Tear Evaporation Considerations and Contact Lens Wear
Miguel F. Refojo
The tear film is essentially composed of three layers: (1) the outermost is the oily layer, which retards water evaporation; (2) the middle, thickest layer, consisting of an aqueous solution of salts, proteins, enzymes, and mucins; and (3) a mucoid layer, consisting of a mixture of glyoproteins, which forms the interface between the hydrophobic corneal epithelium and the aqueous layer of the tear film. A good tear film is essential for the tolerance of contact lenses.
Normally, a substantial amount of the water in the tear film is lost by evaporation. The evaporation rate of normal tears depends on temperature, relative humidity, and air flow over the eye as well as on the palpebral aperture and rate of blinking (Rolando and Refojo, 1983). The rate of complete to incomplete blinks is also a factor in tear evaporation. When a contact lens is placed on the eye, the tear film structure is altered on the surface of the lens and on the ocular surface near the lens. The lens disrupts the three-layer structure of the tear film, and as the lipid layer becomes discontinuous, the rate of tear evaporation increases (Cedarstaff and Tomlinson, 1983).
Contact lenses can be classified as rigid lenses, elastomeric lenses, and hydrogel lenses. All contact lenses alter the tear film, because they interact with the tear components and because they interfere with the lid-cornea congruity. Because of the physicochemical properties and design characteristics that determine the way each kind of lens is fitted on the eye, each type of lens interacts differently with the tear film.
If there is tear film over a contact lens, water from the film evaporates between blinks. When the tear film dries over a contact lens, solid residues
from the tear coat the lens surface. Lipid and protein deposits on lenses transform an originally wettable surface into a surface that the tear film does not wet uniformly. A nonwetting lens lacks the lubricating effect that the tear film provides for lid movement over the lens and will result in an irritated eye and lens intolerance.
Most contact lenses are not covered between blinks with a stable tear film (Doane, 1988, 1989). If the lens is a hydrogel lens, water evaporates from the lens itself. Also, water from the tear film that separates the lens from the corneal epithelium can pervaporate (permeate and evaporate) through high-water thin hydrogel lenses and from silicone elastomeric contact lenses (Refojo and Leong, 1981). The tear film that spreads over rigid contact lenses by the blink retracts immediately upon opening the eyelids. Tear film retraction is caused by the suction force of the tear meniscus at the lids' margins (Doane, 1988).
WATER EVAPORATION FROM HYDROGEL LENSES
A large proportion of the water in a hydrogel is not bound in the polymer network and therefore can be lost by evaporation (Refojo, 1976).
Effects of Lens Hydration
When two hydrogel lenses of low and high water content but of equal thickness are placed on a subject's eye, the lenses dehydrate. The dehydration is slower in low-water lens than in high-water lens. Thus, a high-water-content lens reaches a steady state of hydration on the eye more rapidly than does a lower-water-content lens. Furthermore, more water evaporates in equal time from the lens of higher hydration than from the lens of lower hydration (Andrasko, 1983).
Water evaporates, of course, from the lens surface, but as a hydrogel lens surface dries, more water diffuses from the bulk of the lens to its surface where evaporation continues. Water may also permeate from the tear film that separates the lens from the epithelium. Thus, given two hydrogel lenses of equal thickness but different hydration, water diffusion from the bulk of the lens to its surface will be easier in the high-hydration lens because the polymer network is looser and easier for the water to diffuse than in a tighter network lens. As a lens dehydrates, its base curve becomes steeper. High-water-content lenses dehydrate more than similar lenses of lower hydration under the same conditions, and therefore high-water-content lenses have a greater tendency to tighten on the eye than do lower-hydration lenses.
Effect of Lens Thickness
When lenses of equal hydration but different thickness are used under the same conditions of wear, more water, in absolute terms, evaporates from the thicker lenses in equal time than from the thinner lenses. However, the thinner lenses become proportionately more dehydrated than the thicker lenses and reach a steady state of dehydration on the eye more quickly than the thicker lenses (Andrasko, 1983).
Effect of Abnormally Fast Lens Drying
When the surface of a hydrogel lens dehydrates faster than the bulk moisture of the lens can diffuse to the surface, the polymer at the surface contracts and the lens roll up due to the stresses created in the lens network by the nonuniform swollen state of the lens. These dried, distorted lenses can be easily expelled from the eye by lid motion.
When a hydrogel lens is placed on the eye, it dehydrates to some extent, decreasing the oxygen transmissibility of the lens, increasing the contact lens refractive index, and decreasing the lens thickness and radii of curvature so that the lens becomes steeper. The power of the lens increases with dehydration in plus lenses and decreases in minus lenses.
Pervaporation of Tear Water Through Silicone Elastomeric Lenses
There are two mechanisms of permeation of fluids through membranes: bulk flow, in which the driving force is the difference in hydrostatic pressure of the permeant across the membranes; and activated diffusion, in which the driving force is the difference in concentration (or partial pressure in gases or vapors) of the permeant across the lens. The transmission of gases and vapors through contact lenses is by activated diffusion.
Pervaporation is a type of vapor permeation by activated diffusion in which the membrane separates a liquid from its vapor phase. Water pervaporation through a contact lens occurs when water from the tear film that separated the lens and the corneal epithelium penetrates the lens and evaporates from its surface. This is particularly relevant with silicone rubber lenses due to their extremely high water vapor permeability. When the surface of a silicone rubber lens is not covered by a tear film, moisture that separated the lens from the cornea epithelium pervaporates through the lens, and the lens will adhere to the epithelium (Refojo and Leong, 1981).
EFFECT OF TEAR WATER EVAPORATION ON EYES WITH RIGID CONTACT LENSES
The tear film on rigid contact lenses is very unstable due to their unusual hydrophobic surfaces. In addition, the tear film, which the blink may spread on the lens, is rapidly retracted by the suction force in the tear meniscus, which forms at the lid's edge. Depending on the fitting technique, a rigid lens will, upon opening the eyelids, be in contact with the upper or the lower tear meniscus. Tear evaporation is also a factor in tear drying on rigid lenses (Doane, 1988).
Because a rigid contact lens disrupts the lid-ocular surface congruity, the lids cannot spread the tear film on the ocular surface. The 3 o'clock and 9 o'clock positions are the sections of the cornea under the palpebral aperture that are more exposed to water evaporation and desiccation of the tissue. The result is a localized epithelial damage at the 3 and 9 o'clock positions of the cornea next to the lens.
HYDROGEL CONTACT LENS WEAR IN AIRCRAFTS
Due to comfort and stability of the lens in the eye, it seems at this time that the best choice of contact lenses for aircraft pilots would be hydrogel contact lenses.
It is well known that hydrogel contact lenses dehydrate in the eye and that the degree of dehydration of the lens on the eye depends on the type of contact lens (i.e., high or low water of hydration) and on the thickness of the lenses used. Other important factors on lens dehydration in the eye are the ambient relative humidity, air movement over the lens, palpebral aperture, and rate of blink.
Under normal conditions of wear at 18 percent relative humidity, Andrasko and Schoessler (1980) found that hydrogel contact lenses dehydrate in the eye to about 20 percent below normal hydration of the lens “in the bottle.” When a lens dehydrates, the lens develops a water imbibition pressure, which increases exponentially with the degree of dehydration. The water imbibition pressure for hydrogels, particularly of low hydration is very high, on the order of 4,000–5,000 mmHg for a hydrogel of equilibrium swelling 40 percent H2O, which has dehydrated only to about 5 percent below its normal hydration (Refojo, 1976). A steady state of hydration is obtained when the amount of water that evaporates from the lens equals the amount of water imbibed by the lens from the tears.
Even in the low-humidity conditions of aircrafts, hydrogel contact lenses have been well tolerated, without irritation or dislodgement (Nilsson and Rengstorff, 1979). Therefore, with no air drafts over the face of the pilot (i.e., air from the air conditioner blowing on the pilot's face), in a normal-blinking individual with normal tears, a hydrogel lens will not continue to
dry until it becomes “bone dry,” but rather the lens will acquire a steady state of hydration that is probably not too different from the steady-state hydration of lenses used under normally higher humidity conditions. Although research is lacking on the quantitative dehydration of hydrogel lenses in the eye under the low-humidity conditions of high-altitude aircraft, pilots using hydrogel lenses under these conditions do not seem to have major problems, such as intolerance, loss of lenses, or poor vision. Changes in lens fitting in subjects flying in a commercial aircraft cabin were most noticeable, as the relative humidity was sharply reduced at the beginning of the flight from the normal 47 percent to 11 percent relative humidity (Eng et al., 1982). It is important to recall here that parameter changes on dehydration of hydrogel contact lenses in the eye might be minimal with low-water lenses, but substantial parameter changes will take place in lenses of high hydration (Gundel and Cohen, 1986).
The critical variable in the dehydration of hydrogel lenses in all conditions of wear is blinking, and this is particularly true for lens wearers flying in an aircraft (Corboy and Tannehill, 1973). Patients with hydrogel contact lenses have been found to blink more frequently than patients without lenses (Carney and Hill, 1984). However, after the patients have adapted to the lenses, their blink pattern seems to depend on the visual task being performed (Pointer, 1988). Therefore, a pilot using hydrogel lenses must conscientiously blink (i.e., complete blinks) and frequently blink. Good blinking in subjects with hydrogel lenses is important not only to maintain lens hydration but also to maintain good visual acuity. A pilot needs good visual acuity at all times; therefore he or she must blink to maintain good visual acuity and at the same time will rehydrate the lenses.
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