostic performance.

RESULTS

Ten of 54 parameters reached significant differences between the groups (Mann-Whitney test, P < .05). Neither CRF nor CH differed significantly between the groups. Among the ORA waveform measurements, the best parameters were those related to the area under the first peak, p1area, and p1area1 (AUROC, 0.714 ± 0.064 and 0.721 ± 0.065, respectively). Among the investigator ORA variables, a measure incorporating the pressure-deformation relationship of the entire response cycle performed best (hysteresis loop area, AUROC, 0.694 ± 0.067).

CONCLUSION

Waveform-derived ORA parameters, including a custom measure incorporating the pressure-deformation relationship of the entire response cycle, performed better than traditional CH and CRF parameters in differentiating forme fruste KC from normal corneas.

Ocular response analyzer parameters in healthy, keratoconus suspect and manifest keratoconus eyes

Background:

To evaluate and compare corneal biomechanical indices and their specificity among keratoconus (KC), keratoconus suspect (KCS), and normal eyes (NL) before and after controlling potential confounders.

Materials and Methods:

A total of 160 eyes in three groups were included prospectively: NL, KC, and KCS groups based on clinical examination and topography. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured by the ocular response analyzer. CH and CRF were compared between the three groups by analysis of variances test.

Results:

The three groups consisted of 80 NL, 48 KC, and 32 KCS eyes. The mean CH measured was 10.4 ± 1.25, 7.83 ± 1.28 and 10.17 ± 1.80 mm Hg in NL, KC and KCS eyes, respectively. The mean CRF was 10.23 ± 1.75, 6.5 ± 1.63 and 9.98 ± 2.00 mm Hg in NL, KC and KCS eyes, respectively. Mean CH and CRF were significantly different between the NL and KC (P < 0.05); however after controlling for central corneal thickness and sex; there was no significant difference between NL and KCS (P > 0.05).

Conclusion:

CH and CRF can be helpful in differentiating KC from NL eyes; however, they are not valuable for detecting KCS that is the main concern for refractive surgery. Future studies focusing on more accurate tests for identifying KCS, using a consistent grading scale for defining KC and KCS are still warranted.

Scheimpflug camera in the quantitative assessment of reproducibility of high-speed corneal deformation during intraocular pressure measurement

The paper presents an original analysis method of corneal deformation images from the ultra-high-speed Scheimpflug camera (Corvis ST tonometer). Particular attention was paid to deformation frequencies exceeding 100 Hz and their reproducibility in healthy subjects examined repeatedly. A total of 4200 images with a resolution of 200 × 576 pixels were recorded. The data derived from 3 consecutive measurements from 10 volunteers with normal corneas. A new image analysis algorithm, written in Matlab with the use of the Image Processing package, adaptive image filtering, morphological analysis methods and fast Fourier transform, was proposed. The following results were obtained: (1) reproducibility of the eyeball reaction in healthy subjects with precision of 10%, (2) corneal vibrations with a frequency of 369 ± 65 Hz (3) and amplitude of 7.86 ± 1.28 µm, (4) the phase shift within two parts of the cornea of the same subject of about 150°. The result of image sequence analysis for one subject and deformations with a corneal frequency response above 100 Hz.

Evaluation of Sex Differences in Corneal Hysteresis

Purpose

To study sex differences in corneal biomechanical parameters in Egypt as regards to corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated intraocular pressure (IOPcc).

Methods

This is a cross-sectional observational study that includes 350 eyes of 350 normal subjects (175 men and 175 women) who were evaluated using the Reichert ocular response analyzer (ORA) to measure CH, CRF, IOPg, and IOPcc.

Results

The mean CH in men was 9.69 ± 2.05 mm Hg (range 5.7–14.6 mm Hg) and in women 10.41 ± 1.65 mm Hg (range 7–14.4 mm Hg) with a p value of 0.00038. The mean CRF in men was 9.54 ± 2.16 mm Hg (range 4.9–14.4 mm Hg) and in women was 10.37 ± 1.71 mm Hg (range 7.3–15.5 mm Hg) with a p value of 0.00008. The mean IOPg in men was 14.78 ± 4.15 mm Hg (range 7.2–26.5 mm Hg) and in women was 15.44 ± 3.3 mm Hg (range 8.6–23.7 mm Hg) with a p value of 0.09. The mean IOPcc in men was 16.2 ± 4.08 mm Hg (range 10.9–28.9 mm Hg) and in women was 15.86 ± 3.41 (range 9.7–24.2 mm Hg) with a p value of 0.4. There was a statistically significant difference between men and women in CH and CRF while IOPg and IOPcc showed no statistically significant difference (p>0.05).

Conclusions

There is a statistically significant difference between men and women in CH and CRF, where women show higher values, while no statistically significant difference exists between the groups in IOPg or IOPcc.

Effect of pathological myopia on biomechanical properties: a study by ocular response analyzer

AIM

To evaluate the ocular response analyzer (ORA) measurements of patients with pathological myopia in comparison with those of emmetropic control subjects, and to investigate the correlation between these ORA measurements and spherical equivalent (SE).

METHODS

Measurements of 53 eyes of 53 subjects with pathological myopia (SE>-6.00 D) were compared with those of 60 eyes of 60 emmetropic controls. Corneal hysteresis (CH), corneal resistance factor (CRF), noncontact tonometer intraocular pressure (IOPg), and corneal-compensated IOP (IOPcc) were obtained for each subject. The refractive error value was determined as SE via a cycloplegic refraction test.

RESULTS

The mean age was 54.1±18.9y (ranging from 5 to 88) in the pathological myopic group and 56.2±19.0y (ranging from 6 to 89) in the control group. There were no significant differences between the groups concerning age and sex. CH and CRF were significantly lower in the pathological myopic group than in the control group (P<0.001, P=0.005, respectively). IOPcc and IOPg were significantly higher in the pathological myopic group than in the control group (P<0.001, P=0.009, respectively). There were significantly positive correlations between CH and SE (r=0.565, P<0.001) and between CRF and SE (r=0.364, P=0.007). There were significantly negative correlations between IOPcc and SE (r=-0.432, P=0.001) and between IOPg and SE (r=-0.401, P=0.003).

CONCLUSION

The present study displayed that pathological myopia affected biomechanical properties measured by ORA. The results of corneal biomechanical properties measured by ORA may need to be appreciated by taking refraction into account. Further, pathological myopia might be related with the increased IOP.

Influence of age and gender on corneal biomechanical properties in a healthy Italian population

PURPOSE

The aim of this study was to assess corneal hysteresis (CH) and corneal resistance factor (CRF) in healthy subjects, to evaluate the relationship with age, and to investigate possible associations with other ocular factors.

METHODS

Four hundred Italian subjects (male-to-female ratio, 168:232; mean age, 58.8 ± 17.2 years) were included and divided into 5 subgroups based on age. CH, CRF, and central corneal thickness (CCT) were measured by using the Ocular Response Analyzer and the integrated handheld pachymeter, and their relationship with gender, age, and ocular factors was evaluated.

RESULTS

The mean CH, CRF, and CCT values were 10 ± 1.6 mm Hg, 10.5 ± 1.7 mm Hg, and 532.2 μm, respectively. Women had a lower mean CH (9.9 vs. 10.2 mm Hg; P = 0.04) and CRF (10.3 vs. 10.8 mm Hg; P = 0.03) than did men. The youngest subjects had the highest CH (11.2 ± 1.5 mm Hg), whereas the oldest patients had the lowest CH values (9 ± 1.1 mm Hg). No significant differences in CRF were observed between age groups. CH and CRF showed a positive correlation (r = 0.58; P < 0.001), and both had a positive association with CCT (r = 0.27; P < 0.001 and r = 0.57; P < 0.001, respectively). The strongest correlations were observed between Goldmann-correlated intraocular pressure (IOP) and corneal-compensated IOP (r = 0.68; P < 0.001) and between Goldmann-correlated IOP and Goldmann applanation tonometry (r = 0.88; P < 0.001).

CONCLUSIONS

Gender and advancing age may influence corneal biomechanical properties. In our population, CH decreased with aging, and men demonstrated a higher CH and CRF than women did. Further, CH, CRF, and CCT were significantly related.

Quantitative assessment of responses of the eyeball based on data from the Corvis tonometer

BACKGROUND

The "air-puff" tonometers, include the Corvis, are a type of device for measuring intraocular pressure and biomechanics parameters. The paper attempts to analyse this response and its relationship with other parameters measured in the Corvis tonometer.

METHODS

A number of 13,400 2D images were acquired from the Corvis device and analysed (32 healthy and 16 ill people). A new method has been proposed for the analysis of responses of the eyeball based on morphological transformations and contextual operations.

RESULTS

The proposed algorithm enables to determine responses of the eyeball to an air puff coming from the Corvis tonometer. Additionally, responses of the eyeball have been linked to some selected features of corneal deformation. The results include, among others: (1) distinguishability between the left and right eye with an error of 7%; (2) the correlation between the area under the curve in corneal deformation and the response of the eyeball -0.26; (3) the correlation between the highest concavity time and the maximum deformation amplitude of 0.4. All these features are obtained fully automatically and repetitively at a time of 3.8s per patient (Core i7 10GB RAM).

DISCUSSION

It is possible to measure additional parameters of the eye deformation which are not available in the original software of the Corvis tonometer. The use of the proposed methods of image analysis and processing provides results directly from the eye response measurement when measuring intraocular pressure.

Evaluation of corneal deformation analyzed with a Scheimpflug based device

PURPOSE

To evaluate the correlation between corneal biomechanical and morphological data in healthy eyes.

METHODS

A complete clinical eye examination of naïve eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Linear regression between central corneal thickness (CCT), corneal volume (CV) and anterior corneal curvature measured with Sim'K (SK), versus corneal deformation parameters measured with Corvis ST have been run using SPSS software version 18.0.

RESULTS

Seventy-six eyes of 76 healthy subjects (44 women and 32 men) with a mean age of 36.84 ± 10.74 years and a mean refractive error of -0.55 ± 1.68 D (measured as spherical equivalent) were evaluated. Corneal deformation parameters were weakly correlated with corneal morphological parameters and with spherical equivalent. Although the correlations between deformation amplitude versus SK and between SK versus Velocity of Applanation 2, were higher than the others (R(2) = 0.28 and 0.26 respectively), none of them was statistically significant (p>0.01).

CONCLUSIONS

According with these findings, Corvis ST seems to be able to provide an analysis of corneal deformation independent from corneal morphological characteristics. If these data will be confirmed in further studies, this device could be useful in the management and screening of eyes with corneal diseases.

Automatic method of analysis and measurement of additional parameters of corneal deformation in the Corvis tonometer

INTRODUCTION

The method for measuring intraocular pressure using the Corvis tonometer provides a sequence of images of corneal deformation. Deformations of the cornea are recorded using the ultra-high-speed Scheimpflug camera. This paper presents a new and reproducible method of analysis of corneal deformation images that allows for automatic measurements of new features, namely new three parameters unavailable in the original software.

MATERIAL AND METHOD

The images subjected to processing had a resolution of 200 × 576 × 140 pixels. They were acquired from the Corvis tonometer and simulation. In total 14,000 2D images were analysed. The image analysis method proposed by the author automatically detects the edge of the cornea and sclera fragments. For this purpose, new methods of image analysis and processing proposed by the author as well as those well-known, such as Canny filter, binarization, median filtering etc., have been used. The presented algorithms were implemented in Matlab (version 7.11.0.584-R2010b) with Image Processing toolbox (version 7.1-R2010b) using both known algorithms for image analysis and processing and those proposed by the author.

RESULTS

Owing to the proposed algorithm it is possible to determine three parameters: (1) the degree of the corneal reaction relative to the static position; (2) the corneal length changes; (3) the ratio of amplitude changes to the corneal deformation length. The corneal reaction is smaller by about 30.40% compared to its static position. The change in the corneal length during deformation is very small, approximately 1% of its original length. Parameter (3) enables to determine the applanation points with a correlation of 92% compared to the conventional method for calculating corneal flattening areas. The proposed algorithm provides reproducible results fully automatically within a few seconds/per patient using Core i7 processor.

CONCLUSIONS

Using the proposed algorithm, it is possible to measure new, additional parameters of corneal deformation, which are not available in the original software. The presented analysis method provides three new parameters of the corneal reaction. Detailed clinical studies based on this method will be presented in subsequent papers.

Corneal resistance factor and corneal hysteresis in a 6- to 18-year-old population

PURPOSE

To determine the distribution and normal range of the corneal resistance factor (CRF) and corneal hysteresis (CH) in the 6- to 18-year age range and their relationship with biometric components.

SETTING

Dezful, Iran.

DESIGN

Cross-sectional study.

METHODS

This study of Dezful school children used a multistage, stratified, cluster approach sampling. All students had examinations for biometry, noncycloplegic refraction, and corneal biomechanical properties; the examinations were performed in the same order in all cases.

RESULTS

Of the 864 selected students, 683 participated in the study. The mean CRF and CH was 11.74 mm Hg±1.77 (SD) (95% confidence interval [CI], 11.58-11.89) and 11.49±1.91 mm Hg (95% CI, 11.33-11.65), respectively. In a linear multiple regression model, the CRF significantly correlated with female sex (β coefficient=0.488, P=.013), central corneal thickness (CCT) (β coefficient=.034, P=.001), and keratometry (β-coefficient=0.157, P=.003) and CH significantly correlated with CCT (β coefficient=0.025, P<.001), axial length (β coefficient=-0.303, P=.011), and keratometry (β coefficient=0.11, P=.043). Each year increase in age was associated with a 42-unit decrease in the peak 1 area (P=.003).

CONCLUSIONS

The distribution of CRF and CH in an Iranian population was symmetrical and bell shaped. However, the CRF did not have a normal distribution. The mean CRF and CH were higher than those reported in almost all previous studies.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Quantitative assessment of the impact of biomedical image acquisition on the results obtained from image analysis and processing

Introduction

Dedicated, automatic algorithms for image analysis and processing are becoming more and more common in medical diagnosis. When creating dedicated algorithms, many factors must be taken into consideration. They are associated with selecting the appropriate algorithm parameters and taking into account the impact of data acquisition on the results obtained. An important feature of algorithms is the possibility of their use in other medical units by other operators. This problem, namely operator’s (acquisition) impact on the results obtained from image analysis and processing, has been shown on a few examples.

Material and method

The analysed images were obtained from a variety of medical devices such as thermal imaging, tomography devices and those working in visible light. The objects of imaging were cellular elements, the anterior segment and fundus of the eye, postural defects and others. In total, almost 200'000 images coming from 8 different medical units were analysed. All image analysis algorithms were implemented in C and Matlab.

Results

For various algorithms and methods of medical imaging, the impact of image acquisition on the results obtained is different. There are different levels of algorithm sensitivity to changes in the parameters, for example: (1) for microscope settings and the brightness assessment of cellular elements there is a difference of 8%; (2) for the thyroid ultrasound images there is a difference in marking the thyroid lobe area which results in a brightness assessment difference of 2%. The method of image acquisition in image analysis and processing also affects: (3) the accuracy of determining the temperature in the characteristic areas on the patient’s back for the thermal method - error of 31%; (4) the accuracy of finding characteristic points in photogrammetric images when evaluating postural defects – error of 11%; (5) the accuracy of performing ablative and non-ablative treatments in cosmetology - error of 18% for the nose, 10% for the cheeks, and 7% for the forehead. Similarly, when: (7) measuring the anterior eye chamber – there is an error of 20%; (8) measuring the tooth enamel thickness - error of 15%; (9) evaluating the mechanical properties of the cornea during pressure measurement - error of 47%.

Conclusions

The paper presents vital, selected issues occurring when assessing the accuracy of designed automatic algorithms for image analysis and processing in bioengineering. The impact of acquisition of images on the problems arising in their analysis has been shown on selected examples. It has also been indicated to which elements of image analysis and processing special attention should be paid in their design.

Evaluation of corneal deformation analyzed with Scheimpflug based device in healthy eyes and diseased ones

This study was designed to evaluate the correlation between corneal biomechanical and morphological data in healthy eyes, eyes that underwent myopic photorefractive keratectomy (PRK), keratoconus affected eyes, and keratoconus affected eyes that underwent corneal collagen crosslinking (CCC). Complete clinical eye examination of all eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Differences among Corvis ST (CST) parameters in the different groups have been performed. Linear regression between central corneal thickness (CCT), intraocular pressure (IOP), and anterior corneal curvature measured with Sim'K (KM), versus corneal deformation parameters measured with Corvis ST in the different groups, has been run using SPSS software version 18.0. We evaluated 64 healthy eyes of 64 patients with a mean refractive error of -0.65 ± 1.68 D (measured as spherical equivalent), 17 eyes of 17 patients that underwent myopic PRK for a mean refractive defect of -4.91 ± 2.05 D (measured as spherical equivalent), 16 eyes of 16 patients affected by keratconus (stage 2-3 of Amsler Classification), and 13 eyes of 13 patients affected by keratoconus that underwent CCC. Our data suggest that corneal curvature would have a greater influence on corneal deformation than CCT; in fact KM values are more strongly associated with more CST parameters both about corneal change in shape and both about the corneal ability to come back at original shape.

Ocular biomechanical metrics by CorVis ST in healthy Brazilian patients

PURPOSE

To evaluate ocular biomechanical metrics given by the CorVis ST (Oculus, Inc., Berlin, Germany) in a population of healthy Brazilian patients.

METHODS

An observational and cross-sectional study involving 1 eye randomly selected from 90 healthy patients. Studied parameters (including deformation amplitude, first applanation time, highest concavity time, second applanation time, first applanation length, second applanation length, curvature radius highest concavity, curvature radius normal, velocity in, and velocity out) derived from the CorVis ST were correlated to central corneal thickness from the Pentacam (Oculus, Inc.). Differences between data on the basis of gender were evaluated.

RESULTS

Mean patient age was 35.80 ± 12.83 years (range: 21.07 to 78.84 years). Mean central corneal thickness was 547.50 ± 32.00 μm (range: 490 to 647 μm) and mean spherical equivalent refraction was -3.29 ± 3.69 diopters (range: -9.50 to +10.37 diopters). Mean deformation amplitude was 1.05 ± 0.08 mm (range: 0.91 to 1.26 mm). Highest concavity time was 18.38 ± 0.93 ms (range: 16.95 to 21.07 ms). Intraocular pressure was 16.43 ± 2.15 mm Hg (range: 11.50 to 21.0 mm Hg). First applanation time was 8.32 ± 0.33 ms (range: 7.53 to 9.12 ms) and second applanation time was 23.80 ± 0.44 ms (range: 22.76 to 24.95 ms). First applanation length (max) was 2.07 ± 0.38 mm (range: 1.20 to 3.10 mm) and second applanation length (max) was 2.37 ± 0.47 mm (range: 1.33 to 4.12 mm). Curvature radius highest concavity was 11.09 ± 2.06 mm (range: 7.58 to 15.98 mm) and curvature radius normal was 7.59 ± 0.67 mm (range: 6.82 to 11.02 mm). Velocity in was 0.21 ± 0.05 m/s (range: 0.16 to 0.72 m/s) and velocity out was -0.33 ± 0.07 m/s (range: -0.72 to -0.20 m/s). Studied parameters were not associated with gender.

CONCLUSIONS

Eight of 11 ocular biomechanical metrics given by the CorVis ST were associated with central corneal thickness, but the influence of central corneal thickness on these measurements was low.

Assessment of corneal biomechanical properties and intraocular pressure in myopic spanish healthy population

Purpose. To examine biomechanical parameters of the cornea in myopic eyes and their relationship with the degree of myopia in a western healthy population. Methods. Corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOP), and corneal compensated IOP (IOPcc) were measured using the ocular response analyzer (ORA) in 312 eyes of 177 Spanish subjects aged between 20 and 56 years. Refraction was expressed as spherical equivalent (SE), which ranged from 0 to -16.50 diopters (D) (mean: -3.88 ± 2.90 D). Subjects were divided into four groups according to their refractive status: group 1 or control group: emmetropia (-0.50 ≤ SE < 0.50); group 2: low myopia (-0.75 ≤ SE < 3.00 D); group 3: moderate myopia (-3.00 ≤ SE ≤ -6.00 D); and group 3: high myopia (SE greater than -6.00 D). We analyzed the relationship between corneal biomechanics measured with ORA and SE. Results. CH in the emmetropia, low myopia, moderate myopia, and high myopia groups was 11.13 ± 0.98, 11.49 ± 1.25, 10.52 ± 1.54, and 10.35 ± 1.33 mmHg, respectively. CH in the highly myopic group was significantly lower than that in the emmetropic group (P = 0.07) and low myopic group (P = 0.035); however, there were no differences with the moderate myopic group (P = 0.872). There were no statistically significant differences regarding IOP among the four groups (P > 0.05); nevertheless, IOPcc was significantly higher in the moderately myopic (15.47 ± 2.47 mmHg) and highly myopic (16.14 ± 2.59 mmHg) groups than in the emmetropia (15.15 ± 2.06 mmHg) and low myopia groups (14.53 ± 2.37 mmHg). No correlation between age and the measured parameters was found. CH and IOPcc were weakly but significantly correlated with SE (r = 0.171, P = 0.002 and r = -0.131, P = 0.021, resp.). Conclusions. Present study showed only a very weak, but significant, correlation between CH and refractive error, with CH being lower in both moderately and highly myopic eyes than that in the emmetropic and low myopic eyes. These changes in biomechanical properties of the cornea may have an impact on IOP measurement, increasing the risk of glaucoma.

Relationship Between Corneal Hysteresis and Corneal Resistance Factor with Other Ocular Parameters

PURPOSE

To evaluate the relationship between corneal hysteresis (CH) and corneal resistance factor (CRF) with age, central corneal thickness (CCT), corneal curvature (KM), corneal volume (CV), and refractive error in naïve eyes.

METHODS

105 healthy subjects (58 male and 47 female) were included in this study. The ages ranged from 19 to 82 years (mean 43.1 ± 15.4 years) and refraction between -11 D and +6 D (mean -0.79 ± 2.95 D). CH and CRF obtained with the Ocular Response Analyzer (ORA) were correlated with age, refractive error, Goldmann Applanation Tonometry (GAT), and with CCT, KM, CV obtained with the Pentacam, and with Corneal-Compensated Intraocular Pressure (IOPcc) and Goldmann-correlated intraocular pressure measurement (IOPg) obtained with ORA. A multivariable mixed effect model was used to evaluate associations among these parameters.

RESULTS

CH ranged from 6.9 to 14.6 mmHg (mean 10.26 ± 1.49 mmHg); CRF ranged from 5.8 to 17 mmHg (mean 10.38 ± 1.64 mmHg). Multivariate analysis showed a statistically significant correlation between CH with CCT (p < 0.001), and KM (p < 0.001), and between CRF with CCT (p < 0.001) and GAT (p < 0.001).

CONCLUSIONS

Our findings support the hypothesis that CH and CRF are related to the corneal shape and thickness, and show a decrease of CH with age.

Biomechanical parameters of the cornea measured with the Ocular Response Analyzer in normal eyes

Background

To evaluate the relationships between Reichert Ocular Response Analyzer (ORA) parameters corneal hysteresis (CH) and corneal response factor (CRF) and ocular dimensions, age and intraocular pressure.

Methods

Two hundred and twelve eyes of 212 participants with no ocular pathology had CH and CRF measured with the ORA. Intraocular pressure (IOP) was measured with the Dynamic Contour tonometer and central corneal thickness (CCT) was also evaluated. Partial least squares linear regression (PLSLR) analyses were performed to examine the relationships between each response variable, CH and CRF, and the predictor variables age, corneal curvature (CC), axial length (AL), CCT and IOP.

Results

CH was positively associated with CCT and negatively associated with age (scaled coefficients: CCT 0.62, p < 0.0001; age -0.55, p <0.0001; r² = 0.25). CRF was positively associated with CCT and DCT IOP and negatively associated with age and AL (scaled coefficients: CCT 0.89, p < 0.0001; DCT IOP 0.46, p < 0.01; age – 0.60, p < 0.0001; AL -0.37, p < 0.01; r² = 0.43). There was no significant association between CC and CH or CRF.

Conclusions

The study suggests that age and CCT are strongly associated with CH and CRF, and that the latter is also influenced by AL and IOP. However, the variables studied could explain only 25% and 43% of the measured variation in CH and CRF, respectively, suggesting other factors also affect the values of these measurements.

Anterior segment changes following short-term reading and its correlation with corneal biomechanical characteristics

PURPOSE

To investigate the changes in anterior segment following short-term reading and evaluate the correlation of such changes with corneal biomechanical characteristics (CBC).

METHODS

Thirty-six right eyes of 36 healthy subjects were examined. Anterior segment parameters were measured using the Pentacam before and after 30 min of reading. Ocular Response Analyzer was used to record CBC after reading. The following were recorded: central corneal thickness (CCT), central corneal power (CCP), superior corneal power (SCP), inferior corneal power (ICP), anterior chamber depth (ACD), anterior chamber volume (ACV), anterior chamber angle (ACA), corneal hysteresis (CH) and corneal resistance factor (CRF). Statistical analysis was performed with the paired student t-test and Pearson correlation test in SPSS 16.

RESULTS

There were statistically significant decreases in CCP, SCP, ACD and ACV values following reading (p < 0.05). Our results showed a statistically significant negative correlation between CH and changes in ICP (r = 0.36, p = 0.02). Significant negative correlations were also found between CRF and changes in ICP (r = 0.41, p = 0.01) and SCP (r = 0.34, p = 0.04). On the other hand, statistical analysis indicated no correlation between CBC and other studied parameters (p > 0.05).

CONCLUSION

This study demonstrated significant changes in some anterior segment parameters after reading. Being knowledgeable about these changes may have important implications in high accuracy examinations such as pre-operative assessment of corneal refractive surgery candidates. This could also help researchers have a better understanding of the factors that may influence near work related development of refractive errors.

Central corneal thickness in Iranian congenital glaucoma patients

Purpose:

To compare central corneal thickness (CCT) in subjects with controlled primary congenital glaucoma (PCG) and nonglaucomatous subjects and to investigate the correlation between CCT and intraocular pressure (IOP) in the study population.

Materials and Methods:

Twenty-three consecutive PCG cases with controlled IOP and no clinical evidence of corneal edema comprised the Study Group. There was an interval of at least 2 months between last intraocular surgery and inclusion in the study. Twenty-one subjects with strabismus or lacrimal drainage insufficiency who did not have glaucoma or any history of intraocular surgery or ocular trauma comprised the control group. The Control Group was age and sex-matched. Data from ultrasonic pachymetry and applanation tonometry were analyzed for differences between groups. Correlation of the study parameters was investigated. A P-value less than 0.05 was statistically significant.

Results:

Data from both eyes of subjects in the Study Group and Control Group were included in the original analysis. Mean CCT was statistically significantly higher in the Study Group compared to the Control Group (589.42 ± 53.44 μm vs. 556.14 ± 30.51 μm, respectively; P=0.001). There was a significant correlation between CCT and IOP (r=0.63; P<0.0001). Similar statistically significant outcomes were observed when only one eye per subject was used in a reanalysis of the data for the Study and Control Groups.

Conclusion:

Patients with PCG who had controlled IOP have statistically significantly thicker corneas than nonglaucomatous age and sex-matched subjects The thicker cornea could significantly alter IOP measurement with applanation tonometry. Pachymetry should be considered an essential part of the evaluation for PCG.

Corneal Biomechanical Properties and Anterior Segment Parameters in Forme Fruste Keratoconus

Purpose.

To evaluate the sensitivity and specificity of corneal biomechanical metrics, anterior segment data, and a combination model in differentiating forme fruste keratoconus (FFK) from normal corneas.

Methods.

A total of 50 FFK eyes were identified by calculation of the KISA index and recruited FFK group. Results were compared with 50 normal eyes (NG group) randomly selected from 50 patients. The following parameters were evaluated for their diagnostic capacity by evaluation of their receiver operating characteristic curves (ROC): corneal hysteresis (CH), corneal resistance factor (CRF), corneal astigmatism (Cyl), anterior chamber depth (ACD), corneal volume (CV) at 3 mm (CV3) and at 5 mm (CV5), maximum posterior elevation value (PEL), central corneal thickness (CCT), thinnest corneal thickness (TCT) and its coordinates (TCT x, TCT y), the ratio TCT/CCT, pachymetric progression indexes (PPImin, PPIavg, and PPImax), and Ambrósio's relational thickness (ARTmin, ARTavg, and ARTmax). Logistic regression was attempted for identification of a combined diagnostic model.

Results.

Significant differences were detected in all studied parameters except the Cyl, ACD, TCT x, and CV. Among individual parameters, the highest predictive accuracy was for ARTavg (area under the curve [AUC] 95.4%, sensitivity 90%, specificity 88.9%) and TCT (AUC 95.3%, sensitivity 90.9%, specificity 89%). Sufficient predictive accuracy (AUC 99.4%, sensitivity 98.8%, specificity 94.6%) was identified in a diagnostic model that combined the CRF, ARTavg, and PEL parameters.

Conclusions.

None of the individual parameters provide sufficient diagnostic capacity in FFK. However, diagnostic models that combine biomechanical and tomographic data seem to provide high accuracy in differentiating FFK from normal corneas.