Biomechanical Properties Of Cornea Research Articles

Biomechanical and histological changes associated with riboflavin ultraviolet-A-induced CXL with different irradiances in young human corneal stroma

Keratoconus (KC) is a degenerative condition affecting the cornea, characterized by progressive thinning and bulging, which can ultimately result in serious visual impairment. The onset and progression of KC are closely tied to the gradual weakening of the cornea's biomechanical properties. KC progression can be prevented with corneal cross-linking (CXL), but this treatment has shortcomings, and evaluating its tissue stiffening effect is important for determining its efficacy. In this field, the shortage of human corneas has made it necessary for most previous studies to rely on animal corneas, which have different microstructure and may be affected differently from human corneas. In this research, we have used the lenticules obtained through small incision lenticule extraction (SMILE) surgeries as a source of human tissue to assess CXL. And to further improve the results' reliability, we used inflation testing, personalized finite element modeling, numerical optimization and histology microstructure analysis. These methods enabled determining the biomechanical and histological effects of CXL protocols involving different irradiation intensities of 3, 9, 18, and 30 mW/cm2, all delivering the same total energy dose of 5.4 J/cm2. The results showed that the CXL effect did not vary significantly with protocols using 3–18 mW/cm2 irradiance, but there was a significant efficacy drop with 30 mW/cm2 irradiance. This study validated the updated algorithm and provided guidance for corneal lenticule reuse and the effects of different CXL protocols on the biomechanical properties of the human corneal stroma.

Biomechanical properties of cornea and ocular aberrations in myopic eyes.

To study the correlation of corneal topography, corneal biomechanical properties, and ocular aberrations with the magnitude of refractive error in myopic eyes. All myopic patients attending the clinic for refractive surgery were recruited. Data recorded included visual acuity, axial length (AL), central corneal thickness (CCT), manifest refraction spherical equivalent (MRSE), topography (Pentacam - Oculus), corneal biomechanical factors [ocular response analyzer (ORA)], and optical aberrations (iTrace). They were further categorized into group 1 (suitable for femto-LASIK) and group 2 (unsuitable for femto-LASIK). Sixty eyes (30 myopes) of mean age 22.78 ± 2.71 years were enrolled. A negative correlation of refractive error was noted with AL (ρ = -0.9; P < 0.001), total aberrations (ρ = 0.53; P < 0.001), and lower-order aberrations (LOA) (ρ = 0.54; P < 0.001). A strongly positive correlation was noted between CCT and corneal hysteresis (CH) (ρ = 0.63; P < 0.001), CCT and CRF (ρ = 0.56; P < 0.001), CH and corneal resistance factor (CRF) (ρ = 0.83; P < 0.001), and Goldmann equivalent intraocular pressure (IOPg) and corneal compensated intraocular pressure (IOPcc) (ρ = 0.78; P < 0.001). An increase in higher-order aberrations (HOAs) as well as lower-order aberrations (LOAs) was noted with increasing sim-K [HOA (r = 0.73, P = 0.001); LOA (r = 0.601, P = 0.014)] and increasing CRF [HOA (r = 0.5, P = 0.006); LOA (r = 0.732, P = 0.001)] in group 2. The amount of refractive error, axial length, central corneal thickness, and sim-K were significantly different in the two groups. Increasing degree of myopia is associated with an increase in axial length, total aberrations, and lower-order aberrations. Corneal biomechanical parameters have a strong correlation with each other. Lower corneal biomechanics are noted in high myopes as they have weaker and thinner corneas. Corneal biomechanics and ocular aberrations do not differ significantly between cases suitable for femto-LASIK and cases unsuitable for femto-LASIK.

Patient-specific air puff-induced loading using machine learning.

Introduction: The air puff test is a contactless tonometry test used to measure the intraocular pressure and the cornea's biomechanical properties. Limitations that most challenge the accuracy of the estimation of the corneal material and the intraocular pressure are the strong intercorrelation between the intraocular pressure and the corneal parameters, either the material properties that can change from one person to another because of age or the geometry parameters like central corneal thickness. This influence produces inaccuracies in the corneal deformation parameters while extracting the IOP parametric equation, which can be reduced through the consideration of the patient-specific air puff pressure distribution taking into account the changes in corneal parameters. This air puff pressure loading distribution can be determined precisely from the fluid-structure interaction (FSI) coupling between the air puff and the eye model. However, the computational fluid dynamics simulation of the air puff in the coupling algorithm is a time-consuming model that is impractical to use in clinical practice and large parametric studies. Methods: By using a supervised machine learning algorithm, we predict the time-dependent air puff pressure distribution for different corneal parameters via a parametric study of the corneal deformations and the gradient boosting algorithm. Results: The results confirmed that the algorithm gives the time-dependent air puff pressure distribution with an MAE of 0.0258, an RMSE of 0.0673, and an execution time of 93s, which is then applied to the finite element model of the eye generating the corresponding corneal deformations taking into account the FSI influence. Using corneal deformations, the response parameters can be extracted and used to produce more accurate algorithms of the intraocular pressure and corneal material stress-strain index (SSI). Discussion: Estimating the distribution of air pressure on the cornea is essential to increase the accuracy of intraocular pressure (IOP) measurements, which serve as valuable indicator of corneal disease. We find that the air puff pressure loading is largely influenced by complex changes in corneal parameters unique to each patient case. With our innovative algorithm, we can preserve the same accuracy developed by the CFD-based FSI model, while reducing the computational time from approximately 101000s (28h) to 720s (12min), which is about 99.2% reduction in time. This huge improvement in computational cost will lead to significant improvement in the parametric equations for IOP and the Stress-Strain Index (SSI).

Сell model for experimental research in keratoconus pathogenesis

INTRODUCTION: Certain mineral elements dismetabolism is known to play an important part in the pathogenesis of the corneal degenerative and dystrophic diseases, as metalloenzymes participate in connective tissues metabolism affecting their properties. Thus, in keratoconus corneal tissue is depleted in iron, copper and zinc, what could be the underlying cause of cornea biomechanical properties impairment. Keratoconus modeling is complicated and practically not reproducible in animals, therefore cell models elaboration is very much in demand. Definite mineral elements content must be reduced in order to simulate pathological changes specific for keratoconic corneas.&#x0D; PURPOSE: The aim of this study was an elaboration of a cell model suitable for keratoconus pathogenesis research. This goal achieving involved solving the following tasks: 1) to develop a tissue-engineered system that mimic healthy corneal stroma; 2) to develop a technique for selective depletion of the nutrient medium by mineral elements involved in keratoconus pathogenesis; 3) to assess the possibility of the designed tissue-engineered system growth in the depleted nutrient medium.&#x0D; MATERIAL AND METHODS: The study was carried out with the primary culture of human keratocytes, which were used to build tissue-engineered systems of three types: on silicone, on a membrane, without a carrier. Cell cultures morphology was evaluated by light and electron microscopy. The nutrient media were zinc depleted via decationization of fetal bovine serum using two types of ion-exchange resins; mineral elements concentrations were evaluated by means of inductively coupled plasma mass spectrometry.&#x0D; RESULTS: The tissue system engineered without a carrier in form of cell sheet was chosen as the most convenient model. The decationization of the serum by means of Chelex 100 resin was shown to be a successful method for tenfold zinc concentration reduction in the nutrient medium. Keratocytes cultivation in the form of cell sheet on a zinc-depleted medium was successfully approved.&#x0D; CONCLUSION: Elaborated tissue-engineered system could be considered as a model of the corneal stroma under specific for keratoconus conditions of zinc depletion.

Имплантация кольца myoring для коррекции миопии высокой степени у пациента с нистагмом

Purpose. Analysis of clinical and functional results and quality of life after implantation of MyoRing in a patient with high myopia and nystagmus. Material and methods. Patient M., 29 years old, was implanted a myoRing using a femtosecond laser to correct high myopia. Uncorrected visual acuity (UCVA) on the operated eye before surgery was 0.03, corrected visual acuity (CVA) 0.5, spherical refractive component –10.25 D, cylindrical component –4.00 D. Keratometry OD 41.5 × 45.25 × 174°. Corneal hysteresis (CH) was 7.3 mm Hg and corneal resistance factor (CRF) was 6.6 mm Hg. The thickness of cornea in center was 513µm. The data on higher-order aberrations were in line with the generally accepted norm. Observation period – 2 years. Results. 2 years after the operation, visual acuity without correction at OD 0.6, with correction – 0.7. The spherical component was 0.75 D, cylindrical component was –1.25 D. Keratometry OD 30.5 × 31.5 × 171°. Corneal hysteresis was 8.5 mm Hg, corneal resistance factor was 6.9 mm Hg. The thickness of cornea in center was 518µm. All higher-order aberrations decreased, except for vertical Coma (Z3/–1), which increased by 0.35µm, horizontal Coma (Z3/+1), which increased by –0.06µm. According to the results of the questionnaire, the patient noted a significant improvement in visual functions in the period from 3 to 12 months. Problems with the quality of vision in conditions of reduced contrast and complaints of halos were present only in the first months (3–4 months) after surgery. Conclusion. The MyoRing implantation method for correction of high myopia in a patient with nystagmus is highly effective, safe, providing a refractive effect on the spherical and cylindrical component of refraction and enhancing the biomechanical properties of cornea. Keywords: myopia and complex myopic astigmatism, thin cornea, MyoRing ring, femtosecond laser.

Corneal Biomechanical Changes after Corneal Cross-Linking in Patients with Keratoconus.

To evaluate the changes in biomechanical properties of the cornea using the Corvis ST device after the treatment of keratoconus patients with ultraviolet-A/riboflavin corneal cross-linking (CXL). Thirty-seven eyes from 37 consecutive patients with progressive keratoconus were included in this prospective observational case series. Corneal biomechanical parameters including the length of the applanated cornea (L1 and L2), corneal movement velocity during applanation (V1 and V2) at the moment of the first and second applanation, deformation amplitude (DA), distance between bending points of the cornea points of the cornea (PD), and concave radius of curvature (R) at the point of the highest concavity were recorded using the Corvis ST at baseline, 3 months, and 1 year after CXL. The mean age of the patients was 23.27 years (range, 19-31 years). Among CorVis ST corneal biomechanical parameters, L1, DA, PD, and R at the point of the highest concavity did not change significantly. The length of the applanated cornea at the moment of second applanation (L2) showed a significant change 3 months after CXL, but no significant difference was found between the 3-month and 1-year values of this parameter. Corneal movement velocity during applanation (V1 and V2) did not change 3 months after doing CXL, but the changes in these parameters were significant 1 year after CXL. Although the CorVis ST device may detect changes in some biomechanical properties of cornea after the treatment of keratoconus patients with CXL, many parameters remain unchanged, and this device cannot readily be used to find the effects of CXL.

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.

Effects of intraocular pressure and aspheric transition zone ablation profile on corneal biomechanics after conventional refractive surgery

Our Purpose is to study the effects of intraocular pressure (IOP) and aspheric transition zone (ATZ) on corneal biomechanics after pure hyperopia correction by using the finite element analysis (FEA). The values of IOP were changed, and 1-5# aspheric transition zones were designed in 1-5D hyperopia correction model. Simulate and calculate the wavefront aberration, stress and vertex displacement of cornea. The results show that with the increase of IOP and diopter, defocus increases positively and sphere increases negatively. Diopter and IOP have slight influence on coma. At 22mmHg, the maximum value of defocus was 1.367mm at 5D-1#, and the maximum value of sphere was -0.32mm at 5D-5#. IOP and diopter have great influence on the stress in the marginal region of the anterior corneal surface, and 1D-1 # has the maximum value at 22mmHg. With the increase of IOP and diopter, the vertex displacement of posterior corneal surface increased. The ATZ ablation profile has little effects on the wavefront aberration and displacement. We can draw a conclusion that refractive surgery destroys the physiological structure of cornea and has a great influence on the biomechanical properties of cornea. IOP plays an important role in maintaining the physiological structure of cornea.

Age distribution and associated factors of cornea biomechanical parameter stress-strain index in Chinese healthy population

BackgroundTo investigate the new cornea biomechanical parameter stress-strain index (SSI) in Chinese healthy people and the factors associated with SSI.MethodsA total of 175 eyes from 175 participants were included in this study. Axial length was measured with the Lenstar LS-900. Pentacam measured curvature of the cornea and anterior chamber volume (ACV). Cornea biomechanical properties assessments were performed by corneal visualization Scheimpflug technology (Corvis ST). Student’s t-test, one-way ANOVA, partial least square linear regression (PLSLR) and linear mixed effects (LME) model were used in the statistical analysis.ResultsThe mean (±SD) SSI was 1.14 ± 0.22 (range, 0.66–1.78) in all subjects and affected by age significantly after age of 35 (P < 0.05). In LME models, SSI was significantly associated with age (β = 0.526, P < 0.001), axial length (AL) (β = − 0.541, P < 0.001), intraocular pressure (IOP) (β = 0.326, P < 0.001) and steepest radius of anterior corneal curvature (RsF) (β = 0.229, P < 0.001) but not with ACV, biomechanical corrected intraocular pressure (bIOP), flattest radius of anterior corneal curvature (RfF) or central corneal thickness (CCT) (P > 0.05 for each).ConclusionsSSI increased with age after the age of 35. In addition to age, SSI was positively correlated with RsF and IOP, while negatively correlated with AL.

A Review on Different Tonometers for Intraocular Pressure Measurement After Photorefractive Keratectomy or Small Incision Lenticule Extraction.

This paper aims to review different tonometers for intraocular pressure measurement in eyes after photorefractive keratectomy or small incision lenticule extraction. An Entrez Pubmed search was performed on July 19th, 2020. There were 32 eligible articles investigated in the use of tonometers or biomechanical properties of cornea in post photorefractive keratectomy eyes and 11 articles investigated in post small incision lenticule extraction eyes. Corvis ST and dynamic contour tonometry were found to be accurate for intraocular pressure measurement in post photorefractive keratectomy eyes. For post small incision lenticule extraction eyes, Corvis ST was found to be more accurate than other tonometers. Other tonometers such as Goldmann applanation tonometer, noncontact tonometry, Tonopen, ocular response analyzer can also be used in post photorefractive keratectomy or small incision lenticule extraction eyes in clinical practice; however, it is important for ophthalmologists to take note of the chances of discrepancies.

Keratoconus Prevalence in Families: A French Study

To assess the keratoconus (KC) prevalence among first-degree relatives of affected subjects. We performed large epidemiological, prospective, and observational cohort, which took place in the French KC National Reference Centre, in Bordeaux. We studied data from 94 unrelated patients with KC and at least 2 first-degree relatives assessable. Each subject had a thorough ocular examination including visual acuity, refraction, slitlamp biomicroscopy, indirect ophthalmoscopy, keratometry, corneal topography and tomographer (Topographic Modeling System; Tomey Corporation, Japan; and Galilei G4, Ziemer, Switzerland, respectively), pachymetry, and biomechanical properties of cornea (Ocular Response Analyzer; Reichert, Depew, NY). Additional data were collected about atopy, smoking, and eye-rubbing habits. KC prevalence, multivariate analysis, and familial correlation analysis have been studied. Of the 221 relatives, 9.05% (n = 20) had clinically manifest KC, and 15.4% (n = 31) had subclinical KC. The prevalence of KC was estimated equal to 0.14 [95% confidence interval (CI), 0.07-0.22] among parents, 0.03 (95% CI, 0-0.10) among offspring, and 0.10 (95% CI, 0.04-0.20) among siblings. In multivariable analysis, age and eye-rubbing were associated with an increased risk of KC. The familial correlation of KC among probands' first-degree relatives was estimated at 0.55 among parents, 0.29 among offspring, and 0.49 among siblings. Being a first-degree relative of a subject with KC remains a risk factor for developing KC. This should be taken into account especially when screening for KC in pediatric patients, for refractive surgery candidates, and for patients being evaluated for cataract surgery.

Effects of the LASIK flap thickness on corneal biomechanical behavior: a finite element analysis

BackgroundIt is well known that the biomechanical properties change after LASIK refractive surgery. One reason is the impact of flap creation on the residual stroma. The results have revealed that the change is closely related with the flap thickness in several studies. However, the quantitative relationships between the distributions of displacement and stress on the corneal surface and flap thickness have not been studied. The aim of the study was to quantify evaluate the biomechanical change caused by the LASIK flap.MethodsBy building a finite element model of the cornea, the displacement, the stress and the strain on the corneal surface were analyzed.ResultsThe results showed that the corneal flap could obviously cause the deformation of the anterior corneal surface. For example, the displacement of the corneal vertex achieved 15 μm more than that without corneal flap, when the thickness of corneal flap was 120 μm thick. This displacement was enough to cause the change of aberrations in the human eyes. In the central part of the cornea, the stress on the anterior corneal surface increased with flap thickness. But the change in the stress on the posterior corneal surface was significantly less than that on the anterior surface. In addition, the stress in the central part of the anterior corneal surface increased significantly as the intra-ocular pressure (IOP) increase. Furthermore the increase of IOP had a clearly less effect on stress distribution at the edge of the cornea. Distributions of strain on the corneal surface were similar to those of stress.ConclusionsThe changes in the biomechanical properties of cornea after refractive surgery should not be ignored.

Comparison of intraocular pressure measured by ocular response analyzer and Goldmann applanation tonometer after corneal refractive surgery: a systematic review and meta-analysis

BackgroundAccurate measurement of intraocular pressure (IOP) after corneal refractive surgery is of great significance to clinic, and comparisons among various IOP measuring instruments are not rare, but there is a lack of unified analysis. Although Goldmann Applanation Tonometer (GAT) is currently the internationally recognized gold standard for IOP measurement, its results are severely affected by central corneal thickness (CCT). Ocular Response Analyzer (ORA) takes certain biomechanical properties of cornea into account and is supposed to be less dependent of CCT. In this study, we conducted the meta-analysis to systematically assess the differences and similarities of IOP values measured by ORA and GAT in patients after corneal refractive surgery from the perspective of evidence-based medicine.MethodsThe authors searched electronic databases (MEDLINE, EMBASE, Web of science, Cochrane library and Chinese electronic databases of CNKI and Wanfang) from Jan. 2005 to Jan. 2019, studies describing IOP comparisons measured by GAT and ORA after corneal refractive surgery were included. Quality assessment, subgroup analysis, meta-regression analysis and publication bias analysis were applied in succession.ResultsAmong the 273 literatures initially retrieved, 8 literatures (13 groups of data) with a total of 724 eyes were included in the meta-analysis, and all of which were English literatures. In the pooled analysis, the weighted mean difference (WMD) between IOPcc and IOPGAT was 2.67 mmHg (95% CI: 2.20~3.14 mmHg, p < 0.0001), the WMD between IOPg and IOPGAT was − 0.27 mmHg (95% CI: − 0.70~0.16 mmHg, p = 0.2174). In the subgroup analysis of postoperative IOPcc and IOPGAT, the heterogeneity among the data on surgical procedure was zero, while the heterogeneity of other subgroups was still more than 50%. The comparison of the mean difference of pre- and post-operative IOP (∆IOP) was: mean-∆IOPg > mean-∆IOPGAT > mean-∆IOPcc.ConclusionsIOPcc, which is less dependent on CCT, may be more close to the true IOP after corneal refractive surgery compared with IOPg and IOPGAT, and the recovery of IOPcc after corneal surface refractive surgery may be more stable than that after lamellar refractive surgery.

Biomechanics in DALK: Big bubble vs manual lamellar dissection.

The present study's aim was to compare the biomechanical properties of corneal tissue in patients who underwent deep anterior lamellar keratoplasty (DALK) surgery, with successful big bubble formation and manual lamellar dissection, during failed big bubble formation. This retrospective comparative study included 60 eyes from 60 keratoconus patients who previously underwent DALK surgery. These patients were categorized as big bubble (+) or big bubble (-) based on the success or failure of big bubble formation during the surgery. The big bubble (+) group included 42 eyes, while the big bubble (-) group had 18 eyes. Moreover, the patients were regrouped as 0.25 mm and 0.50 mm to evaluate the effects of the disparity between donor and trephine punches on the biomechanical properties of the cornea. These biomechanical properties, characterized by corneal hysteresis and the corneal resistance factor, were measured using the Ocular Response Analyzer 12 months after the surgery. There was no statistically significant difference between the big bubble (+) and big bubble (-) groups in the biomechanical properties of the cornea (corneal hysteresis: 10.06, 10.25; p=0.716/corneal resistance factor: 10.15, 10.07; p=0.805, respectively). In addition, pachymetry results were not statistically different between the two groups. Multivariate regression analysis revealed that corneal hysteresis and corneal resistance factor were positively associated with central corneal thickness (p<0.001/r2=0.506; p<0.001/r2=0.561, respectively). However, the study did not demonstrate a relationship between any of the punch sizes and corneal hysteresis or between the punch sizes (p=0.673) and the corneal resistance factor (p=0.643). The corneal hysteresis and corneal resistance factor values were similar in big bubble and manual lamellar dissection after DALK. Thus, manual lamellar dissection was not a disadvantage considering the cornea's biomechanical properties.

Biomechanics in DALK: Big bubble vs Manual lamellar dissection.

PURPOSE The present study's aim was to compare the biomechanical properties of corneal tissue in patients who underwent deep anterior lamellar keratoplasty (DALK) surgery, with successful big bubble formation and manual lamellar dissection, during failed big bubble formation. METHODS This retrospective comparative study included 60 eyes from 60 keratoconus patients who previously underwent DALK surgery. These patients were categorized as big bubble (+) or big bubble (-) based on the success or failure of big bubble formation during the surgery. The big bubble (+) group included 42 eyes, while the big bubble (-) group had 18 eyes. Moreover, the patients were regrouped as 0.25 mm and 0.50 mm to evaluate the effects of the disparity between donor and trephine punches on the biomechanical properties of the cornea. These biomechanical properties, characterized by corneal hysteresis and the corneal resistance factor, were measured using the Ocular Response Analyzer 12 months after the surgery. RESULTS There was no statistically significant difference between the big bubble (+) and big bubble (-) groups in the biomechanical properties of the cornea (corneal hysteresis: 10.06, 10.25; p=0.716/corneal resistance factor: 10.15, 10.07; p=0.805, respectively). In addition, pachymetry results were not statistically different between the two groups. Multivariate regression analysis revealed that corneal hysteresis and corneal resistance factor were positively associated with central corneal thickness (p<0.001/r2=0.506; p<0.001/r2=0.561, respectively). However, the study did not demonstrate a relationship between any of the punch sizes and corneal hysteresis or between the punch sizes (p=0.673) and the corneal resistance factor (p=0.643). CONCLUSIONS The corneal hysteresis and corneal resistance factor values were similar in big bubble and manual lamellar dissection after DALK. Thus, manual lamellar dissection was not a disadvantage considering the cornea's biomechanical properties.

Glaucoma vs. Biomechanical Properties of Cornea

Abstract The paper deals with the influence of various biomechanical parameters on the intraocular pressure (IOP). The IOP is a very important factor in more accurate diagnosis and better management of glaucoma. To get a more realistic value of IOP the measurement methodology must reflect the patient’s individual biometric and biomechanical parameters, for example age, sex, race, biomechanical properties of cornea, etc. Many measuring methods are based on the applanation of cornea during its loading by the measuring device - tonometers. These tonometers apply the Imbert – Fick law [1], which determines their size. This work also addresses the current approaches to creating more realistic IOP data.

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

Impact of diurnal IOP variations on the dynamic corneal hysteresis measured with air-puff swept-source OCT

The deformation amplitudes measured with air-puff OCT are sensitive to both (intraocular pressure) IOP and biomechanical properties of the cornea. Analysis of the amplitudes of corneal deformation is challenging due to interrelation of IOP and corneal biomechanics. In this study, we used natural diurnal IOP fluctuations to investigate corneal deformations in a number of subjects whose eyes were measured multiple times during a day. The results of analysis, based on corneal hysteresis, revealed a corneal hysteresis parameter, which remains constant during a day for each individual eye. We hypothesize that above-mentioned metric might correlate with biomechanical properties of the cornea without influence of IOP. Full Text: PDF ReferencesMeek KM, Tuft SJ, Huang Y, Gill PS, Hayes S, Newton RH, Bron AJ, "Changes in Collagen Orientation and Distribution in Keratoconus Corneas", Invest Ophthalmol Vis Sci, 2005. 46(6): p. 1948-56. CrossRef Zimmermann DR, Fisher RW, Winterhalter KH, Witmer R, Vaughan L, "Comparative studies of collagens in normal and keratoconus corneas", Exp Eye Res, 1988. 46(3): p. 431-42. CrossRef Andreassen TT, Simonsen AH, and Oxlund H, "Biomechanical properties of keratoconus and normal corneas", Experimental Eye Research, 1980. 31(4): p. 435-441. CrossRef Heijl A, Leske MC, Bengtsson B, Hyman L, Bengtsson B, Hussein M, "Reduction of Intraocular Pressure and Glaucoma Progression", Arch Ophthalmol, 2002. 120(10): p. 1268-79. CrossRef Chauhan BC and Drance SM, "The influence of intraocular pressure on visual field damage in patients with normal-tension and high-tension glaucoma", Investigative Ophthalmology &amp; Visual Science, 1990. 31(11): p. 2367-2372. DirectLink Gelaw Y, "The impact of central corneal thickness on intraocular pressure among Ethiopian glaucoma patients: a cross-sectional study", BMC Ophthalmology, 2012. 12(1): p. 58. CrossRef Doughty MJ and Zaman ML, "Human Corneal Thickness and Its Impact on Intraocular Pressure Measures: A Review and Meta-analysis Approach", Surv Ophthalmol, 2000. 44(5): p. 367-408. CrossRef Liu J, and Roberts CJ, "Influence of corneal biomechanical properties on intraocular pressure measurement: Quantitative analysis", J Cataract Refract Surg, 2005. 31(1): p. 146-55. CrossRef Ehlers N, Hansen FK, and Aasved H, "Biometric Correlations of Corneal Thickness", Acta Ophthalmol (Copenh), 1975. 53(4): p. 652-9. CrossRef Harada Y, Hirose N, Tawara A, "The Influence of Central Corneal Thickness and Corneal Curvature Radius on The Intraocular Pressure as Measured By Different Tonometers: Noncontact and Goldmann Applanation Tonometers", J Glaucoma, 2008. 17(8): p. 619-25. CrossRef Alonso-Caneiro D, Karnowski K, Kaluzny BJ, Kowalczyk A, Wojtkowski M, "Assessment of corneal dynamics with high-speed swept source Optical Coherence Tomography combined with an air puff system", Optics Express, 2011. 19(15): p. 14188-14199. CrossRef Dorronsoro C, Pascual D, Perez-Merino P, Kling S and Marcos S, "Dynamic OCT measurement of corneal deformation by an air puff in normal and cross-linked corneas", Biomedical Optics Express, 2012. 3(3): p. 473-487. CrossRef Karnowski K, Kaluzny BJ, Szkulmowski M, Gora M, Wojtkowski M, "Corneal topography with high-speed swept source OCT in clinical examination", Biomedical Optics Express, 2011. 2(9): p. 2709-2720. CrossRef A. N. S. Institute, "American National Standard for Safe use of Lasers," (American National Standards Institute, Orlando, FL, 2000) DirectLink David R, Zangwill L, Briscoe D, Dagan M, Yagev R, Yassur Y, "Diurnal intraocular pressure variations: an analysis of 690 diurnal curves", Br J Ophthamlom, 1992, 76(5): p. 280-282 CrossRef Maczynska E, Karnowski K, Szulzycki K, Malinowska M, Dolezyczek H, Cichanski A, Wojtkowski M, Kaluzny BJ, Grulkowski I, Journal of Biophotonics (to be published).

Corneal hysteresis values in normal Egyptian population

IntroductionKnowledge of the cornea's biomechanical properties is important for a wide variety of applications within ophthalmology. Corneal biomechanical variations are known to affect the accuracy of intraocular pressure measurements and may be used to identify early corneal disease and may assist in predicting refractive outcomes following corneal refractive surgery. It has also been suggested that corneal biomechanical properties may reflect globe biomechanics, and thus give an indication of the susceptibility of developing glaucomatous damage.Aim of the workThe purpose of this study was to explore the relationships between Ocular Response Analyzer-generated corneal biomechanical characteristics and age in a sample of Egyptian population.Materials and methodsCorneal hysteresis (CH) and corneal resistance factor (CRF) were measured in 195 Egyptians of different age groups.ResultsThe mean CH value was 10.25 ± 0.12 mmHg (range, 6.5–14.4), and the mean CRF was 10.25 ± 0.15 mmHg (range, 4.9–14.2). The CH value was lower in older eyes, and the difference between the youngest age group (19–40 years) and the oldest age group (40–71 years) was statistically significant (t-test = 0.01). The mean CH in the youngest age group was 11.1 ± 0.14, and that in the oldest age group was 9.8 ± 0.21. The mean CRF in the youngest age group was 10.9 ± 0.18, and that in the oldest age group was 10.1 ± 0.19.ConclusionThis study of corneal biomechanics in normal Egyptian eyes describes the interactions between age and Ocular Response Analyzer metrics. The study reveals that age is significantly associated with CH and CRF. This may help in diagnosis and treatment.

Changes of intraocular pressure and cornea biomechanical properties after cataract phacoemulsification

PurposeCataract phacoemulsification is accompanied by changes in intraocular pressure and biomechanical parameters of the eye. Corneal hysteresis characterizies the visco‐elastic properties of the cornea. Our purpose was to analyse the changes of intraocular pressure and corneal hysteresis during 3 months after phaco.MethodsThe study included 80 eyes of 80 patients who underwent uncomplicated phacoemulsification throught clear corneal incision with IOL implantation. Preoperative examination included standard techniques and measurement of corneal hysteresis, central corneal thickness (CCT), corneal compensated intraocular pressure (IOP) and Goldman IOP by Ocular Response Analyzer, repeated before surgery, 1 day, 2 weeks, 1 and 3 months after surgery.ResultsCorneal hysteresis decreased on the first day after cataract phacoemulsification from 9.90 ±0.24 mm Hg to 8.88 ± 0.26 mm Hg and after 2 weeks to 7.91 ± 1.13 mm Hg (p &lt; 0.05). Then corneal hysteresis increased and after 3 months after surgery returned to preoperative values (9.78 ± 0.29 mm Hg). Intraocular pressure raised after surgery by 3 mm Hg, decreased to preoperative values by 1 month after phaco, and continued to decrease by 1.5–2 mm Hg up to 3 months after phacoemulsification. Central corneal thickness increased after phaco from 539 ± 21 mcm to 559 ± 33 mcm the first day after surgery and 659 ± 28 mcm in 2 weeks. It decreased to 552 ± 26 mcm in 1 month and returned to preoperative values in 3 months.ConclusionsCorneal hysteresis decreased in the early postoperative period after phaco, dropping to a minimum 2 weeks after surgery. Then corneal hysteresis gradually increased, reaching preoperative values at 3 months after surgery. Changes of intraocular pressure are opposite to changes of corneal hysteresis. CCT increased after phaco, reached maximum in 2 weeks and returned to preoperative values in 3 months.