In this day and age, we have a wide variety of soft toric contact lens options at our disposal as eyecare providers. There are numerous brands, replacement modalities, and materials to choose from, not to mention the different designs they each employ.
By better understanding the available lenses, we can work to ensure the stability and clarity of our astigmatic patients' vision. In this article, we'll take a closer look at the array of designs available, and discuss the features that make each lens unique.
Astigmatism in a (football-shaped) nutshell
Astigmatic refractive error occurs when there are different refractive powers in separate, usually perpendicular, planes of the eye. This causes light to land in multiple locations or at separate focal points requiring each to be corrected for clear vision.
Figure 1 illustrates how corneal astigmatism causes refractive error.
While patient symptoms can vary, typical complaints associated with astigmatism can include:
- Halos and glares with lights more often at night
- Blurred vision at distance and near
- Visual discomfort
- Squinting or straining to see objects more clearly
- Hazing or clouding of letters or words
One 2018 article found the prevalence of astigmatism recorded in children to be roughly 15% worldwide and about 40% in adults.1 Astigmatism commonly manifests in either the cornea or lens, and is referred to as corneal or lenticular astigmatism, respectively. Of course, both of these may be present concurrently as well.
Comparing the types of astigmatism
Depending on the orientation of the previously mentioned refractive planes, regular astigmatism can be termed with-the-rule (WTR), against-the-rule (ATR), or oblique. The direction of the steepest meridian, or more-plus power, correlates with the terminology applied.
For example, WTR astigmatism occurs when the vertical meridian (60 to 120°) is the steepest, contrasting to ATR astigmatism wherein the horizontal meridian (150 to 180°) has the steeper curvature. Oblique astigmatism is assigned when the steeper plane falls between 30 to 60° or 120 to 150°.
On the other end of the astigmatism spectrum lies irregular astigmatism. This can describe cases where the refractive planes are not perpendicular to one another or are asymmetrical along their meridians as well as other pattern irregularities.2
While patients with low amounts of irregular astigmatism may be able to use traditional soft toric lenses, these designs are intended to correct regular astigmatism. This article, therefore, will focus on using soft toric lenses for regular astigmatism.
Figures 2a to 2c show corneal topography imaging of WTR, oblique, and irregular astigmatism, respectively.
Figure 2a: Courtesy of Alex Tharman, OD.
Figure 2b: Courtesy of Alex Tharman, OD.
Figure 2c: Courtesy of Alex Tharman, OD.
When can I choose a toric contact lens?
In a report by Morgan et al. released in 2020, it was stated that soft toric contact lenses made up about 28% of all of the soft lens fits in the 24 represented countries.3 Remembering the statistic from before that nearly 40% of adults have astigmatism ≥0.50D, there appears to be about a 12% disparity between these two figures.1
That being said, there obviously can be a number of factors that affect this such as contact lens dropout, the fitting of spherical equivalent contact lens powers, the use of presbyopia-correcting lenses, and more.
Most standard disposable soft contact lenses begin correcting for astigmatism at as little as -0.75D of cylinder power and stretch to about -2.75D.4 However, some manufacturers and labs offer more custom or niche lenses that expand either side of this range to further expand the available fitting options.
As a common rule of thumb, many providers will not prescribe or fit toric lenses when astigmatism is ≤0.75D of cylinder power. This is due to the fact that the spherical equivalent at these powers is roughly 0.25D, and most patients can still maintain acceptable vision with this option.
There are some patients, though, that may have higher visual demands or that may be more sensitive to this mild astigmatic correction that can benefit from being fit with a toric lens. For these patients, performing a trial lens fit of a low cylinder-correcting lens can be beneficial to establish if this correction will prove beneficial in their lifestyle.
The many faces of toric lenses
In most readily available commercial soft toric lenses there exist two main stabilization designs, the prism ballast technique and the use of thin zones. These can of course be combined with one another depending on the manufacturer as well.
Prism-ballast design
Prism-ballast designs feature a thicker bottom portion of the lens than the top, similar to a prism. Gravity and blink pressure of the eyelids then move the thicker portion of the lens inferiorly. With prism ballast lenses, the graduation of thickness exists through the entirety of the lens body, which can induce vertical prism that is noticed visually to the wearer.5
Prism-ballast designs are utilized by lenses such as the Bausch + Lomb Ultra for Astigmatism, as well as CooperVision’s Clariti 1-Day Toric and Proclear Toric/Proclear Toric XR lenses.6
Peri-ballast design
As such, some manufacturers have adopted a peri-ballast system that works similarly to the prism-ballast lenses, with the noticeable difference that the thickness change occurs only at the edge of the lens. This removes the visual disturbances that can occur as the so-called “prism” is out of the visual axis.
While this change can be beneficial visually, there still exist some stability issues with these lenses when head position or is altered. They continue to follow gravity and tend to rotate more to keep the thicker inferior edge closer to the ground, even if this no longer matches the patient's refractive plane locations.
Similar to prism-ballast designs, manufacturers like Bausch + Lomb use this design with their BioTrue OneDay for Astigmatism lens options.6 Other CooperVision lenses, such as the Biofinity Toric, MyDay Toric, and Avaira Vitality Toric as well as Alcon lenses like the Precision 1 for Astigmatism, Dailies Total 1 for Astigmatism, Air Optix for Astigmatism, and the Total 30 for Astigmatism are designed with a variation of prism or peri-ballasting touches.6
Thin-zone design
Finally, other brands utilize thin zone technology in their toric soft lenses. Thin-zone lenses appear somewhat inversely to the previously described designs. Here, the thicker portion of the lens lies more centrally in the vertical meridian. Specifically, around the areas of the lens that interact more with the upper and lower eyelids.
Figure 3 shows the thin-zone and Blink Stabilized design by Johnson & Johnson.
Figure 3: Courtesy of Johnson & Johnson Vision.
Like peri-ballast lenses, there is no vertical prism induction that occurs in this design, which is one of their visual benefits. This provides clear, distortion-free vision and can also be advantageous if a patient requires a toric lens in only one eye.
Also, as the lids are interacting with the lens with every blink these lenses allow for faster and more consistent stabilization of lens rotation.7 Without a thicker edge, these lenses demonstrate less rotation with abnormal head positions or tilting/turning.
Figure 4 depicts the rotational stability of the thin-zone design by Johnson & Johnson in abnormal head positions.
Figure 4: Courtesy of Johnson & Johnson Vision.
For a toric lens, head position or head rotation may influence how a prism-ballasted lens stabilizes on the eye. Our patients today lead unique lifestyles from leaning into the backseat to check on their kids, performing near-acrobatic skills to fix a car, to getting under the desk to find that pesky wall outlet, to the fast-paced demands of playing various sports.
Even something as simple as laying on our sides to watch TV can alter the way a lens rotates. These instances are where thin-zone lenses and Blink Stabilized designs can really shine through and make the difference in a successful toric lens fit.
Conclusions
We understand that no two patients are alike and yet we do our best to provide repeatable and consistent outcomes to everyone we see. After all, it is a pleasure to improve our patients' lives through their vision.
When it comes to soft toric contact lenses and the many options available to us, at the end of the day we want to provide our patients with a lens that is healthy for their eye, provides them clear and consistent vision throughout their use, and is readily available.
The lenses that take advantage of thin zones can do just that for us and while a small change for us, can mean the world to a patient and their experience.
The views and opinions expressed in this article belong to the author themself and do not necessarily reflect those of their associated groups or organizations.
