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Biomedical subjects

D L MacKeen

Publications and source records attributed to D L MacKeen.

15 recordsLinked to original sources
PubMed1992

Tear potassium contributes to maintenance of corneal thickness.

Isolated rabbit corneas were bathed on their endothelial surfaces with normal Krebs bicarbonate Ringer solution, while the epithelial surfaces were bathed in a basic tear solution containing sodium and potassium. When bathed in basic tear solution alone, corneal swelling occurred at an average of 12 microns/h over a 3-hour period. Corneal swelling occurred at a rate of about 21 microns/h when the epithelial solution was switched from normal basic tear solution to an iso-osmotic K(+)-free basic tear solution. Corneal swelling then slowed, and in the final hour of a 3-hour exposure to K(+)-free tear solution, the corneas deswelled at about 10 microns/h. The data indicate that potassium is a necessary solute for the maintenance of normal corneal thickness. The results suggest that a lacrimal dysfunction that would cause a decrease in the potassium content of tears may influence corneal thickness and also suggest that the inclusion of potassium in artificial tears is important.

Animals↗
PubMed1992

A prospective multicenter clinical trial to assess safety and efficacy of Menicon SF-P RGP lenses for extended wear.

A prospective, multicenter, national clinical trial was undertaken to assess the safety and efficacy of Menicon SF-P (melafocon A) rigid gas permeable (RGP) contact lenses for extended wear in healthy human eyes. Ten investigators enrolled 167 patients (334 eyes), of which 143 (286 eyes) were initially fit with lenses for the 12-month study. Most subjects were reexamined at 18 months or more, with findings essentially unchanged from those noted at 1 year. Two hundred-two eyes (71%) completed the study. Seventy-two eyes (25%) discontinued, and 12 (4%) elected to continue extended wear. During the study, no significant adverse clinical reaction attributable to the lens was observed. Surface irritation, occasional punctate staining, and lens binding were very rare. The average wearing time was 6.2 days, mandated partially by FDA restrictions as well as patient response. As expected, keratometric shifts were principally vertical: less than 0.99 D (69%), 1.00-1.99 D (28%), and 2.00-2.99 D (4%). The majority of changes observed were toward sphericity. Two hundred lens replacements were required, 24% for adjustments in power/base curve. Only 16 lenses were replaced for warpage, and none for discoloration or crazing. Of those completing the study, 76% rated overall lens comfort and satisfaction to be very good. These results suggest that RGP lens extended wear can be safe and effective and offer patients an acceptable alternative to extended wear of disposable hydrogel lenses.

Adolescent↗
PubMed1988

Laser-assisted contact angle measurements.

A laser beam, aimed at the solid-liquid-air interface of a sessile drop on a contact lens, produces two lines of diffraction which are theoretically normal to the profile of the fluid and solid surfaces at the point of contact. In the present experiments, the beam and the diffraction patterns were projected on a flat screen perpendicular to the beam, and the angle was measured as a contact angle. The angle was measured on curved surfaces (contact lenses) of 6.5 to 8.9 mm radius and for each material studied, the angle: (1) was independent of the front surface radius, (2) was independent of droplet volume at 2 and 10 microliter, and (3) decreased with time after drop placement. The laser-derived measurements correlated well with contact angles measured by goniometry on contact lenses and a variety of other materials (r = 0.86; p less than 0.0001). Because the diffraction lines are straight and are generated at the point of contact, the angle formed by them is measured easily with a protractor, yielding a convenient and reproducible contact angle measurement. An application of the method was demonstrated by measuring contact angles for saline-containing 0 to 2% bovine serum albumin or bovine submaxillary mucin on Silafocon-A (Polycon II), Pasifocon C (Paragon EW), and polymethyl methacrylate (generic PMMA and Paragon 18) lenses. On each material, physiological concentrations of mucin, but not of albumin, significantly (p greater than 0.01) decreased contact angles by 8 to 15 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena↗
PubMed1980

Aqueous formulations and ointments.

The development of ocular drug delivery systems has progressed greatly during the past four decades. This has resulted from better understanding of the pertinent physiological processes and from the availability of new synthetic polymers. More must be learned about the distribution of polymer solutions during the blink and their interactions with biosurfaces. In addition, awareness of the limitations placed on vehicle characteristics (e.g., viscosity, surface activity) by the variation in human eyes and more accurate extrapolation of data from relatively uncomplicated experimental models to the human eye will result in the development of even more effective ophthalmic vehicles designed for specific purposes and for particular drugs.

Administration, Topical↗
PubMed1976

Chlorampenicol retention on, and penetration into, the rabbit eye.

The retention of the antibiotic chloramphenicol on, and penetration into, the rabbit eye has been compared following administration in two vehicles. Samples were taken of both aqueous humor and a washing fluid applied to the conjunctival surface and the concentration of 14C-chloramphenicol determined. Greater tear film and aqueous humor drug concentrations, which are effective against many organisms, were found with a vehicle which enhanced retention time on the ocular surfaces.

Animals↗
PubMedDate not supplied

Corneal swelling responses with extended wear in naive and adapted subjects with menicon RGP contact lenses.

Overnight corneal swelling responses to Menicon SF-P and EX rigid gas permeable (RGP) contact lenses were determined in 20 human volunteers previously adapted to lens wear and in five volunteers who had no prior contact lens experience. Slit lamp biomicroscopy revealed no evidence of lens adherence in either group. Overnight wear of both lenses produced greater corneal swelling in naive (8.732% +/- 1.50%) than in adapted eyes (5.016% +/- 1.10%) (P less than .00005). These results suggest that, at least for the lenses tested, adaptation to contact lens wear may down-regulate overnight swelling responses to levels at or near no lens wear or daily-wear values for the same lenses. Both the EX and SF-P lenses appear to satisfy Holden's criteria for zero residual swelling, suggesting promise for successful clinical extended wear.

Adaptation, Ocular↗
PubMedDate not supplied

Swelling and deswelling of rabbit corneas in response to rigid gas permeable, hydrogel and elastomer contact lens wear.

We determined diurnal variation in corneal thickness in rabbits prior to and following overnight wear of: (i) selected rigid contact lenses with different Dk values; (ii) hydrogel lenses of low and high water content; and (iii) elastomer lenses. The degree of contact lens-induced corneal swelling observed during 24 hours of lens wear, and the rates of deswelling in the subsequent 24 hours, correlated well with the different oxygen transmissibilities of the individual RGP contact lenses. The greatest swelling (21.6 +/- 5.4%) followed the wear of PMMA lenses. The least swelling, 2.9 +/- 4.0%, followed the wear of rigid gas permeable (RGP) Menicon SF-P (melafocon A) lenses, a value nearly identical to the swelling observed in the morning following sleep without lenses, (0.0 +/- 3.1%). By contrast, low-water content hydrogel soft contact lens use was associated with drastic corneal deswelling rates (-15.1 +/- 4.5%) during the hours after lens wear. The difference between these and control corneas was significant by paired t-test (P less than 0.01). Eyes wearing high water content lenses had less deswelling than eyes with their low-water counterparts. Corneal swelling produced by elastomer lenses was similar to that seen with RGP lenses.

Animals↗