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Chapter 12 – Contact Lenses

Chapter 12 – Contact Lenses

PAUL F. WHITE
CLIFFORD A. SCOTT

DEFINITION
• Contact lenses are visual devices that provide an artificial anterior refracting surface to the human eye and are used for corrective, cosmetic, and therapeutic purposes.

KEY FEATURES
• Lenses are made from various soft and rigid materials and are configured in a variety of designs.
• Special design lenses include disposable, colored, astigmatic, aphakic, presbyopic, and keratoconic.
• Initial fitting procedures should fulfill specific general and individual criteria.
• Follow-up care is essential to provide optimum vision, appearance, comfort, and tissue integrity.

ASSOCIATED FEATURES
• Corneal and conjunctival tissue problems.
• Common categories include superficial punctate keratopathy, edema, microcysts, infiltrates, hyperemia, neovascularization, polymegathism, blebs, and giant papillary conjunctivitis.
• Mechanical or physical problems include spectacle blur, flexure, visual flare, aberrations, magnification, accommodation, convergence, and tear fluid effects.

INTRODUCTION
Contact lenses have changed dramatically since their basic optical concept was described first by Leonardo da Vinci in the 16th century and later by René Descartes in the 17th century.[1] [2] Many large textbooks are devoted exclusively to the subject of contact lenses; in this chapter an overview is presented.
About 32 million people in the United States wear contact lenses, constituting about 20% of those who use refractive correction.[3] The major use of contacts is to correct myopia, but contact lenses are also used to correct hyperopia, astigmatism, presbyopia, and aphakia. Rigid contact lenses are often the best type to correct for irregular corneal surfaces, as found in keratoconus, corneal trauma, and penetrating keratoplasty and, sometimes, after radial keratotomy. Soft contact lenses may be used as a therapeutic bandage for some conditions, such as bullous keratopathy and recurring corneal erosion, and also to improve comfort, vision, and wound healing in the immediate postoperative period after photorefractive keratectomy.[4]
GENERAL LENS AND MATERIAL TYPES
In terms of their overall lens diameter (LD), contact lens types are classified as
• scleral
• semiscleral (soft)
• corneal
Scleral LD is 23–25?mm; the central optic zone diameter is 11.5–13.5?mm, peripheral to which is an annular haptic area. The optic zone is fitted to clear slightly the underlying cornea and limbus, and the haptic area has a flatter curvature to align with the underlying sclera.[5] Either a diagnostic trial set is used when contact lenses are fitted or an impression is made of the ocular surface and a lens molded to conform with this. From their original production in the late 19th century up to the late 1950s, only scleral contact lenses were available. Originally, these were made of glass, but in the late 1930s a plastic called polymethyl methacrylate (PMMA) came into general use.[6] Although PMMA has many excellent properties as a contact lens material, it is almost completely impermeable to the gases, such as oxygen, that are essential for normal corneal metabolism. Some currently available scleral lenses are made from rigid gas-permeable (RGP) materials that allow oxygen permeation. Corneal rigid and semiscleral soft lenses are used by the vast majority of patients, but scleral lenses are preferable for some patients who have severe keratoconus, very irregular corneas, and ocular surface disorders such as occur in Stevens–Johnson syndrome.
The development of corneal contact lenses made from PMMA began in about 1950. The LD of these earlier corneal contact lenses was about 11.5?mm, whereas contemporary LDs are usually in the range 8.5–9.5?mm. In contrast to scleral lenses, corneal lenses are easier to fit because they cover much less of the ocular surface. The smaller area covered also allows better flow of preocular tear fluid and direct access to atmospheric oxygen for the ocular surface not covered by the contact lens. Early corneal contact lenses had a monocurve back surface, or base curve, but this did not allow appropriate conformation to the cornea, which flattens from the apex to the limbus.[7] Contemporary corneal contact lenses usually have bicurve or tricurve back surfaces, wherein the spherical secondary curves are flatter than the back curve toward the periphery of the lens. Aspheric back surface curvatures are sometimes used for better conformation to the aspheric cornea, whose elliptical shape has an eccentricity of about 0.5. Early corneal contact lenses often had a large center thickness; for example, a -3.00D lens had a center thickness of 0.30?mm, which is about twice that of contemporary lenses. Since the mid-1970s, PMMA has been copolymerized with silicone and/or fluorocarbons and polyvinylpyrrolidone (PVP) and/or methacrylic acid (MAA) for use in contact lenses.[8] Silicone is highly permeable to oxygen, but it is flexible and hydrophobic, whereas PVP and MAA are hydrophilic. Almost all current corneal contact lenses are RGP materials, and the various brands are formulated with different percentages of PMMA or similar materials, silicone or fluorocarbons, and povidone or MAA. Accordingly, the resultant contact lenses have different characteristics of oxygen permeability, stability and/or flexure, and surface wetting and/or reactivity. These lenses are termed siloxane acrylates or fluorosiloxane acrylates.
The development of soft (hydrogel), semiscleral contact lenses began in the late 1960s and early 1970s.[9] The LDs of soft lenses fall in the range 12.5–15.5?mm but most often are 13.5–14.5?mm. These lenses are fitted to cover the cornea and limbus and to extend slightly onto the sclera. Soft lenses with LDs similar to those of corneal lenses move excessively, fold, and are uncomfortable.

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With the appropriate LD, soft lenses wrap around the ocular surface, which makes them comfortable and easy to fit. The basic material of most soft lenses is hydroxyethyl methacrylate, which is able to absorb fluid.[10] When dehydrated, a soft lens is hard and brittle; after hydration, the lens swells and softens, its thickness and diameter increase, and its refractive index decreases. The combination of various percentages of hydroxyethyl methacrylate, povidone, MAA, and other monomers produces different fluid absorption capacity, strength, and surface reactivity. Low-fluid soft lenses absorb about 38% of their weight in fluid, medium-fluid lenses absorb about 55%, and high-fluid lenses absorb about 70%.
The U.S. Food and Drug Administration (FDA) gives each contact lens material a generic name.[11] In general, all hydrogel lens generic names have the suffix “filcon” and all nonhydrogel lenses, “focon.” Hydrogel lenses are categorized into four groups by the FDA to enable the evaluation of effects of accessory products upon the lens material ( Table 12-1 ). Lenses with less than 50% water content are considered to be “low water,” and the others are known as “high water.” Less reactive surfaces are termed “nonionic,” and more reactive materials are labeled “ionic.” Disinfection of low water content group 1 and 3 lenses can usually be done safely and effectively using thermal, chemical, or hydrogen peroxide systems. High water content group 2 and group 4 lenses generally should not be disinfected thermally, but chemical and hydrogen peroxide systems usually provide safe and effective disinfection for these.
The Dk (permeability) of hydrogel contact lenses is a function of the water content; materials of lower water content have lower Dk values and those of higher water content higher Dk values. Theoretically, Dk values are an absolute for any given material, but practically the values found by different researchers vary somewhat. A clinically useful approximation is to consider Dk values in three groups ( Table 12-2 ).
It also is important to remember that Dk/L (central transmissibility) and

(average overall transmissibility) are dependent upon lens thickness (L) and are more important than Dk. Minimum center thicknesses for minus-powered soft lenses with different water contents are about 0.03?mm for low water, 0.06?mm for medium water, and 0.12?mm for high water. Thus, the Dk/L is approximately the same, about 30 × 10-9 , for all lenses of minimum center thickness.[12]
In the term Dk, D stands for diffusion and k stands for solubility. The oxygen permeability of soft materials is almost entirely the result of solubility, whereas for RGP materials it is almost entirely the result of diffusion. When fully hydrated, RGP materials absorb less than 1% of their weight in fluid; RGP materials with a Dk less than 20 are low, 20–49 medium, 49–99 high, and over 100 very high. The better materials should have a balance of oxygen

TABLE 12-1 — PROPERTIES OF FOOD AND DRUG ADMINISTRATION GROUPS (FDAS) 1–4
Group
Water Content
Surface Reactivity
1
Low
Nonionic
2
High
Nonionic
3
Low
Ionic
4
High
Ionic

TABLE 12-2 — USEFUL CLASSIFICATION OF Dk VALUES
Water (%)
Dk (permeability)
c. 38
c. 9
c. 55
c. 18
c. 75
c. 36

permeability, surface wettability and/or reactivity, and stability and/or flexure. Materials of medium Dk provide this best.
For over 20 years, manufacturers have tried to develop contact lenses that combine RGP and soft materials. The goal was to maintain the fit and comfort of soft lenses while significantly increasing the Dk/L. The gas permeability of such combinations comes from both the solubility (k) of soft and the diffusion (D) of RGP materials. In the past few years, silicone-hydrogel lenses have become available in Dk values over 100.
SPECIAL LENSES AND USAGE
Daily wear (DW) contact lenses are worn during the day; after removal, they are cleaned and disinfected. Extended wear (EW) contact lens were to be worn day and night for periods of 1–7 days, which had been the maximum continuous FDA-approved wearing period. They must then be removed, cleaned, and disinfected. [13] In 2001, the FDA approved a lens made from a silicone-hydrogel material for continuous wear of up to 30 days and nights. Since then a second silicone-hydrogel material and an RGP material have also been approved for continuous wear of up to 30 days and nights. Mandatory postmarketing surveillance is required as part of the approval in order to determine the true level of safety and efficacy under real-life conditions and for more patients than were involved in the research required for approval. For example, 5000 “subject years” are required to assess the incidence of microbial keratitis during a 1-year follow-up period. Conventional soft DW and EW lens materials are basically the same, and so too are their Dk/L values. Although these values are generally sufficient for DW, they are about one third of those required for EW.[14] The resultant EW hypoxia and insufficient soft lens hydration and cleanliness during sleep increase significantly the probability of infectious and inflammatory tissue reactions in relation to the continuous duration of wear. For example, microbial keratitis is 10–15 times more common with conventional EW lenses than with DW.[15] The vast majority of contact lens clinical researchers advise most patients against conventional soft lens EW, except for occasional periods of short duration. Some RGP materials have a high enough Dk/L value to satisfy the cornea’s oxygen needs with EW; however, owing to problems such as binding and increased corneal distortion, only a small percentage of patients are fitted for RGP EW.
The Dk/L valves of silicone-hydrogels materials seem to be adequate for EW and researchers believe that it is safe for many patients to use them for EW. For example, one of the FDA-approved silicone hydrogels has a Dk of 140 and a water content of 24, whereas another has a Dk of 110 and a water content of 36. The lenses are treated in gas plasma–reactive chambers to transform the hydrophobic to hydrophilic surfaces, which are necessary for good in vivo wetting and resistance to deposit formation.
Disposable Contact Lenses
The use of disposable and programmed replacement soft contact lenses has grown enormously since their introduction in 1986.[16] They, too, are made from the same basic materials as conventional DW and EW soft lenses, and their Dk/L values are also insufficient for EW. Silicone-hydrogel lenses are also available for the programmed replacement regimen. Their uniqueness lies in the manufacturing techniques that produce lenses inexpensively and with relatively good reproducibility; this reduces the per lens cost to patients. Should patients replace their lenses daily, weekly, monthly, quarterly, semiannually, or annually? The answer is different for each patient and is determined by safety, efficacy, economic, and convenience factors.
Colored Lenses
Soft and rigid lenses can have very light tints to improve their visibility when off the eye and to aid the patient in handling

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them. Soft and rigid lenses that alter the apparent eye color are available in cosmetic enhancement tints for people who have lighter eyes and opaque tints for people who have darker eyes. Such lenses typically have a clear central area of about 4?mm for visual purposes and a clear annular peripheral area of about 1?mm that overlies the sclera.
Contact Lenses for Astigmatism
Front surface toric rigid lenses and front or back surface toric soft lenses may be used for vision correction in patients who would have 0.75D or more of residual astigmatism if fitted with spherical lenses. Corneal rigid and semiscleral soft lenses rotate on the eye, and proper meridional orientation of the cylinder axis is established primarily by the incorporation of prism and/or thin zones (slab-off) in the lens. Back surface toric rigid lenses may be used to provide better physical matching for corneas that have 2D or more of keratometric astigmatism.[17] Such lenses also require a front surface toricity for vision correction; they are called bitoric lenses.
The amount of astigmatism that remains uncorrected when a contact lens is worn is referred to as residual astigmatism. Several potential reasons exist for residual astigmatism in an eye. Corneal toricity, which can be measured with a keratometer or other corneal surface analyzer, produces a corresponding amount of astigmatism. However, many individuals manifest a different amount of cylinder in their refractive correction. The difference can be ascribed to astigmatism produced by the internal refractive elements of the eye, specifically, the posterior cornea and the crystalline lens. The eyeglass cylindrical correction (referred back to the corneal plane) represents the total astigmatism of the eye. Reiterated differently, the internal astigmatism (AI ) is the difference between the total astigmatism (AT ) of the eye and the corneal astigmatism (AC ), that is, AI = AT – AC .
A spherical base curve rigid lens neutralizes the corneal toricity. [18] Residual internal astigmatism occurs when the corneal cylinder has been neutralized. The cylinder present in the spherocylindrical overrefraction results from the internal astigmatism of the eye. Patients who have small amounts of uncorrected astigmatism are often symptom free and are left uncorrected. Individuals who have infrequent symptoms can manage successfully with a pair of spectacles that have the required residual astigmatic correction and that are worn in situations known to provoke eyestrain or blurred vision. Patients who cannot tolerate the uncorrected astigmatism can be refitted with an anterior toric rigid lens that incorporates the correcting cylinder. These lenses are difficult to fit in that they must be axis stabilized with a lens configuration designed to prevent lens rotation.
A toric base curve rigid lens is used to match the corneal toricity in cases in which a spherical base curve is unstable. As a result of the induced cylinder at the base curve–tear layer interface, a toric front surface is almost always required to provide accurate correction. Since the anterior surface power is determined by overrefracting the lens in situ, the correction for internal astigmatism is also incorporated, which results in a lens fit with no residual astigmatism.
Residual induced astigmatism from a toric base curve rigid lens may be corrected by designing a spherical power effect lens for that eye. Since the on-eye back surface cylinder power is about one third the cylinder power of the lens in air, an offsetting front surface cylinder of one third the toricity produces a lens whose total power on the eye is a sphere, irrespective of its rotation.[19] In the same way that a spherical base curve spherical power rigid lens works, the spherical power effect lens corrects only the corneal astigmatism, leaving any internal astigmatism uncorrected.
A spherical base curve soft lens drapes the anterior eye; as a result, the front surface of the lens assumes almost the same toricity as the cornea. A spherocylindrical overrefraction yields approximately the same cylinder power and axis as the spectacle correction. Toric soft lenses have become the method of choice to correct bothersome amounts of residual soft lens astigmatism.
A toric soft lens drapes the eye in the same way that a spherical soft lens does, so no appreciable lacrimal lens and no induced astigmatism occurs at the back surface interface. A toric lens that has the amount and axis of the spectacle cylinder (referred back to the corneal plane) corrects the total astigmatism of the eye. Meridional stabilization is achieved by a variety of lens designs that incorporate prism ballast, thin zones, and eccentric lenticulation.
Soft contact lens materials vary in their water content as well as in their stiffness. Some soft lenses are reputed to work better than others to mask corneal astigmatism. This tendency can be enhanced by using lenses in standard thicknesses rather than in ultrathin designs.
Contact Lens Asphericity
Both rigid and soft lenses can be manufactured with aspheric curves. Back surface aspheric lenses were originally designed to provide a closer approximation to the aspheric surface of the cornea. They are also used to provide a progressive “add” effect for use as multifocal lenses. Several different shapes have been used as aspheric base curves. Ellipses most closely represent the shape of the corneal contour. The variable used to select an ellipse is its rate of flattening, the eccentricity. The range of eccentricities is from zero, which represents a circle, to over 1.0. The greater the back surface eccentricity, the greater the amount of plus power that is produced in the midperipheral area of the lens. Often, manufacturers describe the asphericity of their lenses in terms of the amount of add produced.
Other forms of aspheric back curves include the sphere-aspheric, the biaspheric, the sphere-cone, and the offset periphery designs. Soft lenses can be produced with aspheric curves by a spin-cast technique, molding, or lathing. Front aspheric lenses are used to provide a continuous change in power from the center to the periphery of the lens. This gradual steepening produces a progressive multifocal effect in which plus power decreases from the center to the periphery.
Contact Lenses for Presbyopia
The 60-year-old concepts of alternating and simultaneous vision still provide the basis of contact lens design for presbyopia.[20] Most alternating vision bifocal lenses have a prism to stop lens rotation and small optic zone diameters, superior for distance and inferior for near. As with eyeglass multifocal lenses, it is intended that the patient’s fixation alternates between the zones as needed for specific tasks. With simultaneous vision, the patient’s pupils are covered partially and simultaneously with optic zones that contain both the required distant and near powers. One type of simultaneous lens design incorporates a small central zone and an annular zone that surrounds this. The central zone may have either the distance or near power; the annular zone has the opposite power. Newer simultaneous lens designs have a series of four or five concentric zones in which distance, near, or intermediate powers are alternated. The goal is to provide consistent pupillary coverage by these various powers as the lens moves or the pupil diameter changes. A second type of simultaneous design uses either front or back surface aspheric curvature to produce a somewhat progressive power change from the center to the periphery of the optic zone. The third type of simultaneous vision approach uses single vision lenses, with one eye given the distant and the other eye the near correction, that is, monovision. Generally, alternating vision is preferable with rigid corneal lens bifocals and simultaneous vision is preferable with soft lens bifocals. However, the exact positioning and movement required for good vision are not attained for many patients. Success is much greater with distant single vision contact

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lenses and reading glasses or with single vision monovision contact lenses. With monovision, usually the patient’s dominant eye has the distance prescription and the other eye has the near prescription.[21] A fourth type of simultaneous vision uses the optical pinhole concept. A single power between the patient’s distance and near corrections is used to focus vision. The pinhole increases the depth of focus so that all objects from infinity to a practical near distance are imaged adequately on the retina. Pinhole diameter is usually selected from 1.5 to 2.0?mm to balance image clarity, brightness, and visual field. A narrow ring surrounded by a clear zone has been used to create the central pinhole to enhance brightness and field.
Contact lens wear places a demand on the ocular surface, which is not always accepted by presbyopes. The aging presbyopic eye, with its many changes of anatomy and physiology, is even less likely to accept this demand.[22] With aging, a reduction of the quantity and quality of the precorneal fluid typically occurs because of decreased lacrimal aqueous tear production and meibomian gland lipid tear production. Eyelid tonus decreases, which decreases spreading of precorneal fluid and may cause various degrees of ectropion. Pupil diameters decrease, and the pupillary response to stimuli becomes sluggish. The crystalline lens loses transparency, the retina’s nerve fiber layer becomes thinner, some optic nerve fibers atrophy, and the macula may degenerate. These internal ocular changes simultaneously decrease the quantity and quality of light that reaches the retina and its ability to receive and transmit images properly.
Unusual Back Surfaces
Very unusual corneal topography, which can occur with keratoconus, after penetrating keratoplasty, and after radial keratotomy, requires specialized back surface configurations of rigid or soft lenses to conform better to the corneal shape. Specialized back surface configuration rigid lens designs are also available for orthokeratology.[23] This somewhat controversial procedure uses rigid lenses to reshape the cornea and to reduce myopia. Orthokeratology is safe in that it does not produce significant undesired corneal problems, but its efficacy to produce the desired corneal reshaping and myopia reduction is limited. In general, myopia is reduced by less than 2D, the results are not permanent, and so-called retainer lenses must be worn for many hours a week. Newer reverse geometry design lenses may prove to increase myopia reduction to 4 or 5 diopters. Wearing orthokeratology lenses during sleep is being investigated to determine whether this method provides safety and efficacy while eliminating the need to wear lenses during waking hours.
INITIAL FITTING
Two general methods exist to fit contact lenses. First, in the measurement and standard procedures used to determine the parameters, readings are taken of the corneal curvature using keratometry or videokeratography and measurements are made of the horizontal iris diameter, vertical palpebral aperture, and pupil diameter. These findings then are related to nomograms to determine the contact lens parameters to be ordered. Second, in the diagnostic lens procedure, the preceding measurements are made initially. Next, the appropriate lens is selected from the practitioner’s contact lens trial set, inserted on the patient’s eye, and allowed to settle for 15–20?min. With corneal lenses this is necessary so that the initial lacrimation decreases; with soft lenses it is necessary because the temperature on the eye is greater than the ambient temperature, which forces fluid out of the lens (i.e., the lens must equilibrate).[24] Then the position, movement, and relationship between the back surface of the contact lens and the front surface of the eye are evaluated in relation to criteria for a good fit.
Corneal lenses are fitted for either superior positioning or intrapalpebral aperture positioning. For either of these (with primary fixation) the lens should be centered horizontally and its lower edge should be at least 1–2?mm above the lower eyelid. The upper edge with a superior positioning lens should be under the upper eyelid, but not over the superior limbus, and with intrapalpebral positioning the upper edge should be just below the upper eyelid. With blinking, the lens should move 1–2?mm, return to its original resting point, and remain there during the interphase between blinks.
The relationship between the back surface of the lens and the front surface of the cornea is evaluated after sodium fluorescein has been applied. [25] Fluorescein mixes with the preocular film between the lens and the cornea and, when activated with a “black light,” it fluoresces with specific patterns. When the curves of the lens are very different from those of the cornea, the precorneal fluid is deeper and the fluorescence is a brighter yellowish green. Conversely, closer alignment of lens and corneal curves produces a shallow precorneal fluid, less fluorescence, and a bluish black appearance. For corneas that have less than 1.5D of astigmatism, the desired optic zone pattern is blue-black or a very mild, uniform yellow-green and the desired secondary zone pattern is a moderately bright yellow-green ( Figs. 12-1 to 12-3 ).
Soft lenses should cover the cornea fully and be relatively centered around it; they should move 1–2?mm with blinking. Fluorescein is not used with soft lenses as they absorb the dye. Direct evaluation with large-molecule diagnostic dyes (such as fluorexon) has not proved to be a significantly useful method for evaluating the relationship of posterior radius to cornea curve. Indirect methods of evaluation include keratometry, retinoscopy, and the subjective report of stability of vision.[26] The goal is to obtain a fit that provides a clear, consistent mire, retinoscopy reflex, refraction, and vision. A steep lens (too large a sag or vault) between blinks theoretically shows distorted or irregular keratometry mires, a central dark spot in the retinoscopy reflex, and blurred vision. Directly after blinks, these responses improve for a short time and then revert. A flat lens (too small a sag or vault) between blinks theoretically shows slightly distorted keratometric mires, an inferior dark spot in the retinoscopic reflex, and fairly good vision. Directly after blinks, these responses worsen for a short time and then improve. Poor keratometric mires, as well as poor retinoscopy reflex or vision, can result from a dry lens surface. The patient is asked to blink four or five times to help differentiate this condition from an improperly fitted lens.
The edge of the lens may provide information about the sag of the lens. An edge that turns away from the eye between blinks often indicates a flat lens. An edge that bears heavily against the cornea or bulbar conjunctiva often indicates a steep lens; also, using a slit lamp, blanching of the conjunctival vessels or conjunctival “drag” may be observed with a steep lens.
The position and movement of a soft lens provide information about the sag of the lens on the eye; clinically, these data are the most frequently used in this context. A lens that decenters or has excessive movement is often too flat, and the converse applies to a lens that is too steep. If lens position, movement, or back surface fitting relationship does not meet the criteria, trial lenses with different parameters are inserted and the evaluation is repeated. After the appropriate fit is attained, the necessary power is determined. This is done by computation or by refraction over the trial lens in situ.
After the spherical power of a contact lens has been calculated, the effect of vertex distance needs to be considered. In myopia, the strength of a spectacle correction lens is greater than the correction required at the corneal plane. Conversely, the power of a spectacle lens for hyperopia is less than the lens power required at the cornea. The effects are not significant until about 4D of correction is required. Tables that list the appropriate vertex compensation are readily available.
In fitting rigid lenses with a spherical base curve, the “lacrimal lens” fills in the space between the cornea and the back surface of the lens. Because the toricity of the cornea is

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Figure 12-1 Fluorescein pattern of corneal contact lens fitted 1D steeper than “flat K.” Note the central clearance.

Figure 12-2 Fluorescein pattern of corneal contact lens fitted “on K.” Note the central alignment.

Figure 12-3 Fluorescein pattern of corneal contact lens fitted 1D flatter than flat K. Note the central touch.
filled in by the lacrimal lens, thus neutralizing the corneal cylinder, the flattest corneal meridian (flat K) becomes the reference for optical calculations.[27] Compared with the radius of flat K, a lens base curve is “on K” (i.e., the same radius as the flat meridian), “steeper than K,” or “flatter than K.” A lens on K has a plano power lacrimal lens; a steeper than K lens has a plus power lacrimal lens; and a lens that is flatter than K has a minus power lacrimal lens. A rule of thumb commonly used to compare dioptral adjustments from changes in base curve is that a 0.05?mm change in radius produces a 0.25D change in overall refractive power.
In fitting soft contact lenses with a spherical base curve, little or no lacrimal lens exists because the back surface tends to drape the cornea.[28] Therefore very little neutralization of the corneal cylinder occurs. However, soft lenses that may be somewhat stiffer than most, because of their material and thickness, may manifest some mild lacrimal lens effects.
FOLLOW-UP CARE
A contact lens is a foreign, plastic object placed on the ocular surface; therefore, patients must be instructed properly in effective lens care and in the necessity of follow-up visits. These are more frequent during the earlier stages of wear, but visits at least annually are necessary for the duration of lens wear. Some problems may occur during the first few weeks, others during the first 6 months, and still others over years of wear. Follow-up examinations include evaluations of history, vision, lens fit, tissue integrity, patient compliance, and lens physical structure.
A contact lens may be considered to be an optical patch and bandage. [29] As a patch, it reduces the availability of oxygen to and the dissipation of carbon dioxide from the cornea. As a bandage, it creates pressure on the underlying tissues and reduces wetting of the ocular surface and dissipation of material from between the contact lens and the cornea. It may also become contaminated with organic and inorganic deposits and become scratched, chipped, or ripped. The patch effect creates different amounts of hypoxia and interference with the cornea’s normal aerobic metabolic cycle. This leads to edema, decrease of glycogen reserves, and increase of lactic acid dehydrogenase.[30] The last decreases the cornea’s pH, which may result in stromal and endothelial reactions. The bandage effect may lead to problems of desiccation, mechanical abrasion, and chemical reaction with solutions and toxins from the breakdown of trapped debris.
A large body of knowledge of basic science and clinical research has been gathered over the past 30 years concerning the corneal and conjunctival complications caused by contact lens wear. The following information is based on these data. For newer design contact lenses that combine silicone and hydrogel materials and have Dk values over 100, much less research and clinical history are available. This is discussed separately after the general discussion of corneal and conventional tissue problems.

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