Article

CONTACT LENS DESIGN & MATERIALS

IS IT STILL PERMISSIBLE TO USE HYDROGEL SOFT LENSES?

There are many advantages to teaching the next generation of optometrists, not the least of which is being around bright, motivated, and energetic young persons every day. It also allows the occasional glimpse into various topics from the perspective of those who have little or no background to provide the perspective that we might take for granted. One such recent opportunity occurred when an optometry student asked: “Is it OK to still use hydrogel soft lenses?”

Admittedly, my first inclination may have been to somewhat sarcastically suggest that we probably would not have multiple fitting sets of hydrogel lenses on hand in the clinic if it was not OK. But, upon further reflection, the question probably makes a good deal of sense.

Nearly all new introductions of soft contact lenses that these students may have witnessed have been in silicone hydrogel (SiHy) materials. It is likely that didactic lectures have also placed a much higher emphasis on these newer lenses, and on SiHy in general. Furthermore, it is likely that there have been at least a few lectures emphasizing the importance of oxygen to the cornea and the ill effects of corneal hypoxia associated with wearing contact lenses.

While the focus of some of these lectures may have been the oxygen needed for overnight wear, the cumulative effect is to give the impression that SiHy lenses have replaced hydrogel soft contact lenses. If that is true, then why would we still be fitting this “old technology”?

“Old” or “New” Technology?

While hydrogel lens materials cannot supply the level of oxygen needed for overnight wear, they can be a very successful option for many patients using their lenses on a daily wear basis. The best examples of this are the hydrogel materials found in a number of very popular daily disposable (DD) contact lenses.

The classic study to first detail the oxygen levels needed for successful daily wear (Holden and Mertz, 1984) reported that a Dk/t of 24 Fatt units was required. A number of other studies followed, with the most widely accepted re-evaluation of the topic (Harvitt and Bonanno, 1990) determining that a Dk/t of 35 was needed to avoid corneal hypoxia with daily wear.

The four hydrogel DD contact lenses with the highest market share today all fall in the 26 to 42 Fatt units range, well within the ballpark needed to supply acceptable levels of oxygen when used for daily wear only. Therefore, it is reasonable to assume that the most popular DD hydrogel lenses will have sufficient oxygen transmissibility for the majority of patients. They also provide a highly convenient modality and often are very comfortable to wear.

In fact, hydrogel lenses are generally lower in modulus (less stiff) compared to SiHy lenses and may have a lower incidence of infiltrates associated with their wear. Hydrogel DD lenses also typically cost less, making them more economical.

In Summary

So, yes, it is still permissible to fit hydrogel soft contact lenses. And, more than that, it is often desirable to do so. Patients who have high prescriptions or thick lenses, who report often sleeping in their lenses, or who need more oxygen to their corneas than normal may not be good candidates. However, the majority of patients can be safely and successfully fit in DD lenses made in hydrogel materials. CLS

For references, please visit www.clspectrum.com/references and click on document #255.