Film, tear

Clinical experience has shown that certain types of lens materials are more prone to deposit problems. In general, lenses with negatively charged moieties at the surface accumulate greater amounts of lysozyme, the principal tear film protein (10). The introduction and use of disposable lenses make these deposits and their clinical problems less significant. 

The mechanism responsible for improved delivery of lipophilic drugs has not yet been clarified. Absorption of liposomes by cells is unlikely. Adsorption to cells followed by slow release of the drug from the liposome, either via diffusion through the thin aqueous tear film or via direct partitioning from the membrane of the vesicle to the membrane of the cell, was proposed as a possible pathway. 

Previous ocular or eyelid surgery Loose sutures Previous corneal surgery Ocular Surface Disease Misdirection of eyelashes Abnormal lid anatomy or function Tear film deficiencies Ocular infection such as conjunctivitis or blepharitis Systemic Conditions Diabetes mellitus... 

The tear break-up time test assesses the stability of precorneal tear film. Break-up times of less than 10 seconds are considered abnormal. 

Regardless of the cause, the mainstay of treatment for dry eye is artificial tears. Artificial tears augment the tear film topically and provide relief. If a patient uses artificial tears more than four times daily, recommend a preservative-free formulation. Preservative-free formulations are also appropriate if the patient develops an allergy to ophthalmic preservatives. Artificial tears are available in gel, ointment, and emulsion forms that provide a longer duration of relief and may allow for less frequent instillation. Ointment use is appropriate at bedtime.30... 

The lipid layer, which consists of cholesterol esters, phospholipids, and triglycerides, prevents and regulates aqueous evaporation from the tear film. 

VHL Lee, DJ Schanzlin, RE Smith. (1986). Interaction of rabbit conjunctival mucin with tear protein and peptide analogs. In FJ Holly, ed. The Preocular Tear Film in Health, Disease, and Contact Lens Wear. Lubbock, TX Dry Eye Institute, pp 341-355. 

I Tapaszto. (1973). Pathophysiology of human tears. The preocular tear film and dry eye syndromes. Int Ophthalmol Clin 13 119-122. 

Microscopically, the cornea shows a rather simple and multilayered structure that can be divided into six layers the epithelium, basement membrane, Bowman s layer, stroma, Descemet s membrane, and endothelium. The corneal tissue consists of three different cell types epithelial cells, keratocytes (corneal fibroblasts), and endothelial cells. The outermost corneal surface is covered with the preocular tear film, which is functionally associated with the cornea. The epithelial surface must be kept moist and smooth, a role played by the tear film in conjunction with a spreading function of the eyelids during blinking motions. Furthermore, the tear film provides a protection against infectious agents that may gain access into the eye. 

At the corneal surface, two to four layers of polygonal, extremely flat, and terminally differentiated superficial cells can be found. The cell diameters are 40-60 /xm with about 2-6 /xm thickness. The corneal surface is populated with microvilli and microplicae, resulting in surface area enlargement. These structures are associated with the tear film. Superficial corneal epithelial cells are... 

Chow CYC, Gilbard JP. Tear film. In Krachmer JH (ed.) Cornea Fundamentals of Cornea and External Disease, Vol 1. Mosby, St. Louis, pp 49-60 (1997). 

A number of ocular surface disorders collectively termed as Dry Eye Syndromes have also been associated with the conjunctiva. For example, a deficiency and/or imbalance in compositions of the tear film is often found on the ocular surface during keratoconjunctivitis sicca. Since the conjunctiva plays a direct role in the maintenance of the tear fluid stability via secretion of mucin [1] by its resident goblet cells [4] and basal fluid secretion driven by electro-osmotic gradients across the tissue [3], the conjunctiva is a well deserved, but not intensively studied, target of interest in research efforts aimed against combating Dry Eye Syndromes. 

The first stmeture encoimtered by an ocular dosage form is the precorneal tear film, consisting of three layers ... 

The case of the camera is the skull. The forehead and brow partially protect the eye that is enclosed in the bony orbit. To facilitate stereoscopic vision, the adult eyes are positioned such that the focus is able to converge to achieve a binocular view between 20 cm and infinity. The eyelids provide partial protection to sudden changes of light and also serve to compress and spread the tear film maintaining a clear optical surface. Fibrous tarsal plates provide the framework for the movable folds of modified skin (Figure 23.1a and Figure 23.1b). 

The mucins of the eyelid margin trap particulate matter, which on squeezing of the lids move the material onto the base of the eyelashes. Eyelid closure results in compression of the film, temporarily increasing the thickness of the tear film mucins in the tears must support changes in dimensions and anchor the hydrated film to the surface. The elasticity of the mucins, provided by uncoiling regions in the structure coupled with the binding of water, maintains the hydration of the cornea. 

The synchronized movement of the eyelids spreads the precorneal tearfilm across the cornea and pushes it toward the nasolacrimal duct. Precorneal drainage is quite efficient. An aqueous instilled dose leaves the precorneal area within 5 min of instillation in humans. Most of the drug absorbed by transcorneal penetration, without retention modification, is spread across the cornea by the eyelids in the first minutes postdosing. In the precorneal space transcorneal penetration is limited by solution drainage, lacrimation and tear dilution, tear turnover, conjunctival absorption, and the corneal epithelium. Slowing down tear film turnover has well-established benefits to topical ocular drug delivery. 

Chitosan has been shown to increase precorneal drug residence times. The cationic chitosan slows tear drainage by increasing viscosity and by mucoadhesion with the negatively charged mucin. Up to a threefold increase of the corneal residence time has been achieved by the addition of chitosan to topical vehicles [5]. Carbomer gels at 0.3% have also been shown to be effective in prolonging the tear film break-up time [6]. Hyaluronic acid has been reported to... 

The sclera and the cornea are the toughest and outermost layers of the eye and resist the normal internal pressure of 13 to 19 mmHg. This intraocular pressure (IOP) gives the eye its shape and maintains its dimensions that are necessary for sharp vision. The sclera covers 5/6 of the eye s surface and the cornea the remaining 1/6. Although the principal structural element of both tissues comprises of type 1 collagen fibers, differences in size and orientation of the fibers, degree of hydration, and presence of mucopolysaccharides are responsible for differences in transparency. The avascular cornea receives nourishment from the tear film, the aqueous humor, and the limbal vessels. In contrast, the sclera is vascularized and is supplied by several blood vessels, particularly in the uppermost layers (episclera). 

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