Cornea, Replacement

Cornea Replacement for corneal opacity due to congenital diseases, injury, etc. 

Other recently developed biomaterials will be used for quite different purposes in tissue engineering such as artificial pancreas and liver, artificial skin, nerve regeneration, gene therapy vascular grafts, cornea replacement and others.3... 

A study conducted by Wang et al. employed the use of PVA-BC produced by freeze-thawing as a composite material for use as an artificial cornea replacement. 

Biosynthetic corneas were used to restore vision in people with keratoconus, a condition that causes comeal scarring. These biosynthetic corneas replaced rejection-prone, scarce cadaver corneas. 

Injury (either physical or chemical) to the comeal endothelial cells has a marked efiect on occular function as these cells are responsible for maintaining the thickness and clarity of the cornea, yet they cannot be replaced if damaged. Immunohistochemical studies have revealed that enzymatic antioxidant defences, SOD, CAT and GSHPx, are similarly distributed in the corneal epithelium and endothelium (Rao etal., 1985 Attala et d., 1987, 1988). Other antioxidants include ascorbate, carotenoids and vitamin E (Fleath, 1962). 

Located on the edge of the transparent cornea, it maintains the junction with the opaque sclera. On the level of the epithelium, it is the transition between a multilayer scale-like corneal epithelium and a cylindrical conjunctival epithelium with two cellular bases, with continuity of the basement membranes. At the epithelial level, the cells of the comeal epithelium are gradually replaced by a conjunctival epithelium made of two layers of cylindrical cells accompanied by calyciform cells. 

This discovery soon led to the use of PMMA in cornea transplants and in the replacement of damaged skull bones. 

Soft lenses are made of hydrophilic hydrogel polymers that contain 36-74% water. Other plastics and copolymers are added to alter the physical characteristics of the lens. The diameter is 10.5-15.5 mm and the thickness at the center 0.03-20 mm. Soft lenses can correct most optical defects, including myopia, hyperopia, and astigmatism. Bifocal lenses are also available. They can be colored with either transparent hnes or opaque patterns to change apparent eye color or to mask malformations of the cornea or iris. They are available for daily, weekly, and twice-weekly disposable nse, 1-3 months frequent replacement, and annual replacement. 

None of the in vitro alternative eye tests has proven applicable as a valid replacement for the Draize eye irritation test or has been acceptable for regulatory purposes (Table 5), though some are considered either reliable or reproducible. The most frequently used test has been the ex vivo bovine cornea opacity and permeability assay. The newer human corneal equivalents system, an in vitro culture of immortalized human corneal cells that develops into... 

The Ex Vivo Eye Irritation test (EVEIT) is a self-healing system that makes use of rabbit corneas to determine corneal damage and healing [88], Results of a study on one test substance and an artificial tear replacement with increasing concentrations of up to 0.1 % benzalkonium chloride, showed a dose-dependent inhibition... 

Cornea Disease, trauma Epithelial e-PTEE, PMMA, PVA Repair, replacement Elasticity, refractive properties, transparency, curvature [14]... 

In the field of medicine, biomimetic solutions and products span the range from externally worn biomimetic devices that augment the function of sensory organs such as hearing aids and artificial corneas to implantable biomimetic devices. Implantable devices function as (i) simple physical replacements including silicone implants used in cosmetology, dentures and dental implants, and artificial... 

Other biomedical applications of polymers include sustained and controlled drug delivery formulations for implantation, transdermal and trans-cornealuses, intrauterine devices, etc. (6, 7). Major developments have been reported recently on the use of biomaterials for skin replacement (8), reconstruction of vocal cords (9), ophthalmic applications such as therapeutic contact lenses, artificial corneas, intraocular lenses, and vitreous implants (10), craniofacial, maxillofacial, and related replacements in reconstructive surgery (I), and neurostimulating and other electrical-stimulating electrodes (I). Orthopedic applications include artificial tendons (II), prostheses, long bone repair, and articular cartilage replacement (I). Finally, dental materials and implants (12,13) are also often considered as biomaterials. 

Since the simple aromatic alcohols had been shown (23, 24) to act as local anesthetics, Gray replaced the methyl group in the methyl ester of cocaine with aromatic alcohol residues. When tested on the cornea of rabbits, most of these compounds were better than cocaine, and some were superior when tested by subcutaneous injection. The pharmacological results are summarized in Table 3. 

W. Stone Jr., E. Herbert, Experimental smdy of plastic material as replacement for the cornea a preliminary report, Am. J. Ophthalmol. 36 (1953) 168-173. 

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