Corneal Resistance Factor Values Research Articles

The Effect of Diabetic Retinopathy and Blood Glucose Regulation on Corneal Biomechanical Parameters

Purpose The aim of this study was to evaluate the effects of different stages of diabetic retinopathy (DR) and metabolic control of blood glucose levels on corneal biomechanical parameters. Methods Diabetic patients were categorized into three groups: no DR group, nonproliferative DR (NPDR) group, and proliferative DR (PDR) group. Of the 141 eyes examined, 40 belonged to the control group, 34 to no DR group, 34 to NPDR group, and 33 to PDR group. Using an Ocular Response Analyzer to measure corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated IOP (IOPcc). IOP was assessed using a Tono-Pen, while central corneal thickness (CCT) was determined using an ultrasonic pachymeter. HbA1c levels were also recorded. We conducted comparisons among these groups across biomechanical parameters and IOP (tonopen), and CCT, while also investigating the impact of HbA1c levels on these parameters. Results Among any groups show a statistically significant difference in CCT, IOP (tonopen), CH, CRF, IOPg, and IOPcc. In diabetic patients, CRF, CTT, and IOPg values were significantly higher in those with HbA1c levels ≥ 7 mg/dl than in those with HbA1c levels < 7 mg/dl (p = 0.009, p = 0.013, p = 0.038), respectively, while there was no statistically significant difference in IOPcc, CH, and IOP (tonopen). Linear regression analysis showed that CH was positively associated with CCT (p < 0.001) and negatively associated with IOPcc (p < 0.001), while CRF was positively associated with CCT (p < 0.001), HbA1c (p < 0.05), and negatively associated with diagnosis of DR (p < 0.05). Conclusion This study underscores the influence of metabolic control, as reflected by HbA1c levels, on corneal biomechanical parameters in diabetic patients, emphasizing the importance of monitoring and managing glycemic control in this population.

The Effect of Childhood Obesity on Intraocular Pressure, Corneal Biomechanics, Retinal Nerve Fiber Layer, and Central Macular Thickness.

Elevated corneal hysteresis (CH) and resistance factor (CRF) in obese and overweight children imply weight's effect on corneal biomechanics. Increased Goldmann-correlated intraocular pressure (IOPg) in obese children indicates glaucoma risk, emphasizing screening for IOP and retinal changes. To evaluate the effect of obesity on corneal biomechanics, retinal nerve fiber layer (RNFL), and central macular thickness (CMT) in children. In this prospective, cross-sectional, comparative study, 146 eyes of normal-weight, over-weight, and obese children aged between 6 to 17 years were evaluated. The IOPg, corneal compensated IOP (IOPcc), CH, CRF, and the average retinal nerve fiber layer (RNFL), average cup-to-disk ratio (c/d), and central macular thickness (CMT) were measured by Ocular Response Analyser and Spectral-Domain Optical Coherence Tomography (SD-OCT), respectively. There was no statistically significant difference regarding age, gender, IOPcc, average RNFL thickness, c/d ratio, and CMT among the groups ( P ≥0.05). The IOPg was significantly higher in obese children compared with normal-weight children, while CH and CRF values were significantly higher in both obese and over-weight children compared with healthy ones ( P <0.05). There was a positive correlation between BMI percentile and IOPg, CH, and CRF values. In our study, higher IOPg, corneal hysteresis, and corneal resistance factor values suggest that obese children could be potential candidates for glaucoma. Therefore, it would be appropriate to screen them for IOP and retinal alterations. Further investigations with larger sample size and longer follow-up are needed to understand the risk of glaucoma in obese children.

Analysis of potential impact factors of corneal biomechanics in myopia

PurposeTo investigate potential impact factors associated with corneal biomechanical properties in Chinese myopia and further to investigate quantifying corneal biomechanics in clinical work.MethodsThree hundred fifty-five eyes from 181 healthy myopic subjects with a mean age of 25.1 ± 9.4 were recruited in this study. Each patient carried out a comprehensive ophthalmic examination, including corneal hysteresis(CH), corneal resistance factor(CRF), central corneal thickness(CCT), axial length(AL), intraocular pressure(IOP), spherical equivalence(SE) and corneal curvature (K). CH and CRF were measured with the ocular response analyzer(ORA). To analyze the corneal biomechanical characteristics of myopia patients and their association with other parameters.ResultThe multiple linear regression analysis showed that CH was positively associated with CCT, and corneal curvature (all with P < 0.05) and negatively associated with SE and AL)(all with P < 0.05); CRF was positively correlated with CCT, corneal curvature and IOP(all with P < 0.05), but negatively correlated with SE and AL(all with P < 0.05). The CH and CRF values in children group were both higher than adults group (≥ 18 years old) (P < 0.05), but it attributed to that the CCT of children was thicker than adults. Excluding factor of CCT, there was no significant difference in CH and CRF between children group and adult group. Excluding factor of CCT, there was no significant difference in CH and CRF among different stage of age (age 18–48).ConclusionThe CCT played the most important role of affecting the CH and CRF. The SE, corneal curvature, AL and IOP had a certain influence on corneal biomechanics. Whether the CH and CRF values of individual patient are normal in clinical work, it should refer to the CH and CRF values corresponding CCT sectional range and SE.

Corneal Biomechanical Parameters after 60-Year-Old.

To determine the distribution of corneal biomechanical parameters in an elderly population. This cross-sectional study was conducted in subjects above 60 years living in Tehran. The participants were selected using multi-stage cluster sampling. Corneal biomechanical parameters were measured in a randomly selected subsample of this population using the Reichert Ocular Response Analyzer (Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA). Of 470 subjects, the data of 420 participants aged over 60 years were analyzed (mean age: 69.3 ± 6.5 years and range: 61-88 years), 363 (86.4%) of whom were male. The mean and standard deviation of corneal hysteresis (CH) and corneal resistance factor (CRF) were 8.37 ± 1.55 mmHg (95% confidence interval [CI]: 8.02-8.72) and 9.06 ± 1.70 mmHg (95% CI: 8.69-9.44), respectively. The mean CH was 8.27 ± 1.54 mmHg in men and 9.25 ± 1.28 mmHg in women, and the mean CRF was 9.00 ± 1.71 mmHg in men and 9.63 ± 1.37 mmHg in women. According to the results of multiple linear logistic regression analysis, CH had a significant association with younger age (β = -0.05, P = 0.032), female sex (β = 1.83, P < 0.001), reduced maximum keratometry (β = -0.22, P = 0.06), and increased anterior chamber volume (β = 0.01, P = 0.007). CRF had a significant correlation with a younger age (β = -0.06, P = 0.02), female sex (β = 1.01, P = 0.05), central corneal thickness (β = 0.02, P < 0.001), and reduced maximum keratometry (β = -0.39, P = 0.010). The mean CH and CRF values were low in this sample of the Iranian population aged over 60 years indicating the weaker elasticity of the corneal connective tissue.

The Effect of Thyroid Eye Disease on Corneal Biomechanical Properties.

Objectives:The aim of this study was to identify corneal biomechanical parameters measured by ORA in patients with TED compared to the healthy group. The NOSPECS classification of patients is used to assess the relation between biomechanical changes and disease severity.Methods:We included 22 TED patients, diagnosed with TED for more than five years, and 43 healthy participants. The NOSPECS classification was assessed as mild (grade 1-3) and severe (grade 4-6) disease. For each group, corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT), Goldmann-correlated intraocular pressure (IOPg) and corneal compensated intraocular pressure (IOPcc) parameters were measured by ORA.Results:The mean age was 38.8±11.6 years for the TED patients and 42.9±15.58 years for the control group. For TED patients and healthy volunteers, the mean levels of CRF, CH, and CCT were measured as follows: 10.43±2.04 vs 10.28±1.91mmHg, p=0.67; 10.18±1.81 vs 10.21±1.68 mmHg, p=0.90; 550.31±35.73 vs 545.23±37.91 µm, p=0.47, respectively. These values were not significant between groups, but they were significantly higher in females compared to males in TED patients [CRF;10.68 (IQR: 9.49-12.14) vs 8.96 (IQR: 8.04-9.92) mmHg, p=0.002, CH; 10.43 (IQR: 9.48-11.25) vs 8.58 (IQR: 7.90-9.95) mmHg, p=0.003 and CCT; 554.25 (IQR: 536.05-579.52) vs 527.40 (IQR: 492.25-545.90) μm, p=0.014]. CRF values were negatively correlated with NOSPECS score (r=-0.317, p=0.036) and significantly higher CRF was observed in mild patients compared to severe disease (11.43 (IQR: 10.14-12.87) vs 9.46 (IQR: 8.75-10.28) mmHg, p=0.008).Conclusion:We found a significant gender effect on corneal biomechanical parameters of TED patients. CRF, CH and CCT values were significantly higher in females compared to males with TED. The clinical severity score of TED showed negative correlation with CRF. CRF value might be a useful parameter in follow-up of TED patients in clinical practice.

Effect of upper eyelid blepharoplasty on corneal biomechanical, topographic and tomographic parameters 4\xa0weeks after surgery

PurposeTo investigate the effect of “skin-only” upper eyelid blepharoplasty on corneal biomechanics and central as well as peripheral topographic/tomographic parameters before and 4 weeks after surgery.MethodsIn a prospective study, the corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated before and after blepharoplasty. Corneal topographic (maximum simulated keratometry value, inferior-superior value, index of surface variance, index of vertical asymmetry, index of height asymmetry, index of height decentration) and tomographic parameters (corneal thickness, corneal astigmatism and mean 5-mm- and 7-mm-zone keratometry value) were measured by the Pentacam HR. Statistical analysis was performed using a linear mixed model considering correlated data of both eyes.ResultsThis study included 42 eyes of 35 patients (mean age: 64.5 years, range 52–82 years). Four weeks after surgery CH and CRF increased (9.4 ± 2.3 to 10.2 ± 2.2 mmHg and 9.7 ± 2.1 to 10.5 ± 2.2 mmHg) but did not reach statistical significance (P = 0.100 and P = 0.072). A significant increase in central maximum simulated keratometry value (Kmax) from 45.0 ± 2.3 to 45.4 ± 2.2 diopters (D) was observed (P = 0.004). Inferior-superior value (I-S) and index of surface variance (ISV) showed significant changes from 0.32 ± 0.98 to 0.10 ± 0.98 D (P = 0.02) and from 19.98 ± 9.84 to 22.93 ± 11.23 (P = 0.009), respectively. These alterations did not affect the subjective spherical equivalent (-0.09 ± 4.71 to -0.04 ± 4.51 D; P = 0.437) and the best-corrected distance visual acuity of patients (0.11 ± 0.14 to 0.15 ± 0.15 logMAR; P = 0.142). Age, gender and corneal thickness were not correlated with pre and postoperative differences of CH, CRF, corneal compensated IOP, Kmax, corneal astigmatism or I-S.ConclusionThe trend of increasing CH and CRF values might indicate a rise of corneal damping capacity. Despite statistically significant differences of Kmax, I-S and ISV, all other tomographical and topographical parameters did not change 4 weeks after surgery. The corneal steepening with a mean change of 0.4 diopters and the decrease of I-S with a mean of 0.22 diopters do not seem to have a clinically relevant effect for blepharoplasty patients in daily practice.

Corneal Biomechanical Parameters and Central Corneal Thickness in Glaucoma Patients, Glaucoma Suspects, and a Healthy Population.

Purpose: To evaluate and compare corneal hysteresis (CH), corneal resistance factor (CRF), and central corneal thickness (CCT), measurements were taken between a healthy population (controls), patients diagnosed with glaucoma (DG), and glaucoma suspect patients due to ocular hypertension (OHT), family history of glaucoma (FHG), or glaucoma-like optic discs (GLD). Additionally, Goldmann-correlated intraocular pressure (IOPg) and corneal-compensated IOP (IOPcc) were compared between the different groups of patients. Methods: In this prospective analytical-observational study, a total of 1065 patients (one eye of each) were recruited to undergo Ocular Response Analyzer (ORA) testing, ultrasound pachymetry, and clinical examination. Corneal biomechanical parameters (CH, CRF), CCT, IOPg, and IOPcc were measured in the control group (n = 574) and the other groups: DG (n = 147), FHG (n = 78), GLD (n = 90), and OHT (n = 176). We performed a variance analysis (ANOVA) for all the dependent variables according to the different diagnostic categories with multiple comparisons to identify the differences between the diagnostic categories, deeming p < 0.05 as statistically significant. Results: The mean CH in the DG group (9.69 mmHg) was significantly lower compared to controls (10.75 mmHg; mean difference 1.05, p < 0.001), FHG (10.70 mmHg; mean difference 1.00, p < 0.05), GLD (10.63 mmHg; mean difference 0.93, p < 0.05) and OHT (10.54 mmHg; mean difference 0.84, p < 0.05). No glaucoma suspects (FHG, GLD, OHT groups) presented significant differences between themselves and the control group (p = 1.00). No statistically significant differences were found in the mean CRF between DG (11.18 mmHg) and the control group (10.75 mmHg; mean difference 0.42, p = 0.40). The FHG and OHT groups showed significantly higher mean CRF values (12.32 and 12.41 mmHg, respectively) than the DG group (11.18 mmHg), with mean differences of 1.13 (p < 0.05) and 1.22 (p < 0.001), respectively. No statistically significant differences were found in CCT in the analysis between DG (562 μ) and the other groups (control = 556 μ, FHG = 576 μ, GLD = 569 μ, OHT = 570 μ). The means of IOPg and IOPcc values were higher in the DG patient and suspect groups than in the control group, with statistically significant differences in all groups (p < 0.001). Conclusion: This study presents corneal biomechanical values (CH, CRF), CCT, IOPg, and IOPcc for diagnosed glaucoma patients, three suspected glaucoma groups, and a healthy population, using the ORA. Mean CH values were markedly lower in the DG group (diagnosed with glaucoma damage) compared to the other groups. No significant difference was found in CCT between the DG and control groups. Unexpectedly, CRF showed higher values in all groups than in the control group, but the difference was only statistically significant in the suspect groups (FHG, GLD, and OHT), not in the DG group.

Biomechanical properties of the cornea following intravitreal ranibizumab injection.

To evaluate intraocular pressure (IOP) and corneal biomechanical properties changes after intravitreal ranibizumab injection (IVRI). One hundred twenty eyes of 120 patients who underwent IVRI between January and March 2018 in Adnan Menderes University Ophthalmology Clinic were included in study. Corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated intraocular pressure (IOPcc) were measured by Ocular Response Analyzer (ORA) by the same specialist preoperatively (preop), postoperative 1st (postop-1st), postoperative 3rd (postop-3rd), and postoperative 24th hours (postop-24th) after single-dose IVRI. Among the 120 wet age-related macular degeneration subjects, 58 (48.3%) were female and 62 (51.7%) were male. The mean age was 64.8 ± 10.4years. Postop-1st hour IOP cc and IOPg measurements were statistically different from all other measurements (p < 0.001). Postop-1st hour CH measurements were statistically different from preop and postop-3rd (p = 0.022 and p = 0.008, respectively). After intravitreal injection, IOP pressures significantly increased. But, CH and CRF values were significantly decreased. All these changes were temporary.

Changes in corneal biomechanics in patients with diabetes mellitus: a systematic review and meta-analysis.

To determine the changes in corneal biomechanical parameters in patients with diabetes mellitus (DM) in comparison with controls. Pertinent studies were identified by comprehensively search of PubMed, Embase, the Web of Science, the Cochrane Library, Scopus, the China National Knowledge Infrastructure and the Chinese biomedical disc (CBM) databases. Pooling analyses by random models using the D-L method were performed for corneal hysteresis (CH), the corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOPcc) and Goldmann-correlated intraocular pressure (IOPg). A total of 15 studies were included in the final analysis, involving 1506 eyes in the diabetic group and 2190 eyes in the control group. The diabetic group had significantly higher CH, CRF, IOPg and IOPcc values than the control group. The pooled mean differences were 1.34mmHg (95% confidence interval [CI] 0.60-2.08mmHg, P < 0.001) for IOPg and 0.85mmHg (95% CI 0.18-1.51mmHg, P = 0.013) for IOPcc, 0.38mmHg (95% CI 0.01-0.75, P = 0.047) for CH and 0.63mmHg (95% CI 0.27-0.98, P = 0.001) for the CRF. Sensitivity analyses using the leave-one-out method showed a consistent significant difference between the groups (all P < 0.001). Corneal biomechanics changed in the patients with DM. High CH, CRF, IOPcc and IOPg values may be associated factors for diabetes mellitus. Future studies are warranted to clarify the underlying mechanisms and explore the relationship between corneal biomechanics, glaucoma and diabetes mellitus. PROSPERO registration No CRD4201705465.

Characterization of In Vivo Biomechanical Properties in Macular Corneal Dystrophy

To measure and compare corneal biomechanics in patients with macular corneal dystrophy (MCD), versus those who underwent penetrating keratoplasty (PK) for MCD, versus normal subjects. Cross-sectional study. This study enrolled 24 eyes with MCD, 25 eyes that underwent PK in patients with preoperative diagnosis of MCD, and 28 normal eyes. The ocular response analyzer was used to measure corneal biomechanical properties, including corneal resistance factor (CRF) and corneal hysteresis (CH). Intraocular pressure (IOP) was measured using a Goldmann applanation tonometer, and central corneal thickness (CCT) was measured using an ultrasonic pachymeter. The CCT, CRF, CH, and IOP were compared among the study group. CCT was significantly lower in the MCD group (423 ± 47 mμ) than in the PK group (541 ± 31 mμ; P < .001) and the controls (540 ± 26 mμ; P < .001). Meanwhile, CCT was comparable between the PK and control groups (P= .98). CRF did not differ between the MCD (8.34 ± 2.12mm Hg) and the PK (8.66 ± 1.66mm Hg) groups (P= .89); however, both of these groups had lower CRF values than the control (9.76 ± 1.83mm Hg, P= .02) group. No significant differences were observed among the 3 groups in CH (P= .13). IOP values did not differ between the MCD (11.25 ± 1.69mm Hg) and PK (12.0 ± 2.67mm Hg) groups (P= .95); however, the IOP values for both of these groups differed significantly from those of the controls (13.46 ± 2.17mm Hg; P= .006). CRF was significantly reduced in MCD and did not return to normal values even after PK. The reduced corneal rigidity could result in underestimating IOP in patients with MCD and in those undergoing PK for this stromal dystrophy.

Relationships between corneal biomechanics and the structural and functional parameters of glaucoma damage.

To investigate the relationships between (i) thickness of the retinal nerve fiber layer, optic nerve head topography, and visual field parameters and (ii) corneal biomechanical properties in normal controls and patients with ocular hypertension and primary open-angle glaucoma. This observational, cross-sectional study included 68 eyes with primary open-angle glaucoma, 99 eyes with ocular hypertension and 133 control eyes. Corneal biomechanical properties, optic nerve head topographic features, retinal nerve fiber layer thickness, and visual fields were assessed in all cases. Corneal biomechanical properties, retinal nerve fiber layer thicknesses, and optic nerve head topographic features were compared among the groups. The associations between structural and functional measures of glaucomatous damage and corneal biomechanical factors were also evaluated. Significantly lower corneal hysteresis and corneal resistance factor values were observed in the primary open-angle glaucoma and ocular hypertension groups as compared with the control group, but there were no significant differences between the primary open-angle glaucoma and ocular hypertension groups. In the ocular hypertension group, no associations were observed between the corneal hysteresis and corneal resistance factor with values and the structural and functional parameters. In the primary open-angle glaucoma group, positive correlations were observed between the corneal hysteresis values and the global retinal nerve fiber layer thickness (p<0.01, r=0.27), mean retinal nerve fiber layer thickness (p<0.01, r=0.33), and mean deviation (p<0.01, r=0.26), and negative correlations were observed between the corneal resistance factor values, and the cup area (p<0.01, r=-0.39), cup-to-disk ratio (p=0.02, r=-0.28), linear cup-to-disk ratio (p=0.02, r=-0.28), and cup shape (p=0.03, r=-0.26). In the control group, weak correlations were detected between the corneal hysteresis and the cup area (p=0.03, r=0.19), cup-to-disk ratio (p=0.01, r=0.21), and linear cup-to-disk ratio (p=0.01, r=0.22). Distinct correlations were identified between the corneal hysteresis and corneal resistance factor values and the functional and structural parameters in the primary open-angle glaucoma and control groups. Corneal hysteresis and corneal resistance factor may have different roles in the pathophysiology of glaucoma.

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.

Comparative analysis of biomechanical parameters of the corneas following Descemet membrane endothelial keratoplasty and contralateral healthy corneas.

To compare the biomechanical properties of the unilateral operated corneas in patients who had undergone Descemet membrane endothelial keratoplasty (DMEK) for pseudophakic bullous keratopathy (PBK) with those of the contralateral normal corneas. This was a retrospective cohort study conducted at university hospitals (Department of Ophthalmology, Rabin Medical Center, Petach Tikva, Israel, and S. Fyodorov Eye Microsurgery State Institution, Moscow, Russia). Forty eyes of 20 patients who underwent DMEK for unilateral PBK 3.5 to 36months ago and with normal fellow eyes were included in the study. An ocular response analyzer was used to measure the corneal biomechanical properties in the operated and normal fellow eyes. The main outcome measures were corneal hysteresis (CH) and corneal resistance factor (CRF). The mean CH (8.4 ± 1.5mmHg vs. 8.2 ± 1.5mmHg, P = 0.707) and the mean CRF (8.7 ± 1.6mmHg vs. 8.3 ± 1.6mmHg, P = 0.419) values did not show any statistically significant difference between the operated and the normal fellow eyes. In our study, the corneas that underwent DMEK for PBK showed normal values for biomechanical parameters. These findings support the previous studies that have reported near complete visual, functional, and ultra-structural rehabilitation of the corneas following DMEK.

Influence of preoperative astigmatism on corneal biomechanics and accurate intraocular pressure measurement after micro-incision phacoemulsification.

To define the corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) and corneal compensated intraocular pressure (IOPcc) prior to and following coaxial micro-incision phacoemulsification in patients with corneal astigmatism. Of 97 patients with cataracts were enrolled in the study. Group 1 included patients with corneal astigmatism (K1-K2) values of K1-K2<+1.0 D, and group 2 with values of K1-K2 ≥+1.0 D and ≤+2.25 D. Coaxial micro-incision phacoemulsification of a corneal incision of 2.0 mm with intraocular lens (IOL) implantation was performed. CH, CRF, IOPg, IOPcc, waveform score (WS) were measured preoperatively and one week, one month postoperatively using an Ocular Response Analyzer. Axial length (AXL) was calculated by Tomey Optical Biometer OA 2000. Group 1 consisted of 51 patients with mean corneal astigmatism value of +0.49±0.25 D. Group 2 included 46 patients with astigmatism of +1.43±0.43 D. In group 1, CRF (t=2.68, P<0.05), CH (t=2.64, P<0.05) and WS (t=3.51, P<0.05) were significantly lower one week postoperatively, when compared to the preoperative values. CRF significantly decreased (t=3.61, P<0.05) when measured one month following the surgery. In group 2 CH (t=5.92, P<0.05), and WS (t=3.96, P<0.05) were significantly lowered one week after cataract surgery. Moreover, we observed a significant decrease in IOPg (t=2.24, P<0.05), CRF (t=5.05, P<0.05) and CH (t=2.31, P<0.05) one month after phacoemulsification. There was no statistically significant (t=-0.83, P=0.41) difference in AXL between study groups. CRF, CH and IOPg are reduced in patients with preoperative corneal astigmatism equal or higher than +1.0 D and lower than +2.25 D. Hence, bias of IOPg measurement in these patients may cause underestimation of the real IOP both before and after cataract surgery. The measurement of IOPcc allows the precise assessment of IOP pre- and postoperatively, independently on corneal astigmatism, CH and CRF values.

Assessment of corneal biomechanics in young myopic individuals using ocular response analyzer

Objective The aim of this study was to assess corneal biomechanical properties using the ocular response analyzer (ORA) in myopic young individuals with normal topography. Background The development in corneal refractive surgeries raises the necessity of studying corneal properties in detail. Corneal viscoelasticity, in terms of corneal biomechanics, is measured by ORA through the analysis of the corneal dynamic behavior when it is deformed by an air puff. The important parameters measured are corneal resistance factor (CRF) and corneal hysteresis (CH). Patients and methods This study included 116 myopic eyes, which were assessed by clinical examination, best corrected visual acuity examination, and corneal pentacam topography. CH and CRF were measured using ORA. Results The mean age in our study participants was 23.6 ± 2.9 years, and the mean of the spherical equivalent was −4.56 ± 2.4 (moderate myopia). The mean CRF was 9.7 ± 1.4 mmHg, while the mean CH was 10.0 ± 1.3 mmHg. As regards corneal topography, normal participants were recruited with the following measurements: Kmax (maximum keratometry) 44.3 ± 1.4 and the mean central corneal thickness (CCT) was 529.5 ± 12.7 μm. Correlation of CRF and CH values in this moderate myopic group were negatively correlated with the spherical equivalent. Both CH and CRF had positive linear correlation with CCT and Kmax, and negative correlation with age. Conclusion The ORA is used to measure CH and CRF by which the biomechanical changes of the cornea in different degrees of myopia can be detected. It was found that it is important when evaluating the corneal biomechanical properties to take CCT and myopic status into consideration.

Comparison of corneal biomechanical properties of patients with dry eye secondary to Sjögren's syndrome and healthy subjects

The goal of this study is to determine whether any difference in corneal biomechanical properties exists between Sjögren's syndrome dry eye patients and healthy subjects. Thirty-one patients diagnosed with Sjögren's syndrome and associated dry eye manifestations and 44 healthy individuals were included in the study. Ultrasonic pachymetry (UP) was used to measure central corneal thickness (CCT). Corneal biomechanical parameters were obtained using ocular response analyzer (ORA). The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOPg) and corneal compensated IOP (IOPcc). A Student's t-test for independent groups was performed to compare the mean of these variables between both groups. Mean CH values in Sjögren's syndrome and healthy subject eyes were 10.1mmHg and 11.18mmHg respectively, representing a statistically significant difference (P=0.003). No other variable measured differed between cases and controls (P>0.05). Mean CRF values were 9.51mmHg and 10.37mmHg respectively, and mean CCT measured by UP in cases and controls was 527.41μm and 552.51μm respectively. Sjögren's syndrome can influence corneal biomechanical properties, specifically CH. ORA measurements should be considered of interest in the evaluation of Sjögren syndrome subjects.

Higher Intraocular Pressure Levels Associated With Lower Hysteresis In Type 2 Diabetes.

Aim: To investigate the differences of corneal biomechanic characteristics using Ocular Response Analyzer (ORA, Reichert; USA) on type 2 diabetics and healthy subjects.Methods:One hundred eyes of 100 subjects (between the ages of 17-91) who applied to Adnan Menderes University’s Ophthalmology Clinic between January-March 2015 were included in this study, 50 diabetics (Group 1) and 50 healthy controls (Group 2). The eyes included in the study were randomly chosen. Corneal Hysteresis (CH), Corneal Resistance Factor (CRF), Goldmann correlated Intraocular Pressure (IOPg) and corneal compensated Intraocular Pressure (IOPcc) of patients were measured by ORA. Detailed ophthalmological examinations were done for every subject. Kolmogorov-Smirnov test was used to analyze the distribution of quantitative variables and t test was used for the data that were normally distributed. Any p value <0.05 was considered as statistically significant.Results: The mean ages were 63.3±9.0 and 61.7±11.6 in Group 1 and 2, respectively (p=0.459). 25 (50.0%) were female, 25 (50.0%) were male in Group 1 and 26 (52.0%) were female, 24 (48.0%) were male in Group 2 (p=1.000). Mean IOPcc values were 17.8±3.6 (12.1-29.0) and 16.0±3.1 (10.9-23.8) mmHg (p=0.006); mean IOPg values were 16.9±3.5 (10.9-25.9) and 15.4±2.9 (9.0-24.7) mmHg (p=0.032); mean CH values were 9.9±1.5 (6.1-13.3) and 10.5±1.7 (6.5-15.7) (p=0.080) and mean CRF values were 10.4±1.6 (7.5-14.0) and 10.5±1.7 (6.6-15.4) (p=0.730) in Groups 1 and 2, respectively.Conclusions:There was no any statistical difference between the groups in terms of CH and CRF. However, mean CH and CRF values were found less in diabetic group. Corneal biomechanical differences seen in diabetic patients may be associated with a statistically significantly higher IOP measurements.

Evaluation of Changes in Corneal Biomechanics in Patients with Systemic Sclerosis

Our study compares the biomechanical properties of the cornea in patients with long-term pre-existing (for at least 10 years) systemic sclerosis (SSc) with those of healthy suspects. The examination was performed in 14systemic SSc patients and 38 healthy volunteers. Non-invasive assessment of corneal biomechanical parameters, including central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF), was performed in one randomised study eye in accordance with a standardised protocol. Intraocular pressure (IOP) values were analysed using different measuring techniques, including Goldmann applanation tonometry (GAT), dynamic contour tonometry (DCT) and Ocular Response Analyzer® (ORA)-based non-contact tonometry (NCT), GAT-correlated IOP (IOPgat) and CCT-compensated-IOP (IOPcc). When measured with DCT and ORA-based NCT, IOP levels were significantly lower in SSc patients than in the control group (DCT IOP: p = 0.048, NCT IOPgat: p = 0.002, NCT IOPcc: p < 0.001). CCT was also significantly reduced in the SSc (p = 0.001). There were no statistically significant differences between the groups in CH and CRF. The difference between the corrected values (CHcorr - CRFcorr) was negative in the two groups. This was slightly lower for the SSc patients (delta = - 0.83) than for the control group (delta = - 0.66). For SSc patients, there were highly significant negative correlations between CH and CRF and between CHcorr and CRFcorr. In contrast, in the control group there was a non-significant positive correlation between age and biomechanical properties. In the course of the disease SSc leads to various alterations in connective tissue, which can also involve corneal tissue. CCT becomes significantly thinner and simultaneously partially loses elastic properties and gains viscosity. This accounts for reduced IOP values with dynamic contour tonometry as well as with ORA-tonometry (Goldmann-correlated IOP, CCT-compensated IOP). This distinct pre-existing significant negative correlation between age and CH and CRF values in patients with SSc could be due to slight regression of the viscous components and diminution of corneal damping capacity.

Goldmann and error correcting tonometry prisms compared to intracameral pressure

BackgroundCompare Goldmann applanation tonometer (GAT) prism and correcting applanation tonometry surface (CATS) prism to intracameral intraocular pressure (IOP), in vivo and in vitro.MethodsPressure transducer intracameral IOP was measured on fifty-eight (58) eyes undergoing cataract surgery and the IOP was modulated manometrically to 10, 20, and 40 mmHg. Simultaneously, IOP was measured using a Perkins tonometer with a standard GAT prism and a CATS prism at each of the intracameral pressures. Statistical comparison was made between true intracameral pressures and the two prism measurements. Differences between the two prism measurements were correlated to central corneal thickness (CCT) and corneal resistance factor (CRF). Human cadaver eyes were used to assess measurement repeatability.ResultsThe CATS tonometer prism measured closer to true intracameral IOP than the GAT prism by 1.7+/−2.7 mmHg across all pressures and corneal properties. The difference in CATS and GAT measurements was greater in thin CCT corneas (2.7+/−1.9 mmHg) and low resistance (CRF) corneas (2.8+/−2.1 mmHg). The difference in prisms was negligible at high CCT and CRF values. No difference was seen in measurement repeatability between the two prisms.ConclusionA CATS prism in Goldmann tonometer armatures significantly improve the accuracy of IOP measurement compared to true intracameral pressure across a physiologic range of IOP values. The CATS prism is significantly more accurate compared to the GAT prism in thin and less rigid corneas. The in vivo intracameral study validates mathematical models and clinical findings in IOP measurement between the GAT and CATS prisms.