There are approximately 90 million contact wearers worldwide. In the United States alone, data indicate that approximately 13% of the population (>36 million) use contact lenses for vision correction and it is estimated that the United States represents approximately 40% of the number of wearers worldwide. Most contact lens wearers also own a pair or two of spectacles. Data indicate that in 2015, the number of spectacle units (number of eyeglasses manufactured) worldwide was approximately 3.5 billion.
The soft lenses rival rigid lenses in their quality of vision and surpass them in the realm of comfort and ease of adaptation ( Fig. 15.1 ). The two basic types of soft lenses are the hydrogel (hydrophilic) lens, which owes its softness to its ability to absorb and bind water to its structure, and the silicone hydrogel lens, which owes its softness to the intrinsic property of the rubbery material.
History of hydrophilic lenses
In 1960 two young New York lawyers established a company with the unique function of promoting patent exchanges among corporations. Their specialty was combing through the dusty corporate files for idle patents and setting up licensing agreements with other companies interested in putting the dormant ideas to use. In 1965 the men who had established the National Patent Development Corporation suddenly dissolved their patent law business. They had uncovered a patent with so many exciting possibilities that they decided to pick up a license themselves. In effect, they became their own client, eliminating their role as middlemen.
The new material was a plastic that they called hydron. It was developed by Dr. Otto Wichterle, head of the Institute of Macromolecular Chemistry of the Czechoslovakian National Academy of Science and a leading expert on polymer chemistry, and by Dr. Drahoslav Lim. The new material appeared to be like other plastics in that it was a hard transparent substance that could be cut, ground, or molded into a variety of shapes. When placed in water or an aqueous solution, however, the tough, rigid plastic became soft and pliable. In this wet form, it could be bent between the fingers until the edges met or could be turned easily inside out, yet it would snap back into its original shape quickly. When allowed to dry, the supple waterlogged material became as dry as a cornflake and crushed to a powdery dust if smashed. The substance was subjected to rigorous biologic tests and was found to be inert and fully compatible with human tissue. One of its properties was that although highly elastic when wet, it remained strong and able to hold its shape ( Fig. 15.2 ).
The plastic is hydroxyethyl methacrylate (HEMA), a plastic polymer with the remarkable ability to absorb water molecules. Chemically, the polymer consists of a three-dimensional network of HEMA chains crosslinked with ethylene glycol dimethacrylate molecules about once every 200 monomer units. As the water is introduced to the plastic, it swells into a soft mass with surprisingly good mechanical strength, complete transparency (97%), and the ability to retain its shape and dimensions.
Five vigorous years of improvements and clinical trials were conducted on the soft lens before the U.S. Food and Drug Administration (FDA; which considered the lens a drug) approved the lens as a safe prosthetic device of good optical quality. The FDA’s caution, after the thalidomide tragedies in which a drug produced severe deformities in the babies of pregnant women, was understandable. Both the public and the practitioner needed protection. In the first phase of research, the soft lens was tested with laboratory animals to ensure that it was nontoxic; later it was given to selected practitioners and independent research workers for clinical trials on human beings.
It soon became apparent that the soft lens was an innovation of major importance, with widespread application not only as an instrument for treating diseased corneas, but also as a superior contact lens. In the early stages, however, the therapeutic possibilities of soft lenses overshadowed any other consideration because it appeared that these lenses would replace many conventional treatments of external diseases of the eye.
As the number of contact lens companies throughout the world expanded, a search for newer and better lens plastics and lens designs developed. Many modifications were made as new monomers were discovered and crosslinked to create differing polymers; polyvinyl pyrrolidone (PVP) was added to increase oxygen permeability, and methyl methacrylate (MMA) to create more stiffness to aid in handling ease. Non-HEMA hydrogel polymers were introduced.
Research activity was also directed toward lens designs to correct astigmatism, bifocal corrections, and tinted lenses for cosmetic appearance, as well as therapeutic application. Ultrathin and higher-water-content lenses have opened up significant new areas in contact lens development, with increased success rates. In the manufacturing arena, the emphasis is on automated computer-driven systems and advanced molding processes.
The most recent developments have been the addition of ultraviolet-screening agents to soft polymers and the amplification of the concept of biomimesis. Biomimesis is defined as the ability to create or mimic biologic surfaces. Various approaches have been tried to improve the biocompatibility of materials for use in the human body. The surface properties of foreign materials play a critical role in triggering a biologic response and initiate undesirable and unwanted interactions with proteins and other biomolecules at the material surface. The formation of blood clots on surface materials, dental plaque buildup, and contact lens deposits are examples of the same phenomenon. A synthesized phosphorylcholine added to soft lens material to promote biocompatibility has produced the Proclear lens from CooperVision. Current soft lens research aims to produce a contact lens that replicates the tears in the human eye. Materials with water content as high as 92% are under investigation.
With the proliferation of soft materials, it became necessary to classify lenses in several ways. Classification by water content means that the soft lens contains that percentage of water, for example, low water content (37.5%–45%), medium water content (46%–58%), and high water content (59%–92%). The FDA classifies lenses into four basic groups based on water content and ionicity for the purpose of evaluating disinfecting systems with different lens material groups. With the development of new high-DK materials, a fifth classification group is being considered. The process used to manufacture a soft lens is another method of classification. Lenses may be spin cast, lathe cut, or cast molded; 90% of soft lens production in the world today uses the cast-molded method. Finally, soft contact lenses may be classified by design or function (e.g., daily wear, flexible wear, extended wear, or continuous wear).
Silicone hydrogel lenses with higher oxygen permeabilities are the latest development in soft contact lens material ( Boxes 15.1 and 15.2 ).
Material: balafilcon A (36% water)
Manufacturing method: cast molding
Base curve: 8.3 and 8.6 mm
Diameter: 14 mm
Power range: + 6.00 diopters (D) to –12.00 D (8.6 mm base curve) –0.25 D to –6.00 D (8.3 mm base curve)
Center thickness: 0.05 to 0.50 mm
Optical zone: 8.9 mm for –3.00 D
Oxygen permeability: 101 × 10 −11 (cm 3 O 2 [STP] × cm)/(sec × cm 2 × mm Hg) @ 35° C (95° F) Polarographic method (boundary corrected, nonedge corrected)
Visibility tint: light blue
Very high DK values available
Suitable for extended wear
Rapid adaptation
Easy handling because of their rigidity
Low rate of deposits
Advantages
The advantages of hydrophilic contact lenses are as follows:
- 1.
Comfort
- 2.
Rapid adaptation
- 3.
Lack of spectacle blur
- 4.
Disposability
- 5.
Minimal lens loss
- 6.
Minimal overwear reaction
- 7.
Lack of glare and photophobia
- 8.
Difficulty in dislodging
- 9.
Protection of entire cornea
- 10.
Attractive alternative for rigid lens dropouts
- 11.
No serious corneal abrasion on insertion
Comfort
These lenses are exceptionally comfortable from the initial period. It is impressive to witness the rapid tolerance of the cornea to the presence of the contact lens. The lack of awareness of a soft lens is caused partly by the flaccidity, water content, and thin edges of the lens, which mold to the white of the eye (the sclera). Therefore there is almost no interference from the upper lids during normal blinking. The lens hugs the eye so closely that the advancing surface of the eyelid just glides over it ( Fig. 15.3 ). Its supple quality when wet also contributes to the easy acceptance of the soft lens. Being hydrophilic, or water-loving, it forms a cushioned fluid buffer between itself and the cornea. It also contours itself to the unique shape of the individual cornea. With no hard edges to irritate the eyelid edge and no rigid structure to compress delicate living tissue, there are minimal frictional erosions. Because of its soft qualities, a normal tear exchange takes place by diffusion through the lens matrix and under the lens.
A rigid lens has to be fitted according to the precise shape of the cornea. If the fit is poor, or if the laboratory does not make the lens according to exact specifications, a rigid lens will irritate the eye. A soft lens is more flexible on the eye. A wide latitude is possible without corneal injury and less exacting measurements are required.
Rapid adaptation
Tolerance is extremely high compared with that of the rigid lens. The lenses are frequently comfortable to a new patient within 30 minutes. Wearing schedules can be easily increased to full-time day wear within 10 days.
Lack of spectacle blur
Removal of the lenses permits patients to switch directly to their glasses within 5 to 10 minutes without the spectacle blur that occurs with edema induced by polymethyl methacrylate (PMMA) rigid lenses. Spectacle blur is uncommon because of the diffuse nature of any edema, which spreads evenly over the cornea and does not alter its radius ( Fig. 15.4 ).
Disposability
Soft lenses may be replaced on a disposable regimen, for example, replaced daily, weekly, or biweekly, reducing patient discomfort and risk of infection. Soft lenses replaced on a disposable regimen are ideal for occasional or intermittent wear.
Minimal lens loss
With new patients, rigid lenses are sometimes lost in the first 3 months, when handling is still somewhat clumsy. Rigid lenses also dislodge with aggressive sports activities. The technique for removal of a soft lens is such that loss is less frequent than with a rigid lens. The larger size of the soft lens, coupled with the firm adherence of the lens to the cornea with minimal sliding effect, reduces the loss factor considerably. It is rare for a patient to report the loss of a soft lens ( Fig. 15.5 ). The lenses do not fall out.
Minimal overwear reaction
Every ophthalmologist remembers cases of the overwear syndrome experienced by the old PMMA rigid lens wearer, who appeared at the hospital emergency room in the middle of the night with excruciating pain, having worn these lenses longer than the normal time limit. This problem is virtually eliminated with the soft lens. In older low-DK soft lens material, 2% of patients reported slight corneal edema with halos about lights and a burning sensation of their eyes. At the end of the day, no serious disabling disorder has occurred. The edema effect was further reduced by the advent of materials with greater oxygen permeability.
Oxygen is carried to the cornea through the tear film and is replenished through the circulation of tears under the lens and diffusion through the lens. This is the same method by which the cornea receives its oxygen supply under a rigid lens. The evidence of a good tear layer between the soft lens and the cornea has been demonstrated by a French ophthalmologist, Dr. Paul Cochet, who showed spherical particles 1 to 3 μm in diameter passing underneath the lens. When the tear layer has been stained, it has been shown to ripple with the blinking motions of the lids. The respiration of the cornea is provided by tear exchange during blinking.
Lack of glare and photophobia
Glare and light sensitivity are seen almost routinely in the early weeks of rigid lens wear. These symptoms are virtually absent with the soft lens, making it the ideal lens for outdoor athletes, such as golfers and tennis players. Also the generous size of the optic zone means that the pupil is always covered; this minimizes glare.
Difficulty in dislodging
The firm adherence of the soft lens to the eye permits it to be used in body contact sports and reduces embarrassment associated with dislodgment ( Fig. 15.6 ).
Protection of entire cornea
Hydrophilic lenses cover the entire cornea. They can be used to reduce corneal exposure for such conditions as facial paralysis and corneal insensitivity ( Fig. 15.7 ). In this sense, these lenses are used as bandage lenses for entropion, trichiasis, dry eye, and corneal dystrophies.
Attractive alternative for rigid lens drop-outs
A significant percentage of rigid lens patients are unable to persist in wearing their lenses. This intolerance may be the result of dryness as a result of pregnancy, birth control pills, allergies, or a change of environment. Most of these patients readily accept soft lenses and are able to wear them comfortably.
No serious corneal abrasion on insertion
In the rigid lens, incorrect insertion can cause an abrasion of the cornea. This does not occur with the soft lens because of its soft nature.
Cosmetic lenses
Colored and opaque hydrogel lenses to enhance or change eye color have been available since the 1980s.
Disadvantages
The disadvantages of soft contact lenses are as follows:
- 1.
Lack of ability to correct severe astigmatism
- 2.
Variable vision
- 3.
Lack of durability
- 4.
Faulty duplication
- 5.
Deposit formation
- 6.
Modifications impossible
- 7.
Disinfection problems
Lack of ability to correct severe astigmatism
Astigmatism is not easily corrected by conventional soft lenses. The soft lens contours to the eye and corneal astigmatism frequently remain uncorrected. There are, however, a number of special designs of toric soft lenses available to correct astigmatism of dioptric powers up to 4.00 diopters ( Fig. 15.8 ).
Variable vision
Despite good fittings, a small percentage of patients become disenchanted because of either poor or variable vision. These problems may result from fitting failures, uncorrected astigmatism, or a dehydration effect from the water-laden lenses. They may also result from deposit formation and lens spoilage.
Lack of durability
Soft lenses are much more fragile than rigid lenses and any rough handling may scratch or tear them. Even with careful handling, they may be sliced by fingernails and may develop nicks at the edges because of the constant pinching and flexing of the lenses during insertion and removal. However, they are much hardier today. The higher the water content or the thinner the lenses, the more fragile they are.
Faulty duplication
Lenses will break or tear and require replacement. Often a duplicate set may be necessary, but duplication of lenses today is of a high standard. Most hydrogel lenses are disposable or frequently replaced and therefore are mass produced.
Deposit formation
Protein, mineral, and lipid deposits may accumulate on the surface of a soft lens more quickly than on a rigid lens ( Fig. 15.9 ). Although special cleaning and protein-removing agents are available, strict adherence to their schedules for use is required to eliminate these buildups and preserve the life of the lens. New lens developments may minimize deposit adherence.
Modifications impossible
Although a soft lens can be dehydrated to the dry state, it does not form a regular shape in the dry state and accurate modifications are impractical.
Disinfection problems
The routine of disinfecting must be rigidly adhered to, otherwise infection may occur. This applies if the lenses sit in the drawer during illness or vacations, or if there is a temporary “holiday” back to glasses.
Boiling, chemical, and ultraviolet methods of disinfection have their advantages and their drawbacks. In any event, rigid adherence to disinfection procedures is important both for the practitioner who keeps an inventory and for the patient who wears the lenses only occasionally. Fungal growth and bacterial contamination of the lenses may occur because soft lenses can be penetrated more easily by infectious organisms. In addition, protein adhering to the lens surface can harbor bacterial organisms. Another source of infection is the use of stale, dated solutions. Because of the very real possibility of contamination with Acanthamoeba , which could result in a serious protozoan infection, tap water and even distilled water is no longer recommended for use in any way with soft contact lenses.
Patient evaluation
Each patient who visits the office for a contact lens examination requires a complete history and physical examination. History taking should include previous eye disease, recurrence of infection, systemic diseases (e.g., diabetes, neurologic disorders, ocular disorders), current medications, allergies, and general health. In addition, age, occupation, and previous contact lens experience should be recorded. The third factor to consider is the patient’s motivation and capability for compliance. A history of allergies might require the patient to use nonpreserved solutions for rinsing.
External examination consists of careful inspection of the cornea, conjunctiva, and fornices. The lids should be everted to detect any underlying pathologic condition and, in particular, note should be made of papillary formation of the conjunctiva. Routine slit-lamp biomicroscopic examination is performed to detect the presence of any corneal disease or scarring. Schirmer’s test should be performed to evaluate the adequacy of tear formation. The corneal sensitivity test can be administered ( Fig. 15.10 ). The palpebral fissures should be measured; the findings may alter the fitter’s judgment about the diameter of the soft lens to be used. Lid tension can be evaluated by grasping the upper lid between thumb and forefinger and holding it upward. A careful refraction should be performed, along with keratometer readings. The pupil and horizontal visible iris diameter should be measured by a ruler, a pupillometer, or a slit-lamp biomicroscope. These measurements also may affect the diameter of the lens selected.
Patient selection
Soft lenses are the lens of choice today.
They are ideal for the following individuals:
- 1.
Rigid lens drop-outs because of irritation, a bad experience with overwear, or difficulty in maintaining a rigid daily wearing schedule
- 2.
Intermittent wearers, such as public speakers, athletes, or actors, who want to wear their lenses only occasionally
- 3.
Workers who fear losing a lens at work (soft lenses rarely fall out)
- 4.
Aphakic patients
- 5.
Older adults who are impatient with the prolonged routine in following the rigid lens-wearing schedule
- 6.
Patients with low to moderate astigmatism
The decision to dispense contact lenses to any patient is, of course, a professional judgment and should be made by a professional contact lens fitter.
Manufacture
There are various methods of manufacturing soft lenses:
- 1.
Spin casting
- 2.
Lathe cutting with manual or automated lathes
- 3.
Molding methods
Today, the most common method of manufacturing soft lenses is a combination of lathe cut and molding. Most steps today are relatively automated, quality control being performed by random sampling.
Spin-cast lenses
The spin-cast process, devised by Bausch & Lomb, produces a highly reproducible lens with a very smooth surface. This means that the lens is so standardized that all replacement lenses are duplicates of the original regardless of where in the world they are purchased. Although this is not difficult for a Coca-Cola bottle, it is a triumph for contact lens assembly.
Bausch & Lomb manufactures its soft lens by means of the spin-cast method. It is derived from a revolving mold that whirls the liquid plastic at high speed. The mold gives the lens its outside curvature. The inside curvature is formed as a result of the speed of rotation, the various surface tensions of the liquid, and the precalculated mathematic relationship between gravitation and rotation. The result is a parabola with an inside curvature that can shorten or lengthen, depending on the speed of rotation. The procedure is basically a kind of pressureless molding.
Because the posterior surface of a spin-cast lens is aspheric, the traditional K readings and the posterior surface’s relationship to the base curve do not apply with these lenses. The basic fitting system is based on measurement of the horizontal visible iris diameter and selection of a suitable diameter of lens with proper power. The numeric suffix on the label denotes the lens diameter. A label ending in 4 is a 14.5-mm diameter lens, a 3 is a 13.5-mm diameter lens, and the absence of a number is a 12.5-mm diameter lens.
A combination of the spin-cast and lathing processes has been developed by Bausch & Lomb. This design (e.g., Optima), permits more variables in fitting while combining the high quality of the front surface for crisp optics. Other major soft contact lens manufacturers, such as Alcon, CooperVision, and Vistakon have developed their own advanced proprietary manufacturing methods.
Lathe-cut lenses
In the lathe-cut manufacturing process, the lens, in the dehydrated state, behaves like a rigid lens. It is cut on a lathe to exact specifications similar to those of a rigid lens. Automated lathes are in fashion and reduce labor. Initially, the back surface is ground with a diamond tool and then the front surface is polished and edged. In this method, peripheral curves, blends, and even intermediate curves can be cut for better lens design. Lathed lenses are individually or custom made and a wide variety of parameters can be ground for a better fit. The most important factor in the grinding of the soft lens is that one cannot use the usual polishing compounds that contain water; thus the whole process of grinding and polishing must be performed without any contamination by water.
Lathe-cut lenses are produced for both stock and custom orders, with most using high-tech computer-controlled systems for excellent quality and reproducibility. The most important consideration in cutting soft lenses from a dry button that is later to be hydrated into a hydrogel lens is that the entire process of lathing must be performed under very strict climate control; too much humidity in the laboratory can cause variations in the finished product because the button can absorb moisture from the air. After completion of the grinding process in the hard inflexible state, the lens is placed for several hours in a water bath, where it undergoes swelling and expands to its final state. This swell factor must be taken into account in the grinding of the lens in the hard state.
When the finished lens in a dry state is hydrated for final wet inspection or quality control, the difference or swell factor is 20% to 40%, depending on the polymer material and water content. This factor makes it extremely important for the “dry state” lens to be made to exact specifications. In years past, criticisms of lathe-cut lenses included their inconsistency and that reproducibility could be suspect. Today’s lathe-cut lenses, however, compare favorably in quality and reproducibility with those manufactured by any other process.
Molded lenses
The cast-molding process uses precision injection molding of engineered thermoplastic resins to produce lens replica molds. These molds are used in a monomer-casting process to convert crosslinkable lens monomers directly into a finished contact lens form. This process produces an optically finished surface from the mold, thus ensuring accurate reproduction of the lenses.
The FDA receives a constant parade of applications for approval of new lenses. With each lens comes an innovative approach to solving some of today’s contact lens problems.
Inventory versus diagnostic lenses
Soft lenses may be fitted in one of two ways: from an inventory of lenses, by selecting the lens that gives the best fit and the best visual acuity, or from a trial set of standard diameters and base curves to obtain the proper fit. The fitter can then overrefract to obtain the correct power of the lens and order directly. Alternatively, lenses may be ordered after the fitter makes an educated guess according to K readings, horizontal corneal diameter, and refraction, realizing that a few changes of lenses may be required before the correct lens fit is achieved.
As a result of the large diameter of soft lenses and its effect on sagittal depth, their fit must be much flatter than that of rigid lenses. The average diameter of soft lenses used today is 13.8 to 14.5 mm, requiring that they be fitted 1.0 to 1.5 mm (5.00–7.00 diopters) flatter than K . Because of the large diameter of these soft lenses, they should be fitted appreciably flatter than the flattest K reading of the cornea. Lens diameter and base curve are inversely related. To arrive at essentially the same fit, the base curve of the lens selected should be flattened as the lens diameter is increased; for example, a 12- to 13-mm diameter lens usually is fitted approximately 2.00 to 3.00 diopters flatter than K , whereas a 14- to 15-mm diameter lens has to be fitted approximately 3.00 to 5.00 diopters flatter than K .
Lens selection may be based on one of three methods:
- 1.
Selection of soft lenses based on probable corneoscleral profiles, in which the fitter may select a lens diameter based on the horizontal iris diameter and observe how the lens performs on a given eye.
- 2.
Selection of soft lenses with a posterior curvature of radius based on K readings of the cornea, determined by actual measurement of the cornea.
- 3.
Selection of soft lenses based on the sagittal value of the lenses, which requires the K reading of the cornea and takes into account not only the posterior radius of the lens curvature but also the diameter of the lens.
The fitting criteria are similar for all daily wear soft lenses. Some basic guidelines apply:
- 1.
The hydrophilic lenses are fitted as large as or larger than the diameter of the cornea and range in size from 12 to 15 mm.
- 2.
Small eyes require smaller diameters and consequently steeper base curves, whereas larger eyes are fitted with larger lenses and flatter base curves.
- 3.
Soft standard-thickness lenses generally are fitted flatter than the flattest K reading, usually about 2.00 to 3.00 diopters for the smaller lenses and 3.00 to 5.00 diopters for the larger lenses. The thinner soft lenses (≤0.06 mm) are fitted 4.00 to 7.00 diopters flatter than K because they tend to drape themselves over the corneal surface ( Fig. 15.11 ).
Fig. 15.11
Three-point touch. A normally fitting soft lens will rest lightly at the apex and at the periphery of the cornea.
(From Stein HA, Slatt BJ, Stein RM, et al. Fitting Guide for Rigid and Soft Contact Lenses: A Practical Approach . 4th ed. St Louis: Mosby; 2002.)
- 4.
A normal-fitting lens should show a 0.5- to 1-mm lag in the downward direction with each blink and provide good vision before and after blinking.
- 5.
A soft lens that moves excessively (more than 1 mm with each blink) is too flat; a soft lens that moves less than 0.5 mm with each blink is too steep and will limit tear exchange.
- 6.
A soft lens that decenters usually is too flat or too small ( Fig. 15.12 ). To correct this problem, a larger-diameter lens or a steeper lens should be selected.
Fig. 15.12
Decentration. The soft lens does not center properly; it has decentered outward.
- 7.
The fitter may determine increased steepness or flatness from a table showing the relationship of the diameter to the radius (sagittal values; Table 15.1 ). To loosen a lathe-cut lens, smaller diameters in 0.5-mm steps may be fitted or the radius increased in 0.2- to 0.3-mm steps.
Table 15.1
Sagittal relationship of various base curves and diameters
Diameter
Radius
Diameter
Radius
Flatter
14.0
8.1
12.0
8.7
13.0
7.2
12.0
8.1
14.5
8.4
12.5
8.7
13.5
7.5
12.0
8.4
15.0
8.7
12.5
8.4
14.0
7.8
12.5
8.1
14.5
8.1
12.5
7.8
15.0
8.4
12.0
7.8
15.5
8.7
13.0
8.7
13.5
7.2
13.0
8.4
14.0
7.5
13.5
8.7
14.5
7.8
13.0
8.1
15.0
8.1
13.5
8.4
15.5
8.4
14.0
8.7
14.0
7.2
13.0
7.8
14.5
7.5
13.5
8.1
15.0
7.8
14.0
8.4
15.5
8.1
13.0
7.5
15.0
7.5
14.5
8.7
15.5
7.8
13.5
7.8
Steeper
- 8.
The lower the water content of the lens, the more durable the lens becomes. The higher the water content, the more fragile is the lens.
- 9.
The thinner the lens, the greater is the oxygen permeability to the cornea. However, a thin lens tears easily and some optical quality may be lost by wrinkling.
- 10.
Hazy vision caused by oxygen deprivation may occur either from wearing a lens that is too tight or from overwearing the lens.
- 11.
Contact lens decentration can be caused by tight eyelids, large corneas, against-the-rule astigmatism, or asymmetric corneal topography.
- 12.
Routine soft lenses do not correct large amounts of corneal astigmatism. In general, astigmatism greater than 1.0 diopter requires correction by toric soft lenses or rigid lenses.
- 13.
With soft lenses, regular fluorescein cannot be used to study tear exchange because it permeates the lens. High-molecule fluorescein can be used, but it is no more effective than evaluating tear exchange by noting the movement of the lens. Fluorescein is helpful in highlighting and assessing corneal pathologic conditions.
- 14.
Heavier lenses usually have to be fitted larger. The lens weight is influenced by its thickness and water content. Polymers of high water content are usually weaker than those of lower water content and require lenses of greater thickness. The increased gravitational pull on a heavier lens has to be offset by use of a larger diameter.
- 15.
The rigidity of a lens is a function of its thickness, its water content, and the unique properties of the polymer from which it is made.
- 16.
When fitting soft lenses, the fitter should aim at fitting the flattest possible lens that will provide good clear vision, center well, and have no effect on the corneal integrity.
Lens inspection
The following routine is important for checking the quality of the lens received. For all soft lenses, each factor should be assessed. Because of the manufacturing reproducibility of today’s soft contact lenses, lens inspection is typically only done on specialty lenses when there is a concern of an incorrect parameter(s).
Edge and surface inspection
This should be performed with the lens under the microscope. The lens may be held in the hand or placed on a clear glass slide. It may be compared with white paper placed behind it to see whether discoloration has occurred. The lens also can be examined under a hand magnifier.
Diameter
The lens is viewed through a magnifying gauge with a millimeter scale. No pressure must be exerted on the lens to distort the surface.
Base curve
The Soft Lens Analyzer was developed to measure the base curve of hydrogel lenses. In addition, it measures diameter and center thickness and provides close surface and edge inspection on soft lenses as well as rigid lenses. Because hydrogel lenses all contain some percentage of water, accurate measurement is best obtained in the hydrated state. The Soft Lens Analyzer provides a wet cell in which the lens is immersed in saline. The lens is then measured against a series of hemispheric standards with known radii from 7.6 to 9.8 mm in 0.2-mm increments. A beam of light is projected through and around the lens positioned on the standard. This image is projected onto a small built-in screen at × 15. The operator determines the base curve in terms of the lens-bearing relationship to the standard on which it is centered. This system for measuring the base curve of a lens is applicable to all lenses.
Power
This can be determined with a fair degree of accuracy for spin-cast and lathed lenses. The lens is cleaned well and blotted dry with lint-free tissue. It is placed concave side up under a lensmeter. The measurement must be read quickly because this becomes impossible if the lens dries too much. The value of in-office inspection of a soft lens for its power before dispensing it is questionable. Sterility can be compromised and the ability to check lens quality in the practice is poor. Manufacturers use optical, ultrasound, and interferometry to verify the base curve and power of soft lenses.
The evaluation of a good fit depends on the positioning of the lens and its movement on the eye. The basic fitting philosophy is to fit the flattest, thinnest lens that will provide crisp vision before and after the blink, comfortable wear, stable positioning, and minimum metabolic interference.
A well-fitted lathe-cut lens is about 1 to 2 mm larger than the cornea, centers well, and results in a lag of 0.5 to 1 mm or slightly less on upward gaze. If the eye is moved sideways, the lens will lag slightly, but will quickly center and follow eye movements. The eye should be white and the patient able to wear the lenses comfortably all day. The lenses may, however, be too tight or too loose ( Fig. 15.13 ).
Tight lens
A tight lens is really a large, or steep, lens and does not appear to have any movement after a blink ( Fig. 15.14 ). It is uncomfortable to the wearer and may cause circumcorneal injection and indentation of the sclera adjacent to the limbus. The lens centers well, but vision is frequently blurred or fluctuating ( Fig. 15.15 ); after a blink, there may be a temporary clearing that lasts a few seconds. Retinoscopy may reveal a dark shadow in the center, which may momentarily clear after a blink. Keratometry will show distortion of the mires, which clear after a blink. All these signs of a tight lens are caused by a lens that is steeper than the central cornea, which results in a gap that separates the lens from the cornea. During a blink, the lid smooths the lens across the cornea and there is a temporary central adherence of the lens to the latter. Sometimes a lens that fits well initially gradually tightens ( Fig. 15.16 ).
