Contact Lenses
The 7 Secrets Of RGP Success

These steps can take the toil and trouble out of fitting RGPs.

Janice M. Jurkus, O.D., M.B.A. Vera M. Howe, O.D. Chicago

Getting started is the most difficult part of any task. When you look at it as a whole, it seems overwhelming. If you look at it as several smaller steps, it's not so bad.

It's the same way with fitting RGP contact lenses. You select parameters and material. You consider the tear layer, edge lift, tear exchange, oxygen transmissibility, edge contour, secondary and peripheral curves, optical zone diameter, base curve, power and lens thickness.

When you look at these factors together, you might be tempted to put off RGP fitting and concentrate on soft lenses instead. This isn't always in your patient's best interest. If you've put off getting started with RGPs, or perhaps you just want to sharpen your fitting skills, break the task down into these seven smaller steps.

1. Pick Your Parameters
The first lens you place on the patient's eye will guide you to the final fit. Use a fitting philosophy you're comfortable with. You can find different approaches in manufacturers' guidelines or textbooks, or you can work with a lab consultant.

Our fitting philosophy incorporates concepts of sagitta, toricity and size in a rather cookbook fashion.1 We determine the parameters in this order:

• Diameter (mm)/optical zone (mm). We suggest sizes 9.0/7.8, 9.5/8.3 or 8.5/7. 3. We usually reserve the 8.5 size for smaller, steeper eyes. Most often, we recommend the 9.0/7.8 size, at least as a start.

• Base curve (mm). To determine the base curve for a 9mm lens, subtract 0.50D from the average or mid-keratometry reading. The base curve value generally provides an alignment fit for eyes with up to 2.25D of corneal cylinder. Suppose, for example, the patient has K readings of 42.50/44.00 @ 090. The average K reading (43.25) minus 0.50D yields a curvature of 42.75D. Divide this value into 337.5 (or use a conversion chart), and you get a base curve of 7.89mm.

If you select the 9.5/8.3 lens, subtract 1.00D from the mid-K to determine the alignment base curve. If you choose the 8.5mm lens, you'll need a steeper base curve to keep the same alignment. In this instance, use a base curve equal to the mid- or average K.

• Secondary curves (mm). The intermediate curve usually is 1mm flatter than the base curve; the peripheral curve is usually 3-5mm flatter than the base curve. We usually round these radius values to the nearest 10th. For the patient described above, the intermediate curve radius would be 8.90 (7.90 +1.00), and the peripheral curve would be about 10.90 (7.90 +3.00).

• Lens power (D). Generally you'll have a lens power equal to the sphere refraction at the corneal plane when the base curve equals "K," the flattest curve of the cornea.

In some instances you'll have to use the SAM/FAP (steep add minus/flat add plus) rule. Here's how it works. If you fit the lens steeper than K, you'll create a plus lens tear layer power. To compensate for this, you'll need to order more minus power. For example, if the patient's refraction is -3.25 and flat K measures 42.50 and you try on a 43.00D (7.85mm base curve) lens, you'll have +0.50 tear layer power. So, you need to order a lens power of -3.75D.

Suppose you fit the same patient with a lens that's 0.50D flatter than K using a base curve of 42.00D (8.04mm). This time, you'll form a minus tear layer, and will need to incorporate more plus power. So, you'll need to order a -2.75D lens.

If your trial set does not have the exact power you need, select a lens within 4.00D of the desired power. Add the over-refraction to the trial lens power to determine the final power to order.

2. Check the Lens
This step takes a few minutes, but can save you hours of frustration and work later on. Before you put the lens on the patient's eye, verify these variables:

• Power. Gently hold the lens against the lensometer aperture. Most RGP lenses are spherical. If you see fuzzy mires, the lens may be dirty or warped.

• Base curve radius. Traditionally you measure this with a radiuscope. Attachments are also available for your keratometer that allow you to measure the concave curvature with that instrument. Additionally, contact lens attachments for your auto-refractor accurately measure the base curve.2

• Diameter/optical zone. Use a 7x or 10x loupe to measure lens diameter and optical zone. You can also use a channel gauge to measure lens size.

• Edges. A lens with a poorly shaped edge will cause discomfort on the eye. A projection magnifier (shadowscope) is probably the best way to evaluate a lens edge. Or, turning your 7x or 10x loupe around so the contact lens is at the ocular end and will also allow you to view the edge.

Gently rub the lens edge between your thumb and index finger. A properly contoured edge should feel smooth and round.

3. Keep it Clean
 Properly clean and condition a rigid lens to improve comfort, wettability and vision. Lenses may have a hydrophobic haze or pitch on the surface. A laboratory cleaner or a rigid lens cleaning solution will remove the haze. Soak the lens in a conditioning solution or massage it with conditioner before insertion. This will optimize initial comfort.

4. Make it Feel Good
Most patients feel RGP lenses when you first place them on the eyes. To minimize this sensation and the reflex tearing, use one drop of a topical anesthetic in each eye. This allows you quickly to insert the lens and rapidly evaluate the lens fitting relationship.3

5. Check the Fit
 Three things you need to check in this step:

• Fluorescein tear layer pattern. Place a small amount of fluorescein on the superior bulbar conjunctiva. This causes minimal lid irritation and reflex tearing when the patient looks down. Evaluate the lens fit with a slit lamp with cobalt blue filter. Look for apical alignment of base curve over the pupil, bearing evenly distributed in the intermediate zone and clearance at the edge to allow an even tear exchange under the lens.

A well-fit spherical RGP lens should demonstrate a central alignment pattern and minimal mid-peripheral bearing with adequate edge lift in the 0.4-0.5mm periphery.4 Aspheric lenses are similar, but will generally demonstrate slight apical clearance, mid-peripheral bearing and about 0.3mm of edge lift.5

A lens that is too steep shows apical pooling and peripheral touch. This results in seal-off, causing poor tear exchange, possible lens adhesion and trapped bubbles leading to dimple veiling.

Apical touch results from a fit that's too flat. This region of excessive bearing results in central corneal distortion. A significantly flat fit also will show excessive edge lift that produces bubbles under the edge of the lens.

• Centration. The lens should center on the cornea and fully cover the pupil. A lens can decenter three ways. Superior decentration may be caused by a thick lens that the upper lid picks up, a displaced corneal apex or with-the-rule corneal astigmatism. Inferior displacement commonly occurs when a heavy lens falls to the bottom of the cornea or an excessive edge lift pushes the lens down during blink. A high-specific-gravity material and unusual corneal topography also cause inferior decentration. Lateral decentration often results from against-the-rule corneal astigmatism.

• Movement. A rigid lens should move smoothly—about 1-2mm with each blink—along the vertical meridian. If the lens moves too much or not at all, change it.

6. Be a Problem Solver
Here are some common problems you might run into when fitting RGP lenses, and what you can do to solve them:

• Lens fits too steep. The telltale sign is apical pooling with peripheral touch. Decrease the overall diameter and/or optical zone by at least 0.4mm, or flatten the base curve radius by at least 0.1mm. Another solution: Change the secondary and peripheral curve radii by at least 1mm and a width of at least 0.2mm. This will create a looser fitting lens.

• Lens fits too flat. Apical touch and excessive edge lift give this away. A lens with a larger diameter, optical zone and/or a steeper base curve will create a tighter fit.

• Superior decentration. This happens when the lens edge is too thick and the lens positions superiorly owing to lid interaction. If you thin the edge, the lens will center lower. A plus lenticular design or anterior, CN bevel gets the job done.

A lens may also decenter upward if it's too light. Order a thicker lens for increased lens mass. A steeper base curve may help lower a high-riding lens, too. To correct aspheric lenses that decenter superiorly, reduce the lens diameter, steepen the base curve or do both.

• Inferior decentration. Too much lens mass causes this. You can reduce lens mass by providing lenticular designs for higher plus and minus lenses, minimizing center thickness or choosing a lower-specific-gravity material. Looser fitting or lid-attachment designs also can elevate a lens. Reorder aspheric lenses that decenter inferiorly using a minus carrier lenticular design, a steeper base curve or a larger diameter.

• Lateral decentration. Rigid spherical lenses on an against-the-rule cornea may decenter laterally. To improve centration, try using a steeper base curve, a larger diameter or an aspheric design that will more closely mimic the contour of the cornea.

• Lens awareness. Initial lens comfort is one of the most influential factors in RGP adaptation. Poor edge contour or rough posterior lens junctions make the patient more aware of the lens on the eye. Here's how you can determine the cause: Gently hold the lids away from the lens while the patient looks straight ahead. If the lens sensation disappears, the lens edge is the cause. To fix this, thin, shape and polish the lens edges. If the lens sensation remains after that, a sharp posterior junction between curves is your culprit. A heavier blend will improve comfort.

Non-wetting lens surfaces and excessive edge lift also can cause lens awareness. In the case of the former, clean or polish the lenses. With the latter, steeper peripheral curves will reduce excessive axial edge lift.

• Unsatisfactory visual acuity. The lens power is based on an accurate spectacle refraction converted to the corneal plane and the SAM/FAP rule of tear layer power. Even if you've carefully determined the power, the patient may not be happy with his or her vision.

Check to see if the lens surface is wetting evenly. If it's not, clean the lens. If the patient reports seeing a shadow or halo, check for a decentered lens that prevents the patient from looking through the optical zone.

Lens flexure resulting from a lens that's too thin or too much lid pressure may cause subtle vision reduction. Keratometry measurements over the lens will show a toric reading if there's any flexure. If so, try a thicker (at least 0.04mm), or flatter (at least 0.50D) lens or switch to a stiffer material. Theoretically, a 0.5mm decrease in the optic zone might also reduce the flexure.

7. Know Your Polymers
 Each monomer in an RGP material is important for good oxygen permeability, stability and wettability. Silicone has excellent oxygen permeability.6 But it's a softer hydrophobic material that's predisposed to poor wetting and protein deposition. Like silicone, fluorine has excellent oxygen permeability.6 But it too is hydrophobic and prone to deposits, although less than silicone alone.7

Neither of these is suitable for a lens material without added stability components. Ethyl methacrylate, the most commonly used stability agent, adds rigidity, stability, optical clarity and machinability.8 Cross-linking agents in the polymer chains also enhance stability. Ethylene glycol dimethacrylate is a commonly used cross-linker.7

The material you select for an RGP lens can make or break the fit. Certain characteristics affect lens fit, patient comfort and adaptation.

• Oxygen. Oxygen permeability of lens materials is measured as a Dk value ("D" represents diffusion and "k" represents solubility). Oxygen diffuses through the spaces between polymers in a rigid lens. A lens with a higher Dk has more oxygen permeability than one with a lower Dk, but that doesn't necessarily allow more oxygen to get to the cornea.

Another key factor that determines oxygen transmission to the cornea is lens thickness. If two lenses are identical except for their thickness, the thinner lens will allow more oxygen to get to the cornea. Many doctors have a "workhorse" Dk material in the 25-50 range for minus lenses. Hyperopic prescriptions and extended wear require a higher-Dk (51-150) material.

• Wetting characteristics. You can overcome the hydrophobic nature of silicone and fluorine by adding wetting components that bind tears to the lens surface. Electrostatic interaction is the most common way to improve the tear-lens interface. A positively charged molecule is drawn to the negatively charged sites on the polymer. Agents such as methacrylate acid, N-vinyl pyrrolidone, polyvinyl alcohol and HEMA enhance the wetting characteristics of the lens.9

• Specific gravity. This, too, can influence the fit. If a lens decenters inferiorly, you can often correct this by simply changing the lens to a lighter material with a lower specific gravity.

So, after all this information, what lens material would you select for your patient? We most often use a fluorosilicone acrylate material. It combines the properties of silicone and fluorine with a methylmeth-acrylate to provide good oxygen permeability, optical clarity and lens stability.

If your patient is a moderate to high myope, a Dk of about 30 will provide the stability he or she needs for the thinner center thickness. A patient being refit from PMMA will prefer a lower-Dk lens because it's closer to what the patient is used to. Use a higher-Dk material (greater than 50) for hyperopic corrections that have greater center thickness or if a patient desires flexible wear.

You now have the information you need to select the initial RGP lens. Take things step by step, keep a positive attitude and use common sense. This will help you prescribe RGP lenses for your patients efficiently and effectively. Believe it or not, fitting RGPs won't seem like so much work.

Dr. Jurkus is a professor at Illinois College of Optometry. Dr. Howe is the Cornea Contact Lens resident there.

1. Schwartz CA. Specialty Contact Lenses: A Fitter's Guide. 1st ed. Philadelphia: W.B.Saunders Co., 1996:223-226.
2. Jurkus JM, Kelly SA. Automated and manual base curve assessment of rigid gas permeable contact lenses. ICLC 1996;23(4):138-141.
3. Bennett ES. RGP Fitting: How to Increase Comfort. Practical Optometry 1997;8(4):148-152.
4. Schnider CM. An option overlooked: make room in your practice for RGP extended wear. Rev Optom 1995;132(4):51-59.
5. Koetting RA. Are you ready to try aspherics again? Contact Lens Spect 1989;4(1):71-73.
6. Caroline PJ, Ellis EJ. Review of the mechanisms of oxygen transport through rigid gas-permeable lenses. Int Eyecare 1986;2(4):210-213.
7. Weinschenk JI. A look at the components in fluorosilicone-acrylates. Contact Lens Spect 1989;4(10):61-64.
8. Hom MM. Manual of contact lens fitting and prescribing. Boston: Butterworth-Heinemann, 1997:105-111.
9. White P. RGP material and immaterial cliches. Contact Lens Spect 1988;3(11):63-65.

 

| | | | | |