Spotlight: ON TECHNOLOGY AND TECHNIQUE
30-Day Wear: Getting It Right
Thanks to advances in technology, continuous wear contact lenses are finally becoming a reality.
BY CHRISTOPHER KENT, SENIOR ASSOCIATE EDITOR

There's no question that patients want contact lenses they can leave in for weeks at a time. When the first (ill-fated) extended wear lenses were introduced in the 1980s, the number of soft contact lenses wearers almost doubled. With new, high oxygen transmissible continuous wear lenses now becoming available, it's entirely possible that the same thing will happen again.

Of course, some M.D.s are understandably cautious about jumping on the "extended wear" bandwagon a second time. However, the new generation of materials has been developed with extreme care and tested exhaustively. So far, the evidence overwhelmingly supports an excellent prognosis for these lenses -- and their wearers.

Overcoming obstacles

The first of the new lenses to reach the marketplace was CIBA Vision's Focus NIGHT & DAY lenses. As it turned out, creating continuous wear lenses that worked was no easy task. CIBA Vision spent millions of dollars and many years on the project, systematically addressing all the technical issues through intensive research and clinical testing.

Here's how they did it:

Problem 1: oxygen transmission. As you know, the metabolic stress caused by insufficient oxygen produces a thin, damaged epithelium, ideal for the adherence of pathogenic bacteria and the development of microbial keratitis (MK); it also causes edema. Some edema occurs overnight even when no contact lens is worn, so the ideal was to create a lens that produced no additional edema. Research has shown that 125 barrers is the minimum level of oxygen transmission needed in a contact lens to achieve this goal. (The extended wear lenses available in the 1980s only had oxygen transmissibility between 8 and 30 Dk/t, associated with as much as 15% overnight edema.)

Silicone elastomer, introduced in the 1980s, has oxygen permeability in excess of 300 barrers. Unfortunately, lenses made with this material have a hydrophobic surface that can cause problems with lens adhesion. Other lens materials (including conventional hydrogels) have tried to increase oxygen transmission by increasing water content, but even 100% water only has a Dk of 80 to 90 barrers.

To solve the problem, researchers at CIBA Vision found a way to combine polymers with different properties into one lens by incorporating two "phases" into the molecular structure of the lens material. One phase, consisting of fluorosilicone, facilitates oxygen transmission; the other phase, consisting of a hydrogel monomer, helps the lens moves properly on the eye.

Problem 2: surface compatibility. Despite having achieved the desired levels of oxygen permeability, lenses made with a silicone hydrogel material still had problems with wettability, lipid interaction and lens binding.

To resolve these issues, CIBA Vision developed a method of modifying the surface of each lens in a gas plasma chamber. This process creates a permanent, ultrathin hydrophilic surface that's nonionic, resists deposits and lowers bacteria binding.

Research has shown that the resulting lenses move as freely on the eye as ordinary hydrogel contact lenses.

Problem 3: rigidity and fit. Silicone hydrogel lenses are more rigid than ordinary soft contact lenses. This makes the new lenses easy to handle and contributes to proper movement on the cornea, but the stiffness can prevent the material from draping over the cornea. This resulted in a imperfect fit and discomfort for some patients when only one base curve was available.

CIBA Vision now offers a second base curve, which appears to have solved the problem. A recent clinical trial conducted at the University of Waterloo found that 98% of patients achieved a satisfactory fit when both an 8.4 mm and 8.6 mm base curve were available.

Evaluating the result

So how well do CIBA's Focus NIGHT & DAY continuous wear lenses work?

• By the beginning of 2002, a quarter of a million people around the world were wearing the new lenses, but very few cases of MK had been reported. To date, the rate of infection appears to be significantly less than that found among wearers of low-Dk extended wear soft lenses. CIBA Vision is currently engaged in a post-marketing clinical study to determine the true incidence rates of MK with these lenses.
• Overnight swelling while wearing the new silicone hydrogel lenses is similar to having no lens on the eye at all -- about three times less swelling than is caused by wearing a 70% water content low-Dk hydrogel lens.
• Reports of contact lens-induced corneal striae, microcysts, epithelial cell size increase or lens-induced endothelial polymegethism have been rare.
• Limbal vascularization, seen in wearers of previous extended wear lenses, appears to be minimal.

Comparing pros and cons

How do the lenses stack up to alternative methods of vision correction?

Compared with LASIK. Given the option, most patients surveyed would prefer continuous wear contact lenses to surgery on their eyes. And the facts suggest that this is a
reasonable choice:

• The only serious danger associated with wearing contact lenses is MK. (Less serious adverse reactions include infiltrative keratitis, conjunctivitis, GPC, or lens discomfort, such as dryness, mild burning or stinging.)
• LASIK, on the other hand, has been associated with a host of problems, including MK, vision loss from aberrations or surgical complications, corneal ectasia, lamellar keratitis, retinal detachment, bilateral keratitis, optic neuropathy, macular hemorrhage and post-surgery symptoms such as haloes, dry eyes, decreased contrast sensitivity, and the possibility of imperfect outcomes resulting in the need for further correction.
• One study in which patients wore NIGHT & DAY lenses for a year found that 83% reported 20/20 or better visual acuity at every checkup. Some studies of LASIK patients have found that only 65% of subjects matched this criteria.

LASIK isn't reversible; contact lens correction is.

Compared to a low-Dk lens worn for a week. One study involving 1,395 patients compared the health effects of wearing a conventional low-Dk hydrogel weekly replacement lens to wearing the NIGHT & DAY lens for 30 days:

• Despite wearing the NIGHT & DAY lenses four times as long, no statistical increase in ocular complications was reported compared to the other lenses.
• No cases of infectious ulcerative keratitis were reported during the year-long study.
• Fewer NIGHT & DAY wearers reported symptoms of dryness compared to the weekly replacement group.
• No myopic creep was reported among NIGHT & DAY wearers. In contrast, 1.5% of weekly lens wearers had an increase in myopia greater than 1.0D during the year.

Off to a great start

Research at CIBA Vision continues, and forthcoming versions of the new lenses will undoubtedly meet doctors' and patients' needs and desires even more completely. In the meantime, however, most patients are very happy with NIGHT & DAY lenses. In surveys conducted by the Cornea and Contact Lens Research Unit in Australia and the Cooperative Research Center for Eye Research and Technology, 93% of patients rated them as "excellent."

It's hard to argue with a happy patient.

Are you aware of new products or technology that have made (or are likely to make) a significant difference in practice? Contact Christopher Kent at kentcx@boucher1.com  to find out about possible coverage in a future issue.