Lesson 20 - Rigid Gas Permeable Contact Lenses

Question 1

Lenticular

Answer 1

In order to make the lens edges more comfortable on the cornea, a “plus lenticular” edge is often requested so that the thick edge is smoothed down to be made thinner. A “plus lenticular” edge should be requested for a high minus lens and a “minus lenticular” edge should be requested for a high plus lens. The plus contact lens edge could be made thicker by lenticulating it so it will interact with the eyelids to stay centered on the cornea better.

Question 2

Hybrid lens

Answer 2

Has a rigid gas permeable lens in the center surrounded by a soft contact lens material skirt. These offer the clear vision provided by an GPCL while reducing the amount of debris that accumulates under the lens. These lenses give the benefit of correcting corneal astigmatism with the comfort of a soft contact lens edge.

Question 3

Back surface toric design

Answer 3

Back surface toric GPCLs fully correct the corneal astigmatism with the toricity at the back surface of the lens. They are used when the toricity is fairly high, over 3 diopters, and the corneal toricity is the same as the refractive cylinder. The base curve should be the same as the flattest curve, and the second curve on the lens should be the difference between this base curve and the prescription cylinder divided by 1.5.

1. For example, if the prescription is -2.00 -3.00 x 90, and the K’s are 44.00 and 47.00, the flatter of the two base curves is:
2. 00 diopters is the flatter of the two K’s.
3. The second curve is 3 ÷ 1.5 = 2, so 2 diopters from 44 is 46 diopters.

Question 4

Radiuscope

Answer 4

Used to measure the base curve of a GPCL

Question 5

3-9 staining

Answer 5

Fluorescein staining at the 3 o’clock and 9 o’clock positions of the eye. This is usually caused by an edge lift between the lens and the cornea at the horizontal meridians causing the eye to become dry.

Question 6

Bitoric GPCLs

Answer 6

Bitoric GPCLs are used when the cylinder is greater than 3 diopters and there is a significant difference between the corneal cylinder and the spectacle cylinder. Use the flattest K for the primary base curve, and the secondary curve should be 1 diopter flatter than the steepest curve.

Question 7

List and describe the advantages of GPCLs

Answer 7

1. Lower long-term cost. These lenses are custom-made, so their initial cost is higher than the cost of a single pair of soft contact lenses. However, clients don’t need to replace them on any regular schedule. Basically, they’ll last until they become chipped or get lost, or until the client’s cornea curvature changes significantly. Since these are rigid lenses, the cornea doesn’t change much because it generally molds to the curvature of the lenses. It’s also less costly to care for GPCLs. Usually, a single cleaning solution and a wetting agent are all clients need for these lenses.
2. Sharper vision. Many people report that these lenses yield clearer, crisper vision than soft lenses. This is because they don’t contain water and can “smooth out” an irregular corneal surface.
3. Less surface coating buildup. Because they don’t get coated as easily as soft lenses, RGP GPCLs may reduce the risk of ocular infections.
4. Higher oxygen permeability. These lenses are gas-permeable, as their name states, so they don’t block the flow of oxygen to the eye. Thus, users may be less prone to inflammation or other eye problems.
5. Great ease of handling. Unlike soft lenses, GPCLs don’t tear.
6. Lower risk of fungal infection. While all contact lenses have a good safety record, serious fungal infections can occur in some users—and this risk is greater with soft contact lenses than with GPCLs.

Question 8

GPCLs can be used for orthokeratology. What is that?

Answer 8

Orthokeratology is a technique in which a person is fitted with progressively flatter and flatter curved lenses in order to reduce myopia

Question 9

The segmented bifocal GPCL designs are very similar to bifocal spectacle lens designs, and people with ___ often can see better with these than with the progressive soft bifocal designs.

Answer 9

The segmented bifocal GPCL designs are very similar to bifocal spectacle lens designs, and people with presbyopia often can see better with these than with the progressive soft bifocal designs.

Question 10

Given the advantages, why doesn’t everyone choose GPCLs?

Answer 10

Because it takes time to adapt to them. Many people adjust to them within a few days, but some decide that they’re just too uncomfortable. In addition, the small size of these lenses compared to soft lenses allows debris and dust particles from the tear film to get under the lens, which can cause mild to very annoying pain or even a scratched cornea.

Question 11

Manufacturers use many types of materials to make gas permeable lenses. What are the most popular?

Answer 11

Silicone/acrylate—a combination of silicon (allowing for oxygen permeability), methylmetacrylate (which gives the material stability), and some wetting agents

Fluorosilicone/acrylate—a combination of fluorine (which improves the wettability of the lens surface) and the silicone/acrylate material

Menicon Z—a relatively new material that has high wettability

Question 12

GPCLs come in several designs. All of these lenses are designed so they can custom fit them over corneas with varied curvatures, or fit them for presbyopes. List some of the designs.

Answer 12

spherical, lenticular, bifocal, scleral, and quadrant specific.

Question 13

Bifocal GPCLs

Answer 13

A bifocal RGP will have a segment at the bottom of the lens like a flat-top bifocal spectacle lens.

Question 14

Scleral GPCLs

Answer 14

Scleral lens designs have become more popular recently. It is a rigid gas permeable material lens with a diameter larger than the cornea. It can be fit on highly toric corneas or eyes that have very irregular surfaces. They are for occasional wear, about 4-6 hours at a time, and work well on very dry eyes where other lens modalities fail.

Question 15

Quadrant-specific

Answer 15

This design is used with scleral lenses where only a sector or quadrant of the lens is bearing on the cornea. The sector design is made where only a sector of the scleral lens is curved differently than the rest of the lens surface.

Question 16

To fit a gas permeable lens, you will need to consider what?

Answer 16

Corneal diameter

Pupil size

Size of the palpebral aperture or eyelid opening

Eyelid position

Completeness of the eyelid when blinking

Lid tightness

Health of the cornea, lids, and lashes

Tear film

Question 17

The first step in fitting a lens is to accurately measure your client’s corneal curvature. You can use what for this job?

Answer 17

The first step in fitting a lens is to accurately measure your client’s corneal curvature. You can use a keratometer or an autokeratometer (computerized keratometer) for this job.

Question 18

Measuring the corneal curvature correctly is critical, because an incorrect curvature or base curve fit can do what?

Answer 18

Measuring the corneal curvature correctly is critical, because an incorrect curvature or base curve fit can be painful, scratch the cornea, or lead to poor vision.

Question 19

Some autokeratometers also calculate a starting base curve. It’s common to start with a base curve that’s either the same as the ____ keratometer reading (also called the K reading) for that cornea or about ___ diopters flatter. If the cornea has more astigmatism, you’ll need to fit a ___ lens.

Answer 19

Some autokeratometers also calculate a starting base curve. It’s common to start with a base curve that’s either the same as the flattest keratometer reading (also called the K reading) for that cornea or about .25 diopters flatter. If the cornea has more astigmatism, you’ll need to fit a steeper lens

Question 20

To fit a gas permeable contact lens with a spherical or toric base curve, you’ll need to decide what? Many gas permeable contact lens manufacturers and distributors provide ___ for you to borrow or purchase. It is often easier for you to have a ___ so you can save time by having access to a diagnostic lens during a fitting session.

Answer 20

To fit a gas permeable contact lens with a spherical or toric base curve, you’ll need to decide what base curve you will start with for a diagnostic lens. Many gas permeable contact lens manufacturers and distributors provide fitting sets for you to borrow or purchase. It is often easier for you to have a fitting set so you can save time by having access to a diagnostic lens during a fitting session.

Question 21

When you order a GPCL from your lab, you can specify what?

Answer 21

When you order a GPCL from your lab, you can specify the base curve, secondary and peripheral curves, or let your lab calculate the best peripheral curves using their computer algorithm based on your base curve.

Question 22

After inserting the lens in your client’s eye, you’ll put ___ in the eye so you can see how the lens sits on the cornea. You’ll use a ____ with a ____ filter to view the alignment.

Answer 22

With this number in hand, you’re ready to test a trial lens with the base curve you’ve selected. After inserting the lens in your client’s eye, you’ll put some fluorescein stain in the eye so you can see how the lens sits on the cornea. You’ll use a slit lamp with a cobalt filter to view the alignment.

Question 23

After you’ve identified the base curve for the lenses, you’ll need to identify the what?

Answer 23

After you’ve identified the base curve for the lenses, you’ll need to identify the material, oxygen permeability needed, surface wettability, and treatment coatings.

Question 24

How to fit GPCLs

Answer 24

1. Apply the lens, and allow it to settle a few minutes.
2. Assess visual acuity.
3. Using fluorescein, assess the central and peripheral bearing.
4. Assess the centration of the lens and how it moves with the blink.
5. Order lenses with any necessary power and fit modifications.

Question 25

If the cornea has toricity, you will not be able to fit a ___ on it. It’s like if you sliced a grape lengthwise, hallowed it out, and put it on a marble—not a great fit. So front surface toric lenses and back surface torics have ___ to fit toric corneas.

Answer 25

If the cornea has toricity, you will not be able to fit a spherical gas permeable lens on it. It’s like if you sliced a grape lengthwise, hallowed it out, and put it on a marble—not a great fit. So front surface toric lenses and back surface torics have nonspherical surfaces to fit toric corneas.

Question 26

Front surface GPCLs

Answer 26

Front surface GPCLs can correct corneal astigmatism of up to 4.50 diopters. They are stabilized on the eye with a prism ballast, truncating the bottom edge, or dynamic stabilization zones that use the eyelids to stabilize the lens.

Question 27

Multifocal gas permeable contacts

Answer 27

Multifocal gas permeable contacts have a segment at the bottom similar to bifocal glasses lenses so that when the eye moves downward to read, the pupil moves into the segment. When the person looks in the distance, they are to move their eye straight ahead into the top portion of the lens. The lens must sit at the bottom of the lid margin. The segment height should be at the lower pupil margin. If the lens rotates too much, add prism and order a flatter base curve. You can also truncate the bottom of the lens to hold the lens in place during the blink. If the distance vision is blurry, the patient might be looking through part of the segment. The seg height might be too high or the lens may need added prism so the lens will drop down. If the near vision is blurry, the patient might be reading through part of the distance portion of the lens. The patient might be moving their head down instead of their eyes to read, seg height might be too low, it might be rotating too much, or the lens might not be translating.

Question 28

Many GPCL manufacturing labs use computers to help optimize the design of a lens. If you give one of these labs the what the lab can help you determine a good starting gas permeable contact lens to order?

Answer 28

Many GPCL manufacturing labs use computers to help optimize the design of a lens. If you give one of these labs the prescription, the corneal diameter, the pupil size, and the K readings, the lab can help you determine a good starting gas permeable contact lens to order.

Question 29

The parameters of a GPCL lens that need to be verified are what?

Answer 29

Power, base, diameter, center thickness, color, material

Question 30

How to measure the power of the GPCL?

Answer 30

To measure the power of the GPCL, place the contact lens adapter on the lens stop of the lensometer and use the lensometer to measure the lens power just as you would a spectacle lens.

Question 31

How to obtain the diameter of a GPCL?

Answer 31

The diameter is measured using a contact lens loupe.

Question 32

How to measure center thickness of a GPCL?

Answer 32

Center thickness is measured with a contact lens thickness gauge.

Question 33

Color options for GPCLs

Answer 33

Most GPCLs come in brown, blue, ice blue, green, gray, or clear.

Question 34

The gas permeable contact lens modification unit can do what?

Answer 34

The gas permeable contact lens modification unit can polish, clean, reshape the edge, change the secondary and peripheral curves, modify the optic zone, and alter the total power of the lens a little.

Question 35

Modifying a GPCL lens

Answer 35

Polish, clean, reshape the edge, change the secondary and peripheral curves, modify the optic zone, and alter the total power of the lens a little.

Modifying a GPCL lens requires skill, practice, and may be done with hand tools, but more often nowadays it is done with motorized, sometimes computerized, equipment at the lab. The practitioner would need to know how much to change and on what part of the lens in order to give the lab the correct instructions.

Question 36

Practically, most offices send their GPCL lenses back to the lab for modifications. Why?

Answer 36

Practically, most offices send their GPCL lenses back to the lab for modifications, since the machinery and tools needed for GPCL modifications are costly and the job is time-consuming. Most of the newer GPCL materials and designs are very difficult to modify, so it is less risky to send it back to the lab.

Question 37

Possible solutions for 3-9 staining

Answer 37

When you see this problem, you can investigate if these solutions can help:

Thin the contact lens edge to allow the lid to cover and wet the eye better.

Another cause is the edge is too flat, so you need a heavier blend between the optic zone and the peripheral curve.

Reduce lens diameter and steepen the peripheral curve.

Flatten the peripheral curve slightly.

Question 38

Limbal keratitis

Answer 38

This is when you see inflammation and blood vessels at the corneal limbus. Symptoms include highly inflamed, engorged blood vessels at the limbal junction between the cornea edge and the conjunctiva.

Question 39

Possible solutions for limbal keratitis

Answer 39

Reduce the overall diameter and optic zone.

Flatten the peripheral curve.

Have the patient see the doctor for medications.

Question 40

It is important for you to know how to select a diagnostic lens when fitting a patient with gas permeable lenses.

In your own words, calculate the base curve(s) of a diagnostic lens to fit on the right eye of your patient who has a prescription of: OD: -2.00 -3.00 x 80 K’s : OD: 42.25@170/45.25 @080.

Answer 40

In your own words, calculate the base curve(s) of a diagnostic lens to fit on the right eye of your patient who has a prescription of: OD: -2.00 -3.00 x 80 K’s : OD: 42.25@170/45.25 @080.

Since this is a highly toric prescription, you will want to fit a bitoric gas permeable contact lens. To figure out the two curves of the lens you would start with:

The flatter of the two corneal curves is 42.25 diopters. This is the first base curve.

The second base curve is 3 ÷1.5 = 2, so 2 diopters away from 42.25 is 44.25.