|Year : 1975 | Volume
| Issue : 3 | Page : 12-15
Soft lenses v/s hard lenses
A-416, Defence Colony, New Delhi-110024, India
R S Garkal
A-416, Defence Colony, New Delhi-110024
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Garkal R S. Soft lenses v/s hard lenses. Indian J Ophthalmol 1975;23:12-5
Let us begin with considering the problems that we have so far experienced with stable lenses (hard lenses) and how it could be avoided with Gel lenses. Have the Gel lenses come upto our expectation in solving some of our problems with hard lenses ?
When we consider the successful wearer of hard lenses as one who has worn the lenses the whole day with comfort for over six months then perhaps we have had a very high success rate-almost over 95%.
But have we also considered success not only in terms of comfort and patient acceptance but also the avoidence of corneal cedema and corneal integrity? If we do so, we shall realise that the percentage of success comes down to one fourth. Corneal oedema in most of these cases earlier is asymptomatic. The patient then starts complaining of burning sensation in the eyes, tearing and photophobia. A whole day wearer reduces wearing gradually till he says he cannot wear more than 3 to 4 hours a day. These patients must be refitted or asked to give up contact lenses and wear spectacle glasses for some time. All of us know how difficult it is to prescribe glasses to those people correctly. Most of them have distortion of the corneal surface with irregular shadows as seen in retinoscopy and irregular mires as observed with keratometer which persists sometimes for months. Even after this we commonly observe that astigmatism has increased which makes them unsuitable candidates for soft lens fitting.
| Advantages of Gel Lenses|| |
1. No corneal curvature changes. 2. No spectacle blurr which enables the patients to wear Gel lenses and spectacles alternately if they so desire. 3. Comfort of wearing. The initial adaptation is almost non-existent. 4. No over wear corneal abrasions even if there is epithelial oedema due to anoxia. 5. Lenses are rarely displaced from the cornea or lost and thus more suited for sports. 6. Foreign bodies i.e. dust particles etc. are not likely to be trapped behind the lens and thus more suitable for outdoors.
| Disadvantages of Soft Lenses|| |
1. Quality of visual acuity is generally not as good as with Hard Lenses or spectacles.
The visual quality by the soft lenses requires an optimum relationship between the back surface of the lens and the front surface of the vision by compression during each blink, while a too flat lens will also cause fluctuation of the vision due to its excessive movements. It is not an easy task to fit a patient with an optimum-lens-cornea relationship. It requires an experience and a shrewed observation-including the slit lamp microscopic examination etc.
2. Chemical and bacterial contamination of the lenses. This occurs more often with the soft lenses and is another hazard. Meticulous care in handling, sterilisation and storage has to be observed by the patient.
3. Drying phenomenon : This occurs during wearing. The patient comfortable with the lenses during the first few hours becomes uncomfortable during the later part of the day and complains of irritation and burning. The visual activity may also drop with it due to the dryness of the lenses.
4. Durability of these lenses as compared to stable (hard lens) is less : The lenses have tendency to develop cracks, and the edges may get damaged or frayed. The lenses tend to become yellow, brown and even opaque. This is due mainly to proteinceous deposits from eye secretion and bacterial and chemical contamination especially if the handling of the lenses is not hygenic or daily sterilisation of lenses is not carried out properly. Most of these discolorations can be dealt with by special sterilisation technique to restore the lens to its original transparency. In addition we have the problem of physical deterioration in the surface quality of the lenses.
5. Expense involved in soft lenses is much more than conventional hard lenses.
6. Last but not the least is the inability to correct astigmatism. Since the soft lens drapes over the cornea unlike the hard lenses, there is no tear lens to correct astigmatism. An astigmatism, of more than .75 to 1'0 diopter cannot be corrected by soft lenses. It is gross over-generalisation that any patient with 1.50 diopters of astigmatism can be alright with soft lenses. Even if it is subjectively accepted it cannot be clinically acceptable.
| Fitting Procedure|| |
Since it would not be possible to describe the details of the same, I shall summarise a few important points
1. Before a patient is accepted by the practitioner for the soft lens fitting, he must ensure that his patient has clean and hygenic habits. A patient who does not observe these habits is more likely to end up with infected lenses and the eyes. He must carry out meticulously the instructions by the practitioner on the daily cleaning, sterilisation and storage of lenses.
2. The usual examination of the eyes including its adnexa is done in addition to refraction, slit lamp microscopic examination if possible, and `K' reading of the corneal curvature. The measurements of the iris diameter (not the corneal diameter-the iris diameter being larger than the corneal diameter and extends approximately to outside edge of the lumbus of cornea) is taken.
3. An appropriate trial soft lens is selected which should be :
a) approximately 0.4 to 0.6 mm flatter than the flatter `K' reading.
b) Has an over all diameter of about 1.50 mm larger than the iris diameter (as described above).
This lens is inserted in the eye but make sure that the lens is fully hydrated and immediately inserted in the eye after removal from the storage bottle. An interval of about half an hour should be given for the lens to settle down before checking the V.A. and fit.
4. For checking up the fit the following points be noted : -
a) The lens centres itself well on the cornea, b) The lens edge rests about 3/4 to 1.0 mm outside the limbus on the sclera, c) The movements of the lens with the blink are about 2 mm No movement or movement of the less than 1'0 mm signifies a tight fitted lens while an excessive movement signifies a flat or loose lens which also tends to get displaced more often into the upper fornix (under the upper lid). The movement of the lens can best be examined on the slit lamp microscope with the slit beam of light on the sclera outside the lens edge. d) If the fit is found satisfactory- the refraction be done again subjectively as well as with retinoscopy and correct power required be arrived at. A good fit will give a good and crisp retinoscopic reflex. e) A note may be made if there is any variation in the visual acuity due to too tight or loose fitting as mentioned above under disadvantages of soft lenses. f) After the patient has worn the lenses for at least 6 hours -the visual acuity be recorded again as also the fitting. Here it is important to observe the cornea-scleral blood vessels at the edges of the lenses. If the lens is tight and its edge interfering with the corneal blood supply- one can notice the blanching or constriction of vessels inside the edge of the lenses with congestion of the blood vessels outside the edge of the lens. If this happens the lens must be substituted with a flatter lens. I would recommend that a fit should be such that the lens has a draping effect across central 5 to 7 mm of cornea and should definitely have a certain amount of clearance at the peripheral cornea so that there is no compression of blood vessels. Usually this fit is within the range of 0.4 to 0.8 mm flatter than flatter `K' reading of the cornea. The mean being 0.6 mm flatter lens.
| Solution for Soft Lenses|| |
A mention may be made of something of great importance to all contact lens practitioners who are fitting soft lenses and those who are planning to do so. It is with regard to the solution for soft lenses. Soft lenses require two different solutions :
a) Preserving Solution : This is designed to provide hydration conditions for hydrophilic contact lenses during storage and prior to its insertion in the eyes. This solution must provide an eye compatible solution of controlled tonocity (CH) and Ph. It must also contain a certain amount of ba'tericidal agent to maintain the safety of the solution and assist in lens preservation.
It must be remembered that this solution is the one that is absorbed in the lens and thus goes in the eye,-as such its sterility, Ph, and CH must be ensured. In view of this it is a criminal act to use any solution which has not been approved and certified by the Drug Controller of the Ministry of Health. As at present we do not have any solution manufactured in India to conform to above. We shall have to depend upon the import of this solution. This imported solution available for Sauflon lenses is manufactured by C.L. manufacturing London that is at present being used in India is known as Steri-Sal.
b) Cleaning Solution for Soft Lenses : This solution cleans the lens and removes the debris and contamination like dirt, cosmetics, metabolic products etc. which adhere to the lens. The solution has the property of promoting liquid outflow by control of the tonocity of the solution and also by the osmotic effect of the high molecular weight polymer additive in it. This solution available for Sauflon lenses is known as Steri Soly. It must again be emphasised over here, that this solution also must conform to the rules under drug act.
Steri-Solv is used to clean both the surface of the lens at night after removal and the lens is again stored in Steri-Sal or normal saline for atleast a few hours before insertion in the eye.
The future of Soft Lenses
Since the first Czech material for soft lenses 12 years ago various materials by different companies with claims of better performance have been made. We have the Bausch & Lomb lens which is hydroxyethyl material ; Griffin lens which is a hydrophilic plastic primarily poly (N-Vinyl pyrrolidine) and absorbes 55;1 water by weight against 40% of Bausch & Lomb ; Vericon lenses made in Quebec (Canada); N&N lenses made in Vancouver using a plastic produced by the Toyo Contact Lens Co in Japan; Contaflex lenses manufactured by Calcon Laboratory of California. The lenses we are manufacturing and using in India at present are the Sauflon Lenses available in hydration of 55%, 70°% and 85%. The lens blanks are manufactured by Contact Lens manufacturing (London). These lenses are of Hema material and I find these as one of the best soft lenses available today. It is a hydrophilic lens made of complex chain of polymers with hydrophilic sites distributed along the length of the chain. The polymer backbone is provided by a chain of methacrylate molecules which are chemically stable. These lenses have good mechanical properties like tensile strength, flexibility and elastic recovery in addition oxygen permeability essential for the health of the cornea is claimed to be 4 to 5 times that of average available soft lens material. The Silicone lens under experimentation has great feature. This material is so controlled that it can be soft and pliable or semihard. The material has the drawback that it cannot be ground and polished-it can only be moulded and thus has inherent drawbacks though its potential strength is excellent and it can be stretched like a rubber band.
The research in the field of an ideal material for soft lenses still goes on and will continue. Tomorrow's material for a lens will have to be one which gives an ideal vision, comfort, has a good oxygen permeability, is durable and is easy to handle, clean and keep it in sterilized condition. We do hope and believe that in the next few years we shall have such a lens-thanks to all those who have done all the pioneering work in this field so far and those who are continuing to do so. Till such time the hard lenses will still have an important role to play in the contact lens field.