Healthcare and Medicine Reference
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To the thinning of pre-lens layer, a break-up may follow and an
evaporative-dehydration process begins that draws water through the
lens and out of the post-lens tear fi lm toward the environment, leading to
corneal staining (Fonn 2007). Blinking is the natural process that partially
rehydrates the eye for deposition of a new pre-lens fi lm on the lens surface;
after some time a periodic steady state of hydration is reached (Nichols et
al. 2005).
To evaluate the capacity of the pre-lens tear fi lm to maintain a moist
surface of the lens between eye blinks, the relationship between the PLTFTT
and the inter blink interval (IBI) can be measured. If the result of the
relationship is ≥1, then the pre-lens lacrimal fi lm is stable from one blink
to another, maintaining an uniform lens surface wetness; on the contrary, if
the results indicate a relationship lower than 1, the tear fi lm breaks before
the blink and causes a dry zone on the lens surface.
Some experiments (Guillon et al. 2002) demonstrated that the tear
composition infl uences the NIBUT. A low level of phospholipids resulted
in NIBUTs of about 4.5 s; levels higher than 0.06mg/ml were associated to
signifi cantly higher NIBUTs (about 6.5 s). Additionally, higher cholesterol
esters levels were detected in CL wearers with dryness symptomatology.
However, the pre and post lens tear fi lms originated by lens insertion
are thinner than the original three layer tear fi lm and result in a different
composition because chemical/physic interactions of the original
tear solution occur at the lens surface. Eyes appear less hydrated and
infl ammation may easily occur.
Contact Lens Interaction with Tear Film Components
After lens insertion, the tear fi lm is strongly destabilized and interactions
of the lens material with the water of the aqueous medium, with proteins
and lipids dissolved in the aqueous and lipid layers occur. Deposits on lens
strongly compromise the rigid lens optical performance; on the contrary, for
hydrogel lenses adequate blinking and hydration are very crucial factors
for observing clear optical images.
Lens interaction with water is a process mainly dependent on the lens
surface and less on the bulk; water lost through lens material dehydration
is relatively minor compared with water evaporated from the anterior lens
surface (Cedarstaff et al. 1983).
However, the hydration-dehydration mechanism is dependent on the
lens material. For hydroxyethyl methacrylate (HEMA) lens (soft or hydrogel
lenses), the surface-hydration mechanism was explained considering the
chemical composition and spatial arrangement of the lens polymer. It is
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