TORICS FOR ASTIGMATISM
Toric Lenses for Today and Tomorrow
There are toric lenses that will work for most of your patients who have astigmatism.
BY JOHN MARK JACKSON, OD, MS, FAAO
Quality correction of patients’ astigmatism is one of the most important roles we play in vision care. Correction of astigmatism with soft contact lenses is a vital component of this.
I am still surprised at how many patients say that they were told they could not wear contact lenses because of their astigmatism. I hear this from patients who have both low and high amounts of cylinder correction. Truly, for most astigmats, there is a lens option that will work for them. A review of toric lenses appears to be in order.
Torics for Today
Perhaps some of our colleagues who are reluctant to fit soft torics remember the “bad old days.” Early soft toric lenses were often referred to as “snowflake” lenses because no two were exactly alike. Due to inconsistent lathing or other manufacturing processes, it was difficult to get reliable results with early soft torics. Fortunately, those days are long gone. Today’s torics use sophisticated designs and extremely consistent and accurate manufacturing to ensure reliable results.
Do patients who have lower amounts of astigmatism really benefit from toric lens correction? In one study, Dabkowski et al (1992) found that astigmats who have as low as 0.75DC showed improved Snellen acuity and contrast sensitivity with soft torics compared to their habitual sphere correction. Snellen acuity, on average, improved from approximately 20/30 to 20/20 in low-to-moderate myopes who have low astigmatism. Patient questionnaires showed that 71% of the patients preferred toric correction. We have known for quite a while that toric correction really does benefit low astigmats.
More recent research confirms that today’s torics really do perform well for our patients in other ways. Cox et al (2015) tested driving performance in an immersive simulated environment with soft torics and soft spheres. They found that correcting the astigmatism “resulted in significantly safer tactical driving performance” compared to no correction, while correcting just the spherical error did not.
What are the prescribing characteristics for soft torics? One study looking at prescribing habits in seven countries found that torics accounted for approximately 30% of all soft lens fits in 2009 (Efron et al, 2011). This represented an increase of approximately 10% over the previous decade, indicating additional use of torics. However, their data also showed that patients who have 0.75DC were more often fit with spherical lenses.
Data showed a slight increase in toric lens prescribing, but still a discouragingly low usage rate for lower astigmats (Morgan et al, 2013). As clinicians, I believe we can do better for our patients by increasing the use of torics.
One issue that can make fitting toric lenses challenging is accuracy and stability of the lens axis. Older toric lenses also had significant issues with lens rotation. It was not unusual to see lenses with 20° to 30° of rotation and inconsistent rotation as well.
Fortunately, changes in how lenses are designed have made big improvements in lens performance. Newer lens designs have changed the mechanisms for orientation control, so lenses are thinner and more comfortable, yet still provide stable positioning on-eye, with less rotation than previous generations.
One study compared several modern brands of soft torics in terms of their rotational stability and how quickly they would reorient to their “resting” position when manually rotated 45° away. They found that most prism-ballasted designs returned to within 5° of resting position within about 30 blinks, and they returned to resting position within about 60 blinks (Young et al, 2009). Momeni-Moghaddam et al (2014) also found that lenses return to resting position quickly, most within two to three minutes. This is good news for clinicians, as it means today’s torics settle quickly, allowing for less chair time.
Also good news for clinicians: soft toric lenses are available in wider parameters and replacement schedules than ever before, even in mass-produced lenses. No matter what type of lens you want for your patient—daily to monthly replacement, silicone hydrogel, or standard hydrogel materials—there is an option for you to try.
Soft Toric Fitting
Let’s review the fitting of soft toric lenses. The fitting process should begin with determining whether a patient is an appropriate toric candidate. This is mostly based on how much refractive astigmatism the patient has. Remember, all of the patient’s refractive cylinder is in the contact lens itself, not the tear film, as it would be with a GP lens. You do not need to worry about the amount of corneal astigmatism or whether there is a difference between the refractive and corneal cylinder—just the amount of refractive astigmatism.
Soft torics start with a minimum of 0.75DC, with most mass-produced lenses having up to 2.25DC. Two things to keep in mind here: some patients can tolerate uncorrected astigmatism better than others can, so low astigmats (1.00DC or less) may have acceptable acuity with a spherical lens, depending on their visual demands. More likely, patients who have 0.75DC or more are better off in a toric, as noted previously. Also, keep in mind that as the cylinder power gets higher, axis stability becomes more crucial to prevent induced cross-cylinder effects and blur.
As far as lens parameters go, many mass-produced lenses are available in only one base curve (BC) and overall diameter (OAD), typically in the range of 8.7mm BC/14.5mm OAD. The diameter is typically larger compared to a sphere in the same brand because the larger diameter helps to stabilize the lens and limit rotation.
With soft lens fitting, the overall sagittal depth of the cornea is more important compared to the K reading alone. A larger- or smaller-than-average corneal diameter may have a sag depth that is not appropriate for a mass-produced lens. In these cases, a custom lens may provide a better fit and comfort.
Power selection is straightforward. Simply vertex-correct the refractive error to the corneal plane, and select the closest available lens in your diagnostic set. Cylinder powers in most mass-produced lenses are typically available in 0.75DC, 1.25DC, 1.75DC, and 2.25DC, although some are available in 2.75DC.
Most lenses are available with axes in 10° steps around the clock, but some brands are more restricted. Often the kits come in 0.50D steps for the sphere powers to save room in the lens inventory, but they can be ordered in 0.25D steps. This can lead to some confusion about choosing the first lens powers.
As an example, let’s say the vertexed refraction is –4.25 –2.00 x 055. The brand we want to use is as described above, so we can’t obtain this exact prescription from the kit. For the cylinder power, round down to −1.75DC. For the axis, we would have to choose from 050 and 060. Since we haven’t seen a lens on the eye yet, we really don’t know whether the lens will rotate or not, so we could really pick either one to start. With experience, you may find that one brand typically rotates a little nasal, while another tends to rotate a little temporal, and you can use this to help you select your first lens; note: we will discuss compensation for lens rotation later in this article.
For the sphere power, I would likely go slightly more minus, especially since we decreased the cylinder. This will keep the spherical equivalent closer to the original refraction.
While there is a valid concern about over-minusing and accommodation, going the other direction—over-plussing—is going to create distance blur. Remember, most powers under 6.00D are available in 0.25D steps in ordered lenses, so the over-minusing will be temporary. So, for our initial lens selection, we could go with –4.50 –1.75 x 050 or 060.
Allow the lenses to settle for no more than about five minutes for most brands. As noted earlier, today’s torics are remarkably quick to find their position without the excessive wait times of the past (Momeni-Moghaddam et al, 2014).
Should you check acuity first, or lens fit first? I prefer to check lens fit first because a poorly fitting or excessively rotating lens will diminish acuity. Taking time for patients to stumble through the Snellen letters with an off-axis lens doesn’t provide useful information. If the lens fits well (good centration, coverage, movement, and minimal rotation), I would then check acuity, as the lens should provide good vision.
Today’s torics are light years ahead of old designs in terms of stability. I am still surprised at how often modern lenses will have minimal to no rotation. However, some patients’ lid anatomy and lid tightness may induce some lens rotation, so it’s important to understand how to compensate for this. Really, this is the only challenging part of soft toric fitting, and it’s not terribly difficult.
Lens Rotation and Misalignment
Rotation refers to twisting of the lens on-eye. You measure the amount of rotation (if any) by narrowing the slit lamp beam to an optic section and rotating the beam until it aligns with the orientation mark on the lens. Use the protractor scale on most slit lamps to determine the number of degrees of rotation. Note whether the mark has moved to your left (clockwise) or right (counter-clockwise). Figure 1 shows a soft toric lens with a small amount of left rotation, while Figure 2 shows a lens with 15° of right rotation.
Figure 1. A soft toric lens with a small amount of left rotation.
Figure 2. A soft toric lens with 15° of right rotation.
The problem with lens rotation is that it can cause axis misalignment, which is simply the axis of the lens not lining up with the patient’s needed axis. The rotation itself is not the problem; it is the axis misalignment that causes decreased acuity. To prevent axis misalignment, change the axis of the lens to account for the lens rotation using the familiar acronym LARS: Left Add, Right Subtract.
For example, let’s say that the patient’s refractive axis is 140, and we diagnostically fit a lens with axis 140. We look at the lens and note it rotates 10° to the right (counter-clockwise). Due to the lens rotation, the patient is not looking through axis 140. Rather, he or she is looking through axis 150 (Figure 3A and 3B). To compensate for the rotation, we use LARS and subtract 10° from the refractive axis. Our next lens should have axis 130.
Figure 3. Lens #1 with axis 140 (A) results in 10° of misalignment after right rotation (B). Lens #2 with axis 130 (C) has no misalignment after the same right rotation (D).
When we try on the next lens, it should still rotate 10° degrees right. If it does, the patient is looking through axis 140 and the problem is solved (Figure 3C and 3D). Most importantly, note that in this example, after compensation, the lens is still rotated but has no axis misalignment. Understanding that subsequent lenses must still rotate the same amount is key. We aren’t trying to eliminate the rotation itself, we are only trying to compensate for it and eliminate axis misalignment.
How much does axis misalignment impact visual acuity? There are two things to consider here: the amount of cylinder correction and the amount of misalignment. It should be apparent that patients who have higher cylinder need more precision in their axis location compared to those who have lower cylinder. The more axis misalignment we have, the worse acuity is going to be.
Practically, it works out that for every 5° of axis misalignment, one-sixth of the cylinder remains uncorrected (i.e., residual astigmatism) due to cross-cylinder effects. For example, if the lens has a cylinder power of –1.25, and the lens misaligns 10°, one-third of the cylinder power will be present in the over-refraction, or approximately 0.50DC remaining uncorrected. That doesn’t sound so bad, but what if the lens has –2.75DC? That same 10° of misalignment would yield approximately 1.00DC remaining uncorrected. Table 1 shows the effect for various amounts of cylinder powers and axis misalignments.
One tricky scenario occurs when the patient has an axis that is on a 5° point…such as axis 095 or 135. Because most lenses are available in 10° steps, if the lens has no rotation, there is already 5º of misalignment. It may be tempting to look at a lens and see 5° of rotation and not be concerned about it. Assume that we used axis 140 for our patient who needs axis 135. If rotation is 5° left, then there is zero axis misalignment, but if it rotates 5° right, then there is 10° of axis misalignment and a good case for using LARS to compensate.
How much that uncorrected cylinder affects actual acuity will depend on the patients, as they will vary in their sensitivity to blur. One way to see how sensitive they are to blur is to try a “twist-test” when doing their refraction. When you reach the end of your axis adjustment, ask the patient to report to you when they first notice blur, then slowly rotate the axis knob. See how many degrees it takes before they notice the blur.
Again, higher astigmats will be more sensitive to any axis change, but this test can give you an idea of how much they will tolerate axis instability in their soft torics. While you should strive to eliminate any axis misalignment, there will always be a little instability, and this test can help you predict which patients may have more difficulty with soft torics.
Other Toric Options
While soft toric lenses are a great option for astigmats, don’t forget that GP lens options are plentiful, too. Depending on the situation, spherical GP optics can provide great correction (due to the lacrimal lens astigmatic power), and we aren’t limited to corneal lenses for this; hybrid lenses and scleral lenses are also great options to consider, especially for patients who are not satisfied with the stability of their vision with soft torics. Because the cylinder correction is in the tear film, not the lens, rotation of these lenses has no impact on visual acuity.
Toric GP lenses are also available when spherical optics are not the best choice. Bitorics (with a toric base curve) are primarily used when the cornea is too astigmatic for a spherical lens to fit well. When a spherical base curve fits well, but there is excessive residual astigmatism with spherical optics, a front-surface toric optic can be used. I typically reserve this option for sclerals due to the rotational instability of corneal designs.
Be sure to note the difference between a scleral with a toric scleral landing zone (to better fit the sclera) and toric optics (to provide better vision). Because scleral topography is typically non-spherical, a toric scleral zone can often provide better alignment to the sclera and prevent sectorial blanching of conjunctival vessels. As a side benefit, these lenses also have very stable orientation on the eye. That means that if you also need toric optics, the astigmatic axis is very stable—a win-win for the patient!
Torics for Tomorrow
Today’s torics are quite amazing compared to previous generations, but there are still some things that I think we could use to make them even better for our patients. While lens designers have incorporated some great changes in soft lens ballasting in the last few years, making lenses more stable than ever, there is still room for improvement.
I look forward to a day when all soft toric lenses have zero rotation on-eye. While it is not difficult to compensate, it still takes chair time and multiple lenses to refine the fit. I suspect that this extra chair time is one thing that makes some clinicians hesitate to fit toric lenses.
Lens comfort with soft torics has also improved greatly with the newer generation of lenses that offer thinner ballast designs, but there is still room for improvement here as well. This is especially true for patients who wear one sphere and one toric. They can often notice a difference in comfort between the two designs. Given that the torics must incorporate some form of ballasting, there may always be a difference, but I hope lens designers keep working on this.
I would also like to see more parameters available. We used to have at least a few base curve radii to choose from, but many soft designs are only available in one base curve and diameter now. While these have been chosen to fit a wide range of eyes, I am surprised at how often I see patients who need a custom lens due to large or small horizontal visible iris diameter. Having a second base curve or second diameter without having to use a custom lens would be helpful.
Many patients who have astigmatism would like to wear cosmetic colors, but as of this writing, there are no mass-produced colored soft torics. It would be great to have this option available again.
There really has never been a better time to be a contact lens wearer who has astigmatism. Clinicians and patients have so many options to choose from, it would be a shame not to give patients the crispest vision possible. Toric lenses are available in all replacement schedules (including daily disposable), lens materials (including silicone hydrogel), and a wide variety of parameters (with custom lenses correcting higher amounts of cylinder).
While a soft sphere may provide adequate vision to low astigmatic patients, why settle for adequate? Show patients the difference that full astigmatism correction can make, and I believe that you and your patients will be rewarded for the effort. CLS
For references, please visit www.clspectrum.com/references and click on document #249.
Dr. Jackson is a 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 email@example.com.