RGP insights

RGPs: When Oxygen is a Priority

BY EDWARD S. BENNETT OD, MSED
APRIL 1998

RGP lenses are underutilized even though the benefits are many, including quality vision, durability, availability of special designs (torics, bifocals, keratoconus) and the ability to profit from the use of service agreements. other benefits include smaller diameters, which reduce the incidence of peripheral the use of service agreements. Smaller diameters reduce the incidence of peripheral corneal complications such as vascularization and infiltrates, and higher oxygen transmissibility levels supplemented by an active retro-lens tear pump results in fewer long-term corneal hypoxia-related problems with daily and extended wear RGP materials.

Reviewing the Dk Data

Benjamin (1993) has divided contact lens materials into the following five categories based on oxygen transmission: Low (Dk/L<12); Medium (Dk/L=12-25); High (Dk/L=26-50); Super (Dk/L= 51-80); and Hyper (Dk/L>80). The water content of hydrogel lenses typically limits them to the low and medium oxygen groups, but the tear pump activity also plays a role. While rigid lenses exchange 10 to 20 percent of tears with the blink, the soft lens tear pump exchanges as little as one percent.

Hill (1997) found that the hypoxic relief provided by the tear pump is advantageous for the low, medium and high oxygen categories, especially in extended wear. Studies have demonstrated that the first night of soft lens extended wear typically results in corneal swelling of 10 to 15 percent, compared to seven to 10 percent with high Dk/L RGPs and less for super and hyper Dk/L RGPs. The swelling that occurs under high, super and hyper Dk/L RGPs returns to baseline within a few hours after waking, but it's not uncommon for residual swelling or a chronic low-grade hypoxic situation to exist with extended wear hydrogels. This is consistent with the "gold standard" for oxygen transmission -- the Holden-Mertz criterion, which states that a Dk/L of 24 is necessary for no corneal swelling with daily wear and a Dk/L of 34 is necessary for no residual swelling with extended wear.

Oxygen is perhaps just as important a consideration in hyperopic contact lens wear as it is in extended wear. In 1990, Jim Gordon, M.D., and I concluded that the high Dk/L RGPs approximated or exceeded the Holden-Mertz criterion for both extended wear and daily wear in the +3.00D power, while soft lens extended wear materials provided an average of only 40 to 45 percent of the oxygen necessary for edema-free extended wear and approximately 60 percent for edema-free daily wear.

These results are confirmed in a study by Solomon (1996), in which 82 first-time extended wear patients were fitted with soft disposable contact lenses approved for extended wear. Six out of eight hyperopes failed to wear the lenses for one week of extended wear, compared to only three out of 74 myopes. The study concluded that the hyperopic failures were caused by the high level of corneal swelling induced by extended wear.

A Material Selection Protocol

The oxygen needs of our patients are an important consideration when selecting contact lens materials. Based on the research, we can conclude that seven-day soft lens extended wear should be limited to myopic patients and that these patients should be monitored carefully for clinical signs of corneal hypoxia.

Using Benjamin's classification for RGPs, we can also make the conclusions listed in Table 1 regarding appropriate material selection.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis. He is also executive director of the RGP Lens Institute.