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Module 36 |
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Module 36: |
Hard Contact Lenses (Rigid Gas Permeables) |
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Part 2 | ||
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Table of Contents (with bookmarks)
Evaluation of the contact lens fit (continued from Module 35) Flexure Residual astigmatism Comfort Fluorescein pattern Corneal and eyelid integrity punctate staining corneal abrasion 3 and 9 o'clock staining arcuate staining dimple veiling neovascularization polymegathism spectacle blur orthokeratology Diagnostic fitting Empirical fitting Reading the lens power Reading the diameter Reading the base curve (with a radiuscope) Reading the center thickness RGP lens insertion and removal Techniques General instructions Wearing time Daily cleaner Disinfection Enzymatic cleaner All purpose solutions General instructions |
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Evaluation of the contact lens fit (continued from Module 35)
The fit of the RGP contact lens should be evaluated according to the following criteria:
Criteria 1-3 are discussed in Module 35. The discussion is continued here with criteria 4.
4. Flexure: With the patient still behind the phoropter for the spherical over-refraction, it is a good time to test for lens flexure. Flexure occurs when the contact lens "bends" over the astigmatic ridge of the cornea when the patient blinks. Flexure can be observed by performing retinoscopy with the contact lens in place. Flexure is indicated by changes in the reflex after a blink. Flexure can also be tested for using the keratometer. With the lens in place on the eye, observe the pattern of the mires reflected on the lens through the keratometer, particularly after the patient blinks. Flexure will be indicated by changes in the shape of the mires. Flexure can be decreased by slightly flattening the base curve (.5 D) and/or by increasing the center thickness of the lens by .2 to .4 mm.
As you might suspect, flexure is also a function of the amount of corneal astigmatism. The patient with 3+ diopters of corneal astigmatism may be better off with a bi-toric lens design. The bi-toric lens has two different base curves 90 degrees apart, so that the lens fits the shape of the corneal surface. The bi-toric design is complex and is beyond the scope of this module.
5. Residual astigmatism: Another possible cause of imperfect vision with a hard contact lens is residual astigmatism. This is astigmatism that is other than corneal astigmatism, and it is not corrected by the tear lens of the spherical hard contact lens. The usual suspect is lenticular astigmatism. In other words, the shape/optics of the crystalline lens inside the eye is not spherical. This can be measured by performing retinoscopy and a complete refraction (not just spherical) over the contact lens. Residual astigmatism of one diopter or less is usually tolerated by employing the spherical equivalent power. Residual astigmatism not well tolerated may require a re-fit with a toric lens.
A problem with residual astigmatism can, to a degree, be predicted. If the refractive cylinder correction is significantly different from the corneal astigmatism as measured by the keratometer, residual astigmatism must be suspected. For example:
MR OD -4.00-300X180
K OD 46.00 x 90, 44.00 x 180
The amount of refractive cylinder correction is -3.00, but the corneal astigmatism as measured by the keratometer is 2.00 D (46 minus 44). The difference (1 D) may be residual astigmatism. Knowing this may save you the trouble of a trial fit with a spherical RGP lens, but the only way to know for sure if this would be a problem would be to refract over the spherical RGP lens and get feedback from the patient.
6. Comfort: The first time RGP contact lens wearer will not be comfortable until wearing time has been gradually built up. If the established wearer complains of discomfort, you must find the underlying cause of the discomfort. It can be as simple as dryness, which may be alleviated by the use of re-wetting drops. The fit of the lens may need to be evaluated, as well as the integrity of the cornea and the lids.
7. Fluorescein pattern: It is possible to judge the dynamics of the hard contact lens fit by instilling a drop of fluorescein into the eye wearing the contact lens. The fluorescein temporarily stains the tears. The depth of the tear layer under the lens can be judged by the appearance of the fluorescein under the lens. A deeper layer of tears will appear brightly fluorescent when observed with the cobalt blue light of the slit lamp microscope. A shallow layer of tears will appear a lighter shade of green.
The illustration below depicts a contact lens that is fit well to a spherical cornea. There is a large, central pattern that is light green, indicating a close fit of the base curve of the lens to the central cornea. The bright green at the edge of the lens indicates a deeper layer of fluorescein under the peripheral curves of the contact lens.
A contact lens that fits flat will show a smaller area of light green in the center and a larger area of a deeper tear layer at the periphery of the lens. An "alignment fit" lens may normally have some flat fit characteristics.
A steep fit is illustrated below. The central area of the lens vaults the the central cornea, creating a pool of bright fluorescein in the center. An "apical clearance fit" lens may normally have some steep fit characteristics.
A lens fit on a significantly astigmatic cornea will demonstrate a "dumbbell" type pattern, as shown below.
Proper fluorecein pattern evaluation requires experience. The basic idea is to avoid "apical bearing" and "mid-periphery bearing". Apical bearing is caused by the lens rubbing against the corneal apex (too flat). Mid-periphery bearing results from too much apical clearance (too steep) and a lens periphery that is "stuck" against the cornea, not allowing tear circulation.
8. Corneal integrity: This evaluation applies more to the returning contact lens patient rather than to the initial contact lens evaluation, although some problems can show up almost immediately.
The cornea should be evaluated for fluorescein staining patterns. This is done with the slit-lamp biomicroscope. With hard contact lenses, the fluorescein evaluation of the cornea can be done with the lenses in, or with the lenses out. If there is a significant staining pattern, it will persist for some time after the lenses are removed. Soft lenses should not be evaluated in the eye with fluorescein because the soft lens material will stain with the fluorescein.
Corneal staining patterns appear because the corneal epithelium has been disrupted. The dye stains compromised cells and pools in disruptions in the smooth corneal surface. Disruptions can extend into the layers under the corneal epithelium. Staining patterns can be caused by mechanical rubbing, chemical exposure, oxygen deprivation, or infection.
Punctate staining (pictured below) is an area of small, pinpoint like disruptions in the epithelium. The more faint variety is sometimes termed stippling. Common causes are oxygen deprivation or some type of chemical exposure.
A corneal abrasion is an area where the corneal epithelium has been stripped away. It is usually caused by mechanical action and the shape depends upon the agent and the action. Examples include a central area of abrasion caused by a flat lens, or a linear abrasion caused by a foreign body under the contact lens.
3 and 9 o'clock staining is punctate like staining that occurs in these clock hours of the cornea. It can be caused by mechanical rubbing of the periphery of the lens, usually made worse by inadequate lubrication, poor blinking action, and/or over-wearing the lenses.
Arcuate staining occurs in the shape of an arc in the periphery of the cornea. It is caused by rough blends between the peripheral curves.
Dimple veiling has a stippled appearance and is caused by air bubbles trapped under a lens that is either too steep (central staining) or too flat (limbal staining).
Significant corneal edema will show up as central clouding of the cornea, which can be seen without the aid of fluorescein. Corneal edema secondary to contact lens wear can be due to poor corneal metabolism. The contact lenses must be refit for better oxygen transmission or the contact lenses must be discontinued.
If corneal oxygen metabolism is chronically depressed, as can be the case with extended wear soft lenses, new blood vessels may encroach across the limbus and grow into the cornea (neovascularization).
Also of concern is the loss of corneal endothelial cells that can result from poor metabolism secondary to contact lens wear. The problem is that the endothelial cells do not regenerate. The neighboring cells enlarge to fill in the void (polymegathism).
The endothelial cells are pumps that keep the cornea dry (free from edema) and clear. Significant endothelial cell loss can compromise the ability of the endothelium to keep the cornea dry and clear.
Serial K readings (e.g. K readings at least once a year over a period of years) can be used to assess changes in corneal curvature over time. The shape of the cornea can tend to mold to the shape of the contact lens over time. This can be a bad thing or a good thing, depending upon your point of view. Corneal molding an make it difficult for the patient to see well with glasses, and make it difficult to re-fit contact lenses.
So called "spectacle blur" is blurry vision with glasses following the removal of contact lenses. It is caused by the cornea molding to the contact lens and it is common with hard contact lenses. After removing the contact lenses, the cornea tends to revert to the original shape. The changing shape of the cornea causes the blurriness with glasses. It should not last more than 15 to 20 minutes, otherwise the fit may need to be re-evaluated.
A type of spectacle blur can result from corneal edema secondary to contact lens wear. After the lenses are removed, the edema resolves and the vision improves.
Some practitioners use corneal molding on purpose to reduce the patient's near-sighted correction by using contact lenses to flatten the shape of the cornea. So called "orthokeratology" has been around for a long time. In 2002, the FDA approved corneal refractive therapy (CRT). The patient wears a contact lens overnight that gradually changes the shape of the cornea, reducing and sometimes eliminating myopia up to six diopters. If the patient stops wearing the lenses, the cornea gradually returns to the original shape and the myopia comes back.
There are two schools of thought regarding how many RGP lenses to have on hand in your practice. There is the "empirical" fitting school, and there is the "diagnostic" fitting school.
This involves fitting lenses from a fitting set or from a lens inventory. A fitting set can consist of a handful of lenses to over 200 lenses. A lens inventory is just a large fitting set, with perhaps hundreds of lenses. The major advantage of a large diagnostic set is that the patient can immediately experience the optimum vision that an RGP lens can offer. This is a major selling point. Other advantages include the ability to evaluate parameter changes immediately, and the ability to dispense replacement lenses immediately. A fairly complete set from -1.00 to -6.00 in .25 D steps over a range of base curves from 7.42 to 8.13 would number around 200 lenses.
Empirical fitting means that measurements are made (K readings, manifest refraction, pupil size, lid opening, corneal diameter, etc.) and the measurement are sent to a manufacturer. The manufacturer uses nomograms (formulas) to determine the lens parameters. The lens is shipped to you and the patient is scheduled for a return visit at which time the lens is evaluated. If the lens is satisfactory in fit and performance, then the lens is dispensed. If there are problems, then the manufacturer is consulted and changes are made for another go at it. At first blush, this may not seem to be the best strategy, but this method can be successful on a percentage basis, meaning you reach a point where you have few re-fits. The significant time savings can more than make up for the extra costs of re-fitting. Corneal topography manufacturers have come up with software programs that can increase the accuracy of empirical fitting.
Because of the nature of soft contact lens materials, we are not able (practically) to determine the lens specifications with inspection techniques. However, we are able to do this with hard contact lens materials. We are able to verify diameter, base curve, lens power, and center thickness before an RGP lens is dispensed, and we can read these parameters from an "unknown" lens that was dispensed elsewhere.
The edge of every RGP lens should be inspected. The condition of the edge is critical to a comfortable fit. Inspection can be done at the slit lamp microscope by simply holding and rotating the lens with your fingers. Obviously, a lens with a chipped or cracked edge should not be worn, but also inspect the contour of the lens edge. In particular, the anterior edge profile (the surface that the lid goes over) should be nicely rounded and smooth.
RGP lens power can be read with a lensometer. The lens is held with your thumb and index finger against the lensometer port, with the concave side against the port. The power is read just like you would read a glasses prescription. The power will be spherical unless the lens is toric in design. Care must be taken not the flex the lens between your fingers. Flexure may induce a cylindrical reading. A warped lens can also produce a cylindrical lensometer reading.
The RGP lens diameter can be read with a variety of tools that have a millimeter scale. A very handy tool is the millimeter ruler with a "V" groove. The lens is placed in the groove and it slides down until it stops. The diameter is read from the scale reading at the point where the lens stops.
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The RGP lens base curve is read with a radiuscope. This instrument looks similar to a microscope. The lens is placed on a table under a lens. The user looks through an eyepiece to focus mires and to take a reading from a scale.
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1. The lens holder is removed from the table. A drop of water is placed into the depression in the lens holder. The contact lens floats convex side down on the water drop. Care must be taken to have a minimal amount of water under the lens. Excess water can be removed by placing a tissue into one of the groves coming from the depression. | ||
| 2. The lens holder with the contact lens is placed onto the table and the table is moved if necessary to place the center of the lens directly under the green light projecting from the lens of the radiuscope. |
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3. While looking into the eyepiece, the star shaped mire is brought into focus by turning the focusing knob on the right side of the instrument.
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The large part of the knob is for gross focusing, and the smaller part of the knob is for fine focusing.
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| 4. The small knob to the left of the eyepiece is used to move the line on the scale to the "zero" position. |
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| The small knob under the eyepiece is used to focus the scale if necessary. |
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5. Once the star is in focus and the scale line is placed in the zero position, the base curve is measured by turning the focusing knob in the clockwise direction (away from you). The star mire will disappear at first, and then it will come back into focus as you continue to turn the focusing knob. The scale reading will be moving away from zero. Use the fine focus knob once the star mire comes back into focus. |
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When the star mire comes into focus the second time, it may not be centered, with only part of the star visible. | ||
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The star can be centered again by moving the table that holds the contact lens mount. | ||
| 6. The base curve is then read at the position of the line on the scale. The reading will be a "radius of curvature" value in millimeters. It can be converted to diopters using a conversion table. This scale reading is about 8.41. |
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| If the lines on the star focus at slightly different settings, then this indicates that the lens is warped. |
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| If one line comes into focus, with the others blurry, then the lens is probably toric. The line 90 degrees to the first one should come into focus by itself when the focus is changed. |
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Center thickness is read with a thickness gauge. The most common type is the dial gauge. The needle of the instrument is placed over the center of the lens and the thickness is measured from a dial. |
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RGP Lens Insertion and Removal
If this discussion seems familiar, it is because it is very similar to the discussion you read in the soft contact lens modules. RGP lens care is somewhat less complicated because the RGP lens does not soak up water, chemicals, or pathogens like the the soft lens can. RGP insertion is similar to soft lens insertion, but RGP lens removal is somewhat more complicated.
The RGP lens is inserted by positioning the lens on the index finger of the dominant hand. The lids are opened and held by the remaining fingers of both hands. The lens is guided to the cornea until contact is made. It is helpful if all parts of the lens edge make contact with the eye at the same time. The lids are not released until the lens has adhered to the cornea. The lens can usually be centered with a few blinks. It is helpful for most patients to look at themselves in the mirror during the procedure. |
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Yes, this is a soft lens, but the insertion technique is very similar. | ||
| The RGP lens is removed by placing the finger on the skin at the outer canthus and pulling outward. This tightens the lids against the the upper and lower edges of the lens. The patient then blinks, and if all goes well, the lens pops out of the eye. |
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It is best to bend over a towel on a flat surface to catch the lens. After some practice, the patient can pop the lens into the other hand. If the lens does not pop the first time, make sure the lens is centered in the palpebral fissure by looking straight ahead and be sure to apply equal pulling force to the upper and lower lids. An alternative to this method is to use a lens removal tool. This has a small suction cup on the end of a holder. The suction cup is guided to the lens until suction is achieved and the lens is then removed. Use of a wetting drop in the eye prior to using the tool usually helps.
The patient should be taught what to do if the contact lens becomes de-centered. See the instructions in the next section.
Even though your I&R video may cover these points, be sure they are part of a printed handout for the patient. You may want to add to or modify this list as your experience may suggest: |
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Initial wearing time: This varies with the practitioner. The new RGP wearer must gradually build up wearing time. Some practitioners like to start the patient with 2 hours the first day and add 2 hours each day until the lenses are worn for all waking hours if desired.
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It is the goal of every RGP contact lens care system to remove foreign matter and microorganisms from the surface of the lens, to neutralize or kill any remaining microorganisms on the lens, and to properly "wet" the RGP lens prior to insertion. Care systems use various combinations of daily cleaners, disinfectants, and wetting agents.
Daily cleaners work well only if combined with mechanically cleaning the lens. This is accomplished by putting a drop of cleaner on the lens and rubbing the lens gently with a finger in the cupped palm of the other hand, with the convex side of the lens against the palm. Unlike with the soft lens, with the RGP lens, the cleaner can be rinsed with warm tap water. This is best done after the contact lens is removed for the day.
Disinfection can be accomplished by storing the contact lenses overnight in a disinfecting (storage) solution.
Enzymatic cleaners are used to remove tear protein deposits from the surface of the contact lens. These are deposits that may not be removed with a daily cleaner. These cleaners are for the patients who are more susceptible to these deposits than other patients. Excessive protein deposits can block oxygen transmission through the lens and they may trigger a hypersensitivity reaction under the eyelids. This is less of a problem with RGP lenses than it is with soft lenses.
All-purpose RGP lens care systems are currently popular for contact lens care. One or two solutions are used for cleaning, disinfection (storage in the case), and wetting prior to insertion. The idea is to encourage better compliance with a more simple system. Unfortunately, many patients think all they have to do is remove the lenses and place them in the storage solution overnight. Be sure to encourage the patient to clean (rub and rinse) the lenses as described above.
Some patients may become hypersensitive to the preservatives in care systems. Symptoms may include soreness, stinging, foreign body sensation, redness of the conjunctiva, redness of lids. swelling of the conjunctiva and/or lids, and punctate staining of the cornea. A major offender has been the preservative thimerosal. It is best to avoid care systems with this preservative. Switching the patient to a care system with a different preservative may solve the problem.
General instructions to the patient should include:
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