TEAR FILM
Definition:
◦The value of tear fluid in clear cornea has been understood
since ages
◦The fact that blinking action of lids was essential for
spreading tears and maintaining a moist surface on anterior
portion of globe
FUNCTIONS OF TEAR FILM :
◦ 1.The most important functions of tear is to form an almost perfectly smooth optical surface
of the cornea by filling in and smoothening out small surface irregularly in corneal epithelium
◦ 2.It serves to keep the surface of cornea and conjunctiva moist
◦ 3.It serves as lubricant for preocular surfaces and lids thereby decreasing the frictional forces
that are generated during the constant blinking movement of eye lids
◦ 4.It transfers oxyzen from the ambident air to the cornea
◦ 5.It prevents infectIons due to presence of antibactireal substances such as lysozymes
betalysim lactoferrin immunoglobulins and other protiens
◦ 6.It washes away debris and noxious irritants .
◦ 7.It provides a pathway for white blood cells in case of injury
STRUCTURE OF TEAR FILM:
◦ The tear fluid in preserving a clear cornea the fact that the blinking action of lids was essential for
spreading the tears and maintainig a moist surface on anterior portion of globe
◦ Tear film has three layers
◦ 1.lipid layer
◦ 2.aqueous layer
◦ 3.mucin layer
LIPID LAYER:
◦
the outermost superficial oily layer derived from the secretion of
meibomian, zeiss and moll gland covers the entire free surface of the tear
fluid
◦ the thickness of the layer is about 0.1 micrometers depends on the
palpaebrial tissues width it increases when the lids are partially closed the
oily layer of tear film prevents the outflow of tears retards their
evaporation
AQUEOUS LAYER:
◦ THE MIDDLE LAYER of tear film is secreted by the lacrimal glands and the
accessory gland
◦ Main bulk of thickness of tear film is constituted by this layer
◦ Thickness of aqueous layer of pree corneal tear film is uniform over that cornea
is about is about 10 micrometers .
◦ It serves to provide atmospheric oxygen to the epithelium
MUCUS LAYER:
◦ The deepest stratum of the corneal tear film is the mucus coat
◦ Being a highly hydrated ,semi solid state ,the mucus layer is not strictly
apart of fluid film
◦ It plays a vital role is the stability of tear film
PHYSICAL PROPERTIES OF TEAR FILM:
+
◦ 1.Thickness of tear film
◦ 2.volume of tear film
◦ 3.Rate of tear film
◦ 4.Turn over rate
◦ 5.Refractive index
◦ 6.PH of tears
◦ 7.Osmotic pressure
MECHANISM OF TEAR FILM:
◦ Holly(1973) has describe a mechanism of tear film ruputure
◦ The steps involved in the break up of tears film as per hollys also known hollys
and lemp’s mechanism:
◦ First of all the tear film thins uniformly by the evaporation
◦ When the tear film is thinned out to same critical thickness , a significant number
of lipid molecules begin to be attracted by the mucin layer and migrate down to
this layer this migration process is enhanced if there is any spontaneous local
thinning
◦ When mucin layers on the epithelium is sufficiently contaminated by the lipid
migrating down from the top surface of the tear film .
TEAR FILM DYNAMICS :
◦ SECRETION OF TEARS:
◦ Tears are continuously secreted throughout the day by accessory lacrimal glands
◦ [basel secretion +reflex secretion]
◦ Basel secretion is throughout secondary to high temperature stimulation on both
◦ Reflex secretion occur response to sensation from cornea and conjunctiva
◦ Hyper lacrimation occur due to irritation sensation from cornea and conjunctiva
EVAPORATION FROM TEAR FILM:
◦EVAPORATION FROM TEAR FILIM IS ESTIMATED TO
BE ABOUT 10% OF PRODUCTION OF TEAR THAT
MAKES THE EVAPORATION TO 0.12 MICROLETRES
PER MINUTE SINCE TEAR PRODUCTION RATE IS 1.2
MICROLETRES PER MINUTE.
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