Ocular Biomechanical Properties Research Articles

In vivo assessment of the ocular biomechanical properties in patients with idiopathic normal pressure hydrocephalus

PurposeIdiopathic normal pressure hydrocephalus (iNPH) is associated with an increased prevalence of open-angle glaucoma, attributed to variations of the pressure gradient between intraocular and intracranial compartments at the level of the lamina cribrosa (LC). As ocular biomechanics influence the behavior of the LC, and a lower corneal hysteresis (CH) has been associated to a higher risk of glaucomatous optic nerve damage, in this study we compared ocular biomechanics of iNPH patients with healthy subjects.Methods Twenty-four eyes of 24 non-shunted iNPH patients were prospectively recruited. Ocular biomechanical properties were investigated using the ocular response analyzer (Reichert Instruments) for the calculation of the CH, corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and corneal-compensated intraocular pressure (IOPcc). Results were compared with those of 25 eyes of 25 healthy subjects.Results In iNPH eyes, the median CH value and interquartile range (IQR) were 9.7 mmHg (7.8–10) and 10.6 mmHg (9.3–11.3) in healthy controls (p = 0.015). No significant differences were found in IOPcc [18.1 mmHg (14.72–19.92) vs. 16.4 mmHg (13.05–19.6)], IOPg [15.4 mmHg (12.82–19.7) vs. 15.3 mmHg (12.55–17.35)], and CRF [9.65 mmHg (8.07–11.65) vs. 10.3 mmHg (9.3–11.5)] between iNPH patients and controls.Conclusions In iNPH patients, the CH was significantly lower compared to healthy subjects. This result suggests that ocular biomechanical properties may potentially contribute to the risk of development of glaucomatous optic nerve damage in iNPH patients.

Biomechanical analysis of Corvis tonometry parameters in patients with hypotony after glaucoma filtering surgery

Aims/Purpose: Postoperative hypotony is a common complication of glaucoma filtering surgery, particularly with adjunctive use of antimetabolites, such as mitomycin C and 5‐fluorouracil. Precautions may be taken intraoperatively and postoperatively to decrease the likelihood of hypotony. Sometimes, despite these measures, the low‐pressure syndrome still can occur, the management of which can be difficult. The cornea has a complex biomechanical structure that determines its response under stress conditions and there are so many studies related with glaucoma and corneal biomechanical properties. However, in the best of our knowledge, there are no reports concerning the corneal parameters in patients with hypotony. This study aimed to investigate the possible correlation of hypotony risk after a filtering surgery with different corneal parameters obtained by Corvis.Methods: A retrospective case control study of 30 eyes with chronic hypotony (IOP ≤6 mmHg for more than a month) after glaucoma filtering surgery at the Hospital Universitari Germans Trias i Pujol. In all patients, the IOP (represented in mmHg) and following parameters were obtained using the Corvis device: deformation amplitude ratio (DA), Ambrósio's relational thickness (ARTh), stiffness parameter at first applanation (SPA1), Integrated radius (IR), Stress strain index (SSI) and pachymetry (represented in μm).Results: The results will be represented as the value in eyes with hypotony after glaucoma filtering surgery (cases) vs the healthy eye (control). IOP 4.4 ± 1.8 vs 18.1 ± 7.7. Pachymetry 527 ± 53.4 vs 549.1 ± 94. SPA1 65.2 ± 19.1 vs 111 ± 17.1 in controls (p < 0.05). ARTh 495 ± 170 vs 523 ± 222. IR 10.9 ± 2.67 vs 8.4 ± 1.36 (p < 0.05). DA 5.6 ± 0.7 vs 5 ± 0.5 (p < 0.05). SSI 1.3 ± 1.4 vs 1.6 ± 1.4.Conclusions: SP‐A1 value decreased while the IOP decreased, suggesting that the lower corneal stiffness could increase the percentage of hypotony due to the different ocular biomechanical properties in these eyes, so therefore, it could be considered a risk marker for hypotonia in patients who are going to undergo glaucoma filtering surgery.

Influence of corneal biomechanical properties on intraocular pressure measurement in different types of Graves’ orbitopathy

To compare the ocular biomechanical properties of inactive Grave's orbitopathy (GO) patients and healthy subjects and to evaluate the influence of severity and phenotype of GO on these parameters. This was a cross-sectional study. All included inactive GO patients and healthy controls underwent complete ocular examination, including Goldman applanation tonometry (GAT), corneal biomechanical analysis using Ocular Response Analyser (ORA), and corneal epithelial thickness analysis using Optovue. Patients with inactive GO were classified based on the severity and orbital phenotype (predominantly myogenic or lipogenic). Comparison among groups was performed. 60 eyes from 30 inactive GO patients and 30 healthy eyes were examined. Corneal hysteresis (CH) was significantly lower in inactive GO patients (9.6 [p25 8.1; p75 11.2]) compared to controls (10.4 [9.8; 11.5]) (p = 0.012). In GO patients, cornea compensated intraocular pressure (IOPcc) was significantly higher than Goldman applanation tonometry IOP (IOP-GAT) (p = 0.001). A total of 13.3% GO patients were initially classified as having ocular hypertension (OHT; defined as IOP > 21 mmHg with no signs of glaucomatous optic neuropathy) based on IOP-GAT measurement. According to IOPcc, 27.8% of GO patients were classified as OHT. In GO patients, no differences were found in corneal bimechanical properties according to the disease severity or orbital phenotype. CH is significantly lower in inactive GO patients compared to healthy subjects. ORA corrected IOP was significantly higher in GO patients compared to IOP-GAT. No differences in corneal biomechanical properties between mild and moderate-to-severe GO disease and between myogenic and lipogenic orbitopathy were found.

Corneal Stiffness and Modulus of Normal-Tension Glaucoma in Chinese

To assess and compare the corneal biomechanics of normal-tension glaucoma (NTG), high-tension glaucoma (HTG), and normal controls based on stiffness and modulus. The correlations among central corneal thickness (CCT), visual field, retinal nerve fiber layer (RNFL) thickness, and corneal biomechanics in glaucoma eyes were also evaluated. A prospective, cross-sectional, comparative study. This study included 334 eyes of 108 NTG patients, 113 HTG patients, and 113 control subjects at Zhongshan Ophthalmic Center, Sun Yat-Sen University. Corneal biomechanics were evaluated using a corneal indentation device (CID) and corneal visualization Scheimpflug technology (Corvis ST). Visual field and RNFL thickness were obtained using standard automated perimetry and spectral-domain optical coherence tomography. One-way analyses of variance with Bonferroni post hoc tests and a multivariable linear regression analysis with adjustment were conducted. Correlations among corneal biomechanical parameters, CCT, visual field, and RNFL thickness were analyzed. The corneal stiffness of the NTG patients (71.0 ± 10.9 N/m) was significantly lower than that of the HTG patients (77.3 ± 15.6 N/m; P=.001) and the CCT- and IOP-matched normal controls (75.6 ± 11.0 N/m; P=.023). The patients in the NTG group had lower corneal stiffness than those in the control group (β=-4.88, 95% CI -9.002, -0.758; P=.020) after adjusting for confounders. Stiffness was positively correlated with CCT in the NTG group (P=.028) but not in the HTG group (P= .509). There was no significant correlation (P > .05) between corneal biomechanics, visual field, or RNFL thickness. The corneas of NTG patients were softer than those of HTG patients and controls, as assessed by CID, which were associated with thinner CCT. These might suggest different ocular biomechanical properties in NTG and HTG.

Ocular biomechanics and glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. Ocular biomechanical properties have been implicated in glaucoma pathogenesis, but existing commercial devices cannot determine changes in specific tissues. We recently performed a study to compare corneal wave speed (a measure of corneal elasticity) and ocular rigidity (a whole eye measurement of ocular biomechanics) between glaucomatous and normal eyes. Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, intraocular pressure (IOP), and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography in which a small, 0.1 s harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. No significant differences in corneal wave speed between glaucomatous and normal eyes were detected (P = 0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218 ± 0.0033 vs 0.0252 ± 0.0050 μl−1, P = 0.01). While lower ocular rigidity is consistent with altered biomechanics in glaucoma, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role.

Biomechanics of the optic nerve head and sclera in canine glaucoma: A brief review.

Glaucoma is a leading cause of irreversible blindness, a progressive optic neuropathy with retinal ganglion cell (RGC) death beginning in the optic nerve head (ONH). A primary risk factor for developing glaucoma is elevated intraocular pressure (IOP). Reducing IOP is the only treatment proven to be effective at delaying disease progression. Nevertheless, even when patients have their IOP reduced, the majority of them continue to lose vision. There are, in both humans and dogs, significant interindividual variabilities in susceptibilities to IOP-induced optic nerve damage. Vision loss progresses much more slowly in Beagles with open-angle glaucoma (OAG) caused by ADAMTS10mutation. This can be attributed to the mutation-related altered ocular biomechanical properties. The principal site of optic nerve (ON) damage in glaucoma is the ONH. It is suggested that the biomechanical properties of the ONH and the surrounding peripapillary sclera (PPS) contribute to glaucoma development and progression. As far as the beneficial biomechanical properties of the ONH and PPS for a decreased susceptibility and slow progression of glaucoma, data are inconsistent and conflicting. Recent biomechanical studies on beagles with ADAMTS10mutation demonstrated that the mutant dogs have mechanically weak posterior sclera. This weakness was associated with a reduced collagen density and a lower proportion of insoluble collagen. These changes, observed before glaucoma development, were considered intrinsic characteristics caused by the mutation rather than a secondary effect of IOP elevation. Further studies of ADAMTS10-OAG may elucidate the effects of altered biomechanical properties of ONH and PPS in determining the glaucoma progression.

Comparison of Corneal Wave Speed and Ocular Rigidity in Normal and Glaucomatous Eyes.

Ocular biomechanics were compared between treated glaucoma patients and healthy subjects matched for age, intraocular pressure (IOP), and axial length. There was no difference in corneal wave propagation speed, but ocular rigidity was lower in glaucomatous eyes. Ocular biomechanical properties are important in understanding glaucoma pathogenesis but the affected tissues are unclear. In this study, we compared corneal wave speed (a measure of corneal elasticity) and ocular rigidity coefficient between glaucomatous and normal eyes. Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, IOP, and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography. A small, 0.1-second harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. Comparisons were performed using generalized estimating equation models. There were no significant differences in corneal wave speed between glaucomatous and normal eyes (2.16±0.25 vs. 2.07±0.16 m/s, P=0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218±0.0033 vs. 0.0252±0.0050/μL, P=0.01). Corneal wave speed was not correlated with age and IOP in either group (P≥0.23) but was correlated with ocular rigidity (R=0.48, P=0.02) and inversely correlated with axial length (R=-0.53, P=0.01) in glaucomatous eyes. Glaucomatous eyes tend to have lower ocular rigidity than healthy eyes with similar age, IOP, and axial length. However, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role in glaucoma.

The Relationship Between Corneal Biomechanics and Intraocular Pressure Dynamics in Patients Undergoing Intravitreal Injection

A higher "corneal resistance factor" (CRF) was associated with greater intraocular pressure (IOP) elevation after intravitreal injection of bevacizumab. Both higher "corneal hysteresis" (CH) and CRF were associated with more rapid IOP recovery postinjection. The purpose of this study was to evaluate the relationship between measurable corneal biomechanical properties and acute IOP elevation after rapid intraocular volume expansion from the routine intravitreal injection. A total of 100 patients necessitating unilateral intravitreal injection with 0.05 mL of bevacizumab for retinal pathology were analyzed before injection with Goldmann Applanation Tonometry to measure IOP, Ocular Response Analyzer (ORA) to measure corneal biomechanical properties, and optical biometry to calculate globe measurements. IOP and ORA were measured again within 5 minutes of the injection and then IOP measurements were taken every 10 minutes until the IOP was ≤150% of the preinjection IOP. Linear regression and logistic regression were used to test variables associated with acute IOP increase. A Cox proportional hazard model accounting for preinjection IOP and postinjection IOP was used to test the effect of CH or CRF on the time required to return to 150% of baseline IOP. Higher CRF was associated with greater immediate postinjection IOP (P=0.026) elevation. A preinjection IOP>15.5 mm Hg moderately predicted postinjection IOP≥35 mm Hg (area under the receiver operating characteristics curve=0.74). A preinjection IOP>18.5 mm Hg combined with CH poorly predicted postinjection IOP>50 mm Hg (area under the receiver operating characteristics curve=0.67). A higher CH [hazard ratio (HR)=1.24; 95% confidence interval (CI)=1.08-1.42; P=0.002] and preinjection IOP (HR=1.16; 95% CI=1.09-1.22; P<0.001), along with a lower immediate postinjection IOP (HR=0.93; 95% CI=0.90-0.95; P<0.001), were each independently associated with quicker IOP recovery postinjection. Similar results were seen in the Cox model examining CRF and IOP recovery. Higher CRF and preinjection IOP were independently associated with greater postinjection IOP elevations. ORA metrics did not greatly strengthen the prediction of patients who would have postinjection IOP>50 mm Hg. Higher CH and CRF were associated with faster IOP recovery after intravitreal injection, demonstrating the dynamic relationship between ocular biomechanical properties and aqueous outflow pathways.

Changes in ocular biomechanics after treatment for active Graves' orbitopathy.

To evaluate the changes in ocular biomechanical properties in active moderate-to-severe Graves' orbitopathy (GO) after intravenous glucocorticoids (IVGCs), and to clarify correlations between clinical findings and ocular biomechanical properties. A prospective study. A total of 20 consecutive GO patients and 20 age- and sex-matched healthy control subjects were included. GO was diagnosed on the basis of the recommendation by the European Group on Graves' Orbitopathy (EUGOGO), and disease activity was assessed by the clinical activity score (CAS). Patients were assigned to receive once weekly IVGCs (0.5g, then 0.25g, 6weeks each). All participants received a full ophthalmic examination and biomechanical evaluation was performed with dynamic Scheimpflug analyzer (Corvis ST) at baseline and 12th weeks after therapy. The biomechanically corrected intraocular pressure (bIOP) in GO patients was significantly higher than that in healthy subjects. In contrast, the whole eye movement (WEM) in GO patients was significantly lower than in healthy subjects after adjusting for bIOP. The CAS, NOSPECS score, and exophthalmos were significantly positively correlated with the bIOP and negatively correlated with the WEM after adjusting for bIOP, CCT and age. The WEM significantly increased, whereas bIOP significantly decreased after IVGCs (P < 0.001, P = 0.001 respectively). The overall response rate at the 12th week was 85% (17 of 20). The changes of ocular biomechanical properties measured by Corvis ST were an objective indicator of inflammatory activity and severity of GO. Combining CAS and ocular biomechanical properties could better evaluate the therapeutic outcome of active moderate-to-severe GO.

Age and Menopause Effects on Ocular Compliance and Aqueous Outflow

PurposeGlaucoma is the second leading cause of blindness worldwide. Recent work suggests that estrogen and the timing of menopause play a role in modulating the risk of developing glaucoma. Menopause is known to cause modest changes in intraocular pressure; yet, whether this change is mediated through the outflow pathway remains unknown. Menopause also affects tissue biomechanical properties throughout the body; however, the impact of menopause on ocular biomechanical properties is not well characterized.MethodsHere, we simultaneously assessed the impact of menopause on aqueous outflow facility and ocular compliance, as a measure of corneoscleral shell biomechanics. We used young (3–4 months old) and middle-aged (9–10 months old) Brown Norway rats. Menopause was induced by ovariectomy (OVX), and control animals underwent sham surgery, resulting in the following groups: young sham (n = 5), young OVX (n = 6), middle-aged sham (n = 5), and middle-aged OVX (n = 5). Eight weeks postoperatively, we measured outflow facility and ocular compliance.ResultsMenopause resulted in a 34% decrease in outflow facility and a 19% increase in ocular compliance (P = 0.011) in OVX animals compared with sham controls (P = 0.019).ConclusionsThese observations reveal that menopause affects several key physiological factors known to be associated with glaucoma, suggesting that menopause may contribute to an increased risk of glaucoma in women.

Artificial intelligence in glaucoma.

The use of computers has become increasingly relevant to medical decision-making, and artificial intelligence methods have recently demonstrated significant advances in medicine. We therefore provide an overview of current artificial intelligence methods and their applications, to help the practicing ophthalmologist understand their potential impact on glaucoma care. Techniques used in artificial intelligence can successfully analyze and categorize data from visual fields, optic nerve structure [e.g., optical coherence tomography (OCT) and fundus photography], ocular biomechanical properties, and a combination thereof to identify disease severity, determine disease progression, and/or recommend referral for specialized care. Algorithms have become increasingly complex in recent years, utilizing both supervised and unsupervised methods of artificial intelligence. Impressive performance of these algorithms on previously unseen data has been reported, often outperforming standard global indices and expert observers. However, there remains no clearly defined gold standard for determining the presence and severity of glaucoma, which undermines the training of these algorithms. To improve upon existing methodologies, future work must employ more robust definitions of disease, optimize data inputs for artificial intelligence analysis, and improve methods of extracting knowledge from learned results. Artificial intelligence has the potential to revolutionize the screening, diagnosis, and classification of glaucoma, both through the automated processing of large data sets, and by earlier detection of new disease patterns. In addition, artificial intelligence holds promise for fundamentally changing research aimed at understanding the development, progression, and treatment of glaucoma, by identifying novel risk factors and by evaluating the importance of existing ones.

In Vivo Noninvasive Measurement of Young's Modulus of Elasticity in Human Eyes: A Feasibility Study.

Abnormal ocular biomechanical properties may be important for understanding the risk of glaucoma. However, there are no clinical methods for measuring standard material properties in patients. In this feasibility study we demonstrated proof-of-principle for a novel method, ultrasound surface wave elastography (USWE), to determine the in vivo Young's modulus of elasticity of corneas in normal human eyes. In total, 20 eyes of 10 healthy subjects (mean age, 51.4±7.2; ±SD; range, 43 to 64 y) were studied. A spherical-tipped probe (3-mm diameter) was placed on closed eyelids and generated a gentle harmonic vibration at 100 Hz for 0.1 second. Wave speed propagation in the cornea was measured by USWE, and Young's modulus was calculated from the wave speed. Associations between Young's modulus and intraocular pressure (IOP), age, central corneal thickness, and axial length were explored by the Pearson correlation. Statistical significance was determined by using generalized estimating equation models to account for possible correlation between fellow eyes. Mean IOP was 12.8±2.7 mm Hg. Mean wave speed in the cornea was 1.82±0.10 m/s. Young's modulus of elasticity was 696±113 kPa and was correlated with IOP (r=0.57; P=0.004), but none of the other variables (P>0.1). USWE is a novel noninvasive technique for measuring ocular biomechanical properties. Corneal Young's modulus in normal eyes is associated with IOP, consistent with measurements in cadaver eyes. Further work is needed to determine elasticity in other ocular tissues, particularly the sclera, and if elasticity is altered in glaucoma patients.

Cataract surgery causes biomechanical alterations to the eye detectable by Corvis ST tonometry.

PurposeModern cataract surgery is generally considered to bring about modest and sustained intraocular pressure (IOP) reduction. However, the pathophysiological mechanism for this remains unclear. Moreover, a change in ocular biomechanical properties after surgery can affect the measurement of IOP. The aim of the study is to investigate ocular biomechanics, before and following cataract surgery, using Corvis ST tonometry (CST).Patients and methodsFifty-nine eyes of 59 patients with cataract were analyzed. IOP with Goldmann applanation tonometry (IOP-G), axial length, corneal curvature and CST parameters were measured before cataract surgery and, up to 3 months, following surgery. Since CST parameters are closely related to IOP-G, linear modeling was carried out to investigate whether there was a change in CST measurements following cataract surgery, adjusted for a change in IOP-G.ResultsIOP-G significantly decreased after surgery (mean±standard deviation: 11.8±3.1 mmHg) compared to pre-surgery (15.2±4.3 mmHg, P<0.001). Peak distance (the distance between the two surrounding peaks of the cornea at the highest concavity), maximum deformation amplitude (the movement of the corneal apex from the start of deformation to the highest concavity) and A1/A2 velocity (the corneal velocity during inward or outward movement) significantly increased after cataract surgery (P<0.05) while radius (the central curvature radius at the highest concavity) was significantly smaller following cataract surgery (P<0.05). Linear modeling supported many of these findings, suggesting that peak distance, maximum deformation amplitude and A2 velocity were increased, whereas A2 deformation amplitude and highest concavity time were decreased (after adjustment for IOP change), following cataract surgery.ConclusionCorneal biomechanical properties, as measured with CST, were observed to change significantly following cataract surgery.Trial registrationJapan Clinical Trials Registry UMIN000014370

In vivo ocular biomechanical compliance in thyroid eye disease

Background/aimsTo compare the ocular biomechanical properties in patients with thyroid eye disease (TED) and healthy participants using a non-contact Scheimpflug-based tonometer (CorVis ST).MethodsAll eyes were examined by slit lamp biomicroscopy,...

Influence of Age on Ocular Biomechanical Properties in a Canine Glaucoma Model with ADAMTS10 Mutation.

Soft tissue often displays marked age-associated stiffening. This study aims to investigate how age affects scleral biomechanical properties in a canine glaucoma model with ADAMTS10 mutation, whose extracellular matrix is concomitantly influenced by the mutation and an increased mechanical load from an early age. Biomechanical data was acquired from ADAMTS10-mutant dogs (n = 10, 21 to 131 months) and normal dogs (n = 5, 69 to 113 months). Infusion testing was first performed in the whole globes to measure ocular rigidity. After infusion experiments, the corneas were immediately trephined to prepare scleral shells that were mounted on a pressurization chamber to measure strains in the posterior sclera using an inflation testing protocol. Dynamic viscoelastic mechanical testing was then performed on dissected posterior scleral strips and the data were combined with those reported earlier by our group from the same animal model (Palko et al, IOVS 2013). The association between age and scleral biomechanical properties was evaluated using multivariate linear regression. The relationships between scleral properties and the mean and last measured intraocular pressure (IOP) were also evaluated. Our results showed that age was positively associated with complex modulus (p<0.001) and negatively associated with loss tangent (p<0.001) in both the affected and the normal groups, suggesting an increased stiffness and decreased mechanical damping with age. The regression slopes were not different between the groups, although the complex modulus was significantly lower in the affected group (p = 0.041). The posterior circumferential tangential strain was negatively correlated with complex modulus (R = -0.744, p = 0.006) showing consistent mechanical evaluation between the testing methods. Normalized ocular rigidity was negatively correlated with the last IOP in the affected group (p = 0.003). Despite a mutation that affects the extracellular matrix and a chronic IOP elevation in the affected dogs, age-associated scleral stiffening and loss of mechanical damping were still prominent and had a similar rate of change as in the normal dogs.

Ocular biomechanical measurements on post-keratoplasty corneas using a Scheimpflug-based noncontact device.

To analyse ocular biomechanical properties, central corneal thickness (CCT) and intraocular pressure (IOP) in post-keratoplasty eyes, as compared to normal subjects, with a new Scheimpflug-based technology. Moreover, biomechanical data were correlated with the size and age of the donor and recipient corneas. Measurements were conducted on 46 eyes of 46 healthy patients without any corneal pathology (age: 53.83±20.8y) and 30 eyes of 28 patients after penetrating keratoplasty (age: 49.43±21.34y). Ten biomechanical parameters, the CCT and IOP were recorded by corneal visualization scheimpflug technology (CorVis ST) using high-speed Scheimpflug imaging. Keratometry values were also recorded using Pentacam HR system. Scheimpflug measurements were performed after 43.41±40.17mo (range: 11-128mo) after the keratoplasty and after 7.64±2.34mo (range: 5-14mo) of suture removal. Regarding the device-specific biomechanical parameters, the highest concavity time and radius values showed a significant decrease between these two groups (P=0.01 and P<0.001). None of other biomechanical parameters disclosed a significant difference. The CCT showed a significant difference between post-keratoplasty eyes as compared to normal subjects (P=0.003) using the CorVis ST device. The IOP was within the normal range in both groups (P=0.84). There were no significant relationships between the keratometric data, the size of the donor and recipient, age of the donor and recipient and biomechanical properties obtained by CorVis ST. The ocular biomechanics remain stable after penetrating keratoplasty according to the CorVis ST measurements. Only two from the ten device-specific parameters have importance in the follow-up period after penetrating keratoplasty.

Effect of axial length on diurnal IOP in cataract patients without glaucoma.

To evaluate the effect of ocular biometrics on intraocular pressure (IOP) and diurnal IOP fluctuation in nonglaucomatous subjects. We examined 115 subjects from May to December 2007 in Kangbuk Samsung Hospital, Seoul, Korea. Intraocular pressure was measured with a Goldmann applanation tonometer in habitual positions every 2 hours from 9 am to 11 pm. Ocular biometric values including central corneal thickness, central corneal power, axial length, anterior chamber depth, and lens thickness were measured using an ultrasound biometer and keratometer and the refractive state was determined. Two hundred fourteen eyes of 115 patients were included in this study; the mean (±SD) IOP of all eyes was 12.33 (±2.55) mmHg. The mean (±SD) diurnal IOP fluctuation was 2.72 (±1.43) mmHg. Central corneal thickness was positively correlated with the mean IOP (Pearson correlation, r = 0.217, p = 0.002); however, there was no relationship between central corneal thickness and the diurnal IOP fluctuation. Axial length was not related to the mean IOP (Pearson correlation, r = 0.049, p = 0.476) and the diurnal IOP fluctuation (Pearson correlation, r = 0.058, p = 0.395). The mean IOP or diurnal IOP fluctuation was not related to any of the following values: central corneal power, anterior chamber depth, refractive error, lens thickness, or vitreous chamber depth. Central corneal thickness is significantly related to the IOP but may not affect diurnal IOP fluctuation. The axial length was not associated with IOP profiles in this study. Our results can contribute to a broader understanding of the effects of ocular biomechanical properties on the IOP profile.

Collagen: A potential factor involved in the pathogenesis of glaucoma

Numerous studies have been completed on glaucoma pathogenesis. However, the potential and controversial interaction between ocular biomechanical properties and the glaucomatous diseases process has received much more attention recently. Previous studies have found that collagen tissues gain mutation change in glaucoma patients. This study was conducted to determine the role of collagen in the biomechanics of glaucoma in humans. Its changes may be the result of mechanical modifications brought on by intraocular pressure (IOP) fluctuations. More importantly, biomechanics and genetic evidence indicate that the mutation of collagen may play a role in the process of glaucoma. Alteration of collagen in the outflow pathway may alter mechanical tissue characteristics and a concomitant increase of aqueous humor outflow resistance and elevation of IOP. The variations of collagen, leading to inter-individual differences in scleral and lamina cribrosa properties, result in different susceptibility of individuals to elevated IOP. Therefore, this study hypothesized that collagen mutations may be an original cause of glaucoma.

Repeatability of Ocular Biomechanical Data Measurements With a Scheimpflug-Based Noncontact Device on Normal Corneas

To analyze the repeatability of a new device measuring ocular biomechanical properties, central corneal thickness (CCT), and intraocular pressure (IOP) and to investigate these parameters and their correlations in healthy eyes. Three consecutive measurements were performed on each eye using the CorVis ST device (Oculus Optikgeräte, Inc., Wetzler, Germany). Ten specific parameters, CCT, and IOP were measured. Biometric data were recorded with IOLMaster (Carl Zeiss Meditec, Jena, Germany). This study comprised 75 eyes of 75 healthy volunteers (mean age: 61.24 ± 15.72 years). Mean IOP was 15.02 ± 2.90 mm Hg and mean CCT was 556.33 ± 33.13 μm. Intraclass correlation coefficient (ICC) was 0.865 for IOP and 0.970 for CCT, and coefficient of variation was 0.069 for IOP and 0.008 for CCT. ICC was 0.758 for maximum amplitude at highest concavity and 0.784 for first applanation time, and less than 0.6 for all other parameters. The device-specific data showed no significant relationship with age and axial length. Flattest and steepest keratometric values and IOP showed a significant correlation with the 10 device-specific parameters. The CorVis ST showed high repeatability for only IOP and pachymetric values. Single measurements are not reliable for the 10 device-specific parameters. The device allows for conducting clinical examinations and screening for surgeries altering ocular biomechanical properties with some form of averaging of multiple measurements.

Combining Ocular Response Analyzer Metrics for Corneal Biomechanical Diagnosis

To evaluate corneal biomechanical properties in non-keratoconic myopic eyes and to identify descriptors for improving the specificity of the Ocular Response Analyzer (ORA) (Reichert Ophthalmic Instruments, Depew, NY) testing for subclinical keratoconus detection. Observational case series of 52 non-keratoconic non-myopic eyes and 97 non-keratoconic myopic eyes (spherical equivalent < -5 diopters [D]) in dataset 1 and 87 non-keratoconic eyes and 73 eyes with subclinical keratoconus in dataset 2. Examination included corneal topography, tomography, and biomechanical testing with the ORA. Receiver operating characteristic curves and logistic regression were used to identify optimal combinations of biomechanical indices for keratoconus detection. Main outcome measures were corneal thickness-corrected hysteresis (DifCH) and resistance factor (DifCRF), the difference between these two (CH-CRF), and the diagnostic performance of their combinations. Compared to non-keratoconic non-myopic eyes, non-keratoconic myopic eyes with flat corneas (average corneal power < 44.0 D) had reduced DifCH (mean ± standard deviation, 0.11 ± 1.27 vs -0.79 ± 1.50, P < .01) and DifCRF (0.24 ± 1.16 vs -0.70 ± 1.59, P < .01) values, whereas non-keratoconic myopic eyes with steep corneas showed no difference. Keratoconic eyes exhibited lower DifCH and DifCRF values than non-keratoconic myopic eyes. Combinations of DifCH, DifCRF, and CH-CRF had increased specificity (> 80%) for subclinical keratoconus compared to the DifCRF index alone (71%). In biomechanical keratoconus screening, some non-keratoconic myopic eyes show altered ocular biomechanical properties and are identified as false-positive cases. The low specificity of DifCRF when dealing with these non-keratoconic eyes could be improved by considering additional biomechanical descriptors such as DifCH and CH-CRF, which seem to be indicative of the aforementioned biomechanical profile.