Year : 1973 | Volume
: 21 | Issue : 1 | Page : 25--27
The soft plastic contact lenses
S K Dastoor
|How to cite this article:|
Dastoor S K. The soft plastic contact lenses.Indian J Ophthalmol 1973;21:25-27
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Dastoor S K. The soft plastic contact lenses. Indian J Ophthalmol [serial online] 1973 [cited 2021 Jan 20 ];21:25-27
Available from: https://www.ijo.in/text.asp?1973/21/1/25/31423
In an endeavour to provide greater comfort to a corneal contact lens wearer, a variety of materials are being tried out every now and then. Even the most acceptable polymethyl methacrylate has a variety of properties, and this has been made possible by using the same monomer, but polymerising is effected by different catalysts, which have proved better in their own ways in the contact lens wear. This innocuous inorganic substance does not prove harmful when put next to the human cornea and bathed with the human tears, and also provides more dimentional stability in fabricating contact lenses.
Many other materials afford a high refractive index and the Rohm and Hass plastic of a different refractive index as well as the harder material made by some of the manufacturers in the U.S.A. have proved to be of great advantage. Coatings have added new dimensions to the patient's comfort and have solved many a problem of the practitioners.
This paper, however, does not deal with this type of materials, but would confine itself to the much talked about `SOFT LENSES'. Most of us are aware of the Czechoslovakian Gell Lens which is comfortable and is a forerunner to all the soft lenses that have been tried out in various parts of the world so far. Messrs. Bausch & Lomb state in their report that their lenses can he controlled to as thin a thickness as the conventional lenses that we are fitting today, and the amount of hydration also has been altered as compared to the European countries. Very close to Bausch & Lomb, are the recently known P.H.P. lenses, which of course are not allowed to be fitted freely because of the restrictions imposed by the Foods & Drugs Act, until they are proved safe. To enable further research on these P.H.P. lenses in this country, some samples have been made available for fabrication and fitting, and the interesting information gathered through this experience of fabricating and fitting is as under:
It has been found that this soft polymer is both hydrophilic and hyaroscopic. The water content is high and dependent upon the pH of the solution. The lens can be considered to be a dynamic substance with a high diffusion and permeability. The index of refraction depends upon the water content and since water content is controlled by the pH, the consideration is only with the pH near that of the human tear buid, 7.2 being the average. The index of refraction is approximately 1.412.
Hydrated lenses should have diameters from 12.5 mm to 15.0 mm. Extending the lens beyond the limbus area is necessary in order to make full use of the osmotic pressure mechanism of the eye.
Regardless of the size, the wearer of a soft lens has very little feeling and no actual discomfort. The eye moves in a normal and completely restricted manner, without the usual head tilting or squinting of the eye which immediately points out some hard lens wearers. There is no need to worry that the lens will fall out. A properly fitted soft lens moves with the eye with practically no lag such as conventional hard acrylic lens. Soft lenses are worn a full day the very first day and there have been no indications of discomfort or fatigue by any lens wearer.
It has been found that the base curves necessary are from 7.0 to 8.5 mm, in approximately 0.3 mm jumps. Thus it becomes unnecessary to make more than 6 or 7 base curves. Most of the successful fittings are done with lenses measuring from 0.30 mm to 0.45 mm. However, plus lenticulars are successfully made with thicknesses as high as 0.65 mm.
It is necessary that the lens has a point of direct contact in the peri-limbal section that contains the bevel. This point of contact brings out a "Hover-Craft" effect. The anterior chamber of the eye produces a flow of aqueous through five membranes of the cornea. When the lens touches the sclera in the peri-limbal area, it seals itself, so that the pressure is slowly built up inside the lens by the flow of anterior chamber fluid through the corneal membranes. When the lens is on the eye from 5 to 10 minutes, the adhesion of the lens in the perilimbal area is broken by pressure of a continuous flow. The lens thus floats on a thin layer of tears much in the same manner as a "Hover-Craft" hovers supported by a stream of air.
In this manner, utilizing the natural mechanism of the eye, the lens is supported and the epithelial cells beneath it are continuously fed by the fluid that passes from the anterior chamber and through the corneal stroma and membranes. This aqueous fluid contains oxygen and other metabolic solutes that support a continuous and healthy metabolism of epithelial cells underneath the soft lens.
A properly fitted lens should not be too loose or too tight. A loose lens is one which is too flat and a tight lens is one which is too steep. It has also been found that different degrees of astigmatism can be corrected by a certain percentage with hydrophilic soft lenses. Upto 1.00 diopters of corneal astigmatism can be corrected 75% or better, and from 1.00 to 2.00 diopters approximately 50%.
The inspection of the lens on the eye with a slit lamp is mandatory and should be done after the lens is on for at least 10 minutes along with or following the refraction of the selected lens on the patient.
Daily Sterilization and Care by Wearers
Every night after the lenses are removed and placed in their individual punched containers and then into a jar, 3% hydrogen peroxide solution has to be poured over the lenses and allowed to remain for not less than 5 minutes and not more than 10 minutes. Then the containers are removed from the hydrogen peroxide and the hydrogen peroxide solution is discarded and replaced with saline buffered to a 7.2 pH. The lenses in their containers remain in this solution overnight at room temperature. Leave the jar open. In the morning the lenses are then rinsed with fresh sterile buffered saline solution and placed on the eye without discomfort. This procedure is to be repeated DAILY in order to ensure sterility and to avoid any complications.
The wearer must NEVER place a lens on the eye that may contain hydrogen peroxide due to insufficient length of time of exposure to buffered saline.
It has been advised NEVER to use any preparations made for hard contact lenses as these contain preservatives which can damage the epithelial cells.
Usage of Tap or Distilled Water is strictly prohibited as this will cause the lens to stick to the cornea causing great discomfort as well as danger to the epithelial cells, and if the removal of the lens is forced, the epithelial cells may be damaged. If tap or distilled water is used accidentally, allow the lens to remain on the eye until the lens reaches osmotic equilibrium and is. loose enough to be removed.
The soft plastic lens thus promotes all the comfort that a contact lens can give, and there have been many instances where the lenses can be tolerated during all the waking hours from the very first day. However, the cumbersome method of insertion and removal, and the problem of keeping the lenses sterile from time to time without creating fungus, are the factors to be still reckoned with. Nevertheless, these soft lenses are all set to gain a legitimate position in the contact lens field, as they hold a definite promise in increased tolerance and improved vision.