Silicone hydrogel multifocals – Combining two great things to make something even better

Introduced simply as Peanut Butter Cups, and now known as Reese’s Peanut Butter Cups (Hershey Foods Corporation, Hershey, PA), this combination of chocolate and peanut butter has been manufactured for over 80 years.  For many years the advertising campaign for this product revolved around the concept of “two great tastes that taste great together”.  Unless you are an extreme chocolate connoisseur, although a bit unconventional, one could consider the combination of lens material and lens optics introduced in today’s silicone hydrogel multifocal lenses an equally impressive feat, with two rather impressive lens developments that have the potential to work great together to improve comfort and vision for presbyopic contact lens wearers (and chocolate lovers, alike).

Taking a rather simplistic look at it, there are only a few goals that contact lens manufacturers strive to accomplish when designing a contact lens.  In general terms, these are to design a lens that provides the “best” of each of the following attributes: (1) vision, (2) comfort, (3) ocular health and (4) cost.  Although, these goals may in some regards seem rather simple, they are inherently complex, and manufacturers spend millions of dollars trying to define how exactly the term “best” is actually defined, and investigating novel methods to accomplish these desired goals. 

Technological advances over the last decade have led to several advances in lens material and manufacturing technology that have greatly expanded manufacturer’s capabilities to achieve their design goals.  For example, in the late 1990s several spherical silicone hydrogel lenses were initially released (e.g. Bausch and Lomb PureVision and CIBA Night and Day).  Around this same time, multi-axis lathes capable of cutting complex surfaces into lenses, and efficient and accurate molding techniques were developed, aiming to improve not only the optics of contact lenses and resultant vision for the wearer, but also reduce the cost of producing lenses.  With these technological advances, these initial spherical silicone lenses have over the last several years now become the cornerstones for several lines of contact lens products, including spherical, toric, and multifocal lenses.  This article will briefly highlight how the technological feats of combining the enhanced oxygen transmissibility of silicone hydrogel lenses with advanced optical designs have created new silicone hydrogel multifocal lenses now available in today’s marketplace.

When silicone hydrogel lenses were first developed, they were thought to fill somewhat of a niche.  However, in the US today, over half of all patients being fit (or refit) with contact lenses are prescribed a lens composed of a silicone hydrogel material [1].  The superior oxygen transmissibility of silicone hydrogel lenses over conventional hydrogel materials is presumably a major driving factor for this shift in fitting trends.  Much of the market shift is also driven by practitioners wanting to prescribe lenses with the latest technology available, since the market share of patients actually wanting to wear lenses on an extended wear basis has not grown significantly in recent years.
Lens manufacturers have aimed to capitalize on their improved manufacturing capabilities coupled with increased demand for silicone hydrogel lens designs, such that there are currently three silicone hydrogel multifocal lenses available in the marketplace, with more lenses to come.

PureVision Multifocals (Bausch and Lomb, Rochester NY), made with balafilcon A material, were the first silicone hydrogel multifocal lenses on the market.  Launched in the United States in 2006, these lenses incorporate a design roughly comparable to the SofLens Multifocal design.  One unique aspect of this lens design that differentiates it from the SofLens design, however, is that it is built upon an aspheric carrier aimed to help control the induction of spherical aberration.  For example, all spherical lenses contain levels of spherical aberration that vary with lens power [2].  Therefore, generally, high minus lenses contain high levels of negative spherical aberration, and high plus lenses contain high amounts of positive spherical aberration.  However, multifocality is generally accomplished in soft contact lenses by purposefully incorporating additional spherical aberration into a lens design with the goal of increasing the resultant on-eye, eye-plus-lens spherical aberration.  Therefore, in multifocal lenses, it may be possible for the spherical aberration that is typically inherently induced as a function of lens power, to hinder the multifocality effect experienced by the wearer.  In an effort to counteract this, Bausch and Lomb incorporated an aspheric design within the PureVision multifocal (as they have done with the PureVision spherical and toric lenses) that aims to have a more constant level of spherical aberration within the lens regardless of the lens power (e.g. -6.00 D lens aimed to have same levels of spherical aberration as a -1.00 D lens). 

As with SofLens Multifocal, PureVision Multifocal lenses are available in both a low and a high add design, with both designs incorporating a near-center aspheric design surrounded by a wide intermediate zone.  The low add design is intended to be used on patients with a spectacle add of +1.50 D or below, the high add for patients with spectacle adds of +2.00 D and above, and a combination of low and high adds for those patients with a +1.75 D add.  Similar to all lenses within the PureVision line, the lens is designed to have a monthly replacement schedule, and is approved for up to 30 days of continuous wear.

A few years later in early 2009, Vistakon (Jacksonville, FL) released its newest presbyopic lens, ACUVUE Oasys for Presbyopia.  Similar to the other lenses within the Oasys line, this lens is made of senofilcon A material.  Besides the material, there are several innovative design changes in the Oasys for Presbyopia lens that distinguish it from its conventional hydrogel predecessor, the ACUVUE Bifocal lens.  Whereas the ACUVUE Bifocal was a distance center alternating-ring, concentric design, the Oasys for Presbyopia lens adds asphericity to the lens back surface and within each individual concentric ring.  It is available in a low- (up to +1.50 D), mid- (to +1.75 D), and high- (+2.00 D and higher) add design.  Vistakon calls their multifocal design a “stereo precision technology”, designed to “work with the eye’s natural depth of focus”.  Similar to the rest of the ACUVUE line, this lens has a designed two-week replacement schedule, and contains HYDRACLEAR® Plus technology aimed to provide enhanced lens wettability.

Around this same time in early 2009, Ciba Vision (Duluth, GA) launched a new multifocal of their own within their Air Optix Aqua line of lenses.  Comparable to what other major lens manufacturers have done, Ciba Vision also made significant design changes in addition to a material change to their Air Optix Aqua Multifocal lenses, compared to their previous conventional hydrogel multifocal design, the Focus Progressive.  The Air Optix Aqua Multifocal lenses are of a bi-aspheric near center design available in three add powers:  Lo (up to +1.00 D), Med (+1.25 - +1.50 D), Hi (+2.25 and up).  Ciba Vision calls this a “Precision Transition” design, which aims to control the aberration power within the lens in order to extend the depth of focus for the wearer.  As with other Air Optix Aqua , these lenses utilize lotrafilcon B material and incorporate a moisturizing agent within the packaging solution, as well as a surface treatment to enhance comfort.  The lenses are intended to be worn on a monthly replacement schedule, and are approved for up to six nights of extended wear.

Similar to the other major contact lens manufacturers, CooperVision (Rochester, NY) also plans to release a silicone hydrogel multifocal within its Biofinity family of lenses in the summer of 2010.  This lens will be similar in parameters and wearing schedule to the currently available Biofinity spherical lenses, and will have a “Balanced Progressive” multifocal design similar to the currently available Proclear Multifocal lens series. 

Table 1 summarizes the detailed parameters of all the four lenses we have discussed, please click here to view Table 1.

In closing, I must admit that I am actually not a chocolate lover myself.  However, there are presumably many people around the world that do enjoy Reese’s Peanut Butter Cups.  Although little change has been made to the Reese’s Peanut Butter Cup over the last 80 years, given the changes that have occurred over the last 80 years with contact lenses, and given the ingenuity of today’s manufacturers combined with the steadily increasing demand by the contact lens wearing public, I am curious to see what new developments occur with contact lens corrections over the next 80 years.  I think the silicone hydrogel multifocal lenses currently available today are a great start to a promising future.

References

1. Nichols, JJ. Annual Report: Contact Lenses 2008, Contact Lens Spectrum, January 2009
2. Kollbaum PS, Jansen ME, Bradley A, Thibos LN.  (2008) Validation of an off-eye contact lens Shack-Hartmann wavefront aberrometer. Optom vis Sci (85)9: E817:E828.