Tune up Your Toric Fits
FITTING TORIC LENSES
Tune up Your Toric Fits
Patients have many options when it comes to contact lens correction for astigmatism.
BY JOHN MARK JACKSON, OD, MS, FAAO
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 firstname.lastname@example.org.
As clinicians, we are very fortunate to live in a world with so many options for patients who have astigmatism. Our profession revolves around helping our patients to see as well as they possibly can. Unfortunately, I still see patients who were told that they can’t wear contact lenses due to their astigmatism, or who are suffering wearing soft spheres. (I evaluated a patient recently who had –2.00 x 180 and was wearing spheres!) If you need to brush up on your toric contact lens skills, this article will review the basics, as well as some more advanced skills, in astigmatism correction.
Soft Toric Lenses
It’s never been easier to fit soft toric lenses. In the early days of soft torics, they were difficult to manufacture, and no two lenses ended up performing alike, so they were often referred to as “snowflake” lenses. Those days are now long gone, and everything has changed dramatically for the better.
The key to working with soft torics remains management of lens rotation (Figure 1). As all the astigmatic correction is in the lens itself, if the lens rotates, you end up with axis misalignment and reduced visual acuity. Today’s soft toric contact lenses usually rotate very little on the eye, so large adjustments to the lens axis are rarely needed.
Of course, to adjust the axis to compensate for rotational misalignment when needed, the well-known mnemonic LARS (left add, right subtract) is used. If the lens rotates to your left, you add the amount of rotation to the spectacle axis; if it rotates to your right, you subtract. For example, if a patient’s spectacle axis is 030, and the soft toric lens rotates 10 degrees to the right, the required axis is 030 – 010 = 020. For this to be successful, the new lens must rotate to the same position.
TABLE 1 Axis Misalignment & Visual Acuity
|Lens Cylinder Power
||10° of Axis Misalignment Results in Residual Astigmatism of:|
|The amount of residual astigmatism induced by axis misalignment varies with the cylinder power of the lens.|
Figure 1. A soft toric contact lens with about 5° left rotation. Note the three toric lens markings on the inferior portion of the lens.
It is tempting, if patients appear to be satisfied, to ignore small amounts of lens rotation. Unfortunately, many toric lens patients have some amount of fluctuation in their vision due to axis misalignment and assume that their vision can’t be any better. Although it may take a little more chair time, you can really tune up your toric lens fits by closely observing the axis misalignment. Table 1 shows how a small amount of axis misalignment can result in residual astigmatism. You can observe that the higher the amount of astigmatism correction, the more residual astigmatism results from axis misalignment. For a lens with –0.75DC, 10° of misalignment will result in only –0.25DC of residual astigmatism. This small amount of misalignment should not have too much impact on visual acuity. However, the same amount of misalignment with higher cylinder amounts will result in more residual astigmatism.
Misalignment and rotation are not the same thing. For example, if a patient’s spectacle prescription is –1.00 –1.75 x 155, and we apply a soft toric with power of –1.00 –1.75 x 160, there will be 5° of axis misalignment, even if there is no rotation. If this lens rotates 5° to the right, the patient is looking through axis 165, which means there is 10° of misalignment, and the patient will have a little more than 0.50DC residual astigmatism. Changing the lens axis to 150 will yield axis 155 after rotation, which is exactly what the patient needs. In other words, don’t stop when you see the lens is rotating only 5°; analyze the axis misalignment carefully to provide optimal vision.
Figure 2. SCOR axis with 10° right rotation.
Soft Toric Over-Refraction
While LARS is successful in many cases, it is still useful to perform a sphero-cylindrical over-refraction (SCOR) to see how much residual astigmatism is present and to fine-tune the prescription. Many calculators are available (on the Internet and also as mobile device apps) to determine how to change the lens power based on the SCOR. However, manually interpreting the SCOR is straightforward and can give you a quick insight into how the lens is performing.
Assuming that the lens is the correct power, but the lens has rotated and there is axis misalignment, the SCOR power will have a spherical equivalent of plano and will return one-third of the lens’ cylinder for every 10° of axis misalignment. For example, suppose a patient’s spectacle Rx is –2.00 –1.50 x 180. We apply a soft toric with a power of –2.00 –1.50 x 180. If the lens rotates 10° to the right, there is 10° of axis misalignment, and we would expect the SCOR to be +0.25 –0.50 x ____. Note that the spherical equivalent is plano, and the cylinder is one-third of the original amount. If the SCOR had been +0.50 –1.00 x ____, we would know the lens had rotated 20°, because we have two-thirds of the original cylinder.
Figure 3. SCOR axis with 20° right rotation.
Figure 4. Flowchart for selecting type of GP lens.
I left the SCOR axes blank in the previous examples, as the axis requires a little more explanation. The SCOR axis will be 45° away from halfway between the spectacle axis and the on-eye lens axis, on the side of the spectacle axis (whew!). In the first example with 10° of rotation to the right, the lens on-eye has an axis of 010. Halfway between 180 and 010 would be 005; 45° from there, moving in the direction of the spectacle axis, would be axis 140 (Figure 2). In the second example, with 20° of rotation, the lens’ axis on-eye would be 020. Halfway is 010, and 45° from there would be axis 145 (Figure 3).
Note that the farther the lens was off-axis, the closer the SCOR axis was to the patient’s refraction. In fact, if the lens is only a few degrees off axis, the SCOR axis is 45° away from the spectacle axis. This result can be confusing if you aren’t expecting it, which could lead you to conclude that something is erroneous with the SCOR when the results are actually to be expected.
Figure 5. A bitoric GP lens on a 4.50DC cornea.
Rigid Lens Options
Soft toric lenses work great for many patients, but there will always be a need for the optics of a rigid lens to correct astigmatism. Rigid lens options can get slightly overwhelming, as there are so many from which to select. Navigating these options is much easier if you understand the fundamental reasons for picking each one. To choose the best one, you have to be able to answer two questions: 1) Which type of base curve is needed to fit the patient’s cornea? and 2) How well will this option correct the patient’s vision?
Figure 4 shows a flowchart that can help you determine which type of lens to use for different corneas.
Before getting into toric rigid lenses, let’s do a quick review of non-toric options and when to use them.
Spherical GPs What kind of astigmatic patient is a good candidate for a traditional spherical GP lens? Thinking of our criteria of fit and vision, you first need a patient who has a low amount of corneal toricity for the spherical base curve to fit properly. For optimal lens fit, a spherical base curve will work when the cornea has about 2.00DC or less of corneal astigmatism. This can be determined with keratometry or corneal topography. If the cornea has more than this amount of toricity, there can be too much compromise in how the lens fits. Because the two meridians are so differently curved, the lens will rock too much on the cornea, which can make the lens uncomfortable, lead to lens flexure and variable acuity, and could result in unwanted corneal molding.
The second criterion, optimal visual correction, will be met when a patient’s corneal astigmatism is very close to the refractive astigmatism. As the lens has spherical power, all of the astigmatic power is provided by the tear lens. The tear lens cylinder power will be equal to the amount of corneal astigmatism, essentially “correcting out” the corneal component of the patient’s refraction. As there is likely some lenticular astigmatism in most patients, there will usually be some amount of residual astigmatism. As long as the corneal and refractive cylinders are within approximately 0.75DC, patients will have a reasonable amount of residual astigmatism and good visual acuity. For patients who have borderline toricity for a spherical lens to fit, an aspheric base curve can also be used to improve lens centration (see Figure 4).
Hybrids and Sclerals One good option for patients who need rigid lens optics, but better comfort compared to that provided by corneal GP lenses, is the SynergEyes Duette HD hybrid contact lens. The soft skirt stabilizes the central rigid portion, which makes the lens center well and avoid flexure, even with higher amounts of corneal astigmatism. The lens can fit well for up to about 6.00DC of corneal astigmatism. As with any spherical GP optics, the corneal and refractive astigmatism must be a close match for the lens to provide optimal vision correction.
Scleral lenses can also be successful for patients who have astigmatism. Traditionally reserved for irregular corneas and dropouts from other designs, mini-sclerals (about 15mm to 16mm diameters) are becoming more commonly used for normal corneas as well. One potential disadvantage is that the tear film is much thicker compared to that with corneal lenses, and this can result in tear film astigmatism that is less than predicted by the keratometry values.
The best way to determine how well a scleral lens will correct a patient’s astigmatism is to just apply one and perform a SCOR. If the amount of residual astigmatism is excessive, consult with the manufacturer to determine the best course of action. Sometimes residual astigmatism can result from flexure, and making the lens a little thicker can prevent this. Many designs can also be made in a front-surface toric design to correct the remaining astigmatism (see information on front-surface torics below).
Front-Surface Toric GPs A front-surface toric GP lens has a spherical base curve, but cylinder correction on the front of the lens. Thinking of our two criteria again, a patient needs a front-surface toric GP when a spherical base curve will fit well, but a spherical power will result in excessive residual astigmatism. If the keratometry cylinder is more than 0.75DC different than the refractive astigmatism, a spherical GP will likely result in excessive residual astigmatism.
In practice, this design is not often used in a corneal lens design due to the inherent instability of the design. Because the lens has cylinder correction on the front, any rotation of the lens will result in axis misalignment and degrade visual acuity. The lens is prism-ballasted to prevent rotation, but the small diameter of the lens means that there will be unwanted rotation even with the prism. From a clinical perspective, therefore, a soft toric is a better option than a corneal front-surface GP because the larger diameter of the soft lens will make the lens more stable.
For diagnostic fitting, use a spherical GP set to determine the best base curve, then over-refract that lens. If the residual astigmatism is excessive, the residual cylinder can be added to the lens to make a front-surface toric.
For example, if your trial lens is –3.00DS, and you over-refract –1.00 –1.25 x 090, the lens to order would be –4.00 –1.25 x 090. Your laboratory consultant can help with determining other lens parameters, such as the recommended amount of prism needed to prevent lens rotation.
As mentioned earlier, a front-surface toric can work reasonably well with a scleral design. This is because the large diameter of the scleral lens prevents lens rotation. It can be a challenge for patients, however, to apply the lens in the correct orientation for the cylinder axis to be correct. Placing a dot on the lens can assist with this. Compare this to a soft toric lens, in which the lens will self-correct and rotate to the proper orientation (eventually).
Back-Surface and Bitoric GPs Back-surface toric (BST) and bitoric GPs both have a toric base curve. So considering our first criterion, optimal lens fit, either of these could be needed for a patient who has a large amount of corneal astigmatism. In general, patients who have more than 2.00DC of corneal cylinder may benefit from a toric base curve, and those who have more than 3.00DC will most likely require one. This is especially true if the corneal toricity is limbus-to-limbus, as a spherical base curve will rock excessively on the cornea.
Choosing between a BST and a bitoric lens comes down to vision correction. A BST will be successful in a very specific situation: when a patient’s refractive cylinder is 1.50 times greater than the corneal cylinder. For example, if a patient’s keratometry values are 44.00/41.00 @ 090, and the refraction is –1.00 –4.50 x 090, the patient has 3.00DC of corneal cylinder; 3.00 x 1.50 = 4.50, which is equal to the refractive cylinder. Since the rule is met, a BST would work well for this patient. As this is a relatively rare occurrence, most of the time a bitoric is needed instead.
A bitoric lens, as the name implies, has toricity on both surfaces of the lens. Practically, this means that you can put whatever power you need in the lens to optimally correct vision—a huge advantage for this lens design.
Although these lenses can seem intimidating, they are actually fairly straightforward to fit. Because most practices won’t have a bitoric fitting set, empirical ordering is the way to go. If you have a corneal topographer that includes a lens-fitting package, you can design the base curves quite accurately to obtain an alignment relationship for each meridian. By inputting the refraction, the topographer will also calculate the appropriate meridian powers as well. If you don’t have a topographer, call the laboratory with the keratometry values and refraction, and a consultant will help you design the lens.
When you apply the lens, the fluorescein pattern will resemble the way a spherical lens looks on a fairly spherical cornea; it should look like it has approximately the same alignment in all meridians (Figure 5).
As you can see, patients have many options when it comes to contact lens correction for astigmatism. The wide range of toric lenses (as well as spherical GP designs) means that there is a lens for just about every patient. I hope this article helps you sort out the options and provide exceptional vision for your astigmats. CLS
Contact Lens Spectrum, Volume: 29 , Issue: February 2014