Stability: The New Paradigm in Toric Contact Lenses
BY ALLAN S. TOCKER, OD
Toric lens wearers have always been among our most challenging patients. Historically, with multiple refits and typically some compromise in vision and/or comfort, the experience was challenging for both practitioners and patients.
Since I began fitting Acuvue Advance for Astigmatism (Vistakon) lenses I've found that my astigmatic patients can quickly achieve comfortable, good vision in all positions of gaze. In fact, my expectations of the result and the time and effort involved are now the same for an astigmat as they are for a spherical patient.
The stability of a prism-ballast lens is largely dependent on gravity. The technology works perfectly when the wearer is upright and gazing straight ahead. But when the forces of gravity are not aligned with the direction of the lid forces, the lenses tends to rotate, producing visual instability and blur.
Another disadvantage of this design is that patients often experience dryness and some discomfort where the thicker, ballasted part of the lens rests on the eye.
Many astigmats have been willing to accept these limitations, but I believe they no longer have to. Acuvue Advance for Astigmatism uses Accelerated Stabilization Design, which is designed to harness the equal and opposite forces of the eyelids to help maintain the lens position, regardless of head position.
A prism-ballast lens may take several minutes to settle on the eye as the thicker portion of the lens slowly rotates to the inferior position. This newer design tends to move into position quickly, with just a blink or two, which makes it much easier to fit.
A recent study demonstrates that my anecdotal experience isn't unique. In a double-blind, randomized study, Zikos and colleagues (2007) compared the rotational stability of prism-ballast and Accelerated Stabilization Design lenses in four tasks designed to mimic real-world viewing conditions.
Twenty contact lens wearers age 23 years to 55 years were randomly fitted with either the Acuvue Advance for Astigmatism lens or the prism-ballast SofLens Toric (Bausch & Lomb). Subjects completed a series of tests, then rested with the lenses out for 10 minutes before repeating the tests with the other lens.
The lenses were marked with a small black dot. A custom-designed, head-mounted infrared camera captured the position of the mark immediately after lens application and continuously during the testing at 30 frames per second.
The four tasks included:
• Settling time
• Reading a newspaper with wide text
• Visual search for a number and specified paragraph on a page of newspaper print
• Large versional tasks in which patients were instructed to blink while gazing centrally, then look away at a target several feet away.
The results showed that lenses using the Accelerated Stabilization Design were significantly more stable than the prism-ballast design during settling time and for large versional tasks that required the eyes to move synchronously and symmetrically in the same direction. Performance was similar for the other tasks.
Putting it into Practice
I believe Acuvue Advance for Astigmatism provides an opportunity not only to reduce some of the visual compromises experienced by our patients, but also to introduce more astigmatic patients to toric lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #154.
Dr. Tocker is in private practice in Wilmington, Del. and is a Professional Affairs Consultant for Vistakon. Contact him at (302) 731-8456 or firstname.lastname@example.org.