Changes In Corneal Biomechanical Properties Research Articles

Implantation of Intracorneal Ring Segments in Keratectasia: Effects on Corneal Biomechanics in 112 Eyes.

The aim of this study was to analyze changes in corneal biomechanical properties after implantation of intracorneal ring segments (ICRSs) in keratectasia. This retrospective single-center study included 112 patient eyes that underwent femtosecond laser-assisted ICRS implantation (Intacs SK; Addition Technology Inc, Des Plaines, IL) for keratectasia. Biomechanical analysis was performed using the Ocular Response Analyzer (ORA; Reichert Inc, Depew, NY), with determination of corneal resistance factor, corneal hysteresis, and Keratoconus Match Index, as well as by Corvis ST (OCULUS, Wetzlar, Germany), with determination of stiffness parameter A1, Ambrosio relational thickness to the horizontal profile (Arth), integrated radius, deformation amplitude ratio, and stress-strain index as well as Corvis Biomechanical Index and Tomographic Biomechanical Index. Data collection was performed preoperatively and 6 months postoperatively for ORA and Corvis ST and additionally after 1 and 2 years for ORA. The corneal resistance factor decreased significantly postoperatively (5.8 ± 1.7 mm Hg) compared with preoperatively (6.75 ± 3.7 mm Hg; P = 0.021) and increased again during follow-up (6.2 ± 1.9 mm Hg; P = 0.024), without regaining preoperative values. Corneal hysteresis and Keratoconus Match Index did not change significantly. Stiffness parameter A1 ( P = 0.045) increased significantly after ICRS implantation and Arth decreased significantly from 181 ± 85 to 150 ± 92 ( P = 0.016). However, there was no significant postoperative change for others Corvis parameters. Corneal biomechanical properties showed inconsistent changes after ICRS implantation. Classical corneal biomechanical parameters (using single central air-puff tonometers) do not seem to be suitable for follow-up after ICRS implantation.

Changes in Corneal Biomechanical Properties after Laser-Assisted in situ Keratomileusis and Photorefractive Keratectomy in Myopia.

To compare changes in corneal biomechanical properties after laser-assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) in low and moderate myopia by Ocular Response Analyzer. Quasi-experimental study. Place and Duration of the Study: Armed Forces Institute of Ophthalmology (AFIO), Rawalpindi, Pakistan, between September 2020 and April 2022. Myopic correction was done in forty-six eyes of twenty-three patients by PRK, and forty-seven eyes of twenty-four patients by LASIK. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured using Ocular Response Analyzer (ORA), pre-operatively, and then 1, 3, and 6 months, postoperatively. The relationship between the amount of myopia treated and biomechanical properties was also studied. CRF and CH were decreased significantly after LASIK and PRK. A significantly larger decrease in CRF was observed after LASIK as compared to PRK at 6 months (Mann-Whitney U test: CRF, p = 0.02); however, decrease in CH was not statistically significant between LASIK and PRK at 6 months period (Mann-Whitney U test: CH, p = 0.388). A significant correlation was observed between the changes in biomechanical properties and extent of myopic correction after LASIK and PRK. Biomechanical strength of the cornea was significantly reduced by PRK and LASIK, which was also dependent on the spherical equivalent (SEQ) of myopic correction. A significantly larger change in CRF was observed after LASIK as compared to post PRK. Myopia, Laser in situ keratomileusis, Photorefractive keratectomy, Cornea, Biomechanics.

Effect of eye rubbing on corneal biomechanical properties in myopia and emmetropia.

Purpose: To investigate short-term changes in corneal biomechanical properties caused by eye rubbing in myopia and emmetropia and compare the different responses between the two groups. Methods: This was a prospective observational study of 57 eyes of 57 healthy subjects aged 45years and younger. The participants were divided into myopia and emmetropia groups. All the subjects underwent eye rubbing by the same investigator using the same technique. Biomechanical parameters were recorded using the Corvis ST device before and after 1min of eye rubbing. One week later, all the participants underwent the test again. Statistical methods were employed to compare the differences between the data from before and after the 1min of eye rubbing and demonstrate the different responses of the two groups. Results: After 1min of eye rubbing, smaller SP-A1 (p < 0.001), higher deformation and deflection amplitudes (p < 0.001, p = 0.012), higher peak distances (p < 0.001), earlier A1 times (p < 0.001), faster velocities (p < 0.001), and lower maximum inverse radii (p = 0.004) were observed. According to the automatic linear modeling analysis, the refractive states (B = -5.236, p = 0.010) and biomechanically corrected intraocular pressure (bIOP) (B = 0.196, p = 0.016) had influenced a decrease in the stiffness parameter at the first applanation (SP-A1). The central corneal thickness (CCT) had decreased only in the myopia group (p = 0.039). The change of SP-A1 in amplitude was larger in the myopia group than in the emmetropia group (p < 0.001). All the parameters returned to the baseline level 1week later. Conclusion: Eye rubbing appears to alter corneal biomechanical properties temporarily and make the cornea softer, especially for myopic young patients.

Biomechanical effect of intracorneal ring segment implantation

AbstractBackground: Previous studies have reported significant differences in corneal biomechanics between keratoconic and healthy corneas. Corvis ST® (CST – OCULUS, Wetzlar, Germany) and Ocular Response Analyser® (ORA – Reichelt Inc., Depew, NY, USA) are two single central air‐puff tonometers and are commonly used to analyse the corneal biomechanics. Implantation of intracorneal ring segments (ICRS) using a femtosecond laser represents a reliable surgical option to widen the spectrum of the stage‐related therapy of keratoconus for selected patients. Our purpose was to analyse the change in corneal biomechanical properties after ICRS implantation in keratectasia.Methods: This retrospective monocentric study included 118 patient eyes that underwent femtosecond laser‐assisted ICRS implantation (INTACS SK®, Addition Technology Inc., Des Plaines, IL, USA) for keratectasia. Biomechanical analysis was performed using both ORA, with determination of Corneal resistance factor (CRF), Corneal hysteresis (CH) and Keratoconus Match Index (KMI), as well as by CST with determination of stiffness parameter A1 (SP‐A1), Ambrosio's related thickness to the horizontal profile (Arth), integrated radius (IR), deformation amplitude‐ratio (DA‐ratio) as well as Corvis Biomechanical Index (CBI) and Tomographical Biomechanical Index (TBI). Data collection was performed preoperatively and after 6 months postoperatively for ORA and CST and additionally after 1 and 2 years for ORA.Results: The CRF decreased significantly postoperatively (5.8 ± 1.7 mmHg) compared to preoperatively (6.8 ± 3.7 mmHg; p &lt; 0.001), and increased again during follow‐up (6.2 ± 1.9 mmHg; p = 0.001), without regaining preoperative values. CH and KMI did not change significantly. SP‐A1, IR and DA‐ratio increased significantly after ICRS implantation (p &lt; 0.001). Arth decreased significantly from 190 ± 88 to 157 ± 108. However, there was no significant postoperative change for CBI and TBI.Conclusions: Corneal biomechanical properties showed inconsistent changes after ICRS implantation. The classical corneal biomechanical parameters (using single central air‐puff tonometers) do not seem to be suitable for follow‐up after ICRS implantation.

Biomechanical properties analysis of forme fruste keratoconus and subclinical keratoconus.

To analyze the biomechanical properties of the eye in patients with unilateral keratoconus with normal (forme fruste keratoconus [FFKC]) or abnormal topography (subclinical keratoconus [SKC]). This study included 153 eyes of 153 participants, including 95 eyes of patients with unilateral keratoconus, and 58 eyes of 58 healthy controls. Contralateral eyes with unilateral keratoconus were divided into two groups according to clinical manifestations and global consensus: FFKC (n = 30) and SKC (n = 65). The biomechanical characteristics were analyzed using non-parametric tests; further analysis thereof was performed after adjusting for confounding factors (i.e., intraocular pressure, age, and corneal thickness). Receiver operating characteristic curve (ROC) was used to analyze the ability of the biomechanical parameters to distinguish FFKC from SKC. Statistically significant differences between the FFKC and SKC groups were found in 9 of the 18 corneal biomechanical parameters analyzed using non-parametric tests. After adjusting for confounding factors, the multivariate analysis still revealed significant statistical differences in A1-time (P = 0.017), integrated radius (IR) (P = 0.024), and tomographic and biomechanical index (TBI, P < 0.001) between the FFKC and SKC groups. Stiffness parameter at first applanation (SP-A1) (Area under ROC [AUROC] = 0.765) demonstrated the strongest distinguishing ability, except for TBI (AUROC = 0.858) and Corvis Biomechanical Index (AUROC = 0.849), however, there was no statistically significant difference in SP-A1 (P = 0.366) between FFKC and SKC. Biomechanical parameters A1-time and IR have a high diversity between FFKC and SKC, besides TBI, and may reflect more subtle changes in corneal biomechanical properties (BPs) preceding SP-A1. The BPs of SKC are weaker than FFKC, which might be a basic and clue for the classification and diagnosis of the severity of early keratoconus in terms of biomechanics.

Measurement of corneal biomechanical properties in diabetes mellitus using the Corvis ST.

We sought to assess changes in corneal biomechanical parameters in patients with diabetes mellitus (DM) in comparison with those among healthy controls using Corvis ST (CST). The study group included 209 eyes from healthy control subjects and 33 eyes from diabetic subjects, respectively. Following an ophthalmological examination, measurements with CST were taken. Additionally, hemoglobin A1c and blood glucose values were collected. Results were then compared to those of the control group after adjusting for potential confounding factors, including age-, intraocular pressure (IOP)-, central corneal thickness (CCT)-, spherical equivalent (SE)- and axial length (AL). After adjusting for potential confounding factors, including the age, IOP, CCT, SE, and AL, patients with DM presented significantly lower whole-eye movement (WEM) (ms) values than patients without DM (21.71 ± 0.84 vs. 22.15 ± 0.64 ms; P < .001). There was a significant and negative correlation between WEM (ms) and hemoglobin A1c in DM patients (r = -0.733; P = .001). In univariate and multivariate general linear mixed model (GLMM) analyses, IOP (P < .001 and P < .001, respectively) and the presence of DM (P = .001 and P < .001, respectively) significantly affected WEM (ms). In DM, significant changes in corneal biomechanical properties were detectable. The DM group showed significantly less deformable cornea and sclera than did the normal controls, even after adjusting for age, IOP, CCT, SE, and AL. These findings may cause misinterpretation of IOP measurements in diabetic patients. Therefore, the measurement of corneal biomechanics should be taken into consideration in clinical practice.

The relationship between corneal biomechanical parameters and treatment outcomes of orthokeratology lenses

BackgroundTo evaluate changes in corneal biomechanical properties after long-term orthokeratology (OK) treatment and the factors affecting treatment outcomes.MethodsTwenty-four myopic teenagers who wore OK lenses for more than 1 year were included. Twenty-three individuals of the same age and with the same spherical equivalent wearing single-vision spectacles (SVS) were enrolled as controls. After routine eye examinations, corneal biomechanical properties and axial length were measured. Parameters were compared between groups.ResultsLess axial elongation (AE) occurred in the OK group (P = 0.021). The OK group experienced a statistically significant decrease in the A1 deformation amplitude (P = 0.02), whole eye movement maximum (P = 0.026), and Ambrósio’s relational thickness to the horizontal profile (ARTh) (P < 0.001), and a statistically significant increase in the pachyslope (P < 0.001) and Corvis biomechanical index (P < 0.001). Smaller ARTh and a larger highest concavity deflection area resulted in a better refractive state. The inhibitory effect of AE was better for older patients with smaller ARTh.ConclusionsLong-term OK treatment slowed myopia progression by reshaping the cornea. Smaller ARTh after OK lens wear indicated a better refractive state and slower AE and could predict OK lens treatment outcomes.

Evaluation of the Relationship Between the Changes in the Corneal Biomechanical Properties and Changes in the Anterior Segment OCT Parameters Following Customized Corneal Cross-Linking.

PurposeThis study aimed to investigate the relationship between changes in corneal biomechanical properties and changes in anterior segment optical coherence tomography (AS-OCT) parameters preoperatively and following customized corneal cross-linking (C-CXL) in eyes with progressive keratoconus.Patients and MethodsThis study included 44 eyes of 44 patients (33 men, 11 women; average age 22.8 ± 6.4 years) who underwent C-CXL for progressive keratoconus. Scheimpflug-based tonometer (SBT) and AS-OCT findings were evaluated preoperatively and 3 months following CXL. Parameters related to changes in SBT parameters were examined by multiple regression analysis using the stepwise method.ResultsRegarding SBT parameters, significant changes were observed in the integrated area under the curve of the inverse concave radius (pre, 12.19 ± 1.95/mm; post, 11.26 ± 1.89/mm; p < 0.0001), maximum inverse radius (pre, 0.24 ± 0.04/mm; post, 0.23 ± 0.04/mm; p = 0.0053), deformation amplitude ratio max 2 mm (pre, 5.53 ± 0.81; post, 5.29 ± 0.71; p = 0.0048), and stress–strain index (pre, 0.74 ± 0.16; post, 0.84 ± 0.20; p < 0.0001), pre and post C-CXL. Regarding AS-OCT parameters, significant changes were observed in average keratometry (pre, 47.87 ± 3.61 D; post, 47.56 ± 3.29 D, p = 0.0104), steep keratometry (pre, 49.61 ± 4.01 D; post, 49.25 ± 3.59 D; p = 0.0115), maximum keratometry (pre, 55.44 ± 6.22 D; post, 54.68 ± 5.56 D; p = 0.0061), and thinnest corneal thickness (pre, 450.43 ± 41.74 μm; post, 444.00 ± 39.35 μm; p < 0.0001), pre and post C-CXL. Multiple regression analysis demonstrated that when the change in the deformation amplitude (DA) ratio max (2 mm) was the dependent variable, age, change in average keratometry, and change in the thinnest corneal thickness were selected as explanatory variables. When changes in the stiffness parameter at applanation 1 and stress–strain index were the dependent variables, change in the intraocular pressure (IOP) was selected as the explanatory variable.ConclusionChange in the SBT parameters following C-CXL could be related to the age, change in the IOP value, change in average keratometry, and thinnest corneal thickness.

Changes in Corneal Biomechanical Properties in PRK Followed by Two Accelerated CXL Energy Doses in Rabbit Eyes.

To evaluate whether photorefractive keratectomy (PRK) combined with the two commonly delivered energy doses in accelerated corneal cross-linking (A-CXL) could help the cornea maintain its preoperative stiffness level. A total of 72 corneas of 36 healthy white Japanese rabbits were randomly divided into four equal groups. The groups included an untreated control group and three that had undergone PRK. After tissue ablation, one of the latter three groups (PRK group) was left untreated, whereas the other two were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) with the same irradiation (30 mW/cm2) but different CXL energy doses of 1.8 J/cm2 (PXL group) and 2.7 J/cm2 (PXH group). Dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH) measurements of stiffness parameter at first applanation (SP-A1), Stress-Strain Index (SSI), and other dynamic corneal response parameters were taken 3 days preoperatively and 1 month postoperatively. Subsequently, ex vivo inflation testing was performed and the tangent modulus of each specimen was estimated using an inverse analysis process. In comparison to the control group, the tangent modulus at a stress of 10 kPa decreased by 8.9% in the PRK group and increased by 10.6% and 22.4% in the PXL and PXH groups, respectively. SP-A1 decreased postoperatively in the PRK group (P < .05), indicating an overall stiffness reduction of -7.4, -3.5, and -5.3 mm Hg/mm in PRK, PXL, and PXH groups, respectively. The material stiffness parameter SSI remained almost unchanged in the PRK group (P = .989), increased slightly in the PXL group (8.3%, P = .077), and increased significantly in the PXH group (11.1%) (P < .05). Biomechanical deterioration following PRK was significant and could not be fully compensated for by ACXL with either 1.8 or 2.7 J/cm2 doses. The increased value of corneal overall stiffness was higher in A-CXL with 2.7 J/cm2 energy than with 1.8 J/cm2 energy. [J Refract Surg. 2021;37(12):853-860.].

Анализ изменения биомеханических свойств роговицы на приборе CORVIS ST (Oculus, Германия) у пациентов с миопией и миопическим астигматизмом после операции ФРК

Purpose. To analyze changes in corneal biomechanical properties after refractive surgery of photorefractive keratectomy (PRK) in patients with myopia and myopic astigmatism. Material and methods. The study included 10 patients (20 eyes) aged 23 to 51 years, planned for keratorefractive surgery for myopia and myopic astigmatism. Spheroequivalent of myopic refraction was -2.5 to -9.25 diopters, corneal astigmatism: -0.5 to 4.25 diopters. Patients underwent keratotopography using Pentacam scanning projection topograph (Oculus, Germany) and study of cornea biomechanical properties using Corvis ST non-contact tonometer (Oculus, Germany) at baseline and 1 month after surgery using the PRK technique. Results. Initially, in 3 patients (30%), keratotopogram showed elevation along posterior corneal surface (up to +26) and increase in corneal curvature in lower sections. Tomographic Biomechanical Index in all 10 patients ranged from 0.01 to 0.46, with 3 of them exceeding 0.3. Central Corneal Thickness in all patients averaged 514 ± 31 µm, of which 7 patients (70%) had 500 µm or less. Month after surgery only two indicators revealed statistically significant difference: Inverse Concave Radius (on average 5.95 ± 0.79 mm before surgery and 7.02 ± 0.70 mm after) and the Central Corneal Thickness (514 ± 31 µm before surgery and 410 ± 41 µm after). Conclusion. When selecting patients for keratorefractive surgery, it is imperative to take into account the initial corneal biomechanical properties to select the optimal technique in each specific case. Key words: corneal biomechanical properties, photorefractive keratectomy, myopia, myopic astigmatism.

Depth-resolved Corneal Biomechanical Changes Measured Via Optical Coherence Elastography Following Corneal Crosslinking.

PurposeTo evaluate depth-resolved changes of corneal biomechanical properties in eyes with corneal ectasia after corneal crosslinking (CXL) using optical coherence elastography.MethodsIn a prospective pilot series of eyes with corneal ectasia, a custom high-speed swept source optical coherence tomography system was used to image the cornea before and 3 months after CXL during a low-speed applanating deformation while monitoring applanation force. Cross-correlation was applied to track frame-by-frame two-dimensional optical coherence tomography speckle displacements, and the slope of force versus local axial displacement behavior during the deformation was used to produce a two-dimensional array of axial stiffness (k). These values were averaged for anterior (ka) and posterior (kp) stromal regions and expressed as a ratio (ka/kp) to assess depth-dependent differences in stiffness. CXL was performed according to the Dresden protocol with a system approved by the U.S. Food and Drug Administration.ResultsFour eyes from four patients with keratoconus (n = 3) or post-LASIK ectasia (n = 1) underwent optical coherence elastography before and 3 months after CXL. The mean ka/kp was 1.03 ± 0.07 before CXL compared with 1.34 ± 0.17 after the CXL procedure. All four eyes demonstrated at least a 20% increase in the ka/kp.ConclusionsPreferential stiffening of the anterior stroma with the standard CXL protocol was demonstrated with optical coherence elastography in live human subjects.Translational RelevanceAlthough ex vivo studies have demonstrated anterior stiffening effects after CXL using various destructive and nondestructive methods, this report presents the first evidence of such changes in serial live human measurements.

Comparison of safety and efficiency of corneal topography-guided photorefractive keratectomy and combined with crosslinking in myopic correction: An 18-month follow-up.

To compare the safety and efficiency of simple corneal topography-guided (T-CAT) photorefractive keratectomy (PRK) and T-CAT PRK combined with crosslinking (CXL) to correct myopia with borderline suspicious tomography.Eyes with suspicious tomography (not classified as forme fruste keratoconus) underwent PRK combined with CXL. The suspicious tomography showed irregular posterior corneal morphology or maximum elevation value of the central 6 mm zone of the posterior surface (MEL) >15 μm, or Belin/Ambrósio Enhanced Ectasia Index (BAD-D) was >1.6. The PRK group was generated and matched within 2 μm for MEL, 0.3 for BAD-D value, and 0.5 D for manifest refractive spherical equivalent (MRSE) compared with the PRK-CXL group.PRK-CXL exhibited a larger MRSE (0.09 ± 0.22 D vs −0.03 ± 0.24 D, P = .02) and a larger sphere (0.14 ± 0.22 D vs 0.01 ± 0.21 D, P = .002) compared with PRK alone at 18 months postoperatively. The magnitude change and relative change rate of stiffness parameter A1 in the PRK-CXL were smaller than in the PRK group (−15.72 ± 14.56 vs −19.95 ± 14.37, P = .04, for magnitude change and −0.16 ± 0.15 vs −0.20 ± 0.14, P = .02, for relative change rate). In the PRK-CXL and PRK groups, 4.8% and 6.9% of eyes suffered grade 0.5 haze at postoperative 18-month. No cases of ectasia were reported in either group.PRK in combination with prophylactic crosslinking showed comparable safety and efficacy, but higher biomechanical stability compared to PRK alone, thus, the additional CXL plays a measurable role in reducing the change in corneal biomechanical properties after PRK in suspicious eyes.

The changes of corneal biomechanical properties with long-term treatment of prostaglandin analogue measured by Corvis ST

BackgroundTo investigate the corneal biomechanical changes in primary open angle glaucoma (POAG) patients treated with long-term prostaglandin analogue (PGA).MethodsOne hundred eleven newly diagnosed POAG patients, including 43 high tension glaucoma (HTG) and 68 normal tension glaucoma (NTG), were measured by Corvis ST to obtain intraocular pressure (IOP), central corneal thickness (CCT) and corneal biomechanical parameters at baseline and at each follow-up visit after initiation of PGA treatment. The follow-up measurements were analyzed by the generalized estimate equation model with an exchangeable correlation structure. Restricted cubic spline was employed to estimate the dose–response relation between follow-up time and corneal biomechanics.ResultsThe mean follow-up time was 10.3 ± 7.02 months. Deformation amplitude (β = -0.0015, P = 0.016), the first applanation velocity (AV1, β = -0.0004, P = 0.00058) decreased and the first applanation time (AT1, β = 0.0089, P < 0.000001) increased statistically significantly with PGA therapy over time after adjusting for age, gender, axial length, corneal curvature, IOP and CCT. In addition, AT1 was lower (7.2950 ± 0.2707 in NTG and 7.5889 ± 0.2873 in HTG, P = 0.00011) and AV1 was greater (0.1478 ± 0.0187 in NTG and 0.1314 ± 0.0191 in HTG, P = 0.00002) in NTG than in HTG after adjusting for confounding factors.ConclusionsChronic use of PGA probably influences the corneal biomechanical properties directly, which is to make cornea less deformable. Besides, corneas in NTG tended to be more deformable compared to those in HTG with long-term treatment of PGA.

Factors Influencing Corneal Biomechanics in Diabetes Mellitus.

Diabetes mellitus (DM) induces changes in corneal biomechanical properties. The influence of disease-specific factors was evaluated, and a novel DM index was created. Eighty-one patients with DM and 75 healthy subjects were matched according to age, intraocular pressure, and central corneal thickness. Information on the disease was collected, and measurements with the Ocular Response Analyzer and the Corvis ST were taken. Results were compared between the groups, and the influence of disease-specific factors was evaluated. From dynamic corneal response parameters, a DM index was calculated. In DM, corneal hysteresis was higher than in healthy subjects (10.5 ± 1.9 vs. 9.7 ± 1.9 mm Hg, P = 0.008). In addition, dynamic corneal response parameters showed significant differences. Among others, highest concavity (HC) (17.212 ± 0.444 vs. 16.632 ± 0.794 ms, P < 0.001) and A2 time (21.85 ± 0.459 vs. 21.674 ± 0.447 ms, P = 0.017) as well as A1 (0.108 ± 0.008 vs. 0.104 ± 0.011 mm, P = 0.019) and A2 deflection amplitudes (0.127 ± 0.014 vs. 0.119 ± 0.014 mm, P < 0.001) were increased in DM. In DM type 1, HC deformation amplitude (1.14 ± 0.19 vs. 1.095 ± 0.114 mm, P = 0.035) was higher than in type 2. The time of deflection amplitude max correlated with the severity of retinopathy (R = 0.254, P= 0.023). In case of diabetic maculopathy, A1 velocity (0.155 ± 0.018 vs. 0.144 ± 0.019 ms, P = 0.043) and A2 time (22.052 ± 0.395 vs. 21.79 ± 0.46 ms, P = 0.04) were increased. Deformation amplitude max (R = 0.297, P = 0.024), HC time (R = 0.26, P = 0.049), HC deformation amplitude (R = 0.297, P = 0.024), and A2 deformation amplitude (R = 0.276, P = 0.036) were associated to disease duration. The DM index revealed a sensitivity of 0.773 and a specificity of 0.808 (area under the curve of receiver operating characteristic = 0.833). In DM, changes in corneal biomechanics were correlated with disease-specific factors. The DM index achieved reliable sensitivity and specificity values.

Biomechanical profile of refractive surgery procedures

AbstractThe impact of corneal biomechanics continues to be increasingly recognized as an integral part of refractive surgery outcomes. Corneal biomechanical properties are of major importance in laser refractive surgery and must be taken into consideration to reduce the risk for iatrogenic ectasia. Some in vivo corneal biomechanical alterations are possible to quantify with ORA and Corvist ST but are difficult to interpret and use in clinical practice due to their dependence of IOP, CCT, refractive status, time from surgery and age. Development and refinement are needed if ORA and Corvis ST should be implemented in a screening procedure of the biomechanical strength before refractive surgery.Flap‐creation technique plays a major role in changing the biomechanical stability of the cornea. Nevertheless several studies suggest that the type of procedure is not as important as the native charac‐teristics of the patient’s cornea in determining the risk for ectasia. Although we may have reached a plateau of technological improvement in excimer laser tissue ablation, the introduction of small incision lenticule extraction (ReLEx SMILE), has changed the face of corneal refractive surgery in particular in its effect on corneal biomechanics. Mathematical analysis and finite‐element models suggest that SMILE may preserve corneal biomechanical properties better than LASIK. Furthermore, the current in vivo studies performed with ORA and Corvis ST indicate that SMILE is equal or superior to LASIK in terms of preservation of the postoperative biomechanical strength.However CH and CRF were reduced after refractive surgery but no differ‐ences were found between SMILE and flap‐based (FLEx and FS‐LASIK) refractive surgeries in the RCTs. Studies have demonstrated that in LASIK and PRK, the amount of refractive error corrected is related to the changes in corneal biomechanical properties. Considerable reduction in the corneal biomechanical properties observed even in ‘flap‐less’ procedures like PRK and SMILE,Brillouin microscopy provides safe and non contact method for measuring longitudinal elastic modulus of ocular tissues. It provides three‐dimensional resolution of the imaged tissue revealing spatial variations in mechanical properties. Its clinical usefulness has already been demonstrated in corneal diagnostic and treatment monitoring and may eventually be a valuable tool for surgical planning of refractive and therapeutic procedures.question both in vivo and ex vivo, and by mathematical or finite‐element models of the cornea

Intraocular pressure measurements in diabetes mellitus.

To investigate the impact of diabetes mellitus-induced changes on intraocular pressure measurements using Goldmann applanation tonometry, Ocular Response Analyzer, and Corvis ST. Measurements were done using Goldmann applanation tonometry, Ocular Response Analyzer, and Corvis ST in 69 diabetic patients. Biomechanical-corrected intraocular pressure values by Ocular Response Analyzer (IOPcc) and Corvis ST (bIOP) were used. In addition, biometry and tomography were performed and information on diabetes mellitus specific factors was collected. Results were compared to an age-matched group of 68 healthy subjects. In diabetes mellitus, Goldmann applanation tonometry intraocular pressure (P = 0.193) and central corneal thickness (P = 0.184) were slightly increased. Also, IOPcc (P = 0.075) and bIOP (P = 0.542) showed no significant group difference. In both groups, IOPcc was higher than Goldmann applanation tonometry intraocular pressure (P = 0.002, P < 0.001), while bIOP was nearly equal to Goldmann applanation tonometry intraocular pressure (P = 0.795, P = 0.323). Central corneal thickness showed a tendency to higher values in poorly controlled than in controlled diabetes mellitus (P = 0.059). Goldmann applanation tonometry intraocular pressure correlated to central corneal thickness, while IOPcc and bIOP were independent from central corneal thickness in both groups. All intraocular pressure values showed significant associations to corneal biomechanical parameters. Only in diabetes mellitus, bIOP was correlated to Pachy slope (P = 0.023). In diabetes mellitus, Goldmann applanation tonometry intraocular pressure was slightly, but not significantly, increased, which might be caused by a higher central corneal thickness and changes in corneal biomechanical properties. However, intraocular pressure values measured by Ocular Response Analyzer and Corvis ST were not significantly different between diabetes mellitus patients and healthy subjects. The bIOP showed a higher agreement with Goldmann applanation tonometry than IOPcc and was independent from central corneal thickness.

Four‐year changes in corneal biomechanical properties in children

BackgroundTo determine four‐year changes of corneal biomechanical parameters in Iranian children aged seven to eleven years and their correlation with optical components.MethodsIn 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).ResultsAll 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).ConclusionAge 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.

Pneumotonometer Accuracy Using Manometric Measurements after Radial Keratotomy, Clear Corneal Incisions and Lamellar Dissection in Porcine Eyes

ABSTRACT Purpose/Aim of the study Measured intraocular pressure (IOP) after corneal incisions may not be reflective of the true IOP because of changes in corneal biomechanical properties. The purpose of this study is to investigate the effect of various corneal incisions on pneumotonometer accuracy in enucleated porcine eyes. Materials and Methods A pneumotonometer was used to measure IOP (IOPp) at manometrically controlled pressure levels of 10, 20, 30 and 40 mmHg in enucleated porcine eyes. IOP measurements at each level were repeated after one of the following corneal incisions: radial keratotomy (8 eyes), lamellar dissection (10 eyes), clear cornea standard phacoemulsification incisions (10 eyes). The pneumotonometer error, defined as the difference between IOPp and manometric pressure (IOPm), was calculated for each pressure level. The error before the corneal incisions was compared to the error after the corneal incisions to assess the accuracy of the pneumotonometer. Results The pneumotonometer underestimates true IOP at all pressure levels, both before and after the corneal procedures. There was a statistically significant greater underestimation of IOP after radial keratotomy incisions at pressure levels of 20, 30 and 40 mmHg (p = .013, 0.004, and 0.002, respectively). There was no statistically significant difference in the amount of pneumotonometer underestimation error after lamellar dissection or standard cataract incisions. Conclusion The pneumotonometer underestimates true IOP in enucleated porcine eyes at all pressure levels between 10–40 mmHg. Radial keratotomy incisions caused a statistically significant greater underestimation error in pneumotonometry measurements at pressures of 20–40 mmHg. Lamellar dissection and clear corneal cataract incisions did not cause an additional error in pneumotonometry measurements in enucleated porcine eyes.

Changes in Corneal Biomechanical Properties After Descemet Membrane Endothelial Keratoplasty.

To evaluate the course of corneal biomechanical properties (corneal hysteresis and corneal resistance factor) after Descemet membrane endothelial keratoplasty (DMEK) in patients with Fuchs endothelial dystrophy (FECD). In this prospective study, we consecutively included 49 patients with FECD with an indication for DMEK. Central corneal thickness, measured using the Oculus Pentacam, and corneal hysteresis (CH) and corneal resistance factor (CRF), both measured using the Reichert Ocular Response Analyzer, were examined preoperatively, 3 months, and 6 months after DMEK. CH was significantly increased 6 months postoperatively (8.66 ± 2.50 mm Hg) compared with preoperative baseline (7.43 ± 1.56 mm Hg). Preoperative CRF (7.89 ± 1.68 mm Hg) increased significantly 6 months after DMEK (8.49 ± 1.71 mm Hg). Preoperative central corneal thickness showed a significant decrease from 629 ± 58 μm to 550 ± 40 μm after 3 months and 535 ± 40 μm after 6 months postoperatively. DMEK affects both measured corneal biomechanical properties in patients with FECD: CH and CRF. This has to be taken into account for the accuracy of intraocular pressure measurements.

Changes in corneal biomechanical properties before and after operation in different types of glaucoma

Objective To compare the differences of corneal biomechanics in different types of glaucoma and its related influence factors, and to investigate the change trends of cornea biomechanics after the operations. Methods In a prospective comparative case series, 61 eyes of 61 acute primary angle-closure glaucoma (APACG) (remission or chronic phase)patients, 94 eyes of 94 chronic primary angle-closure glaucoma (CPACG) patients, 70 eyes of 70 primary open angle glaucoma (POAG) patients and 64 eyes of 64 age-related cataract (ARC) patients were recruited in Tianjin Eye Hospital from January to July, 2017.Corneal biomechanical properties were measured by using Corvis ST.The differences of corneal biomechanics in different types of glaucoma and the change trends after surgeries were assessed; the association between parameters and the age, intraocular pressure after correction (IOPcc), central cornea thickness (CCT), axial length (AL), average corneal diopter (K) were assessed by multiple linear regression analysis.This study followed the Declaration of Helsinki.This study protocol was approved by Ethic Committee of Tianjin Eye Hospital (TJYYLL-2016-17). Written informed consent was obtained from each subject prior to any medical examination. Results Compared with the ARC group, the first applanation velocity (A1V) and second applanation time (A2T) were both smaller in preoperative CPACG, POAG groups (all at P 0.05). Conclusions CPACG and POAG have poorer ability of corneal deformation, which gradually returns to normal after surgery; because the intraocular pressure of APACG (remission or chronic stage) return to normal after transient elevated stage, the corneal biomechanics is basically similar to ARC, which has no difference between pre-and post-operation.The parameters of corneal biomechanics are affected by IOPcc and K. Key words: Glaucoma; Cornea biomechanical properties; Glaucoma surgery; Cornea refractive power