Contact lenses have become a routine part of our armamentarium for visual rehabilitation of the eye. Their use and demand are constantly increasing. More than 20% of the North American population is myopic and many of these people depend on visual correction. Coupled with the increasing use of contact lenses, many myths and fallacies have arisen regarding their indications and contraindications. At the very minimum, the ophthalmic assistant should be able to discuss with the patient the function of a contact lens, its purpose, and its limitations. The ophthalmic assistant can also be of value in some of the technical aspects of contact lens wear, such as method of insertion and removal and, particularly, proper care and storage of the lens itself. This chapter deals with the practical aspects of management of the patient who desires contact lenses, and in particular, rigid contact lenses.
As early as the 16th century, Leonardo da Vinci conceived and sketched prototypes of modern contact lenses. He experimented by neutralizing his own refractive error by placing his face in a container of water. In the following century, René Descartes described and illustrated a glass type of scleral contact lens ( Fig. 14.1 ). However, the first practical type of contact lens was produced in 1887. This lens consisted of a glass capsule containing gelatin that was placed in contact with the cornea, with the glass being molded to correspond to the shape of the eye.
In 1932 the first major advance in the design of the contact lens was made. Investigating impressions made from the human eye, Dr. Joseph Dallos found that no two were identical. From this, he concluded that it was impossible to fit a contact lens manufactured to a preconceived formula. Dallos then developed a technique of making negative casts of the anterior segment of the living eye. However, his lenses could be tolerated for only limited periods of time because of the excessive weight of the glass, and they were difficult to manufacture. In 1938 the first molded scleral contact lens that overlaid the sclera was made from a plastic material called polymethyl methacrylate (PMMA). This lens had many advantages over glass because it was lighter, shatterproof, and easily moldable.
It was not until 1948 that Kevin Tuohy introduced the first fluidless corneal lens, which was designed to rest on the corneal tear layer. These were large, but later smaller microcorneal lenses were introduced, which made possible a great step forward in the successful wearing of contact lenses.
Further developments in rigid lens manufacture came with the introduction of intermediate curves, the practice of refining the edges, and the development of toric lenses. Rigid lens technology made another leap forward with the introduction of silicone and fluorocarbon. When combined with PMMA, these materials make the plastic material gas permeable ( Fig. 14.2 ). As a result of these new materials, in most countries today PMMA rigid lenses are rarely used.
Advancement in rigid gas-permeable (RGP, or GP) technology involves improving the biocompatibility of materials to prevent unwanted deposits from tears. Polysulfone has been used in contact lens polymerization. New crosslinking technology stabilizes thin lens designs for comfort and durability. Advancement in manufacturing techniques and new lens material have reintroduced scleral lenses. There are at least 48 different RGP contact lens materials available by 11 different manufacturers. These manufacturers provide their GP material to labs all across the world that produce numerous GP lens designs.
The rigid contact lens, for all practical purposes, eliminates the cornea as a major source of refractive error of the eye. The index of refraction of the RGP lenses is slightly greater than the tear layer. The fluid interface between the spherical contact lens and the cornea fills out irregularities in the contours of the anterior corneal surface, converting an astigmatic cornea to a sphere. Thus the fluid may be considered a forward extension of the cornea. If the radius of curvature of the back surface of the contact lens is the same as that of the front surface of the cornea, the fluid lens will be zero. The change in refractive power is produced by altering the curvature of the contact lens, as well as changing the total contact lens power. Change in the contact lens base curve alters the fluid lens power.
How the corneal contact lens works
A contact lens rests on the cornea just as a small fragment of paper adheres to the wet fingertip by just touching it. The natural moisture on the surface of the cornea is sufficient to create a surface tension and permit the lens to adhere quite strongly.
The back surface of the lens is contoured so that it is very similar to the curvature of the cornea. The corneal curvature can be measured by instruments, such as an ophthalmometer (or keratometer) and topographer. It is vital that these measurements be exact because if there is any contact or touch between the cornea and the contact lens, then a scratch, abrasion, or erosion can occur in the superficial layers of the cornea. The contact lens therefore rests on a liquid cushion (tear film) and never on the eye itself. Injury to the cornea is one of the most damaging complications that can result from a poorly fitted contact lens. Not only does it produce a painful red eye that obscures vision, but it also provides a portal of entry for bacteria and other pathogenic organisms to form a corneal ulcer.
The difference between the front surface curvature and the back surface curvature of a contact lens produces the power of the lens.
The edge of an RGP contact lens is thin and polished so that it can gently slide underneath the lid without being dislodged and prevent lid irritation when blinking.
To appreciate contact lens technology, one should have an understanding of contact lens jargon. The following terminology applies to rigid as well as soft lenses.
The ophthalmometer is an instrument designed to measure the corneal curvature by using the cornea as a front surface mirror. The instrument is most commonly referred to as the keratometer, even though this is actually a trade name of Bausch & Lomb.
In astigmatism with-the-rule, the vertical corneal meridian has the steepest curvature, whereas in astigmatism against-the-rule, the horizontal meridian has the steepest curvature ( Fig. 14.3 ).
In performing keratometry some authors record the flattest meridian first and the steepest meridian next so that a keratometer ( K ) value of, for example, 44.00 diopters × 46.00 diopters × 85 indicates that the horizontal meridian has a radius of 44.00 diopters and that the vertical meridian has a radius of 46.00 diopters with the axis at 85 degrees. Other authors prefer to always record the horizontal meridian first, regardless of which is the flattest. This value may be expressed in either diopters or millimeters of radius. Table 14.1 gives a comparative value of the K reading in diopters and millimeters. Each 0.05 mm is equivalent to approximately 0.25 diopter, so that a 0.5-mm radius equals approximately 2.50 diopters. Expressed another way, each 1.00 diopter change in the K reading equals approximately a 0.2 mm radius change.
|Keratometric reading (D)||Radius convex (mm)|
In contact lens work, one usually first considers the flattest K reading. If the back surface of the lens is to be the same radius as K , this is referred to as fitting on K (i.e., if K readings are 44.50/45.00, the base curve of the lens would be 44.50 or 7.58 mm). RGP lenses may be fitted on K , flatter than K , or steeper than K . This may depend on the size of the lens and the corneal astigmatism. Soft contact lenses are generally fitted 3.00 to 5.00 diopters flatter than K .
The corneal cap is the central zone of the cornea. This has a radius of approximately 4 to 6 mm and has a relatively constant spherical radius of curvature ( Fig. 14.4 ). The peripheral or paracentral zone of the cornea is the area surrounding the corneal cap and extending to the limbus. It has a much flatter curvature than does the central curve. The rate of flattening does not conform to a mathematic progression, that is, the cornea is not a true ellipse. It is generally described as being aspheric.
Most rigid corneal lenses are either bicurve or tricurve. A bicurve lens has one base curve and one peripheral secondary curve ( Fig. 14.5 ). A small lens is usually bicurve. An intrapalpebral fit is one that fits within the palpebral fissure limits and is bicurve. This type of lens is small and steep, with narrow peripheral curves of 0.2 mm and small diameters of 7.5 to 8.8 mm.
A multicurve lens has a base curve and three or more peripheral curves. A tricurve lens usually has a large diameter ( Fig. 14.6 ). A contour lens is basically a tricurve lens with a narrow intermediate curve. The blend is the point of transition between the radii of curvature from one curve to another. The sharp junction is removed by making the zone of transition with a curved tool that has a radius value between the values of the two adjacent curves.
An intermediate curve is a curve between the base curve and the peripheral curve.
The total lens diameter or the chord diameter is the measurement from one edge of the lens to the opposite side. This is a linear measurement and is not related to the circumference of the lens. Most corneal rigid lenses used today have a chord diameter between 8 and 10 mm, whereas the chord diameter of a soft lens usually ranges from 12 to 15 mm. Depending on which end of the scale they fall into, lenses may be referred to as small or large.
The peripheral curve width is the diameter from one edge of a secondary curve to another.
The central thickness of a lens is the separation between the anterior and posterior surfaces at the geometric center of a lens. The higher the minus power, the thinner is the center, whereas the higher the plus power, the thicker is the center.
Tints refer to the coloring available in a lens. They may be blue, brown, gray, or green for rigid lenses. They may be numbered 1 to 3, with number 1 the lightest and number 3 the darkest shade of each color.
A ballasted lens, often referred to as a prism ballast lens, is one that is weighted with a heavier base that orients inferiorly when the lens is worn. A truncated lens is one that is cut off to form a horizontal base. The amputation of the base is usually at the inferior pole of the lens. Truncation is frequently used to add stability to a rigid alternating bifocal lens to prevent rotation.
Back vertex power refers to the effective power of the lens from the posterior surface. The distance from the back surface of the lens to the focal point is the back focal length; its reciprocal is the back vertex power.
The primary base curve, as well as all other curvatures of a lens, may be expressed in terms of millimeters of radius of curvature. It can also be expressed in diopters: a primary base curve of 43.25 diopters is equal to 7.8 mm. The primary central posterior curve of a lens is designed to conform to 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 of the cornea.
Toroidal or toric lenses (derived from Latin torus , “a bulge”) are lenses with different radii of curvature in each meridian. The meridians of the shortest and longest radii are called the principal meridians, and they differ by 90 degrees. These lenses are used to correct astigmatism.
A front surface toric lens has an anterior surface that has two different radii of curvature but a central posterior surface that is spherical. Usually a prism ballast is required for orientation.
A back surface toric lens has a posterior surface that has two different radii of curvature and an anterior spherical surface.
In a bitoric lens, the anterior and posterior surfaces have two curvatures on the front and back surfaces. The axes of the anterior and posterior toroidal surfaces may coincide or be oblique to one another, but usually they coincide.
The lenticular bowl refers to the diameter of the optical portion of a lens and is used with higher-power lenses.
The posterior apical radius (PAR) refers to the radius of curvature of the back surface of a lens at its apex. This is the area of curvature that will conform to the front surface of the apex of the cornea. Lenses are labeled by the posterior radius at the apex of the lens.
When a base curve of a lens is said to be made steeper, this means that the posterior radius of curvature is decreased (e.g., from 8.4 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 (e.g., from 8.1 to 8.4 mm), so that the curvature is now flatter.
The sagittal depth or 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 produces a greater “vaulting” of the lens and in effect is steeper. 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 radius of the lens. If the diameter is kept constant, by changing the radius to a longer radius (e.g., from a 7.8- to an 8.4-mm radius), the sagittal vault or sagittal height of the lens becomes shorter and the lens becomes flatter. The converse is also true ( Figs. 14.7 and 14.8 ).
If the central posterior curve (radius) of the lens remains the same but the diameter is made larger (e.g., from 13 to 15 mm with soft lenses or from 8 to 9 mm with rigid lenses), the sagittal vault or sagittal height of the lens is increased and the lens becomes steeper. The converse is also true ( Fig. 14.9 ).
For example, if we consider the lens as being part of a similar circle ( Fig. 14.10 ) and we take two parts of the circle with different diameters or chords, the portion of the circle with the larger diameter will have a greater sagittal vault.
The wettability of a surface is measured in terms of its contact or wetting angle. In the case of a contact lens, this is the angle formed between the tangent to the edge of a drop of water and the surface of a contact lens. The wetting angle, called theta, is expressed as zero if the material is completely wetted by the liquid ( Fig. 14.11 ). As the wetting angle becomes greater than zero, the solid surface cannot be completely wetted. As an extreme, if a drop of mercury were to be placed on a glass slide, it would not spread out on the surface, but would form a small blob whose angle would be greater than 90 degrees (see Fig. 14.11 ). Some plastics have smaller wetting angles than others; the lower the wetting angle of the plastic, the better the tears will spread evenly over the surface of the contact lens. Wetting solutions reduce this angle and permit better flow of tears and comfort. The lower the wetting angle, the greater is the spread of tears.
Terminology pertaining to oxygen studies has taken on increasing importance in the contact lens literature because of the development of extended-wear lenses 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. 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 material obtained in a given condition at a given temperature in the laboratory only. A rule of thumb is that the higher the DK of the material, the more flexible will be the lens that is made from this material. The oxygen flux refers to the amount of oxygen that passes 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 (P). DK/L is often referred to as oxygen transmissibility.
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, which involve the total lens and account for not only the material, but also the design of the lens and its thickness in the center and the periphery. This measurement is called the equivalent oxygen percentage (EOP). An EOP of 9% is considered the minimally safe level for daily-wear contact lenses, whereas an EOP of 12% is considered the minimally safe level for extended-wear contact lenses. An EOP of 18% is considered the ideal level for extended-wear contact lenses, as this is the level for normal physiologic corneal swelling that occurs in the closed eye during sleep.
Sophisticated corneal contact lens technology has made possible the manufacture of numerous widths, thicknesses, curvatures, and edge designs to aid the modern practitioner.
The contact lens can vary in diameter from 6 to 12 mm. Most people wear lenses with a diameter of 8 to 10 mm. They are generally smaller than soft lenses. The optic zone of the lens varies widely in diameter and may range from 5 to 8 mm, depending on the overall diameter of the lens. Contact lenses are available in monocurve, bicurve, tricurve, multicurve, and aspheric designs.
A secondary, or intermediate, curve is often put on a lens next to the base curve of the optic zone to accommodate the flatter periphery of the cornea. This intermediate curve may be 2.00 to 7.00 diopters flatter than the base curve. A tricurve lens is designed to have secondary curves. The width of these curves is relatively small, such as 0.2 mm. Occasionally, multicurve lenses are designed.
The resurgence of aspheric designs in rigid lenses has aimed to mimic the peripheral flattening of the cornea in a controlled and reproducible manner, thereby eliminating the need for progressively flattening peripheral curves.
The peripheral curve is the outermost curve and is much flatter than the other curves of the lens to conform to the flatter periphery of the cornea. The peripheral and intermediate curves permit a free flow of precorneal fluid under the lens.
To eliminate the sharp edges of the junction lines of these curves, the junctions are blended to give smoothness to the transition of the different curvatures. The blend may be light (the zones are readily identified) or heavy (the zones blend into each other).
Fenestration of hard lenses is valuable only from a historical perspective. Most RGP lens materials cannot be fenestrated because this increases the fragility of the lens. The ability of RGP lenses to transmit gases negates the functional need of fenestration.
Central thickness is an important factor in contact lens comfort. The thinner the lens, the more comfortable but also the less stable is the lens and the more likely it is to warp and break. The thinner the lens, the greater is the oxygen transmission through the plastic.
Edge design of a contact lens is important and is frequently the cause of patient rejection. The edge must be carefully designed, rounded, and polished. If the edge is too thick, it will irritate the eyelid margin. If it is too thin, it will be too sharp (knife edge) and irritate.
Contact lenses can also have cylinder ground into the back surface (back toric lenses), into the front surface (front toric lenses), or into both the front and back surfaces (bitoric lenses). Prism ballast can be incorporated in a contact lens to give weight to the lens and thus prevent rotation. Bifocals of varying design are also available (see Ch. 16 )
Although a careful eye examination is mandatory before a practitioner decides whether a patient is a suitable candidate for contact lenses, certain areas must be delineated in more detail. A complete history of the eye is needed, as well as a history of systemic medications, such as oral contraceptives or the possibility of pregnancy. During the inquiry, an assessment should be made not only of the person’s reasons for desiring contact lenses, but also of personal hygienic habits and the ability to look after the lenses and persevere with the required lens care routines. For obvious reasons, the patient’s manual dexterity should be evaluated. Also the patient’s temperament and emotional maturity should be evaluated so that introduction of contact lenses will ensure success rather than frustration and problems for both the practitioner and the patient. In those patients who were dissatisfied with previous contact lenses but wish to try again, one should establish the reason for the previous failure.
More than 95% of patients can wear contact lenses. The following instances preclude their use:
Chronic blepharoconjunctivitis from any cause, such as seborrhea or acne rosacea
Pterygium formation: with small pterygia, contact lenses can be fitted, but larger pterygia may require removal first
Seventh nerve palsy (Bell’s palsy)
Poor hygiene: people who do not keep their hands clean will not keep their lenses clean. Clean fingernails trimmed short are mandatory.
Industrial hazards: people who deal with highly volatile acids or bases (alkalis)
Severe allergies: most allergic reactions can be suppressed by the use of local antihistamines. However, allergic reactions can reduce wearing time in soft lens wearers and cause papillary conjunctivitis
Age: if the need is present, age is no barrier. One-day-old newborn children who have had cataract surgery have been fitted with contact lenses. Older adults, until intraocular lenses became popular, routinely wore aphakic contact lenses
Low tear film: serious dry eyes with low tear film.
The TearScope (Eagle Vision and Keeler) analyzes the tear film in vivo to assess its stability and to determine whether there is a dry eye syndrome requiring punctal plugs before contact lens wear. The TearScope measures the break-up time of tears and tear film with fluorescein-stained tears.
A careful slit-lamp microscopic examination of the cornea before the patient begins contact lens wear permits detection of small scars or opacities of the cornea, which cannot later be blamed on the contact lens. The eyelids, and in particular, the undersurface of the upper eyelid should be examined for follicles, papules, and signs of inflammation. The lid margins must be free of blepharitis. Any ghost vessels or signs of new vessel formation on the cornea should be noted and their cause determined.
The patient’s eyes should be refracted and the best possible visual acuity recorded. Any muscle imbalance should be measured by prisms, and particular attention should be paid to the phorias.
The corneal diameter can be measured with a handheld ruler, a slit-lamp eyepiece reticle, pupillometer, or a contact lens of known diameter. The most accurate method of measuring a corneal diameter is by applying a contact lens of known diameter or by using a slit-lamp eyepiece reticle. The size of the pupil may be measured by a ruler under room lighting conditions or estimated by a pupillary gauge. The height of the palpebral fissure and the area where the upper eyelid crosses the cornea should be recorded. These factors have an important bearing on the lens design. Lid tension can be estimated by grasping the lid between thumb and forefinger, pulling slightly down and letting go. This will give a rough indication of whether the eyelid is loose or tight. The practitioner should observe the patient to see if the blink rate is reasonably normal. Adequacy of tear production may be measured by the filter paper (Schirmer’s) test. Keratometry (described later) is an important basis for the initial lens design required.
Fitting corneal contact lenses
The following measurements are required to fit a contact lens:
The refractive error of the eye
The dioptric curvature of the cornea
The contact lens diameter
The lens thickness
The optic zone
The peripheral curve blending
The refractive error of the eye is determined by conventional methods. Prescription for distance vision is determined in minus cylinders because all contact lenses are manufactured in minus cylinder form. When fitting rigid lenses, the corneal cylinder is disregarded by the fitter because up to 3.00 diopters of with-the-rule corneal astigmatism can be easily corrected with a rigid spherical lens. The vertex distance of the spectacles should also be recorded because the dioptric power of a plus lens is increased and that of a minus lens decreased when the spectacle prescription is converted for contact lenses. These changes in the power of the lens are related to the distance from the original spectacle to the cornea and the power of the lens itself.
The dioptric power of the cornea is determined by keratometer readings ( Figs. 14.12 and 14.13 ). For keratometer measurements, the patient is seated before the keratometer, with the chin placed on the chin rest. The room should have dim illumination. Two illuminated targets, called mires, are positioned so as to be reflected from the center of the cornea. The observer views the cornea through the telescopic system of the keratometer. This light is split by a prism into the images of the vertical and horizontal axis.
The keratometer should be calibrated on a steel ball of known radius at least once every 2 months. Before measurements are taken, the eyepiece must be adjusted to the observer’s prescription. The patient should be comfortably seated and the keratometer placed so that the patient must lean slightly forward to place the chin in the chin rest. The patient is then instructed to press his or her head firmly against the forehead rest and to grasp the base of the instrument firmly with both hands to steady the head and to fixate the eyes steadily. The patient is encouraged to open the eyes widely and to blink occasionally. The fitter should be seated with both feet flat on the floor, to be able to operate the instrument without having to strain the neck or slump forward. Once the instrument has been aligned, the mires are positioned so as to be reflected from the center of the cornea. The fitter should constantly keep one hand on the knob that controls the clarity of the mire images and the other hand on the knob that controls the separation of the mire images; otherwise the instrument may be out of focus at the moment the final setting is made.
The Bausch & Lomb keratometer is typical of other keratometers in use ( Box 14.1 ). The illuminated targets, or mires, consist of three illuminated circles, with one circle having a plus sign as an appendage. The central circle has both a plus and a minus sign. At first, the central circle appears doubled ( Fig. 14.14A ) until the focusing knob is used to produce a single central circle ( Fig. 14.14B ). The black control cross should always be in the center of the right bottom circle. The next step is to rotate the axis of the keratometer so the plus and minus signs are aligned ( Fig. 14.14C ). Then the horizontal measuring drum is turned so that the plus signs overlap and become single ( Fig. 14.14D ). This is the first K reading. The vertical measuring drum is then turned so that the minus signs overlap ( Fig. 14.14E ). This is the K reading of the second curvature. In exceptionally flat or steep corneas, readings cannot be taken without accessory lenses to extend the range of the keratometer.
Adjusting the eyepiece.
Position the occluder.
Turn the eyepiece cap counterclockwise as far as possible; the user should see a blurred cross.
Look through the eyepiece and turn the eye focus. Note the reading on the outer periphery of the eyepiece. Repeat the same set several times to verify the results.
Seat the patient comfortably before the instrument and fit the chin securely on the chin rest with the head against the headrest.
Level the keratometer to the patient’s eye. Set the instrument at 90 and 180 degrees. Put the instrument to one side to align it with the eye. Raise or lower the instrument until the silver pin on the side of the lamp house is lined with the patient’s pupil. The patient’s head must be positioned vertically and not tilted.
Patient fixation: turn the instrument so that it points directly at the eye to be examined. One should see a tiny bright ring (the mire) in the center of the cornea. The mire should be aligned with the pupil and the patient should see the reflection of his or her own eye.
Take the reading: to obtain the proper measurement, Steps A through E should be followed.
Focus the instrument carefully with the focusing knob. The double circle is seen with black crosshairs near the center when out of focus (see Fig. 14.14A ). By turning the knob you should see a single clear circle with a clear cross in the center (see Fig. 14.14B ).
Rotate the instrument to locate the axis plus signs tip to tip. The axis of the cylinder is found when the tips of the two plus signs just touch. Turn the horizontal measuring drum until the plus signs are barely separate and the lines of the plus cylinders appear to be continuous (see Fig. 14.14C ).
Measure the horizontal meridians. Turn the left measuring drum until the plus signs are superimposed (see Fig. 14.14D ).
Measure the vertical meridians. Turn the right measuring drum until the minus signs are superimposed (see Fig. 14.14E ). Actual diopter power of the corneal curvatures can be obtained.
The difference between the two measurements is the amount of corneal astigmatism. Read the axis on the scale and record.
It is common practice to record the horizontal reading first. The difference between the horizontal meridian and the vertical meridian constitutes the corneal astigmatism. As mentioned previously, when the horizontal meridian is flatter than the vertical meridian, the corneal astigmatism is referred to as with-the-rule. If the horizontal meridian is steeper than the vertical meridian, the corneal astigmatism is referred to as against-the-rule. When the corneal astigmatism of the eye differs from the refractive cylinder of the prescription, this difference is referred to as residual astigmatism.
Because the keratometer measures the central zone, or optic cap, of the cornea, which has a diameter of 5 to 7 mm and includes the visual axis of the cornea, this reading should be accounted for the base curve of choice ( Fig. 14.15 ). However, the periphery of the contact lens actually rests on the intermediate zone of the cornea, which is somewhat flatter than the optic cap.
Automated keratometers also provide the dioptric K value of the central cornea ( Fig. 14.16 ). The corneoscope is another instrument that provides a photographic representation of the curvature of the portions of the cornea central to peripheral.
Topographic corneal analysis is the most sophisticated way of analyzing the corneal dioptric values. By a computerized readout with color analysis, a person can determine 6000 keratometry points on the cornea. This provides considerable new information and data on the cornea, which in turn can help determine how much flatter the periphery of the lens should be to the central radius of curvature. Details of this topographic analysis are addressed in Chapter 44 .
The size of the rigid contact lens is important. The topogometer is an instrument designed to measure the optic cap or apex of the cornea. In spherical corneas this optic cap is the same in all diameters, but in astigmatic corneas there is a difference in curvature at different meridians. RGP contact lenses are ordered on the basis of the flattest radius of the optic cap. Other factors that determine the size of the lens are the width of the palpebral fissure, the prominence of the globe, and the spasticity of the eyelids.
The thickness of a lens is an important factor in comfort. Again, thinner rigid lenses are more comfortable, but are less stable and more likely to warp and break. Lenses should be ordered with minimal thickness and be verified by a thickness gauge. In powers greater than −6.00 or +2.50 diopters, a lenticular design is frequently used to reduce the thickness, and consequently, the weight of the lens. Some ultrathin lenses flex and provide less irritation.
The optic zone is determined by the difference of the lens diameter and the width of the peripheral curves, which are flatter than the base curve. The peripheral curves are designed to allow tear flow under the lens at the flatter corneal periphery.
Trial lenses are simply a set of corneal contact lenses of known diameter, power, base curves, and peripheral curves. Most experienced practitioners use a fitting set of corneal contact lenses in conjunction with a keratometer. The use of a fitting set practically eliminates the need for exchanges of lenses. Most experienced fitters agree that the best instrument to evaluate a fit is a contact lens on the eye.
Materials and manufacture
Corneal rigid contact lenses were for many years constructed of PMMA. This material absorbs fluid minimally (<2%) compared with the soft lenses, which vary in hydration from 25% to 80%. PMMA has excellent optical qualities, but requires some adaptation because of some discomfort associated with its use and its lack of oxygen permeability. These lenses are fitted smaller than the visible iris diameter, but larger than the pupillary diameter, and are known as corneal lenses.
The PMMA lens is hardly used today and has been superseded by RGP contact lenses. To date, RGP lenses have been made of: (1) cellulose acetate butyrate (CAB) (currently out of use), (2) silicone acrylate, (3) styrene, (4) silicone resin, (5) fluoropolymer, and (6) fluoronated silicone acrylate combinations (fluorosilicone acrylates [FSAs]). These lenses are often referred to as semisoft or flexible rigid contact lenses. These lenses are much more flexible than standard PMMA lenses and have greater oxygen permeability, which is not found in the standard PMMA lenses. The FSA polymers have advanced in terms of oxygen permeability as well as the wetting angle since their introduction. They have the advantage of being able to correct several diopters of corneal astigmatism and, because of their gas permeability, are able to maintain corneal deturgescence and relieve some of the symptoms of corneal hypoxia. RGP is now being replaced by GP to reflect the high oxygen permeability and other features of these newer materials.
Fitting gas-permeable lenses
The ideal fit for any GP lens is one in which the lens position is high, even when the lens overlaps the superior limbus ( Fig. 14.17 ). The upper lid should cover a portion of the lens during the full cycle of each blink. The purpose of the high-riding lens is to tuck the edge of the lens under the lid to avoid lid impact. This “full-sweep” blink also enhances lens surface wetting. The engagement of the upper lid margin with the edge of the lens is a major cause of discomfort. Even a soft hydrogel lens is uncomfortable if it is made small and interpalpebral.