CONTACT LENSES

Prepared by Feyza Selçuk

I. INTRODUCTION

i. History of Contact Lenses

ii.Characteristics of An Ideal Contact Lens

II. POLYMERS AT CONTACT LENS PRODUCTION

i. Polymerization

ii. Cross-linking

iii. Hydrogels

III. POLYMERS IN CONTACT LENS INDUSTRY

i. HEMA(Hydroxyethylmethacrylate)

ii. PMMA (Polymethylmethacrylate)

IV. CONTACT LENS MANUFACTURING

V. IMPORTANT PARAMETERS IN CONTACT LENS DESIGN

VI. TYPES OF CONTACT LENSES

i.Soft Contact Lenses

ii. Hard Contact Lenses

iii. Cellulose Acetae Butyrate (CAB) Lenses

iv. Silicone Rubber Lenses

v. Extended Wear Contact Lenses

VII. DRAWBACKS OF CONTACT LENSES

IIX. CONCLUSION

REFERENCES

I. INTRODUCTION

Contact lenses are not implanted devices, they are used in very close contact with the eye, and their physiological performance depends greatly on the material used to make the lens. Contact lenses are optical devices, usually made of a synthetic polymer, that are placed over the cornea of the eye and remain between the lids and the cornea when the eye blinks. The main purpose of contact lenses is vision correction, but certain types of lenses are often used medically to treat corneal diseases or to protect the cornea in patients with certain eye problems. In the latter cases, the contact lenses are called therapeutic or bandage lenses. Contact lenses have been used to manage corneal disease, in postoperative care, and in vision rehabilitation after disease, surgery, and trauma by serving as a vehicle to deliver drugs, as a bandage, “optically” as a new corneal surface, and as a prosthetic device. A further use of hydrogel contact lenses, which takes advantage of their absorbing properties, is for prolonged delivery of drugs to the eye.

i. History of Contact Lenses

The history of contact lenses begins in 1508 with the description of the principal effect of the contact lens by Leonardo Da Vinci. First use of contact lenses was by Sir John Herschel. He described the use of a glass contact lens on a distorted cornea

Early contact lenses for clinical purposes were from glass(rigid) that permit extremely short wearing times and have very limited uses like for the correction of aphakia, keratoconus and injury.

In 1936 first partially plastic lens was marketed with central portion made of glass and the peripheral portion from PMMA. Later, between 1938-1940 first all plastic lenses were made from methacrylate, a plastic that has excellent light transmission and low toxicity.

Professor Otto Wichterle and his assistant Drahoslav Lim thought a new polymer hydroxyethylmethacrylate (HEMA) could be used to make contact lenses. This was in the mid-1950s. Principal advantage of this new material was that fewer parameters needed to fit the lens to the human cornea and that practitioners would be able to stock lenses and dispense them directly from their office.

The history of contact lenses has occurred in the latter half of the 20^th century. In particular, events in the 1970s through the 1980s related to the invention of soft, hydrogel contact lenses have revolutionized the contact lens industry and the eye care attached to it. 1970s was the period when first gas permeable materials were introduced.

Silicone elastomer contact lenses were initially introduced to the market by Breger’s Mueller-Welt Company as the Silcon lens. In 1972 this was reformulated as a coated silicone rubber lens in relatively small diameters for aphakia (Silsoft) and myopia (Silsight). Although initial experience was reportedly quite positive, silicone elastomer lenses generally were not well-tolerated by myopic patients with frequent complaints of poor wetting and discomfort. B&L acquired the lens in 1985 and continues to provide the lenses in aphakic powers only. This lens has become the lens of choice for pediatric aphakia.

In 1978 cellulose acetate butyrate lenses were used for both daily- and extended-wear, but they were approved by FDA only for daily-wear. They were eventually discontinued due to problems with dimensional stability. In1979 a gas permeable polymer silicone/acrylate was developed and used at the production of first widely used gas permeable lens known as Polycon. The early siloxane/acrylate lenses provided limited oxygen permeability . These lenses were also more difficult to make, resulting in marked differences in laboratory-specific quality of product. Due to the siloxane in the materials, surface wetting problems were common, and patients were frequently complained of end-of-day dryness.

Recognition that extended-wear of hydrogel lenses carried an increased risk for serious complications, in part, led to a dramatic shift in the soft lens market. The prevailing clinical opinion of the 1980s was that lens handling played a substantial role in the risk of microbial keratitis. In part, this general opinion led to the introduction of the disposable soft lens in 1987 by Vistakon. The Acuvue disposable lens was initially introduced as a 2-week extended-wear lens to be worn and then thrown away. Initial claims were that this lens would be safer because lens handling was reduced to a minimum and the lens was replaced before potentially harmful contaminants could adhere to lens. Vistakon’s venture changed the face of the soft contact lens industry, and frequent replacement soft lenses are now the predominant lens type in the US.

In the mid 1980s a new class of gas permeable lenses were designed named as “high Dk” siloxane/acrtylate lenses that have a higher oxygen permeability. The increased permeability was primarily achieved through the addition of higher amounts of siloxane to the polymer. Although this was a successful strategy for elevating the oxygen flux through the material, it also made the lenses less wettable and hence, less well-tolerated by the wearer.

The last advances in contact lens production came in 1990s by the addition of fluorine to the polymer mix wetting properties of the lens were enhanced and comfort, visual performance and permeability were improved.

ii.Characteristics of An Ideal Contact Lens

Although the modern fitter and wearer have many choices of types of contact lenses the ideal lens has not yet been devised. The ideal contact lens would have the following characteristics:

(a) It would fully correct the refractive error with good optics.

(b) It would not produce physiological or pathological changes in the eye. In other words, there would be no edema, no staining, no vascularization and corneal warping.

(d) It would be comfortable from the moment it was first placed on the eye, and it would continue to be comfortable. The lens would be capable of continuos wear if desired.

(e) It would be easy to insert, remove, center and handle. Its care including storage, wetting, cleaning and sterilization would be simple.

(f) Its manufacture would be simple, with its specifications accurately reproducible. It would be inexpensive; cosmetically acceptable and durable. It would have therapeutic capacity.

II. POLYMERS AT CONTACT LENS PRODUCTION

Polymers are promising class of biomaterials that can be engineered to meet specific end-use requirements. They can be selected according to key ‘device’ characteristics such as mechanical resistance, degradability, permeability, solubility and transparency, but the currently available polymers need to be improved by altering their surface and bulk properties. Compared to other types of biomaterials, such as metals and ceramics, polymers offer the advantage that they can be prepared in different compositions with a wide variety of structures and properties.

The choice of the polymer is primarily governed by the end use of the basis of physical and chemical properties but also on the extensive biochemical characterization followed by specific preclinical testing of the chosen material.

There are numerous applications where polymer materials interact with biological components such as cells, tissues and extracellular fluids. These interactions are dependent on the nature and morphology of the surface. If they are to be avoided, the surface should be smooth like in the case of contact lenses.

Important mechanical properties of polymers for contact lenses are the elastic modulus, the ultimate tensile strength, the strain at break and the thermal propagation energy (defined as the energy required to tear a film with a predetermined cut.

i. Polymerization

Polymerization is the basic problem when preparing medical-grade polymers. Three methods may provide excellent solutions to the problem of preparation of ultra pure, medical-grade polymers. Gaseous polymerization is expensive and it has been used with success for only a few monomers such as ethylene. Plasma polymerization is the most promising technique, and it is now used to prepare medical-grade polymers in small quantities in impurity-free forms. Irradiation induced polymerizations can produce pure polymers of somewhat irregular structure, although the cost is rather high for efficient industrial production.

Bulk polymerization is the method of choice in contact lens production, since it produces polymers free from solvent, emulsifiers and catalysts. It can be initiated by peroxides, AIBN or ionic initiators which are added in small amounts.

ii. Cross-linking

Formation of three-dimensional networks of macromolecular chains by reaction of active functional groups of these chains is cross-linking. Crosslinking imparts improved mechanical strength and it can be used to form polymers that swell in water.

Hydrophilic methacrylate monomers that have been used as starting materials for the production of contact lenses can polymerize in the presence or absence of water or other polar solvents and always in the presence of a small quantity of cross-linking agent to form hydrophilic, cross-linked polymers.

The most commonly used cross-linking agent in the preparation of PHEMA is EGDMA, although cross-linking agents such as tri- and tetraethylene glycol dimethacrylate, and so on have also been employed. Typical initiators employed include benzoyl peroxide and azobisisobutyronitrile (AIBN); reaction temperatures vary between 50^oC and 90^oC.

On chemical cross-linking, small quantities of unreacted cross-linking , small quantities of unreacted cross-linking agents may remain in the polymer. In addition to their potential migration to surrounding tear fluid, unreacted cross-linking agents may react with the polymer chains at a future stage of contact lens use, creating internal stresses, dimensional changes and warping.

Structural parameters that unfavorably affect the refractive index of polymers are the degree of cross-linking and the degree of crystallinity. Increase of the values of both parameters leads to relatively turbid or translucent materials. Mechanical or environmental degradation of carelessly prepared polymers with chemically unstable bonds is the cause of contact lens discoloration. Plasticizers and additives must be avoided for the same reasons.

Increased degrees of crystallinity lead to mechanical strength as observed by increased values of the modulus, ultimate tensile strength and strain at break. Extensive cross-linking usually leads to increased modulus and tensile strength but decreased strain at break. Therefore mechanically weak, soft contact lenses may be reinforced by cross-linking and crystallization processes.

The swelling properties of hydrogels prepared from water-soluble hydrogels are controlled by the amount of crosslinking agent used in preparation. As the amount of crosslinking agent decrease, the degree of swelling increases, thus facilitating the permeation of oxygen through hydrogels. Highly swollen hydrogels, however, exhibit very poor mechanical properties. Consequently, in the preparation of hydrogels for soft contact lenses it is necessary to reach a compromise between the diffusivity characteristics and the mechanical properties of the hydrogels by controlling the amount of crosslinking agent incorporated in the copolymerization medium.

iii. Hydrogels

Polymers for use in ocular devices have to be aqueous or lipid soluble and to have good gel-or-film forming ability and adequate mechanical stability. Hydrogels are permeable to water and water-soluble molecules. For soft contact lenses, additional requirements include transparency, durability, sterilizability, hydrophilicity and water insolubility. Hydrophilic polyvinylalcohol based gels are suitable and their retention time, mechanical stability and permeability can be improved by cross-linking, making them insoluble.

The design of new polymers is not an easy task , instead it requires careful consideration of all chemical, physical and biological phenomena related to the use of polymers in contact with the eye and proper optimization of a variety of required properties.

Acrylic and methacrylic polymers are important starting materials for the development of hydrogels that can be used in the preparation of contact lenses. The usefulness of these materials for these applications arises, in part, from the facilities they present to incorporate hydrophilic groups into their structure, and also from the ease with which they may be copolymerized with a series of comonomers, including multifunctional crosslinking agents. It is often important to use a semipermeable membrane or hydrogel capable of acting as a barrier to the host’s immune system.

In recent years various types of polymeric materials have been used with success for hard and soft contact lens applications. The two basic categories of polymers are homopolymers and certain copolymers of methylmethacrylate (MMA) and cross-linked homopolymers and copolymers of hydroxyethylmethacrylate (HEMA).

III. POLYMERS IN CONTACT LENS INDUSTRY

i. HEMA(Hydroxyethylmethacrylate)

HEMA; a hydrophilic polymer that has been fashioned into a contact lens by spin-casting technique. The first contact lens was a HEMA product which had a fairly low water content of 31 to 55%. By adding polyvinyl perolidone, which is hydrophilic, cross-linkage was decreased. Second generation soft lenses had about 55% water content and were relatively thick. The third generation lenses had a water content of 70 to 85 or were very thin with good gas transmission. These lenses were a combination of hydrophilic and hydrophobic monomers.

When HEMA is dry, it is much more brittle than PMMA. But when fully hydrated, a lens made of the material is so flexible that it cannot be removed from the eye even with a suction cup unless air is allowed to get under it. It recovers its original shape almost regardless of any deformation it may undergo. In addition, it is biologically inert and compatible with human tissue. Bacteria and fungi are claimed to be incapable of penetrating the surface of the hydroyethylmethacrylate lenses. Lenses made from HEMA are stable, clear, nontoxic, nonallergenic and optically desirable for spherical correction.

HEMA-based hydrogels have always offered a certain degree of gas permeability. Refinements in the polymer formulations, including adding methacrylic acid, had the effect of elevating the water content of the lens polymer. Oxygen permeability is directly related to the water content of the polymer and is inversely proportional to the thickness of the material.

ii. PMMA (Polymethylmethacrylate)

When, in 1938, PMMA appeared on the scene, it brightened markedly the future of contact lenses. This material is readily fabricated by lathe cutting and by a variety of casting and molding techniques. Adjustments can be made on a lens after its original fabrication and its light weight, optical clarity and relative chemical and physical stability are highly desirable qualities. A PMMA lens may have an indefinite life on the eye. PMMA is stable, clear, nontoxic, nonallergenic, easily worked, and optically desirable.

PMMA, can be prepared by radiation or chemically induced polymerization of MMA in the presence or absence of solvents. Various grades of modified PMMA that are widely used for contact lens materials are branched or lightly cross-linked PMMA and blends of PMMA with small amounts of additives or other polymers, such as polyethylmethacrylate.

IV. CONTACT LENS MANUFACTURING

There exists two basic methods for the development of hydrogels with high oxygen permeability. The first approach involves the development of high-water content hydrogels. The high-water content lens material increases the supply of oxygen to cornea; the higher the water content, the higher the oxygen permeability of the hydrogel. The second approach for the development of high-oxygen permeable hydrogels involves the design of silicone-based hydrogels. Polydimethylsiloxane (PDMS), due to its low modulus of elasticity, optical transparency, and high oxygen permeability, is an ideal candidate for use in contact lens materials. PDMS possess an oxygen permeability that is about 50 times higher than the oxygen permeability of the hydrogel poly(HEMA) and 15 times higher than the high-water content hydrogels.

There are, however, several limitations to overcome before designing hydrogels based on PDMS. The primary obstacle is that PDMS is hydrophobic and insoluble in hydrophilic monomers. Thus, when attempts are made to copolymerize methacrylate-functionalized siloxanes with hydrophilic monomers opaque phase-separated materials are usually obtained. In addition , the copolymerization of methacrylate functionalized silicones with hydrophilic monomers results in materials with a reduction in water contents, a loss of surface wettability, and an increase in lipophilic character. Lipid uptake can lead to a loss in material wettability.

Copolymers of methacrylate end-capped fluoro-substituted siloxanes with varying concentrations of fluorinated methacrylates resulted in transparent, oxygen-permeable, low-water materials possessing a low affinity for lipids. The higher concentrations of fluoro side chain and fluoro methacrylate in the copolymer formulation, resulted in a dramatic reduction in lipid uptake.

Also copolymerization of methacrylate end-capped siloxanes containing hydrophilic side chains, with high concentrations of hydrophilic monomers, resulted in transparent hydrogels possessing high levels of oxygen permeability without the use of a solubilizing cosolvent.

It was shown that the fluorinated side chain siloxanes, when copolymerized with hydrophilic monomers, such as DMA, resulted in transparent, wettable, and oxygen permeable hydrogels.

Methacrylate end-capped fluoro side chain siloxanes containing a terminal –CF2-H group were synthesized and evaluated for potential use as hydrogels for contact lens application. The preparation of the fluoro side chain siloxanes was accomplished in two relatively simple synthetic steps: ring-opening polymerization to prepare a methacrylate end-capped silicone hydride containing polydimethylsiloxane, followed by hydroxylation of a perfluorinated allylic ether. Radical copolymerization of the fluoro side-chain siloxanes with DMA resulted in transparent hydrogels possessing high oxygen permeability, excellent hydrolytic stability, and mechanical properties suitable for contact lens wear.

V. IMPORTANT PARAMETERS IN CONTACT LENS DESIGN

Chemical stability

The term chemical stability encompasses characteristics of the backbone macromolecular chains as well as the type of functional groups of the chain ends and substituents, which are related to the chemical inertness of a polymer under the physiological conditions of the eye. The –C-C- and –Si-O- bonds of most presently available contact lenses are relatively stable, and they do not degrade under the mild chemical conditions of tear fluid.

Hardness

This property is essential for a polymer to be manufactured by lathe cutting. Surface quality polishability, resistance to warpage during cutting and scratch resistance are also dependent on hardness. As a general rule any material with a Shore D hardness of 83 units or greater will be able to be lathed into a contact lens.

Contact Angles

Contact angles of lens plastics may be measured by one of three methods including the sessile drop, captive bubble and Wilhemy plate techniques.

Modulus

The mechanical properties of a plastic which are chooses to be measured depends upon the lens type. The flexural modulus is measured on RGP materials because the ability to withstand the force of the eyelid is important to clinical function. For soft lenses, the tensile properties of the hydrogels are measured. These include Young’s modulus, percent elongation, tensile strength at break and tear propagation strength. The Young’s modulus (hydrogels) and the flexural modulus (RGP) play a critical role in manufacturing and clinical performance. A lens with a low modulus will warp during lathe manufacture and flex during wear, yielding poor optical performance. To combat this problem, polymers with low modulus must be cut thick, reducing comfort and oxygen delivery to cornea.

Oxygen Permeability

It is clear that all contact lenses produce a barrier to the oxygen available on the cornea. This barrier will decrease the partial pressure of atmospheric oxygen at the corneal surface, and hence, will result in a reduced flow into the cornea. The cornea is avascular tissue that metabolically maintains its thickness and transparency by consuming oxygen. When a contact lens is on the cornea, atmospheric oxygen is available through two primary pathways: pumping of oxygenated tears around and under the lens and diffusion through the lens. Tear pumping has been determined to be supplementary route for providing oxygen to cornea. Normally, oxygenation of the corneal surface takes place through the tear film that supplies the cornea with oxygen from the air. When the eyelids are closed, as during sleep, oxygenation is supplied to the corneal surface from the blood capillary vessels of the palpebral conjuctiva which comprises the posterior part of the eyelids.

The lack of adequate oxygen permeability of certain polymer films has been the single most important reason of failure of many promising contact lenses. No Dk value is too high, but the higher Dks are often incompatible with other properties, and compromises must be made. The lenses should also pass CO₂ easily since CO₂ accumulation is also detrimental to corneal health.

Elongation

Elongation at break is one important measure of lens durability. Stretching often occurs during lens handling and cleaning and represents a significant challenge to lens integrity.

Tear Strength

Handling, as well as insertion and removal from the eye and from cleaning devices (often in a partially hydrated state) can lead to cuts, cracks, scratches and edge tears; the propagation of which will eventually lead to lens replacement. As is well known, high modulus, elongation, and tear strength are often mutually exclusive and again a compromise in formulation must be struck.

Light Transmission, Optical Clarity and Color

While the concern for these aspects is easily understood, they are not always easy to achieve. Many of the best polymers for most other properties are opaque. Monomer and oligomer incompatibility can lead to polymer inconsistency with resultant light scattering which will reduce contrast sensitivity for the wearer.

Refractive Index

The more that the refractive index of a polymer differs from that of the tear fluids, the less the curvature of the lens surface must deviate from each other to yield a given power. With less deviation of curvature, there will be less change in thickness across the lens. Reduced lens thickness gives greater comfort and oxygen transmission. The higher the refractive index, the thinner the lens can be made.

Wettability and Lubricity

While there is a correlation between wetting angle and lens wetting, it is not absolute. Some lenses with poor wetting angles apparently interact with tear mucin to give superior in-eye. Furthermore, some lens materials appear to have bioadhesive properties which cause lid sensation and corneal damage on removal.

Stress Free, Isotropic and Stable Structure With High Tg

A polymerization method must be available that yields polymers which result in lenses with predictable expansion factors (hydrogels) and which will not change shape with time. This stability must be unfailing despite repeated cycles of stress, temperature and hydration.

The glass transition temperature; Tg is measured on RGP plastics because it is an indication of the polymer stability. In general, the higher the Tg of a plastic, (more then 90^oC), the more stable the materials.

Minimal Response to pH, Osmolarity, Temperature and Humidity

In the eye, lenses are subjected to temporary changes in each of the above factors. If the material changes in dimension in response to any of these changes, optical performance, durability and fit will be compromised.

Pore Size

Present thinking is that the pore size for polymers should be small enough to exclude invading organisms, cellular debris and tear protein. Tear proteins may become trapped in the lens matrix and denatured, yet be unreachable by even the most aggressive dehydration rate.

Biomaterial Deposition

The deposition of lipids and proteins on contact lens materials in the eye determines how long a lens can be worn before replacement. Protein deposition is the largest single cause of lens replacement. Lysozyme, the most common culprit, is bound strongly to lenses with a negative charge, but other tear proteins with isoelectric points below seven may be attracted to positively charged surfaces. A lens material free of protein deposition may still suffer from a similar fate if it is attractive to tear lipids. The problem, though less common than protein deposition, appears to be related to polymer composition and individual tear chemistry. As yet, a clear definition of the surface chemistry which is required to avoid both protein and lipid deposition is not available.

Toxicity

Contact lenses reside within microns of one of the most sensitive and delicate tissues; the cornea. Even minute quantities of slightly toxic materials can disrupt the function of the corneal epithelium. The presence of any suspect compounds, such as Plasticizers, initiators, monomers, solvents, oligomers, degradation products, dyes... must be investigated in a full battery of toxicity tests. Any threat, real or perceived, will increase the risk of regulatory rejection. The need for extensive testing for residuals will increase the cost of manufacture and the concern over product liability.

The structure plays a very important role in toxicity and carcinogenicity of biomaterilas in general. It is believed that toxic reactions with the tissue are the results of functional groups of the polymer or of residual impurities or products of partial degradation, which can be leached to the surrounding fluid.

Disinfection

Lens surface chemistry may be such that it is particularly amenable to habitation by various pathogenic organisms, or it may interfere with the action of the commonly available disinfection products. Demonstration of acceptable lens disinfection under standard conditions is now required for all new lens materials.

Wetting, soaking and cleaning solutions are commonly used with contact lenses.

The functions of wetting solutions are:

1. To convert the hydrophobic lens surface to a hydrophilic surface more easily covered by lacrimal fluid. This in turn increases comfort by providing a cushioning and lubricating effect between the inner surface of the eyelid and the lens surface and between the cornea and the lens surface.

2. To provide a viscous protective coating over the lens surface so that the lens does not come into direct contact with the fingers during insertion. This prevents transfer of oily, sebaceous deposits that are normally present on the skin, to the lens surface.

3. To stabilize the lens on the fingertip and thus promote easier insertion.

Wetting solutions usually contain a viscosity-increasing additive such as methylcellulose, a preservative such as benzalkonium chloride or thimersol and a wetting agent such as polyvinyl alcohol.

Most commercially available soaking solutions consist primarily of inert ingredients that is sterile water and various preservatives.

Th uses are:

1. To maintain the lenses in a permanent state of hydration

2. To leach out the chemical compounds that accumulate on the lenses during the wearing period

3. To maintain the sterility of the lenses (microorganisms can be transferred to the eye by a contaminated lens.

Fabrication

Soft lens hydrogels are usually manufactured into lenses in the dry state. Therefore, a candidate material must have dry to wet expansion factors in all three dimensions which do not vary by more than ±2%. Expansion is measured using an optical microscope with a linear indicator gauge to determine dry and wet dimensions. A contact lens hydrogel must also have expansion parameters which do not vary by more than ±2% from batch to batch. Greater variability will result in poor manufacturing yield due to unpredictable parameter targeting.

Permeability

Gas transport properties, particularly oxygen permeability are among the most important and controversial aspects of contact lens material. Oxygen permeability is necessary for good eye physiology. RGP materials are oxygen permeable primarily as a result of their silicone/fluorine content while hydrogels are oxygen permeable as a result of their water content.

Lens thickness contributes to comfort and gas permeability; however, lenses must have a certain minimal thickness to be able to refract incoming light rays.

Sterilization

Sterilization procedures depend on the surface properties and degree of swelling of the contact lens. Most heat treatment techniques are to be avoided because of the potential change of the mechanical properties and the considerable extent of thermal degradation of most biomaterilas at temperatures above 100 ^oC. Of the chemical sterilization techniques, very few can work with soft lens materials because of the potential permanent adsorption of sterilizing agents on the hydrogel surface. A preferred sterilization technique is irradiation at low dose levels, although it is well understood that this method may cause some change in the degree of cross-linking of the polymer.

Together with these factors monomer cost, availability, reproducibility, patentability, rapid approval, reliance on standard manufacturing methods, lens designs and fitting methods must be considered also.

VI. TYPES OF CONTACT LENSES

The contact lens market is divided into hard lenses, rigid gas permeable lenses, soft lenses and hybrids. These lens categories are further divided into daily wear, provisional wear, extended wear, disposable, spherical, bifocal, toric and cosmetically tinted lenses.

i.Soft Contact Lenses

Soft contact lenses are less stable than hard lenses, producing more variables and a greater likelihood of change. Soft contact lenses last an average of 2 years and must then be replaced. Visual acuity with soft contact lenses is generally not as clear as with hard lenses. This is often due to the poor optical quality of the high water content plastic in comparison to the material from which hard lenses are made.

Soft contact lenses require adequate tears for acceptable wear. In fact, they may require more tears than hard lenses since the wearer is required not only to keep his own eyes moist, but to keep the soft contact lenses moist as well. Soft or hydrophilic contact lenses are characterized by the ability to absorb water, by elasticity, and by flexibility. This compound originally consisted of HEMA linked with ethylene glycol dimethacrylate (EDMA).

Poly (2-hydroxyethylmethacrylate) (PHEMA) and its copolymers, in the form of cross-linked networks swollen in water or saline up to thermodynamic equilibrium (constant weight) at 34^oC., are the materials of choice for soft contact lens applications. Hydrophilic polymers vary in water content, mode of manufacture (spin cast or lathe-cut), lens parameters and associated cleaning and storage procedures.

There are basically two types of soft lenses that have been investigated so far: those achieving their softness because of the intrinsic properties of the material (low glass transition temperatures) and those whose softness is attributable to hydration of an otherwise glassy polymer. The best known examples of the former are the silicone polymers, whereas the latter are examplified by the Bausch & Lomb Soflens or the Griffin Naturalens.

The Bausch & Lomb lens is consisted of a homopolymer of HEMA lightly cross-linked with EDMA. EDMA, when swollen in water, has an upper limit of about 45% water content. The Griffin polymer is formed by polymerizing HEMA containing some EDMA and about 20% PVP:

ii. Hard Contact Lenses

Most successful hard contact lenses are based on linear, branched or modified PMMA and its copolymers. Hard lenses vary in hardness and the manufacturer’s special emphasis is on design characteristics such as flexibility and thinness. Gas permeable hard lenses are made from a mixture of compounds such as PMMA and silicone in the hope of utilizing the advantages of both materials.

iii. Cellulose Acetate Butyrate (CAB) Lenses

CAB is a gas-permeable organic compound. Aliquot samples are tested for purity, stability and physical properties. It is flexible, better wetted, stable, nontoxic, nonallergenic, optically good, easily worked and permeable to oxygen and glucose. Since it is hydrophobic, the surface must be treated to make it hydrophilic.

The CAB lens is fabricated from commonly found materials: cellulose from wood and cotton, acetic acid from vinegar and butyric acid from natural gas.

The main advantage of CAB lenses is increased oxygen permeability. This property of the material varies with temperature, pressure and thickness. For a given thickness, the CAB lens is much more permeable than the PMMA lens and about 40% water content. A hydrogel lens of equal thickness and a water content of over 45% will be more oxygen permeable. Compared to a soft contact lens, the CAB lens is 40 times more permeable to oxygen and 320 times more permeable to carbon dioxide.

iv. Silicone Rubber Lenses

They are recently developed and they are also gas-permeable. Silicone lenses are semiorganic polymers with the highest oxygen permeability of any lens material currently available. The silicone lens has the unusual ability to return to its original state on bending and can be broken or split like rubber if one folds it too hard. This property of elasticity is responsible for the good visual acuity one can obtain with it in the thickness in which the lens is fabricated.

The constraints do not permit a lens to be made that can be tolerated or that will survive the rigors of handling or wearing that delivers the oxygen levels needed in the cornea under closed-eye conditions. Therefore, novel materials were needed that would not be limited by these boundaries. There are two such polymers up to this time. These are balifilcon A and lotrafilcon A. Both of these polymers are referred to as silicone hydrogels, and each exceeds 90 barres of permeability.

v. Extended Wear Contact Lenses

There are certain characteristics of the ideal extended wear contact lenses. It does not interfere with the nutrients of the eye. It is gas permeable, easy to and gives good vision. It is durable enough to remove, clean and withstand spoilage during needed handling. It can asepticized repeatedly with heat. It resists deposits and does not alter the bacteria of the eye or contribute to infection. It is comfortable, convenient, inexpensive and practical for fitter.

Extended wear contact lenses bear some advantages over other lenses. They cause less corneal edema. Since oxygen is available at levels sufficient to maintain corneal integrity without corneal edema. They have reduced loss of corneal sensitivity. The corneal sensitivity is impaired with prolonged hard or soft contact lens wear. There is less of this change with use of an extended wear lens. There is less mass to the lens, which reduces lid sensation and improves comfort. There is less lid influence on centering. They are safe and convenient.

On the other hand extended wear contact lenses also have certain disadvantages. Extended wear lenses are more difficult to handle. Inversion of the thin lens is more difficult to detect. There is higher risk of lens damage or loss. Cleaning procedures may be inefficient since these lenses are more difficult to clean than other lenses. These lenses dehydrate more easily in a dry and windy environment and in air-conditioned buildings. They are expensive and follow-up visits are required.

In fitting, extended wear lenses generally are chosen for their ability to give adequate oxygen transmissibility to the cornea and good vision.

Lenses available for extended wear can be listed as:

1. Silicone: silicone has the highest oxygen permeability of any lens material currently available.

2. CAB. CAB lenses have been worn successfully by aphakic patients on an extended basis.

3. Polycon. Polycon has about the same gas transmissibility as CAB, although the Polycon lens is slightly thinner.

4. Extended wear soft contact lenses that have a low water content are very thin. These lenses include:

i. Hydrocurve II (Bufilcon A): it is a thin and yet durable to be worn for either daily or extended wear. It has a 45% water and center thickness from 0.06 to 0.07 mm.

ii. CSI (Crofilcon A): It has the lowest water content (39%)and the thinnest central lens thickness of all extended wear lenses.

iii. Softcon (Vifilcon-A): It is 55%water and has a center thickness between 0.28 and 0.70 mm.

iv. Bausch and Lomb (Polymacon): It is used only for extended wear in the treatment of myopia. It is 39% water and has a center thickness of 0.35 mm.

5. High water content extended wear soft lenses.

i. Sauflon PW (Lidofilcon-D): It is lenticular in shape, has a large number of N-vinyl perolodone molecules which are hydrophilic. It is 70 to 85% water and has a center thickness of about 0.5mm.

ii. Permalens (Perfilcon-A): It is a very pliable lens. 69 to 72% water and has a center thickness from 0.24 to 0.43 mm. By changing from 70 to 80% water, the hardness is decreased 10 times but the fragility also is increased 10 times.

Hard PMMA lenses are not designed or intended for extended wear: However, some patients have worn these lenses without removal. The longest known continuos wearing of hard lenses is 21.5 years.

VII. DRAWBACKS OF CONTACT LENSES

Over the past 25 years, great strides have been made in understanding the pathophysiology of the adverse events found with contact lens wear. These events can be grouped into four major categories: hypoxia mediated events, immune events, mechanical events and osmotic events.

Hypoxia increases the risk of bacterial infection. Interestingly, new high oxygen flux lenses, both soft and rigid, appear to reduce bacterial adhesion in eyes wearing high Dk lenses compared to lower Dk materials.

Immune mediated adverse responses are common ocular complications to lens wear. Important examples of this class of complications include contact lens –associated superior limbic keratoconjuctivitis, peripheral corneal ulcers, central sterile infiltrates and giant papillary conjuctivitis.

Mechanical injury can be subclinical with microtrauma to corneal epithelial cells. Stresses in the lens materials imposed by repeated application, removal or eye movement can cause irreversible deformation or fracture, resulting in loss of optical performance, user discomfort or even complete disintegration.

Environmental aging may be the result of slow oxidation or hydrolysis, influence of UV light, temperature changes and chemical reactions. It frequently leads to polymer degradation and considerable change of the mechanical strength of polymers.

The irreversible migration of organic and inorganic molecules from the tear solution to the lens surface is the underlying phenomenon in contact lens spoilage. Tear solutes include proteins (e.g. albumin, globulin, Lysozyme), amino acids, mucin, glycoproteins, glucose and lipids (e.g. phospholipids, neutral fats, fatty acids, cholesterol and its esters) calcium, potassium, chloride, bicarbonate, phosphate and urea. A contact lens absorbs tears, proteinaceous debris, and oils, which can form a residue on the contact lens. This residue can act as a growth medium for bacteria and lead to infection as well as impair visual acuity. The purpose of contact lens cleaning is to remove this filmy residue.

Probably the most chemically complex manifestations of ocular incompatibility are discrete elevated deposits, the so-called white spots found on the anterior lens surface. They are also referred to as mucoprotein-lipid and mucopolysaccharide deposits.

Protein deposition on biomaterials is an important problem that has been studied extensively over the past 25 years. The phenomenon is especially serious in hydrogels, for example, those used for soft contact lenses where deposition is hypothesized to lead to ocular disease. Therefore, there is the need for deposit-resistant materials, and methods to evaluate new materials for such a property.

IIX. CONCLUSION

The correction of vision using contact lenses has grown into a billion dollar industry over the past 40 years. Starting out with small made-to-order laboratories in the 1950s, the industry has increased several fold in volume and is dominated by companies such as Ciba-Geigy, Johnson & Johnson, Schering-Plough and Bausch & Lomb. Lenses are now mass-produced using automated manufacturing processes. The number of types of lenses available has tripled over the past 15 years. The patient can now select from a long list of materials including silicone acrylate and fluoroacrylate hard lenses, and hydrogels of mid-water and high-water contents. There are currently 60 brands of daily wear soft lenses, 31 brands of extended wear hydrogel lenses and 27 brands of hard lenses. Research and development efforts on new materials and manufacturing processes continue to expand. Several research groups are vigorously pursuing the design of the 30-day extended wear lens, and the development of a fully automated lens manufacturing process by Johnson & Johnson has led to the introduction of the first daily disposable contact lens.

Today there are more than 25 hydrogel polymers on the US market alone and scores of contact lens manufacturers with revenues in the hundreds of millions of dollars supplying millions of US contact lens wearers. Worldwide there were approximately 75 million contact lens wearers as of 1998, according to the International Association of Contact Lens Educators. Estimates by region indicate that 17 million people wear contact lenses in Asia, 17 million in Europe, 0.5 million in Australia, and million in the US.

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