TORIC LENS DESIGNS
Soft Toric Lenses: Design Matters
Achieving good comfort and vision with toric soft contact lenses is all a matter of design.
World-renowned architect Frank Lloyd Wright is famous for saying that "Form follows function," which means the respective functional requirements of a structure or building will drive its design. The power of this statement arises from the simple elegance of its inherent truth.
I believe that we can equally apply Wright's statement and its truth to eyecare professionals who are striving to find a balance between vision and comfort for astigmatic patients. We must choose a lens design to meet the needs of our patients. I'll explain toric lens design elements and how they affect comfort and visual acuity.
Many of us have experienced the frustration of fitting toric soft contact lenses frustration stemming from lens designs that provide quality vision for patients but aren't comfortable, or from alternative designs that feel fine but fail to adequately meet patients' astigmatic needs.
Over the years, as I've strived in my practice to fit toric soft contact lenses that balance comfort and vision, this frustration has taken the form of a two-word phrase: "Design matters." I'm convinced that design matters when fitting toric soft contact lenses because it affects aspects of both vision and comfort, which are critically important to patients. In fact, a much quoted Gallup study in 2003 found that the number-one reason for contact lens dropout is discomfort, followed by inadequate visual acuity. If we want to keep astigmatic patients within the contact lens franchise, then we need to find that balance between a functional comfort and a form of design that delivers acceptable visual outcomes.
The bottom line is that patients won't be happy if they can see great but feel like they have hubcaps in their eyes. Nor will they be happy if they have a comfortable toric soft contact lens that rotates excessively every time they blink, impairing their visual quality.
Early Soft Toric Design Flaws
Until recently, the problems associated with fitting toric soft contact lenses included the following:
Lack of stability, resulting in unpredictable and variable vision
Lack of centration
Difficult to fit
Vision not significantly better than masking with spherical lens
The reality of these inferior, early designs forced many of us to mask low-cylinder astigmats with spherical equivalent prescriptions in our favorite single vision spherical (SVS) lenses as a way to keep the patients in contact lenses, or we would fit more severe astigmats with spectacles, forcing them out of contact lenses altogether. Neither option was ideal for patients or practitioners.
However, recent design developments within the toric soft contact lens category allow us to offer patients the best of both worlds when it comes to visual quality and comfort.
My Success Story
While I've had the opportunity to fit patients in my practice with all available designs of toric soft contact lenses, I've had the greatest success fitting Bausch & Lomb's SofLens66 Toric lens. The latest publicly reported market share data from HPR shows that this lens continues as the market leader and has the strongest growth in the segment (Table 1).
Astigmatic patients whom I've fit in this lens have generally experienced excellent visual outcomes and typically don't express any negative issues concerning lens comfort.
According to B&L, several design features of the SofLens66 Toric account for its success. Elements of its Lo-Torque design include the following:
Prism ballasting for rotational stability. Toric lenses must resist the torsion effects of the eyelids during the blink, while at the same time using those effects to maintain alignment and vertical movement.
Refined optic zones for consistency across powers. B&L says it maintains the design across all cylinder powers to provide consistent fitting.
Balanced vertical thickness profile for stable vision. The design must maintain a uniform midperipheral thickness at the apex, center and base of the optic zone.
Rounded posterior edge profile. A rounded edge reduces pressure on conjunctival tissue, facilitating easy movement and contributing to all-day comfort.
In my practice, I've found that these design elements result in a lens that's rotationally stable and offers consistent centration as well as lens comfort that's comparable to SVS lenses.
In terms of performance, a toric lens provides significantly better visual acuity than SVS masking. B&L recently conducted the following clinical trials that generated some interesting findings.
In the first study, researchers switched more than 800 astigmatic patients wearing SVS contact lenses to SofLens66 Toric lenses. After using SofLens66 Toric lenses for two weeks, the patients completed a forced-choice questionnaire and chose product preferences for specific attributes. The result: nine out of 10 preferred SofLens66 Toric for crisp, clear vision.
In a separate, controlled clinical study, 20 randomly assigned low-cylinder astigmatic patients wore a SofLens66 Toric lens with –0.75D cylinder correction for one week and Acuvue (Vistakon) spherical correction for another week. After one week, patients switched into the other lens. Researchers evaluated visual acuity at the initial and one-week visits. According to B&L, the results showed statistically significant differences in all acuity and visual function measures with SofLens66 Toric. Furthermore:
SofLens66 Toric improved visual acuity an average of one line vs. spherical lenses.
Nearly all low-cylinder patients achieved 20/20 vision with SofLens66 Toric.
Three times as many low-cylinder patients achieved 20/15 vision with SofLens66 Toric vs. spherical lenses.
Over the years I've developed some fitting pearls for the SofLens66 Toric that I'd like to share with you:
Demonstrate a patient's prescription in the phoropter with and then without the cylinder correction so he can see the visual difference with his toric correction. Try using hand-held cylinder lenses over SVS contact lenses as well.
Fit low-cylinder patients empirically.
Fit based on the spectacle prescription. Clinical studies involving thousands of patients have shown that you may correctly fit more than nine of 10 astigmatic patients this way.
Make the recommendation. Your patients trust you to provide them with the best vision correction. Don't hesitate to recommend a toric lens as their best contact lens option, even for low astigmats.
Use toric trial lenses so patients can "test drive" toric lenses in their prescription.
Same Design, New Material
B&L will launch its PureVision Toric lens in the US market in the second half of this year. It's been available outside of the United States since June 2004, and it combines SofLens66 Toric's Lo-Torque design features with the following modifications:
PureVision Toric incorporates slightly more prism ballasting than SofLens66 Toric (1.2D vs. 1.0D) for increased rotational stability and centration.
A fully rounded edge profile, created by edge polishing, helps facilitate adequate lens movement on-eye. This edge is similar to that of the PureVision spherical lens.
B&L has optimized the PureVision Toric design to improve oxygen performance throughout the parameter range.
B&L designed the thickness profiles to leverage the high-Dk of the balafilcon A material.
PureVision Toric features aspheric anterior optics to control spherical aberration and improve visual quality, similar to the PureVision spherical lens.
Many of PureVision Toric's design elements are shared with PureVision sphere, to allow a seamless refit of low cylinder patients currently in PureVision, as well as to allow unilateral toric lens fitting; sphere on one eye, toric on the other.
Table 2 summarizes these design elements. We've had such great success with the SofLens66 Toric over the years that it's exciting to have these design attributes available in a silicone hydrogel material. PureVision's balafilcon A offers exceptional oxygen transmissibility as well as effective fluid transport, material elasticity, excellent wettability and lens movement, and deposit resistance. I think it's great that I can offer patients a proven toric design in a healthy and comfortable silicone hydrogel option.
I�m sure you�d agree that your astigmatic patients deserve every opportunity to experience the best visual outcome with the most comfortable toric soft contact lens for them. Obviously, no single lens works for every patient on every occasion, but you can increase the odds in your favor as you strive to balance vision and comfort by trying to fit patients with the SofLens66 Toric � because ultimately, �Design Matters.�
Dr. Robinson is in private group practice in Indianapolis, IN. She frequently participates in FDA and industry studies. She's also an education consultant for Bausch & Lomb, teaching optometry students about contact lenses. Dr. Robinson is a frequent lecturer and author and currently serves as president of Women of Vision (wovonline.org) and as a contributing editor to Optometric Management.