Article Date: 3/1/2004

Finding the Comfort Zone
WITH THE NEWEST SILICONE HYDROGEL TECHNOLOGY
VISTAKON®'s new lens combines the best qualities of traditional hydrogels and silicone hydrogels.
BY ROBERT B. STEFFEN, O.D., M.S., and KEVIN P. McCABE, B.S.

Patients' number-one priority when being fit with a new contact lens is comfort. ACUVUE® ADVANCE™ Brand Contact Lenses with HYDRACLEAR™ deliver the long-lasting comfort patients desire while meeting their most critical clinical needs. In surveys, large majorities of patients report their overall vision, comfort, lens handling and the whiteness of their eyes are "excellent" or "very good" with this lens (see Fig. 1, "Exceptional Long-lasting Comfort" and Fig. 2, "Critical Needs").

ACUVUE® ADVANCE™ with HYDRACLEAR™ represents the first successful marriage of the advantages of traditional hydrogels and those of silicone hydrogels, yet without their negative characteristics. ACUVUE® ADVANCE™ lenses provide the fit and feel of traditional hydrogels with the oxygen-rich nature of silicone hydrogels owing to a Dk/t of 85.

This article will provide insights into the technological innovations that make ACUVUE® ADVANCE™ lenses with HYDRACLEAR™ a preferred option for patients looking for healthy, comfortable contact lens wear.

TRADITIONAL VS. SILICONE HYDROGELS

Because ACUVUE® ADVANCE™ lenses bring together many of the qualities of traditional hydrogels and silicone hydrogels, the pros and cons of the two modalities are worth noting. (See Fig. 3, "Pros and Cons of Traditional and Silicone Hydrogels.")

Among the advantages of traditional hydrogel contact lenses are their soft, flexible material, their hydrophilic nature and their affordability. A frequent replacement schedule curtails the incidence of complications. After many years of experience, traditional hydrogels rest on a proven track record.

Traditional hydrogels have their downsides. One is their low Dk/t, limited oxygen transmissibility. Also, traditional hydrogels are prone to dehydration. Patients tend to experience dryness toward the end of the day.

As for current silicone hydrogels (lotrafilcon A and balafilcon A), their advantages include a higher Dk/t with less dehydration. Plus, silicone hydrogels collect fewer protein deposits.

Yet, these silicone hydrogels have their disadvantages as well. Their higher modulus, or stiffness, tends to cause an increased risk of contact lens-related papillary conjunctivitis (CLPC) and superior epithelial arcuate lesions (SEALs). Also linked to increased modulus are the development of mucin balls and an initial adaptation period. Some silicone hydrogels require surface coating or treatment for wettability, which is one factor that makes them more costly. In addition, they tend to invite increased lipid deposits.

HYDRACLEAR's™ BENEFITS

HYDRACLEAR™ technology is VISTAKON's® brand name for what distinguishes ACUVUE® ADVANCE™ lenses from other brands and makes possible their beneficial qualities.

HYDRACLEAR's™ chemical component comes from the polyvinyl pyrrolidone (PVP) family. But HYDRACLEAR™ is more than just a chemical compound. Its technology is threefold:

HYDRACLEAR™ functions both as a humectant (moisture-loving agent) and a lubricant. Its humectant nature provides the capacity to bind moisture and presents a wettable lens to the ocular surface. Therefore, HYDRACLEAR™ makes it possible to incorporate silicone to increase oxygen transmissibility without sacrificing the lens's wettability. (See Fig. 4, "Wettability -- Advancing Dynamic Contact Angle.")

Humectant characteristics can be illustrated by the lens's wettability or dynamic contact angle. Wettability refers to the lens's wetting effect when it comes in contact with a liquid. The dynamic contact angle is a measure of a lens's wettability expressed in degrees.

COEFFICIENT OF FRICTION

A lens's coefficient of friction is a measure of its lubricant properties. ACUVUE® ADVANCE's™ coefficient of friction measurements represent the amount of friction created at the lens surface from a load roughly equal to eyelid forces and oscillated at frequencies to mimic the normal blink. The values in Fig. 5, "Coefficient of Friction -- Lubricity," have been adjusted for material deformation and the fluid interface due to measuring a wet lens. HYDRACLEAR's™ lubricant qualities result in a soft, flexible feel in the hand and the initial comfort and fitting characteristics of today's best hydrogel lenses. There's no need to change fitting habits or ask patients to adapt to the lens's initial comfort.

OXYGEN TRANSMISSIBILITY -- OCULAR REDNESS

A clear benefit of ACUVUE® ADVANCE™ lenses is the increased oxygen transmission vs. traditional hydrogels lenses. ACUVUE® ADVANCE™ lenses deliver three times the oxygen of the leading hydrogel brand. ACUVUE® ADVANCE's™ oxygen transmissibility exceeds the Holden-Mertz criteria based on corneal swelling during daily wear (Dk/t = 24) and the Harvitt-Bonnano criteria based on corneal acidosis during open-eye wear (Dk/t = 35) at all points on the lens. (See Fig. 6, "Oxygen Transmissibility Data.")

A clinical study of 15 new lens wearers using ACUVUE® ADVANCE™ lenses showed that bulbar redness and limbal hyperemia were virtually unchanged from the pre-wear appearance over a month with 2-week replacement. (See Figs. 7 and 8, "Redness examples.") ACUVUE® ADVANCE™ will allow patients' eyes to be as white as wearing no lens at all.

MECHANICAL PROPERTIES

The mechanical properties of current silicone hydrogels (lotrafilcon A and balafilcon A) differ from those of traditional hydrogels. These silicone hydrogels have been associated with an increased risk of SEALs, mucin ball formation and CLPC.

MODULUS DATA

An additional benefit of ACUVUE® ADVANCE™ lenses is their lower modulus. (See Fig. 9, "Modulus Data.") Modulus is a material's stress divided by its strain -- or how much a material gives for each pound of load applied. Traditional hydrogels typically have a very low modulus, on the order of 0.20 to 0.50 megapascal (MPa). ACUVUE® ADVANCE's™ modulus is similar to that of traditional hydrogels. VISTAKON® achieved this lower modulus through proprietary technology involving the selection and processing of appropriate monomers.

UV BLOCKING

ACUVUE® ADVANCE™ lenses block more than 90% of UVA and 99% of UVB rays. (See Fig. 10, "UV-blocking Data.") This marks the first time a hydrogel lens is capable of blocking UV radiation to this extent. It also means ACUVUE® ADVANCE™ lenses are the first soft contact lenses to meet the ANSI and ISO standards for Class 1 UV protection.

A PROVEN DESIGN

A final benefit of ACUVUE® ADVANCE™ is its design, based on the proven performance of ACUVUE® 2 Brand Contact Lenses. This design, combined with HYDRACLEAR's™ properties and other factors such as the silicone's oxygen permeability, the lens's low modulus and its UV blocking, illustrate that ACUVUE® ADVANCE™ brings together the best of both worlds of traditional and silicone hydrogel contact lenses.

Robert B. Steffen, O.D., M.S., is a senior research optometrist for VISTAKON® Materials R&D. Kevin P. McCabe is an associate engineer for VISTAKON® Materials R&D.

 

 

ACUVUE® ADVANCE'S™ PRODUCT SPECIFICATIONS

 

THE ACUVUE® ADVANCE™ BRAND CONTACT LENS WITH HYDRACLEAR® IS MADE OF GALYFILCON A, A 47% WATER-CONTENT MATERIAL CLASSIFIED AS GROUP 1, LOW WATER AND NON-IONIC. (SEE FIG. 11 FOR MORE DETAILS.) HYDRACLEAR® IS THE INTERNAL WETTING AGENT.

THE ACUVUE® ADVANCE™ LENS COMES IN BASE CURVES OF 8.3 MM AND 8.7 MM, WITH A 14.0-MM DIAMETER AND 0.070-MM CENTER THICKNESS IN ITS MINUS POWERS. ITS POWERS RANGE FROM +4.00D TO -6.00D (IN 0.25D INCREMENTS).

THE LENS'S OXYGEN PERFORMANCE IS EXPRESSED AS A DK OF 60 (EDGE-CORRECTED) AND A DK/T OF 85. THE LENS BLOCKS GREATER THAN 90% OF UVA AND GREATER THAN 99% OF UVB. IT COMES WITH AN INSIDE-OUT MARK OF 1-2-3 AND FEATURES A LIGHT-BLUE VISIBILITY TINT.

DATA ON FILE WITH VISTAKON®.

 

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Contact Lens Spectrum, Issue: March 2004