PubMed Health⌕ Search

Biomedical subjects

I Fatt

Publications and source records attributed to I Fatt.

At least 55 records · Page 3Linked to original sources
PubMed1980

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.

Adhesiveness↗
PubMed1979

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.

Contact Lenses↗
PubMed1978

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.

Contact Lenses, Hydrophilic↗
PubMed1978

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.

Adult↗
PubMed1978

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.

Adult↗
PubMed1977

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.

Contact Lenses↗
PubMed1977

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.

Contact Lenses↗
PubMed1977

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.

Animals↗
PubMed1976

Measurement of intraocular pressure over a gel lens.

Tonometric measurements of albino rabbits were determined with and without the use of a gel lens while the intraocular pressure (IOP) was manometrically controlled. The IOP was evaluated over a 50-mm Hg range of pressures. A high degree of correlation between measurements made with and without a gel lens was found. The results suggest that the technique may have clinical applications on patients for whom a corneal anesthesia is contraindicated or on whom repeated tonometry or tonography might cause corneal trauma.

Animals↗
PubMed1976

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.

Contact Lenses, Hydrophilic↗
PubMed1976

Oxygen tension under a soft or hard, gas-permeable contact lens in the presence of tear pumping.

The oxygen tension under hard or soft, oxygen-permeable contact lenses is calculated. The calculation recognizes delivery of oxygen to the tear fluid under the contact lens by diffusion through the lens material and by entrance of fresh, air-saturated tears at each blink. The results show that oxygen diffusion through the lens material is much more effective than tear pumping in maintaining a useful oxygen tension under the lens.

Contact Lenses↗
PubMed1976

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.

Biophysical Phenomena↗
PubMed1975

Effects of solutes on the swelling pressure of the corneal stroma.

The chemical environment of bare, excised rabbit corneal stroma was varied to determine the effects of solutes on the swelling pressure. Measurements of changes in the thickness of the stroma are converted to hydration units which are in turn converted to a change in swelling pressure at the initial hydration. The order of effects on swelling pressure are compared to the Hofmeister series for polyelectrolyte gels and the results are analyzed in the context of a structural theory of the swelling pressure of corneal stroma proposed by Hart and Farrell and based on the gel properties of the stroma.

Animals↗