prescribing for astigmatism
A Daily Disposable Soft Toric Contact Lens
BY PETER D. BERGENSKE, OD, FAAO
Given the incredibly wide number of parameter variations normally available in a soft toric lens, one might view the idea of a daily disposable soft toric as a manufacturing and inventory nightmare. On the other hand, if you had to choose the least number of parameters to provide a useful range that would apply to a high percentage of astigmatic patients, it starts to look manageable after all. We know from the literature, and from clinical experience, that most patients have a relatively small amount of astigmatism. Nearly three-quarters of myopic astigmats have cylinder component of 0.75D to 1.50D. Of these, 80 percent have an axis that we could comfortably generalize as "with the rule" (180 degrees ± 20) or "against the rule" (90 degrees ± 20). Furthermore, if we look specifically at patients seeking contact lens correction, 75 percent have astigmatic component of 1.25D or less.
One Cylinder, Two Axes
Now let's add to this our knowledge that the error induced by rotation is directly proportional to the amount of cylinder, and that clinically, we find that under-correcting a cylinder is much less problematic than over-correcting. Given all this, is it reasonable to think we could fit a lot of astigmatic patients with one cylinder power and two axes? Before you answer, consider how many astigmatic patients we fit with spheres!
CIBA Vision has invested in and investigated these questions, and has recently released the first daily disposable soft toric contact lens, Focus Dailies Toric. It may seem like a bold move after all the attempts to refine toric correction with small axis increments and the precision of over-refraction calculators. Dailies Toric is available in just 46 variations, making it a manageable project. (Keep in mind the original launch of Acuvue (Vistakon) featured just 23 powers, one base curve and diameter). Dailies Toric is available in sphere powers of 0.50D to 6.00D. There is just one sagittal depth (diameter 14.2mm, radius 8.6mm). The sole cylinder is 0.75D at either 180 degrees or 90 degrees. At this point, no scribe markings indicate axis location. This may become desirable in the future as more parameters become available, but for now it is hardly necessary. (In fact, CooperVision's ToriTrack calculator makes rotation observation all but obsolete, so maybe we will see more lenses without a marking).
The lens material is identical to that used in the other Dailies products, and makes use of the same technology used to produce that family of lenses. Reproducibility has always been a key issue with toric lenses, and this process is extremely well suited to consistent production.
The lens design uses a back-surface cylinder with double thin zone stabilization, similar to the design used in some Wesley-Jessen torics, CIBA Torisoft and Vistakon Acuvue Toric. This does provide for a particularly thin lens for a toric, and good stability.
Dailies Toric launched in Europe during the summer and recently debuted in the United States. It is a welcome addition to the options we can offer our astigmatic patients.
Dr. Bergenske, a Past Chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. Email him at: email@example.com.