Module 33

	Module 33
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Module 33:	Soft Contact Lenses:
	Part 2

Fitting Spherical Soft Contact Lenses

Table of Contents (with bookmarks) Anterior Segment Examination health tear production lids pupil size hygiene Lens Type Diameter Base curve Power Fit Evaluation coverage centering drape movement power Ordering the contact lens Insertion and removal Initial wear time Soft contact lens care


Anterior segment examination: A new contact lens patient typically has a complete eye exam before contact lenses are fit. The ophthalmologist or optometrist will pay particular attention to the following details of the examination: Health of the lids, conjunctiva, and cornea - patients with corneal diseases or chronic blepharitis are not good contact lens candidates. A pterygium or pinguecula may be irritated by contact lens wear. Tear production - poor tear production alone may cause a patient to discontinue contact lens wear. A Schirmer tear test and tear break-up time (BUT) may be performed.
Lids - the lids must be elastic and fit closely to the globe. This picture would obviously be an exaggeration of a loose lid. The lid aperture must be wide enough for the patient to be able to insert the contact lens.
Giant papillary conjunctivitis may be in the history of a former contact lens wearer wanting to wear lenses again. This is characterized by large bumps, or "papillae", which cover the upper tarsal conjunctiva. It is usually caused by wearing contact lenses that are covered with deposits. The lid must be "everted" to view the extent of the condition. The patient must usually discontinue wear for the condition to subside. Some patients can resume contact lens wear with a frequent replacement schedule.
	Pupil size - if the diameter of the pupil approaches or exceeds the optical zone diameter of the contact lens, then annoying distortion and flare occur. If the patient has unusually wide pupils in a well lit room, she may not be a good contact lens candidate.

Hygiene - take a look at the patient's appearance during the exam. Dirty fingernails are a good indicator of a contact lens problem child.

Lens type: Before the actual contact lens evaluation begins, a lens manufacturer and a particular lens model must be selected. Some practitioners have a favorite lens that they fit the majority of patients with. The choice may depend upon patient preference for daily, flexible, or extended wear, frequency of replacement, power limitations, cosmetics (color), and other factors. Commonly, a daily wear lens with a light blue (visibility) tint is a good choice, with replacement every 1 to 3 months. The spherical soft contact lens has three variable parameters: diameter, power, and base curve. Diameter: Most manufacturers have a standard diameter for a particular lens model (e.g. 14.0mm, or 14.5mm). A larger or smaller diameter may be available for special situations. Base curve: As discussed earlier, for a given diameter, the fit of the lens is adjusted by changing the base curve. There are usually three base curves available: steep, medium, and flat. Most manufacturers identify the base curve by radius of curvature. Examples would be 8.9 (flat), 8.6 (medium), and 8.3 (steep). Some use the Vault system: Vault I (flat), Vault II (medium), and Vault III (steep).
	The starting point for choosing a base curve is keratometry. The manufacturer gives guidelines regarding which base curve should be used for a given K reading. An average K reading of 43.50 might call for a base curve of 8.6 in our 14.0 diameter trial contact lens.

Power: Since we are fitting a spherical soft contact lens, the eye should not have more than .75 D of astigmatism. If it does have more astigmatism than that, a toric soft lens or a hard gas permeable (HGP) lens is indicated for optimum vision. Thicker spherical soft contact lenses are thought to be able to "mask" a low amount of astigmatism. A thin lens however will drape over the curves of the cornea and the residual astigmatism will limit the visual acuity obtained with the spherical lens. If only one eye has significant astigmatism, it is sometimes acceptable to the patient to wear spherical lenses in both eyes, especially if the better seeing eye is the dominant eye.

If the eye being fit has astigmatism, the refraction is converted to minus cylinder, and the spherical equivalent power is used. For example: OD MR = -6.00+1.00x180 Minus cyl. = -5.00-1.00x90 Spherical equivalent = -5.50

The spherical equivalent power must then be adjusted for vertex distance. This is because the refractive power is measured at approximately 12 to 13 mm away from the cornea. The contact lens, of course, will be on the corneal surface. For powers 4 diopters or less, this calculation makes little difference. The greater the lens power is, the more difference this calculation makes in the contact lens power. Most contact lens publications have a table for figuring this adjustment. You will need to remember that, for minus lenses, power is taken away from the refractive lens power to arrive at the contact lens power. For plus lenses, power is added to the refractive power to arrive at the contact lens power.

Pictured to the right is a hyperope corrected with a glasses lens. VD is the vertex distance, which is measured from the back of the glasses lens to the cornea. FD is the focal length of the compound lens system (the glasses lens plus the cornea and the natural lens.
Pictured here is the same hyperopic eye corrected with a contact lens. The VD is zero and the focal length is shorter. Because the focal length is shorter, the plus powered contact lens will need to be proportional stronger than the glasses lens to focus light on the retina. Thus the vertex power adjustment increases the power of the plus powered contact lens compared to the power of the glasses lens.
	Picture to the left is a myopic eye corrected with a glasses lens. The vertex distance (VD) and the focal distance (FD) are marked. Remember that a minus lens diverges light so that the focus is lengthened for the nearsighted eye.
	Pictured here is the same myopic eye corrected with a contact lens. The VD is zero and the focal distance is shorter. Since the focal distance is shorter, the minus lens will need to be less powerful to achieve the same focus. Thus the vertex power adjustment decreases the power of a minus powered contact lens compared to the power of the glasses lens.
The clinical optics calculator that we give away free on this website has a vertex distance calculator. We would enter -5.00-1.00x90 into the calculator, along with a "present vertex distance" of 13 and a "new vertex distance" of zero. The calculator gives us a spherical equivalent, vertex adjusted contact lens power of -5.10, which we would round off to -5.00. This would be the power that we would choose for our trial lens. If a -5.00 D lens is not available in the fitting inventory, choose the closest power available. Fit evaluation: If the patient has no contact lens experience, you will need to insert the lenses for the evaluation. After insertion, wait at least 10 minutes before you evaluate the lens. This allows time for patient acclimation and time for the water content of the lens to stabilize. The lens will loose water after insertion, and it will fit tighter as a result. During this time, if the patient complains of a foreign body sensation, remove, rinse, and reinsert the lens. Start the 10 minute wait again after reinsertion.

Evaluate the fit of the trial lens with the slit-lamp microscope, keeping the following points in mind: Coverage - the lens should cover the entire surface of the cornea in all meridians and should go slightly beyond the limbus. A very loose lens may ride low, exposing an upper portion of the cornea. A solution may be a tighter lens. A lens that is loose may be too small. A larger diameter may solve the problem. Remember that a larger diameter lens will fit tighter if the base curve is kept the same. Centering - the lens should center well on the cornea. A very loose lens may ride low. A high plus lens, particularly an aphakic lens, may ride low. A larger diameter lens may solve the problem. Drape - the lens should fit closely to the cornea without any pockets between the lens and the cornea. This is rarely a problem unless the contact lens parameters are not close to what they should be. Movement - the lens should show some movement. A thin lens will usually show less movement than a standard thickness lens. The standard for movement will depend upon the practitioner. Some doctors like to see .5 to 1 mm of movement with a complete blink. To evaluate this, watch an edge of the lens with the slit-lamp as the patient blinks. Other doctors think that a lens that does not move with a blink is not necessarily too tight. However, every lens should show movement if it is mechanically pushed. To evaluate this at the slit-lamp, have the patient look slightly upward. With your finder, push on the patient's lower lid so that the lid pushes against the lower edge of the lens. The lens should move, if not, the lens is too tight. The fit can be loosened by going to a flatter base curve and/or a smaller diameter. A lens that is really tight will actually indent the conjunctiva at the edge of the lens. This degree of tightness usually only shows up on extended wear patients at the end of the wear period. A lens can also demonstrate too much movement. Excessive movement may give the patient a foreign body sensation, and the vision will be disturbed as the lens moves on the cornea. The fit can be tightened by going to a steep base curve and/or a larger diameter. The fit could also be tightened by using a thinner lens. Before deciding that a lens is too loose, be sure to allow enough time for stabilization, perhaps longer than the minimum 10 minute period.
Evaluate the power of the trial lens - your power calculation should have you close to the optimum power, if not dead on. Evaluate the power by performing an over-refraction with the phoroptor or loose lenses.

Most of the time, a spherical over-refraction will be sufficient. The exception might be the patient with significant astigmatism (>.5D) who is being fit with a spherical soft lens. Any spherical error found during the over-refraction is simply added or subtracted from the trial lens power when finalizing the contact lens prescription. Ordering the contact lenses - if the lenses are being fit from an inventory, and the patient's optimum prescription is on hand, then the patient can leave the office wearing the lenses. Even if the optimum Rx is not on hand, if there are lenses in inventory close to the optimum prescription, then the patient can leave with those lenses and the correct Rx can be ordered. This always makes the patient happy, and we do like to make the patient happy.

Insertion and Removal (I&R) - This routine is usually only for the new contact lens wearer, although some experienced patients can use the review. This duty can be drudgery, because it is always the same thing, over, and over, and over again. To minimize the drudgery, have the patient watch a videotape of I&R procedures. This procedure mainly has to do with the patient getting over the fear of sticking her finger in her eye. Once that is overcome, it is usually smooth sailing. Soft contact lenses can be flipped inside-out. They are usually most comfortable, and vision is usually the best, when the lens is inserted correct-side out. Although with thin lenses, it doesn't seem to make much difference. The "taco test" is used to determine which way is the correct way.

The lens is slightly pinched between the thumb and forefinder, to make it look like a taco. It helps if the lens is slightly dry. The edge of the lens is observed. If the edges slope inward, then the lens is in the correct position.

		If the lens edges reverse and curve outward, then the lens is inside-out. This phenomenon can usually be observed without pinching the lens.

Some manufacturers print initials near the edge of their lenses. A keen observer (or someone using a slit-lamp) can then tell if the lens is inside-out.

The soft 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, if not, the lens can be pushed toward center with a finger. It is helpful for most patients to look at themselves in the mirror during the procedure. Alternatively, the contact lens can be placed on the conjunctiva below the cornea as the eye looks upward. The lens is then moved onto the cornea with a finger.

		The soft lens is removed by pinching the lens between the thumb and index finger of one hand. As with insertion, the lids are held open with the remaining fingers. It is helpful to look slightly upward, and pinch the lower edge of the lens. Some find it helpful to first slide the lens downward from the cornea before pinching the lens.

Most contact lens patients eventually become very adept at insertion and removal. Many are able to use a one handed technique that simply pulls the lower lid down for insertion and removal. It is best to teach the two handed technique and let them improvise on their own.

The patient should be taught what to do if the contact lens becomes de-centered. Knowing ocular anatomy, we know that the lens will not become "lost behind the eye", but the patient may not know this. Finding a de-centered lens is just a matter of searching the conjunctival area after the lids have been pulled away from the globe. The lens is then pinched, removed, and then re-inserted. Even though your I&R video may cover these next 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: Always wash your hands before handling a contact lens. Short fingernails are better than long nails. Care must be taken so that long nails do not tear the lens. Hand lotions and creams should not be used prior to contact lens handling. It is best to use hair spray before contact lenses are inserted. It is best to apply makeup after contact lenses are inserted and remove the contact lenses before makeup is removed. Only use appropriate care products for cleaning, wetting, and rinsing contact lenses. Saliva and tap water should not be used as wetting or rinsing agents. Eye redness, discomfort, or blurry vision should be reported to your doctor immediately. If the contact lens becomes de-centered from the cornea, it cannot travel behind the eye. Using a mirror, try to locate the lens on the white part of the eye, or under the upper or lower lid. Move the lens with your finger if necessary. Remove the lens by pinching it with your fingers. Re-insert the lens in the usual manner. If you cannot find the lens, it may have fallen out. It is not a good idea to wear contact lenses while swimming. The lenses can easily become dislodged and/or soft lenses may soak up some chlorine and irritate your eyes. Do not use eye drops with contact lenses, except for lubricating drops approved for your lenses. Do not sleep with your contact lenses unless they are approved for overnight wear. Wear safety glasses over the contact lenses in appropriate situations.
Initial wearing time: This varies with the practitioner. Many patients tolerate soft contact lenses right away. Some practitioners like to start the patient with 4 hours the first day and add 2 hours each day until the lenses are worn for all waking hours if desired.

Soft contact lens care It is the goal of every contact lens care system to remove foreign matter and microorganisms from the surface of the lens, and to neutralize or kill any remaining microorganisms on the lens. This is usually called "cleaning" and "disinfection". Care systems use various combinations of daily cleaners, rinsing agents, and disinfectants.

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. The lens can be flipped inside out so that the other surface can be cleaned. A rinsing agent is used to wash the daily cleaner and other matter from the surface of the lens. A rinsing agent should be used whether or not a daily cleaner is used. Rinsing the lens will obviously be much more effective if a daily cleaner is used. Disinfection can be accomplished by storing the contact lenses overnight in a disinfecting 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. More frequent replacement of the lenses also reduces this problem. All-purpose soft contact lens care systems are currently popular for contact lens care. One solution is used for cleaning, rinsing, and for disinfection (storage in the case). 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 solution overnight. Be sure to encourage the patient to rub and rinse the lenses as described above. The all-purpose solutions are expensive. The cost for the patient can be reduced if the all-purpose solution is only used as the cleaning agent and the disinfecting agent (storage). A much less expensive contact lens saline solution can be purchased to rinse the lens during cleaning and to rinse the lens before insertion. Some patients may become hypersensitive to the preservatives in chemical 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 chemical care system with a different preservative may solve the problem. Alternatives to chemical care systems are daily disposables or a hydrogen peroxide care system as discussed below. A hydrogen peroxide based care system requires more steps for the patient, but it may be the only choice for the patient who is hypersensitive to the preservatives in other care systems. The peroxide is an efficient antimicrobial and it has some protein cleaning activity as well. The system requires an exposure step of about 10 minutes and a neutralization step that lasts from 20 minutes to a few hours, depending on the system. General instructions to the patient should include: Only use products that are compatible with your lenses. Check with your doctor's office. Do not mix care product brands unless recommended by the doctor's office. Wash hands before handling lenses. Do not skip steps in your lens care routine, as instructed by the doctor's office. Keep the lens care environment clean (case, counter, storage bag, etc.). Keep care product bottle tops from touching any surface. Work over a clean surface. Use paper towels if in a public restroom. If you drop your lens prior to insertion, rinse the lens well before insertion. Notify your doctor's office if you experience eye or lid redness and/or irritation.

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