Back to Basics: Soft Lenses for Astigmatism
SOFT TORICS BASICS
Back to Basics: Soft Lenses for Astigmatism
With better lens designs and reduced chair time, it's a good time to revisit soft torics if you're not fitting them.
By John Mark Jackson, OD, MS, FAAO
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Dr. Jackson is an associate professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at jjackson@sco.edu. |
Astigmatism is a mystery to most of our patients. It certainly should be clear to us as clinicians how it impacts their vision. Correcting astigmatism is a critical step in helping many patients achieve their best possible vision.
In the last decade, soft (toric) lenses for astigmatism have improved by leaps and bounds. The designs are more stable, more comfortable, and have better optics. Silicone hydrogel (SiHy) materials and daily disposable options have made them a healthier choice for patients as well.
Yet I still have patients who recently were told that they can't wear soft lenses because they have astigmatism. I have met clinicians who are reluctant to fit them because they believe that these lenses take too much chair time or are too expensive. This article is a refresher on how to fit soft torics and how to manage fitting issues. I hope it will shed some light on a few confusing points and encourage you to fit more of these great lenses.
Basic Lens Mechanics
To understand how soft toric lenses do their job, it's important to compare them to the alternative: GP contact lenses.
GP lenses have an important advantage over soft lenses in that the tear layer between the lens and cornea (lacrimal lens) has astigmatic power. Spherical GP lenses can correct astigmatism because the rigid material keeps its shape on the eye, and the lacrimal lens provides the same amount of correction as the amount of corneal astigmatism. For example, if the K values are 42.00/43.00 @ 090, and a spherical GP lens is applied to the eye, the lacrimal lens will provide –1.00 x 180 of astigmatism correction. As a bonus, no matter how the lens rotates on the eye, it provides the same amount of correction.
Soft torics are at a disadvantage here because the soft lens conforms its shape to that of the cornea, effectively eliminating any contribution from lacrimal lens power. Therefore, all of the astigmatic correction has to be in the lens itself. A spherical soft lens cannot correct astigmatism at all (although a small amount of “masking” may occur of about 0.25DC, it is not reliably useful). Soft toric lenses have cylinder power built into the lens to correct astigmatism. Unlike spherical GP lenses, if a soft toric lens rotates on the eye, the astigmatism correction does change. As the lens rotates, the lens' cylinder axis shifts away from the desired location. This effect is exactly the same as having a spectacle lens made with an incorrect axis—the patient's acuity will decline the farther the lens is from the needed axis.
Soft toric lenses can do a fine job of correcting astigmatism as long as this limitation is understood and accounted for. Your number-one job then is to choose a lens that minimizes any rotation on-eye to keep the axis where it's needed.
Lens manufacturers have devised a variety of ways to prevent lens rotation. The primary method has been prism ballasting. Prism ballasting makes the bottom of the lens thicker compared to the top. The idea is that both gravity and pressure from the upper lid squeeze the thicker portion of the lens out from under the upper lid, and the lid pressure helps keep it in place.
Newer designs modify this idea in several ways (Figure 1). The central idea remains the same: thicker areas of the lens are held between the lids to inhibit lens rotation.
Figure 1. Some newer designs (Air Optix, Alcon; Acuvue Oasys, Vistakon) modify the regions of greater thickness (shown in red) compared to traditional prism ballasting.
The newer designs in SiHys also help with oxygen transmissibility. Fewer thick areas increases the average Dk/t (transmissibility) of a lens. Previous lens designs that were thick at the bottom could lead to neovascularization at the lower corneal limbus. Of course, SiHy materials themselves have been beneficial in preventing neovascularization as well.
Another factor in preventing lens rotation is the diameter. In general, larger diameters are less likely to rotate compared to smaller diameters. Most brands are made with 14.2mm to 14.5mm diameters, with some brands going up to 15.0mm diameters.
Note that all soft torics will rotate slightly when the eye blinks. The forces on the lens as the lids close create torque, causing the lens to rotate slightly. This is not generally a problem as long as it is only a few degrees and the lens moves back to its original position after the blink is completed.
Fitting: Initial Steps
The first step in selecting a lens power is to vertex-adjust the prescription for powers that are 4.00D or higher. For example, if the spectacle prescription is –7.00 –2.00 x 180, both meridians need to be adjusted, and for a 12mm spectacle vertex distance, the power would be –6.50 –1.50 x 180 at the corneal plane. If the spectacle prescription is +7.00 –2.00 x 180, the power would be +7.50 –2.25 x 180.
Note that the myopic correction has less cylinder power at the cornea, while the hyperopic correction has more cylinder power at the cornea. You can look up vertex changes in tables or use one of many ophthalmic calculators that can complete this adjustment for you.
The second step is to choose a lens based on your vertex-adjusted prescription. Lenses are typically available in limited parameters for cylinder power and axis. For example, many “stock” lenses (such as monthly and two-week lenses from the major manufacturers) come in cylinder powers of –0.75DC, –1.25DC, –1.75DC, and –2.25DC. Axes are usually available around-the-clock in 10-degree steps, depending on the design. Choose the closest available lower cylinder power and the closest available axis from your trial lens inventory and apply the lens.
For patients needing higher cylinder powers, you may need to use a custom lens. Many of the independent contact lens labs also make custom soft toric lenses, even in SiHy materials. These are often available with any cylinder power and axis steps in 1-degree increments.
After allowing a lens to settle on the eye for a few minutes, check the patient's visual acuity and assess the fit of the lens. Unlike toric lenses of the past, today's lenses equilibrate fairly rapidly. The lens should have aligned however it is going to within five to 10 minutes. Ideally, the lens will have minimal rotation and the reference marking will be at the 6 o'clock position (Figure 2), depending on the design. If so, the patient's visual acuity should be comparable to his best acuity with spectacles. If it isn't, or the lens has rotated, you need to perform some troubleshooting.
| Soft Toric Clinical Pearls |
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• Don't settle for 20/foggy. Try to improve to 20/crispy.
• LARS is Left Add, Right Subtract to adjust the lens axis for rotation.
• Always take LARS from the spectacle axis, not the trial lens.
• Don't try to “fix” the rotation; fix the axis misalignment caused by rotation.
• For fluctuating acuity, consider a different stabilization design.
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Figure 2. (top image) A soft toric contact lens exhibiting no rotation, and (bottom) a soft toric contact lens rotating 15 degrees to the right.
Fortunately, most designs these days are very good at preventing lens rotation, and the initial lens should work quite well. If the lens needs to be adjusted to improve vision, it often only takes one more lens to achieve a successful fit, which then reduces overall chair time.
Troubleshooting Lens Rotation
Of course, soft toric lenses don't work perfectly 100 percent of the time. The most common issue with soft torics is lens rotation. Due to individual differences in lid anatomy and tension, the lenses will align differently from patient to patient. Your goal is for the lens to be stable in its rotation—to have the same alignment orientation every time the patient wears the lens. As mentioned, the lens will rotate slightly between blinks. That is fine as long as it goes back to its original resting position after the blink.
Your goal is to account for any resting-state lens rotation by adjusting the lens axis. Suppose the patient's spectacle axis is 10 degrees and you apply a soft toric contact lens with axis 010. If the lens has no rotation, this lens axis should work fine. But if it rotates and remains stable, there will be axis misalignment. This means the patient is no longer looking through the correct axis. For this example, if the lens rotates 20 degrees counter-clockwise or CCW (the mark moves 20 degrees to your right), the patient will end up looking through axis 030 instead of axis 010. Thus, the lens rotation resulted in 20 degrees of axis misalignment.
Note that rotation and axis misalignment are not the same thing. If I had instead applied a soft toric lens with axis 170 and it rotated 20 degrees CCW, the patient would end up looking through axis 010! So for our patient who needs to see through axis 010, 20 degrees of rotation means the lens has no axis misalignment! In fact, that's exactly how lens rotation is dealt with clinically: the lens axis is adjusted so that there is no misalignment after lens rotation.
The usual way to remember how to adjust the lens axis is the mnemonic LARS: Left Add, Right Subtract. If the lens rotates 10 degrees to your left (clockwise), then add 10 degrees to the patient's spectacle axis and vice versa for rotation to the right. An important and often misunderstood point is that after you adjust the axis with LARS and apply a new contact lens with that axis, the lens must rotate the same amount and direction as before. Thus, the scribe marks on the lens should be in the exact same position as those of the first diagnostic lens. The whole point of using LARS is to eliminate the axis misalignment resulting from soft toric lens rotation—not to eliminate the rotation.
How much axis misalignment is clinically meaningful? At what point should you use LARS? It depends on the situation. The more cylinder power the lens has, the more that axis misalignment will degrade acuity. For example, five degrees of misalignment in a –0.75DC lens will have less impact on acuity compared to a lens with –2.25DC. Axis misalignment causes a cross-cylinder effect, resulting in residual astigmatism for the patient. Table 1 shows how the amount of residual astigmatism changes with different amounts of lens cylinder power.
| TABLE 1 Effects of Different Amounts of Lens Cylinder Power on Residual Astigmatism* |
| Lens Cylinder Power |
10° of Axis Misalignment gives residual astigmatism of…. |
| –0.75 DC |
–0.25 DC |
| –1.25 DC |
–0.42 DC |
| –1.75 DC |
–0.58 DC |
| –2.25 DC |
–0.75 DC |
| –2.75 DC |
–0.92 DC |
| * The amount of residual astigmatism induced by axis misalignment varies with the cylinder power of the lens. |
Even in low cylinder powers, it's important to adjust for axis misalignment if possible. Do not accept “20/foggy” if you can change it to “20/crispy.” Patients usually appreciate the difference even if they can't read more letters on the chart.
Here are a few examples of using LARS to adjust for axis misalignment. Patient 1 has a spectacle prescription of +2.00 –1.25 x 150. Having that same power and axis in one of my soft toric trials, I applied this lens and found that the resulting visual acuity was 20/30. By rotating the beam on the slit lamp and using the built-in protractor, I measured the lens rotation as 20 degrees left and stable at that rotation. Using LARS, I added 20 degrees to the spectacle axis and changed it to axis 170. I applied a second lens with a power of +2.00 –1.25 x 170, and the visual acuity improved to 20/20+2. Note that I should still obtain 20-degree left rotation when I look at the lens—LARS doesn't “fix” the rotation, but compensates for it so there is no axis misalignment.
Patient 2 has a spectacle prescription of –1.00 –1.50 x 045. The closest soft toric diagnostic lens I had available was –1.00 –1.25 x 050. The axis is only five degrees off, which should be acceptable in this fairly low cylinder power. Without knowing how the lens might rotate on the eye, that's as close as I could come. The patient's visual acuity with this lens was 20/20-2, although he exhibited a tendency to slowly call out the letters. The lens was rotating five degrees to the right (CCW). This didn't appear like much rotation, and the lens I was using is available only in 10-degree steps anyway, so the temptation would be to leave everything alone.
However, this rotation means that the patient is looking through axis 055—10 degrees off-axis from what he needs. With this much cylinder power, you would not accept a pair of glasses being off by 10 degrees, and you shouldn't with soft torics either, if it can be fixed. Using LARS, I subtracted the five-degree rotation from the spectacle axis of 045 to obtain a new lens axis of 040. Assuming the new lens rotates the same five degrees to the right, the patient will end up looking through axis 045 and have no axis misalignment. This should improve his visual acuity to a stronger 20/20.
Lens Fit and Rotational Stability
Adjusting the axis to reduce misalignment only works if the lens rotation is stable. If it isn't consistent, then visual acuity will always fluctuate as the axis shifts around every time the patient blinks.
A loosely fitting lens will tend to exhibit large amounts of rotation with each blink. Normally, lenses will rotate about five degrees with each blink, but a loose lens will move more than that. It will still tend to drift back to its original position if the patient doesn't blink, however. In this case, you should change to a steeper base curve or larger diameter if these options are available.
A tightly fitting lens will tend to not move much between blinks, but won't always stabilize to the same position each time it is worn. If you manually rotate the lens using the lids, it will tend to stay at that new position. You would need to change to a flatter base curve or a smaller diameter if available.
Many brands of toric lenses now come in only one base curve and diameter. If the lens is inconsistently rotating, the only choice would be to switch brands. It might help to switch to a lens with a different type of mechanism for preventing rotation. Check the manufacturer's promotional materials if you are unsure of the lens design.
Using Over-Refraction Data
In addition to using LARS, a spherocylindrical over-refraction (SCOR) can help to sort out how a toric lens is performing. Ideally, with the proper lens power and axis, the over-refraction would be plano. If there is axis misalignment, then the SCOR provides clues as to how to fix the lens. LARS is sufficient most of the time, but if you are unsure of the rotation, or using LARS didn't fix the problem, the SCOR may provide a more accurate result.
It is most likely easiest to use a contact lens over-refraction calculator to adjust the prescription using the SCOR. Several of these were recently evaluated in the March 2012 Contact Lens Practice Pearls column. To use the calculators, you input the spectacle Rx, contact lens power, and the SCOR, and it outputs the correct lens power to order.
It may help to understand the basic principles of SCORs. In general, the more the axis is misaligned, the farther the SCOR will be from plano. Axis misalignment results in crossed-cylinder effects, which can be challenging to decipher. The easiest way to analyze them is by looking at the amount of cylinder in the SCOR. Assuming the lens power itself is correct, the amount of cylinder tells you the amount of axis misalignment. It works out that for every 10 degrees of misalignment, the SCOR will have one-third of the lens cylinder power. For example, if the lens has a power of –2.00 –1.50 x 180 and the axis misaligned on-eye by 10 degrees, the SCOR will have a cylinder power of one-third of the –1.50 or –0.50DC. If it misaligned by 20 degrees, the SCOR will have a cylinder power of two-thirds the original or –1.00DC. The spherical equivalent of the SCOR will be plano when the power is correct, so for the 20-degree example, the SCOR will be +0.50 –1.00 x ___. The SCOR axis is the most challenging part to understand. It will be oblique to the original axis, however. For complete details on the axis, see the June 2007 Prescribing for Astigmatism column.
Alternative Options
Some patients don't get satisfactory vision with soft torics. Even with the best-fitting lenses, they are bothered by inconsistent rotation or even the small rotations between blinks. This is especially true as the amount of astigmatism increases, as even small amounts of axis misalignment can degrade acuity. Other patients may have dryness symptoms with soft lenses, preventing comfortable lens wear.
Options for these patients include spherical GPs, bitoric GPs, hybrid lenses, and mini-scleral designs. All of these are less influenced by lens rotation, and GP lens wearers may avoid some of the dryness associated with soft lenses. The hybrid lenses and mini-sclerals are great options for those who can't adapt to or don't want to try standard GP designs. Keep these options open for those challenging patients.
In Summary
Soft lenses for astigmatism have made great strides over the last decade. Improvements in lens designs, especially changes in the way the lenses stabilize on-eye, have made what used to be a challenging lens modality much less frustrating and time consuming. Expansion of lens parameters and increased brands to choose from have made it easier to get good results with more challenging prescriptions. Take that extra step and bring some extra clarity to your soft lens wearers who have astigmatism. CLS
Contact Lens Spectrum, Volume: 27 , Issue: June 2012, page(s): 28 - 32