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Lasers in Medicine: Healing with Light
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
What happens when laser light with a visible wavelength enters the eye (Figure 3.30b)? Since the lens and cornea of the eye are both transparent to these wavelengths, visible laser light can pass through them to the inside of the eye without being significantly absorbed, and hence without damaging either structure. The vitreous humor also transmits visible light. Because the eye is adapted to utilize as much light as possible, the retina strongly absorbs the visible laser light. Hence, for surgery on the retina, visible laser light can be shone directly into the eye without incisions, using external lenses plus the lens and cornea of the eye itself for focusing. Ultraviolet-wavelength light, such as that emitted by the excimer laser, is strongly absorbed by the cornea and lens, permitting laser surgery on these areas as well (Figure 3.30c).
Medical retina
Published in Mostafa Khalil, Omar Kouli, The Duke Elder Exam of Ophthalmology, 2019
The vitreous is the largest cavity, 4.0–4.4 mL in adulthood, of the eye that contains viscoelastic gel made of mostly water. Besides water, the vitreous humour contains mainly hyaluronic acid and type II collagen.
Ageing – the biology of growing old
Published in David Beales, Michael Denham, Alistair Tulloch, Community Care of Older People, 2018
Typical age-associated changes in the eye include slight shrinkage of the eyeball to produce long-sightedness, and alterations to the proteins of the cornea and the lens making them more likely to scatter light. The curvature of the cornea changes making astigmatism more common in the elderly. The lens also ‘hardens’ making it more difficult to focus light effectively. Loss of nerve cells from the retina might take place and metabolic changes in the retinal pigment epithelium could predispose to damage. The vitreous humor, which is partially composed of connective tissues, undergoes some deterioration which may eventually lead to a detachment of the retina. Consequently, old people usually need brighter conditions in order to be able to see well, although very bright lights can lead to problems of ‘dazzle’.
The Roles of Vitreous Biomechanics in Ocular Disease, Biomolecule Transport, and Pharmacokinetics
Published in Current Eye Research, 2023
Richard H. Luo, Nguyen K. Tram, Ankur M. Parekh, Raima Puri, Matthew A. Reilly, Katelyn E. Swindle-Reilly
The vitreous humor is a gel-like, soft, and transparent ocular tissue located between the lens and the retina, occupying 80% of the eye’s volume.1 The vitreous contains very few cells, mostly phagocytes, which remove unwanted cellular debris in the visual field, and hyalocytes, which turn over hyaluronic acid. However, the vitreous humor is also surrounded by many different types of cells, including fibroblasts, astrocytes, macrophages, white blood cells, lens epithelial cells, retinal pigment epithelial cells, and ciliary epithelial cells.2 Other noncellular components of the vitreous include water (98–99%) and a framework of collagen fibers and hyaluronic acid. The hyaluronic acid coils are interspersed in a network of collagen type II which is loosely crosslinked with collagen type IX bridges.3 The viscoelasticity of the vitreous is maintained by a balance between collagen, which provides elasticity, and hyaluronic acid, which provides viscosity, to the vitreous.
Topical drug delivery to the retina: obstacles and routes to success
Published in Expert Opinion on Drug Delivery, 2022
The space between the cornea and the iris, the anterior chamber, contains a clear aqueous liquid called aqueous humor [64]. It has relatively low viscosity and its chemical composition resembles that of blood plasma although it has lower protein content (and less drug protein binding). Its total volume in humans is between 200 and 300 μl. It is continuously secreted by the ciliary body and then drained by the trabecular meshwork and the uveoscleral pathway. The rate of aqueous humor turnover follows a circadian rhythm and varies between 1.5 (at night) and 3.0 µl/min (in the morning) [65]. The vitreous humor (about 4 ml) consists of mainly (99%) of water in a network of collagen and hyaluronic acid [66]. It is a rather firm hydrogel that helps to keep the retina in place and maintain the spherical shape of the eye. Dissolved drug molecules permeate the vitreous via an aqueous pathway between the collagen and hyaluronic acid mesh. After intravitreal injection, hydrophilic and high-molecular-weight drugs (e.g. proteins and peptides) are removed from the vitreous humor via the anterior route to the aqueous humor, while small lipophilic drugs that easily pass the retina are removed via the posterior route with the choroidal blood flow [34]. The half-life of a dissolved drug in the vitreous humor is typically less than 10 to 24 hours, where small molecules have shorter half-life than larger ones like proteins [8,67]. Consequently, intravitreal drug delivery systems most frequently consist of polymeric particles and inserts that can release controlled amounts of drugs for several weeks or even months.
Glutathione Improves the Antioxidant Activity of Vitamin C in Human Lens and Retinal Epithelial Cells: Implications for Vitreous Substitutes
Published in Current Eye Research, 2021
Nguyen K. Tram, Rayna M. McLean, Katelyn E. Swindle-Reilly
The vitreous humor is a fragile hydrogel composed of water, collagen fibers, and hyaluronic acid that occupies 80% of the volume of the eye.1 Due to its proximity to the lens and retina, the vitreous humor has important roles in protecting other ocular tissues. One of its functions is to create and maintain an oxygen gradient with a high level of oxygen near the retina and a low level of oxygen near the lens, protecting the lens from oxidative damage. Tissues in the eye are particularly susceptible to oxidative damage, especially the lens.2,3 Common ocular complications such as retinal detachments and tears typically require vitrectomy, or complete surgical removal of the vitreous. With the removal of the vitreous and replacement of the tissue with current vitreous substitutes such as saline solutions and silicone oil, the lens is no longer protected from oxidative damage that might cause cataract formation. In fact, up to 95% of patients require cataract extraction within 2 years post-vitrectomy.4–8 There is a clear need for a new generation of vitreous substitutes that can replace not only the physical roles but also the biochemical functions of the natural vitreous humor.