Staying in shape

June 19, 2013

A new contact lens material (nesofilcon A), which combines the best features of conventional hydrogels and silicone hydrogels, offers high water content to combat end-of-day dehydration and resulting discomfort.

 

Patients and clinicians alike know what a lost or discarded contact lens looks like. It shrinks, becomes brittle and crumples, losing its clarity and spherical shape. Loss of water content causes this transformation. So, for a lens to be viable, it must remain hydrated.

Something akin to this dynamic lies at the heart of end-of-day comfort concerns, experts believe, albeit on a much lesser scale. Applied in the morning, a contact lens has its full water content intact. At this time of day, patients report the highest levels of comfort. But, as the hours pass, lenses slowly dehydrate and comfort levels decline, reaching their nadir around dinnertime when many patients opt to remove their lenses and rely on spectacles until turning in for the night.

The big question: What’s the connection between loss of water content and loss of comfort? Current theory-and theoretical it is, because these processes remain difficult to evaluate in vivo-holds that on a much smaller scale the same transformation seen in discarded lenses occurs in worn lenses. That is, dehydration causes the material to begin to crumple up and lose its spherical shape. This very slight loss of optical shape accounts for the dry, pinching, scratchy foreign-body sensation, as well as the blurred vision reported by end-of-day wearers, researchers believe.

The design of a new daily disposable hydrogel contact lens, the Biotrue ONEday (Bausch + Lomb [B + L]), addresses hydration by retaining enough moisture to hold its shape throughout the day, according to the company, which also said the lens is proving to be a hit with patients and practitioners.

Bio-inspiration

Biotrue is made from HyperGel (nesofilcon A), a new material that combines the best features of conventional hydrogels with silicone hydrogels. It offers high water content-78%, the same percentage present in the human cornea-while delivering more oxygen than a traditional hydrogel and without sacrificing hydrogel comfort, according to the company.

While designing the material, B + L scientists looked to the natural model of the human eye; consequently, the terms “bio-inspired” and “bio-mimicry” are often applied to Biotrue ONEday. This is not an uncommon approach in product design. Famously, the inventors of Velcro were inspired by the burrs that stick to clothing after a walk in the woods, for example.

In this case, researchers looked to how the cornea retains moisture. They included a polymer in the material that slowly migrates to the surface of the lens throughout the day, forming a barrier that guards against moisture evaporation, similar to the way the lipid layer of tear film guards against evaporation.

At the same time, the material provides a high level of oxygen transmissibility, with a Dk/t value of 42 at the center of a -3.00 D lens. This will not match the best Dk/t values of silicone hydrogel material, but is far above average for a conventional hydrogel.

Increased late-day comfort stands as the main selling point of this lens, a vital characteristic because comfort remains the number one reason cited by contact lens dropouts. Biotrue has received mostly positive reviews since its entry into the market.

High contentment

“We were using some specialty lenses, such as Safigel 1Day, known for their wetting properties,” said David I. Geffen, OD, FAAO, a private practitioner in San Diego. “We’ve switched most of those patients over to Biotrue. They have been happier than ever because not only do they get the wetter lens, they also get a lens with fantastic optics.”

Paradoxically, previous lenses with high water content tended to become the most dry toward the end of the day, Dr. Geffen noted. “They dehydrated so badly that they became like potato chips on the eye. We had a lot of complaints from patients about these lenses. On top of everything else, because the lenses were so fragile, we were ripping them, too. We would split the lens while pulling it out of the vial,” he said. The high tensile strength of the Biotrue is another virtue appreciated by practitioners and patients.

Dr. Geffen added that he has had positive experiences fitting former contact lens dropouts with Biotrue. “One patient, a 5.00 D myope, was having dry eye problems with moderate SPK across the whole cornea. Even after she discontinued lens wear, the dryness continued,” he said. But, after wearing Biotrue lenses for several days, her corneas healed. “She was having exposure problems, and the lenses acted as a bandage, keeping the moisture in. Now she was telling me how comfortable her eyes were.”

Although Biotrue is the go-to daily disposable for Jason Jedlicka, OD, FAAO, Dr. Jedlicka said it’s not a silver bullet for every patient. “This lens is sturdy, which makes handling nice, and most people do not mind the thickness. But I have had a few patients prefer a thinner lens design when given the option,” said Dr. Jedlicka, in group practice in Edina, MN.

Biotrue might not be the best choice for patients with particularly large or steep corneas, Dr. Jedlicka added. Also, patients will typically require about -0.25 D more than their spectacle refraction calls for, he said. “But, as long as we know that going in, it’s not a problem.”

Aspheric optics

Like other B + L lenses, Biotrue offers what the company calls high-definition vision. This correction strategy is based on the notion that all human eyes contain a small amount of spherical aberration, averaging about +0.15 µm per eye. The company’s proprietary spherical aberration correction algorithms correct for such naturally occurring asymmetry. The result is most noticeable during nighttime vision when glare and halos are reduced. The company says the Biotrue lens goes one step further than its previous products, customizing algorithms for each 0.25 D change in lens power.

“It’s a great attribute and increases vision quality,” said Dr. Geffen. “When you have patients who show 20/20 vision in your office but still have difficult night symptoms, this lens can possibly eliminate some of those problems.”

Daily disposables are now the fastest growing segment of the contact lens market, according to Dr. Geffen. Many eyecare professionals see Biotrue as the perfect workhorse lens for this demographic.

"As long as the patient’s prescription falls within the available parameters, I think a daily disposable option is the best replacement schedule for most patients,” said Joseph P. Shovlin, OD, FAAO, in group practice in Scranton, PA. “For years, we’ve been hearing that the most compatible lens will be the one that best mimics the cornea and supports the tear film. The HyperGel material mimics the water content of the cornea. This product seems to deliver on our wish list, without using silicone, which has its impediments.”ODT

 

 

AUTHOR INFO

Frank Celia is a freelance healthcare writer based in the Philadelphia area. Contact him at frankcelia@aol.com.

 

TAKE-HOME MESSAGE

A new contact lens material (nesofilcon A), which combines the best features of conventional hydrogels and silicone hydrogels, offers high water content to combat end-of-day dehydration and resulting discomfort.