Most eye care professionals will agree that the introduction of mass-produced silicone hydrogel contact lenses has been one of the most important developments for contact lenses in the past 30 years. According to Robert W. Baird & Co., an equity research firm, silicone hydrogel contact lenses are expected to account for two thirds of total US contact lens sales by 2009. The rapid growth of this category of contact lenses, at the expense of conventional hydrogels, indicates that more and more prescribing eyecare professionals are understanding and accepting the value proposition these products bring to the profession, and more importantly, your practice and patients.

Understanding the unique properties and characteristics that this new generation of contact lenses has can better prepare contact lens fitters and dispensers when answering patient questions. When discussing silicone hydrogel contact lenses with new wearers (or potential wearers), it is important to be able to explain how these lenses differ from conventional hydrogels. While most patients don’t need or won’t want a detailed biochemistry lesson, being able to discuss the properties of this new class of lenses in terms that relate directly to the patient wearing experience (plain English) will go a long way in setting appropriate expectations and enhancing your role as a trained eyecare professional (trusted advisor).

Oxygen Transmissibility

Conventional hydrogel lenses rely primarily on the amount of water in the polymer to regulate the amount of oxygen that can pass through the lens. The higher the water content, the more oxygen can be transmitted to the cornea through the lens.

However, for conventional hydrogels, higher water content translates to a lens that is less durable (rips and tears more easily) and is subject to greater on-eye dehydration (the water in the lens evaporates). And even the highest water-content conventional hydrogels don’t approach the Dk/t values (oxygen transmissibility) that are available with silicone hydrogels.

Silicone hydrogels, because of the inherent properties of the silicone molecules, allow more oxygen to permeate the lens, resulting in fewer hypoxia-related (the depravation of adequate oxygen supply) complications compared to conventional hydrogel contact lenses. In fact, many silicone hydrogel contacts allow sufficient oxygen to the cornea to approximate the corneal oxygen levels of non-contact lens wearers.

Modulus

The inclusion of silicone into the contact lens polymer mix has had an impact on lens modulus: a measure of rigidity or flexibility. First generation silicone hydrogel lenses (such as CIBA Vision’s Night & Day contact lenses) generally have a higher modulus than their high water content conventional hydrogel predecessors. This results in a lens that feels stiffer on the eye. For patients who are transitioning from a low modulus lens to a silicone hydrogel, there may be an extended adaptation period (expect lens awareness to be more pronounced).

Managing patient expectations here is critical. Most patients can successfully adapt to a higher modulus lens if they gradually increase their wearing time over a two week period (wearers will get used to the new sensation and the awareness will diminish over time). And for many patients this may not even be an issue (particularly if they have not worn contact lenses in the past and do not have a “comfort” reference point to compare the new experience to). If stiffness is an issue, consider the Acuvue Advance or the Acuvue Oasys lenses as they have the lowest modulus of the silicone hydrogels currently available.

Lens Deposits

Experience has shown that silicone hydrogel contact lenses have a greater propensity to attract and collect lipid deposits. Lipid deposits on a lens can reduce the lens’s refractive ability resulting in reduced visual acuity for the wearer. Lipid deposits can also affect on-eye comfort. More important, lipid deposits can result in pathologically adverse events (though these are more common in patients who are indicated for 30-day continuous wear). As a practitioner, it is important to teach silicone hydrogel wearers (all contact lens wearers) the importance of lens cleaning and maintenance.

It is good practice to instruct daily-wearers of silicone hydrogel contacts to place the contact in the palm of their hand and rinse with a liberal amount of solution and, with the tip of their finger, rub the lens (in a back and forth motion) against the palm of their hand for 15-20 seconds. This may be standard operating procedure in your practice, but we all know that most contact lens wearers fall somewhere short of fulfilling this requirement. Talk to your patients about lipid deposits and stress the importance of rubbing the lenses.

Hydrophobia

Silicone is a hydrophobic material. This means it repels water. This is generally not a good thing for contact lenses. To counter the hydrophobic nature if silicone, lens manufacturers have developed various ways to make silicone hydrogel lenses more hydrophilic (water loving).

Silicone hydrogel lenses made by Bausch & Lomb and CIBA Vision use a plasma treatment on the surface of the lens to create a hydrophilic surface that masks the hydrophobic nature if the silicone. Vistakon (J&J) attacked the problem differently. They created a proprietary molecule, called “Hydroclear”. The Hydroclear is added to the silicone, and it binds to the hydrophobic tail and replaces it with a hydrophilic (water-loving) one. Hydroclear was first introduced on the Acuvue Advance and the amount of Hydroclear was doubled for the launch of Acuvue Oasys. I will not opine on which approach is better in this article; suffice it to say that each approach has its pros and cons.