contact lens materials
Lens Material Enhancements: Different…but the Same
BY NEIL A. PENCE, OD, FAAO
When a new contact lens enters the market, we realize that it has undergone extensive development, testing and clinical trials to receive FDA approval. And for however many years that lens is available, we tend to view it as a fixed, static, unchanging entity. While generally true, this may not always be the whole story.
At each of the many tasks involved in producing a lens, there are certain tolerance ranges. As long as manufacturers stay within these tolerance ranges, the potential exists for slight variances in quite a number of steps. Small alterations (both intended and unintended) can affect lens performance, which can be clinically relevant (see sidebar).
Minor changes in the selection and handling of material components and their subsequent mixing can alter the final properties of that material. The material chemistry and material name remain the same, because changes are within specified tolerances. It's believed that's how Bausch & Lomb was able to reduce the modulus of its PureVision lens material. The modulus changed, but the material is still balafilcon A and has the same silicone and water content and the same oxygen transmission.
Likewise, CIBA Vision acknowledged manufacturing process improvements for both Night & Day and O2Optix lenses. According to the company, these manufacturing innovations enhanced the consistency and comfort of these lenses. Performance may be slightly different, but the materials and oxygen transmission remain the same.
|The Same…but Different|
Our patient reported for an annual exam wearing PureVision Toric (Bausch & Lomb) lenses with no symptoms or problems. Testing revealed no changes, and we ordered a year's supply in the exact same parameters.
After wearing a new pair, the patient reported blur with the right eye, which remained when she tried the next lens from that box. We instructed her to try a right lens from the second box, and when that didn't help we scheduled her for a visit.
Corrected acuities were now 20/25– OD, 20/20 OS. The right lens was rotated 20 degrees temporal, the left 5 degrees temporal. On the previous visit, and at the fitting and follow-up the year before, both lenses were noted at 5 degrees temporal. The patient's spectacle and contact lens were both axis 140. A trial lens of identical power but axis 160 rotated 20 degrees temporal OD and yielded 20/20+.
It's possible some undetected lid changes altered the lens positioning. The patient had her old right lens box (empty), which showed an expiration of 01-08. The new boxes had expirations of 01-10, indicating they were made in the newer material. Therefore, the small modulus change in the material may have led to the slightly altered lens performance.
We thought we were ordering exactly the same lenses as before. They may instead have been the same…but different.
Be on the Lookout
If a company's continual testing and monitoring of both new and existing products identifies opportunities for improvement, it's reasonable to think these refinements would be applied. Therefore, a lens might in fact be changed in a number of small ways without need of further approval or review by the FDA. Keep in mind the possibility a lens could be the same…but different. CLS
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana.