Basics of Soft Contact Lens Fitting
HAROLD A. STEIN, RAYMOND M. STEIN, MELVIN FREEMAN and LYNN MAUND
Table Of Contents
ADVANTAGES AND DISADVANTAGES OF SOFT LENSES|
REQUIREMENTS FOR PROPER FIT
BASIC GUIDELINES FOR FITTING NORMAL SOFT LENSES
EVALUATION OF FIT
CLINICAL HIGHLIGHTS OF FITTING PROBLEMS
COMPLICATIONS FROM THE USE OF SOFT LENSES
|Soft contact lenses are essentially flexible lenses that mold to the corneal
topography. The majority of these soft lenses are hydrophilic (sometimes
referred to as hydrogel) lenses that owe their softness to their ability to absorb and bind water.|
The advent of soft contact lenses almost four decades ago greatly expanded the number of patients who could wear contact lenses. This is largely because soft lenses are very comfortable, easy to adapt to, and do not require a rigid wearing schedule. This chapter discusses the properties of the many hydrogel (hydrophilic) soft contact lenses available today. Aspects of soft contact lens patient selection are also described.
The development of the type of plastic for making soft contact lenses was first achieved by a group of Czechoslovakian chemists under the leadership of Wichterle at the Institute for Macro-molecular Chemistry of Prague. Two patent attorneys from New York actually purchased the rights to the spin-cast method of producing soft lenses and subsequently licensed the Bausch & Lomb Company to manufacture and market these soft lenses throughout the world. This manufacturing process, in which the polymerization process took place in a spinning mold, was unique for contact lenses. The optical power of these lenses was determined by the speed at which the mold was spinning and the subsequent centrifugal force that was developed at a given speed. The higher the speed, the greater were the centrifugal force and the displacement of the reaction mixture, and consequently the higher the minus power.
Since that time, a number of other methods of producing lenses have been developed. Chief among these has been the lathe-cut method. In this process, lens buttons are cut from a long, slender, circular hard rod. These lens buttons are then lathed. Automated systems have been developed that can now mass produce lathed lenses.
A third method of producing soft contact lenses, called molding, has been developed and has been employed for making hydrogel lenses and rigid gas-permeable lenses. This method is an injection technique similar to the one used to produce children's plastic toys. Spin casting, automated lathing, and molding are very economic manufacturing techniques because they are not labor intensive, yet give rise to a lens that is highly refined and reproducible.
|ADVANTAGES AND DISADVANTAGES OF SOFT LENSES|
For many patients, comfort is the chief advantage of soft contact lenses. Because of its nonrigid structure and lack of hard edges, a soft lens is much less likely to cause corneal compression, corneal abrasions, or irritated eyelid edges. The soft lens is larger, and thus its upper edge fits under the upper lid margins. On blinking, the eyelid glides over a soft lens without dislodging it (Fig. 1). The lens hugs the eye, molding to the shape of the individual cornea on which it rests. In addition, the thin edges of a soft lens mold to the sclera. This molding action has an effect on soft lens fitting. Precise corneal measurements are less crucial than in rigid lens fitting: a poorly fitted soft lens is much less likely to cause irritation than a poorly fitted rigid lens.
In addition to comfort, an advantage of soft lenses is the rapidity with which the eye adapts to wearing them. Soft lenses are often well tolerated from the initial insertion; many patients report that they are comfortable within 30 minutes. This contrasts with the days or weeks required for adaptation to rigid lenses.
Other advantages of soft contact lenses include the following:
The following are among the disadvantages to the use of soft lenses compared with rigid lenses:
|A lens may be rigid or soft, depending on the nature of the material of which it is composed. Gas-permeable lenses are lenses that permit the passage of oxygen and carbon dioxide through
the material, and the term is usually confined to rigid lenses. Cosmetic lenses are colored lenses used to cover an unsightly blind eye; this term is
sometimes applied to lenses that are used for nonpathologic conditions
to enhance cosmetic appearance as a substitute for eyeglasses. A bandage lens is a contact lens that is used over the cornea to protect the cornea from
external influences and to permit healing of underlying corneal disorders. Scleral contact lenses cover not only the cornea, but also most of the exposed conjunctiva overlying
the sclera. Corneal contact lenses are confined to the cornea. Semiscleral lenses bridge the limbus and lie partially on the conjunctival tissues overlying
the sclera adjacent to the limbus.|
DESIGN OF LENSES
The base curve of a lens is the curvature of the central portion of the back surface of a lens and is measured in millimeters of radius of a curvature. This is the central posterior curve (CPC). The primary base curve and all other curvatures of a lens may be expressed in terms of millimeters of radius of curvature. It can also be expressed in diopters (D)—a primary base curve of 45.00 D is equal to 7.5 mm. The primary CPC of a lens is usually designed to approximate the curvature of the optic zone of the cornea.
The optic zone of a lens is the central zone that contains the refractive power and generally corresponds to the central corneal cap.
When a base curve of a lens is said to be made steeper, this means that the posterior radius of curvature is decreased, for example, from 8.4 mm to 8.1 mm, so that the curvature is now steeper. When the base curve is said to be made flatter, it means that the posterior radius of curvature of the lens is increased, for example, from 8.1 mm to 8.4 mm.
Most rigid corneal lenses used today are either bicurve or tricurve. A bicurve lens has one base curve and one secondary curve. A small lens is usually bicurve.
A multicurve lens has a base curve and three or more peripheral curves. A tricurve lens usually has a large diameter. A contour lens is basically a tricurve lens with a narrow intermediate curve. These latter lenses have an outer peripheral posterior curve (PPC) and one or more intermediate posterior curves (IPC). At each junction of these curves, there is a junctional zone. One can obtain smoothness of this junction by blending the zone to remove the sharp junctional ridge between zones.
Not every lens has a smooth spheric back surface. Some lenses have an aspheric back surface that is not of uniform radius, but rather is shaped like a parabola. The radius of curvature of these aspheric lenses must be measured at the apex or center of the lens; this measurement is called the posterior apical radius (PAR). This is particularly applicable to spin-cast lenses whose posterior curvature is not uniform.
Every contact lens has a diameter, or chord diameter, which represents the width or linear measurement from one edge of the lens to the opposite edge. Each of the curves referred to previously, the CPC, the PPC, and the IPC, also has its own curve width.
The central thickness of a lens is the separation between the anterior and posterior surfaces at the geometric center of the lens. The higher the minus power, the thinner the center, whereas the higher the plus power, the thicker the center.
A ballasted lens has a cross-sectional shape with one side heavier than the other so that the heavier side becomes oriented inferiorly when the lens is worn. It is usually prism ballasted: that is, a prism wedge is used to weight the lens. A truncated lens is cut off to form a horizontal base. The lens is usually amputated at the inferior pole, although superior truncations have been used as well. Truncation is frequently employed to add stability to a soft toric lens. A truncated edge is usually beveled to blunt the sharp edges of amputation.
The back vertex power of a lens refers to the effective power of a lens measured from the back surface. The wetting angle of a lens is the angle that the edge of a bead of water makes with the surface of the plastic. The smaller the angle, the greater the wetting ability.
TYPES OF LENSES
Toric (or toroidal) lenses (derived from the Latin “torus,” meaning a bulge or cushion) are lenses with different radii of curvature in each meridian. The meridians of the shortest and longest radii are called the principal meridians and differ by 90°. These lenses are used to correct astigmatism and to improve the fit on very astigmatic corneas.
A front surface toric lens has an anterior surface with two different radii, but a central posterior surface that is spheric. A back surface toric lens has a posterior surface that has two different radii and an anterior spheric surface. In a bitoric lens, both the anterior and posterior surfaces are toric.
Higher-power plus lenses are often designed with a lenticular bowl or central lens area that has the appearance of an upside-down bowl sitting on the underlying lens.
The sagittal depth or apical height of a lens is the distance between a flat surface and the back surface of the central portion of the lens. Thus, for two lenses of the same diameter but of different sagittal depths, the lens of the greater sagittal depth would produce a greater vaulting of the lens and in effect would be steeper (Fig. 3). This is often referred to as the sagittal vault.
There are two important variables in understanding the mechanism of loosening or tightening a lens. These variables are the diameter and the CPC (radius) of the lens. If the diameter is kept constant, with the CPC being changed to a longer radius (e.g., from 7.8-mm to 8.4-mm), the sagittal vault or sagittal height of the lens becomes shorter and the lens becomes flatter. The converse is also true (Figs. 4 and 5).
If the CPC of the lens remains the same but the diameter is made larger (e.g., from 13 mm to 15 mm with soft lenses; 8 mm to 9 mm with rigid lenses), the sagittal vault or sagittal height of the lens is increased and the lens fits as though it were steeper. The converse is also true (Fig. 6). For example, the lens is considered as being part of a circle similar to the one shown in Figure 6, and two parts of the circle with different chord lengths are taken, the portion of the circle with the larger chord length will have a greater sagittal vault.
Terminology pertaining to oxygen studies has taken on increasing importance in the contact lens literature because of the development of extended-wear and gas-permeable contact lenses.
The oxygen transmission through a given material is a laboratory measurement, often referred to as the DK value, where D is the diffusion coefficient for oxygen movement in lens material and K is the solubility coefficient of oxygen in the material. It should be noted that a coefficient is a measure of a physical or chemical property that is constant for a system under specific conditions. The DK, or permeability, is characteristic of a given material obtained in a given condition at a given temperature in the laboratory only. It is inversely proportional to the thickness of the lens at the center. The oxygen flux is the amount of oxygen that will pass through a given area of the material in a given amount of time driven by a given partial pressure difference of oxygen across the material. It is a function of the DK of the material, the lens thickness (L), and the pressure drop across the lens (ΔP):
In this equation, L is the thickness of the central optic zone, D is the diffusion coefficient for oxygen movement, K is the solubility of oxygen in that material, and ΔP is the pressure drop across a lens.
Low flux materials include PMMA, hydroxyethylmethacrylate (HEMA), cellulose acetate butyrate (CAB); medium flux materials are hard silicone, ultrathin hydrogels or those with a high water content, and silicone copolymers; and high-flux materials are pure silicone resins and elastomers, as well as hydrogels with a very high water content.
When a lens is made thinner, more oxygen will pass through the material, so the thinness or thickness of a lens becomes an important aspect of lens performance. The term oxygen transmissibility is used to indicate the oxygen permeability (DK) divided by the thickness of the lens, L, so that
Of more clinical importance is how much total oxygen passes through a lens and is permitted to reach the cornea. These are in vivo measurements that involve the total lens and take into account not only the material, but also the design of the lens. This measurement is called the equivalent oxygen performance (EOP). A thicker center (e.g., in a hyperopic lens) or a thicker periphery (e.g., in a myopic lens) will alter the performance.
|To fit any given patient's eye, the lens fitter today has a very large
selection of lenses from which to choose. The various lens products
marketed do exhibit some differences both in their design and their
approach to fitting. Modern lenses have variations in diameter, posterior
radius of curvature, and central thickness. The majority of available
lenses are designed to fit the average eye. Specialty lenses may
be required to fit the atypical eye. Most lenses are essentially designed
to fit spheric corneas; however, toric soft contact lenses are available
to fit eyes with corneal toricity varying from 1.00 to 4.00 D.|
Product selection may be difficult for the average lens fitter. No one product is fully capable of providing correction for all the patients he or she will see. Both optical and anatomic considerations must be taken into account, as well as the availability of lenses and the supportive services by the manufacturer. A practitioner who is just beginning to fit soft contact lenses will find it advantageous to start with a product of one of the major manufacturers. Larger companies have the financial resources to support a staff of experienced field personnel to assist the inexperienced fitter. In addition, their dominant position in the marketplace indicates an extensive clinical expertise with their products, which can be beneficial to the neophyte fitter. As the fitter becomes more experienced, he or she may want to select alternative products with different sizes or base curves in order to provide more adequate fitting and better visual function. Also, as the fitter becomes more experienced, inventory systems will offer better service to his or her patients and avoid the inconvenience of delays in arriving at an optimal fit. If the lenses can be fitted and dispensed on the same day, both the fitter and patient are spared an additional office visit. A suitable inventory would consist of several different products to provide a range of diameter size from approximately 13 mm for small corneas, 13.5 mm to 14 mm for typical eyes, and greater than 14 mm for larger corneas or unusual fitting problems.
|REQUIREMENTS FOR PROPER FIT|
|The geometric form of the anterior segment of the eye consists of a combination
of two curved structures: the cornea and sclera. The radius of
curvature of the sclera averages 4 to 6 mm larger than the cornea. Contact
lens fitters usually describe the sclera as being “flatter” than
the cornea or, conversely, the cornea as “steeper” than
The combination of these two curved structures (Fig. 7) makes the fitting of a lens a difficult task. In the early years of contact lens development, the problem was circumvented by molding the anterior segment of the eye with rubber-like dental impression material and then forming the glass lens over a dental stone positive of the eye. Only the most skilled craftsmen were able to make lenses that provided good visual acuity. Unfortunately, the wearing time of the lens was limited to 3 to 4 hours.
Within 5 to 6 years, the manufacturing procedures were simplified. Plastics that were machinable became available, and with these materials a lens design was developed with a hemispheric posterior surface that geometrically aligned with the corneal apex and with a flatter peripheral flange to align with the sclera. This development provided the basis for (1) the lathe-cutting technique for plastic lenses, which is still being used to manufacture soft lenses; and (2) current soft lens designs. The mathematical model to illustrate the geometric relationship between a hemispheric lens and the anterior surface of the eye is depicted in Figure 7.
The center of the hemispheric lens configuration contacts the anterior surface of the eye at the apex of the cornea, while its periphery is in contact with the sclera. The limbus is vaulted over with this design, and only the center of the lens and its periphery are bearing surfaces of the lens in situ. For a given chord length or lens size, only one spheric radius of curvature will pass through those points.
If the lens is designed to fit only the cornea, the posterior curvature of the lens (often referred to as its base curve) will be determined by the size and central curvature (keratometer reading) of the cornea. This determinant establishes the extent to which the cornea vaults above its chord and is referred to in mathematical terms as the sagitta of the cornea (i.e., the distance between the apex of the cornea and the midpoint of its chord; see Fig. 7).
Most soft lenses are designed as semiscleral lenses, with the periphery of the lens extending beyond the limbus onto the paralimbal sclera. With this design, the sagitta for the lens is determined by the corneal sagitta plus a small sagittal increment contributed by the sclera. The sum of these two values is the fitting sagitta, representing the distance from the midpoint of the scleral chord to the apex of the cornea. The scleral contribution is somewhat of a constant value for a given lens size in a normal population. Although the range of scleral curvatures is 12 to 14 mm in such a population, the extreme values of this range actually have a negligible effect on the fitting characteristics of a lens.
The diameter of the lens is important in fitting. As the diameter of the lens is increased, so is the sagittal height. This affects the fitting of a soft lens as though the base curve were increased. It causes the lens to fit tighter even though the radius of curvature on the base curve is not altered.
The predictability of this relationship is not linear because there are other considerations to the fitting of the lens. An important element is the lens “drape,” or its floppiness. Some lenses are more flaccid than others, because of either their water content or their polymer formation. But the flaccidity of the lenses affects the fit: the more flaccid and draped the lens is on the eye, the less its displacement. However, neither lens drape nor lid adherence can be measured. Again, a tight-fitting lid has more traction or drag than a looser lens, but it is not a quantity that can be assessed.
The foregoing discussion does not consider the fact that the flexibility of the soft lens causes the lens to conform to the shape of the eye to a varying degree. The periphery of the lens tends to flare outward into a flatter curvature than the central posterior portion. Some means must be provided to achieve peripheral adherence, or the lens will simply roll up at the edges and thus be unstable on the eye.
One way to stabilize the periphery is to fabricate a hemispheric lens with a thin edge whose flexibility enables it to achieve paralimbal alignment. This design is usually referred to as a monocurve because the curvature is constant over the entire posterior surface. Lenses with this configuration are usually of a 13-mm chord length and undergo minimal peripheral flattening on the typical eye. With corneas of average size (12.5 to 13 mm), these lenses are actually corneal and have a secondary bearing surface on the paracentral cornea instead of on the sclera. This design concept works well with corneas of average size and shape, but can cause problems in the atypical fitting situations discussed later in this chapter. Moreover, such a design is dependent on uniformity of size, thickness, and base curve to achieve reproducible fitting characteristics. These minor criticisms do not detract from the utility of the monocurve configuration. Problems, when they occur, can be readily resolved with other designs, including alternatives offered by the two laboratories that use the monocurve design.
Another way to achieve peripheral stability is to use a bevel that is an added curve in the periphery that rests on the sclera. The posterior peripheral bevel with a different radius of curvature is flatter than the base curve of the central lens and rests on the flatter peripheral cornea. To provide alignment on the average sclera, most of the manufacturers use a posterior peripheral bevel with a radius of curvature of 12.5 to 13 mm. The bevel thus becomes a secondary bearing surface for the lens, causing the periphery of the lens to remain in contact with the sclera, rather than rolling away from it.
With atypical scleras, the stabilizing effect of the posterior peripheral bevel may not be achieved, and the edge of the lens tends to roll away from the sclera. This geometric incompatibility can be controlled by decreasing the lens size to minimize the adverse effect of the sclera. However, such cases will occur rarely if the fitter pays close attention to the lens size considerations discussed below.
The cornea's geometry has the greatest effect on the fitting characteristics of a lens on a given eye. Thus, there must be some way to assess the corneal contribution and use it in a practical sense for the selection of a lens to fit that eye. If the lens fitter had precise knowledge of all of the geometric components of the anterior segment, he or she could achieve a satisfactory fit for each patient. For clinical use, however, it is not practical to go to the expense and effort to obtain such data when a few very simple measurements enable most fitters to achieve better results.
The two measurements required are (1) the central keratometer readings (K readings), and (2) an estimate of corneal size from the horizontal visible iris diameter (VID). K readings are routinely taken in all contact lens fittings, but the VID is rarely measured. However, VID measurements are quickly made with a metric rule to the nearest 0.5 mm and, although they are approximately 1 mm smaller than the cornea, can be used to determine their relationship with the central K reading.
The VID measurement is basically a convenient guideline. There is no method of clinically assessing the total radius of curvature of the cornea or its diameter. The cornea is ellipsoid: that is, flatter peripherally but not uniformly flatter. The temporal slope of the cornea differs from the nasal slope. Furthermore, the keratometer measures only between 3 and 3.50 mm, depending on the keratometer used. The diameter of the cornea is also not uniform, being less in the 90° axis than in the 180° axis. What should be measured is the chord diameter of the cornea (i.e., measurement from limbus to limbus). Because the limbus can be 1 mm or greater, depending on the eye and the location, one can readily see that even the chord diameter measured with calipers is not precise. So the VID is employed because it is easy to measure and does not create patient unrest or discomfort.
Clinical experience has taught us that there is a geometric relationship between central K readings (flattest meridian) and VID. Generally speaking, flat corneas are large and steep corneas small. Atypical relationships occasionally occur. These atypical situations are the primary cause of problems in fitting. For example, small, flat corneas require lenses that are flatter than those required by the average case that has the same central K reading. In such a situation, the corneal vault is small because the sagittal depth of the cornea is primarily a function of corneal size and central curvature. If the corneal diameter decreases, with a constant radius of curvature, then the corneal vault height also decreases. Similarly, if the central K reading flattens and the diameter remains unchanged, the sagitta decreases. Conversely, the sagitta increases when the central K reading steepens or the corneal diameter increases.
Many fitters use these two readings as a guide for the initial lens selection. Some fitters ignore the K readings completely and fit primarily according to lens size. If the cornea is large (based on VID), they will use a large lens with a large optic zone in a relatively flat type lens for their initial lens selection. If the lens is too loose, they do not change the base curve but merely select a lens with a larger diameter.
Use of Sagitta
Fitters are not accustomed to working with sagitta because most of the literature on contact lens fittings concentrates on K readings; consequently, they do not recognize the influence of this factor. Most fitters are comfortable in dealing with the more common method of determining the relative steepness and flatness of the cornea from central K readings, and therefore it is probably best to equate the sagittal influence with these conditions. Thus, steepening the lens increases its sagitta and flattening the lens decreases its sagitta.
How does the fitter decide on the size of a lens to fit a given eye? In the past, there were only a few manufacturers of soft lenses, and they offered limited sizes because of the limitations of their production capabilities. Fitters were forced to “make do” with what was available, and consequently many wearers were marginally fitted. These limitations in available sizes made size selection simple, but at the cost of optimal results. A wide choice of products with sizes ranging from 12.5 to 16 mm is available. This may complicate the fitting process and cause the fitter to feel considerable frustration in determining which lenses to use, but the ultimate beneficiary will be the patient, who will enjoy better fitting lenses with better long-term results from wearing such products.
The shape of the patient's eye should determine the size of the lens. It has already been shown that in matching the geometry of the lens with that of the anterior segment, the size of the cornea is a significant factor along with the central K readings. It therefore follows that the measurement of these two factors must also be instrumental in the initial selection of a lens for a given eye.
Corneal size can be easily approximated by measuring the diameter of the iris along the horizontal meridian. The measurement of the horizontal VID can be accurately done with a slit lamp equipped with a measuring ocular. For practical purposes, however, an ordinary metric scale used to measure pupillary distance is sufficient, and the measurement can be easily made to plus or minus 0.5 mm. The actual corneal chord length is larger than the horizontal VID, but that does not detract from the utility of the measurement. This factor is taken into consideration by selecting a lens whose size is at least 1 to 1.5 mm larger than the VID. If the lens has a posterior peripheral bevel that is less than 0.3 mm in width, the 1-mm value is used; if the posterior bevel is wider than 0.3 mm, the 1.5 mm value should be used. The width of the bevel is usually given in the literature accompanying the product; if not, it can be obtained from the manufacturer.
It is important to consider the size of the lens relative to the width of the posterior peripheral bevel to minimize the potential for excessive tightness caused by lenses that are too big. Moreover, excessive size seems to be a contributing factor to pathologic problems such as giant papillary conjunctivitis, neovascularization, corneal ulcers, and other conditions that occur with the use of soft lenses and that will be discussed later in this chapter. For these reasons and because the manufacturer can now supply a wider range of sizes, this factor is being stressed to an increasing degree in the literature.
When small-diameter lenses appear to be acceptable, the final decision rests with two minor considerations: (1) the size of the palpebral aperture, and (2) the relative lid tension. Patients who have wide apertures that are close to the size of the lens or have tight lids generally obtain poorer results with small lenses than with slightly larger lenses. Under these conditions, the lenses do not display optimal stability on the surface of the eye; they tend to curl at the edges and often decenter. A good rule to follow is to select a lens size that is at least 2 mm larger than the palpebral aperture. This will allow good corneal alignment, with the upper lid covering the edge of the lens and minimizing the decentering effect caused by lid tension.
A third consideration for lens size is centration. Lenses that are 1 mm to 1.5 mm larger than the VID should completely cover the cornea to avoid arcuate staining. If any of the cornea is uncovered because of decentration, a large lens should be used. However, those lenses that are more than 1.5 mm larger than the VID are semiscleral lenses, and such a design should cover the perilimbal sclera so that the periphery of the lens clears the limbus by at least 1 mm. A lesser amount of clearance may prevent the periphery of the lens from achieving proper alignment with the surface of the eye.
Adjusting the lens size is not necessarily the answer to decentration of a lens. Decentration may be caused by the following:
Toric cornea: A posterior toric soft lens may be the best solution.
Decentration of lens should be avoided because it leads to variable vision, arcuate staining of the cornea, punctate staining, and corneal vascularization.
All manufacturers provide the fitter with guides and tables to aid in selecting the most likely parameters (i.e., size and base curve) for the initial lens to try on the patient's eye. It should be pointed out that these data are only suggestions, and they are based on typical eyes. The more atypical the patient's eye, the further the deviation will be from the suggested parameter. The following paragraphs review a basic scheme for fitting and discuss methods to increase the success of this initial fitting by detecting atypical situations and compensating for them before the selection of the diagnostic lens.
The basic concept of a geometric relationship between lens and eye makes base curve selection a simple process. It has already been shown how corneal size and central curvature affect the geometry of the anterior segment. In the case of the contact lens, its geometry is a function of the size of the lens and its central posterior curvature. Thus, for any given chord length we are actually matching sagitta: that of the anterior segment of the eye with that of the lens. If the base curve of the lens is steepened, its sagitta is increased and vice versa. Similarly, if size is increased, the sagitta is increased and vice versa. Thus, for any given eye, as the size of the lens increases it is necessary to use a proportionately flatter base curve to prevent the lens from becoming too tight.
The approximation of a geometric match-up between lens and eye provides alignment of the lens and the corneal apex, with only a very thin layer of tears underneath. Most of the lenses mentioned fit in this manner, and therefore it is possible to fit them with a basic scheme predicated on their size. This scheme uses the flattest meridian of the K readings (in millimeters), which is fitted flatter by the increments per predetermined lens size shown in Table 1. Thus, if the patient has a flatter K reading of 7.71 mm (43.75 D) and a VID of 12 mm (which suggests a lens of 13.5 mm), the initial base curve would be calculated as follows: 7.71 + 0.9 = 8.61 mm. If the calculated radius falls midway between two available base curves, the flatter of the two should be chosen.
Ultrathin lens designs allow a little more latitude than the lenses in Table 1 because of their greater flexibility. For instance, suppose the patient had a flat K reading of 8.04 mm (42.00 D). The base curve required would be calculated as follows: 8.04 + 0.9 = 8.94 mm. However, an ultrathin lens with a base curve 0.3 to 0.4 mm steeper will usually fit satisfactorily in such cases.
Lenses with high water content are usually fitted more steeply than the tabular values because they are thicker (to reduce breakage) and less flexible than lenses with lower water content, thus gripping the cornea less tightly than more flexible lenses. It is axiomatic that the more flexible the lens, the flatter it is fitted to offset the tighter grip exerted on the eye. Conversely, the less flexible the lens, the tighter it is fitted. For example, if a lens is being used, the 14-mm lens should be fitted 0.5 mm flatter than the K reading and the 14.5-mm size 0.6 mm flatter than the K reading.
The Soflens (Bausch & Lomb, Rochester, NY) with its aspheric back surface, is another exception. Its 13.6-mm size is sometimes fitted either as a corneal lens on large corneas or as a semiscleral lens on corneas when the VID is 12 mm or less. If fitted as a corneal lens, its posterior apical radius is selected 0.85 mm flatter than the K reading. As a semiscleral lens, it is fitted with a posterior apical radius that is 1.15 mm flatter than the K reading. If there is any doubt as to which classification applies in a given case, the fitter should select a lens based on the 1.15-mm value.
The fit should be flatter or smaller when any of the following conditions is present:
The palpebral fissures are large
The normal soft lens is generally fitted flatter than the flattest K measurement: approximately 2.00 to 3.00 D flatter (approximately 0.4 to 0.6 mm) for smaller lenses and 3.00 to 5.00 D flatter (0.6 to 0.9 mm) for larger lenses.
|BASIC GUIDELINES FOR FITTING NORMAL SOFT LENSES|
|In fitting a normal soft lens, the following should be kept in mind:|
In view of the lack of complete control over soft lens fitting dynamics, the best device for assessing a fitting is a diagnostic soft contact lens taken from inventory or from a fitting set. Because it is impossible to analyze all the variables, the fitter should attempt a dynamic overview in which fitting of the lens is viewed directly.
|EVALUATION OF FIT|
|The general tendency among fitters is to fit lenses that are marginally
tight. Lenses that are either excessively loose or excessively tight
are readily detected, but the marginally tight-fitting lens is very difficult
to diagnose. It is easy to be influenced by the patient into fitting
a lens that is marginally tight because such a lens provides considerable
initial comfort to the wearer. The patient usually responds
that a particular lens feels better than others that might have been
tried. In addition, the more comfortable lens may appear to fit in that
it appears to move and center in an optimal fashion (see later discussion). The
combination of spontaneous response from the patient and apparent
optimal appearance of the lens has lured many an unwary fitter
to abandon a cardinal rule in contact lens fitting: Never allow the patient to fit the lens!|
The suggestive influence of a patient's declaration of comfort is stronger when the patient is experiencing lens wear for the first time. The notion of total unawareness of the lens is a mythical situation that all potential wearers seek. The marginally tight lens satisfies their desires in this regard because it does not initially move as much as a properly fitted lens and hence the patient displays less awareness. A correctly fitted lens does not always initially display its optimal relationship. Excessive tearing may cause it to float around and mimic a loose lens, but a properly fitted lens will “settle down” after 1 or 2 days of wearing experience and will become as comfortable as the marginally tight lens is initially. After 2 or 3 weeks of wear the marginally tight lens will become obviously tight and require refitting, whereas the properly fitted lens will continue to be functional as long as it receives proper care.
The initial evaluation of the fit is done through observing four factors: (1) movement, (2) centration, (3) peripheral compression, and (4) retinoscopy.
The lens should move with each blink and with rotation of the eye. An incomplete blink will cause poor movement, which has nothing to do with the dynamics of the lens on the eye. Following movements are slow (e.g., following a finger). In such a test, the lens may have enough drag to stay on the cornea. The fitter should test fast movement (i.e., a saccade), whereby the patient looks from one held finger to the other, and should note the position of the lens with each movement.
Passive movement of the lens should also be tested. With the lens in place, the index finger is placed under the lower lid and the lens is gently nudged. With this gentle pressure, displacement of the lens upward should occur. If it does not, the lens is probably too tight.
This factor also differs between small lenses and large lenses. If the lens is 1 to 1.5 mm larger than the VID, it should completely cover the cornea while the eye is in primary gaze. Failure to do so will result in arcuate staining of the paracentral cornea. If a larger lens is to be fitted, it should be at least 2 to 3 mm larger than the cornea, and at no point should the edge of the lens be closer than 1 mm from the limbus while the eye is in primary gaze. Encroachment of the edge onto the limbal area will result in chronic injection, peripheral staining, and a high potential for the formation of a neovascular pannus.
Many fitters make the mistake of attempting to improve centration by using a steeper base curve; however, this results in a tight lens fit. A far better approach is to use a larger lens with the appropriate base curve, being careful to select a lens that also has a larger optic zone.
The fitter should recall that the geometric and visual axes of the eye do not always correspond. In some cases, the corneal apex can be sufficiently decentered so as to affect the centration of a small lens over it, resulting in poor visual acuity. It is not unusual to see the reverse situation in which a lens centers perfectly over a centrally located corneal apex, but the patient obtains poor acuity without having evidence of residual astigmatism, which is not improvable with overrefraction. In both situations, poor vision results because both axes are not contained within the optic zone of the lens. Thus, in addition to parametric changes, the fitter should consider an alternative that provides for a larger optic zone.
At times, the lens may be centered at the time of the fitting, but a month later, the lens may be decentered. In such cases, any of the following may have occurred:
Protein may have accumulated on the lens and changed its shape and fitting
The cornea may have become edematous, causing the lens to no longer to fit.
The following are symptoms of a loose fit:
Check for a loose lens by having the patient glance downward as the upper lid is held; a loose lens may move up 2 to 4 mm. A very loose lens may be marked by edge roll-out; blinking may dislodge the lens. The lens should be left on the eye for at least 15 minutes to see if edge lift-off develops.
A basic fitting principle is to seek the flattest acceptable fit. To correct for a loose lathe-cut lens, the base curve should be made 0.2 to 0.3 mm steeper or the diameter increased by 0.5 mm (up to 15 mm). For a Bausch & Lomb spin-cast lens, the diameter should be increased. (The Bausch & Lomb spin-cast lenses are fit by measuring the horizontal VID and selecting an initial lens that is 1 mm larger than the horizontal VID.) The fitter should be aware that spin-cast lenses will have different fitting characteristics if large changes in power are made, because the inside apical radius increases or decreases 0.05 mm for every 0.25-D increase in power.
TIGHT LENS SYNDROME
Fitting manuals often warn that excessively tight lenses may cause compression of the paralimbal conjunctiva. Sometimes the circulation is impaired and chronic circumlimbal injection occurs with a clearly defined margin to indicate the point of maximum compression. Usually this area corresponds to the thickest and least flexible part of the overlying lens. In some cases, the congestion does not occur and instead a circumlimbal groove develops in the conjunctiva, causing the eye to appear to be still wearing a lens after the lens has been removed. This finding is often referred to as “ring around the sclera” (Fig. 8).
Compression is the result of excessive tightness due to a base curve that is too steep, a lens that is too large, or a combination of the two. Such a situation is easy to recognize even before these signs appear because tight lenses show little or no movement and the acuity is usually poor.
The marginally tight lens is much more difficult to detect because it requires that the fitter pay attention to small details. Careful biomicroscopic examination is necessary to detect compression caused by marginally tight lenses. Particular attention should be paid to the periphery of the lens and the effect of the lens on the conjunctival vessels. In most cases, a marginally tight lens (especially in a large size) exerts enough pressure on the conjunctiva that the apparent movement of the lens is actually conjunctival movement. In such cases the conjunctival vessels will be seen to move with the lens. Unless the conjunctiva is very loose, the properly fitted lens will move freely over it and not affect the vessels as a consequence. In the marginally tight lens, the conjunctival movement mimics a properly fitted lens in terms of apparent optimal movement and centration. Couple this with a patient's enthusiastic response to the lack of awareness of the lens, and the net result will be a marginally tight fit.
Lens compression on the fine circumcorneal vessels causes blanching of the vessels. The vessels distal to the lens will have a larger diameter beyond the point of compression. However, blanching can occur in patients with dilated conjunctival vessels in a normal fitting situation. These vessels are permanently dilated and congested before the lens is even placed on the eye. Patients with chronic inflammation from ocular allergies or from chronic irritation as seen with cigarette smoking show this kind of configuration. Patients with diabetes frequently display turgid conjunctival vessels. Therefore, chart the normal before deciding that a fitting problem exists. The size of the superior limbus should be noted. It may be very prominent—up to 2 mm—and be normal, but it may later be mistaken for corneal vascularization.
The following are signs and symptoms of a tight fit:
The practitioner should be aware that the symptoms of a tight lens may be similar to those of a loose lens. The lack of three-point touch may cause the tight lens to move excessively and to ride low on the cornea. The fit of soft lens may tighten over a period of several months. The fit should always be checked at follow-up visits. If the fitter has difficulty removing an overly tight lens, saline drops should be used to avoid damage to the corneal epithelium.
The symptoms of tight fit and loose fit may be similar. A tight fit is corrected by reducing the apical vault of the lens to allow for three-point touch. This is achieved by flattening the base curve or reducing lens diameter. For Bausch & Lomb spin-cast lenses, a lens that is either thinner or has a smaller diameter should be tried. As noted earlier, a tighter fit is more likely to be acceptable in a thinner lens. To correct a tight fit, flatten (i.e., increase) the base curve or reduce diameter.
In fitting soft lenses, the situation may be encountered where the diameter required is not available in the specific lens being fit and a different base curve does not solve the problem. One possible solution is to find a manufacturer who can produce a custom soft lens.
When a rigid lens patient is making the transition to soft lenses, a temporary soft lens (preferably a thin, highly permeable one) should be fit. Only after the keratometry readings have stabilized (i.e., identical readings are obtained on subsequent visits) should a final lens selection be made. This may take as long as several months.
Soft lens fitters should always follow the lens manufacturer's instructions. Lenses from different manufacturers can have the same base curve and diameter but fit differently on the eye. For this reason, trial lens sets from the same manufacturer as the final lens give the most reliable fitting data. Finally, patients with residual astigmatism will probably not achieve acceptable vision with soft lenses; rigid lenses will likely become the lenses of choice.
For some unexplained reason, most fitters avoid using the retinoscope to evaluate the fit of the lens; yet it is by far the most sensitive indicator of a marginally tight lens. Its value becomes readily apparent when the way in which a soft lens fits the eye is considered. If the geometry of the lens matches that of the eye, there is an area of alignment over the apex with a very thin, underlying tear layer. Soft lenses work by giving this area of the cornea a new, regular refracting surface, and therefore this apical alignment is mandatory to provide good visual function.
A lens that is too tight lacks apical alignment and has a tendency to lift away from the apex. When this occurs, the radius of curvature of the lens at the apex changes and impairs visual acuity. The retinoscope is so sensitive to this response that it will display a distortion in the central part of the reflex regardless of whether the spot or the streak retinoscope is used. It is not necessary to use a + 1.5-D lens because we are concerned only with the quality of the reflex, not with a retinoscopic refraction to determine emmetropia. It is axiomatic that good visual acuity is consistent with a sharp, clear retinoscopic reflex because only then is the optical system in optimal condition. Thus, a lens that does not fit properly also fails to perform optically in the desired manner.
The retinoscope is probably the best device to determine minimal protein on a lens. A lack of symmetry in the reflex with or without scattered opacities is frequently the earliest sign of protein debris.
SUMMARY OF FITTING STEPS
The fitting of a soft lens can be summarized as follows:
|CLINICAL HIGHLIGHTS OF FITTING PROBLEMS|
|Loose lens is identified by (1) excess movement with eye rotation or blink; (2) poor
centration; (3) variable vision, especially after a blink; and (4) discomfort. The
treatment for a loose lens is a steeper base
curve or larger lens diameter.|
Tight lens is identified by (1) fluctuating vision that clears after a blink; (2) circumcorneal compression; (3) minimal movement after a blink; (4) circumcorneal indentation; and (5) difficulty removing the lens. The treatment for a tight lens is a flatter base curve in a lathe-cut lens; with spin-cast lenses, switch to thinner or a smaller-diameter series.
When fitting soft lenses it is best to fit from the flatter side. Soft lenses may tighten with time as a result of either dehydration or protein accumulation on their surfaces.
Spin-cast lenses have an aspheric posterior surface. Therefore, K readings are not as useful as with lathe-cut lenses. A better indication of lens selection can be based on the measurement of the horizontal visible iris diameter. The fit of the lens should be assessed 20 minutes after insertion because the base curve can steepen after 5 to 10 minutes of wear.
LENS CLASSIFICATION SYSTEMS
Soft contact lenses are classified in several ways. The most common classification is based on water content. Lenses are said to be of low water content when they contain 37.5% to 45% water; these lenses are usually suitable for daily wear. Lenses that are of medium water content contain 46% to 58% water; they are suitable for either daily or extended wear. Lenses with a high water content consist of 59% to 79% water and are primarily used for extended wear.
Soft lens materials have been classified by the US Food and Drug Administration (FDA) into four groups based on water content and ionicity. The purpose of the FDA classification system is to evaluate the safety and efficacy of the various disinfection systems and solutions with different preservatives.
Another classification system is based on the process used to manufacture the lens. The first soft contact lenses were produced by spin casting. Bausch & Lomb adopted and perfected the technology and still produces many lenses this way. In spin casting, the liquid lens material is dropped into a small mold that spins at various controlled rates of speed. The shape of the mold and the rate of spin determine the final characteristics of the lens. Lathe cutting, developed later, involves cutting solid polymer rods to form lenses. The control available in lathe cutting makes possible the production of complex lens types, and lathe cut lenses are available in a range of designs from single-cut to toric and lenticular configurations. A method of cast molding, is now used by some manufacturers because it provides lenses with smooth surfaces, free of the marks that may be produced in the lathe-cutting process.
Finally, lenses are classified by their design (e.g., spheric, toric) or function (e.g., daily wear, extended wear). This classification describes lenses in such terms as daily wear spheric, extended wear spheric, daily or extended wear toric, bifocal, or therapeutic. Of course, different lens classification schemes can be combined; thus it is possible, for example, to describe a particular lens as being a low water content, lathe cut, daily wear spheric lens. In the following sections some of the major categories are examined.
Daily Wear Spheric Soft Contact Lenses
Daily wear spheric soft contact lenses can correct up to 1.25 D of astigmatism. They are manufactured in a wide range of diameters to allow the fitter to optimize alignment and centration and to accommodate varying lid apertures and sensitivities. The center thicknesses available range from 0.4 to 1 mm. (Center thickness is important to both patients and fitters. Thinner lenses are more comfortable and allow for greater oxygen transmission; thicker lenses provide better correction of astigmatism and are easier for the patient to handle.) Spin-cast, lathe-cut, and cast-molded lenses are available to meet the fitting characteristics required.
Daily wear spheric lenses are available in tints that enhance eye color and facilitate handling. Daily wear spheric soft lenses are ideal for patients who wear their lenses on a limited basis, such as for sports or social activities.
Extended Wear Spheric Soft Contact Lenses
Extended wear spheric lenses are available in low-, medium-, and high-water-content models. All other things being equal, the higher the water content the greater the oxygen permeability of the lens. Most of the extended wear soft contact lenses fitted today are of relatively high water content. As with daily wear lenses, extended wear lenses are available in tints and may be spin cast, molded, or lathe cut. In general, extended wear soft contact lenses of spheric design will not correct more than 0.5 D of astigmatism.
Single vision spheric soft contact lenses may be the correction mode of choice for presbyopic patients who want extended wear, because there are no extended wear bifocal soft contact lenses on the market yet. Presbyopes who will not tolerate eyeglasses for reading but who still want extended wear may be best served by a monovision fit.
Bifocal Soft Contact Lenses
When discussing bifocal lenses, visual design available as lens design must be considered. Two bifocal visual designs are available: alternating vision and simultaneous vision. In alternating vision, light that reaches the patient's retina passes through either the near vision or the distance vision portion of the lens, depending on the direction of the patient's gaze. In distance gaze, the patient views through the upper distance portion of the lens. In near gaze, the lens translates (moves) up so that the viewing is through the lower part (near segment) of the lens (Fig. 9). In simultaneous vision designs, light reaches the retina from both the distance and near vision portions of the lens at all times. Some of the light entering the eye is focused for near and some is focused for distance, and the two foci are formed simultaneously. The brain then selects the image of regard. Aspheric and diffractive bifocals, while technically simultaneous vision designs, are so different from standard simultaneous vision lenses that they are often treated separately.
A number of bifocal lens designs are available. Segmented lenses have two visual regions: the distance vision portion is in the top of the lens, and the near vision portion is in the crescent-shaped segment on the inferior portion of the lens (see Fig. 9). The key to success with segmented bifocal lenses is positioning. The lens cannot be allowed to rotate, yet it must translate up and down in order to position the segments. One way to prevent lens rotation is to add weight to the inferior portion of the lens through insertion of a wedge shape (prism ballasting) or thickening the lower edge (periballasting-ballasting). Truncation (Fig. 10) involves cutting off the lower portion of the lens; this creates “corners” that hold tightly to the sclera. The flat edge also helps maintain orientation by aligning with the lower lid.
The advantage of translating (segmented) bifocals is the quality of sight they provide when properly fitted, giving good vision for both distance and near. The main problem with segmented lenses is fitting them to move properly on the eye. If the lens does not translate, it will not function as a bifocal. Quick movement is required to prevent blurring. Tight lids are needed to make the lens move properly. Obtaining a good fit requires skill and patience.
In annular (or concentric) lenses, the different portions of the lens are configured in concentric circles or rings (annular means “ring shaped”). That is, if the distance correction is in the center of the lens, the near vision part of the lens forms a concentric ring around the center (Fig. 11). Alternatively, the distance correction may be located in the concentric ring and the near correction located in the center. Unlike segmented lenses, annular lenses are not affected by lens rotation problems.
The advantage of the annular design is that the lens does not have to translate, as both foci are formed simultaneously. (Because lens orientation and position are less critical than with segmented designs, annular lenses are easier to fit.) A disadvantage of the design is that it is dependent on pupil size. For example, in the center-near design, the pupil will constrict in bright sunlight, so that only the center portion of the lens is used; this effectively limits the lens to near vision. Another disadvantage is that visual contrast is reduced, becasue only a portion of light reaching the retina is in focus for each image. Finally, many patients have difficulty with annular lenses because they are unable to adjust to simultaneous vision.
Multifocal (or aspheric) lenses have a gradually flattening (anterior or posterior) curve from the center to the edge of the lens. The result is a progressively increasing add power from the center to the periphery (Fig. 12). In effect, the multifocal lens is a concentric design in which there is no sharp zone of transition between the near and distance portion of the lens.
Bifocal designs are either anterior or posterior, depending on manufacturer and type. Anterior designs have the power manufactured into the front surface of the lens; posterior designs have the power manufactured into the back surface.
An alternative for the correction of presbyopia is monovision. Monovision techniques employ either one or two single-vision lenses to correct near and distance vision separately. That is, one eye is corrected for near vision, and the fellow eye (with or without correction) is used for distance. Because there are no lenses designed specifically for monovision, standard single vision lenses are used.
With bifocal soft contact lenses, the in-office verification of lens power and lens design is often difficult. In slit-lamp examination, multifocal lenses appear as spheric single-vision lenses. A few bifocal lenses have features that allow them to be distinguished.
Toric Soft Contact Lenses
There are a wide variety of toric soft contact lenses on the market, and fitters must familiarize themselves with what is available on a lens-by-lens basis. In the following paragraphs, specific lenses are not described because there are excellent manufacturers' fitting guides available; however, a word of caution is offered: before choosing a specific lens for fitting, the fitter must make sure that the lens is available in parameters to match the patient's refractive error.
Some “off-the-shelf” toric lenses are available, but only in a limited range of cylinders and axes. Manufacturers make and stock lenses in the most commonly requested power ranges, and these are available immediately on request. Off-the-shelf lenses are available in powers from -6 D to + 4 D, with cylinder powers of -0.75 D, -1 D, -1.25 D, -1.75 D, and -2 D. (Lenses with -3 D of cylinder are available, but usually in a more limited axis range, generally 20° on either side of the 90° or 180° meridians.) Limited fitting sets are available. A diagnostic lens of the selected toric design must be evaluated on the patient's eye for fit and position of axis.
For custom work, one must also use a fitting set. The fitting lenses are spheric designs, but with the diameters and orientation systems of the toric lenses the patient will eventually wear. Toric lenses that can be fit in this manner are available in powers from -20 D to + 20 D, with axes of rotation available in 5° increments. A few manufacturers provide made-to-order toric lenses with even greater powers and cylinders and with axes of rotation in 1° increments.
There are several design alternatives used to maintain the orientation of toric lenses. Double slab off, the creation of thin zones on the inferior and superior parts of the lens, allowing the eyelids to hold the lens in position (Fig. 13). As with bifocal soft contact lenses, prism ballasting (Fig. 14), periballasting, and truncation are also used to maintain orientation.
Toric lenses have orientation (or scrib) marks near the lens edge: some at the 3- and 9-o'clock positions, and others at the 6-o'clock position. The manufacturer is sometimes identified by these marks (or by laser marks on the lens).
Therapeutic Soft Contact Lenses
Therapeutic soft contact lenses have many uses and are made from a variety of materials. In most cases, this kind of lens is used as a short-term corneal “bandage” to protect the eye during treatment for nonhealing corneal ulcers or recurrent erosions. Occasionally, therapeutic lenses are used to cover small corneal lacerations that do not require suturing. In this case, extended wear lenses must be used.
Therapeutic lenses are generally of low to medium water content; however, lenses of higher water content, although not designed specifically as therapeutic lenses, are sometimes worn by patients with bullous keratopathy. Tinted lenses (usually of low to medium water content) can be used to improve cosmesis when the cornea is scarred.
The use of therapeutic contact lenses is contraindicated in patients with corneal exposure and dryness, because in these situations therapeutic lenses may simply aggravate the problem. It is mandatory that therapeutic-lens patients be carefully selected and monitored.
Aphakic Soft Contact Lenses
Soft contact lenses, particularly extended wear lenses, can be a great help for aphakic patients who do not achieve correction with intraocular lenses. Aphakic soft contact lenses are available in spin-cast and lathe-cut designs and with medium and high water contents. The lathe-cut lenticular designs have the best stability on the eye and provide the sharpest and most stable vision (Fig. 15).
As with therapeutic soft contact lens patients, aphakic patients must be carefully fitted and followed. Aphakic patients (who tend to be elderly) have an above-average incidence of microbial corneal ulcers associated with contact lens wear. With careful patient selection and fitting, however, it is possible to rehabilitate an aphakic patient quickly and without further surgery.
Soft Contact Lens Candidates: Patient Selection
Soft lenses are the contact lenses of choice for the following classes of patients:
Extended Wear Soft Contact Lenses
Extended wear soft contact lenses for the correction of myopia were first approved by the FDA in 1980. Since then, their popularity has increased, but so has the awareness of problems associated with extended wear. The fitting of extended wear lenses is not difficult; however, it requires a basic understanding of the physiologic needs of the cornea and the potential complications of extended wear.
The cornea adapts in different ways to different types of contact lenses. For example, the movement of non—gas-permeable PMMA contact lenses that occurs on blinking promotes tear exchange beneath the lens, bringing dissolved oxygen to the corneal surface. (PMMA lenses are also small in diameter, which minimizes the area affected.) Despite tear exchange under the lens, overwear of PMMA lenses will produce corneal edema. Rigid gas-permeable and hydrogel lenses attempt to solve the problem by allowing gases to pass directly through the lens to the cornea. Movement of these lenses helps to bring nutrients to and remove waste products from the corneal epithelium.
At night, when the eyes are closed for sleep, the cornea gets the oxygen it needs by diffusion from capillary blood vessels in the palpebral conjunctiva. However, the oxygen available from the conjunctival capillaries is limited and just barely sufficient to meet corneal needs. Gas-permeable (hard and soft) contact lenses that allow an adequate supply of oxygen to the cornea when the eyes are open may not provide an adequate oxygen supply when the eyes are closed, and the amount of oxygen available is greatly reduced. The major challenge of extended wear is creating lenses that allow sufficient gas transfer during the closed-eye state.
Other challenges of extended wear include producing a lens (1) with adequate movement to promote tear exchange and removal of debris; and (2) that resists spoilage from deposit formation.
What allows some hydrogel contact lenses to be worn on an extended wear basis is primarily their high degree of oxygen transmissibility (i.e., their ability to let oxygen pass through to the cornea). Enough oxygen must pass through the lens to allow normal or near-normal corneal metabolic activity, even with the eyes closed during sleep. In addition, adequate lens movement is required to remove metabolic waste products from the corneal surface.
Most daily wear soft contact lenses do not supply enough oxygen to the cornea to allow them to be used for extended wear. However, modifications of the lens polymer or the lens design can improve the lens' oxygen transmission characteristics. Specifically, oxygen transmission through a soft contact lens may be increased by:
Based on the means used to enhance oxygen transmission, hydrogel soft contact lenses for extended wear can be divided into three types:
In short, the higher the water content and the thinner the lens, the greater the gas exchange through the polymer. One must consider that lens thickness is usually measured in the center of the lens. In a high minus power lens (thin in the center and thick at the edge) the edge thickness may be such that oxygen transmission is considerably decreased, causing corneal edema at the periphery. Conversely, central corneal edema may occur if insufficient oxygen is reaching the cornea through the thick center of a high plus lens. (This is a consideration in fitting the high plus power lenses needed by aphakic patients.)
LENS SELECTION. Each of the various extended wear soft contact lens types has certain advantages and drawbacks. Lenses with a high water content tend to be more comfortable, particularly when the eye is slightly dry; however, these lenses are also more easily damaged than those with a low water content, are more prone to deposit formation, and may offer less stable vision. Ultra-thin lenses with a low water content present handling problems and are susceptible to damage during care procedures.
Thin (not ultra-thin) lenses with a medium (46% to 58%) water content offer something of a compromise: more durability and more stable vision than high-water-content lenses, as well as a lower tendency toward deposit formation. Medium-water-content lenses are durable enough to be used for daily wear.
PATIENT SELECTION. Success in fitting extended wear soft contact lenses is a function of the proper screening of prospective lens wearers. General health, anterior segment health, tear function, hygienic habits, motivation, visual requirements, manual dexterity, and financial commitment represent a partial list of patient selection criteria. Patients considering extended wear must understand and accept the fact that this mode of lens wear carries considerable added risk of complications.
Because extended wear is associated with a greater risk of infection and other serious ocular conditions, extended wear patients must be seen by a practitioner more frequently than daily wear patients. Therefore, before fitting a patient with extended wear lenses, the practitioner must assess the patient's willingness to keep follow-up appointments. Also of great importance is the patient's willingness and ability to remove the lens at the earliest sign of redness or ocular irritation. Before the fitting procedure is performed, the patient must be informed that failure to follow the practitioner's instructions may lead to a serious compromise of ocular health.
Realistic patient expectations with regard to lens wear are also important. The patient must understand that extended wear does not mean the ability to leave the lens in place on the eye for unlimited periods of time. Rather, the eyes must be given regular “rest periods,” during which they can resume their normal metabolic activity.
Cost is an additional factor. Compared with most daily wear lenses, the contact lenses used for extended wear are more prone to deposit formation and to damage during handling. These factors shorten lens life and increase cost to the patient. The greater frequency of practitioner visits that is necessary with extended wear also increases the cost. While the extended wear patient may spend less on lens care solutions than the daily wear patient, the greater number of office visits and the greater frequency of lens replacement combine to make extended wear more costly than daily wear.
In summary, the extended wear soft contact lens patient must:
The ideal candidate for extended wear is an experienced daily wear lens patient who is already adept at insertion and removal techniques. If the patient is a first-time lens wearer, it is recommended that there be a daily wear adaptation period of at least 6 weeks. This promotes familiarity with insertion, removal, and care procedures and decreases the patient's fear of lens manipulation.
Available data suggest that extended wear lenses should be worn for a maximum of 6 days and removed on the seventh day to permit reestablishment of normal aerobic metabolism. Each patient must be evaluated on an individual basis to determine the appropriate wearing schedule. The lens care regimen should be compatible with the lens type being dispensed (e.g., heat disinfection may be damaging to high-water-content lenses).
The extended wear contact lens should remain off the eye for at least overnight before reinsertion. Planned lens replacement every 3 months is recommended as a form of “preventive maintenance” that tends to decrease complications associated with extended wear lenses. Disposable extended wear lenses are available for weekly replacement.
The critical need for hand washing and proper lens hygiene should be emphasized. In addition, one must communicate to the patient the need for immediate lens removal in the event of ocular redness or pain. The large number of reported infections (particularly those involving Pseudomonas aeruginosa) among extended wear patients compel the practitioner to counsel the patient in strict lens care procedures.
PROBLEMS. Recent studies suggest that no currently available extended wear soft contact lens provides adequate oxygen transmission for continued normal aerobic corneal epithelial metabolism. As a result, one sometimes sees in extended wear soft contact lens patients (1) microcysts and (2) changes in the appearance of the endothelium (morphometric changes). The long-term significance of these effects is unknown, and further research into the effects of soft contact lens extended wear is warranted. Any of the following findings is cause for immediate concern:
Any of the these findings should immediately lead the practitioner to discontinue the patient's extended wear routine and change the patient to daily wear, which is something that can be done without refitting or dispensing new lenses.
IS THERE REALLY EXTRA RISK IN REGULAR EXTENDED WEAR? The answer to this question is a clear-cut “yes.” In a study of the safety of extended wear sponsored by the Contact Lens Institute, that was published in the New England Journal of Medicine in 1989, Poggio and coworkers found the following rates of ulcerative keratitis in their very large sample group:
Extended wear: 20.9 per 10,000
Thus the rate of this serious complication was five times greater with extended wear.
This doesn't mean that extended wear is not an acceptable mode of lens use. It does mean that great care must be taken in choosing suitable candidates for extended wear, and then educating patients to minimize its dangers. Patients who choose extended wear should do so knowing the added risk.
APHAKIC EXTENDED WEAR SOFT CONTACT LENSES. Aphakic patients without intraocular lenses require strong plus power lenses for visual functioning. Although aphakic eyeglasses are available, the quality of the vision these provide is limited and often unsatisfactory for tasks that require good peripheral vision, such as driving. A number of extended wear soft contact lenses are available in high plus powers for aphakic patients.
Because the aphakic population is largely elderly, particular care must be given to patient selection for extended wear. Elderly patients may not have the manual dexterity to remove, clean, disinfect, and reinsert their lenses. Before fitting such patients, it must be ascertained that if the patient cannot perform care routines alone, there is someone else at hand to render assistance. Even more important, removal of extended wear contact lenses at the very first sign of trouble can prevent (or greatly minimize) subsequent problems. If the patient cannot do this (and assistance is not readily available), the risk of serious ocular complications is greatly increased.
Diminished tear function is more likely to be present in an elderly patient, as are disorders of the corneal epithelium. Aphakic patients should be evaluated for these conditions before being fitted with extended wear contact lenses.
Because of the questions that have been raised about extended wear of contact lenses, some practitioners have decided to stop recommending extended wear to their patients and will prescribe only daily wear lenses. A middle ground exists, however, between extended wear and daily wear. This is the concept of flexible wear, which refers to the combination of daily wear with occasional or intermittent extended wear. An example of a flexible wear schedule is 5 days of daily wear followed by 1 day of extended wear. The lenses are cleaned and disinfected every time they are removed.
Contact lenses to be worn on a flexible wear schedule must be approved for extended wear. However, the more fragile extended wear lenses should be avoided in flexible wear because of the increased handling they will receive. Rigid gaspermeable extended wear lenses or thin, medium-water-content soft lenses (as opposed to ultra-thin or high-water-content soft lenses) are potentially suited to flexible wear.
It must be emphasized that contact lenses that are not approved for extended wear must be used only on a daily wear basis. Extended wear lenses can be used for daily wear, with the caveat that because of their fragility, some extended wear lenses may not stand up to the increased handling in daily wear.
Disposable/Frequent Replacement Lenses
A new era is at hand with the development of disposable extended wear contact lenses. Lenses such as the Vistakon Acuvue, Bausch & Lomb SeeQuence, CIBA Vision New Vues, Ocular Sciences/American Hydron Biomedics, and Wesley-Jessen Fresh Look have been positioned in the market as lenses that can be worn on an extended wear schedule for 1 week and then thrown away. (Alternatively, these lenses may be worn on a daily wear basis for 2 weeks and then discarded.) Recently true daily-wear disposable lenses such as Vistakon's 1-Day Acuvue and Bausch & Lomb's New Day have been introduced. These lenses are designed to be worn for 1 day. They are discarded at the end of the day. It is important that routine screening procedures be applied to candidates for disposable lenses. Ideally, these patients should be monitored as daily wear patients for up to 6 weeks and then monitored for potential extended wear over a 6-month period. Advantages are as follows:
It must be stressed that any time a disposable lens is removed from the eye with the intention of reinserting it, the lens must be disinfected. Some practitioners recommend that disposable lenses be worn on a daily wear basis; in this case, the lenses are removed and disinfected at the end of each day.
Disadvantages include the following:
Finally, the disposable lenses in current use are not greatly different from normal extended wear soft lenses. Although problems with solutions and deposits may be helped by disposing of the lens after a relatively brief wearing period, the basic physiologic questions about long-term extended wear remain unanswered and will not be solved simply by disposability.
Frequent/Planned Replacement Contact Lenses
A growing proportion of soft lens patients are wearing their lenses on a frequent-replacement basis. The aim of frequent replacement is to achieve better vision with fewer contact lens—related complications. In addition, frequent replacement allows practitioners increased patient contact and better control of patient lens wear. (The terms planned replacement and programmed replacement are used interchangeably with frequent replacement.)
Frequent replacement seeks to minimize the problems related to prolonged wear of a soft contact lens. Many soft lens patients use the same pair of contact lenses for a year or more. Among the problems related to the long-term use of the same lenses are the following:
Most of these problems are related to the formation of deposits on soft lenses. Regular use of surfactant and enzymatic cleaners helps a great deal, but does not fully eliminate deposits. In addition, deposit-related complications may begin well before decreased visual acuity or discomfort make the patient aware of the deposits. A major risk is the development of ulcerative keratitis caused by P. aeruginosa. Deposits on soft lenses may act as a focus for the growth of this organism.
Although deposits begin to form almost immediately on new soft contact lenses, frequent replacement allows for the discarding of soft contact lenses before deposit problems manifest. At the least, frequent replacement should serve to reduce deposit-related complications. Frequent replacement can be thought of as preventative care.
Incorporating the frequent-replacement concept into contact lens fitting requires the practitioner to make several choices. Decisions to be made include whether to use frequent replacement with all or just a portion of one's patients. Frequent-replacement lenses give the practitioner some options: the practitioner can dispense at one time a full year's supply of lenses or, alternatively, the patient can pick up the lenses quarterly.
Where frequent-replacement lenses are being considered, patient evaluation is performed the way one would with any soft lens candidate. A complete history and examination are required. If the patient is currently a contact lens wearer, the lenses should be examined for deposits. Tear film evaluation should be performed; this should include a break-up test and Schirmer tests. Blink rate should be at least 12 per minute.
Frequent-replacement lenses can be divided into two categories. The first is disposable lenses, which are approved for continuous wear for up to 6 days. Disposable lenses when used in this way require no lens care. This assumes that a lens that is removed from the eye will be discarded and a new, sterile lens inserted.
The second category is programmed replacement of daily wear or flexible wear lenses. This type of frequent-replacement lens must be cleaned with a surfactant and disinfected every time the lens is to be reinserted into the eye. (Enzyme cleaning may not be necessary for patients who are not heavy depositors.) Programmed replacement may be the method of choice for patients who tend to tear or lose lenses as well as those who are not candidates for extended wear. Programmed replacement is also often less expensive than disposable wear. One month is commonly used as the replacement interval for programmed replacement lenses, but intervals from 2 weeks to 6 months are also used.
The importance of patient compliance with the lens replacement schedule must be stressed when prescribing frequent-replacement lenses. Incentives such as including lens cleaning and disinfection products with dispensed cases may help increase compliance. Some practitioners have patients sign an agreement that outlines the responsibilities of the both patient and practitioner. The practitioner may reserve the right to terminate the agreement in the case of patient noncompliance.
|COMPLICATIONS FROM THE USE OF SOFT LENSES|
DEBRIS. Debris can make a lens hydrophobic and can alter its clarity and fit. Debris may consist of protein and, to a lesser extent, calcium or lipids. Every lens soon becomes coated but not all coatings are sufficiently dense to create disability and pathology. Debris can cause giant papillary conjunctivitis from analgesic factors. The best solution to avoid accumulation of debris is aggressive finger cleaning of the lens at home combined with at least once-weekly cleaning with an enzyme cleaner.
DISCOLORATION. This usually results from vapors such as hair sprays. Nasal or ophthalmic drops with epinephrine may cause brown adenochrome deposits. Nicotine from a smoker's finger will turn the lenses brown.
TEARS IN THE LENS. A long fingernail is usually the culprit in causing tears in the lens. Also, the edges of the lens may get snarled by the carrying case top.
SPONTANEOUS BREAKAGE IN THE EYE. This is usually the result of an undetected tear or chip in the lens that has extended. Replace the torn lens with a new lens. Any break in the surface of a lens will permit bacteria to enter it.
VASCULARIZATION. This is usually superficial and is invariably caused by a tight fit. It is associated with limbal compression and epithelial edema. Many corneas are normally vascularized at the limbus between 10 and 2 o'clock. The appearance of a prominent limbal vasculature should be noted before the lenses are fit. If true vascularization occurs (1% to 2% of patients), one of the following remedies can be taken:
INFILTRATES. Infiltrates are dot-like, translucent opacities of the epithelium or subepithelial layer. They may interfere with comfort or vision, or both. If they are small and few in number, they may not create any clinical problems. The infiltrates are believed to be composed of chronic inflammatory cells. Solutions are application of steroid drops and new lenses.
If the infiltrates are not clinically an issue, they can be noted, followed, and treated if they become irritating. They do not change the corneal dimensions, nor do they cause vascularization of the cornea. Infiltrates may persist for up to 3 months.
ENDOTHELIUM CHANGES WITH CONTACT LENSES. A number of endothelial changes—acute, transient, or chronic—can be seen with contact lens wear. Some changes may be irreversible.
Endothelial blebs are changes that are seen with specular microscopy and appear as scattered black spots throughout the endothelial mosaic. The endothelial blebs occur almost immediately on insertion of a contact lens and resolve within a 2-hour period. These changes are most pronounced in the nonadapted contact lens wearer compared to the long-term wearer, suggesting that in the adapted wearer a physiologic compensation occurs in response to the metabolic disturbance imposed on the endothelium by contact lens wear.
Changes in the size and shape of the endothelial cells (polymegathism and pleomorphism) without cell loss have been documented with either daily or extended wear lenses after several years' wear. These endothelial changes may not be reversible after the patient stops wearing the contact lenses. It is not known, however, whether these anatomic changes in the endothelial cells reflect an alteration in the cell's ability to function.
ARCUATE STAINING. Arcuate staining is seen as arc-shaped areas of abrasion on the cornea, usually created by a lens that is decentered or becomes decentered with blinking. It is particularly common to see this kind of abrasion with toric lenses that have been truncated.
EPITHELIAL EDEMA. Epithelial edema may be difficult to detect with soft lenses because no alteration in radius of curvature occurs with the appearance of edema. With a hard lens, corneal edema always creates a central steepening of the spheric portion of the cornea, so the visual changes are profound. Patients with hard lenses and corneal edema develop induced myopia and spectacle blur. With a soft lens, however, the edema is present from limbus to limbus without any major shift of corneal contour (Fig. 16).
Edema with a soft lens generally requires a 6% to 8% increase in corneal thickness before it is clinically detectable with the slit lamp or until symptoms occur. Slit-lamp findings are best appreciated through retroillumination as a gray central disciform area in relief.
The best instrument for detecting edema is a corneal pachymeter, which can detect even a 1% change in corneal thickness. Clinical signs indicating edema are as follows:
If corneal edema is present, it means that hypoxic changes are occurring in the epithelium. Either the lens does not deliver enough oxygen, so that the equivalent oxygen performance is not sufficient to meet the needs of the cornea, or the lens is too tight and the byproducts of metabolism accumulate.
These complications require attention to the causal factors. If the lens is tight, obviously a flatter fit is required. If the lens moves well, then the oxygen performance of the lens should be increased by either reducing the thickness of the lens or by employing a lens with a higher water content.
The most common cause of corneal edema with a soft lens is protein debris on the surface of the lens. The protein is hydrophobic and may cause chaffing of the cornea, or it may, if abundant, interfere with the oxygen permeability of the lens. However, some experts deny that coating of a lens has any effect on oxygen permeability.
Small microcysts of the cornea, a sign that precedes overt stippling, may be due to hypoxia or germicide toxicity of the cornea. If fine dot stippling is present, the easiest change to make is to alter the germicide used.
Chronic corneal edema may also change the curvature of the cornea in soft lens wearers. The keratometric change is toward making the cornea steeper. Such patients become increasingly myopic. Indeed, this is a common finding with soft lens wearers. At times, pseudomyopia occurs with protein debris on the lens, which changes the shape of the lens and increases its sagittal vault. A replacement lens of the same power or a refraction without a lens in place will usually uncover the true situation. If the myopia is occurring and systemic entities such as diabetes or drugs (e.g., sulfonamides) have been ruled out, then it must be assumed that a change in the corneal contour has occurred. Such an event in a mature adult requires a new approach to this complication of hypoxia. A gas-permeable hard or soft lens must be employed. In cases where the change in corneal shape has led to an irregular corneal surface, a silicone or silicone acrylate lens should be used. The soft lens should be chosen only if significant increases in permeability can be affected and the refraction error has remained essentially spheric.
In attempting to transfer between soft and gas-permeable lenses, there are major gains and losses for both. If the shift is from soft lenses to gas-permeable lenses, there will be satisfaction in the vision attained, the durability of the lens, and the relative ease of management of the lens. However, approximately 25% of patients do not appreciate the loss of total comfort that is almost universal among soft lens wearers. On the other hand, the switch to soft lenses from silicone acrylate lenses may be accompanied by loss of clarity, variable vision, lack of durability of the lens, and increased maintenance. Neither system is ideal.
CORNEAL TRAUMA. The most common traumatic event is inferior abrasion of the cornea caused by lens removal. Sometimes the lens is sliced by the fingernail, which also cuts through the cornea. However, what normally happens is that by the end of the day, the lens becomes slightly dehydrated because of lack of blinking. The lens becomes adherent to the cornea, and the patient tries to pinch off this tight lens. In doing so, a sliver of epithelium is also removed. This area may not heal well, especially if the patient continues to tear away epithelial cells. After the lens is removed there may be discomfort; however, there is no pain as the lens is worn, because the lens acts as a bandage. Patients who are incomplete blinkers, who work in a dry environment, who read most of the day, or who watch a visual display terminal should be warned against this complication.
To avoid damage to the cornea, the lens should be lubricated before removing. This can be done by blinking repeatedly or by using some lubricant drops to ease the suction of the lens on the epithelium.
A foreign body may get trapped under a lens and produce a variety of linear scratch marks on the cornea. The total irregularity of these wavy abrasions is the clue to the culprit.
A soft lens offers no protection whatsoever against blunt trauma to the eye. At the same time, it does not cause any added jeopardy in the case of eye trauma. An eye injured by a fist or ball will not be adversely affected by the presence of a soft lens. In industry, a soft lens is not a substitute for safety glasses.
The most common pitfall in the matter of lens care is the inability to keep lenses clean of deposits. These deposits can consist of protein, lipid, or calcium. They can cause discomfort, pseudomyopia, loss of vision, displacement of lenses (usually in an upward direction), and corneal erosions. Unfortunately, it is impossible to remove all the deposits 100% of the time, so build-up does occur despite the best intentions and practice. After the patient cleans the lens, he or she has no means of assessing whether any residue still remains on the lens.
GIANT PAPILLARY CONJUNCTIVITIS. Giant papillary conjunctivitis may occur in as many as 20% of soft lens wearers. It is usually found by everting the upper lid, where giant papillary hypertrophy of the tarsal conjunctiva is found, more marked at the upper border of the tarsal plate. This condition is considered an antigen-antibody allergic type of inflammation or the mechanical irritation of a foreign body. It responds to local steroids or nonsteroidal medication, such as cromolyn drops. A period of not wearing the lenses for 2 weeks or longer, vigorous cleaning of the soft lens, or replacement with fresh lenses may be required. At times, switching to a new type of soft lens such as a glycerol-based lens or a rigid lens is indicated. This condition affects a smaller proportion of patients with rigid lenses. Occasionally, reducing the diameter of the soft lens is sufficient to break the cycle of inflammation, renewed and excessive protein formation, heavy protein deposits on the lens, and more inflammation.
SOLUTION SENSITIVITY. Solution sensitivity may occur as a complication of soft lens wear. Thimerosal (sodium ethylmercurithiosalicylate) is the major offender in producing a delayed hypersensitivity response. It causes corneal erosions, conjunctival hyperemia, or corneal deposits. Many thimerosal-free solutions are now being prepared.
Chlorhexidine, another antibacterial preservative, can cause increased binding of the protein in the tear film to the surface of the lens. In reduced strengths, this is not as much of a problem.
Hydrogen peroxide, if not neutralized, can cause corneal pitting and marked circumcorneal injection.
|Today, the practicing ophthalmologist must endeavor to be a comprehensive
provider of eye care and must be prepared to deliver the highest quality
of eye care to the public. This commitment to the patient requires
that we continue to improve our knowledge and skill in all areas of
eye care, including the fitting of contact lenses, which plays an integral
role in today's practice of ophthalmology.|
Soft contact lenses used for regular replacement of eyeglasses, excluding aphakia, have been shown to be safe, effective, and convenient. Although corneal ulcers occasionally occur, these are few when one considers the millions of patients who currently wear soft contact lenses. Problems are often related to some basic factor in fitting or in the lens care regimen.
Still, all is not perfect with soft contact lenses. The lens has a short life and requires frequent replacement. The high-water-content lenses tend to create an osmotic shift in marginally dry eyes, thus causing discomfort. The water in low-water-content lenses evaporates quickly in dry places such as Arizona, and vision and comfort are thereby decreased. Finally, when contact lens wearers rub their eyes, the lenses may become wrinkled or displaced. In extended wear lenses, one should prescribe a flexible program of removal, requiring the patient to wear lenses no longer than 7 days without removing, cleaning, and disinfecting. In our view, extended wear lenses currently are still safer and more reliable than surgical alternatives for myopia.
Requirements for achieving a successful practice with soft lenses boil down to three essential factors:
Allansmith MR, Baird RS, Grenier JV: Contact lens-associated giant papillary conjunctivitis as a model for vernal conjunctivitis. In Silverstein AM, O'Conner GR (eds): Immunology and Immunopathology of the Eye, p 346. New York, Masson, 1979