Repeatability of a novel corneal indentation device for corneal biomechanical measurement.

PURPOSE: The aim of the study was to evaluate the repeatability of a new device for measuring corneal biomechanics. METHODS: Twenty-nine normal subjects aged 20-28 years (23.4 ± 1.7 years) underwent measurements of corneal stiffness and tangent elastic modulus using a novel corneal indentation device. Corneal topography, axial biometry and Goldmann applanation tonometry were also performed during the visit. Subjects returned after about 1 week, at approximately the same time, and with the corneal biomechanics, corneal topography and Goldmann applanation tonometry measured again. Both the intrasession and intersession repeatability was assessed. RESULTS: Both the corneal stiffness and tangent elastic modulus demonstrated good intrasession repeatability (corneal stiffness: coefficient of variation = 7.32%, intraclass correlation coefficient = 0.75; tangent elastic modulus: coefficient of variation = 7.34%, intraclass correlation coefficient = 0.84). The mean modulus after normalised to normal intraocular pressure of 15.5 mmHg was 0.755 ± 0.159 MPa. There was no significant difference between the two visits (paired t-tests: p > 0.05). The repeatability [1.96 times the standard deviation (S.D.) of the intersession difference] of the corneal stiffness and the tangent elastic modulus was 0.0022 N mm(-1) and 0.197 MPa, respectively. CONCLUSION: The corneal indentation device has good intrasession and intersession repeatability. It has good potential to measure corneal biomechanics clinically, even at different corneal regions.

Comparative study of corneal tangent elastic modulus measurement using corneal indentation device.

The aim of this study is to examine the corneal tangent modulus measurement repeatability and performance of the corneal indentation device (CID). Twenty enucleated porcine eyes were measured and the eyes were pressurized using saline solution-filled manometer to 15 and 30 mmHg. Corneal tangent moduli measured using the CID were compared with those measured using high precision universal testing machine (UTM). The within-subject standard deviation (Sw), repeatability (2.77×Sw), coefficient of variation (CV) (Sw/overall mean), and intraclass correlation coefficient (ICC) were determined. The mean corneal tangent moduli measured using UTM and CID were 0.094±0.030 and 0.094±0.028 MPa at 15 mmHg, and 0.207±0.056 and 0.207±0.055 MPa at 30 mmHg, respectively, with a difference less than 0.13%. The 95% limit of agreement was between -0.009 and 0.009 MPa. The Sw, repeatability, CV and ICC of corneal tangent moduli measured by the CID were 0.006 MPa, 0.015 MPa, 4.3% and 0.993, respectively. The results showed that the corneal tangent moduli measured by the CID are repeatable and are in good agreement with the results measured by the high precision UTM.

Soft wearable contact lens sensor for continuous intraocular pressure monitoring.

Intraocular pressure (IOP) is a primary indicator of glaucoma, but measurements from a single visit to the clinic miss the peak IOP that may occur at night during sleep. A soft chipless contact lens sensor that allows the IOP to be monitored throughout the day and at night is developed in this study. A resonance circuit composed of a thin film capacitor coupled with a sensing coil that can sense corneal curvature deformation is designed, fabricated and embedded into a soft contact lens. The resonance frequency of the sensor is designed to vary with the lens curvature as it changes with the IOP. The frequency responses and the ability of the sensor to track IOP cycles were tested using a silicone rubber model eye. The results showed that the sensor has excellent linearity with a frequency response of ∼8 kHz/mmHg, and the sensor can accurately track fluctuating IOP. These results showed that the chipless contact lens sensor can potentially be used to monitor IOP to improve diagnosis accuracy and treatment of glaucoma.

Noninvasive measurement of scleral stiffness and tangent modulus in porcine eyes.

PURPOSE: We investigated an indentation technique to measure the scleral stiffness and tangent modulus of porcine eyes. METHODS: The scleral load-displacement responses were measured with a universal testing machine as a function of IOP in 15 porcine eyes ex vivo using a 5-mm diameter cylindrical flat-punch indenter. The scleral radius of curvature and scleral thickness were measured using a DSLR camera (Alpha 900) and a camera-mounted stereomicroscope (M205C), respectively. The relationships between scleral stiffness, tangent modulus, and IOP were examined. RESULTS: The mean local scleral radius of curvature and scleral thickness were 7.86 ± 0.49 and 1.03 ± 0.14 mm, respectively. The average scleral stiffness and scleral tangent modulus of porcine eyes were 0.13 ± 0.02 N/mm and 0.20 ± 0.04 MPa at 15 mm Hg, respectively. The scleral stiffness and scleral tangent modulus were correlated positively with IOP (scleral stiffness, 0.989 < r < 0.999, P < 0.001; scleral tangent modulus, 0.989 < r < 0.999, P < 0.001). CONCLUSIONS: The scleral indentation technique can provide a noninvasive approach to measure scleral stiffness and tangent modulus.

Individual-specific tonometry on porcine eyes.

Intraocular pressure (IOP) monitoring is important in the diagnosis and management of glaucoma. The measurement of IOP is affected by corneal properties, but the effect of corneal stiffness on IOP measurement is unaccounted for in pressure measurement instruments such as the Goldmann Applanation Tonometer (GAT). A new instrumented non-invasive indentation tonometry that can measure IOPIST, a corneal stiffness-corrected intraocular pressure is developed. The inter-individual corneal variations of 12 porcine eyes ex vivo were independently characterized; and their true intraocular pressure, IOPT's, were set using a manometer before indentation using the new indentation tonometry. Analyses of the load-displacement data showed that porcine corneal stiffness varied more than five times from 0.045 to 0.253N/mm. Analysis showed that, without individual stiffness correction, inter-individual variation of IOPGAT can vary up to 8mmHg from IOPT at 15mmHg; the error becomes larger at high IOPT. In comparison when corneal stiffness is accounted for, IOPIST has a significantly smaller error of 1.82±1.70mmHg for IOPT between 12 and 40mmHg than IOPGAT. The results showed that the new tonometry successfully accounted for inter-individual variations in IOP measurement.

Characterization of pressure reduction in coil-filled aneurysm under flow of human blood with and without anti-coagulant.

Filling aneurysms with embolization coils is a widely used part of the treatment to stop intracranial aneurysm from rupturing. However, the effect of coiling on aneurysmal pressure has not been established. In this study, the effect of intra-aneurysmal coiling on pressure reduction was characterized. Coil deployment in the aneurysm will disturb flow and may induce aneurysmal coagulation. These effects were experimentally examined in this study using silicone rubber saccular aneurysm models. Changes in aneurysmal blood pressure under pulsatile flow were characterized. With coils in the aneurysm, results showed that flow reduction of anti-coagulated blood in the aneurysm did not reduce aneurysmal pressure. Significant pressure reduction was observed only when the blood's coagulation ability is restored to normal. These results suggest that blood coagulation is pivotal to pressure reduction and concomitant with rupture risk reduction in treatments of aneurysm with coils.

Characterization of corneal tangent modulus in vivo.

PURPOSE: Intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) changes with individual's corneal properties, but the method to measure the in vivo corneal material properties to account for individual variation in GAT IOP is not available. In this study, a new method to measure the IOP-dependent corneal tangent modulus in vivo is developed to address this research gap. METHODS: Instrumented indentation and analysis were developed to measure the corneal tangent modulus. The validity of the method and procedure was verified using model silicone eye pressurized to different IOP. In addition, 15 porcine eyes and 3 rabbit eyes were tested using the corneal indentation at different set intraocular pressure and different indentation rates. RESULTS: The results from silicone eye showed that the measured tangent modulus is in good agreement with the standard silicone rubber modulus. The results on the porcine eyes and rabbit eyes showed that the method can be used to measure corneal tangent modulus in vivo in the human range of intraocular pressure from 10 to 40 mmHg. CONCLUSIONS: An indentation method to measure the corneal tangent modulus in vivo was developed, and the IOP dependence of the corneal tangent modulus was characterized. The developed indentation method provides a new means to measure the in vivo corneal tangent modulus to account for individual and pressure variations in measurement of intraocular pressure.

Effect of age-stiffening tissues and intraocular pressure on optic nerve damages.

Age-stiffening of ocular tissues is statistically linked to glaucoma in the elderly. In this study, the effects of age-stiffening on the lamina cribrosa, the primary site of glaucomatous nerve damages, were modeled using computational finite element analysis. We showed that glaucomatous nerve damages and peripheral vision loss behavior can be phenomenologically modeled by shear-based damage criterion. Using this damage criterion, the potential vision loss for 30 years old with mild hypertension of 25 mmHg intraocular pressure (IOP) was estimated to be 4%. When the IOP was elevated to 35 mmHg, the potential vision loss rose to 45%; and age-stiffening from 35 to 60 years old increased the potential vision loss to 52%. These results showed that while IOP plays a central role in glaucomatous damages, age-stiffening facilitates glaucomatous damages and may be the principal factor that resulted in a higher rate of glaucoma in the elderly than the general population.

Partial contact indentation tonometry for measurement of corneal properties-independent intraocular pressure.

Inter-individual differences in corneal properties are ignored in existing methods for measuring intraocular pressure IOP, a primary parameter used in screening and monitoring of glaucoma. The differences in the corneal stiffness between individuals can be more than double and this difference would lead to IOP measurement errors up to 10 mmHg. In this study, an instrumented partial-contact indentation measurement and analysis method that can account for inter-individual corneal difference in stiffness is developed. The method was tested on 12 porcine eyes ex vivo and 7 rabbit eyes in vivo, and the results were compared to the controlled IOPs to determine the method's validity. Analyses showed that without corneal stiffness correction, up to 10 mmHg of measurement error was found between the existing approach and the controlled IOP. With the instrumented indentation and analysis method, less than 2 mmHg of differences were founded between the measured IOP and the controlled IOP. These results showed that instrumented partial-contact indentation can effectively account for inter-individual corneal variations in IOP measurement.