Soft lenses clearly dominate the contact lens world, but there are still patients who would benefit from GP lenses. While much of our clinical attention in the GP realm has moved to scleral and hybrid lenses, corneal GPs are still a great option for many patients.
Once you have decided to explore corneal GPs for your patients, how do you get started? In this article, I break down the fitting process into manageable steps to guide you through the process.
CHOOSING A GP LENS DESIGN
For simplicity, I categorize patients with normal corneas as either those who can wear a spherical base curve (BC) or those who need a toric BC. The easiest way to determine this is by looking at the amount of corneal toricity, using a manual or automated keratometer or a corneal topographer.
The greater the mismatch between the major corneal meridians, the worse the fit of a spherical BC will be. When an eye has higher amounts of toricity, a spherical lens will not align well with the cornea to provide an acceptable fit. Corneas with toricity of 2.00DC or less can usually be fitted with a spherical BC; those with 3.00DC or more will need a toric BC; and those between 2.00DC and 3.00DC are in a gray area. For those corneas, try a spherical BC first, and if the fit is unacceptable, attempt a toric BC instead.
Corneal GPs are well suited for with-the-rule (WTR) corneas (steeper along the vertical meridian). This shape allows unimpeded vertical movement of the lens when the patient blinks. Fitting against-the-rule (ATR) corneas often results in horizontal decentration, as the lens tends to follow the steeper corneal meridian.
Keep in mind that keratometry (K) readings represent only the central 3 mm or so. A topographer can refine the choice of lens type. Somewhat counter to what I stated above, a spherical BC may work well for some higher toricity corneas with apical patterns (Figure 1), and lower toricity corneas that are limbus-to-limbus may need a toric BC (Figure 2). The elevation difference at the 8-mm chord also can be used. If the elevation difference along the major meridians is more than about 60 microns, a toric BC should provide a better fit.
SPHERICAL GP LENSES
If you determine a spherical BC will fit well, you must consider how well the patient will see with a spherical lens. With a spherical lens, all astigmatic correction occurs in the lacrimal/tear lens, and it will equal the patient’s corneal astigmatism. If that matches the refractive astigmatism, a spherical lens will provide ideal vision correction. More mismatch means more residual astigmatism and poorer acuity.
Step 1: Determine Initial Lens Parameters
|CORNEAL CYLINDER (DC)||BASE CURVE|
|0 to 0.50||0.50D flatter than K|
|0.75 to 1.25||0.25D flatter than K|
|1.75 to 2.00||0.25D Steeper than K|
|2.25 to 2.75||0.50D Steeper than K
(consider toric BC)
- Overall diameter (OAD). Typically, the lens diameter is determined by a combination of corneal diameter and lid positioning. Start by measuring the horizontal visible iris diameter (HVID), and consider an OAD about 2 mm smaller than the HVID. For example, if the HVID is 11.8 mm, choose an initial lens with a 9.8 mm OAD.
Next, modify the OAD based on whether the lids promote lid-attachment or intrapalpebral positioning (Figure 3). An upper lid that covers the limbus promotes lid attachment, where the upper lid provides some support for the lens positioning. This generally enhances comfort. If the upper lid is near or above the limbus, the lens will likely drop more toward the center and be an intrapalpebral fit instead. Modify the diameter accordingly. A lid-attached lens should be consistent with the initial OAD calculation; an intrapalpebral lens should be about 0.4 mm smaller. If you wish to achieve a lid attachment fitting relationship and the lower lid covers the limbus, adjust the OAD down by about 0.4 mm.
The next two parameters assume you are using diagnostic lenses in your office prior to ordering lenses. You could also perform an empirical fit and call the laboratory to order lenses based on your OAD, K readings, and refraction. You can fine-tune the first lens order by performing a diagnostic fitting.
- Base curve radius (BCR). If you are using K values, use Table 1 to select your BCR. Note that as the corneal toricity increases, the BCR values become steeper to decrease the amount of lens rocking along the steep meridian.
Table 1 assumes an OAD of approximately 9.8 mm. For every 0.4 mm away from 9.8 mm OAD, adjust the BC to compensate. If you are using an OAD of 9.4 mm, for example, make the BC steeper by 0.25D/0.05 mm. If a larger OAD is used, go flatter instead. An intrapalpebral lens may also need to be somewhat steeper to aid centration.
- Power. A GP diagnostic kit likely has one power for each BCR, typically –3.00D. If you have extra lenses in plus powers and your final lens will be a plus power, use those. After finding the correct BCR by checking the lens fit (see below), do a spherical over-refraction (SOR) to determine the final lens power. Be sure to vertex-adjust the SOR first, then add it to the trial lens power. For example, if the trial lens is –3.00D and the SOR is –6.00D, at the corneal plane the SOR value would be –5.50D, and the final power is (–5.50) + (–3.00D) = –8.50D. If the visual acuity is not as expected with the SOR, perform a spherocylinder over-refraction (SCOR) to determine if residual astigmatism is causing the issue.
Step 2: Fit Analysis
A properly fit corneal GP lens should position and move well and have good alignment with the cornea.
|TO RAISE A LOW-RIDING LENS||TO LOWER A HIGH-RIDING LENS|
|Flatten base curve||Steepen base curve|
|Increase the diameter||Decrease the diameter|
|Flatten the peripheral curves||Steepen the peripheral curves|
|Use a lighter material||Use a heavier material|
|Decrease center thickness||Increase center thickness|
- Evaluate lens position and movement. A lens with good lid attachment will remain tucked under the upper lid during and between blinks. It will pick up with the lid after a blink, and move to a centered or slightly superior position relative to the pupil. It should stay well-centered horizontally. An intrapalpebral fit should drop to a central position after a blink.
- Evaluate lens-to-cornea relationship. Use sodium fluorescein dye (NaFl) and a cobalt blue light with the slit lamp for this evaluation. A yellow Wratten filter makes it easier to see the NaFl pattern. Use a small amount of NaFl at first; too much will flood the eye and alter the movement and positioning of the lens. It is easier to add more dye if needed than to wait for too much to dissipate.
A good fit should show a relatively even glow of green through most of the lens with a brighter band under the edge, approximately 0.5-mm wide. Figure 4 shows NaFl patterns that are aligned, flat, and steep. There is some tolerance as to what is considered an acceptable pattern; a lens that moves and positions well but is slightly flat or steep is acceptable. Some practitioners prefer a slightly steep pattern to bridge the center of the cornea.
- Evaluate lens movement. Lens movement can also be a clue as to whether the lens is too flat or too steep. A too-flat lens tends to drop from the lid in a circular pattern, skating along the limbus; it may also have excessive lid attachment. A steep lens will tend to have poorer lid attachment and drop straight down.
- Troubleshoot the fit. Table 2 shows recommendations for adjusting lenses to improve the fit. These are listed in the best order to try them.
- Choose the final power. After determining the correct BCR, perform a SOR to establish the final lens power. Be sure to vertex-adjust the SOR first, then add it to the trial lens power, as described above.
- Place the order. Advise your GP laboratory of the final OAD, BCR, and power. The lab will also want to know the lens set you used, the material to be used, and any modifications to the design, such as changes to the edge lift. If you are unsure about any of these, your lab consultant can provide valuable expertise to guide you. Photos and videos of lens fits are also helpful tools to provide to your lab consultant. This is easy to perform today with the many slit lamp iPhone adaptors commercially available.
- Dispense the lenses. Confirm the lens fit, check vision, and perform a SOR to confirm the lens power is correct. If all is well, dispense the lenses and have the patient return for a progress check. If needed, you can consult with the laboratory on how to improve the lens fit.
If the cornea can be fit with a spherical BCR, but there is excessive residual astigmatism with spherical optics, you can try a front-surface toric. The extra cylinder correction needed is ground onto the front of the lens. This design is prism-ballasted to put the lens axis at the correct location. The laboratory can help you choose the amount of prism and determine the lens power needed based on your SCOR of a spherical lens. With the exception of scleral lenses, these lenses are less frequently used because of difficulty with axis stabilization, and prism ballasted lenses do not lid attach well, possibly decreasing comfort. A soft toric may be a better choice in a case like this.
BITORICS AND BACK-SURFACE TORICS
Both of these designs have toric BCs, enabling you to independently fit each meridian on higher-toricity corneas. Although these designs may seem intimidating at first, it is actually easier to achieve a good fit with them, and they are only a little more challenging than spherical lenses. In fact, the primary difference pertains to having two tear lens calculations — typically the horizontal meridian and the vertical meridian — as opposed to one with spherical lenses, but these values can be determined empirically with online calculators (See “Empirically Design Your Toric Lenses” on page 38). Figure 5 shows the fit of spherical and toric BCs on a high-toricity cornea. Residual astigmatism is less of an issue as well, as you choose the needed powers in each meridian.
Compared with soft toric lenses, these lenses typically provide better vision for patients with higher astigmatism (typically greater than 2.00D of corneal cylinder). Lens rotation has less impact on visual acuity, making them a better optical choice.
Bitoric Lens Designs
Most of the time, a bitoric is the design that is needed. Bitoric designs have toricity on both sides of the lens for reasons I will discuss later. Fitting a bitoric lens follows all of the steps previously covered in terms of OAD, lid attachment, and so on. The significant difference is that you are selecting two BC radii instead of one. The method described here is one that has been successful for me, although other methods have been published. This method is successful for ATR corneas, too.
- BCR. There are two fitting strategies with toric BCs: saddle fit and low toric simulation (LTS). Saddle fit means each meridian is fit with the same alignment; LTS means the vertical meridian is fit somewhat flatter than the horizontal to allow for unimpeded vertical movement. This simulates how a spherical lens aligns on a low-toricity WTR cornea (hence the name). Use LTS when the corneal and refractive cylinder are within approximately 1.00DC of each other. Use a saddle fit when the difference is greater.
To select the BCR, first determine the BCR in the more horizontal meridian. For larger OADs (in the 9.8-mm range), make the BCR equal to the flatter K reading. Next, choose the vertical meridian depending on whether you will use the saddle or the LTS fitting strategy. If you choose saddle, fit the vertical meridian with a BCR equal to the steeper K reading; if LTS, fit it about 1.00D flatter than the steeper K reading. For smaller diameters, adjust each meridian 0.25D/0.05 mm steeper.
For example, if using LTS and the Ks are 44.00/47.00 @ 090 and a larger lens, the horizontal BCR is 44.00D and the vertical BCR is 46.00D. If LTS and a smaller lens, the BC radii are 44.25 (H)/46.25 (V).
- Power. Once the BC radii are known, the power in each meridian is chosen to properly correct each one. Figure 6 uses optical crosses to show the powers needed based on the lacrimal/tear lens powers in each meridian. Note that in Figure 6, the lens specifications for BC radii and powers are written as “drum” readings rather than spherocylinder values.
Back-surface Toric (BST)
These lenses have toricity only on the back. Why? It is the result of an optical quirk. It would seem logical that a lens that has toricity only on the back has cylinder power equal to the BC toricity. For example, if the BCs are labeled as 43.00/47.00, that lens should have 4.00DC. Wrong! These numbers are not the true powers of the back surface. They are the powers of the front of the lacrimal/tear lens! When we calculate the true surfaces using the index of the lens and not the tears, the powers are closer to 64 and 70, a difference of 6.00DC! The lens has 1.5 times the cylinder we think we should get. This is why bitorics are used. Cylinder is placed on the front to neutralize the “extra” cylinder we do not want.
When do you use a BST then? Optically, it works when the refractive cylinder is approximately 1.5 times the corneal cylinder. If you are using a toric BCR and have this relationship, the lens will be a BST. Practically, this makes little difference to clinicians, but it helps to understand the difference. I recommend you follow the same principles covered for bitorics and determine the BCR and powers for each meridian as before and relate those numbers to the laboratory. A BST should be performed as a saddle fit.
EMPIRICALLY DESIGN YOUR TORIC LENSES
Several calculators are available if you want to design toric GP lenses empirically. The most well-known calculator is the Mandell-Moore Guide. In addition, the GP Lens Institute Toric and Spherical Lens Calculator provides graphics to view the tear film powers and calculations. This guide also offers design recommendations in addition to BC radii and powers. It also recommends a back-surface toric when indicated and describes how to empirically design a spherical lens.
For a comprehensive guide to empirical bitoric design, the Newman GP Toric Guide is a good resource. All three of these empirical guides are available at the GP Lens Institute website: www.gpli.info . Finally, EyeDock has a useful bitoric calculator on its website: www.eyedock.com .
For both bitoric and BST designs, look for the same fitting relationships as you did before: good positioning and appropriate lid interaction, an even distribution of NaFl under the lens, and appropriate edge clearance. If these are not ideal, discuss necessary changes with your laboratory consultant. If the lens was designed correctly, the cylinder correction provided to the patient will be correct and a SOR should be all that is needed. If acuity is not as expected, a SCOR can help the laboratory refine the powers.
Corneal GPs are still a great option for patients with normal corneas. Hopefully, this refresher course will inspire you to consider this option more frequently. Great vision for your patients awaits! CLS