contact lens materials
Rotational Characteristics of Silicone Hydrogel Toric Lenses
BY NEIL A. PENCE, OD, FAAO
Toric silicone hydrogel contact lenses can greatly benefit astigmatic patients. Soft toric lenses are thicker than their spherical counterparts, so the significantly higher oxygen transmission of silicone hydrogel materials can reduce the likelihood of hypoxic complications.
The two currently available silicone hydrogel toric lenses in the U.S. market are Acuvue Advance for Astigmatism (Vistakon) and the PureVision Toric (Bausch & Lomb). A third lens, the O2Optix Toric (CIBA Vision), is currently in very limited use but is slated for general release early this year as Air Optix for Astigmatism.
The Acuvue Advance for Astigmatism design features four midperipheral thickness zones in the central third of the lens for stabilization. The PureVision Toric lens uses the same prism-ballasted design as B&L's SofLens 66 Toric hydrogel lens.
I'll discuss the rotational characteristics of Acuvue Advance for Astigmatism and the PureVision Toric.
Amount of Rotation
With the introduction of any new contact lens, it's useful to analyze its performance after a number of patients have successfully worn the lens. Both Acuvue Advance for Astigmatism and PureVision Toric offer only one base curve, so fitting choices are limited to lens power and cylinder axis. Myself and colleagues at Indiana University reviewed data from the records of the first 125 eyes fit in our university clinic with Acuvue Advance for Astigmatism and the first 175 eyes fit with PureVision Toric. Of particular interest is how stable the lenses tend to be as well as what their particular rotational tendencies might be.
Both lenses demonstrated very little rotation in the majority of patients. In more than 95 percent of eyes fit in either design, the lenses rotated 10 degrees or less. More than 80 percent were within 5 degrees of zero. These results seem to attest to the stability of both designs on the eye.
Also of interest are the rotational tendencies of each toric lens design. The most common rotation with both silicone hydrogel toric lenses was 0 degrees. However, for the Acuvue Advance for Astigmatism lens, when the rotation was not 0° it tended to be slightly nasal in direction. With the PureVision Toric, typical rotation was either 0° or slightly temporal.
Knowing the rotational tendencies of toric lenses is very helpful when selecting an initial trial lens or ordering empirically. It's especially useful when a patient's axis falls between the normal 10-degree axis increments available for both lenses (Table 1). While not true in all cases, making fitting decisions based on the most typical rotations may help reduce chair time when fitting or refitting patients into silicone hydrogel toric lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #146.
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana.