Abstract: PURPOSE: To measure corneal hysteresis (CH) in unilateral chronic primary angle-closure glaucoma (CPACG) patients to determine if it was affected by high intraocular pressure (IOP). DESIGN: Prospective interventional case series. METHODS: CH and Goldmann-correlated IOP (IOPg) were obtained with the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Dephew, New York, USA) and central corneal thickness (CCT) was measured by optical coherence tomography. Baseline CH, IOPg, and CCT were measured in 40 CPACG eyes and compared to the fellow eyes and 40 normal controls. Reduction of IOPg in CPACG eyes was achieved medically, followed by trabeculectomy and peripheral iridectomy. Measurements were repeated at 2 and 4 weeks posttherapy. RESULTS: IOPg decreased significantly from 31.55 +/- 10.48 mm Hg (mean +/- standard deviation) before therapy to 11.47 +/- 4.71 mm Hg, and CH increased significantly from 6.83 +/- 2.08 mm Hg to 9.22 +/- 1.80 mm Hg at 2 weeks, with no further changes after that. However, the CH in the treated eyes remained significantly lower compared with that of fellow and normal eyes. Before treatment, CH was negatively correlated with IOPg; however, there was no correlation after treatment. CCT was not affected by the reduced IOPg in the CPACG eyes. CONCLUSIONS: CH was significantly lower in CPACG patients, and partial recovery occurred after successful IOP-lowering therapy. Alternations affecting corneal biomechanical properties appear to occur during glaucoma development.

Abstract: PURPOSE: To compare parameters of biomechanical response of the human cornea measured as corneal hysteresis (CH) and corneal resistance factor (CRF) in patients with diabetes mellitus and healthy control subjects. SETTING: Department of Ophthalmology, Assaf Harofeh Medical Center, Zerifin, Israel. METHODS: In the right eye of each participant, the CH, CRF, Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated intraocular pressure (IOPcc) were measured with the Ocular Response Analyzer. Central corneal thickness (CCT) was measured by ultrasonic pachymetry and intraocular pressure by Goldmann applanation tonometry (IOP GAT). Findings were compared between the 2 groups (control and diabetic). RESULTS: Forty diabetic patients (17 women, 23 men) and 40 healthy subjects (19 women, 21 men) were prospectively recruited. The mean CH was 9.3 mm Hg +/- 1.4 (SD) and 10.7 +/- 1.6 mm Hg and the mean CRF was 9.6 +/- 1.6 mm Hg and 10.9 +/- 1.7 mm Hg in the control group and diabetic group, respectively (both P < .0001). Diabetic corneas were significantly thicker (P = .019); the mean CCT was 530.3 +/- 35.9 microm in the control group and 548.7 +/- 33.0 microm in the diabetic group. The CH and CRF remained significantly different in multivariate analysis that included CCT. There was no statistically significant difference between the 2 groups in IOPcc, IOPg, or IOP GAT measurements. CONCLUSIONS: Diabetes mellitus affected biomechanical parameters of the human corneas, including increased CH, CRF, and CCT. Whether this observation has implications in the clinical management and understanding of corneal ectasia and glaucoma requires further study.

Abstract: AIM: Several methods permit the measurement of geometric parameters of the cornea, but until now biomechanical conditions of the cornea have been ignored (e.g. in refractive corneal surgery). Besides the geometric condition, biomechanical properties of the cornea have been shown to influence applanation measurement of intra-ocular pressure (IOP) and epidemiological studies have identified corneal thickness as an independent risk factor for the development and progression of glaucoma. The aim of this investigation was to characterize the biomechanical properties of the cornea using the ocular response analyzer (ORA). METHODS: The ocular response analyzer (ORA) is a new method available for non-contact measurement of the biomechanical properties of the cornea. We evaluated the reproducibility of measurements, the difference between static and dynamic factors and the impact of independent factors (e.g. IOP, age, CCT, swelling of the cornea) on 2,500 measurements of corneal hysteresis (CH) and corneal resistance factor (CRF). RESULTS: In a large sample size we observed changes in CH and CRF after refractive surgery procedures (LASIK, UV-A cross-linking, keratoplasty) and in other corneal disorders (keratoconus, corneal dystrophies). CONCLUSIONS: CRF and CH changes may reflect structural changes of the cornea. Thus, the ORA provides valuable information for a better understanding and characterization of the biomechanical condition of the cornea, especially with regard to diseases such as keratoconus and glaucoma.

Abstract: PURPOSE. To investigate the effects of Fuchs corneal dystrophy (FCD) on corneal biomechanical properties, and the results of IOP readings in relation to changes of corneal hysteresis (CH) and central corneal thickness (CCT). METHODS. Corneal biomechanical properties, including CH, corneal resistance factor (CRF) and CCT were measured in eleven eyes of 11 patients with clinically confirmed FCD and twelve eyes from 12 healthy subjects with the ocular response analyzer (ORA). The ORA also determined the values of intraocular pressure (IOPg) and corneal compensated IOP (IOPcc). Goldmann applanation tonometry (GAT) was also measured. RESULTS. CH was 10.3+/-1.6 mm Hg (range, 8.7-13.8) in normal eyes compared with 6.9+/-1.8 mm Hg (range, 4.6+/-11.7) in FCD eyes (p=0.001). CRF in the normal and FCD eyes was 10.5+/-1.5 mm Hg (range, 8.5-13.3) and 8.1+/-1.9 (range, 4.5-11.2), respectively (p=0.005). The CCT was higher in FCD eyes, 606.0+/-20.0 microns (578-635), than that in normal eyes, 538.4+/-24.9 microns (495-575) (p=0.0001). IOPg was 16.2+/-2.2 mm Hg (range, 13.5-18.7) in controls compared with 17.6+/-2.7 mm Hg (range, 12.8-18.6) in FCD eyes (p=0.201); however, IOPcc in the FCD group (21.8+/-4.6 mm Hg; range, 12.8-29.0) was higher than that in the controls (16.5+/-3.4 mm Hg; range, 11.9-23.9) (p=0.006). GAT in the normal and FCD eyes was 16.7+/-2.1 mm Hg (range, 12.8-18.6) and 16.9+/-2.3 mm Hg (range, 13.1-19.0), respectively (p=0.205). CONCLUSIONS. FCD lead to a change of corneal biomechanical properties. CH and CRF were significantly lower in FCD eyes compared to that in normal eyes. IOPcc was significantly higher in FCD eyes than in the controls. These values may be useful besides CCT when assessing corneal rigidity. Thus, FCD may cause an underestimation error in IOP measurement.

Abstract: PURPOSE: To investigate the biomechanical properties of the normal cornea, and correlate them with central and peripheral corneal thickness and age. METHODS: Seventy-six right eyes of volunteers were measured with Ocular Response Analyzer (ORA), ICare rebound tonometry and an ultrasound pachymeter at corneal center and at 4 mm from corneal center in the nasal and temporal directions. RESULTS: ICare readings were significantly correlated with central and peripheral corneal thickness and corneal biomechanical properties. Corneal resistance factor was the biomechanical parameter with the higher correlation with ICare intraocular pressure (IOP) values. ICare tonometry at center and Goldmann equivalent IOP obtained with ORA were significantly higher for thicker than thinner corneas (P<0.05). IOP compensated for corneal properties with the ORA was lower than the remaining IOP values measured in the study. Higher correlation was found between Goldmann equivalent IOP with ORA and ICare IOP values. CONCLUSIONS: IOP values obtained with the rebound tonometer are higher in thicker corneas and are positively correlated with biomechanical corneal parameters, namely corneal resistance factor. Although corneal thickness plays a significant role in rebound tonometry, elastic and viscous properties of the cornea seem to play a significant role in the interaction of the tonometer probe with the ocular surface. However, the mechanism behind this process is presently unknown.

Abstract: OBJECTIVE: The aim of this study was to investigate the corneal hysteresis (CH) and corneal resistance factor (CRF) in normal eyes and to determine their relationship between central corneal thickness (CCT), corneal curvature, sphere equivalence (SE) and intraocular pressure (IOP). METHODS: Cross-sectional study. This study included 205 normal eyes (205 patients). Ocular Response Analyzer (ORA, Reichert Ophthalmic Instruments, New York, USA) was used to measure CH and CRF. Multivariate regression analysis was applied to study their relationship between CCT, corneal curvature, SE, IOP (noncontact tonometer). RESULTS: The values of CH and CRF in 205 eyes presented normal distribution and concentrated on 8.1 to 11.0 mm Hg (1 mm Hg = 0.133 kPa). No statistical significant difference was found in both CH and CRF of different gender and ages (< 25 years or > or = 25 years) (P > 0.05). 95% reference interval of CH and CRF in 205 eyes were 6.8 to 13.0 mm Hg and 6.6 to 13.6 mm Hg. The correlation of CH and CRF with CCT, corneal curvature, SE and IOP were significant (P < 0.05). CONCLUSION: CH and CRF measure different biomechanical aspects of cornea, they may reflect the combined effect of CCT, corneal curvature, rigidity, hydration and IOP. These values of CH and CRF in normal eyes established foundation to further investigate their relationship with eye diseases.

Abstract: OBJECTIVE: Acquired pit-like changes of the optic nerve head (APON) are characteristic of glaucomatous damage and may be a sign of a localized susceptibility of the optic nerve. Thus, it is possible that biomechanical properties of the ocular tissues may play a pressure-independent role in the pathogenesis of glaucoma. Corneal hysteresis (CH) appears to provide information of the biomechanical properties of the ocular hull tissues. The purpose of this study was to compare CH of patients with primary open angle glaucoma (POAG) with and without APON. METHODS: A prospective case control study was done. POAG patients with and without APON were measured using the Ocular Response Analyzer by masked investigators. Patients in both groups were matched for sex, age, corneal thickness, and type of glaucoma according to maximal IOP (NTG or POAG). Statistical analysis was done using ANOVA. RESULTS: Corneal hysteresis of 16 glaucomatous eyes with APON and 32 controls (glaucoma without APON) was measured. The mean (+/-SD) CH in the APON group was 8.89 (+/-1.53) and 10.2 (+/-1.05) in the control group. The difference is statistically significant (p=0.005). CONCLUSIONS: Corneal hysteresis in POAG patients with APON was significantly lower than in patients that did not have such structural changes of the optic disc. These findings may reflect pressure-independent mechanisms involved in the pathogenesis of such glaucomatous optic nerve changes.

Abstract: PURPOSE: To investigate, in healthy Asian subjects, the diurnal changes of the corneal biomechanical properties of corneal hysteresis (CH) and its relation to fluctuations of intraocular pressure (IOP) and central corneal thickness (CCT) after sleeping. METHODS: Forty eyes of 20 healthy Asian volunteers (4 males and 16 females) with a mean age of 19.7 +/- 1.1 years were measured with a modified retinal optical coherence tomography and an ocular response analyzer (ORA). CH, CCT, Goldmann-equivalent intraocular pressure (IOPg), corneal resistance factor (CRF), and corneal compensated intraocular pressure (IOPcc) were obtained. Repeated measurements were conducted on both eyes during a 24-h period, including immediately before and after sleeping. RESULTS: There were no significant differences in any measured variable between right and left eyes [Repeated measurement analysis of variance (Re-ANOVA), p > 0.05]. Diurnal variations (Re-ANOVA, p = 0.001) of CCT, IOPg, IOPcc, and CRF were found during the 24-h period. Significant increases occurred a few minutes after overnight sleep compared with the baseline values at 10:00 PM before sleep (post hoc tests, p = 0.001). However, there were no differences of CH over time during the study period (Re-ANOVA, p = 0.11). A few minutes after awakening, the increase of CCT was not related to the changes of CH, IOPg, IOPcc, or CRF. However, the increase of CRF was significantly correlated with the changes of IOPg. CONCLUSION: Diurnal variation of CH was not detectable in healthy Asian eyes. The diurnal fluctuation of IOP appears not to be associated with CH. However, CRF was increased a few minutes after sleeping, a change that was correlated to the changes of IOPg.

Abstract: PURPOSE: The Ocular Response Analyzer (ORA) measures corneal biomechanical properties: corneal hysteresis (CH) and the corneal resistance factor (CRF). The Pascal Dynamic Contour Tonometer (PDCT) measures the ocular pulse amplitude (OPA), which represents the systolic-diastolic variation in intraocular pressure (IOP). Both ORA and OPA values are lower in glaucoma patients than in normal patients. Our purpose was to assess whether there is a correlation between CH, CRF, and OPA values in healthy subjects. DESIGN AND PARTICIPANTS: Prospective observational study including 81 eyes of 41 healthy adult volunteers. Patients with ocular eye disease, previous refractive or intraocular surgery, contact lenses, or topical or general medications were excluded. METHODS: Patients underwent four ORA, three OPA, and two Goldmann Applanation Tonometry (GAT) measurements. For each device, the mean of all measurements were considered for this study. The statistical analysis by Spearman rank correlations was performed for right (RE), left (LE), and both eyes. The correlation between the two eyes of each subject was taken into account and canonical correlations were calculated using the SAS statistical software to improve the power of the analysis. RESULTS: The mean age was 43.6 +/- 14.6 years. The mean central corneal thickness was 545 +/- 32 microm for RE, 553 +/- 34 microm for LE, and 550 +/- 32 microm for both eyes. The mean IOPGAT values for RE, LE, and both eyes were, respectively 14.3 +/- 2.9 mmHg, 14.0 +/- 2.9 mmHg, and 14.2 +/- 2.9 mmHg. Spearman correlations were not statistically significant for separated RE and LE analysis. For both eyes, these correlations were rS=0.210, p=0.193 for OPA/CH and rS=0.388, p=0.013 for OPA/CRF. The Spearman correlation was statistically significant for OPA/CRF. This result was confirmed by canonical correlations (p=0.002) and in addition, CH was statistically correlated to OPA in the latter analysis (p=0.010). CONCLUSION: A statistically significant correlation was found between corneal biomechanical property values as measured by ORA and ocular pulse amplitude values as measured by PDCT in healthy subjects.

Abstract: PURPOSE: The dynamic contour tonometer (DCT; Pascal tonometer) and the Ocular Response Analyzer (ORA) are novel tonometers designed to measure intraocular pressure (IOP) independent of corneal properties and central corneal thickness (CCT), respectively. We wanted to compare the corneal compensated IOP (IOPcc) as measured by ORA with IOP values measured by DCT and Goldmann applanation tonometry (GAT) with respect to IOP readings and the influence of corneal hysteresis (CH) and CCT in glaucoma patients. PATIENTS AND METHODS: In a study of 94 glaucomatous eyes, IOP measurements by ORA, DCT, and GAT were compared, and the effects of CCT and CH were analyzed. All measurements were taken by 1 of the authors only. RESULTS: The Mean CCT was 550+/-44 mum and mean CH 10.24+/-3.3 mm Hg. The mean value for IOPcc was 17.94+/-5.9 mm Hg, 15.14+/-3.7 mm Hg for DCT, and 14.3+/-4.3 mm Hg for GAT. The mean difference was 3.65+/-3.85 mm Hg between IOPcc and GAT and 2.80+/-4.9 mm Hg between IOPcc and DCT (P<0.001) and -0.85+/-3.3 mm Hg between GAT and DCT (P=0.015). None of the methods was related to CCT. In contrast, CH was highly statistically significant related to IOPcc (P<0.0001), whereas GAT and DCT showed only weak relation to CH (P=0.05). CONCLUSIONS: IOP measurements with the ORA are significantly higher than DCT or GAT with no apparent linear correction factor. To date, we cannot estimate the impact of differences in CH in relation to IOP and its measurement.

Abstract: PURPOSE: To study the effects of aging on corneal biomechanical properties and their impact on 24-hour measurement of intraocular pressure (IOP). DESIGN: Experimental study. METHODS: Fifteen older volunteers with healthy eyes (age range, 50 to 80 years) were housed for one day in a sleep laboratory with a 16-hour diurnal or wake period and an eight-hour nocturnal or sleep period. Every two hours, sitting corneal hysteresis, corneal resistance factor, and IOP were measured. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. Data were compared with previous observations in 15 healthy younger volunteers (age range, 20 to 25 years). RESULTS: Variations in 24-hour corneal hysteresis and corneal resistance factor were not significant in the older subjects, but there were time-dependent variations in CCT and IOP. The nocturnal CCT was thicker than the diurnal CCT, but the IOP difference between the diurnal and nocturnal periods was not significant. Cosine-fits of CCT and IOP showed synchronized 24-hour rhythms. The phase timing of CCT rhythm appeared significantly earlier than the phase timing of IOP rhythm. Compared with younger subjects, older subjects had a lower mean 24-hour corneal hysteresis and corneal resistance factor, but not a lower CCT. Phase timings of 24-hour rhythms of CCT and IOP were significantly delayed by aging. CONCLUSIONS: Aging may lower corneal hysteresis and corneal resistance factor, but neither parameter shows a significant 24-hour variation. Aging may not change CCT significantly, but can shift its 24-hour rhythm. The 24-hour IOP pattern in this group of older subjects is not an artifact resulting from a variation in corneal hysteresis, corneal resistance factor, or CCT.

Abstract: PURPOSE: Hysteresis is a viscoelastic property characterized by the difference in behavior under loading and unloading. The aim of the study was to determine corneal hysteresis using experimental means. METHODS: Twenty-nine human corneas with 50-95 year age were subjected to cycles of pressure loading and unloading. Two pressure application rates were adopted to approximate static and dynamic loading conditions. RESULTS: The behavior under both loading and unloading was found to stiffen with increased age. The unloading behavior appeared to be largely independent of the pressure level at which unloading started. The difference between the behavior patterns under loading and unloading was quantified and used as a measure of corneal hysteresis. The hysteresis area was significantly larger with faster loading and with decreased age. CONCLUSIONS: The trend for hysteresis to decrease with age is in agreement with previous clinical observations. Hysteresis was also found to increase with faster pressure application.

Abstract: INTRODUCTION: Intraocular irrigating solutions remain for several hours beyond the actual time of surgery in the eye. The irrigating solution ought to resemble biochemically aqueous humor and vitreous and offer protection for sensitive structures of the eye, such as the corneal endothelium. Impairment of the corneal endothelium may lead to corneal oedema and biomechanical alterations of the cornea. PATIENTS AND METHODS: 54 eyes after pars-plana vitrectomy (PPV) in elective macular surgery were evaluated by measuring corneal thickness (CCT) using ultrasound pachymetry (20 MHz) and corneal hysteresis (CH) using the ocular response analyser (Reichert Ophthalmic Instruments, Buffalo, NY, USA). Measurements were performed not earlier than 2 weeks prior to surgery and 1 to 3 days after surgery. Results were compared to a control group (n = 39) and to 101 eyes after clear cornea cataract extraction (KAT). RESULTS: The two groups (PPV and KAT) did not differ with respect to age (p = 0.555). Corneal thickness has increased significantly in both groups (p <or= 0.001, mean difference: PPV group 34.95 +/- 23.57 microm and KAT group 23.76 +/- 26.0 microm, respectively) whereas corneal hysteresis decreased significantly postoperatively (p <or= 0.001; 1.93 +/- 3.15 mmHg after PPV and 1.15 +/- 2.65 mmHg after cataract surgery, respectively). Postoperative changes were significantly higher in the PPV group (CCT p <or= 0.01 and CH p = 0.011, respectively). CONCLUSION: Surgically induced changes of the cornea may not only be characterized by anatomic parameters such as corneal thickness but also by biomechanical parameters such as corneal hysteresis.

Abstract: Purpose To examine the relationship between corneal biomechanical properties and retinal vascular caliber in Singaporean children. Design Cross sectional study of 257 normal subjects from the Singapore Cohort Study of Risk Factors for Myopia (SCORM). Methods Corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT) and intraocular pressure (IOPcc) were measured with the ORA. Digital retinal photography was performed and retinal vascular calibre was measured using custom software. The central retinal arteriolar and venular equivalents (CRAE and CRVE) were calculated, representing the average arteriolar and venular calibers. Spherical equivalent refraction(SE), axial length, height, weight, and mean arterial blood pressure (MABP)were measured. Results The mean age of the study subjects was 13.97+/-0.90 years. The mean CH was 11.80+/-1.55mmHg, the mean CRF was 11.83+/-1.72mmHg, mean CCT was 578.76+/-34.47microm and mean IOPcc was 15.12+/-2.84mmHg. The mean CRAE was 151.70+/-15.54microm and the mean CRVE was 227.51+/-22.82microm. After controlling for age, sex and ethnicity, and then further for BMI, father's educational level, MABP, IOP and SE, there was a significant increase in CRAE by 1.40(95% CI:0.17,2.61;p=0.03)microm for every 1.55mmHg increase in CH, and by 1.68(95% CI:0.21,3.15;p=0.03)microm for every 1.72mmHg increase in CRF. There were no significant associations between CRVE and CH, CRF, CCT or IOP. Conclusion Lower CH and CRF are associated with narrower retinal arterioles in Singaporean children. .

Abstract: AIMS: To estimate the heritability of intraocular pressure (IOP) by performing a classical twin study and to determine whether the use of different instruments influences calculation of eye IOP heritability. METHODS: Twin pairs were recruited to participate from the TwinsUK Adult Twin Registry at St. Thomas' Hospital London. IOP was measured using Goldmann applanation tonometry (GAT). A subset of twins also had their IOP measured using the Ocular Response Analyser (ORA; Reichert, Buffalo, NY) and the Dynamic Contour Tonometer (DCT, Pascal; Swiss Microtechnology AG, Port, Switzerland). We compared the covariance of IOP within monozygotic (MZ) and dizygotic (DZ) pairs using genetic modelling techniques to determine the relative contribution of genes and environment to the variation in IOP seen in this population. RESULTS: Data for 422 twin pairs (211 MZ; 211 DZ) were analysed. The mean IOP for GAT was 15.4 (SD 2.7) mm Hg (range: 8.7-26.2 mm Hg). The MZ correlations were significantly higher than DZ for IOP measured by GAT, DCT and ORA (correlation coefficients: GAT: 0.57:0.39, DCT: 0.62:0.36, Goldmann-correlated ORA (IOPg) 0.73:0.47, for MZ:DZ twins, respectively). Modelling suggested heritability for GAT IOP of 0.62, with individual environmental factors accounting for 0.38 of the variation. CONCLUSION: This study demonstrated that genetic effects are important in determining IOP in this twin population. IOP readings differed depending upon the instrument used, and this resulted in different heritability values; genetic factors explained 62%, 63% and 74% of the variation in IOP using GAT, DCT and ORA IOPg, respectively. Environmental factors determined the remainder of the variation.

Abstract: AIM: The aim of this study was to evaluate the practicality and accuracy of tonometers used in routine clinical practice for established keratoconus (KC). METHODS: This was a prospective study of 118 normal and 76 keratoconic eyes where intraocular pressure (IOP) was measured in random order using the Goldman applanation tonometer (GAT), Pascal dynamic contour tonometer (DCT), Reichert ocular response analyser (ORA) and TonoPen XL tonometer. Corneal hysteresis (CH) and corneal resistance factor (CRF), as calculated by the ORA, were recorded. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. RESULTS: The difference in IOP values between instruments was highly significant in both study groups (p<0.001). All other IOP measures were significantly higher than those for GAT, except for the Goldmann-correlated IOP (average of the two applanation pressure points) (IOPg) as measured by ORA in the control group and the CH-corrected IOP (corneal-compensated IOP value) (IOPcc) measures in the KC group. CCT, CH and CRF were significantly less in the KC group (p<0.001). Apart from the DCT, all techniques tended to measure IOP higher in eyes with thicker corneas. CONCLUSION: The DCT and the ORA are currently the most appropriate tonometers to use in KC for the measurement of IOPcc. Corneal factors such as CH and CRT may be of more importance than CCT in causing inaccuracies in applanation tonometry techniques.

Abstract: PURPOSE: To evaluate corneal biomechanical metrics with tomographic parameters (given by the Oculus Pentacam) and refractive data in a population of healthy Brazilian patients. METHODS: Observational, cross-sectional study of 150 consecutive patients (53 men and 97 women; 260 eyes). Age, gender, central keratometric readings (central K), central corneal thickness (CCT), anterior chamber depth (ACD), spherical equivalent refraction, corneal hysteresis, and corneal resistance factor (CRF) were assessed and analyzed. RESULTS: Mean patient age was 46.5+/-21.04 years, average central K was 43.59+/-1.54 diopters (D), CCT was 545.05+/-35.41 μm, ACD was 2.96+/-0.52 mm, spherical equivalent refraction was -1.16+/-3.48 D, corneal hysteresis was 10.17+/-1.82, and CRF was 10.14+/-1.8 (range: 5.45 to 15.1). Mean CRF and corneal hysteresis were distinct among gender: CRF 10.326 in women and 9.810 in men (P=.0266); corneal hysteresis 10.421 in women and 9.727 in men (P=.0031). A negative correlation was found between both CRF and corneal hysteresis with age (r=-0.1255, P=.0434;and r=-0.2445, P=.0001, respectively). No association was found between CRF and average central K (r=0.0633, P=.3086), ACD (r=-0.0474, P=.4498) or spherical equivalent refraction (r=0.1028, P=.1061). Corneal hysteresis was not associated with age and average central K (r=0.0572, P=.3573), ACD (r=0.0060, P=.9236), or spherical equivalent refraction (r=0.0975, P=.1253). Corneal resistance factor and corneal hysteresis were positively associated with CCT (r=0.5760, P=0; and r=0.4655, P=0, respectively). CONCLUSIONS: Corneal biomechanical metrics of healthy Brazilian patients were associated with CCT, gender, and age. Corneal steepness, ACD, and spherical equivalent refraction did not affect corneal hysteresis and CRF values in the studied population.

Abstract: BACKGROUND: To evaluate factors affecting corneal hysteresis (CH) in normal eyes. METHODS: We examined 86 normal eyes of 43 healthy volunteers (age, 39.1 +/- 14.5 years (mean +/- standard deviation); range, 19 to 68 years; gender, 26 men, 60 women; manifest refraction, -2.25 +/- 2.89 diopters (D); range, - 9.13 to 3.88 D). We quantitatively assessed the value of CH using an Ocular Response Analyzertrade mark (Reichert Ophthalmic Instruments). We carried out this measurement three times, and the average value was used for statistical analysis. Multiple regression analysis was used to assess the relevant factors of the CH. RESULTS: The mean CH was 10.2 +/- 1.3 mmHg. Explanatory variables relevant to the CH were, in order of magnitude of influence, the central corneal thickness (CCT) (partial regression coefficient B = 0.022, p < 0.0001), and the intraocular pressure (IOP) (B = -0.119, p = 0.04). No significant correlation was seen with other clinical factors such as age, gender, manifest refraction, or mean keratometric readings. CONCLUSIONS: Eyes with thinner CCT and eyes with higher IOP are more predisposed to have lower CH. Refractive surgeons should, from a biomechanical viewpoint, take not only CCT but also IOP into consideration before performing keratorefractive surgery.

Abstract: PURPOSE: To evaluate corneal viscoelastic and intraocular pressure (IOP) changes measured by an ocular response analyzer (ORA) after phacoemulsification and intraocular lens (IOL) implantation. SETTING: Yeditepe University Department of Ophthalmology, Istanbul, Turkey. METHODS: Fifty-one eyes scheduled for cataract surgery were included in the study. Corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOPcc), and Goldmann-correlated IOP (IOPg) were measured by ORA preoperatively and 1 week and 1 and 3 months postoperatively. Central corneal thickness (CCT) was measured using the ORA's integrated handheld ultrasonic pachymeter. RESULTS: The mean preoperative CCT (537 microm+/-46 [SD]) did not change significantly by the end of 1 month postoperatively. The mean preoperative IOPcc (17.2+/-3.0 mm Hg) decreased significantly by 3 months postoperatively (15.2+/-3.7 mm Hg) (P= .018). The mean CH decreased from 10.36+/-1.48 mm Hg preoperatively to 9.64+/-1.26 mm Hg at 1 week (P= .028); it increased to preoperative values at the end of 1 month (10.20+/-1.70) and 3 months (10.74+/-1.54) (P>.05). The mean CRF decreased from 10.94+/-2.54 mm Hg preoperatively to 9.99+/-1.77 at 1 week (P= .026); it increased to preoperative values at 1 month (10.26+/-1.59) and 3 months (10.35+/-1.46) (P>.05). CONCLUSIONS: Although CH and the CRF decreased in the early postoperative period, the parameters increased and reached preoperative values by 3 months postoperatively, showing that corneal biomechanical properties are influenced by phacoemulsification and IOL implantation.

Abstract: The ocular response analyzer (ORA) (Reichert, Inc.) was used in the case of a middle-aged man who developed unilateral corneal ectasia after bilateral laser in situ keratomileusis (LASIK). The preoperative refraction was similar in the 2 eyes. Post-LASIK ectasia was central in the left eye; topography was oblate in the right eye. The ORA values consisted of the mean of 4 measurements. Corneal hysteresis and corneal resistance factor were almost equal in the ectatic eye and the nonectatic eye. However, significant between-eye differences in the morphology of the signals were noted, most prominently in the lower amplitude of the applanation peaks in the ectatic eye. The shape of the applanation signal yielded important information in addition to corneal hysteresis and corneal resistance factor.

Abstract: PURPOSE: The Ocular Response Analyzer (ORA) is a newly introduced tonometer that uniquely measures and then integrates corneal biomechanical data into its intraocular pressure (IOP) estimates in an effort to improve accuracy of IOP assessment. This study was devised to investigate whether ORA-derived IOP and corneal biomechanical variables might be useful in discriminating between subjects with and without primary open-angle glaucoma (GLC). METHODS: All patients seen in the Albuquerque VAMC eye clinic over a 10-week period who demonstrated acceptable ORA signal profiles were retrospectively identified. In subjects classified as normal (NML), ocular hypertension (OH), glaucoma suspect (GS), and GLC, the following variables were compared: age, ethnicity, Goldmann IOP, central corneal thickness (CCT), and ORA-derived data: Goldmann-correlated IOP (IOPg), corneal-compensated IOP (IOPcc), corneal resistance factor (CRF), corneal hysteresis (CH), and difference between IOPcc and IOPg (DIOP; IOPcc - IOPg). RESULTS: Right eyes in 71 NML, 58 OH, 70 GS, and 99 GLC subjects were studied. Using analysis of variance, higher mean age, higher mean DIOP, and lower mean CH were found in the GLC group compared with OH, GS, and NML groups. In multivariate regression analyses, factors that independently discriminated between groups were: age, IOPcc, and DIOP (GLC vs. NML); age and IOPcc (GLC vs. GS); age and CRF (GLC vs. OH). When DIOP was left out of the models, CH replaced DIOP in the GLC vs. NML analysis with nearly equal statistical power. CONCLUSIONS: Our results suggest that ORA-generated parameters may be useful for differentiating subjects with and without GLC. Furthermore, the discriminatory power of each ORA variable seems to depend on the diagnostic groups that are being compared. Finally, our findings also suggest that measured IOP may be significantly underestimated in glaucoma patients compared with non-glaucoma patients.

Abstract: PURPOSE: To evaluate the intraoffice hour variability of intraocular pressure (IOP) and in vivo biomechanical properties of the cornea, as measured with the Ocular Response Analyzer (ORA). METHODS: The right eye of each of 58 young healthy subjects (14 men, 44 women) was measured at 1-hour intervals from 9.00 a.m. until 7.00 p.m. with the ORA. Subjects' age ranged from 19 to 45 years (average +/- standard deviation, 25 +/- 6 years). RESULTS: Average intraoffice hour variability of IOP and corneal biomechanical parameters were not statistically significant, showing a stable profile during the hours of the day studied. An interesting finding was that although there were no significant diurnal variations in any of the parameters, minor changes in corneal-compensated IOP and a Goldmann-equivalent IOP were significantly correlated with the intraoffice hour variability of corneal biomechanical parameters corneal hysteresis (CH) and the corneal resistance factor (CRF). Corneal-compensated IOP changes were the most strongly associated with intraoffice hour variability in CH for all measurement times (r = 0.598; p < 0.001). Changes of Goldmann-equivalent IOP were strongly correlated with intraoffice hour variability of corneal resistance factor (r = 0.849; p < 0.001). CONCLUSIONS: The present study confirms that intraoffice hour values of CH and corneal resistance factor are quite stable among a population of young healthy adults. However, average changes in these parameters over time correlate well with the changes in IOP values obtained with the same instrument, suggesting that diurnal variations of IOP could be at least in part related with changes in the biomechanical behavior of the cornea against the mechanical stimuli used by current non-contact tonometers. The most stable period to measure IOP and biomechanical parameters with ORA was in the afternoon.

Abstract: A precise assessment of the intraocular pressure (IOP) is crucial for diagnosis and decision making regarding treatment modalities in patients with glaucoma. Recent epidemiologic studies show that a difference of only 1 mm Hg in the mean IOP may be critical enough to determine the visual field prognosis in patients with glaucoma. However, the Goldmann applanation tonometer, which is current gold standard, is not precise enough to measure the true IOP within an error of 1 mm Hg. There are many clinically proposed correction algorithms to correctly measure IOP. However, corrections using only the central corneal thickness and curvature may not be sufficient in each individual case. In this article, previously reported theoretical equations about the effects of corneal topography, modulus of elasticity, and tear film on Goldmann applanation tonometric IOP readings were reviewed, and their discrepancies with clinical or experimental data were analyzed. Thereafter, new tonometers such as the dynamic contour tonometer, the rebound tonometer, and the ocular response analyzer were compared with the Goldmann applanation tonometer and other popular tonometers.

Abstract: PURPOSE: To develop a method for evaluating viscosity and elasticity of the cornea and to examine the effect that both properties have on hysteresis. METHODS: A three-component spring and dashpot model was created in Simulink in Matlab to represent the purely elastic and viscoelastic behavior of the cornea during a measurement using device called an ocular response analyzer (ORA). Values for elasticity and viscosity were varied while sinusoidal stress was applied to the model. The simulated stresses were used to determine how hysteresis is affected by the individual components of elasticity, viscosity, and maximum stress. To validate the model, high-speed photography was used to measure induced strain in a corneal phantom during ORA measurement. This measured strain was compared with the strains simulated by the model. RESULTS: When the spring in the viscoelastic portion of the model was stiffened, hysteresis decreased. When the spring in the purely elastic element was stiffened, hysteresis increased. If both springs were stiffened together, hysteresis peaked strongly as a function of the viscosity of the viscoelastic element. Below the peak value, lower elasticity was associated with higher hysteresis. Above the peak value, higher elasticity was associated with higher hysteresis. In addition, hysteresis increased as the air maximum pressure was increased. Measurements from phantom corresponded to predictions from the model. CONCLUSIONS: A viscoelastic model is presented to illustrate how changing viscosity and elasticity may affect hysteresis. Low hysteresis can be associated with either high elasticity or low elasticity, depending on the viscosity, a finding consistent with clinical reports.

Abstract: Purpose The effect of topical corneal anaesthesia on corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal compensated IOP (IOPcc) was measured by ocular response analyzer (ORA). Design Observational, cross-sectional study. Method We examined both eyes of 23 healthy volunteers. Patients with external eye disease, previous refractive surgery, contact lenses or topical medication were excluded. ORA parameters were first measured in both eyes. Oxybuprocaïne 0.4% eye drop was instilled in the right eyes (RE) and physiologic saline in the left eyes (LE) as a control to rule out the lubrication effect. After 2 min, the ORA measurements were performed again. Goldmann applanation tonometry (GAT) was finally done. Data are expressed as mean +/- standard deviation (SD). The medians of the four CH, CRF, IOPg, and IOPcc values measured before and after instillation were compared by using Wilcoxon signed ranks tests for RE and LE. Results The mean age was 39.5 +/- 11.6 years. The mean GAT was 13.1 +/- 2.5 mmHg for RE and 12.8 +/- 2.5 mmHg for LE. In the RE, the respective values for the two sequences were IOPcc = 15.6 +/- 2.6 mmHg and 15 +/- 2.8 mmHg (P = 0.036); IOPg = 15.3 +/- 3.3 mmHg and 15.4 +/- 3.5 mmHg (P = 0.806); CH = 11 +/- 1.3 mmHg and 11.1 +/- 1.4 mmHg (P = 0.563); CRF = 11.1 +/- 1.8 mmHg and 10.9 +/- 1.9 mmHg (P = 0.053). In the LE, the respective values for the two sequences were IOPcc = 15.4 +/- 2.6 mmHg and 15.6 +/- 2.8 mmHg (P = 0.903); IOPg = 15.5 +/- 3.5 mmHg and 15.4 +/- 3.4 mmHg (P = 0.208); CH = 10.8 +/- 1.4 mmHg and 10.7 +/- 1.7 mmHg (P = 0.494); CRF = 10.7 +/- 1.8 mmHg and 10.7 +/- 2.2 mmHg (P = 0.626). Conclusion Two minutes after instillation, topical corneal anaesthetic slightly decreases IOPcc and also-but not statistically significantly-CRF. We did not find any statistical significant difference in CH or IOPg before and after topical corneal anaesthesia. Further investigation with more patients should be advised.

Abstract: AIMS: To determine the associations between corneal biomechanical parameters as measured by the Reichert Ocular Response Analyser (ORA) and disc morphology and retinal nerve fibre layer thickness (RNFL) measured by the Heidelberg Retinal Tomograph (HRT) II in Singaporean children. METHODS: This is a cross-sectional study conducted on a subset of children enrolled in the Singapore Cohort Study of the Risk Factors of Myopia (SCORM). Corneal hysteresis (CH), corneal resistance factor (CRF) and central corneal thickness (CCT) were measured with the ORA. Optic disc morphology and RNFL thickness were assessed by the HRT II. Cycloplegic refraction and ultrasound A-scans were also performed, and disc tilt was assayed from stereo photographs. RESULTS: 102 subjects (mean age 12.01 (SD 0.57) years; range 11-14 years) were included in the study. The mean CH was 12.00 (1.40) mm Hg, the mean CRF was 11.99 (1.65) mm Hg, and the mean CCT was 581.12 (33.53) mum. Eyes with tilted discs had significantly longer axial lengths and more myopic refraction than eyes without tilted discs. There were no significant correlations between CH, CRF or CCT and the HRT II parameters, after the application of the Bonferroni correction. When stratified for disc tilt, however, the global disc area was significantly correlated with CCT (r = -0.49, p = 0.001). CONCLUSION: Corneal biomechanical properties as measured with the ORA do not vary with optic disc parameters or RNFL. Central corneal thickness is correlated with disc area in Singaporean schoolchildren with tilted discs. This relationship may influence glaucoma risk in myopic subjects.

Abstract: PURPOSE: To determine corneal biomechanical properties in patients with high myopia. DESIGN: Observational study. METHODS: High myopia patients (n=45, age: 37.0+/-12.6 years) with refractive errors of spherical equivalent (SE) greater than -9.00D were recruited in this study along with healthy subjects (n=90, age: 33.7+/-12.4 years) with refractive errors of SE ranging from 0D to -3.00D. Only the right eye was studied. Central corneal thickness (CCT) was measured by optical coherence tomography (OCT). Metrics of corneal biomechanical properties, including corneal hysteresis (CH) and corneal resistance factor (CRF), were measured with the Ocular Response Analyzer (ORA). The ORA also determined the values of intraocular pressure (IOP(g)) and corneal compensated IOP (IOP(cc)). RESULTS: No significant differences of CCT and CRF were present between the two groups (P=.15 and 0.35 for CCT and CRF, respectively); however, CH in the high myope group was lower than that in the controls (P<.01). IOP(g) and IOP(cc) were both significantly higher in the high myopes compared to the controls. In both groups, there were significant correlations between CH and CCT and between CRF and CCT. CH was not significantly correlated with age in either the control group or the high myope group (P>.05). There was a significant correlation between CH and SE when the two groups were combined for analysis. CONCLUSION: CH, but not CRF, was significantly lower in high myopia patients compared to that in normal subjects. The results indicate that some compromised aspects of the biomechanical properties of cornea may exist in people with high myopia.

Abstract: AIM: To examine corneal hysteresis (CH) and corneal resistance factor (CRF) in normal and ectatic corneas. METHODS: CH and CRF were measured using the Reichert Ocular Response Analyzer in patients with clinically diagnosed keratoconus (KC), forme fruste KC (FFKC) and normal eyes. RESULTS: 21 eyes (13 patients) with clinically diagnosed KC and 30 eyes (18 patients) with FFKC were included in the study. Mean CH and CRF in FFKC did not differ from that in pachymetry-matched normal eyes. KC eyes had significantly lower CH and CRF than normal and FFKC eyes. CONCLUSION: A significant overlap in CH and CRF values among the 3 groups was evident. Our findings do not indicate a role for CH and CRF measurement as a single test to aid in the detection of early ectasia. It may be of use when used in conjunction with other parameters such as aberrometry. CH and CRF values may prove to be useful in monitoring ectasia progression.

Abstract: PURPOSE: To determine the efficacy of the Reichert Ocular Response Analyzer (ORA) to measure intraocular pressure (IOP) following corneal laser refractive surgery. METHODS: Intraocular pressure was measured using Goldmann applanation tonometry preoperatively and 3 months following LASIK and LASEK for all levels of myopia and low levels of hyperopia. In LASIK eyes, 120-microm flaps were cut using the Hansatome XP. The ORA was used to measure Goldmann correlated IOP and corneal compensated IOP 3 months postoperatively. Postoperative central corneal thickness, hysteresis, and corneal curvature were assessed to determine their relationship with postoperative change in Goldmann applanation tonometry IOP. Efficacy of ORA IOP measurement was analyzed by comparing Goldmann applanation tonometry IOP with Goldmann correlated IOP. RESULTS: LASEK was performed on 35 eyes, and LASIK was performed on 90 eyes. In the LASIK group, mean Goldmann applanation tonometry IOP decreased 3.7+/-2.3 mmHg postoperatively (P=.00). Postoperative Goldmann applanation tonometry did not differ significantly (P=.06) from postoperative ORA Goldmann correlated IOP (10.2+/-2.1 mmHg). In the LASEK group, mean Goldmann applanation tonometry IOP decreased 3.9+/-2.3 mmHg (P=.00). Postoperative Goldmann applanation tonometry did not differ significantly (P=.6) from postoperative ORA Goldmann correlated IOP (10.7+/-2.5 mmHg). Postoperative decrease in Goldmann applanation tonometry IOP did not correlate with age, ablation depth, pre- and postoperative central corneal thickness or corneal hysteresis, or postoperative decrease in corneal curvature. CONCLUSIONS: Intraocular pressure measurements were similar using both the ORA Goldmann correlated IOP and Goldmann applanation tonometry following keratorefractive surgery.

Abstract: PURPOSE: To examine the roles of genetic and environmental factors in corneal hysteresis and ocular pulse amplitude by performing a classic twin study. DESIGN: Cross-sectional twin study. PARTICIPANTS AND/OR CONTROLS: Two hundred sixty-four twin pairs: 135 monozygotic (MZ) and 129 dizygotic (DZ). METHODS: Corneal hysteresis was measured using the Reichert Ocular Response Analyzer (ORA; Reichert, Buffalo, NY), and ocular pulse amplitude was measured using the Pascal Dynamic Contour Tonometer (DCT; Swiss Microtechnology AG, Port, Switzerland). MAIN OUTCOME MEASURES: Contribution of genetic and environmental effects on corneal hysteresis and OPA among MZ and DZ twins. RESULTS: The mean corneal hysteresis was 10.24+/-1.54 mmHg and the mean ocular pulse amplitude was 2.88+/-0.97 mmHg. The MZ correlations were higher than DZ for both corneal hysteresis and ocular pulse amplitude (correlation coefficients, 0.75:0.42 and 0.59:0.32 for MZ:DZ twins, respectively). Modeling suggested heritability of corneal hysteresis of 0.77 (95% confidence interval [CI], 0.70-0.82), with the remaining proportion of variance because of individual environmental effects of 0.23 (95% CI, 0.18-0.30). For ocular pulse amplitude, the heritability was 0.62 (95% CI, 0.51-0.70), with the remaining proportion of variance the result of individual environmental effects of 0.38 (95% CI, 0.30-0.49). CONCLUSIONS: This study demonstrated that additive genetic influences explained most of the individual differences in corneal hysteresis and ocular pulse amplitude among these twins.

Abstract: PURPOSE: To evaluate the association between corneal hysteresis and axial length/refractive error among rural Chinese secondary school children. DESIGN: Cross-sectional cohort study. METHODS: Refractive error (cycloplegic auto-refraction with subjective refinement), central corneal thickness (CCT) and axial length (ultrasonic measurement), intraocular pressure (IOP), and corneal hysteresis (Reichert Ocular Response Analyzer) were measured on a rural school-based cohort of children. RESULTS: Among 1,233 examined children, the mean age was 14.7 +/- 0.8 years and 699 (56.7%) were girls. The mean spherical equivalent (n = 1,232) was -2.2 +/- 1.6 diopters (D), axial length (n = 643) was 23.7 +/- 1.1 mm, corneal hysteresis (n = 1,153) was 10.7 +/- 1.6 mm Hg, IOP (n = 1,153) was 17.0 +/- 3.4 mm Hg, and CCT (n = 1,226) was 553 +/- 33 microns. In linear regression models, longer axial length was significantly (P < .001 for both) associated with lower corneal hysteresis and higher IOP. Hysteresis in this population was significantly (P < .001) lower than has previously been reported for normal White children (n = 42, 12.3 +/- 1.3 mm Hg), when adjusting for age and gender. This difference did not appear to depend on differences in axial length between the populations, as it persists when only Chinese children with normal uncorrected vision are included. CONCLUSIONS: Prospective studies will be needed to determine if low hysteresis places eyes at risk for axial elongation secondary or if primary elongation results in lower hysteresis.

Abstract: PURPOSE: To assess the reproducibility of the ocular response analyzer (ORA) in nonoperated eyes and the impact of corneal biomechanical properties on intraocular pressure (IOP) measurements in normal and glaucomatous eyes. METHODS: In the reliability study, two independent examiners obtained repeated ORA measurements in 30 eyes. In the clinical study, the examiners analyzed ORA and IOP-Goldmann values from 220 normal and 42 glaucomatous eyes. In both studies, Goldmann-correlated IOP measurement (IOP-ORAg), corneal-compensated IOP (IOP-ORAc), corneal hysteresis (CH), and corneal resistance factor (CRF) were evaluated. IOP differences of 3 mm Hg or greater between the IOP-ORAc and IOP-ORAg were considered outcome significant. RESULTS: Intraexaminer intraclass correlation coefficients and interexaminer concordance correlation coefficients ranged from 0.78 to 0.93 and from 0.81 to 0.93, respectively, for all parameters. CH reproducibility was highest, and the IOP-ORAg readings were lowest. The median IOP was 16 mm Hg with the Goldmann tonometer, 14.5 mm Hg with IOP-ORAg (P < 0.001), and 15.7 mm Hg with IOP-ORAc (P < 0.001). Outcome-significant results were found in 77 eyes (29.38%). The IOP-ORAc, CH, and CRF were correlated with age (r = 0.22, P = 0.001; r = -0.23, P = 0.001; r = -0.14, P = 0.02, respectively), but not the IOP-ORAg or IOP-Goldmann. CONCLUSIONS: The ORA provides reproducible corneal biomechanical and IOP measurements in nonoperated eyes. Considering the effect of ORA, corneal biomechanical metrics produces an outcome-significant IOP adjustment in at least one quarter of glaucomatous and normal eyes undergoing noncontact tonometry. Corneal viscoelasticity (CH) and resistance (CRF) appear to decrease minimally with increasing age in healthy adults.

Abstract: PURPOSE: To investigate relationships between acute intraocular pressure (IOP)-induced optic nerve head surface deformation and corneal hysteresis and thickness in glaucomatous and nonglaucomatous human eyes. METHODS: This was a prospective experimental study of 100 subjects (38 with glaucoma, 62 without glaucoma). Data collected included spherical equivalent, optic disc diameter, central corneal thickness (CCT), axial length, cylinder, Goldmann IOP, Pascal IOP, and ocular pulse amplitude and ocular response analyzer (ORA) measurements of corneal hysteresis (CH). Elevation of IOP was induced in the right eye of each subject with a modified LASIK suction ring to an average of 64 mm Hg for less than 30 seconds. Heidelberg Retina Tomography II (HRT) was used to map the optic nerve surface before and during IOP elevation. Mean cup depth was calculated using built-in HRT data analysis software. Change in optic disc depth during IOP elevation was calculated for all right eyes, and tests for correlation with the parameters listed were performed. RESULTS: Both CH and CCT were lower in the glaucoma group (8.8 mm Hg and 532 microm) than in the control group (9.6 mm Hg, P = 0.012; 551 microm, P = 0.011, respectively). There were no statistically significant differences in spherical equivalent, cylinder, axial length, optic disc size, or ocular pulse amplitude between the glaucoma and the control groups. There was no difference between the amount of IOP elevation between the two groups (P = 0.41), and the average difference in mean cup depth between baseline (mean cup depth, 247 microm) and during IOP elevation was 33 microm (29.8 microm in glaucoma and 36.1 microm in control; P = 0.5). Multiple variable analysis, controlling for age and sex, showed that CH was correlated with mean cup depth increase (P = 0.032). This relationship persisted (P = 0.032) after controlling for glaucoma status in addition to age and sex. Other factors, including CCT (P = 0.3), axial length (P = 0.9), ocular pulse amplitude (P = 0.22), and spherical equivalent (P = 0.38), were not significant in this model. CONCLUSIONS: In the glaucoma patients but not the control patients, CH but not CCT or other anterior segment parameters was associated with increased deformation of the optic nerve surface during transient elevations of IOP. (ClinicalTrials.gov number, NCT00328835.).

Abstract: PURPOSE: To analyze the correlation between corneal hysteresis (CH) measured with the Ocular Response Analyzer (ORA, Reichert) and ultrasonic corneal central thickness (CCT US) and intraocular pressure measured with Goldmann applanation tonometry (IOP GA). SETTING: Bordeaux 2 University, Ophthalmology Department, Bordeaux, France. METHODS: This study comprised 498 eyes of 258 patients. Corneal hysteresis, corneal resistance factor (CRF), and IOP corneal-compensated (IOPcc) were provided by the ORA device; CCT US and IOP GA were also measured in each eye. The study population was divided into 5 groups: normal (n = 122), glaucoma (n = 159), keratoconus (n = 88), laser in situ keratomileusis (LASIK) (n = 78), and photorefractive keratectomy (n = 39). The Pearson correlation was used for statistical analysis. RESULTS: Corneal hysteresis was not strongly correlated with IOP or CCT US. The mean CH in the LASIK (8.87 mm Hg) and keratoconus (8.34 mm Hg) groups was lower than in the glaucoma (9.48 mm Hg) and normal (10.26 mm Hg) groups. The lower the CH, the lower its correlation with IOPcc and IOP GA. A CH higher than the CRF was significantly associated with the keratoconus and post-LASIK groups. CONCLUSIONS: Corneal hysteresis, a new corneal parameter, had a moderate dependence on IOP and CCT US. Weaker corneas could be screened with ORA parameters, and low CH could be considered a risk factor for underestimation of IOP. The CCT US should continue to be considered a useful parameter.

Abstract: BACKGROUND: The ocular response analyzer (ORA) uses an air-pressure-triggered, dynamic, bi-directional corneal applanation method to measure biomechanical parameters of the cornea. Corneal hysteresis (CH) is defined as the difference in intraocular pressure recorded during inward and outward applanation. CH is therefore an indicator for the viscoelastic properties of the cornea. PATIENTS AND METHODS: CH was recorded in non-glaucoma patients (80 eyes) as well as in patients with primary open angle glaucoma (POAG, 82 eyes). The correlation between CH and central corneal thickness (CCT) was analyzed. RESULTS: Mean CH was 10.6+/-2.2 mmHg in the non-glaucoma group and 9.3+/-2.2 mmHg in patients with POAG (p<0.01). CH and CCT showed a positive correlation in non-POAG patients, however no such correlation was found in the POAG group. CONCLUSION: Patients with POAG show an alteration of biomechanical corneal parameters with a significant decrease in corneal hysteresis. A positive correlation between CH and CCT, which was seen in the non-glaucoma group could not be detected in the POAG group.

Abstract: INTRODUCTION: Goldmann applanation tonometry (GAT) has been the gold standard for measuring intraocular pressure (IOP) for about 50 years. However, it depends on central corneal thickness (CCT) and is, therefore, prone to being incorrect. Dynamic contour tonometry (DCT) has recently been introduced to measure IOP independently of CCT; however, DCT is costly and difficult. IOP measurement using the ocular response analyzer (ORA) offers noncontact tonometry with declaration of the corneal-compensated IOP (IOPcc), which takes corneal hysteresis (CH) into account and is supposed to be independent of CCT. PATIENTS AND METHODS: Using the ORA instrument, IOPcc was determined in 192 glaucoma eyes and 59 nonglaucoma eyes. Subsequently, measurement by DCT and GAT was performed. IOP measurements were compared and analyzed with respect to CCT and CH. RESULTS: Average values were as follows: IOPcc, 18.38+/-6.3 mmHg; GAT, 14.69+/-4.5 mmHg; DCT, 15.17+/-3.9 mmHg; CH, 9.96+/-2.5 mmHg; CCT, 552+/-57 mum. Neither CCT nor CH differed between the two groups. There was a positive correlation between GAT and CCT that did not exist for IOPcc and DCT values. However, IOPcc and DCT differed significantly in Bland-Altman analysis (p<0.01). Furthermore, these two IOP values differed significantly with respect to CH and the level of IOP. CONCLUSION: Because IOPcc is not a primarily measured variable but also takes CH into account, a direct comparison of DCT and IOPcc values is not acceptable, and a simple correction factor may not be valid.

Abstract: PURPOSE: To study the short-term corneal response to corneal refractive therapy for myopia and correlate it with corneal biomechanical properties as measured with the ocular response analyzer. METHODS: Eight eyes from 8 young subjects were fitted with a reverse geometry contact lens, attempting a myopic correction of -4.00 D. Corneal resistance factor and corneal hysteresis (CH) were measured before contact lens fitting with the ocular response analyzer. These parameters were correlated with the degree of change in apical curvature, simulated keratometry, and central corneal thickness after 3 hours of contact lens wear (effect) and 3 hours after lens removal (recovery). RESULTS: There was a trend toward a faster effect and faster recovery of the orthokeratologic effect for corneas with less resistance in terms of biomechanical properties. Corneal resistance factor did not correlate significantly, however, with any of the topographic and pachymetric parameters. Conversely, CH was significantly correlated with changes in steep keratometry (0.758; P = 0.029) and central corneal thickness (0.755; P = 0.030) during lens wear and with changes in steep keratometry (-0.835; P = 0.010) during recovery. Overall, higher values of CH meant slower effect and recovery of the orthokeratologic effect. CONCLUSIONS: Short-term response of human cornea to corneal refractive therapy is correlated with the biomechanical properties of the cornea. Of the different theories supporting such involvement of corneal response to reverse geometry contact lenses, the most likely one seems to be the one assuming a faster response and faster recovery for corneas with lower resistance. Larger sample studies would be needed to clarify the involvement of corneal biomechanical properties on corneal response to orthokeratology.

Abstract: BACKGROUND: The Ocular Response Analyser (ORA, Reichert Ophthalmic Instruments) is a non-contact applanation tonometer, providing two measures of intraocular pressure (IOP) - IOPg which represents a Goldmann equivalent IOP measure and IOPcc, representing a measure of IOP independent of corneal effects. In addition, the device provides two measures believed to represent corneal biomechanical properties: corneal hysteresis (CH) and corneal resistance factor (CRF). The aim of this study was to assess the repeatability of these measurements. PATIENTS AND METHODS: One randomly chosen eye from 49 healthy volunteers was measured four times consecutively with the ORA prior to Goldmann applanation tonometry (GAT). The repeatability coefficient (RC), the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) were calculated as a measure of intrasession repeatability. RESULTS: CH was the most variable and IOPg the most repeatable measure, with an RC of 2.61 and 1.97, respectively, and ICC of 0.86 and 0.92, respectively. CV ranged between 5.73 % for IOPg and 12.38 % for CH. ORA IOP measurements were higher than GAT (IOPcc = 17.43 +/- 3.23; IOPg = 17.53 +/- 3.0; GAT = 15.75 +/- 2.77 mmHg). CONCLUSIONS: ORA measurements show good short-term repeatability in normal volunteers. Thus, this device appears to be applicable in clinical practice.

Abstract: PURPOSE: To determine if corneal hysteresis (CH) was associated with increased central corneal thickness (CCT) induced by wearing soft contact lenses during eye closure. DESIGN: A prospective laboratory investigation. METHODS: CCT was measured with a modified optical coherence tomography (OCT), and CH was measured with a Reichert Ocular Response Analyzer (ORA) [Reichert Ophthalmic Instruments, Depew, New York, USA]. The ORA also determined values for intraocular pressure (IOP), corneal compensated IOP (IOPcc), and corneal resistance factor (CRF). One randomly selected eye of 20 non-contact lens wearers (four males and 16 females, age 19.7 +/- 1.1 years) was patched during three hours of soft contact lens wear. Measurements were made before lens insertion, immediately upon removal, and every 20 minutes thereafter for 100 minutes. RESULTS: Immediately after contact lens removal, CCT was increased by 13.1 +/- 2.2% (mean +/- SD) compared with baseline (post hoc, P = .001). After 100 minutes, it remained elevated by 2.4 +/- 1.6% (post hoc, P = .001). However, there were no significant differences of CH at any time after lens wear (analysis of variance [ANOVA], P = .9). Immediately after lens removal, there were significant increases in IOP (post hoc, P = .003) and corneal resistance factor (CRF) (post-hoc, P = .015), but not in IOPcc (post hoc, P = .07). After lens wear, there were significant but weak correlations between the percentage change of CCT (CCT%) and IOP (r = 0.32, P = .001) and IOPcc (r = 0.29, P = .001). However, there was no significant correlation between CCT% and CH (r = 0.07, P = .458). CONCLUSION: CH as measured by ORA was not associated with corneal swelling induced by soft contact lens wear in this study group.

Abstract: PURPOSE: To assess the changes in corneal hysteresis (CH) as measured by the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, New York, USA) to describe the influence of clear corneal cataract surgery on corneal viscoelastic properties and intraocular pressure (IOP) measured by noncontact tonometry (NCT) and Goldmann applanation tonometry (GAT). DESIGN: Retrospective, interventional, comparative study. METHODS: One hundred and one eyes of 101 consecutive patients who underwent routine clear corneal cataract surgery were evaluated. CH, NCT, and central corneal thickness (CCT) were measured by ORA before surgery and at postoperative day 1. A control group of 48 pseudophakic eyes (surgery >3 months previously) was included. RESULTS: CCT increased from 556.82 +/- 32.5 microm before surgery to 580.26 +/- 45.5 microm after surgery (P < .001; control, 555.16 +/- 42.33 microm). Mean CH decreased from 10.35 +/- 2.5 mm Hg before surgery to 9.20 +/- 1.9 mm Hg after surgery (P < .001; control, 10.47 +/- 1.63 mm Hg). NCT values rose from 17.85 +/- 3.8 mm Hg before surgery to 20.10 +/- 6.3 mm Hg after surgery. GAT values were 14.85 +/- 2.8 mm Hg before surgery and 15.24 +/- 4.1 mm Hg after surgery (P = .52). There was no significant difference of CCT or CH between the preoperative values and the values of the control group (CCT, P = .986; CH, P = .166), in contrast to the difference between postoperative values and the values of the control group (CCT, P = .005; CH, P = .031). CONCLUSIONS: At day 1 after clear corneal cataract surgery, CH is diminished, whereas CCT is increased significantly. Postoperative corneal edema leads to a change of corneal viscoelastic properties, resulting in a lower damping capacity of the cornea. It is supposed that GAT and NCT measurements are significantly different because of postoperative changes in viscoelastic properties of the cornea.

Abstract: PURPOSE: To compare hysteresis, a novel measure of ocular rigidity (viscoelasticity) in normal and keratoconic eyes. METHODS: The study consisted of 207 normal and 93 keratoconic eyes. Eyes were diagnosed as keratoconic based on clinical examination and corneal topography. The hysteresis was measured by the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, NY). The data were recorded by Generation 3 software for the ORA. Central corneal thickness (CCT) was measured with a handheld ultrasonic pachymeter in the midpupillary axis. RESULTS: The mean hysteresis was 10.7 +/- 2.0 (SD) mm Hg (range, 6.1-17.6) in normal eyes compared with 9.6 +/- 2.2 mm Hg (range, 4.7-16.7) in keratoconic eyes. The difference was statistically significant (P < 0.0001, unpaired t-test). Mean CCT in the normal and keratoconic eyes was 545.0 +/- 36.4 microm (range, 471-650) and 491.8 +/- 54.7 microm (range, 341-611), respectively; the difference was significant (P < 0.0001, unpaired t-test). CONCLUSIONS: Hysteresis was significantly higher in normal than in keratoconic eyes. It may be a useful measurement in addition to CCT, when assessing ocular rigidity, and may be of particular importance when trying to correct intraocular measurements for increased or decreased ocular rigidity. Long-term studies of change in hysteresis may provide information on the progression of keratoconus.

Abstract: The corneal biomechanical properties following descemetorhexis with endokeratoplasty (DXEK) and in normal subjects were studied in 100 eyes (12 DXEK and 88 age-matched normal subjects). Corneal hysteresis measurements were significantly lower in DXEK vs normal subjects. DXEK has a direct effect on corneal hysteresis and corneal biomechanical properties of the human cornea.

Abstract: PURPOSE: To compare the preoperative and postoperative measurement of corneal biomechanical properties and intraocular pressure (IOP) using Goldmann applanation tonometry (GAT), the ocular response analyzer (ORA), and the Pascal dynamic contour tonometer (PDCT) in eyes undergoing myopic laser in situ keratomileusis (LASIK). DESIGN: Prospective, nonrandomized clinical trial. METHODS: IOP was measured in 66 myopic eyes before and after LASIK by GAT, ORA, and PDCT in a randomized sequence. Metrics of corneal biomechanical properties (corneal hysteresis [CH], corneal resistance factor [CRF], and ocular pulse amplitude [OPA]) were recorded. RESULTS: After LASIK, there was a reduction in mean corneal pachymetry of 90.2 mum and in IOP measurements with GAT (Delta = -1.8 +/- 2.8 mm Hg; P < .01), ORA-Goldmann (Delta = -4.6 +/- 2.8 mm Hg, P < .01), and ORA-corneal compensated (Delta - 2.1 +/- 2.6 mm Hg; P < .05). However, there was no statistically significant difference between preoperative and postoperative IOP measurements taken by PDCT (Delta = -0.5 +/- 2.6 mm Hg). Postoperatively, CRF decreased by 28.6% (P < .01), CH by 16.2% (P < .01), and OPA by 1.8% (P = .32). CONCLUSIONS: Measurement of IOP with PDCT appears to be relatively immune to changes in corneal biomechanics and pachymetry after LASIK, in comparison to GAT and ORA measures of IOP. PDCT and ORA both showed statistically lower variation in measurement than GAT. LASIK produced a marked decline in CH and CRF, which may reflect respective changes in the viscous and elastic qualities of the post-LASIK cornea. In contrast, there was no statistical change in OPA.

Abstract: PURPOSE: To compare the biomechanical properties of normal, post-laser in situ keratomileusis (LASIK), and keratoconic corneas evaluated by corneal hysteresis and the corneal resistance factor measured with the Reichert Ocular Response Analyzer (ORA). SETTINGS: Instituto Oftalmológico de Alicante, Vissum, Alicante, Spain. METHODS: Two hundred fifty eyes were divided into 3 groups: normal (control group), post-LASIK, and keratoconus. The corneal biomechanical properties were measured with the ORA, which uses a dynamic bidirectional applanation process. The main outcome measures were intraocular pressure, corneal hysteresis, and the corneal resistance factor. RESULTS: The control group had 165 eyes; the LASIK group, 65 eyes; and the keratoconus group, 21 eyes. In the control group, the mean corneal hysteresis value was 10.8 mm Hg +/- 1.5 (SD) and the mean corneal resistance factor, 11.0 +/- 1.6 mm Hg. The corneal hysteresis value was lower in older eyes, and the difference between the youngest age group (9 to 14 years) and oldest age group (60 to 80 years) was statistically significant (P = .01, t test). One month after LASIK, corneal hysteresis and the corneal resistance factor decreased significantly, from 10.44 to 9.3 mm Hg and from 10.07 to 8.13 mm Hg, respectively. In the keratoconus group, the mean corneal hysteresis was 7.5 +/- 1.2 mm Hg and the mean corneal resistance factor, 6.2 +/- 1.9 mm Hg. There were statistically significant differences in both biomechanical parameters between keratoconic eyes and post-LASIK eyes (P<.001, t test). CONCLUSIONS: The corneal hysteresis and corneal resistance factor values were significantly lower in keratoconic eyes than in post-LASIK eyes. Future work is needed to determine whether these differences are useful in detecting keratoconus when other diagnostic tests are equivocal.

Abstract: PURPOSE: To compare intraocular pressure (IOP) obtained from the ocular response analyzer (ORA) and Goldmann applanation tonometer (GAT) on a group of normal Chinese. METHODS: One hundred twenty-five normal subjects were recruited, with one eye randomly selected for this study. Each eye was measured first with the noncontact tonometer ORA, followed by the GAT and ultrasound pachometry, in a randomized order. Four readings were obtained from the ORA, and three measurements were taken with the GAT. The mean was used for analysis. The ORA provided a Goldmann-correlated IOP (IOPg) and a corneal-compensated IOP (IOPcc). Three central corneal thickness (CCT) values were measured using an ultrasound pachometer, and the mean was used for analysis. RESULTS: IOP obtained from the ORA was similar to that from the GAT (IOPg minus GAT: mean difference = 0.33 mm Hg, 95% limits of agreement = 4.55 to -4.44 mm Hg; IOPcc minus GAT: mean difference = 0.24 mm Hg, 95% limits of agreement = 4.83 to -5.07 mm Hg). CCT was positively associated with corneal hysteresis (CH) (r2 = 0.30, p < 0.01), corneal resistance factor (r2 = 0.38, p < 0.01), GAT (r2 = 0.09, p < 0.01) and IOPg (r2 = 0.16, p < 0.01). IOPcc was not associated with CCT (r2 = 0.01, p = 0.33). CONCLUSIONS: Both IOPg and IOPcc have good agreement with GAT on normal subjects. The influence of CCT on IOPcc was insignificant.

Abstract: PURPOSE: To measure prospectively the early changes in corneal hysteresis, topography, and pachymetry after the creation of a stromal flap cut without laser photoablation. METHODS: A 37-year-old man was referred for a bioptic procedure to correct for compound myopic astigmatism in the left eye. A 159-μm-thick 8 8.5-mm superior hinged flap was created with a mechanical microkeratome in the left cornea. Changes in the corneal hysteresis, corneal resistance factor, Goldmann correlated intraocular pressure (IOP), corneal compensated IOP, anterior and posterior topography, and optical and ultrasound pachymetry were monitored prospectively before and at 1 hour, 1 day, 5 days, and 25 days after the flap creation. The right eye served as a control. RESULTS: In the left eye, corneal hysteresis and corneal resistance factor decreased immediately after the flap cut and remained lower than preoperatively at 1 hour, 1 day, 5 days, and 25 days. Corneal compensated IOP varied significantly less than Goldmann correlated IOP in both eyes. Central flattening of the horizontal meridians was observed on the difference topography maps. The values of the left eye posterior best fit sphere increased after the flap cut. Increased central corneal thickness occurred immediately after the flap cut and decreased over time without returning to its preoperative value. CONCLUSIONS: The creation of a stromal flap can modify the biomechanical properties of the cornea, including a reduction in corneal hysteresis. The topographic changes were consistent with previously reported cases of flap cut in normal corneas.

Abstract: INTRODUCTION: To evaluate the accuracy of ocular response analyzer (ORA) parameters (corneal hysteresis (CH), corneal resistance factor (CRF), and ocular tension (IOPcc and IOPg)) and the correlation with corneal biometry measured with the Orbscan topographer. MATERIAL: and methods: The Ocular Response Analyzer is a new instrument that measures ocular rigidity/elasticity (CH and CRF) and intraocular pressure (IOPcc), which is assumed to be independent of corneal pachymetry. We compared the results of the ORA in 100 eyes with normal slit lamp examination, except ametropia, with the results from corneal measurements obtained with the Orbscan. RESULTS: A high correlation and a low difference between the right and left eyes show the accuracy of measurements (rS=0.84, p<0.001 and 8.52%). The mean CH and CRF were 10.25+/-1.6 mmHg (range, 6.5-14.4) and 10.25+/-1.85 mmHg (range, 4.9-14.2), respectively with a Gaussian distribution in normal eyes. The relationship between CH and CRF was significant with corneal pachymetry but not with IOPcc (corneal corrected). CH and CRF were related, although to a lesser extent, with corneal diameter and astigmatism power, but not to keratometry, sex, age, or spherical equivalent. CONCLUSION: The mean and the distribution of biomechanical factors are similar to the values found in the literature but the relationship to pachymetry seems to be stronger (pachymetry and corneal diameter) than what has been reported in previous publications. The new IOP corrected for the cornea is independent of pachymetry but is not significantly different from Goldmann IOP or IOP measured with a standard noncontact tonometer.

Abstract: PURPOSE: To estimate the relationships between ocular parameters and tonometrically measured intraocular pressure (IOP), to determine the influence of ocular parameters on different instrument measurements of IOP, and to evaluate the association of ocular parameters with a parameter called hysteresis. METHODS: Patients presenting at a glaucoma clinic were recruited for this study. Subjects underwent IOP measurement with the Goldmann applanation tonometer (GAT), the TonoPen, and the Reichert Ocular Response Analyzer (ORA), and also measurements of central corneal thickness (CCT), axial length, corneal curvature, corneal astigmatism, central visual acuity, and refractive error. Chart information was reviewed to determine glaucoma treatment history. The ORA instrument provided a measurement called corneal hysteresis. The association between measured IOP and the other ocular characteristics was estimated using generalized estimating equations. RESULTS: Among 230 patients, IOP measurements from the TonoPen read lowest, and ORA read highest, and GAT measurements were closest to the mean IOP of the 3 instruments. In a multiple regression model adjusting for age, sex, race, and other ocular characteristics, a 10 microm increase in CCT was associated with an increase of 0.79 mm Hg measured IOP in untreated eyes (P<0.0001). Of the 3 tonometers, GAT was the least affected by CCT (0.66 mm Hg/10 mum, P<0.0001). Hysteresis was significantly correlated with CCT with a modest correlation coefficient (r=0.20, P<0.0007). CONCLUSIONS: Among parameters related to measured IOP, features in addition to CCT, such as hysteresis and corneal curvature, may also be important. Tonometric instruments seem to be affected differently by various physiologic characteristics.

Abstract: BACKGROUND: The influence of central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) has been discussed extensively in recent years. The problem, however, has not been solved so far. In addition to CCT there are probably further biomechanical properties that play a role in IOP measurement. We wanted to find out whether these properties are related to Goldmann applanation tonometry (GAT), noncontact tonometry (NCT), or CCT. MATERIAL AND METHODS: Biomechanical properties of the cornea such as corneal hysteresis (CH) and corneal resistance factor (CRF) can be measured with the Ocular Response Analyzer (ORA, Reichert Ophthalmic Instruments, Depew, NY, USA). Furthermore, a corneal compensated IOP (IOPcc) is given. We examined 156 normal eyes of 80 patients who did not show corneal pathology nor glaucoma. In each eye GAT, NCT, and ORA data as well as CCT were measured. Data were statistically analyzed with respect to agreement and the influence of CH and CRF on IOP measurement. RESULTS: In our patients the following average values were calculated: GAT 14.8+/-3.0 mmHg, NCT 16.4+/-3.9 mmHg, IOPcc 16.2+/-4.1 mmHg, CH 10.6+/-2.3 mmHg, CRF 10.9+/-2.4 mmHg, and CCT 557+/-36 microm. IOPcc was not related to CCT in normal eyes and the only IOP value related to CH (p<0.01). CRF, however, was related to GAT and NCT values (p<0.01). DISCUSSION: In our group of normal eyes IOPcc, i.e., the value that is adjusted by measurement of viscoelastic properties of the cornea, in contrast to GAT and NCT does not depend on central corneal thickness. Corneal hysteresis and corneal resistance factor provide further information about biomechanical properties of the cornea beyond central corneal thickness.

Abstract: PURPOSE: To examine corneal hysteresis in children with normal eyes and congenital glaucoma and assess intraocular pressure (IOP) measurement with the Reichert Ocular Response Analyzer (RORA). DESIGN: Observational, cross-sectional study. METHODS: setting: Clinical practice. patients: Corneal hysteresis and intraocular pressure (Goldmann correlated [IOPg] and corneal compensated [IOPcc]) were recorded with RORA. IOP was also measured by Goldmann applanation tonometry. RESULTS: Mean corneal hysteresis of 12.5 mm Hg was recorded in 81 [corrected] normal eyes of 42 children. It was markedly lower (mean 6.3 mm Hg) in 11 eyes of eight children with congenital glaucoma. No correlation was found between age and corneal hysteresis (r = -0.08). IOPg did not differ significantly from Goldmann applanation tonometry (P = .27). IOPcc was statistically significantly greater than IOPg (P = .014). RORA measurement was not possible in the presence of nystagmus but was possible with applanation tonometry. Cooperation with IOP measurement was much higher with RORA (89.8%) compared with applanation tonometry (78.7%). CONCLUSIONS: Corneal hysteresis in children is similar to that reported in adults. No correlation was found with age. In the presence of nystagmus, IOP measurement was possible with Goldmann applanation tonometry but not RORA. Cooperation with IOP measurement was better with RORA than with Goldmann applanation tonometry.

Abstract: PURPOSE: To study 24-hour changes of corneal biomechanical properties and their influences on measurement of intraocular pressure (IOP). METHODS: Fifteen healthy young volunteers (age range, 20-25 years) were housed for 1 day in a sleep laboratory. Sitting and supine central corneal thickness (CCT) were measured every 2 hours with an ultrasonic pachymeter. Sitting IOP and corneal hysteresis, an indicator of viscoelasticity, were measured with a noncontact tonometer. RESULTS: There were consistent 24-hour variations of CCT and IOP for the group. Nocturnal mean CCT and nocturnal mean IOP were significantly higher than the diurnal mean CCT and diurnal mean IOP, respectively. The peak CCT occurred at 1:30 to 5:30 AM and the trough CCT at 1:30 PM. The peak IOP occurred at 5:30 AM and the trough IOP at 9:30 PM. Cosine fits of each subject's 24-hour CCT and IOP data showed synchronized rhythms. The phase timing of 24-hour CCT rhythm was significantly earlier than the phase timing of 24-hour IOP rhythm. Twenty-four-hour variation of corneal hysteresis was inconsistent and cosine fits of 24-hour data of corneal hysteresis did not display a 24-hour rhythm. CONCLUSIONS: In healthy young adults, CCT was thicker, and IOP was higher during the nocturnal period than during the diurnal period. Nocturnal peak CCT occurred a few hours earlier than did nocturnal peak IOP. The twenty-four-hour change in corneal viscoelasticity was not significant. There was no evidence that the 24-hour change in IOP was due to the change in corneal biomechanical properties.

Abstract: BACKGROUND: This study was conducted to assess the diurnal variation in ocular hysteresis, as measured by the Ocular Response Analyser to establish a relationship between diurnal hysteresis variation and diurnal intraocular pressure (IOP) variation. METHODS: Forty-two normal eyes of 21 colleagues and staff in a teaching hospital in Birmingham, UK, were recruited. The IOP and hysteresis were measured by the Ocular Response Analyser. The central corneal thickness (CCT) was measured using a hand-held ultrasonic pachymeter in the mid-pupillary axis. RESULTS: The mean ocular hysteresis at 8 am was 12.7 +/- 2.3 mmHg, at 11 am was 12.2 +/- 2.0 mmHg, at 2 PM was 12.7 +/- 2.1 mmHg and at 5 PM was 12.7 +/- 1.7 mmHg; the difference between the values at any time of measurement was not statistically significant (P > 0.9, repeated measures). IOP as measured by non-contact tonometry was 18.4 +/- 2.8 mmHg, 17.9 +/- 3.3 mmHg, 16.9 +/- 3.1 mmHg and 16.8 +/- 3.2 mmHg, respectively, for the same time period; the difference between the values in the morning and afternoon was statistically significant (P < 0.0001, repeated measures). The CCT was 548.8 +/- 29.5 microm, 547.0 +/- 31.4 microm, 548.2 +/- 29.6 microm and 548.6 +/- 29.4 microm, respectively; the difference between the values was not statistically significant at any time points. Multiple regression analysis showed the relationship between IOP and hysteresis was not statistically significant (P = 0.9). CONCLUSION: The ocular hysteresis reading was almost constant throughout the day, whereas the IOP readings showed highest values in the morning with a reducing trend being lowest in the afternoon. The CCT values were almost stable throughout the day. IOP appears to vary independently of a variation in hysteresis or CCT.

Abstract: PURPOSE: We sought to measure the impact of central corneal thickness (CCT), a possible risk factor for glaucoma damage, and corneal hysteresis, a proposed measure of corneal resistance to deformation, on various indicators of glaucoma damage. DESIGN: Observational study. METHODS: Adult patients of the Wilmer Glaucoma Service underwent measurement of hysteresis on the Reichert Ocular Response Analyzer and measurement of CCT by ultrasonic pachymetry. Two glaucoma specialists (H.A.Q., N.G.C.) reviewed the chart to determine highest known intraocular pressure (IOP), target IOP, diagnosis, years with glaucoma, cup-to-disk ratio (CDR), mean defect (MD), pattern standard deviation (PSD), glaucoma hemifield test (GHT), and presence or absence of visual field progression. RESULTS: Among 230 subjects, the mean age was 65 +/- 14 years, 127 (55%) were female, 161 (70%) were white, and 194 (85%) had a diagnosis of primary open-angle glaucoma (POAG) or suspected POAG. In multivariate generalized estimating equation models, lower corneal hysteresis value (P = .03), but not CCT, was associated with visual field progression. When axial length was included in the model, hysteresis was not a significant risk factor (P = .09). A thinner CCT (P = .02), but not hysteresis, was associated with a higher CDR at the most recent examination. Neither CCT nor hysteresis was associated with MD, PSD, or GHT "outside normal limits." CONCLUSIONS: Thinner CCT was associated with the state of glaucoma damage as indicated by CDR. Axial length and corneal hysteresis were associated with progressive field worsening.

Abstract: PURPOSE: The Ocular Response Analyzer (ORA) proposes to measure corneal biomechanical properties in vivo by monitoring and analyzing the corneal behavior when this structure is submitted to a force induced by an air jet. The purpose of this study was to evaluate the relationship between corneal biomechanical properties and corneal-compensated intraocular pressure (IOPCC) measurements as obtained by the ORA and Goldmann applanation tonometry (GAT) measurements. DESIGN: Observational clinical study. METHODS: The study included 153 eyes of 78 subjects. All subjects underwent IOP evaluation with the ORA and GAT, and also measurements of central corneal thickness (CCT), corneal curvature, and axial length. Univariable and multivariable regression analysis were used to evaluate the associations between IOP (as measured with GAT and ORA) and CCT, corneal curvature, axial length, and age. Bland and Altman plots were used to evaluate the agreement between IOP measurements obtained by GAT and ORA. RESULTS: GAT IOP measurements were significantly associated with CCT (P=0.001) and corneal curvature (P<0.001), whereas ORA IOPCC measurements were not associated with any of the ocular variables. The difference between GAT and IOPCC measurements was significantly influenced by corneal thickness. Patients with thicker corneas tended to have higher GAT IOP measurements compared with IOPCC, whereas in patients with thin corneas, GAT IOP measurements tended to be lower than IOPCC. CONCLUSIONS: ORA IOPCC measurements seem to provide an estimate of IOP that is less influenced by corneal properties than those provided by GAT.

Abstract: PURPOSE: The Ocular Response Analyzer (ORA) is a new instrument that measures the corneal biomechanical response (corneal hysteresis, CH) to rapid indentation by an air jet. CH is the difference in applanation pressures (P1, P2) between the rising and falling phases of the air jet. The investigation had two parts: a characterization study and a validation study. In the characterization study, the purposes were to investigate the intraocular pressure (IOP)-dependence of CH and to characterize the performance of the ORA. In the validation study, the purposes were to investigate the association between CH and both age and central corneal thickness (CCT) and the agreement between ORA and Goldmann applanation tonometer (GAT) IOP measurements. METHODS: For the characterization study, data were collected from 105 untreated subjects (45 ocular hypertensive patients and 60 normal subjects; mean age, 60 years, range, 26-82). GAT and ORA measurements were performed before and after IOP lowering of one randomly selected eye with apraclonidine drops. The change in P1 and P2 (arbitrary units) in relation to change in GAT IOP was analyzed to calibrate the instrument. The relation between P1, P2, and CCT was explored and ORA IOP was derived from the analyses. For the validation study, ORA and GAT IOP and CCT were measured in 144 eyes of 144 untreated subjects (mean age, 58 years; range, 19-83). The characterization calculations were applied to the dataset and values of CH and ORA IOP were calculated. The relationship between CH and both subject age and CCT was determined. The associations between CH and CCT and between ORA and GAT IOPs, were investigated by linear regression analysis. The agreement between measuring devices was calculated. RESULTS: In the characterization study, P1 changed by 6.41 arbitrary units for every 1-mm Hg change in GAT IOP. CH (P1 - P2) changed by -1.60 arbitrary units for every 1-mm Hg change in GAT IOP. For each unit change in P2, P1 changed by 1.27 units. From this association a new IOP-independent corneal factor was derived [P1 - (P2/1.27)] and is termed the corneal constant factor (CCF; mm Hg). ORA IOP normalized for CCF was defined as P2 - CCF (mm Hg). The CCF (mm Hg) was associated with CCT (micrometers) and with age: CCF = [(0.036 . CCT) - (0.028 . age)] + 1.06 (adjusted r2 = 0.34; P < 0.0001 for CCT, P = 0.007 for age). Normalized ORA IOP measurements were not associated with CCT. GAT IOP was associated with CCT and CCF-more strongly with the latter: GAT IOP = (0.03 . CCT)+1.52 (r2 = 0.06, P = 0.002); GAT IOP = (0.65 . CCF) + 4.5 (r2 = 0.13, P < 0.0001). The mean difference (95% limits of agreement) between GAT and normalized ORA IOP was 0.1 (-6.6 to +6.8) mm Hg. CONCLUSIONS: The CCF describes an IOP-independent biomechanical property of the cornea that increases with thicker CCT and decreases with greater age. It is moderately strongly associated with CCT and yet explains more of the interindividual variation in GAT IOP than does CCT. Normalized ORA IOP measurements are not associated with CCT.

Abstract: PURPOSE: To study the results of an ocular response analyzer (ORA) to determine the biomechanical properties of the cornea and their relationship to intraocular pressure (IOP). SETTING: Reichert Inc., Depew, New York, USA. METHODS: The ORA (Reichert) makes 2 essentially instantaneous applanation measurements that permit determination of corneal and IOP effects. RESULTS: Measurements of several populations indicate that corneal hysteresis, a biomechanical measure, varied over a dynamic range of 1.8 to 14.6 mm Hg and was only weakly correlated with corneal thickness (r(2)=0.12); this is related to the observation that some subjects with relatively thick corneas have less-than-average corneal hysteresis. Corneal hysteresis changes diurnally, presumably as a result of hydration changes. Keratoconus, Fuchs' dystrophy, and post-LASIK patients demonstrated low corneal hysteresis. CONCLUSION: The corneal hysteresis biomechanical measure may prove valuable for qualification and predictions of outcomes of refractive surgery and in other cases in which corneal biomechanics are important.