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Biomedical subjects

I Fatt

Publications and source records attributed to I Fatt.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1982

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.

Aphakia, Postcataract↗
PubMed1981

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.

Contact Lenses, Hydrophilic↗
PubMed1981

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.

Contact Lenses, Hydrophilic↗
PubMed1981

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.

Aphakia, Postcataract↗
PubMed1981

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.

Aphakia, Postcataract↗
PubMed1981

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.

Animals↗
PubMed1981

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.

Adult↗
PubMed1980

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.

Animals↗