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Biomedical subjects

I Fatt

Publications and source records attributed to I Fatt.

At least 19 recordsLinked to original sources
PubMed1992

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).

Contact Lenses↗
PubMed1992

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.

Blinking↗
PubMed1992

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.

Contact Lenses↗
PubMed1991

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.

Contact Lenses↗
PubMed1990

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).

Contact Lenses↗
PubMed1990

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.

Bandages↗
PubMed1989

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.

Contact Lenses, Hydrophilic↗
PubMed1989

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.

Contact Lenses↗
PubMed1988

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.

Contact Lenses↗
PubMed1987

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.

Contact Lenses↗
PubMed1986

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.

Body Temperature↗
PubMed1986

Measurement of skin mobility in the upper back.

On manual testing for skinfold tenderness greater resistance has been reported in patients with marked skinfold tenderness. On objective measurement of skin mobility, by raising a skinfold with a vacuum pump and by establishing a stress/strain curve, no difference in skin mobility was found between subjects with and without skinfold tenderness. Furthermore, contrary to manual testing, the suction testing causes no pain in subjects with clinical skinfold tenderness. In a second series of suction tests, comparing skin mobility in a subject with relaxed and contracted underlying muscles, it was found that muscular contraction reduces skin mobility by 50%. The conclusion is that resistance felt by manual skinfold testing is not inherent in the structures, but is caused by contraction of underlying muscles because of pain caused by the manual skinfold test.

Back↗
PubMed1984

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.

Adult↗
PubMed1983

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.

Capillaries↗
PubMed1982

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.

Atmosphere↗
PubMed1982

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.

Contact Lenses, Hydrophilic↗