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.

A review of the theoretical concepts, measurement systems and application of contact lens oxygen permeability.

The interest in rigid gas-permeable contact lenses for extended wear has promoted a re-evaluation of the theoretical and practical aspects of the oxygen transport characteristics of contact lens materials. Oxygen permeability has been considered by some authors to be a parameter which is dependent upon the measurement conditions; however, this paper demonstrates that an intrinsic permeability coefficient may be derived for a given material when the basic principles of the transport mechanisms are considered. Two basic methodologies have been described for measuring oxygen permeability--dual-chamber and electrochemical methods, the latter being the more popular. Although consistency in measured oxygen transmissibility of a given lens may be achieved among laboratories, current analytical techniques do not allow the intrinsic permeability coefficient of materials to be determined. Future research should be directed towards developing methods to fully correct for surface and edge effects, and to achieve standardization among laboratories.