In vivo tear-film thickness determination and implications for tear-film stability

PURPOSE

Previous measurements of tear-film thickness in vivo are limited and cannot be easily applied in a clinical setting. A novel technique to measure tear-film thickness indirectly is introduced here, requiring only a slit lamp, video camera, and computer. A recent fluid mechanical theory relates tear-film thickness h, to the tear meniscus radius R, tear surface tension or, tear viscosity mu, and upper lid velocity U. This theory yields the result that h/R = 2.12 (microU/sigma)2/3. All parameters except h/R are taken as known physical constants, and R was measured for each subject, allowing the above equation to establish h. Tear-film breakup was also evaluated and correlated with tear-film thickness.

METHODS

A clinical study was performed in which aqueous tear-film thickness was determined for 45 subjects, including 24 non-lens subjects, 15 hydrogel contact lens wearers, and 6 RGP lens wearers. R was measured by instilling fluorescein dye in the form of an eyedrop and videotaping the tear meniscus in profile. Tear-film breakup was videotaped through the ocular port of the slit lamp and evaluated based on a severity scale.

RESULTS

Aqueous tear-film measurements are in the same range as literature values, with most measured values falling between 6 and 12 microm. Average tear-film thicknesses for non-lens, hydrogel, and RGP subjects are 10.4, 6.5, and 5.8 microm, respectively. Tear-film breakup is most severe in subjects with thin tear films, especially in contact-lens wearers.

CONCLUSIONS

Tear-film thickness is an important parameter that varies among individuals. These variations correlate with differences in tear-film stability.

Non-steady-state diffusion in a multilayered tissue initiated by manipulation of chemical activity at the boundaries

Diffusion of ionic and nonionic species in multilayered tissues plays an important role in the metabolic processes that take place in these tissues. To create a mathematical model of these diffusion processes, we have chosen as an example hydrogen-bicarbonate ion pair diffusion within the mammalian cornea. This choice was based on the availability of experimental data on this system. The diffusion coefficient of the hydrogen-bicarbonate ion pair in corneal stroma and epithelium is calculated from the observed change in pH in the stroma when conditions at the corneal anterior epithelial surface are changed while the posterior surface is continually bathed with a Ringer's solution in equilibrium with a CO2-gas air mixture. Matching experimental results to a mathematical model of the cornea as a two-layer diffusion system yields, at 37 degrees C, a diffusion coefficient of the hydrogen-bicarbonate ion pair of 2.5 x 10(-6) cm2/s in the stroma and 0.4 x 10(-6) cm2/s in the epithelium. Application of the Nernst-Einstein equation to these data gives the following diffusion coefficients in the two layers: 1) stroma, D(H+) = 11.8 x 10(-6) cm2/s; D(HCO3-) = 1.5 x 10(-6) cm2/s; and 2) epithelium, D(H+) = 1.9 x 10(-6) cm2/s; D(HCO3-) = 0.22 x 10(-6) cm2/s.

Comparative study of some physiologically important properties of six brands of disposable hydrogel contact lenses

PURPOSE

This study examines the water content, refractive index, oxygen permeability (Dk), oxygen transmissibility (Dk/L), and lens thickness profile for six brands of disposable hydrogel contact lenses.

METHODS

Refractive index was measured with a Bausch & Lomb Abbe Model 3L laboratory refractometer. Water content was read from the scale in the refractometer eyepiece. Oxygen permeability and transmissibility were measured by a Rehder polarographic unit using the procedure described in International Standards Organization (ISO) Standard 9913-1.2. Lens thickness profiles were measured on a Rehder Electronic Thickness Gauge equipped with a rotating ball anvil.

RESULTS

Water content ranged from 38% to 62%. Lower water content lens materials had an oxygen permeability of about 7 x 10(-11) (cm2/sec)(mL O2/mL x mm Hg), whereas the permeability for the higher water content materials ranged from 20 x 10(-11) to 30 x 10(-11).

CONCLUSIONS

In theory, it is possible to design contact lenses of approximately the same transmissibility-when starting with materials of different permeabilities-by adjusting lens thickness. This was not the case with the six brands studied here. Some brands of contact lenses had significantly higher transmissibilities than others, but all had FDA approval. The clinical significance of different contact lens transmissibilities based on different definitions of thickness is discussed.

Deposition and Thinning of the Human Tear Film

The exposed part of the eyeball is covered by a tear film, which is vital for the proper function of the eye. The film thickness has been measured to be roughly 10 μm; however, how a tear film of this thickness is generated has not been clearly explained. It is proposed that the tear film is deposited analogous to a coating process by the rising meniscus of the upper lid during a blink. A coating model is formulated that not only predicts correctly the film thickness, but also captures the postblink lipid spreading commonly observed in experiments. A deposited tear film thins rapidly near the tear meniscus surrounding the film. Numerical simulation of this thinning reveals that the minimum film height obeys a power law. When the minimum height reaches the effective range of dewetting intermolecular forces, the film ruptures. The thinning time therefore defines a breakup time, and the thinning law shows explicitly how this breakup time is related to tear viscosity, surface tension, meniscus radius, and initial and final film thicknesses. The calculated breakup time agrees with those observed experimentally.

New physiological paradigms to assess the effect of lens oxygen transmissibility on corneal health

PURPOSE

New paradigms are needed to allow an assessment of the physiological performance of a contact lens. Oxygen transmissibility (Dk/L) is not adequate in several respects. The oxygen supply to the cornea cannot be calculated from Dk/L because the oxygen tension of the cornea-contact lens interface is not known. Also, oxygen delivery to the cornea is not a linear function of Dk/L. Doubling the Dk/L of high Dk/L lenses does not double the oxygen supply to the cornea.

METHODS

We propose a new term, Biological Oxygen Apparent Transmissibility (BOAT). Oxygen supply to the cornea through a lens of known BOAT can be calculated by multiplying BOAT by the oxygen tension in the environment at the anterior surface of the lens. Oxygen supply to the cornea through the lens is linearly related to BOAT. The numerical values for BOAT of any lens of known Dk/L can be read from a graph or calculated from a simple formula. A second paradigm introduced here concerns point-to-point oxygen transmissibility of an optically powered contact lens. We propose that the health of epithelial cells is governed by the oxygen transmissibility of that portion of the contact lens immediately anterior to any small group of epithelial cells. Finally, we propose that, whereas epithelial cell health is governed by the oxygen transmissibility at a point on the lens, the swelling of the cornea is governed by the total oxygen supply to the epithelium. This total oxygen supply is controlled by the harmonic average oxygen transmissibility of the lens.

CONCLUSIONS

These new paradigms suggest that the clinician who is concerned about the patient's epithelium should choose a lens in which the portion with the lowest oxygen transmissibility (i.e., the thickest portion) supplies an adequate supply of oxygen to the epithelial cells. If corneal swelling is the clinical concern, then a lens with the highest harmonic average oxygen transmissibility should be chosen.

Presbyopic drivers' vision through a convex rear view mirror

BACKGROUND

Monovision is a commonly used modality for the presbyope who wants to wear contact lenses. If the add for near vision is in the left eye, driving an automobile creates a circumstance whereby the lens wearer cannot obtain a clear image in that eye of distant objects seen in the driver's side outside rear view mirror.

METHODS

We have discovered that a stick-on convex mirror eliminates this problem by neutralizing either some or all of the power of the add.

RESULTS

The driver wearing an add in the left eye can then obtain a clear image in that eye of objects behind and to the side of the automobile.

A measurable subjective patient response to the dry eye

Patients with "dry eyes" are often poor prospects for successful contact lens wear. Screening out these patients could improve contact lens practice. Currently used diagnostic tests, such as the Schirmer strip test or the time for tear film breakup, are not very effective in identifying the dry eye patient who may have trouble wearing a contact lens. A new test is described here that asks the patient to report the time to disappearance of the pillar-like flare image seen above a disk light source. A flare image is created by the tear prism in the lower lid margin. This test, when it was applied to a small group composed of young, middle-aged, and elderly adults, was able to correlate rapid disappearance of the flare image with the presence of typical dry eye syndrome complaints in two subjects.

Contact lenses affect corneal stromal pH

Recent studies suggest that corneal acidosis may alter corneal structure and function. We determined whether the range of oxygen transmissibility (Dk/L) provided by a hydrogel lens could affect corneal stromal pH. Stromal pH was measured using a noninvasive fluorometric technique under both open- and closed-eye conditions on subjects who wore hydrogel lenses made of the same material but with different lens powers. Under closed-eye lens wearing conditions, central stromal pH was reduced substantially and there was no relation between degree of acidosis and lens Dk/L. Under open-eye lens wearing conditions, central stromal pH was also reduced but the degree of acidosis was dependent on lens Dk/L. These results suggest extended wear of hydrogel lenses can produce marked decreases in stromal pH which could remain reduced even after the eyes are opened.

Areal differences in oxygen supply to a cornea wearing an optically powered hydrogel contact lens

If there is little lateral diffusion of oxygen in an optically powered hydrogel contact lens, then point-to-point differences in the thickness of the lens can result in different areas of the cornea receiving different amounts of oxygen. Specifically, areas under the thicker portions of the lens will receive less oxygen. Cells in these oxygen-deprived areas may produce metabolic products that spread laterally in the cornea resulting in cornea-wide edema. The fact that most of a contact lens transmits adequate oxygen to the cornea does not lead to satisfactory lens performance, if some part of the lens is too thick. We address the problem of lateral diffusion of oxygen in a contact lens and describe the methods for quantifying the point-to-point thickness of optically powered hydrogel lenses. Areal differences in oxygen supply to the cornea are found to be almost totally dependent on optical power; water content and lens design have far less effect.

Influence of polarographic cathode diameter on measured oxygen transmissibility of hydrogel contact lenses with optical power

The oxygen transmissibility of a contact lens is defined as its oxygen permeability (Dk) divided by its thickness (L). Transmissibility can be obtained from separate measurements of Dk and L, or from a single measurement of oxygen flux through the lens (as for example by the polarographic method). Dk/L of hydrogel contact lenses with optical power measured by the polarographic method was compared with Dk/L calculated from separate measurements of Dk and L. Polarographic sensors of different cathode diameters were used to show the effect of the area over which the flux is measured on the observed Dk/L. Dk/L from oxygen flux measured by the polarographic sensor was found to be a function of the optical power of the lens and cathode diameter. Dk/L calculated from separately determined Dk and L for optically powered lenses was found to be a function of Dk and the choice of L (central or average).

Architecture of the lid-cornea juncture

The details of the lid-cornea-tear film juncture influence the encounter of the lid with the top edge of an interpalpebrally fit rigid gas permeable contact lenses. During blink, fluid circulation at right angles to the globe can be hypothesized in the tear prism of the upper lid. This circulation prevents formation of a stratified tear layer, although lipid may always be present at the air interface. Water soluble or dispersed materials will be uniformly distributed in the tear film of the open eye. The collection of tears in the upper margin during a blink is shown to favor movement of tears away from the center of the upper lid. Tears on the temporal side of the center move along the upper lid to the fornix and then along the lower lid to the lower punctum. Tear fluid on the nasal side of center moves directly to the upper punctum.

Polarographic oxygen permeability measurement of silicone elastomer contact lens material

The flexible silicone elastomer is a contact lens material of substantial potential, primarily because of its high theoretical oxygen permeability (Dk). Review of the previous literature indicates some difficulty in precisely determining the Dk of the silicone elastomer, and quantification of this permeability value ranges from 50 to 340 x 10(-11) cm2 ml 02/sec ml mmHg by various techniques. (The exponential term 10(-11) and the units of Dk will be omitted hereafter in this text). We herein report use of the single-chamber polarographic technique, with edge and boundary corrections, to arrive at a value of 190 with a standard deviation (SD) of 79 for the Dk of one silicone elastomer contact lens material. The excessive SD suggests that an improved method to evaluate contact lens materials with Dk values in excess of 30-50 should be determined.

Observations of tear film break up on model eyes

Models of the human eye's anterior surface were constructed from polymethylmethacrylate. Some models had a rigid gas permeable contact lens cemented to the corneal apex; others had strips of plastic or metal cemented at the location of the lids in the open human eye. When the level of water was lowered in the bath surrounding the upward-gazing eye model, tear film break up could be observed. Covering the model with mucin (from saliva) changed the pattern of tear film break up. On the mucin-covered model eye, the tear film break up resembled break up observed in the human eye. These studies on model eyes suggest that tear film break up occurs when tension in the tear film becomes greater than the tensile strength of the film. The mucin layer reduces the thickness of the tear film at break up and the thickness of the retreating film.

The Dk project: an interlaboratory comparison of Dk/L measurements

The oxygen transmissibilities (Dk/L) of a set of 48 contact lenses made from 8 different materials were measured by 4 laboratories. The L/Dk measurements from each laboratory were compared and correlated. Samples which were not masked with a fixed front surface aperture during measurement were corrected for edge effects. This paper shows that provided L/Dk is calculated for each lens using the same technique and Dk is derived using a graphical method of calculation, similar results can be obtained by all laboratories. However, the agreement was less good for materials of Dk greater than 70 x 10(-11) (cm2/s) (ml O2/ml x mm Hg).

Oxygen permeability of collagen shields

The oxygen permeability (Dk) of ten 24-hr collagen shields was measured directly by polarographic methodology at approximately 2 hr of hydration. Edge and boundary effects were included in the calculations. Dk was found to be approximately 26 x 10(-11) cm ml O2/sec ml mmHg at 35 degrees C. Mean water content of the shields was 65.7% (SD = 1.0%) as measured by a hand refractometer. Therefore, the projected oxygen transmissibility of collagen shields is expected to be compatible with normal corneal metabolism.

Cancellation of the boundary and edge effects by choice of lens thickness during oxygen permeability measurement of contact lenses

Edge and boundary effect correction factors have been proposed to modify the oxygen transmissibility and permeability values obtained by polarographic measurement of contact lenses. These two correction factors are opposite in sign: the boundary effect causes oxygen transmissibility and permeability to be underestimated; the edge effect causes the reverse. Two methods are used to define lens thickness values where the two effects should be quantitatively equal and therefore cancel for both rigid gas permeable and hydrogel contact lenses.

Stacking samples while measuring oxygen transmissibility of hydrogel contact lenses

It is necessary to measure several samples with different thicknesses of the same material to be able to determine a value for the oxygen permeability (Dk incm2 ml O2/s ml mm Hg) of that material. Some current contact lens materials are not available in multiple thicknesses, but it might be possible to "stack" several samples of the same thickness as an alternative procedure. This study demonstrates that measurements of Dk for one particular midwater-content hydrogel (Methafilcon, a nominally 55% water-content ionic material) give statistically indistinguishable results whether single samples of various thicknesses (Dk = 20.52 x 10(-11)) or thinner stacked samples to attain similar thicknesses (Dk = 20.05 x 10(-11)) are used in this measurement.

In situ oxygen transmissibility of rigid gas-permeable contact lenses

On the eye a contact lens is bathed in tear fluid, which increases its resistance to oxygen flux. For rigid gas-permeable lenses, this effect should be small during open-eye wear because a large amount of oxygen is provided by air-saturated tears that are pumped under the lens. However, under closed-eye conditions this study suggests substantial decrease in overall lens system oxygen transmissibility when lens transmissibility itself is greater than 20 x 10(-9) cm ml O2/s ml mm Hg and when the average thickness of the tear layer is greater than about 20 micron.

The presence of a contact lens induces a very small increase in the anterior corneal surface temperature

The anterior corneal surface temperature beneath a contact lens covering the cornea was determined from measurements of the average heat flow from the cornea to the atmosphere (efflux) in a group of 13 healthy young subjects. The average heat efflux was 1.1 X 10(-2) cal X cm-1 X sec-1. The mean corneal surface temperature of the same group was 34.5 degrees C for the open eye and 36.2 degrees C for the closed eye conditions. The anterior corneal surface temperature beneath a 0.07 mm thick hydrogel contact lens (40% water content) was found to be 34.6 degrees C (rise of 0.1 degree C) and 34.9 degrees C (rise of 0.4 degree C) beneath a 0.3 mm thick hydrogel contact lens (40% water content), using the measurement of corneal heat efflux and taking the contact lens to have a slight insulating effect. Contact lenses of higher water content caused a smaller rise in anterior corneal surface temperature than lenses of lower water content.

Prentice Medal lecture: contact lens wettability--myths, mysteries, and realities

The author shows that the wetting of contact lens on the eye is a complex process. Measuring the contact angle is possible by several techniques but knowing the contact angles (advancing and receding) is not enough to define wettability in scientific terms. A contact lens on the eye, bathed in lacrimal fluids and subjected to a number of pressures, presents a challenging problem in physics as well as in clinical vision care.

The relation of conjunctival PO2 to capillary bed PO2

A polarographic oxygen sensor in contact with the palpebral conjunctiva makes a measurement of oxygen tension related to that in the underlying capillary bed but not equal to it. A mathematical analysis based on the equation describing oxygen diffusion in an oxygen-consuming tissue indicates that the measured oxygen tension will be 5-10 torr below that in the capillary bed.

Corneal oxygen uptake under a soft contact lens in phakic and aphakic eyes

A simple and comfortable procedure is used to measure the decay of oxygen content of a soft contact lens on a human cornea when the anterior surface of the lens is blocked from further supply of oxygen. Calculations from this measurement give the steady-state oxygen tension under the soft contact lens and the oxygen flux into the cornea when the eye is open but wearing a contact lens. When the procedure is applied to phakic and aphakic human eyes, the results indicate that for both states, the oxygen uptake by the cornea is about the same when the uptake levels are above 1 microliter/cm2 . hr and oxygen tensions under the lens are above 10 mm Hg.

Oxygen flux through a soft contact lens on the eye

The only practical means of making a clinical measurement of oxygen flux into a cornea under a contact lens is by pressing a Clark-type oxygen sensor to the anterior surface of the lens. The earlier limitation to lenses of low water content has been removed and a general method suitable for soft lenses of all transmissibility is presented here. The procedure allows an estimate of oxygen tension under the lens and oxygen flux into the cornea. The method requires a minimum amount of calculation and does not require a prior calibration on the contact lens wearer, but instead gives a direct, absolute measurement of oxygen tension and flux.

External hypoxia and corneal hydration dynamics

Optical pachometry was used to monitor thickness increases in six human corneas exposed to an anoxic external environment. Corneal swelling was observed to occur to an extent consistent with an earlier study (5%/hr or 25mu m/hr). Use of Darcy's law allows analysis of the "pump-leak" equilibrium, which leads to the conclusion that any direct interference by surface anoxia affecting corneal hydration must be at the endothelial side of the cornea.

In vivo aqueous humor oxygen tension--as estimated from measurements on bare stroma

Aqueous humor oxygen tension of the in vivo rabbit eye was estimated by a relatively atraumatic procedure. The anterior corneal surface of the eye was first scraped free of its epithelial layer. A polarographic oxygen electrode was then used to measure the oxygen tension at the bare stromal surface. Aqueous humor oxygen tension can be estimated from this measured steady-state stromal surface oxygen tension by correcting the measured oxygen tension for the tension drop across the stroma and endothelium. When animals breathed room air (155 mm Hg oxygen tension), the oxygen tension of the aqueous humor was 13 mm Hg; this rose to 150 mm Hg when the inspired oxygen tension was 713 mm Hg.

Corneal swelling and oxygen flux through a soft contact lens

After reviewing the oxygen needs of the cornea, we calculate the increase in corneal swelling which is predicted to occur for oxygen flux through a contact lens at the rate of less than 1.5 microliters/cm(2)/hr. The amount of corneal swelling is inversely related to the oxygen flux into the anterior corneal surface. To keep corneal swelling below 4% requires a oxygen tension of about 11 mm Hg which is equivalent to a flux of 2 microliters/cm(2)/hr.

The osmotic component of swelling under extended wear soft contact lenses

Swelling of the cornea during sleep when wearing an extended wear contact lens has a osmotic and hypoxic component. The normal swelling of the cornea during sleep, caused by the change from hypertonic tears of the open eye to the isotonic tears of the closed eye, is about 3%. Under an extended-wear contact lens, there is an additional 6 to 8% swelling of the cornea in the closed eye because the cornea under the contact lens is at an oxygen tension substantially below that present in the palpebral conjunctiva. Data on human subjects wearing extended wear contact lenses are consistent with this allocation of the 8 to 12% total corneal swelling observed upon awakening.

Design criteria for a high plus soft contact lens that fulfills the oxygen needs of the cornea in the aphakic eye

A soft contact lens can be designed that will satisfy simultaneously in an optimum manner the optical, mechanical, and physiological requirements of the aphakic eye. The relationship fo water content to both refractive index and oxygen transmissibility allows incorporation of the latter quantity into the equation for back vertex power. Starting with the patient's requirement of back vertex power, base curve, and optic zone diameter a contact lens is designed to be mechanically strong, by having minimum water content, and to supply sufficient oxygen for the cornea's needs.

Diffusion of oxygen in human corneas in vivo

No oxygen was detected on the corneal surface of human subjects breathing air or almost pure oxygen. These results suggest that the epithelium obtains all its oxygen fro the air or the palpebral conjunctiva and that the endothelium is supplied with oxygen solely by the aqueous humor.

Corneal edema in phakic and aphakic eyes

Explanations are given for the physiological behavior of the cornea in the phakic and aphakic eye and for a cornea covered by a contact lens that causes epithelial anoxia. The normal, constant hydration of the cornea is a result of active ion transport by the endothelial layer. The hypoxic epithelium under a contact lens produces lactate ion that migrates to the posterior portion of the cornea where it modifies the normal stromal balance between osmotic and inhibition pressures. A consequence of this modification can be corneal swelling. A reduction in cell count of the endothelial layer, together with a possible increase in aqueous humor oxygen tension is believed to be responsible for the smaller amount of swelling caused by epithelial hypoxia in the aphakic eye.

Forces retaining a contact lens on the eye between blinks

The forces retaining a contact lens on the eye between blinks are analyzed using quasi-static equilibrium equations. The main forces are: (1) the surface tension forces around the contact lens periphery pulling the lens toward the cornea, (2) the reaction pressure force underneath the lens countering the effects of the surface tension, and (3) the weight of the lens. The reaction pressure force on the posterior side of the contact lens arises as a consequence of lubrication phenomenon. The centered lens can never be in static equilibrium. However, since the time period associated with the inertia effect in a thin-film system is much longer than the time between consecutive blinks, a quasi-static analysis is applied.

Thickness of the corneal epithelium during anoxia

Thickness changes in the epithelium and stroma of the cornea of a living rabbit eye were observed with the specular microscope. Previous experiments on the excised cornea using this technique have failed to show any thickening of the corneal epithelium due to anoxia at the anterior surface. In the living eye, the same result was obtained. The results confirm that corneal edema due to anoxia is caused almost entirely by swelling of the stroma, the epithelium undergoing very little thickness change.

The definition of thickness for a lens

The thickness of a lens depends upon the direction of the measuring vector. The thickness as commonly used from the sagittal height formula does not give the path length for movement of gas, water, and heat through a lens. Furthermore, the amount of gas, water, or heat moving through a lens depends upon an areal average thickness, not central thickness. A formula is derived for an approximate areal averaged sagittal height thickness. This average thickness has the advantage of being easily calculated from a sagittal height table. The equation for the areal average thickness along a radius of curvature is also derived. Sample calculations are made to show how this average thickness is a function of lens dimension. It is recommended that an areal average thickness be used when oxygen transmissibility of a lens is calculated from oxygen permeability of the lens material.

A microliter oxygen electrode system for sperm suspensions

A new polarographic oxygen electrode system is described which can accept sperm suspensions with volumes as low as 50 microliters. In this configuration, the electrode tip forms the base of the suspension reaction chamber, and there is no stirring. Consequently, structured fluids such as cervical mucus can be applied. Measurements can be performed in as little time as 10 minutes. The system reliably measures oxygen consumption rates as low as 0.1 mm Hg/minute, so that mammalian sperm suspensions with concentrations of the order 1 X 10(6)viable cells/ml can be applied. Sample experiments, comparing the oxygen uptake of human spermatozoa in semen and in Tyrode's solution, are described.

Measurement of oxygen flux into the cornea by pressing a sensor onto a soft contact lens on the eye

A polarographic oxygen sensor pressed against a soft contact lens worn on the eye gives data from which can be calculated the oxygen flux into the cornea. The procedure requires knowledge of the diffusion coefficient and solubility of oxygen in the contact lens material. The procedure works best when used with contact lenses of about 40% water content.

Oxygen flux into the human cornea when covered by a soft contact lens

Corneal thickness was measured in human subjects who wore gel lenses that varied only in center thickness. The amount of corneal swelling that developed was a function of center lens thickness. Based on these data and the results of a previous study, the minimum oxygen tension under the lens necessary to maintain normal corneal hydration was determined to be 10 mm Hg. The critical oxygen flux into the cornea at this oxygen tension is 2 microliter/cm2 x hr. Gel-lens center thickness and oxygen permeability can be altered to increase the oxygen tension at the tear/lens interface and resultant oxygen flux into the cornea by increasing the oxygen transmissibility of the lens.

Water flow conductivity and pore diameter in extended-wear gel lens materials

The water flow conductivity of 3 high-water-content (70 to 85%) gel materials proposed for extended-wear contact lenses was measured. Pore diameter was calculated from these flow conductivities. High-water-content gels were found to have pore diameters in the range of 20 to 30 A compared with about 8 A for the lower-water-content HEMA gel. The larger pore diameter of extended-wear lenses will permit entry of drugs, hormones, and enzymes and thereby may pose new problems for wearers.

Oxygen-transmissibility considerations for a hard-soft contact-lens combination

A combination formed by covering a soft contact lens on the cornea with either an oxygen-permeable or an impermeable hard lens offers some advantages over a soft lens or a hard lens alone. Several procedures can be used to fit such combinations to the eye. The amount of oxygen that is delivered to the cornea will depend strongly on the fit on the soft lens to the cornea and the fit of the hard lens to the soft lens; the amount of oxygen delivered by 4 different combinations is described here. The results show that the proper choice of lenses and fit will permit an adequate supply of oxygen to reach the cornea in daily wear.

Hydraulic flow conductivity of the vitreous gel

The hydraulic flow conductivity of rabbit and bovine vitreous gel has been measured by a method that gives better results than the one previously used. The vitreous gel has a hydraulic flow conductivity that would be expected for a highly hydrated connective tissue. The "pore" size (the spaces availabel for water flow) are estimated to be about 4,000 A in diameter.

Oxygen tension under a hard, gas-permeable contact lens

Oxygen tension under a hard, gas-permeable contact lens is calculated. The gas permeability of the lens is taken from the engineering literature and is shown to be 10 times smaller than claimed in the optometric literature. The engineering data are supported by new measurements of gas permeability. Calculations using the new data show that the oxygen tension under the lens is at the lower end of the range of acceptable values.

A model study of facial injury caused by impact while wearing an ophthalmic frame

Damage to facial tissue caused by an ophthalmic frame when there is impact to the face was studied by means of a paraffin-covered mannequin head. Under mild conditions of impact it was easily observed that metal frames and metal components do more tissue damage than is done by a zylframe. A series of the commonly dispensed frames was compared for facial injury potential caused by impact to the face.

A lubrication theory model of tear exchange under a soft contact lens

Tear exchange due to squeezing of a soft contact lens onto the cornea by the lid is calculated from lubrication theory. The theory predicts that for a normal blink and the usual tear film thickness (8 to 10 microns) there will be 10 to 20% tear exchange at each blink.