The aqueous humor is a clear fluid located at the front part of the eye. The aqueous humor also drains out any excess material and waste from the eye. The vitreous fluid, or vitreous humor, is a colorless, transparent, gel-like material. Vitreous humor is located between the retina and the lens. It is mainly composed of water with additional levels of protein, salts electrolytes , sugar glycosaminoglycan and collagen.
It is a water-like fluid between the cornea and iris. The aqueous humor must enter and be drained from the eye at an equal rate, exiting the eye from a structure called the trabecular meshwork. This tissue lets fluid drain through it. The aqueous humor needs to flow free continuously.
Interference with the proper flow can lead to intraocular pressure, which can result in optic nerve damage and vision impairment. Regular eye check-ups can help your doctor detect any issues with the aqueous humor and curate an adequate remedy. The better your health, the more you can reduce the risk of intraocular pressure inside your eye. Some helpful tips:. Even if you take excellent care of your eye health, increased intraocular pressure is linked with age, conditions like diabetes, a family history of glaucoma, and certain ethnicities.
This is a type of liquid that is also clear. It mostly consists of sugar, salt, collagen, hyaluronic acid, and water. The main difference between the vitreous humor and the aqueous humor is that there is a set amount of the vitreous humor in your eye, and it does not move freely about between the two chambers. It remains in the posterior chamber. In children, the vitreous humor is milky and has a gel-like consistency.
The gel-like liquid becomes clearer as you grow older, and in fact begins to liquefy. People over the age of 50 may become affected by vitreous detachment as the vitreous humor dwindles. This causes the vitreous humor to change in consistency and become fibrous. Symptoms are:. A clear fluid at the frontal part of the eye, the aqueous humor provides nutrients to parts of the eye that do not have blood supply. The clear aqueous humor also drains out at an equal rate to remove waste.
It makes sure your eye is the right shape and maintains the right amount of pressure in the eye at all times. This clear fluid is located between the front part of the eye and the retina.
It does not become replenished if some is lost. It plays a more important role in the early years of life, providing structure to the eye and protecting the retina. The aqueous humor is constantly drained and replenished, but problems with the system that allows the aqueous humor to flow properly can increase intraocular pressure. Increased pressure in the eye can damage your optic nerve and lead to vision loss. Glaucoma is often associated with imbalances in the aqueous humor.
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In recent years, shear rheometry has become a common technique for testing the vitreous humor, since this technique can report various rheological properties pertaining to the viscoelasticity of the vitreous gel. In particular, the rheological parameters of interest are storage modulus and loss modulus, which describe the solid and viscous qualities of a material, respectively.
Due to the scarcity of human tissues, only a handful of studies reported human vitreous mechanical data. Previous data on the vitreous humor have been commonly reported on porcine, bovine, ovine, or rabbit eyes. While the vitreous is well-known to liquefy with age, age-related changes in the mechanical properties of the vitreous remain elusive. Colter et al. The authors claimed that the storage and loss moduli of the adult ovine vitreous were lower than those of the infant vitreous and higher than those of the premature vitreous.
However, there were no significant differences between the ages tested. Additionally, the vitreous was tested whole and assumed to be homogeneous. Since the hallmark of vitreous liquefaction is phase separation of the vitreous humor, which results in pockets of liquid inside the vitreous, a different approach to mechanically test the vitreous could be used to capture its heterogeneity.
In this study, both the solid and liquid phases of the vitreous were mechanically tested and correlated to changes with age.
This study utilizes a technique common in characterizing the material properties of hydrogels, shear rheometry, allowing for a broader comparison of our results to previous studies. This study also fills in this scientific gap by using this technique to quantitatively measure human vitreous rheological properties, with an emphasis on the differences between vitreous samples from young and old tissue samples on both the solid and liquid phases of the vitreous.
The sclera was cut along the nasotemporal meridian around the posterior end avoiding the cornea. Blunted surgical forceps were used to remove the vitreous through the corneoscleral shell, and the vitreous was immediately placed on ice.
The vitreous was extracted from the porcine eye within 2 h post mortem and tested immediately, after 2 h, or after 4 h same sample to determine the effect of storing dissected vitreous on its material properties. A number of intact eyes were reserved, dissected, and tested 24 or 48 h later, to determine the effect of post-mortem time on the material properties of the vitreous. Vitreous samples were collected using the same aforementioned technique for porcine eyes within 48 h post mortem and tested within 4 h after dissection.
Approximately 0. A mm parallel plate geometry was lowered onto the vitreous sample to a working gap of 1 mm, which was determined to provide good contact between the geometry and the vitreous sample without damaging the vitreous zero normal force.
To prevent the slippage effect, grit sandpaper was applied to the test geometry of the rheometer Yoshimura and Prud'homme, ; Sharif-Kashani et al. The liquid portion of the vitreous humor sample was also tested using the parallel plate geometry with a 0.
The solid phase of the vitreous was the cohesive portion of the tissue that could be picked up with forceps, while the liquid phase could not be picked up with forceps and was transferred to the testing stage using a dropper. Figure 1. Left Intact human vitreous humor sample with the lens, ciliary body, and iris tissues attached.
Right Experimental set-up. The vitreous sample is sandwiched between the parallel plate geometry and testing stage.
A humidifying chamber filled with phosphate buffered saline was used to prevent dehydration of the vitreous sample. First, amplitude sweep tests were conducted on the solid phase of the vitreous at frequencies of 0.
Thereafter, frequency sweep tests with strain amplitude of 0. It was found that once the frequency passes 1 Hz, the inertia effect of the geometry dominates the response of the vitreous sample, resulting in an artificial increase in modulus of the vitreous, which has been previously reported Nickerson et al.
Therefore, only data from frequencies below 1 Hz were reported. Creep tests were conducted at constant shear stress of 1 Pa for 1, s to cover the different viscoelastic responses as described in Sharif-Kashani et al.
Compliance was calculated as strain over stress and fitted to a viscoelastic spectra model with two Voigt-Kelvin elements and a dashpot in series as described in Sharif-Kashani et al. The mathematical expression of this model is. In a few cases, experimental or sample artifacts gave values that were distinctly different from the majority of the sample values.
In these cases, the outliers were eliminated as previously described Havilcek and Crain, ; Lee et al. Briefly, the mean and standard deviation of each variable were calculated. This process was repeated until all data points were within the 2. This procedure removed no more than two data points per variable, except for one case where 4 outliers were removed Supplemental figure.
It was determined that the presence or absence of these outliers does not affect the results presented, therefore the outliers were eliminated. Statistical analyses were implemented with Minitab software version Spearman correlations were used to determine the correlation between rheological properties and age. The difference between young and old samples was set at 65 years of age old age as defined by the American Heart Association.
A two-tailed Student's t -test was used to compare rheological data from the liquid component of the young and old human vitreous. One-way ANOVA, with post-hoc pairwise comparison using Tukey test, was used to compare the rheological data of the solid component of human vitreous samples from both age groups and porcine vitreous samples.
ANOVA was also used to compare the rheological results of porcine eyes from the post-mortem and post-dissection time studies. The null hypotheses stated that there is no difference between the groups for each test. An alpha value of 0. Post-mortem time and post-dissection time do not have an effect on the mechanical properties of the porcine vitreous Figure 2.
No statistically significant differences were detected in these studies. The porcine moduli and viscosity did not significantly change if tested within 48 h post-mortem and within 4 h after extraction compared to fresh samples. All human samples were tested within 48 h post-mortem and 4 h of extraction. Figure 2.
The data were collected at 1 Hz and 0. No significant differences were detected when the vitreous samples were harvested within 48 h post-mortem and used within 4 h after extraction compared to fresh samples.
Figure 3 shows the results from amplitude sweep experiments. The complex, storage, and loss moduli and the viscosity were larger at higher frequency 1 Hz compared to those at lower frequency 0.
As a result, 0. The moduli, viscosity, and loss tangent appeared to be less dependent on strain amplitude at 1 Hz frequency. Figure 3. Amplitude sweep results of the solid phase of human vitreous samples at frequencies of 0.
The complex modulus A and viscosity B are larger when the amplitude sweep experiment was done at a higher frequency. Figure 4 shows the results from frequency sweep experiments.
As the frequency increased, the moduli increased while the loss tangent decreased, suggesting that the vitreous behaves more as a solid body with increasing frequency. The complex modulus ranged from 1 to 7 Pa for the solid phase of the vitreous and from 0. The complex viscosity ranged from 1 to 10 Pa-s for the solid phase and from 0.
The loss tangent ranged from 0. No significant differences were found between the right and left eyes 9 OD vs. Figure 4. Frequency sweep results of the solid and liquid phases of the human vitreous samples at 0. A Complex modulus of solid and liquid phases of vitreous. B Complex viscosity of solid and liquid phases of vitreous.
C Moduli and loss tangent of solid vitreous. D Moduli and loss tangent of liquid vitreous. The moduli and viscosity of the solid vitreous are higher than those of the liquid vitreous. As the frequency increases, the moduli increase while the viscosity and loss tangent decrease, suggesting that the vitreous behaves more as a solid body with increasing frequency.
Figure 5 shows the Spearman correlation analysis of human vitreous rheological properties as a function of age. The closer rho is to zero, the weaker the correlation between the factors. This figure looks at the effects of age at high and low frequencies on the properties of the vitreous humor. Complex modulus, storage modulus, and complex viscosity were found to positively correlate with age at low frequency 0.
White boxes represent no significant correlation. In this study, loss modulus did not correlate with age. Figure 5. Rheological properties correlation analysis. A Complex modulus, storage modulus, loss modulus, and complex viscosity were compared against age using Spearman correlation analysis. Left: y-axis variables complex viscosity, loss modulus, storage modulus, complex modulus ; Bottom: x-axis variable age ; Top: sample phase solid vs.
Data were obtained at 0. The movement of the geometry at high frequencies can cause an inertial effect that artificially increases the measured modulus of the vitreous, which has been shown in previous studies, so only data from frequencies below 1 Hz are shown. The parameters from solid vitreous are positively correlated with age at low frequency, while those from liquid vitreous are negatively correlated with age at high frequency.
Loss modulus did not correlate with age. B Exponential fit to data with significant Spearman correlations. Figure 6 further analyzes the correlations shown in Figure 5. For the liquid component of the vitreous at high frequency, the rheological results from the old human samples were significantly smaller than those from the young human samples.
Due to the absence of liquid separation in the porcine vitreous samples, the liquid portion of the vitreous could not be tested. Figure 6. The data were collected at 0.
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