Oxygen-permeable Contact Research Articles

Contact lenses for the treatment of ocular surface diseases.

Contact lens wear is useful in ocular conditions such as high refractive errors, irregular astigmatism, corneal ectasias, corneal dystrophies, post-keratoplasty, post-refractive surgeries, trauma, and ocular surface diseases. The new innovations of highly oxygen-permeable contact lens materials have broadened the applications of contact lens suitability. Therapeutic contact lenses are medically used in the management of a wide variety of corneal conditions and ocular surface diseases. These lenses aid in pain relief, enhance corneal healing, maintain ocular homeostasis, and act as a drug delivery system. Drug delivery applications of contact lenses hold promise for improving topical therapy. The modern rigid gas permeable scleral contact lens provides symptomatic relief in painful corneal diseases such as bullous keratopathy, corneal epithelial abrasions, and erosions. It has been useful in therapeutic management as well as visual rehabilitation by enhancing the ocular surface and protecting the cornea from adverse environmental conditions. This review provides a summary of contact lenses used for the treatment of ocular surface diseases based on the current evidence available in the literature. This can help enhance the understanding and management of ocular surface diseases with respect to contact lens use in our day-to-day ophthalmology practice.

The Ophthalmic Performance of Hydrogel Contact Lenses Loaded with Silicone Nanoparticles.

In this study, silicone nanoparticles (SiNPs) were prepared from polydimethylsiloxane (PDMS) and tetraethyl orthosilicate (TEOS) via the sol-gel process. The resultant SiNPs were characterized by dynamic light scattering (DLS), transmission electron microscope (TEM), and scanning electron microscope (SEM). These SiNPs were then blended with 2-hydroxyethylmethacrylate (HEMA) and 1-vinyl-2-pyrrolidinone (NVP) before polymerizing into hydrogel contact lenses. All hydrogels were subject to characterization, including equilibrium water content (EWC), contact angle, and oxygen permeability (Dk). The average diameter of SiNPs was 330 nm. The results indicated that, with the increase of SiNPs content, the oxygen permeability increased, while the EWC was affected insignificantly. The maximum oxygen permeability attained was 71 barrer for HEMA-NVP lens containing 1.2 wt% of SiNPs with an EWC of 73%. These results demonstrate that by loading a small amount of SiNPs, the Dk of conventional hydrogel lenses can be improved greatly. This approach would be a new method to produce oxygen-permeable contact lenses.

Transmission of colour and acuity signals by parvocellular cells in marmoset monkeys

The red-green axis of colour vision evolved recently in primate evolutionary history. Signals serving red-green colour vision travel together with signals serving spatial vision, in the parvocellular (PC) division of the subcortical visual pathway. However, the question of whether receptive fields of PC pathway cells are specialized to transmit red-green colour signals remains unresolved. We addressed this question in single-cell recordings from the lateral geniculate nucleus of anaesthetized marmosets. Marmosets show a high proportion of dichromatic (red-green colour-blind) individuals, allowing spatial and colour tuning properties of PC cells to be directly compared in dichromatic and trichromatic visual systems. We measured spatial frequency tuning for sine gratings that provided selective stimulation of individual photoreceptor types. We found that in trichromatic marmosets, the foveal visual field representation is dominated by red-green colour-selective PC cells. Colour selectivity of PC cells is reduced at greater eccentricities, but cone inputs to centre and surround are biased to create more selectivity than predicted by a purely 'random wiring' model. Thus, one-to-one connections in the fovea are sufficient, but not necessary, to create colour-selective responses. The distribution of spatial tuning properties for achromatic stimuli shows almost complete overlap between PC cells recorded in dichromatic and trichromatic marmosets. These data indicate that transmission of red-green colour signals has been enabled by centre-surround receptive fields of PC cells, and has not altered the capacity of PC cells to serve high-acuity vision at high stimulus contrast.

Biocompatibility of Nanofilm‐Encapsulated Silicone and Silicone‐Hydrogel Contact Lenses

A material placed in or in contact with a biological system, that causes the minimum perturbation that can be tolerated by the host biological system, can be considered to be biocompatible. The major effects caused by a contact lens can be considered to be 1) the blocking of the natural supply of oxygen to the epithelium cells of the cornea, 2) interference with the normal evaporation of water from the tear fluid layer, and 3) hindrance of the normal functions of blinking for replenishing oxygen-saturated tear fluid and removing dead epithelium cells, which result from apoptosis. The encapsulation of a highly oxygen-permeable contact lens by a nanofilm with an imperturbable surface state minimizes all these terms. The encapsulated contact lens can be worn safely in extended wear.

Metabolic changes in the corneal epithelium resulting from hard contact lens wear.

The metabolic state of rabbit corneas was monitored in vivo using the noninvasive method of corneal redox fluorometry. The autofluorescence signals of reduced pyridine nucleotides (PN) and oxidized flavoproteins (Fp) were measured in the corneal epithelium with and without contact lens wear. The PN/Fp ratio, which is related to the metabolic status of the tissue, was then calculated for each of these conditions. After application of polymethylmethacrylate (PMMA) contact lenses having an oxygen transmissibility (Dk) of less than 0.1, the PN signal increased and the Fp signal decreased. The PN/Fp ratio, generally a more precise indicator of metabolic state than either of these two quantities alone, was 1.93 +/- 0.78 without contact lenses, and increased to 2.78 +/- 0.86 (p less than 0.0001) with contact lenses. When oxygen-permeable silicon contact lenses (Dk = 12.5) were placed on the corneas, the PN/Fp ratio was found to increase slightly, but not as much as with the PMMA lenses. Newly developed highly oxygen-permeable contact lenses (Dk = 58.8) did not increase this ratio. Our findings indicate that redox fluorometry can be valuable in determining the effects of contact lens wear on corneal metabolism.

Improvement of visual acuity and corneal physiology in keratoconus by fitting aspherical, high oxygen-permeable contact lenses

Keratoconus is a condition in which the cornea assumes a complex irregular curvature caused by central corneal thinning. The abnormal topography of the cornea in combination with central corneal scarring results in an impaired visual acuity. Even in mild cases spectacles do not correct vision adequately. The use of hard contact lenses with a spherical geometry in the past has already given a marked therapeutic improvement. The use of these lenses however, is complicated by hypoxia and mechanical trauma of the cornea. These complications could theoretically be avoided by fitting elliptical lenses with a high oxygen transmission. To investigate this hypothesis we compared low oxygen-permeable spherical lenses with high oxygen-permeable elliptical contact lenses in a group of twenty patients with mild keratoconus. Our results showed both a marked subjective and objective visual improvement after fitting elliptical lenses with a low incidence of complications.

Towards An Artificial Cornea: Surface Modifications of Optically Clear, Oxygen Permeable Soft Contact Lens Materials by Ammonia Plasma Modification Technique for the Enhanced Attachment and Growth of Corneal Epithelial Cells

The advent of high water content, oxygen permeable contact lens materials has made the intracorneal implants more feasible. A major obstacle encountered is the regrowth of a stable epithelium over the implant. Therefore, ammonia gaseous plasma modification technique was used to modify the surface chemical properties of soft contact lens material such as poly(2-hydroxyethyl methacrylate and methacrylic acid), PHEMA-MAA copolymer, in an attempt to enhance the cell attachment and growth of rabbit corneal epithelial cells.

Oxygen Tension Uder 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.

Biochemical Aspects of Contact Lens Wear

Glycogen, adenosine triphosphate, and hydration were measured in rabbit corneal epithelium to determine whether the corneal epithelium glycogen decrease, increase in epithelial hydration, and decrease in epithelial adenosine triphosphate stores, seen as a result of contact lens wear, were secondary only to anoxia or may also have resulted from mild trauma, with no interference to oxygenation. Conventional contact lens wear, trauma, and oxygen-permeable contact lens wear caused metabolic changes, showing trauma as well as anoxia may play an important role in the corneal epithelial response to contact lens wear.

Oxygen tension under an oxygen-permeable contact lens.

The oxygen tension at the anterior corneal surface under an oxygen-permeable contact lens is shown to be a function of the oxygen transmissibility and the thickness of the lens. The oxygen transmissibilities of several gel contact lenses and a silicone rubber lens are reported together with the procedure for making this measurement. Several of the lenses are shown to give, for an open eye, an oxygen tension at the corneal surface sufficient for maintenance of normal corneal thickness. None of the lenses tested will do this for the closed eye.