Soft Toric Advances and Fitting Trends
A look at designs and fitting tips to help you incorporate toric options into your practice.
By Thomas G. Quinn, OD, MS, FAAO
Dr. Quinn is in group practice in Athens, Ohio. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry, an advisor to the GP Lens Institute, an area manager for Vision Source, and a contributing editor for Contact Lens Spectrum. He has served as an advisor or consultant to CooperVision, CIBA Vision and Vistakon and has received research funding from Allergan, AMO, Bausch & Lomb, CIBA Vision, CooperVision and Vistakon.
Great advancements have taken place in soft toric lens designs, leading to more stable vision, better comfort and enhanced ocular health for our patients. Nearly all new toric design innovations have been incorporated into silicone hydrogel materials. The high oxygen transmissibility feature of these materials (Table 1) is a natural match for toric designs because it is necessary to add thickness to a toric lens to prevent rotation on the ocular surface.
Brennan (2008) has shown that all of today's silicone hydrogel toric lenses meet the cornea's basic oxygen needs under open-eye conditions. Under closed-eye conditions, all silicone hydrogel torics perform similarly, differing only in the regions affected by reduced oxygen levels. These regions correspond to the areas of greater thickness inherent in each lens design.
Silicone hydrogel materials dehydrate at a slower rate and to a lesser extent than do hydrogel materials. This characteristic may lead to less tightening of lenses made of these materials. This feature offers particular benefits when fitting toric lenses as it may serve to enhance the speed of achieving rotational stability.
Silicone Hydrogel Design Details
The PureVision Toric (Bausch & Lomb) lens employs the same prism ballasted design proven successful with the hydrogel SofLens Toric lens. It further offers anterior aspheric optics for spherical aberration correction. PureVision Toric has been approved for daily wear or up to one month of continuous wear and is recommended for monthly replacement.
Both Acuvue Advance for Astigmatism and Acuvue Oasys for Astigmatism (Vistakon) prevent unwanted rotation by employing four areas on the anterior lens surface near the 2 o'clock, 4 o'clock, 8 o'clock and 10 o'clock positions. These areas rest within the palpebral aperture and interact with the lids during the natural blink cycle to help reinforce rotational stability. Acuvue Advance for Astigmatism is approved for daily wear and is recommended for two-week replacement. Acuvue Oasys for Astigmatism is approved for up to two weeks of daily wear or for up to six consecutive nights/seven days of extended wear.
Air Optix for Astigmatism (CIBA Vision) offers ballasting at 8 o'clock and 4 o'clock, providing a thin lens design inferiorly to enhance oxygen transmissibility in this area as well as to aid comfort and stability. Air Optix for Astigmatism is approved for daily wear and up to six nights of extended wear and is recommended for monthly replacement.
CooperVision is scheduled to release the Biofinity Toric in April. Its design is based on the horizontal ISO thickness design of the Biomedics Toric, but fabricated with comfilcon A, the material in the Biofinity sphere.
The Biofinity Toric offers a very attractive Dk/t of 116, allowing it to be worn on either a daily wear or extended wear basis. It is recommended for monthly replacement.
The consistent lens performance provided by today's toric soft lenses allows us to correct even low levels of astigmatism with great success.
Numerous studies have demonstrated the inadequacy of attempting to correct astigmatism of 0.75D or more by masking with spherical and aspheric soft lenses.
Conversely, other studies have shown a high success rate when fitting even low astigmats with soft toric lenses. One study of 150 subjects with astigmatic errors of 0.75D or 1.00D achieved a 97-percent success rate with toric lenses. Ninety-one percent of these subjects needed only one diagnostic fitting to achieve success.
Visual Benefits for Low Astigmats
Although it is apparent that we can fit low astigmats with toric soft lenses with a high rate of success, does it have any real impact on visual performance?
The answer is yes. One study found that patients with less than or equal to 1.00D of astigmatism achieved a three letter improvement in acuity under high-contrast photopic conditions. The benefits increased to 5.5 letters, on average, under high-contrast mesopic conditions.
Another study found that 62 prior spherical lens wearers' visual performance improved an average of nearly one line on the Snellen acuity chart when refit with a toric soft lens.
A study exploring toric soft lens performance on 96 Asian eyes found that 73 percent of subjects preferred toric lens correction versus 21 percent preferring spherical correction. More than 75 percent of subjects achieved 20/20 vision while wearing toric lens designs compared to 20 percent when wearing a spherical design.
Soft Toric Versus GP Lenses
What's the best contact lens option to utilize when fitting an astigmat with contact lenses? In addition to patient disposition, environment and visual sensitivity, let the spectacle prescription guide your choice between soft and GP correction of astigmatic patients.
Cylinder Orientation GP lenses are a great option for patients whose cylinder axis is at 180 degrees ±20 degrees (in minus cylinder form). GP lenses tend to position and move along the meridian of flattest fitting relationship. In with-the-rule astigmatism this direction is vertical, which works synergistically with the natural blink cycle to achieve ideal lens dynamics on the eye.
Patients who have against-the-rule astigmatism (spectacle axis in minus cylinder form is at 90 degrees ±20 degrees) may perform better with a soft toric lens. In addition to lens fitting issues, many against-the-rule astigmats have astigmatic errors that do not match corneal toricity. In such cases, the tear lens created by a spherical GP lens would not provide the proper astigmatic correction, resulting in residual astigmatism.
Cylinder and Sphere Power The magnitude of an astigmatic patient's spherical error can serve as a guide in choosing between a GP and soft toric design. Essentially, the higher the astigmatic error and the lower the spherical error, the more likely the patient will suffer visual disturbance with rotational changes of a soft toric lens. GPs in a spherical or toric design (if the astigmatic error is 2.00D or greater) may be the best option for these patients.
Hybrid Lenses Hybrid lenses, such as the SynergEyes A (SynergEyes, Inc.) lens, offer the optics of a GP lens with comfort and centration comparable to soft lenses. This can be a great option when GP lenses don't center well or are uncomfortable and a soft toric doesn't provide adequate visual clarity.
The Phoropter is Your Friend
Don't dance around the need to correct a patient's astigmatic error. In three quick steps, the phoropter can help you educate a patient about astigmatism and guide you on the proper approach to correcting it. See details in the "Phoropter 3-Step", below.
Vertex Effects on Cylinder Power
Before making your final selection of toric power, inspect the final refraction and make a quick calculation of the spectacle power in each meridian. If the power in either meridian is equal to or greater than –4.00D, then moving the correction to the corneal plane will affect toric power. The needed contact lens cylinder power will decrease for myopes and increase for hyperopes when vertex effects are taken into consideration.
Ideally, a toric lens does not rotate at all. This provides the benefit of selecting a contact lens axis that matches the spectacle axis.
A study by Pence et al (2005) found that Acuvue Advance for Astigmatism positions within 5 degrees of zero rotation in 82 percent of patients. The same study found that if rotation was present, there was a slight tendency for nasal rotation (36 percent nasal versus 20 percent temporal). This can be helpful for cases in which the spectacle axis falls between available contact lens axes. For example, if fitting Acuvue Advance for Astigmatism on a patient's right eye that has a spectacle axis at 15 degrees, choose a contact lens axis of 10 degrees, which will provide correction at 15 degrees after the lens rotates nasally 5 degrees.
In a subsequent study observing Air Optix for Astigmatism, Pence and Kovacich (2008) found that 98 percent of lenses were within ±5 degrees of zero rotation. They observed a slight tendency for temporal rotation (24.5 percent temporal versus 19.5 percent nasal).
Of course, it is permissible for a toric lens to rotate as long as it does so consistently in the same direction by the same degree. The axis can be adjusted using the LARS rule (if rotating Left, Add to the axis; if rotating Right, Subtract from the axis) or by combining cross cylinders with an over-refraction.
There are a number of great, free toric calculators available on the Internet, many at manufacturer's Web sites. These calculators arrive at a new toric lens power by combining cross-cylinders of a trial lens and a sphero-cylinder over-refraction, while taking into account lens rotation. Lens rotation can be observed directly, or calculated by taking into account spectacle correction. The Contact Lens Calculator Plus, in addition to other functions, will calculate resultant power using both observed and calculated rotation, allowing you to compare results for consistency (Figure 1). You can purchase it for a small fee at www.palmgear.com.
Figure 1. The Contact Lens Calculator Plus.
|The Phoropter 3-Step|
|Step 1: Demonstrate to Educate|
Most patients interested in contact lens correction assume that they will get spherical disposable soft lenses. Any time an alternative approach is needed, it is helpful to educate patients about their unique visual condition. I find the following an effective way of educating patients about the importance of correcting their astigmatism to achieve clear vision.
At the completion of the refraction, occlude one eye and instruct the patient to view the distance acuity chart. As the patient observes the chart, remove the cylinder component from the phoropter. The patient will note a drop in clarity, and appreciate an improvement when the cylinder is re-introduced into the instrument. Repeat with the other eye.
Step 2: Rotate for Mislocation Sensitivity
Some patients are simply more sensitive to blur than others. Assess a patient's individual sensitivity by using the following technique. With the final spectacle prescription in place, occlude one eye and instruct the patient to view the distance acuity chart. Ask the patient to report when he notices blur as you rotate the cylinder component (The Becherer Twist Test). Repeat with the other eye.
Highly sensitive patients may not tolerate even small degrees of toric lens rotation and would perhaps be better served by a GP. Others may be tolerant and achieve success with even modest toric lens fitting performance.
Step 3: Reduce Cylinder
The higher the cylindrical component in a toric soft lens, the greater the visual disturbance suffered when it is not properly aligned with the astigmatic error. It is therefore desirable to prescribe the least amount of astigmatic power that achieves clear vision.
Again, with the final spectacle prescription in place, occlude one eye and instruct the patient to view the distance acuity chart. Reduce the cylinder power until the patient reports blur. Increase minus in the spherical component to obtain the best subjective acuity. Bergenske (2003) has demonstrated that this approach is a reliable predictor of patient response to a toric contact lens of the same power.
Toric soft lens rotation is usually measured directly by aligning a slit lamp beam with the scribe mark on the lens surface (Figure 2), or by simply observing the scribe mark and estimating the degree of rotation.
Figure 2. Toric soft lens rotation is usually measured directly by aligning a slit lamp beam with the scribe mark on the lens surface.
It has been shown that experienced clinicians can estimate rotation within eight degrees 95 percent of the time. However, accuracy decreases with greater degrees of rotation. This suggests that estimating rotation can be a viable time saver when working with lenses of low cylinder value that rotate little on the eye. Conversely, it is advantageous to actually measure rotation when working with higher amounts of astigmatism or greater degrees of rotation.
The linchpin of success with toric soft lenses is rotational stability, which translates into consistently clear vision. Take time to thoroughly assess stability at the outset to save time and hassle with troubleshooting later.
Start by observing lens rotation as the patient looks straight ahead. Then ask the patient to look up, blink, then look straight ahead. Observe the scribe mark. Repeat in other directions of gaze. The goal is to see minimal shifting of the scribe mark during these movements. Video clips of this procedure are available by clicking Video 1 and Video 2.
Studies have demonstrated that this technique can detect differences in performance between lens types. Studies further suggest that rotational mislocation is most likely to occur when gazing in oblique directions.
A final step to thoroughly assessing lens stability is to manually rotate the lens on the eye and observe its speed of return to its original orientation. A slow return, or absence of returning to its original location, suggests that you should make a change in lens design. A rapid return suggests that the lens will provide consistently clear vision.
Rather than the common one-week follow-up visit, consider seeing your silicone hydrogel patients, toric and sphere, after two weeks of wear. The higher modulus of some of these materials sometimes necessitates a longer adaptation period.
Even with best practices, there will be times when toric patients present with visual complaints. The key question to ask is, "Is the blur constant, or fluctuating?" If the vision is fluctuating, change the lens fit or design. If the blur is constant, adjust the lens power or axis.
One-Day Toric Lenses
Recent surveys suggest growth in the use of daily disposable lenses. CIBA Vision's Focus Dailies Toric and CooperVision's ClearSight 1 Day Toric allow our astigmatic patients to enjoy the benefits of daily disposability.
The Focus Dailies Toric, which has no rotation indicator mark, is available in sphere powers of +4.00D to –8.00D and in both –0.75D and –1.50D of astigmatic correction at axes of 90 degrees and 180 degrees. The ClearSight 1 Day Toric is available in sphere powers from plano to –10.00D and in toric powers of –0.75D and –1.25D at axes of 180 degrees, 160 degrees, 90 degrees and 20 degrees, with a rotation indicator mark at 6 o'clock.
Today's toric soft contact lenses provide exceptional vision, comfort and safety. Low astigmats can be fit easily and effectively.
Although the new, silicone hydrogel toric lenses offer exceptional performance, some powers are not yet available in these designs. CooperVision offers an extensive line of hydrogel toric contact lens options in a wide range of powers, including a multifocal toric. A number of companies also offer the opportunity to custom design toric soft lenses. In summary, there's no reason not to step up and provide your patients with their best possible vision by correcting their astigmatism. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #160.