I
Volume 68 January 1975
51
Section of Ophthalmology President E C Zorab FRCS
Meeting 9 May 1974
Contact Lenses Mr Montague Ruben (Contact Lens Department, Moorfields Eye Hospital, London WC])
Uhat is a Soft Lens? One may well ask oneself what is a soft lens as opposed to a hard lens. The terms are relative. The human sensation of softness is purely tactile. It expresses the ability of a substance to show a low resistance to pressure. By scientific methods the rigidity of a material and its hardness of surface can be investigated and measured. The interrelated properties of elasticity and plasticity then become apparent. Many materials at high temperatures show such properties but we are only interested in those materials which are soft at a few degrees above body surface temperature (33°C) down to a few degrees below, as low as 0WC, this range being the common human environment. Contact lens materials when compared in engineering terms show polymethyl methacrylate (PMMA) (hard) at one end and wet hydrophilic acrylates at the other. In relative units, given that PMMA=100, then hydrophilic acrylates=2.50. Silicon rubber is 5 and from here to 100 exist only a few experimental materials. It is an area for future development (Table 1). The other problem yet to be solved is that, whilst a hard lens is misfitted by a calculated degree to permit precomeal tear flow and gaseous exchange, a soft lens provides a close-fitted occlusive membrane. The soft material's physicochemical properties will determine whether it can become a functional lens. A completely occlusive soft membrane is not acceptable but one that permits good gas exchange has proved to be wearable for long periods, even several months. Ophthalmologists and applied scientists have to develop the use of such membranes not only for therapeutic purposes but for the correction of simple refractive errors. Whilst much progress has been made
in use of established materials and lens design, there are many problems remaining. With hard lenses, even at a thickness of 0.1 mm, lens shapes were under control. The tolerances of manufacture were acceptable. We were able to determine a fitting back curve and an optical front curve with a high degree of reproducibility. But with soft lenses the back surface is determined by the front of the eye to which it moulds. Therefore imperfections of the front contact lens surface that result from moulding to the eye shape lead to visual problems. Add to these fitting problems, the fact that changes in temperature, pH, tonicity and water equilibrium all change the dimensions of the hydrophilic lens, then we can readily understand that variations in acuity do occur, fortunately most within the tolerances of the average individual. Even with the materials now in use, which are higher than cornea refractive index, the correction of all refractive errors, especially astigmatism, is not possible. But the clinician must understand that many individuals
Table I Relative hardness of contact lens materials Material Relative hardness 100.00 Dry polymethyl methyacrylate Wet Bioplex pH4 8.00 Wet Bioplex pH7 6.00 Wet Bioplex pH9 5.00 semi-soft 5.00 Japon acrylate Sauflex (nylon) 5.00 Silicon rubber (dry state) 5.00 2.50 Optalya (allyl polyhema) 2.00 Hydron (polyhema) Sclera (cadaver), anterior surface 2.50 2.00 Sclera (cadaver), posterior surface 2.00 Sauflon II (polyvinyl pyrollidone + acrylate) 1.50 Bausch & Lomb (polyhema) Cornea (cadaver), anterior surface 1.50 Cornea (cadaver), posterior surface 1.50 0.50 Sauflon I (polyvinyl pyrollidone + acrylate)
Indentation pressure =index of hardness. Standard =indentation 0.125 mm. Needle=Garrard sapphire stylus. Tolerance=- 0.25 g
Proc. roy. Soc. Med. Volume 68 January 1975'
2
are happy with a vision less than 6/4, provided the benefits in field, binocular single vision (BSV) and image size on the one hand and the cosmetic on the other are sufficiently valuable. At this time we have several hydrophilic acrylates with water contents from as low as 3 % to as high as 85%. These can be made into several lens forms. The higher water content lens is softer and has a high oxygen flow. In general terms, the high water content lenses can make large lenses, the low small thin ones. Whenever the limbal zone is to be completely covered, then a high oxygen flow material is necessary, the converse being true for small lenses. Thus even with PMMA of water content 0.2% constant wear without ill effects is possible in some rare instances with lenses of 7-8 mm diameter. The balance between trauma and anoxia is held by' the eyes' ability to adapt. Whenever this balance is upset then we see abnormal changes. Whilst I accept superficial minor changes in the eye I do not think one can accept either for the normal or diseased eye changes in the deeper structure. I mean by this changes involving the regenerating layer of epithelium, stroma and endothelium. To evaluate these changes requires a high degree of clinical ability with slit-lamp microscopy. At this time I doubt whether more than 20 % of the many practitioners involved in contact lens practice qualify. I am sure teaching authorities and better instrumentation will put this right in time. The ultimate in contact lens practice is a soft material, either hydrophilic in substance or surface (e.g. treated silastic elastomer) that can be worn for prolonged periods constantly. With certain qualifications we have reached this stage with lenses used for therapeutic work. But one must realize that the metabolic requirements of abnormal eyes are not those of the normal eye. It remains to be seen whether the normal eye with our present materials and lens designs can achieve constant wear without secondary eye disease. For normal eyes the criteria chosen for safety standards of materials, lens design and hygiene must be high because when disease occurs secondary to contact lens wear the practitioner may be concerned with a patient having some permanent loss of vision. Those of us who are concerned with supervision of patients must not therefore ignore the potential danger. This evening's papers are presented more as cameos for your interest than as complete coverage of soft contact lens practice. Since over 90% of the public wearing soft lenses have low errors of refraction some time must be given to this group. From their behaviour we learn what to expect in the treatment of eye disease. Therapeutically the soft lens has been used as the panacea for all incurable anterior segment con-
ditions. It is a fault of this appliance that it is too easy to fit and therefore often used indiscriminately as a treatment. Even amongst contact lens practitioners there is disagreement as to their true value. It is often unwise to inflict upon some patients a long period or indeed a lifetime of contact lens management if the gain is slight. On the other hand, the future treatment of keratoplasty, aphakia and keratoconus looks much more interesting and the routine use of such appliances even at the end of operations will be possible. The physicochemical properties of hydrophilic lenses and their implications on wear and function will also be discussed. The study of materials after contact with the eye for several months determines their role in contact lens practice. The future depends upon many such studies.
52
Mr Nicholas A Brown and Mr David Lobascher (Contact Lens Department, Moorfields Eye Hospital, London WCJ)
Complications of Soft Contact Lens Use in the Correction of Simple Refractive Errors One hundred and one patients (202 eyes) have been studied with simple refractive errors (mostly myopes), but without any significant ocular pathology. An initial ophthalmic examination was performed prior to soft contact lens and follow-up examinations at three, six and twelve months. All patients now reported had attended for at least the three months follow up, and the majority had attended throughout. Some patients had previously used other types of contact lens. Symptoms
Complaints of mild degrees of irritation, watering, photophobia and redness were elicited on direct questioning in the majority of patients. These symptoms tended to increase in frequency and severity with lens spoilage but were seldom sufficient to cause the patient to cease lens use.
Eyelid signs: Blepharitis was noted frequently prior to soft lens use and showed a tendency to appear after lens use. It was never a clinical problem. Conjunctival signs: A mild degree of conjunctival hypernmia was comnmonly seen at the first
Volume 68 January 1975
51
Section of Ophthalmology President E C Zorab FRCS
Meeting 9 May 1974
Contact Lenses Mr Montague Ruben (Contact Lens Department, Moorfields Eye Hospital, London WC])
Uhat is a Soft Lens? One may well ask oneself what is a soft lens as opposed to a hard lens. The terms are relative. The human sensation of softness is purely tactile. It expresses the ability of a substance to show a low resistance to pressure. By scientific methods the rigidity of a material and its hardness of surface can be investigated and measured. The interrelated properties of elasticity and plasticity then become apparent. Many materials at high temperatures show such properties but we are only interested in those materials which are soft at a few degrees above body surface temperature (33°C) down to a few degrees below, as low as 0WC, this range being the common human environment. Contact lens materials when compared in engineering terms show polymethyl methacrylate (PMMA) (hard) at one end and wet hydrophilic acrylates at the other. In relative units, given that PMMA=100, then hydrophilic acrylates=2.50. Silicon rubber is 5 and from here to 100 exist only a few experimental materials. It is an area for future development (Table 1). The other problem yet to be solved is that, whilst a hard lens is misfitted by a calculated degree to permit precomeal tear flow and gaseous exchange, a soft lens provides a close-fitted occlusive membrane. The soft material's physicochemical properties will determine whether it can become a functional lens. A completely occlusive soft membrane is not acceptable but one that permits good gas exchange has proved to be wearable for long periods, even several months. Ophthalmologists and applied scientists have to develop the use of such membranes not only for therapeutic purposes but for the correction of simple refractive errors. Whilst much progress has been made
in use of established materials and lens design, there are many problems remaining. With hard lenses, even at a thickness of 0.1 mm, lens shapes were under control. The tolerances of manufacture were acceptable. We were able to determine a fitting back curve and an optical front curve with a high degree of reproducibility. But with soft lenses the back surface is determined by the front of the eye to which it moulds. Therefore imperfections of the front contact lens surface that result from moulding to the eye shape lead to visual problems. Add to these fitting problems, the fact that changes in temperature, pH, tonicity and water equilibrium all change the dimensions of the hydrophilic lens, then we can readily understand that variations in acuity do occur, fortunately most within the tolerances of the average individual. Even with the materials now in use, which are higher than cornea refractive index, the correction of all refractive errors, especially astigmatism, is not possible. But the clinician must understand that many individuals
Table I Relative hardness of contact lens materials Material Relative hardness 100.00 Dry polymethyl methyacrylate Wet Bioplex pH4 8.00 Wet Bioplex pH7 6.00 Wet Bioplex pH9 5.00 semi-soft 5.00 Japon acrylate Sauflex (nylon) 5.00 Silicon rubber (dry state) 5.00 2.50 Optalya (allyl polyhema) 2.00 Hydron (polyhema) Sclera (cadaver), anterior surface 2.50 2.00 Sclera (cadaver), posterior surface 2.00 Sauflon II (polyvinyl pyrollidone + acrylate) 1.50 Bausch & Lomb (polyhema) Cornea (cadaver), anterior surface 1.50 Cornea (cadaver), posterior surface 1.50 0.50 Sauflon I (polyvinyl pyrollidone + acrylate)
Indentation pressure =index of hardness. Standard =indentation 0.125 mm. Needle=Garrard sapphire stylus. Tolerance=- 0.25 g
Proc. roy. Soc. Med. Volume 68 January 1975'
2
are happy with a vision less than 6/4, provided the benefits in field, binocular single vision (BSV) and image size on the one hand and the cosmetic on the other are sufficiently valuable. At this time we have several hydrophilic acrylates with water contents from as low as 3 % to as high as 85%. These can be made into several lens forms. The higher water content lens is softer and has a high oxygen flow. In general terms, the high water content lenses can make large lenses, the low small thin ones. Whenever the limbal zone is to be completely covered, then a high oxygen flow material is necessary, the converse being true for small lenses. Thus even with PMMA of water content 0.2% constant wear without ill effects is possible in some rare instances with lenses of 7-8 mm diameter. The balance between trauma and anoxia is held by' the eyes' ability to adapt. Whenever this balance is upset then we see abnormal changes. Whilst I accept superficial minor changes in the eye I do not think one can accept either for the normal or diseased eye changes in the deeper structure. I mean by this changes involving the regenerating layer of epithelium, stroma and endothelium. To evaluate these changes requires a high degree of clinical ability with slit-lamp microscopy. At this time I doubt whether more than 20 % of the many practitioners involved in contact lens practice qualify. I am sure teaching authorities and better instrumentation will put this right in time. The ultimate in contact lens practice is a soft material, either hydrophilic in substance or surface (e.g. treated silastic elastomer) that can be worn for prolonged periods constantly. With certain qualifications we have reached this stage with lenses used for therapeutic work. But one must realize that the metabolic requirements of abnormal eyes are not those of the normal eye. It remains to be seen whether the normal eye with our present materials and lens designs can achieve constant wear without secondary eye disease. For normal eyes the criteria chosen for safety standards of materials, lens design and hygiene must be high because when disease occurs secondary to contact lens wear the practitioner may be concerned with a patient having some permanent loss of vision. Those of us who are concerned with supervision of patients must not therefore ignore the potential danger. This evening's papers are presented more as cameos for your interest than as complete coverage of soft contact lens practice. Since over 90% of the public wearing soft lenses have low errors of refraction some time must be given to this group. From their behaviour we learn what to expect in the treatment of eye disease. Therapeutically the soft lens has been used as the panacea for all incurable anterior segment con-
ditions. It is a fault of this appliance that it is too easy to fit and therefore often used indiscriminately as a treatment. Even amongst contact lens practitioners there is disagreement as to their true value. It is often unwise to inflict upon some patients a long period or indeed a lifetime of contact lens management if the gain is slight. On the other hand, the future treatment of keratoplasty, aphakia and keratoconus looks much more interesting and the routine use of such appliances even at the end of operations will be possible. The physicochemical properties of hydrophilic lenses and their implications on wear and function will also be discussed. The study of materials after contact with the eye for several months determines their role in contact lens practice. The future depends upon many such studies.
52
Mr Nicholas A Brown and Mr David Lobascher (Contact Lens Department, Moorfields Eye Hospital, London WCJ)
Complications of Soft Contact Lens Use in the Correction of Simple Refractive Errors One hundred and one patients (202 eyes) have been studied with simple refractive errors (mostly myopes), but without any significant ocular pathology. An initial ophthalmic examination was performed prior to soft contact lens and follow-up examinations at three, six and twelve months. All patients now reported had attended for at least the three months follow up, and the majority had attended throughout. Some patients had previously used other types of contact lens. Symptoms
Complaints of mild degrees of irritation, watering, photophobia and redness were elicited on direct questioning in the majority of patients. These symptoms tended to increase in frequency and severity with lens spoilage but were seldom sufficient to cause the patient to cease lens use.
Eyelid signs: Blepharitis was noted frequently prior to soft lens use and showed a tendency to appear after lens use. It was never a clinical problem. Conjunctival signs: A mild degree of conjunctival hypernmia was comnmonly seen at the first
