Corneal Biomechanical Properties Research Articles

Evaluation of Corneal Biomechanical Properties in Patients With Thyroid Eye Disease Using Ocular Response Analyzer.

To assess variations in the corneal biomechanical properties in thyroid eye disease (TED) patients using ocular response analyzer (ORA). In this observational cross-sectional study, 75 patients with TED and 57 healthy subjects were enrolled. The mean age of the patients and healthy subjects were 47.50±1.55 and 43.6±1.23 years, respectively (P=0.06). All study participants underwent comprehensive ophthalmologic examination, Goldmann applanation tonometry (GAT), corneal pachymetry, and corneal biomechanical analysis using ORA. Corneal hysteresis (CH), corneal resistance factor (CRF), cornea-compensated IOP value (IOPcc), and Goldmann-corrected IOP value (IOPg) were measured with ORA. Central corneal thickness (CCT) in patients group (536.18±36.20 μm) and control group (539.22±28.83 μm) were not significantly different (P=0.1). In TED group, the IOPcc (20.23±0.54 mm Hg) was significantly higher than both IOP-GAT (17.54±0.49 mm Hg, P<0.001) and IOPg (18.35±0.52, P<0.001). CH in TED patients (9.01±0.20) was significantly lower compared with CH in healthy subjects (10.45±0.21) (P<0.001). Although CRF was lower in TED patients (10.06±0.16) in comparison with normal subjects (10.42±0.29), this difference was not statistically significant (P=0.25). Both CH and CRF were positively correlated with CCT (r=0.52, P<0.001 and r=0.62, P<0.001, respectively) in TED group. In TED patients, CH seems to be lower than in normal subjects. GAT might underestimate the IOP due to differences in corneal properties of cornea in TED patients.

Assessment of corneal biomechanical parameters in myopes and emmetropes using the Corvis ST

BackgroundPrior studies have demonstrated conflicting results regarding the relationship between corneal biomechanical properties and refractive error. Thus, the purpose of this study was to compare the corneal biomechanical parameters of myopes and emmetropes.MethodsNinety‐four subjects with varying degrees of myopia (aged 29 to 74-years, spherical equivalent [SE] −0.5 to −17.5 D) and 25 emmetropes (aged 19 to 75-years, SE: −0.5 to +0.5 D) presenting at the Queen Mary Hospital, Hong Kong were recruited sequentially for this prospective study. All patients were phakic with no history of coexisting ocular disease. The corneal biomechanical parameters of the right eye of each subject were analysed using the Corvis ST non‐contact tonometer. Intraocular pressure (IOP) was measured using both the Corvis ST and the Topcon Non‐Contact Tonometer CT‐80. Refractive error was measured by non‐cycloplegic subjective and objective refractometry.ResultsHigh myopes (SE greater than −6.00 D) demonstrated greater mean outward applanation velocities (p < 0.001) and peak distance measurements (p = 0.009) compared to both low to moderate myopes (SE −0.50 to −6.00 D) and emmetropes. Both outward applanation velocity and peak distance were moderately correlated with refractive error (p ≤ 0.001), strongly correlated with IOP and weakly correlated with central corneal thickness. There were no statistically significant differences in age, IOP or central corneal thickness among emmetropes, low to moderate myopes or high myopes.ConclusionWithin this study of Chinese subjects, high myopes demonstrate greater corneal mean outward applanation velocity on Corvis ST testing, than emmetropes. In particular, those with high myopia (SE greater than −6.00 D) show a distinct corneal biomechanical profile relative to those with either emmetropia or low to moderate myopia using the Corvis ST.

Editorial: Update on Glaucoma Diagnosis and Management.

Glaucoma is one of the most dreadful ocular conditions that affects millions of people worldwide and is one of the major causes of irreversible blindness. Although glaucoma is a complex and poorly understood disorder, the primary goal of therapy is lowering Intraocular Pressure (IOP), which is not only one of the most significant risk factors in development of the disease, but also the only modifiable one. Therefore, a precise measurement of IOP is essential for accurate diagnosis and effective monitoring of glaucoma. Goldmann applanation tonometry is the gold standard method for IOP measurement. It has been known that IOP measurement by this method is affected by corneal properties; typically, thick corneas overestimate and thin corneas underestimate the IOP. Therefore, several methods had been introduced to correct IOP based on the central corneal thickness. However, those methods had oversimplified the problem, and ignored important factors such as corneal viscosity and elasticity. New generation of devices such as Ocular Response Analyzer (Reicherts®) and CorVis ST (Oculus Inc.®) have been devised that can account for corneal biomechanical properties when measuring IOP. It is claimed that these methods are particularly useful for eyes with inherently abnormal and surgically altered corneal biomechanics such as those who have undergone laser vision correction or keratoplasty [1]. Around two-thirds of patients with glaucoma may have their highest IOP readings beyond regular clinic hours, especially during the night. Actually, these undetected IOP spikes have been suggested to play a role in progressive glaucomatous optic neuropathy in a subset of glaucoma patients with apparent normal office-based IOP readings. Therefore, determining the role of IOP fluctuation in glaucoma warrants more extensive research. Accordingly, a 24 hour continuous (or frequent) IOP monitoring would be a commendable goal, especially for those with unexplained progression [2]. In addition to IOP fluctuation, the potential role of corneal characteristics, pathologies, and prior operations on IOP measurement should be considered when interpreting patients’ IOP. While there is no single method which provides an easy, quick and accurate result in all cases, the practitioner should adopt a patient-oriented strategy to select the most appropriate method, based on established strengths and limitations of each device [1]. Antiglaucoma eye drops comprise the mainstay of treatment in most cases of glaucoma. However, some patients still need laser and surgical procedures to prevent glaucoma progression. Nowadays, similar to other aspects of ophthalmology, utilization of laser plays a tremendous role in glaucoma treatment. Typical laser surgeries for glaucoma include laser peripheral iridotomy, laser iridoplasty, laser trabeculoplasty, and laser cyclophotocoagulation. There are also emerging methods of glaucoma laser therapy using advantages of novel inventions such as micropulse laser and pattern delivery [3]. Several surgical procedures have been introduced to help in achieving normal IOP where topical medications are inappropriate or inadequate. For several decades, filtering surgeries (trabeculectomy and shunt) have been used successfully to control IOP in a variety of cases with glaucoma. Although the primary concept has not been changed since then, recent growing knowledge about wound healing allows using modified techniques and novel adjuvants to achieve more efficient filtering blebs with less associated complications [4]. In contrast to open angle glaucoma, small incision cataract surgery with or without goniosynechialysis may be a good alternative for medications or even filtering surgeries in selected cases with angle-closure glaucoma. It can efficiently widen the iridocorneal angle, evades anatomical predisposition to angle closure, and significantly improves IOP control without long-term complications of trabeculectomy [5]. The 5 solicited review articles of this issue deals with the most recent advancements in glaucoma diagnosis and treatment and update the readers with useful and practical information in this regard.

Analysis of the effect of the fluid-structure interface on elastic wave velocity in cornea-like structures by OCE and FEM

Air-pulse optical coherence elastography (OCE) is a promising technique for quantifying biomechanical properties of the cornea. This technique typically involves imaging and analysis of the propagation of the air-pulse induced elastic waves to reconstruct corneal biomechanical properties using an analytical model. However, the effect of the fluid-structure interface (FSI) at the corneal posterior surface on the elastic wave velocity is not accounted for in many models. In this study, we examined the effect of the FSI with OCE experiments on contact lenses with and without fluid in the posterior gap. Finite element models (FEM), also with and without the FSI, were constructed to simulate the elastic wave propagation based on the OCE measurements. The FEM and OCE results were in good agreement demonstrating the feasibility of the method. To further investigate the effect of the FSI, OCE experiments and subsequent FEM simulations were conducted on in situ rabbit corneas before and after rose bengal/green light corneal collagen cross-linking (RGX). Both the OCE experiments and the FE simulations demonstrated that the FSI significantly reduced the group velocity of the elastic wave, and thus, should be considered when determining corneal biomechanical properties from an appropriate mechanical model. By matching the FEM-calculated velocity to the OCE-measured velocity, the corneal elasticity was quantified. The Young’s modulus of the rabbit cornea before RGX was E = 65 ± 10 kPa at a controlled intraocular pressure (IOP) of 15 mmHg. After RGX, the Young’s modulus increased to E = 102 ± 7 kPa at the same IOP.

Effects of topical travoprost 0.004% on intraocular pressure and corneal biomechanical properties in an animal model.

To determine whether topical application of travoprost 0.004% induces changes in corneal biomechanical properties affecting intraocular pressure (IOP) values in rabbits. Both eyes of 10 New Zealand rabbits were measured 3 times with the Ocular Response Analyzer (ORA) before treatment. Each measurement included corneal hysteresis (CH), corneal resistance factor (CRF), corneal-corrected IOP (IOPcc), and Goldmann equivalent IOP (IOPg). A drop of travoprost 0.004% was applied once daily in right eyes for 3 months; left eyes received no treatments. After 3 months of treatment both eyes of all rabbits were again measured 3 times. After complete keratectomy of both eyes, tissues prepared with hematoxylin-eosin stain were analyzed by means of light microscopy. The mean pre- and post-treatment IOPg, respectively, for right eyes was 9.92 ± 5.64 mm Hg and 7.62 ± 2.99 mm Hg (P = 0.027); IOPcc, 19.81 ± 5.25 mm Hg and 17.79 ± 4.09 mm Hg (P = 0.063); CRF, 1.65 ± 1.63 mm Hg and 2.18 ± 2.50 mm Hg (P = 0.266); and CH, 2.79 ± 1.74 mm Hg and 2.64 ± 2.08 mm Hg (P = 0.72). Mean post-treatment right and left eye IOPg values were, respectively, 7.62 ± 2.99 and 10.30 ± 4.40 (P = 0.002); IOPcc, 17.79 ± 4.09 mm Hg and 20.37 ± 4.32 mm Hg (P = 0.009); CRF, 1.65 ± 1.63 mm Hg and 2.17 ± 2.47 mm Hg (P = 0.274); and CH, 2.79 ± 1.74 mm Hg and 2.54 ± 2.08 mm Hg (P = 0.575). No difference in CH and CRF was observed between treated and untreated eyes. Post-treatment reduction of IOP in treated eyes was a direct hypotensive effect of travoprost 0.004% and was not affected by changes in corneal biomechanical properties (CH and CRF), resulting in real lower IOP values.

Associations with Intraocular Pressure in a Large Cohort: Results from the UK Biobank

PurposeTo describe the associations of physical and demographic factors with Goldmann-correlated intraocular pressure (IOPg) and corneal-compensated intraocular pressure (IOPcc) in a British cohort.DesignCross-sectional study within the UK Biobank, a large-scale multisite cohort study in the United Kingdom.ParticipantsWe included 110 573 participants from the UK Biobank with intraocular pressure (IOP) measurements available. Their mean age was 57 years (range, 40–69 years); 54% were women, and 90% were white.MethodsParticipants had 1 IOP measurement made on each eye using the Ocular Response Analyzer noncontact tonometer. Linear regression models were used to assess the associations of IOP with physical and demographic factors.Main Outcome MeasuresThe IOPg and IOPcc.ResultsThe mean IOPg was 15.72 mmHg (95% confidence interval [CI], 15.70–15.74 mmHg), and the mean IOPcc was 15.95 mmHg (15.92–15.97 mmHg). After adjusting for covariates, IOPg and IOPcc were both significantly associated with older age, male sex, higher systolic blood pressure (SBP), faster heart rate, greater myopia, self-reported glaucoma, and colder season (all P < 0.001). The strongest determinants of both IOPg and IOPcc were SBP (partial R2: IOPg 2.30%, IOPcc 2.26%), followed by refractive error (IOPg 0.60%, IOPcc 1.04%). The following variables had different directions of association with IOPg and IOPcc: height (−0.77 mmHg/m IOPg; 1.03 mmHg/m IOPcc), smoking (0.19 mmHg IOPg, −0.35 mmHg IOPcc), self-reported diabetes (0.41 mmHg IOPg, −0.05 mmHg IOPcc), and black ethnicity (−0.80 mmHg IOPg, 0.77 mmHg IOPcc). This suggests that height, smoking, diabetes, and ethnicity are related to corneal biomechanical properties. The increase in both IOPg and IOPcc with age was greatest among those of mixed ethnicities, followed by blacks and whites. The same set of covariates explained 7.4% of the variability of IOPcc but only 5.3% of the variability of IOPg.ConclusionsThis analysis of associations with IOP in a large cohort demonstrated that some variables clearly have different associations with IOPg and IOPcc, and that these 2 measurements may reflect different biological characteristics.

Determining in vivo elasticity and viscosity with dynamic Scheimpflug imaging analysis in keratoconic and healthy eyes.

The paper presents a novel analysis method of corneal elasticity and viscosity based on corneal visualization Scheimpflug technology (CorVis ST) for keratoconus diagnosis. Methods for air puff force measurement and corneal imaging boundary extraction were proposed. Corneal biomechanical properties, described as tangent stiffness coefficient (STSC ) and energy absorbed area (Aabsorbed ), were assessed using the curves of the applied air puff force with corneal displacement to form a loading-unloading cycle. Twenty-five patients with keratoconus and 34 healthy control subjects, matched for intraocular pressure (IOP), were enrolled in this prospective study. The results showed that the intraclass correlation coefficients (ICC) of the STSC and Aabsorbed were 0.941 and 0.878 in Healthy group; and were 0.891 and 0.809 in Keratoconus group, respectively. Both STSC and Aabsorbed of keratoconus patients were significantly different from that of controls (both probability value P < 0.001). ROC curve analysis showed that the area under curve for STSC was 0.918 and for Aabsorbed was 0.894, which reached a good level of predictive accuracy for detecting keratoconus. Our results demonstrated that this new analysis method could be used to characterize the biomechanical properties of corneas. (a) The air puff force of CorVis ST was measured by a custom-designed force detection system. (b) Corneal displacement was extracted from CorVis ST using a proposed imaging analysis. (c) With the utilization of the air puff force and corneal dynamic displacement, an analysis method was developed to introduce new corneal biomechanical parameters - STSC and Aabsorbed .

Effect of selective laser trabeculoplasty on corneal biomechanics

To evaluate the effect of selective laser trabeculoplasty (SLT) on ocular biomechanics as measured with the ocular response analyzer (ORA, AMETEK Inc. and Reichert Inc.). In a prospective case series, 52 eyes of 52 patients (aged 66.6±9.6years) with insufficient intraocular pressure (IOP) control despite maximum tolerated medical therapy underwent SLT (Solo(™) SLT, Ellex Inc.) for further IOP reduction. Goldmann-correlated IOP (IOPg), corneal-compensated IOP (IOPcc), corneal hysteresis (CH) and the corneal resistance factor (CRF) were measured with the ORA prior to and at least 4weeks following SLT. IOPg decreased statistically significantly from 18.0±6.4 to 14.8±3.8mmHg and IOPcc from 20.2±6.5 to 16.7±3.4mmHg (p<0.001). CH increased from 8.53±2.03 to 9.12±1.83mmHg (p=0.028) and CRF decreased from 9.58±2.18 to 9.1±2.1mmHg (p=0.037), which was statistically significant. Correcting the CH and CRF data for the influence of IOP reduction with a covariance analysis, however, resulted in an unchanged CH (IOPg adjusted: p=0.318; IOPcc adjusted: p=0.468) and CRF (IOPg adjusted: p=0.320; IOPcc adjusted: p=0.464) after SLT. Selective laser trabeculoplasty does not change corneal biomechanical properties as measured with the ORA in already pretreated patients with glaucoma. The measured increase in CH and decrease in CRF can be solely explained by IOP reduction.

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.

The Short-Term Effect of Prostaglandin Analog Monotherapy on Corneal Biomechanical Properties in Normal Tension Glaucoma Patients

Purpose: To evaluate the effects of short‐term prostaglandin analogues treatment on the corneal biomechanics of patients with normal tension glaucoma. Methods: This study included 52 eyes of 52 patients who were diagnosed with normal tension glaucoma. All patients were divided into two groups; one group (27 eyes) received tafluprost while the other group (25 eyes) received travoprost. Intraocular pressure, Biomechanical properties were measured by using goldmann applanation tonometer, ocular response analyzer before treatment and at 8‐week after treatment. Results: The mean decrease in intraocular pressure, Goldmann‐correlated IOP (IOPg), corneal‐compensated intraocular pressure by using Goldmann applanation tonometer, and Ocular response analyzer were statistically significant in total patients, tafluprost, and travoprost group after using prostaglandin analogues (p < 0.001, p < 0.001, p < 0.001, respectively). Corneal hysteresis showed no statistical differences after treatment in total, tafluprost and travoprost group but corneal resistance factor (CRF) showed statistically significant decrease after using prostaglandin analogues in total, tafluprost, and travoprost group (p < 0.001, p = 0.025, p < 0.001). Upon multivariate analysis, the higher initial IOPg and the lower initial CRF checked, the variation of CRF (CRF in baseline – CRF at 8 weeks) got higher (β = 0.134, p = 0.017). Conclusions: It is needed to carefully monitor and evaluate the effects of prostaglandin analogues on intraocular pressure associated with initial intraocular pressure and the changes of CRF after prostaglandin treatment in normal tension glaucoma patients. CRF is sensitive factor to short‐term changes of intraocular pressure after prostaglandin analogues treatment, and it is required to consider the properties of CRF when we evaluate between progression of glaucoma and corneal biomechanical properties. J Korean Ophthalmol Soc 2016;57(3):477-484

Detection of the early keratoconus based on corneal biomechanical properties in the refractive surgery candidates.

Context:Subclinical keratoconus is contraindication to refractive surgery. The currently used methods of preoperative screening do not always allow differentiating between healthy eyes and those with subclinical keratoconus.Aim:To evaluate biomechanical parameters of the cornea, waveform score (WS), and intraocular pressure (IOP) as potentially useful adjuncts to the diagnostic algorithm for precise detection of the early keratoconus stages and selection of refractive surgery candidates.Settings and Design:Department of Ophthalmology and prospective cross-sectional study.Patients and Methods:Patients enrolled in the study were diagnosed with refractive disorders. We assessed parameters of corneal biomechanics such as corneal hysteresis (CH), corneal resistance factor (CRF), Goldman-correlated IOP (IOPg), corneal compensated IOP, WS, and keratoconus match index (KMI). They were classified into one of three groups based on the predefined KMI range: Group 1 (from 0.352 to 0.757) – 45 eyes, Group 2 (from −0.08 to 0.313) – 52 eyes, and Group 0 - control group (from 0.761 to 1.642) – 80 eyes.Results:In both study groups, IOPg, CRF, and CH were decreased when compared to control (P < 0.0001). In control group, there was positive correlation between CH and KMI (P < 0.05), with no correlations in any of the two study groups. CRF correlated positively with KMI in control (P < 0.0001) and in Group 2 (P < 0.05).Conclusions:CH and CRF, together with WS and IOPg, consist a clinically useful adjunct to detect subclinical keratoconus in patients referred for refractive surgery when based on KMI staging.

Glokom Şüphesi Olan Hastalarda Su İçme Testinin Korneanın Biyomekanik Özellikleri ve Santral Kornea Kalınlığı Üzerine Etkisi

Glokom Şüphesi Olan Hastalarda Su İçme Testinin Korneanın Biyomekanik Özellikleri ve Santral Kornea Kalınlığı Üzerine Etkisi

Corneal Biomechanical Properties in Aspirin Users

Purpose: To analyze corneal biomechanical properties in aspirin users using an ocular response analyzer. Methods: This study included 80 eyes of 40 aspirin users and 80 eyes of 40 individuals who did not use aspirin. Corneal hysteresis (CH), the corneal resistance factor (CRF), Goldman-correlated intraocular pressure (IOPg), and corneal compensated intraocular pressure (IOPcc) were measured in all participants. The independent samples t test was used to compare measurements in the aspirin users and nonusers in the total study population, and in the diabetic patient subgroup. Pearson's correlation analysis was used to examine the relationship between the measured variables in the aspirin users and nonusers. Results: Aspirin users (59.08 ± 11.83 years) were older than nonusers (39.82 ± 12.97 years; p < 0.001). The mean CH was significantly lower in the aspirin user group than in the nonuser group (p = 0.013). Mean IOPg and mean IOPcc were also significantly higher in the aspirin user group (p = 0.027 and p = 0.002, respectively). The mean CRF was lower in the aspirin user group, but not significantly (p = 0.70). There was a positive correlation between CH and CRF (r = 0.767, p < 0.001), and between CRF and IOPg (r = 0.680, p < 0.001), and a negative correlation between CH and IOPcc (r = -0.415, p < 0.001). Conclusions: Aspirin should be taken into account when interpreting the results of corneal biomechanical measurements.

In Vivo Corneal Biomechanical Properties with Corneal Visualization Scheimpflug Technology in Chinese Population.

Purpose. To determine the repeatability of recalculated corneal visualization Scheimpflug technology (CorVis ST) parameters and to study the variation of biomechanical properties and their association with demographic and ocular characteristics. Methods. A total of 783 healthy subjects were included in this study. Comprehensive ophthalmological examinations were conducted. The repeatability of the recalculated biomechanical parameters with 90 subjects was assessed by the coefficient of variation (CV) and intraclass correlation coefficient (ICC). Univariate and multivariate linear regression models were used to identify demographic and ocular factors. Results. The repeatability of the central corneal thickness (CCT), deformation amplitude (DA), and first/second applanation time (A1/A2-time) exhibited excellent repeatability (CV% ≤ 3.312% and ICC ≥ 0.929 for all measurements). The velocity in/out (V in/out), highest concavity- (HC-) radius, peak distance (PD), and DA showed a normal distribution. Univariate linear regression showed a statistically significant correlation between V in, V out, DA, PD, and HC-radius and IOP, CCT, and corneal volume, respectively. Multivariate analysis showed that IOP and CCT were negatively correlated with V in, DA, and PD, while there was a positive correlation between V out and HC-radius. Conclusion. The ICCs of the recalculated parameters, CCT, DA, A1-time, and A2-time, exhibited excellent repeatability. IOP, CCT, and corneal volume significantly influenced the biomechanical properties of the eye.

İridokorneal Endotelyal Sendromlu Bir Olguda Korneanın Biyomekanik Özellikleri ve Korneal Dansitometri

Iridocorneal endothelial (ICE) syndrome is a rare disorder which includes iris nevus syndrome, Chandler syndrome and essential iris atrophy; and represents a spectrum of the similar disorder process predominantly affecting the corneal endothelium, anterior chamber angle and iris. ICE syndrome is often under-diagnosed especially in early stages when there is no apparent corneal edema and glaucoma. The accurate diagnosis is mainly based on clinical examination and in-vivo confocal microscopy by showing epithelium-like transformation of the corneal endothelium with irregularly shaped cells and hyperreflective nuclei. We hereby report corneal biomechanical properties and densitometry in a 49-year-old patient with ICE syndrome and discuss role of these parameters for the diagnosis and follow-up of ICE syndrome.

Effect of corneal biomechanical properties on surgically-induced astigmatism and higher-order aberrations after cataract surgery.

To investigate the relationship between biomechanical properties of the cornea and postoperative refractive changes in patients with low-level astigmatism after cataract surgery. This prospective study recruited patients undergoing cataract surgery involving 2.8-mm superior incisions. Biomechanical properties of the cornea were evaluated preoperatively using the Ocular Response Analyzer, and corneal profiles were evaluated using a Scheimpflug system (Pentacam HR). Topographic astigmatism, total corneal aberrations (TCA) and higher-order corneal aberrations (HOCA) analyses were performed preoperatively and during 1- and 3-month postoperative exams. The incidences of surgically-induced astigmatism (SIA) and HOCAs were calculated using vector analyses. Associations of the preoperative biomechanical properties of the cornea with SIA and HOCAs were evaluated. This study included 28 eyes of 28 patients. The preoperative corneal hysteresis (CH) was 8.68 ± 1.86 mmHg, and the corneal resistance factor (CRF) was 8.66 ± 1.61 mmHg. At the 1-month postoperative evaluation, significant changes were observed in HOCAs (p=0.023), TCAs (p=0.05), astigmatism (p=0.02), and trefoil (p=0.033); in contrast, differences in coma (p=0.386) and spherical aberration (SA) were not significant (p=0.947). At the 3-month visit, significant changes were only observed in TCAs (p=0.02) and HOCAs (p=0.012). No relationships between the preoperative corneal hysteresis and corneal resistance factor and postoperative SIA and HOCA were identified, other than a positive correlation between the 3-month postoperative incidence of corneal hysteresis and spherical aberration. Despite the observed lack of relationships of preoperative biomechanical properties of the cornea with SIA and postoperative aberrations (except for SA), further studies involving larger patient groups are needed to explore the unexpected refractive deviations after cataract surgery.

Changes in Corneal Biomechanical Properties After Descemet Stripping Automated Endothelial Keratoplasty for Pseudophakic Bullous Keratopathy

To compare corneal biomechanical properties and intraocular pressure (IOP) in eyes with pseudophakic bullous keratopathy (PBK) before and after Descemet stripping automated endothelial keratoplasty (DSAEK). This prospective nonrandomized intrasubject comparative study was conducted on 44 eyes of 22 patients with the diagnosis of PBK who underwent DSAEK in one eye. IOP was measured by Goldmann applanation tonometer, and central corneal thickness was measured by ultrasound pachymetry. The ocular response analyzer was used to measure corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-related IOP, and cornea-compensated IOP. The same measurements were performed in the normal fellow eyes which served as controls. All measurements were performed preoperatively and 6 months postoperatively. The mean patient age was 67 ± 14 years. Mean preoperative CH and CRF values in the DSAEK group were 5.77 ± 2.94 and 6.39 ± 2.72 mm Hg, respectively, which were significantly lower than those measured in the control group (8.2 ± 2.47 and 8.43 ± 2.49 mm Hg, respectively, P = 0.001 for both comparisons). Postoperatively, CH and CRF demonstrated a significant increase (7.09 ± 3.68 mm Hg, P = 0.05 and 8.21 ± 3.84 mm Hg, P = 0.03, respectively) in operated eyes approaching the normal values measured in the control eyes. Corneal biomechanical parameters were significantly lower in PBK eyes than in the normal fellow eyes. These metrics significantly increased after DSAEK and reached values measured in the normal fellow eyes.

The Safety of Orthokeratology—A Systematic Review

Objectives:The aim of this review is to evaluate the ocular safety of orthokeratology (OrthoK) treatment of myopia correction and retardation.Data Sources:Clinical studies published in English and Chinese were identified from MEDLINE, EMBASE CNKI, CQVIP, and WANFANG DATA (all from 1980 to April 2015). The reference lists of the studies and the Science Citation Index were also searched.Selection Criteria:Relevant clinical studies including case series, case reports, patient/practitioner surveys, retrospective and prospective cohort studies, and clinical trials were all included in the review. The material of OrthoK lenses was limited to gas-permeable lens.Main Results:This review incorporated a total of 170 publications, including 58 English and 112 Chinese literature. The risk of microbial keratitis in overnight OrthoK was similar to that of other overnight modality. The most common complication was corneal staining. Other clinically insignificant side effects included epithelial iron deposit, prominent fribrillary lines, and transient changes of corneal biomechanical properties. There was no long-term effect of OrthoK on corneal endothelium.Conclusions:There is sufficient evidence to suggest that OrthoK is a safe option for myopia correction and retardation. Long-term success of OrthoK treatment requires a combination of proper lens fitting, rigorous compliance to lens care regimen, good adherence to routine follow-ups, and timely treatment of complications.

Intraocular pressure and biomechanical corneal properties measure by ocular response analyser in patients with primary congenital glaucoma.

To measure the differences in corneal hysteresis (CH) and corneal resistance factor (CRF) in primary congenital glaucoma (PCG) and in control subjects using ocular response analyser (ORA) and also to compare intraocular pressure (IOP) measurements given by ORA against IOP given by Perkins tonometer, a handheld version of Goldman applanation tonometer (GAT), to determine correlation. One hundred and eighteen eyes of 78 patients with PCG (group I) and 103 eyes of 53 controls (group II) were evaluated using ORA. In all participants, IOP was measured using the Perkins tonometer. The ORA device uses applanation pressure peaks to generate the corneal-compensated IOP (IOPcc), which is reportedly independent of corneal thickness, and the measurement of Goldman-correlated IOP (IOPg), which is influenced by corneal thickness. The measures in group I were as follows: IOPcc 20.92±5.33; IOPg 18.87±6.67; CH 8.51±2.25; CRF 9.85±3.03; and IOP measured by Goldman 18.32±5.13. The measures in group II were as follows: IOPcc 14.33±2.91; IOPg 14.77±3.00; CH 11.37±1.61; CRF 11.02±1.74; and IOP measured by Goldman 13.74±2.42. The differences of all parameters compared between both groups were statistically significant (p<0.001 for CH and for all IOP measures and p=0.001 for CRF). The values of IOPcc, IOPg and IOP measured with Goldman were higher in group I than the values in group II. However, CH and CRF values were lower in group I. A decrease in CH and CRF has been observed in patients with PCG compared to controls. Future research should assess how these parameters are modified in PCG and whether they could provide more information about progression.

The research advance of measuring techniques on corneoscleral constitutive parameters

The occurrence and development of myopia and keratoconus is closely related to the changes of scleral and corneal biomechanical properties. The accurate measurement of biomechanical properties for corneoscleral tissure is very important on diagnosis of eye diseases, improvement of ocular operation, ocular biological parameter measurement and invention of ophthalmic instrument. Corneoscleral tissue, composed of bundles of compact and staggered collagen fiber and extracellular matrix, constitute the outer surface of the eyeball. The inhomogeneous distribution of the diameter, gap and amount of collagen fiber, makes its biomechanical characteristics really complex, characterized by nonlinear, viscoelastic, anisotropic, regional variation and age-related variation and etc. With the development of medical diagnostic technology, the importance of the ocular biomechanical property measurement is increasingly recognized. Nevertheless, measuring technology on ocular biomechanics properties are still not well understood by the majority of ophthalmologists. In order to facilitate the researchers to select a suitable measuring platform and method, the development of international corneoscleral biomechanical propertiy measuring technology was reviewed in this article.